PROBLEMS  OF  PHOTOGRAPHIC  IMAGERY  ANALYSIS 
./ 
^^v*S5P#J£^re$!?»«^^ 
hilalV<&>'  -»!•'>  ^>.*W  -<^iJ;.  • 
26  February  1968 
7?i^     O^7/ 
yg *?y>   '•-  T».-<»?«<V» Ti-,?an*>»^»*»i5*so^-1jWT»  si^fjm^Ki 
»•  •"«■•««?  flSv-V.'-*5  r-: 
.^v«v^wj»<«^.--.>i?.SLiivi;A?^VwJV-y^~:ii^ 
■P»T^i1w»W|^^^i»^Wi»fpi 
■^»V  >»«'  f
PK.    -EMS  OF  PHOTOGRAPHIC  IMAGERY  A'^  \YSIS 
r 
INTRODUCTION    -  $ 
SECTION  I:     PRACTICAL  PROBLEMS  -S 
A. 
-  C. 
0. 
SECTION 
A. 
Imagery  Interpretation 
1.  Small  Objects  Near  the  Limits  of  Imagery  -7  _ 
2.  Fine  Detail  Beyond  the  Range  of  Human  Vision  «/2L 
3.  Imagery  with  Weak  Photo  Contrasts  -  /£ 
4.  Imagery  of  Highly  Reflective  Surfaces  -/? 
5.  Problems  of  Orientation  and  Viewing  Angle  -•** 
Performance  5kl  1 1  -2/        "   .. 
Communication  -21. ,.  .  _  ~    •..'._. 
Summary  -  3  '    *"  * 
II:  BASIC  IMAGERY  RESEARCH-  3  2 
The  Film  v- 33  . 
1.  Composition  and  Characteristics  -.33       .  # 
2,  Processing  Techniques   •?£ 
3*    Resolving  Power  and  Ground  Resolution-?? 
.^-«..-_ 
B; 
C. 
D. 
E. 
F. 
Field  Conditions  -4? 
I.  The  Nature  of  Light  -jf 
*  2.  Light  In  the  Atmosphere  -#7     . 
3,  Reflection  and  Imagery-./? 
Vision  -SZ. 
1.  Its  Relation  to  Light  -SX  » 
2.  Visual  Acuity  -S4-° 
3.  Stereoscopic  Vision  -SL 
Characteristics  of  the  Imagery-iV 
•  - 1  •  -  Tone  -  T% 
2.  Brightness  -  id 
3.  Surround  -  £  2-  "  %  • 
4.  Sharpness  of  Definition  -44    ) 
5.  Contrast    -66  l 
6.  Lighting  Defects  -«? 
Object  Recognition  and  Identification  -7-2- 
Summary*  71  . 
<£> 
-<»<"V--: 
:.::,W-../'^.:v-
(  FIGURES 
n 
1  -  Imagery  of  Roads  and  Wireline  Poles  and  Shadows 
2  -  Mlcroderislty  Trace  Across  Assault  Guns  and  Tanks 
3  -  Isodenslty  Tracing  of  Assault  Guns 
4  -  Imagery  Revealing 'Power  Cables'  .  : 
5a  -  Absorption,  Transmission,  and  Reflection  of  Light  Rays  Striking  a 
Transparent  Surface       ■■['.■    *  •,  .-;..•>..  *  -  '■/.'•  ~  {.-.■ 
5b  -  Diffuse  Reflection  from  Surface"  Which  Is  Irregular  Compared  with 
Dimensions  of  Light  Waves 
*        *  * 
5c  -  Specular  or  Regular  Reflection  ..." 
.-•.■•.•■•     •  ■■ 
■       •  *  .     ■  *     - 
6  -  Love  Field,  Dallas,  Texas,  V.  .   <:,..,..„.   y..„:ii*.ifS 
7  -Misshaped  images  of  Miss 1 1 esT   ''..'.-"  V 
8  -  Typical  Characteristic  Curve     ^ 
9  -  Gamma-Density  Relationship  "V  -. 
10a  -  Diffraction  Pattern  Produced  by  Perfect  Lens 
I  Ob  -  Resolution  Between  Two  Close  Images 
II  -  Wavelength-frequency  Relationship  for  the  Visible  and  Adjacent  Spectrums 
for  Light  Traveling  In  Free  Space 
12  -  Spectral  Sensitivity  of  the  Normal  Human  Eye 
13  -  Relative  Visibility  of  the  Normal  Human  Eye  at  Normal  and  Greatly 
Reduced  Light  Levels,  Showing  the  Purklnje  Shift  -"  
. 
14  -  Relationship  Between  a  Ray  Incident  on  a  Plane  Surface  and  the  Reflected 
and  Refracted  Rays  Which  Result 
15  -  Imagery  of  Ralls  and  Ties  on  Railroad  Bed 
16  -  Missile  Site  and  Support  Area  £ 'J.  '  J..,..      .  i^)\ 
17  -  Diagram  Illustrating  the  Difference  Between  Irradiation  and  Halation  \J^y 
18  -  Imagery  of  the  Edge  of  a  Razor  Blade  from  Fine  and^Co^rse  Grained  Emulsions
PRO(  MS  OF  PHOTOGRAPHIC  IMAGERY  ANAPCSIS 
•INTRODUCTION 
.The  purpose  of  aerial  photography  Is  to  bring  Into  the  laboratory  for 
study  *.nd  evaluation  the  most  revealing  and  Informative  coverage  of  specific 
locations  on  the  earth's  surface  upon  which  nature  or  man  has  placed  some- 
thing of  Interest  to  the  analyst.  Just  how  revealing  and  Informative  the 
coverage  may  be,  and  how  satisfactory  the  study  and  evaluation,  depend  on 
the  capability  of  the  series  of  media  which  comprise  the  photographic  process 
>  to  transmit  the  appearance  of  the  object  on  the  ground  to  the  brain  of  the 
-laboratory  analyst.  The  series  of  transmitting  services  consist  of  light 
or  radiant  energy,  the  optical  properties  of  the  atmosphere,  the  optical 
system  of  the  camera,  the  emulsion,  and  the  reproductive  quality  of  the 
photographic  film,  as  well  as  the  reproducing  processing,  the  optical 
Instruments  used  In  laboratory  study,  and  the  capability  of  the  human  eye 
to  recognize  details  of  the  object's  Image  on  the  resulting  photography* 
Each  of  the  transmitting  services  has  discrete  characteristics.  Each,  In  '-'" 
turn;  degrades  the  Image,  detracting  from  Its  visual  clarity,  modifying  Its 
'tone,  masking  Its  composition,  or  otherwise  making  the  Image  Jt- transmits 
something  less  true  and  less  Intelligible  than  the  Image  It  received.  The  . 
Job  of  the  photo  analyst  Is  to  read  as  completely  as  possible  the  nature 
.  and  significance  of  the  object.  His  ablWty  to  do  so  Is  largely  a  measure 
*of  his  training  and  experience  In  his  trade.  The  capability  with  which  he 
explains  to  his  audience  what  he  perceives  and  also  the  reasons  for  limitation 
"of  his  perception  depend  In  no  small  fraction  on  the  analyst's  understanding 
of  the  basic  physical  phenomena  that  determine  the  nature  of  the  photo  Image. 
Only  by  Judiciously  evaluating  tone  variations  oh  the  photography  does  the 
.analyst  detect  an  image  In  the  first  place.  Only  by  expertly  employing 
effective  laboratory  equipment  can  he  distinguish  as  fully  as  possible  the 
complex  of  these  tone  variations,  and  only  by  his  ability  to  distinguish  the 
causes  and  values  of  these  variations  does  he  ciseate  the  mental  image  and  the 
object  description. 
Photographically  the  object  of  Interest  Is  an  Inert  and  passive  thing. 
In  order  to  be  Identifiable  It  must  exhibit  material  aspects  such  as. size, 
.  shape,  scalar  values,  and  composition.  An  object  Is  recognized  because  It 
'"  has  a  different  tone  or  brightness  from  Its  surroundings  and  because  of  the 
'  variations  of  brightness  and  contrast  over  Its  surface.  It  may  be  associated 
with  other  objects',  re  I  ated  to  Its  function;  Its.  surround  tngs-  may  :be.  re  levant ;  _ 
and  the  light  on  It  may  or  may  not  be  advantageous  to  Its  Identification. 
Whatever  the  Inheren*  characteristics  of  the  object  and  its  locale,  the  -    
Important  aspect  of  the  photography  Is  the  quality  of  the  Image  which  light. 
H?/iA//  r/ze-d   c  #/°y  *
••."•■    . «-,   .       •:    .  V  /   . 
.  •■■■*.  *" 
• 
failing  upon  the  object,  carries  skyward  by  means  of  Individual  reflected 
light- rays.  These  In  turn  are  transported  through  the  atmosphere  to  the 
aerial  camera,  which  Is  some  distance  overhead..  If  a  desirable  amount  of 
light  Is  reflected  from  the  object  through  a  clear  atmosphere,  it  can  be 
expected  that  a  high  quality  Image  will  reach  the  camera.  The  camera  lens 
'  and  body  may  adversely  affect  the  Image  falling  upon  the  film,  which  Is  able 
-  to  capture  the  Image  only  within  the  limits  of  Its  composition  and  purpose. 
In  the  process  of  laboratory  reproduction  to  positive  photography,  certain 
features  of  the  Image  may  be  Ideal ly  reproduced  at  the  expense  of  other 
jx>rtIons  of  the  total  Image.  In  the  human  eye  the  visual  process  may  be 
"  Insufficient  for  detecting  all  the  details  carried  thus  far  on  the  image. 
The  fir.ai  proiuct  tezszis  en  tre  analyst's  ability  to  cc-runlcate  what  he 
•  . 
It  Is  the  purpose  of  this  report  to  consider.  In  t*c  p*r!-s,  the 
photographic  process,  from  the  object  on  the  earth's  surface  to  its  descrip- 
tion In  the  analysts  report,  with  the  Intention  of  Identifying  the  physical 
and  optical  properties  of  each  transmitting  service  and  the  functions  of 
each  which  limit  the  final  quality  of  the  Image  and  Its  identification  and 
evaluation.  Consideration  will  be  restricted  to  black-and-white,  or 
monochromatic,  duplicate  positive  transparency,  which  Is  the  common  media 
subject  to  direct  laboratory  study  arid  analysis. 
\ 
I 
•**•«■»■-*,.  «-  1—  »J~lV»v  -*—  *■ 
. ,  **.  *y,  ■ 
£''.•»>.  *;-,  ~i&ili''*r-?x*'?x\St
SECTION   I 
PRACTICAL  PROBLEMS     £>  F     Z*/J> 
IMAGERY  INTERPRETATION 
«  ;<«.  .  ••. 
'  There  Is  an  endless  variety  of  examples  to  which  one  can  turn  to 
Illustrate  the  business  of  reading,  analyzing,  presenting  and  communicating  . 
'  the  substance  that  photographic  imagery  has  to  offer  In  support  of  man's  .    
world-wide  endeavor  to  know  what  is  there,  what  is  happening,  and  what  may 
"be  the  next  situation  to  be  considered  in  any  locality.  .  The  task  Involved 
Is  simply  ability  to  assess  the  site  and  prepare  whatever  communication  Is 
expected  of  the  analyst.  Difficulties  arise  when  coverage  is  poor  or  of 
small  scale,  for  that  is  when  almost  anything  becomes  a  sma 1 1  object.  •  In ■"---.- 
other  words,  the  mere  term  "small,  object"  Is  a  comparative  concept, .and  -V  "  — 
the  practical  problems  Involved  depend  on  what  is  expected  in  the. way  of     ". 
analysis  and  reportage.  .  ■  ..;.. 
\            ""**'        
The  Imagery  of  small  objects,  in  the  total  concept  In  which  the  term 
.  wasorlglnal  ly  expected  to  be  handled  In  this  project,  can  be  separated  Into"  ~~ 
'five  classes,  each  of  which  requires  a  background  knowledge  different  from 
.  that  needed  for  the  others.  -First,  there  are  those  which  are  real ly  small"" :  _:* 
'.on  the  coverage,  which  are  near  the  limits  of  human  vision..  .There,  are  those'  "  ~" 
"whose  finest  detail  is  beyond  the  range  of  human  vision  but  Is  detectible ..-   
..  In.  brightness  differences  that  can  be  mechanically  or  electronically  .  .-._;,.--•.- 
"distinguished.  There-are  those  which  are  4arge  enough  to-see-but  which  are —  . 
too  weak  in  contrasts  for  visibility  of  pertinent  details.  There  are  images 
."with  highly  reflective  surfaces  which  are  Ilghtstruck  and  therefore  distorted i 
And  lastly,  there  are  the  endless  variations  of  camera  angle,  sun  or  lighting 
angle,  and  target  orientation,  providing  an  image  for  which  the  analyst  must 
Imaginatively  fill  in  the  Invisible  detail  andVeconci  le  its  concept,  some- 
.  times  in  order  to  prove  or  disprove  one  or  another-of  alternate  choices  in   
Identity.  Each  of  these  classes  of  image  variations  will  be  discussed  In 
turn  to  elaborate  background,  essential  factors,  and  possible  points  of  ": 
departure  In  acquiring  solutions,  or,  lacking  them,  a  limited  but  reasonable 
understanding  of  the  insoiution.  Altogether  they  require  a  thorough  tinder-"  -■" 
;  standing  of  the  mechanical,  aspects  of  vision  and  optics,  of  photography  and-  —  - 
"  film,  of  Imagery  and  analysis,  and  of  geometries  and  complexity -of -photography 
* 
-  Thus  far  we  have  followed  the  analyst  in  deriving  the  information  from 
the  Imagery,  In  the  acquisition  of  knowledge.  At  this  point  his  job  is  only 
half  done.  His  communication  of  that  knowledge  to  others  is  still  ahead.
The  effectiveness  with  which  the  commun frcation  task  as  a  whole  is  visualized 
often*  determines  whether  the  new  knowledge  catches  on.  In  common  practice 
communication  Is  a  team  effort,  and  the  analyst  may  find  that,  for  several 
reasons,  his' work  as  an  analyst  Is  communicated  by  others  who  have  made  the 
effort  to  develop  communication  as  a  skill.  There  Is  no  objection  to  this 
type  of  teamwork,  except  that  no  one  else  can  transmit  the  analyst's  ideas. 
as  well  as  he  might  with  a  little  more  awareness  and  effort.  Teamwork  with 
graphic  and  mensuration  specialists  Is  often  desirable.  The  real  problem 
here  Is  one  of  knowing  the  tools  of  communication,  reasoned  analysis, 
succinctly  stated  ideas,  an  apt  vocabulary,  and  a  perspective  of  the  greater 
concept  Into  which  his  contribution  is  to  fit.  The  opportunity  to  improve 
In  the  role  of  communicator  Is  a  real  live  Issue  for  the  Imagery  analyst. 
Exploitation  of  each  of  these  limitations  In  analysis  and  communications 
must  concern  both  practical  and  theoretical  approaches.  We  will  first-address 
ourselves  to  the  practical  problems.  These  are  briefly  summarized  in  the 
first  section  of  this  report,  and  the  broader  background  presented  In  the 
subsequent  section.  The  Individual  who  Is  interested  only  In  the  reasons 
for  the  difficulties  need  not  go  beyond  the  first  section.  For  those  who 
are  Interested  tn  fully  understanding  photoanalysis  It  is  suggested  that  the 
.latter  section  will  be  the  more  interesting  and  informative. 
'  .-.   ■  .  .  .  •  .:   .-  •..».---"-. 
■dj 
v
o 
.  •   •      •         .  ■      •  •  .#         •  _  _  
•• 
Small  Objects  Near  the  Limits  of  Imagery  * 
t 
The  human  eye  Is  very  sensitive  to  light.  It  has  been  said  that  the 
mechanical  energy  produced  by  a  pea  falling  from  the  height  of  an  inch  would, 
If  transformed  Into  luminous  energy,  be  sufficient  to  give  a  faint  impression 
of  light.  Either  too  little  or  too  much  light  can  destroy  an  Image. fa I J I ng 
upon  the  eye.  It  Is  only  within  certain  visual  limits,  and  within  the  visible 
-wavelengths  of  light,  that  an  Image  Is  satisfactorily  registered  In  the  retina' 
of  the  eye  and  that  real  seeing  occurs.  But  there  are  several,  other  involvements 
:*  In  reading  photography.  "*         .  . 
•  ••---  There  Is  a  limitation  of  object  size,  or  visual  acuity..  Standard  visual-   
acuity  Is  defined  as  the  ability  to  see  an  object  which  subtends  only  one    
minute  of  arcat-.the.eye,  such  objects  being  white  on  black,  or  black -on  white  .- 
background..  As  this  has  been-appl  led  to-photography-to-xepresent^.ts-resolvlng 
power.  It  has  come  "to  Imply  that  the  ground  resolution  figure  would"  be'  a"      
criterion  of  what  can  or  cannot  be  seen  on  a  photograph.  But  this -Is  not   ^_-™ 
-the-case.  Imagery- on  photography  deviates -from -this -standard -in -two  distinct-  -- 
directions.  First,  very  few  Images  on  photography  come  from  sharply  black-  _1;  
and-white  contrasted  objects  as£~  Y'\resolut ion -targets  are*.  -Theoretically, 
-«s  such  contrast  is  decreased  the  unit  Image  loses  def InitIon,-making  It  less-  •  ■'•  - 
clear  and  requiring  that  the  size  of.,  the  object,  be ••! ncreased_In.order . that   ■.   -. 
It  be  visible.  Secondly,  the  camera  often  registers  Images  of  objects  that 
,   are  nowhere  near  as  large  as  the  ground  resolution  figure  indicates  they  must 
.  be  In  order  to  be" .definable  unit  Images. .Tor. e^mpie^.Jll  .has  been  po"ssIJbJjg^_ili_..i 
over  a  period  of  years,  to  distinguish  on  some  ^"*"\^  * '"'""'^whether  or  not 
there  are  rails  on  railroad  track  beds.  -It-Is  often  found  possible  to  see  - 
•perimeter  fence  rows*   —  ■••  —    /and  median  strips  on  highways,  and- : •  •-=-i--~ 
"these  dlmensions\         w   „._   ;  ~"  ~.  .  .'".."       •■-—•---._   
have  no  comparison  to  a  minute  of  arc  at  the  eye.  in  imagery  analysis  visual 
•detection  reaches  beyond  the  mechanically  verifiable.--  In-order-to  understand — r  - 
this  we  need  to  know  how  an  Image  becomes  visible  in  the  first  place,  and—  -rzrr„.; : 
how  we  actually  see  It. 
— The  photographic  "system  Is  made  up  of  two  imagery. transmitting- components,-. — 
the  lens  or  camera  system  and  the  film.  It- Is  evident  that  for  whatever  Image 
Is  to  be  captured  by  the  film  the  representative  light  rays  must  first  pass  * 
through  the  lens  and  then  affect  the  film.  In  each  case  with  sufficient  clarity 
to  register  distinguishable  differences  In  contrast  and  resolution.  If  an 
inferior  lens  is  used  the  imagery  will  be  poor ^regardless  of  the  quality  of 
the  film,  and  vice  versa.  If  both  are  of  high  quality  we  get- high  quality 
Imagery,  and  under  certain  circumstances  we  get- imagery  that -far  surpasses 
the  expected  ground  resolution.  Here  we  should  say  not  onI.y_why,.J>ut  also.   _  '. 
*hy  not.  •  Why  not  get  such  imagery  as  an  expectable  thing?.  If  we  can  see 
—,  n ■■r-**   •■   *•■   ••  —•!-•■-•  .4-  «»-  -»-_»■;  »  •.»  -%^  jn#» 
:i.#*<4»,\.>  &  ■>, :  -•    :-  ■  '■■■■   ...  ..'?/•.;...•.•■*•*  -j  ,  ■  ■-».•^1A■'^.•>•Wi^>?alW•v■V^^^*v''~w■•««'J•■..•.l*'♦''■'«'•■
•  •  •  . 
telephone  poles  on  the  desert,  cables  at  a  power  facility,  and  the  shadows 
of  fence  posts  {  _  .1  /  .'I:^L7  none  °*  +h«se  a  foot  In  diameter,  why 
do  we  not  see  them  everywhere? 
A  second  important  question  needing  clarification  here  is  the  manner 
In  which  we  see  an  image,  which  in  essence  means  how  the  mind  handles  the'. 
Image  transmitted  through  the  eye.  Regardless  of  the  appearance  of  similarity, 
the  eye-mind  system  is  only  partly  analogous  to  the  lens-film  system.  The 
latter  Is  purely  objective.  It  registers  whatever  comes  through  to  the  film, 
the  clear  line  and  the  blurred,  making  no  rejection  and  no  analysis.  Not 
so  with  the  eye-mind  system.  The  mind  tries  to  make  sense  of  whatever  ft 
attempts  to  see,  often  rejecting  and  requiring  a  second  look,  and  possibly 
misinterpreting  the  signal.  Furthermore,  in  Imagery  analysis  the  mind  Is 
not  dealing  with  a  primary  image,  but  rather  the  image  of  an  Image,  for  which 
It  has  to  fill  in  the  gaps  and  supply  reason  In  order  to  substantiate  or 
•  Identify  an  object.    •  '•  '  "  **  '"" 
With  small  objects  the  problem  is  one  of  seeing  and  identifying  rather 
than  measuring.  The  sizes  of  objects  on  a  single  photograph?  are  not 
'"necessarily  comparable.  'We  may  "take  as**an  example  a  view  of  wire' lines"   
beside  roads  in  a  desert  area  (Figure  I).  The  roads  are  for  tracked  vehicles, 
the  poles  the  simple  vertical  implanted  pole  with  cross  arms  and  Insulators. 
"fe^ll"?: "'i  rrif'' ■'■  '"'■'.'.'.'.. *~.-   "'._...*'  '..'.\l+  cannot  be  expected  that  either  can- 
be  used  as  i i  control  for  deriving  the  width  of  the  other.  The  width  of  the 
wider  Une,  the  road.  If  It  is  clearly' Imaged,  may  be  fairly  well  derived 
from  the  photographic  scale,  but  not  the  narrower  line,  the  pole,  because 
It  Is  beyond  the  limit  of  resolution.  If  subjective  evaluation  is  required, 
.  reasoned  dimension  for  such  an  object  must  suffice. 
».r<*"'vv^  *.»  .•-* 7  
■•■  *j* ■ » - ■"  «■  —  ."■  —  ■••  "t*  #-<*«*■»*•  •/••«**■„**.-.»  p.«**  •£*_*,+  ...,_  - 
'V"--»  .<'.•'•''••*■.''••  .'  -**i'.-V ' ■,'.\".ii :\'v-cA-
o 
-..  *  • 
>  -■ 
-"-•*■;.— .r* 
•.  V.  ,*•  ••.  * 
.  -';-■••  •• 
"  "-"  r      FIGURE  I     ..-.;... 
Photograph: 
Roads  and  wireline  poles,  and  shadows. 
\
Figure  2.  Microdensity  trace  across  assault  guns  and  tanks 
±i±E 
'   !   t 
4+ 
rr #mrtt 
tHrH-ri- 
i 
7-H-v 
*_i-J-J 
.-•—  1- 
#%•#»#*.#»•* 
">mi    'u(M>nio    mr>«"f»<"»<
-  «v 
k    -/■.-.-... 
■  :'■    ■    -."  "■•  •  •V'-.-»:« 
Figure  3.  Isodensity  tracing  of 
• ..     • 
.       *  -    .•:■•     '.-     *  .-' a 
>»■»    •  •  •  •  « 
;*•• 
>  ■*■ 
•v. 
1*  •■  ■ 
;v*^- 
•?\*f.*-**"?^*'*i' 
,»*n»-*s'W*e1
( 
o 
Fine  Detail  Beyond  the  Range  of  Human  Vision 
•A  critical  phase  of  photoana lysis  comes  where  we  took  for  things 
beyond  the  limits  of  human  vision,  as  In  cases  where  the  photoanalyst 
seeks  to  distinguish  detail  he  Knows  Is  there.  /    .   „.....„,«,v^r.v. -«  --| 
^i-..-ivi/  To  this  end  there 
Is  assistance  possible  from  mechanical  and .electronic  analysis. 
ingui 
Mlcrodenslty  traces  (Figure  2)  are  derived  by  mechanical  curvilinear 
"mensuration,  and  edge  gradient  analysis  Is  performed  wherein.  In  tracing-  .-.._. -^ 
density  variations,  attempt  Is  made  to  distinguish  most  precisely  the  con- 
figuration of  an  object  and  location  of  the  edge  or  contact  between  the 
--object.  J.mage_and  Its  surround.  Continued  research  by  photogrammetrists 
with  better  imagery  and  betteFc6ntroIs~1sexpected-to-produce  "the.  method-^.— .-.; 
ology  necessary  for  satisfactory  determination* of* differences  that  human 
vision  cannot  resolve.  .  ':■"■'  ~  ~  *   -  ••    *  •-••  -  --...- — _  . 
Isodenslty  traces  are  offered  as  photogrammetrlc  support  to  photo 
--analysis  for  the  purpose  of  distinguishing  differences  in  density  too  weak 
for  detection  by  the  human  eyef..~ '-"■" "_'."*  * '■'" " V  V  ;".■..  ^   +°  be  apparent.  They 
are  electronically  derived  plots  of  differences  in  density  and  represent 
-  brightness  differences  of  the  Imagery  (Figure  3).  The  patterns  they  form, 
generally  highly  magnified,  commonly  show  the  Invisible  details  of  the  film 
-jweas.under  investigation.  Only  the  trained  Isodenslty  analyst  can  Interpret 
a  tracef  which  may  be. said' to  have  the  characteristics  of  a  topographic  map,  ... 
with  like  densities  Instead  of  like  elevations  being  represented  by  the 
patterns  derived  from  electronic  scanning  of  photographic- area.  Because 
their  differences  vary  with  actual  contrasts  in  the  photography  they  have 
a  coincident  relationship  to  the  imagery  which  the  photoanalyst  seeks  to 
Interpret.  The  two  analysts  work  as  a  team,  each  seeking  to  derive  informa- 
tion not  available  to  the  other,  each  depending  on  the  other  to  recognize  In 
hts  own  medium  Its  peculiar  variations,  and  each  offering  for  consideration 
points  which  may  add  up  to  significant  conclusions.  The  Isodenslty  study  Is 
not  a  substitute  or  a  verification  of  the  photoana I yst's  subjective  Inter- 
pretation, but  rather  serves  as  an  independent  analysis  of  objective  variations 
beyond  the  visual  realm  of  the  photoanalyst.  ...-•.. 
Isodenslty  analysis,  as  a  support  function".  Is  not  requested  for  the 
majority  of  photoanalysls  problems.  It  Is  specifically  desirable  In  the 
detection  of  minutiae  which  are  beyond  the  limits  of  resolution  of  the 
Imagery  of  slight  differences  In  scale  or  detail  that  make  possible  the  more 
accurate  descriptions  vital  to  distinguishing  critical  features  or  capabilities 
beyond  visual  recognition.  As  one  result  of  this  role  the  Isodenslty  anj ' IZ 
■.*  •  ■.-*;»  *.*;  m  *■"-;'•?,•»***'..•/ i>  <■ 
8^:3^
Is  most  often  called  upon  to  aid  In  analysis  of  poor  Imagery.  He  rarely  sees 
the  best  coverage  because  there  the  photoanalyst  has  become  accustomed  to 
depending  on  his  own  visual  acuity  to  reveal  all  the  necessary  Imagery 
details.  Oftentimes  he  Is  unaware  that  In  the  Imagery  there   Is  stni  a 
certain  reserve  that  Is  visually  Indetectable  but  nevertheless  there  In 
Infinitesimal  changes  in  character  pattern  of  film..  It  has  probably  been 
too  little  understood  that  Isodensity  support  In. the  study  of  good  coverage 
could  probably  add  to  many  requirements  where  derivation  of- small  detail 
might  provide  useful  information.....  .      •'/.'..■.  '..."__ 
Another  use  of  density  analysts  in  preference  fo  high  magnification  .'   
.Imagery  studies-  Is  related  tcrgrajn  noise  In  the  imagery.  In. high  optical   
magnification  the  grain  noise  Is  magnified  along  with  Imagery  detail, 
finally  destroying  the  latter.  In  Isodensity  derivations  it_4s  possible   — 
to  detect  density  differences  fjirst  andthen  enlarge  the  pattern  with  the 
-result  that  grain* noise  falls  somewhat  behind.  " 
In  the  Incr'easedquality  of  film  that  has  come  because  of  improvement." 
iere  has  ccme  opportunity  to  settle  some  of  the  recent  problems  in  inter-. 
pretatton  of  small  objects. 
%-
r~*vv?r:<~* *••'."*■? '■.*■■*•■:,—>  ,-•■■ 
•  \  . '-"-**  '* 
^^^  But  this  does  not  mean;  that  the.  smalt..objects.L  problem-  wl  1 1  go  away. 
For  every  step  Tn  finprovement  of  Information  derivation  there  wiil  come,  as 
better  Imagery  is  produced,  an  increased  demand  for  more  fine  detail.  '  ' 
ii^^^l^KiHP^vio/Ufr'ic^i^V— >V«V,>«.J^.' \V»--  .'I»t'.>.  ..>— . 
■  «.«.  -Jfr--rV.  .»•  * !•  a 4 »>.-*»■*-?»•:-' 
:  - .A>-'i^sv'-:*i"-*'»-  •«< 
Occasionally  unexpectedly  small  objects  are  readily  discerned  on  imagery 
•for  which  ground  resolution  does  not  approach  anywhere  near  so  small  an 
object.  One  of 
•lines  between  two 
d  resolution  does  not  approach  anywhere  near  so  smau  an 
the  Interesting  examples  here  is  the  set  of  cables  or  power-;" 
•.-.-.-:-.■.  ^..v  -  ....  ..:■  "aSthey  have  been  alternately  seen  and  unseen 
•>»•»—■.,  <»■■«»•, 
■  ■\v^yN^.r^«^«^'e^*^i*c!»jJ«-*j(: 5iwjP^.»«TO^'?'W^^Sif?»J.spi'^,'*l!  <  : *  ■
( 
without  the  irradiation  effect,  both  as  light  lines  on  dark  background  and 
as  dark  lines  on  lighter  ground.  The  exact  reason  for  their  visibility  lies 
In  the  quality  of  the  emulsion,  but  the  exact  physical  reason  for  their 
discerniblllty  does  not  reconcile  Itself  with  Inability  to  see  such  ?Tne- 
detall  elsewhere.  ~     • 
I 
;  ;  fw  ,.  ■  _•_  ,  .'  .      '..._'.■!■     '  No  device  has 
been  produced  which  "can  'either" vert fy  or  refute  the  Interpretation  given 
by  a  photoanalyst  or  b'y  analysts  who  may  not  agree  on  a  specific  Interpreta- 
tion.  In  visual  mensuration  cooperation  is  achieved  between  the  two 
analytical  skills.  Where  the  machine  operator  Is  unsure  of  the  limits  of 
the  object  being  measured  by  such  Instruments  VL.f.Jl.  .1."-..   ..  ..'the 
analyst  Is  often  called  upon  to  verify  the  points  for  po int-to-point 
mensuration.  This  he  Is  happy  to  do  In  order  to  forego  the  time  consuming 
operation  of  the  machine  Itse'lf.  There  Is  convincing  evidence  that  the 
two  skills  are  mutually  beneficial.  Perhaps  at  this  point  it  is  also 
quite  obvious  that  the  two  should  be  kept  separate  rather  than  that  the 
photoanalyst  should  be  expected  to  do  tfie  most  exacting  types  of  mensuration. 
The  field  of  knowledge  required  Is  too  extensive  for  analysts  in  general  to 
becontent  with  attempting  to  perform  with  maximum  quality  In  both  fields. 
Aj
r 
.  <  •• 
•  -»»:.. 
.•  .  •■«• 
»      • 
*.**■  .'. 
--~r.  -'.   ~  FIGURE  4  "--  '•*-'--  "■■- 
Photograph  revealing  power  cables. 
-i'*\ 
"j'i>    ) 
i  f  *■*■{■•.*•**  t*.  *»"**;i»i  f  **-\r.* 
..VV'Srl.
V. 
<t*S>  ••■>  /"foa^f 
Imagery  with  Weak  Photo  Contrasts  •* 
Much  of  the  problem  of  object  Identification  or  visualization  Is  not 
a  matter  of  size  but  of  image  clarity.  If  all  photography  were  distinctly 
black  and  white,  such  as  ground  resolution  targets  are,  this  would  be  a 
standard  problem  with  standard  expectabi titles,  but  most  Imagery  consists 
of  gradations  of  gray  tones  that  seem  to  merge  Imperceptibly.  To  some 
degree  this  is  inherent  in  the  use  of  granular  emulsion. 
.Field  conditions  'are  a  major  cause  of  such  poor  contrasts.  Light 
reaching  the  camera  lens  Is  attenuated  by  atmospheric  humidjty,  haze,  or 
overcast;  the  image  may  be  blurred* because  of  lack  of  perfect  coordination 
between  film  and  vehicle  motion;/   "•"'"^■'""'^•:''     •  ."™™?~7;j 
f^^r^-^ry-  • — "■-*•.   "T^™...  ;,.    ,.;..,..-...;.„..    :  /or  vfsJbTfify  imay 
be  poor  because  of  "poor  lighting  "and Tlghting  angles  at  Inappropriate  times 
of  day  or  season,  £Z^l.l^^ZJ^ZZZZ^ZZl^^:^C^"r  V.^"   Snow  cover  often 
masks  ground  surface  detai Is,  but  it  may  be  revealing  because  It  sharpens 
contrast  and  can  date  activity.  Obliquity  of  coverage  may  be  a  problem 
because  of  poor  viewing  angle  but  it  too* is  advantageous  in  many  Instances. 
In  cases  where  one  or  more  of  these  conditions  Is  dominant,  an  object,  even 
one  that  Is  large,  may  be  Imperceptible,  or  If  identifiable  may  still  be  too 
poorly  imaged  for  full  and  satisfactory  description.  These  are  the  usual 
conditions  under  which  imagery  Is  acquired. 
The  real  problem  of  Identifying  the  fine  detail  of  photographed  objects 
Is  generally  one  of  distinguishing  between  closely  related  tones  produced  in 
the  relatively.  ?lear  Images  of  objects  whose  overall  appearance  is  r.ot  black 
and  white  but  the  in-between  shades  or  colors  that  come  out  gray  on  so-called 
black  and  white  photography.  The  large  difficult  objects  fall  Into  two  .  v 
categories:  (I)  those  which  occur  In  shadow/ 
' 
of  a  nearby  object;  and  (2)  those  whose  gray  tor\es  closely  match  the  surround. 
If  the  gray  tones  were  effectively  contrasted  resolution  could  be  good  .-eve* 
£5;;r ';-.        ..•*■-,.- 
,an(|  05jects  0f  low  scalar  size  distinguished  btT  if 
the  tones  are  closeiy'aYike  or  merge  Imperceptibly  object  definition  is  apt 
to  be  poor.  There  has  not  been  reported  any  coherent  body  of  research  Into 
the  expectable  relationships  between  vision,  visual  acuity,  and  identification 
and  description  of  objects  only  faintly  contrasted  In  their  included  detail 
or  In  association  with  surrounds,  at  least  no  research  that  Is  of  direct 
application  to  the  j>hotoanalyst.  In  this  phase' of  hi6  work  he  is  operating 
In  an  area  for  which  no  physical  parameters  have  been  developed  as  guidelines 
or  goals,  with  the  result  that  whatever  he  accomplishes  Is  open  to. criticism 
and  greater  expectations. 
■  •.*  -  .1;  -y  ■.»-?,Vv.*V";'"""'-'  ■  •  '''  '  "-  ''  '  ■   "'" '"
the  photoanalyst,  personally,  either  sees  an  Image  well  enough  to 
recognize  and  describe  It,. or  he  does  not.  In  which  case  he  may  be  able 
to  get  help  from  available  technical  services.  The  Image  contrast  can 
sometimes  be  Increased  by  developing  "techniques  that  enhance  film  density 
differences.  It  Is  worth  while  for  the  analyst  to  discuss  such  contrast 
problems  with  the  skilled  staff  of  the  photo  reproduction  laboratory,  or 
with  the  photogrammetrlst  In  case  an  exact  edge  presents  the  problem. 
These  techniques  are  most  valuable  In  object  mensuration,  and  they 
sometimes  also  lead  to  corroboration  of  Identity, 
••  ■   •      •    •    ;."•••:/■■•■'-':>•'  •  :'-:  .  *r.  *-w**'-     •   .  -  ••■■■     ••-'••'. 
•        ■  •— -  '  -    ••  v  :•    '.        *.•«  •  --»  •  •  •■.  .  .  •  .. 
•  .  "    .» 
'"''SVfy/'tij'&frL-  i£'>^V0'j '';v'<-W  v>  '*f.&:  ...:'£'<:  -  ""'   >;. 
W-$gz
-"     C  -  ■■:        ':--:'0 
Imagery  of* Highly  Reflective  Surfaces 
Metal  Objects,  such  as  aircraft  and  uiipalnted  oil  or  chemical  tanks, 
are  generally  as  readily  seen  and  measured  as  other  types  of  construction, 
but  occasionally,  and  often  then  In  critical  situations,  the  amount  of  light 
reflected  from  such  objects  Into  the  aerial  camera  is  so  bright  as  to  partly 
efface  object  outline  and  detail.  The  analyst  can  most  often  distinguish 
the  nature  of  such  large  objects  by  distinct  outlines  of  portions  of  the 
Image,  or  by  conclusions  derived  from  the  surround.  Exact  measurement  or 
description  of  small 'detail,  especially  that  required  to  distinguish  between 
similar  appearing  aircraft,  for  Instance,  may  be  impossible,  although  this 
bit  of  Information  may  be  one  of  the  most  Important  goals  of  an  aerial  mission. 
The  majority  of  statements  on  such  overbrlght  light  effects  attribute 
the  cause  to  glare,  which  In  conversation  Is  frequently,  though  mistakenly, 
referred  to  as  halation.  The  real  cause  of  overbrlghtness  therefore  needs 
discussion  and  clarification.  Three  questions  Invarlbly  express  the  problems 
of  those  concerned  with  such  Identification  assignments.  They  are:  (I)  under 
what  circumstances  does  such  overbrlghtness  affect  imagery?;  (2)  what  is  the 
amount,  percentage,  or  factor  of  erroY  in  mensuration  of  such  Image  distortion?; 
and  (3)  what  can  be  done  to  avoid  Its  occurrence? 
The' first  of  these  questions  Is  fairly* easy  to  answer  although  It  requires 
detailed  objective  explanation.  Overbrlghtness  Is  almost  wholly  caused  by 
natural  and  normal  concentration  of  light  reflection  from  the  metallic  surface 
Itself.  To  understand  this  we  need  to  review  basic  explanations  of  the 
behavior  of  light,  Its  distribution,  scatter,  or  concentration,  depending  upon 
the  media  through  which  It  passes,  the  surfaces  through  which  It  cannot  go, 
and  the  manner  In  which  It  Is  directed  away  from  those  surfaces. 
It  is  theorized  that  light  travelling  from  the  sun  toward  the  earth  Is   ,  ~< 
propagated  in  direct  rays  until  It-hits  the  earth's  atmosphere,  whereupon      % 
some  of  It  Is  diffused  or  scattered  by  minute  particles  contained  therein. 
Much  of  the  sunlight  falling  through  haze,  overcast,  and  smog  strikes  the 
surface  of  the  earth  or  the  objects  thereon,  whence  It  can  proceed  no  farther. 
Some  of  the  light  Is  absorbed  and  turns  Into  heat  energy.  If  the  surface  Is 
translucent  some  of  the  light  will  be  i , di.smltted  through  It  (Figure  5a).,  as 
through  a  window,  but  that  which  is  reflected  (turned  back  by  the  object/ 
carries  an  image  to  whatever  optical  system  Is  within  viewing  position.  If 
the  surface  Is  rough,  either  coarse  or  fine  (Figure  5b),  the  reflected  light 
Is  diffused  or  scattered  in  all  directions,  and  the  object  Is  generally  easy 
to  see  and  easy  to  look  at,  although  excessive  diffuse  light  produces  a 
washed-out  appearance  which  is  readily  apparent'" 
•.■%  ^H  ......    w
T~7T 
(.•> 
.* 
Incident  beam 
•  •. 
X     Transmitted  rays 
•.  -  ■.■.•»'•• 
i  ;„ 
■  <  • 
»-..•. -..J- 
*£tcAeet  or 
/  granular  surface 
Oiffvse  Reflection 
i.  -  *-  *.  *  -  ■ 
ktctofent  beam 
Reflected  beam 
v.  .  _••  ^.•;»-    ••  ...-»•- 
.Smooth,  potfstied 
f    Surface 
Specular  or  Regular  Reflection 
...  ...  .      \   • 
figure  5.  a-  Absorption,  transmission,  and  reflection  of  light  ray's" 
striking  a  transparent  surface.  -  ■ 
b-  Diffuse  reflection  from  surface  vhich  is  irregular  - 
--compared  vith  dimensions  of  light  vaves.     .--;;;■.*;. " 
v  *; — *..'.©«,  Specular  or  regular  reflection.       ..   ..,<.-.,...;..;., 
(49 
^
<  ■  o 
• 
•The.  smoother  the  surface  may  be  the  greater  the  proportion  of  reflected 
light  that  Is  oriented  in  a  particular  direction  (Figure  5c).  That  direction 
Is  determined  by  the  dtrection  from  which  the. light  comes  and  the  orientation 
of  the  bit  of  surface  from  which  each  light  ray  Is  reflected.  This  relation- 
ship Is  clearly  explained  In  the  physical  law  which  states  that  the  angle  of 
Incidence  of  the  light  Is  equal  to  the  angle  of  reflection,  these  angles 
being  measured  from  a  line  normal,  or  vertically  projected,  to  the  reflecting 
surface.  The  law  Is  well  Illustrated  In  the  brightness  of  sunlight  reflected 
Into  the  eyes  of  a  westbound  driver  In  the  evening  when  the  sun  Is  low,  or  In 
the  brightness  of  a  smooth  water  surface  as  one  looks  into  or  out  over  It 
while  facing  the  sun  on  a  bright  day-  If  one  faces  away  from  the  sun  the 
water  appears  dark,  not  because  the  sun  is  not  shining  upon  It  also  but 
because  the  viewer  Is  In  the  same  direction  as  the  sun.  In  the  diagrammatic 
direction  of  the  angle  of  Incidence,  while  the  concentrated  reflection  .Is 
directed  Into  the  air  out  beyond  the  viewer  in  the  opposite  direction. 
For  smooth  surfaces  this  light  concentration  Is  called  specular  reflection. 
A  mirror  Is  such  a  surface.  Any  smooth  metal  surface,  such  as  the  body  of  a 
car  or  the  curve  of  an  aircraft  wing  has  such  a  reflective  capability.  The 
brightness  of  the  specularly  reflected  light,  and  the  image  it  may  carry. 
Is  said  to  be  equal  to  the  brightness  of  the  source  of  light,  viewed  on  a  line 
through  the  point  of  incidence,  times  the  reflecting  power  of  the  surface, 
which  may  be  less  than  0.1  for  water  or  glass,  but  as  much  as  0.98  for  some 
metals.  Thus  It  is  that  polished  metals'are  used  for  mirror  surfaces,  and 
that  the  brightness  of  reflection  from  metal  equipment,  such  as  tanks  and 
aircraft  bodies,  sometimes  appears  too  bright  for  comfortable  visual  observation. 
The  brightness  from  such  an  object  may  be  too  great  for  the  camera  film  to 
capture  a  good  clear  Image;  this  is  the  reason  why  these  items  often  appear  as 
overbright  blobs  of  light  or  only  partially  discernible  images  on  aerial  film. 
The  second  question,  concerning  the  amount/  percentage,  or  factor  of 
error  in  the  mensuration  of  light  distorted  images,  has  no  single  or  specific 
answer.  E*>ch  item  is  a  case  by  itself.  The  amount  of  distortion  regis, ered 
on  the  film,  for  example,  will  depend  not  only  upon  the  nature,  curvature, 
and  angle  of  the  reflecting  surface,  but  also  upon  redirection  and  control 
of  the  light.  For  each  object  there  is  not  necessarily  a  unit  reflection. 
This  is  well  Illustrated  in  the  photography  of  an  airfield  at  Dallas  where 
several  aircraft  were  parked  (Figure  6).  Of  these  only  one  was  subject  to 
oyerbrightness,  but  for  It  the  forward  part  of  the  fuselage  between  wmg^and 
nose  was  distorted  beyond  definition,  while the  .rear  o^  the  same  aircraft 
was  clearly  outl Ined. f  ..';". '". i" **"*j*'].'.^" L;-----.-**-..**--'--.-. ■■■■    .  ;  .  .  .-. -.  +    ■ 
■■reblsma,  and  some  means  have  been  achieved  in  avoiding  undesirable  light
it 
ill. 
r 
O 
FIGURE  6 
_  Efcotograph: 
m 
f*  ...... 
N 
QI 
?■  ji 
to 
4* 
Love  Field,  Dallas,  Texas,  { 
f  '" 
:V^£:r!27?'T#SF,W!15''** 
WVW.1WI  %.  •»".«*.•«  «KM 
-j»*'">v
( ■  ■      o        • 
effects.  First  among  these  may  be  mentioned  the  use  of  polarizing  filters 
to  help  reduce  excessive  diffuse  light  and  to  minimize  reflectivity  while  * 
otherwise  producing  good  quality  film.  In  the  photogrammetric  and  reproduc- 
.tlon  laboratories,  too,  microdenslty  and  film  reproduction  research  Is. 
underway  to  help  solve  such  problems.  But  this  laboratory  work  cannot 
erase  from  the  negative  the  overbrlghtness  already  registered  there  nor 
restore  an  Image  that  never  reached  the  film.  •'•.' 
We  now  come  to  the  question  of  what  can  be  done  to  avoid  the  occurrence 
of  specular  reflect I oh.  Let  us  look  at  this  In  two  parts,  the  general  and 
the  particular.  As  a  general  subject  in  photography  we  need  concern  ourselves 
very  little  about  Its  occurrence.  AM  of  us  as  experienced  ground  photographers 
know  that,  except  for  special  lighting  effects  and  with  great  discretion.  In 
order  to  get  good  photographs  we  turn  our  backs  toward  the  sun  and  by  observa- 
tion guard  against  including  overbright  objects.  By  this  same  deduction  we 
.  could  also  conclude  that  aertai  photography  taken  with  the  same  caution  could 
equally  well  avoid  specular  reflection.  But  aerial  photography  Is  not  quite 
so  simple. 
In  order  that  a  photographic  misslonjre  most  ^successful.  It  Is  ca^efuMy^ 
programmed  before  the  mission  Is  f  Iowa,  l~:3^-  IT._ :...„  ,.;. ;'' "  ..3^*^^S^S3 
*P!I^#£B^^  -*  And  because  there  are  no  possible  means  of  knowing  all  the 
reflecting  surfaces  in  the  path  of  the  mission,  the  laws  of  chance  practically 
guarantee  that  for  a  long  mission  some  smooth-surfaced  object  will  be  Imaged 
In  the  very  angle  that  will  produce  specular  reflection.  However,  If  the 
object  Is  known  to  exist,'  and  an  Image  of  such  an  object  Is  a  purpose  of  the 
mission,  there  should  be  no  reason  why  the  photography  could  not  be  secured 
from  a  satisfactory  angle. 
It  could  be  possible  that,Y~~~   7  .  .   ""         there  Is 
only  a  stngle  chance  to  acquire  a  photograph  of^'a  desired  object,  and  that, 
regardless  of  the  uncertainty  as  to  the  sun,  object,  and  camera.  It  Is  deemed 
best  to  take  that  chance.  In  this  case  an  exceedingly  small  variation  in  the 
relative  position  of  the  three  could  insure  a  good  view  or  an  unsatisfactory 
one.  It  is  worth  the  try.  But  If  the  object  in  quest ton,  an  airfield  for 
'  example.  Is  one  of  the  prime  targets  f\    .'  .  ..\  and  +ne  presence  or  probable 
presence  of  highly  reflective  objects  (in  this  case  either  oil  tanks  .or  air- 
craft) Is  expectable,  the  programming  should  take  account  of  possible  specular- 
reflection  to  the  point  of  guaranteeing  against  it  by  careful  placement  i     \ 
Wui."-.''  ''■'■'. '■"".  ':'.'"*■  ■'.'•'•. WJZZ7 "7T7  """1  Va  Posl+lon  which  not  necessarily 
approaches  the  angle  of  Incidenceof  light "on  the  object  but -which  Is  In  the 
same  aztmuthal  orientation.  This  would  minimize  the  probability  of  good 
"-*■•»•,
shadows.  It  Is  In  sharp  contrast  to  the  long-standing  belief  that  objects 
are  most' clearly  seen  when* the  shadow  falls  toward  the  observer.  The  really 
-  analytical  photoanalyst  finds  that  a  good  look  at  the  sunny  side  of  an 
object  provides  details  usually  associated  with  obliquity,  and  often  revealing 
of  Image  details  not  captured  on  the  shadowy  side.  No  matter  how  carefully 
such  programming  Is  established  there  may  still  be  times  when  an  aircraft, 
oriented  at  an  arbitrary  angle  and  with  a  curved  surface  reflecting  sunlight 
from  a  fortuitous  portion  of  Its  complex  exterior,  will  direct  a  bright  enough 
amount  of- specularly  reflected  light  to  cause  poor  J I  neat  Ion  over  a  fraction 
-of  the  Image.  If  such  overbrightness  destroys  a  critical  detail  of  the  . 
.  Image  It  may  be  Impossible  to  distinguish  that  aircraft  as  being  one  or  the 
"other  of  two  closely  like  models,  or  to  Identify  .a  known  model  from  a*  possible' 
-closely  resembling  prototype  that  has  not  been  previously  observed.  Although" 
such  an  Instance  seems  wholly  unlikely,  the  possibility  should  not  be  6yer-_- 
•-•  looked,  and  cannot  be  completely  avoided.  "*  *  * 
•  • 
In  most  Instances  where  clear  photography  Is  vital  It  Is  possible  to   
.arrange  for  more  than  a  single  frame  of  coverage  of  the  vital  object. 
the  chances  are  very  good  _ 
-fl^  at  least  one  frame  J f  not 
both  will  have  the  object  clearly  Imaged.  If  neither  Image  Is.clear  or  only-' 
a  single  poor  Image  Is  acquired,  there  Is  some  value  In  knowlng^that  an  .... 
:  Interesting  object  Is  at  least  poorly  Imaged  and  that- Its  Identity  may  be 
clarified  when  another  mission  Is  flown. 
I-. 
"^^^SM^^^^Smm^i&mmmmmmm^^^
Problems  of  Orientation  and  Viewing  Angle 
t  Overhead  photography  provides  the  analyst  with  imagery  that  varies 
with  lighting  angle,  viewing  angle,  and  target  orientation.  First  of  all 
the  geographic  location  of  an  object  determines  whether,  on  a  particular 
date,  there  can  be  sufficient  light  at  a  site  to" a! low  clejsr-photographv. 
.;'•:....  .'..■;   „•'  ,^__*^.^,.,.  ■.  ,-   ../"Exceptions 
"nadV'fo  this  condition  are  Yh^rud'e^'Vn'mlssFons  wTth*  f fie  Yu I :l  knowledge  that 
certain  targets  are  better  viewed  under  poor  light  than  not  at  allT£S2£2»*"~~ 
^->^"'  r      *   .   >  In  cases  where  obscurity  occurs  the  reasons  are 
"generally  other  atmospheric  conditions  or  terrain  or  structure  shadows. 
,     The  matter  of  sun  angle' should  not  be  left  without  pointing  out  the    ' 
seasonally  greater  quantity  of  light  In  the  sky.  In  summer  than  in  winter. 
If  the  sunfs  course  were  related  to  a -cylinder  Instead  of  to  i-he  spherical 
"shape  of  Earth  there  would  be  no  differences  In  this  respect.  But  anyone 
reflecting  on  the  subject  recognizes  that  the  immediately  pre-sunrlse  and  __ 
post-sunset  light  in  summer  Is  far  greater  than  in  mid-winter. 
In  lower  latitudes  it  can  happen  that  with  high  seasonal  sun  mid-day 
photo-coverage  yields  imagery  tnat  is  almost  shadowless,  generally  an 
undesirable  feature.  On  the  other  hand.  In  the  latitudes  between  the 
tropics  (Cancer  and  Capricorn),  coverage  can  be  secured*  seasonal ly  with 
shadows  falling  either  north  or  south  as  well  as  dally  either  east  or  west. 
This  allows  unshaded  views  of  alt  sides  of  an  object  of  interest  and  provides 
guidelines  for  targeting  in  order  to  acquire  the  desired  look.  \ 
■:*.' 
A  full  discussion  of  these  properties  of  the  imagery  should  also 
Include  unit  target  shape  and  complexity,  and  the  nature  of  its  surface. 
But  these  are  so  intimately  related  to  the  basic  aspects  of  vision  and  imagery 
that.  If  discussion  is  deemed  desirable,  it  should  follow  after  a  full  reading 
of  the  second  section  of  this  report.  | 
W^i^-r   '  •■--■-•  -■•....        _ 
/*■■•■*  ■" 
/ 
.1%*   .'...
A  j>hotoanalyst  has  mixed  feelings  about  shadows.  He  learns  early  that 
photography  Is  easier  to  look  at  If  oriented  so  that  shadows  fall  toward    * 
him.  For  vertical  photography  this  Is  simply  done,  but  for  oblique  photo- 
graphy, where  obliquity  should  also  be  slanted  In  the  direction  of  the  viewer, 
there  are  times  when  a  choice  has  to  be  made  between  the  two.  And  then  there 
.  Is  no  single  answer  or  simple  rule  to  follow.  The  situation  varies  not  only 
with  the  obliquity  and  shadow  angles  but  with  Image  quality  and  even  the 
'nature  of  the  Image  itself.  The  analyst  In  such  cases  may  find  It  useful 
•  -■  to  look  at  the  photography,  the  site  location,  from*several  angles,  .and  often  . 
•  finds  something  revealing  In  viewing  the  Imagery  from  different  angles. 
.  The  angle  permitting  the  most  penetrating  analysis  Is  often  found  to  depend  ..... 
"•  on  target  orientation,  or  or.  whether  or  not  the  object  to  be  Identified  Is   
a  single  unit  or  a  small  factor  In  a  more  detailed  and  complex  site. 
:?  •  •-.  Target  orientation"  Is  also  a  dual  or  triple  concept.  In  the  first    
•  place  It  may  make  a  difference  In  judgment  whether  one  views  a  30-foot 
-missile  or  a  freight  car  from  the  end,  the  side,  or  some  point  in.  between. 
It  may  or  may  not  be  important  whether  It  Is  seen  from  the  direction  In 
-  which  the  shadows  fall  or  In  the  direction  of  obliquity.  If  the  target    ~  — 
has  a  highly  reflective  surface  the  UghtVeturn  from  It  will  depend  on 
..*_the  relationships' of  the  camera's  look  angle  to  the  angles  of  incidence  ~7~~~- 
rr  -end  reflection  (Figure  7).  These  are  aH  orientation  factors  that  .-aid  or  .."*."' 
-  hamper  the  look  at  the  Imagery.  But  target  orientation  with  respect  to 
--acquisition  of  the  imagery  is  also  important.  This  Is  immediately  obvious  . 
"*•  In  consideration  of  ground  resolution  targets,  where  tests  show  that  the 
^--target  oriented  parallel  to  the  flight  path- of  the  vehicle,  has  a. lesser ....  A--ff 
•""resolution  than  the  "target  oriented  perpendicularly;  the  reason' being  that  r:. 
""there  Is  a  si I'ghrly  greater  sensitizing  of  the  emulsion  in  the  direction  of   • 
-flight,  with  the*  film  traversing  that  flight  path.  In  some  Instances  the 
difference  may  be  Important. 
.*
('     ^ 
y. 
1      *•*•'.       •• 
"■     »   .      - 
FIGURE  7 
x\..;\y-.  '• 
••  t    • 
.•  «„■•  •   ••- 
Photograph:* 
Misshaped  Images  of  missiles r 
I 
-2.C 
y 
V-    \
PERFORMANCE  SKI LL 
•  .  .  ••     -••■•.- 
Imagery  analysis  is  not  just  a  matter  of  object  definition.  As  much 
as  its  concern  over  objects  may  be  emphasized,  the  imagery  analyst  is 
responsible  also  for  recognizing  action  or  the  immediate  condition  of  a  -•  —  -• 
capability  that  Is  being  developed.  Ke  Is  most  often  expected  to  be  able 
to  draw  on  an  intensive  background  knowledge  of  some  one  or  several  diverse 
and  technically  complex  fields  of  man's  industrial,  political,  economic  and 
military  capabilities.  To  the«degree  that  it  is  pertinent,  he -acquires  a  - 
wide. acquaintance  with  the  geographic  assets  and  limitations  of  a  site,  and  - 
the  history  of  the  activity  there.   It  is  expected  that  he  keep  himself 
Informed  of .whatever. may  transpire -there,  or  elsewhere,  that  may  be  related   
to  the  function  in  question. 
-     In  the  process  he  becomes  skilled  in  the  techniques  he  finds  most*     ""/" 
productive  of  satisfaction  in  his  work.  Such  skills  may  take  the  form- of  -  •'•" 
II  lust  rati  on_pf_a  single  site  or  feature,  of  photographic  comparisons  between 
similar  but  unrelated  sites.  Or  he  may  have  a  special  interest  in  verbal 
communication.  In  which  case  his  word  pictures  become  increasi-ngly  apt  and 
meaningful.  But  In  all  probability  he*  will  not  be  highly  skilled  In  all    --• 
collateral  techniques.  : 
..*....-       .  .  •       ;   ......      +.$ck  on  J 
Most  particularly  he  wi  1 1,  probably  jotT-be-extremely-Iiiterested-nor— ever- 
hJgh+y  competenfsiin  the  mechanical  aspects  of  imagery  derivation  and  presenta- 
tion... He=may=do -a-t-*nc  fcbny  analysts  are  graphic  artists,  but  the  average 
analyst,  will  neither  enjoy  nor  perform. we!  J  st  graphic  support. or  the  mensura- 
tion of  his  own  small  objects*  analysis  problems.  This  Is  not 'because  he' lacks 
interest  in  the  data  such  oanipulationcan  provide  for  him  but  because  fie  Is  * 
then  moving  from  the  subjective  analysis,  which  he  enjoys  and  therefore -does 
^Well,"  to  objective  skill  that  by  comparison  is  a  subdominant  accomplishment   
and  therefore  not  able  to  command  the  place  in.his  interest  which  is  - 
necessary  for  real  skill.  In  the  process  of  becoming  self-sufficient  he  '--—•- 
-becomes  isolated,  a  hermit  if  you  will,  lacking  the  support  of  discussion  ;   *" 
and  differences  of  opinion  and  purpose  which  in  photoanaiysis  are  highly 
productive.  *      ' 
_.   -The  techniques  of  subjective- and  objective  analysis  of  photography  are — .-- 
..distinctly  separate  concepts,  both  as  to  philosophy  and  practice.  -The— ------ - 
individual  who  is.concerned  with  the  objective# techniques  will  become  a  ; 
..photographer,  a  photo-lab  technician,  a  photogrammetrist,-or  an  optical   
specialist  perhaps,  but  he  will  not  be  Interested  in  verbally  describing  the  -
{ 
o 
leg  of  an  electronics  site  or  the  chemical  output  of  a  rallslde  Installation. 
His  interest  Is  In  the  mechanical  perfection  of  the  photographic  product, 
and  having  achieved  the  highest  quality  under  ."a*  given,  set  of  mechanical^ 
conditions,  he  then  attempts  to  improve  some  small  component  of  these  """"-  . 
conditions  for  another  go-around.  The  imagery  analyst,  on  the  other  hand. 
Is  Interested  In  the  game  of  deriving  expertise  from  the  visual  potentials. 
His  skill  differs. from  the  mechanical  or  mensurable,  and  when  he  is  asked 
to  do  both  he  Is  in  a  position  analogous  to  that  of  a  test  pllctfbeing  asked 
to  .take  a  place  In  the  assembly  line  of  an  aircraft  factory.  -&a  there  is  a 
c£e&\   need-f  or  -objective  analysis  of  small  object  photography,  -It  should  not 
be  expected  that  this  be  done  by  the  .subjective  analyst.  True,  he  will,  or 
should,  understand  the  objective  background,  but  he  needs  the  support  of  the 
skilled  and  knowledgeable  objective  analyst  to  produce  the  best  products.  . 
*  * 
In  addition.  It  will  probably  be  discovered  that  the  objective  work  can 
be  done  better  by  the  objective  specialists,  that  they  will  do  It  faster  and 
less  expensively  than  the  subjective  analyst  can,  that  he  thereby  will  be 
free  to  accomplish  a  greater  amount  of  "the  work" he  does  best,  and  that  as"  a 
total  result  more  work  will  be  done  more  economically  and  more  people  will 
be  happier.  «.        •    - 
'£• 
^ $ 
T*  •"i--'...,?-.;.-.- 
»'»'.■■'»-  .;'•..'.'.■■
COMMUNICATION 
Communication  of  Ideas  Is  mainly  a  language  skill.  Our  words  must  be 
wall  chosen  to  say  exactly  what  we  think  If  we  are  to  convey  to  others., 
precisely  what  we  mean.  The  phrase,  "be  specific/'  rings  In  our  ears.  This 
does  not  mean  that  graphics,  either  annotated  photographs  or  objective  Illus- 
trations, do  not  have  a  place  in  putting  across  salient  points  In  presentation, 
.  or  that  numbers  are  not  highly  effective  In  conveying  magnitudes.  These  are 
"  useful  after  the  fact,  The  concepts  they  convey  must  first  have  had  to  be 
verbally  defined.  In  company  with  visual  acuity" a'nd  analytical  skill  In 
studying. the  Imagery,  idea  formulation  and  vocal Ity  are  the  means  by  which 
the  analyst  communicates.  To  do  this  well  he  must  know  the  nature  of  the 
"*  media  with  which  he  works,  and  the  descriptive  terms  which  effectively  carry.~-=r- 
hls  message  to  others.  "  The  first  of  these  Is  covered  In  the  second  section-  .  - 
~  of  this"  report.  Here  we  will  concern  ourselves  with-  Introducing  the  analyst's  - 
practical  problem  of  communicating  what,  he  knows.  •'..'.,.   
* 
-"  .  As  In  any  profession,  the  body  of  photoanalysis  terminology  has  been 
-"accumulated  slowly  as  the  science  has  ^rown..  Certain  terms  that -have  been—   
well  chosen  stand  the  test  of  time.  Others,  less  apt,  may  be  carried  in  active  ... 
use  because  of  priority,  because  their  real  meaning  is  overlooked,  because 
"'"their  use  is  expected  and  even  required, -or  because-more -adequate  terms  have- 
not  been  Introduced."  The  photoanalyst,  as  a  vocal Izer,  might  sharply  increase  - 
•  his  ability  to  handle -words  if  this  phase  of  his  activity* were  brought  to  . 
•  attention  as  a  whole  field  of  habit  that  needs  evaluation.  Unless  a  good  look 
~  "and -an  openmind  are  brought  to  focus  on  the  problem  it  might  better.. be  d J sr„„ "_:__: 
regarded.  ._ 
The  prime  weakness,  poor  word  handling  (which  here  also  Includes    —   
phraseology),  resolves  itself  Into -two  parts,  words  poorly  used  and  words 
-poorly  understood.  Kords  poorly  used  are  most  commonly  the  general  or 
non-professional  tern*  no  logy  that  has  been  incorporated  and  thereby  .endowed   ._  . 
with  pseudo-scientific  status.  "Poor  Image  quality"  Is  such  a  term.  Many 
of  these  terms  are  ambiguous  and  misleading.  They  include  terms  which  mean 
one  thing  to  one  group  of  analysts,r  "          ^  bu.t.so5*et-h,n3  else  \ 
-to  others. \   '  ''" .'"."',Z.'S  '"  \  ■  .  
  ".—-:. 
""•Terms  Which  connote  different  things  in  several  aspects. of  Image_productibn,  .-™ 
such  as- speed  or  flare,  most  commonly  are  used  precisely  In  one  sense,  which 
should  be  the  only- usage,  and  In  all  other  cases  should  be  replaced  by  terms 
which  really  mean  what  Is  implied.  In  a  few  cases,  as  contrast  and  exposure 
used  by  photogrammetrlsts  In  one  sense  and  photoanalysts  In  another,  two  - 
different  usages  are  justifiable. 
2f. 
!.*■»- 
?'.»■>:  .:'"?"/•*
••(.:••-     .  r  {  \ 
*.•  •  •  •      \       .• 
»  •  •  -  *  •• 
Among  words  poorly  understood  we  find  roost  commonly  the  borrowed 
sclenttflc  terms,  which  in  the  borrowing  have  been  poorly  chosen,  and  In 
the  retention  have  come  to  denote  something  outside  the  original  meaning 
which  may  be  a  valid  term  within  the  bounds  of  .the  profession.  Halation 
fits  Into  this  group.-  We  find  a  simple  term;  defined  precisely  In-physical 
science,  used  loosely  and  conveying  no  useful  Impression  when  substituted 
Into  the  position  of  a  related,  significant  idea.  There  is,  for  Instance, 
a  custom  of  using  "resolving  power"  as  a  criterion  In  Imagery  when  "ground 
resolution"  Is  the  Important  va4ue.  .  On  the  other  hand,  we  may  find- ourselves 
poorly  understood  because  our  audience  or  readership  falls  to  catch  the  full 
-significance  of  terms  we  choose*  In  this  case  we,w  however,  have  the  choice 
of  using  simple  vocabulary  so  carefully  presented  as  to  convey  the  more 
complex  meaning,  or  of  explaining  exactly  what  an  unfamiliar  term  means. 
._.  The  photoana I yst..shou Id  b®  aware  .of  .these .weaknesses  In  verbal  .  *  __; __'_._ 
communication; ■  He  may  find  value- In  this  amount  of  definition.  To  go  .  — ^_ 
beyond~th!s  point  Is  not: with! n-the  scbpe-of~aivJ ntroduct ion. to_ the  problem.  .. 
•,.  .T.-  .■»  -«.  *•»,..  ■
~^'^«*&tt£teb^l^ 
^^&$&&&i)^  •"'**" 
■  r~.  «*'-*■  *»*».■*"*«'.#»-•.< 
•  * 
*   Given  two  reels  of  photography  from  film  with  equal  resolving  power, 
used  In  two  different  but  supposedly  equal  cameras,  exposed  under  two 
Jwl f«5fn+, Supposedly  equal)  sets  of  atmospheric  conditions,  photographing 
two  actual  (and  supposedly  equal)  set*  of  scenes,  developed  under  Supposedly 
equal  conditions,  and  viewed  by  two  different  (supposedly  equally  capable) 
Imagery  ana  ysts.  It  Is  difficult  to  conceive  that  definition  and  description 
of  dent i ca I  small  objects  would  be  equal.  There  are  too  many  variables  for 
full  equality.  #£^^3*1?^^  variaoies  Tor 
^^JX^A^^l^^^^0^^^^^   Preanalysis,  and  with  its 
products  W^^^st^iJ^ii&^aS^  comprehended   that  Imagery,  analysis  is  not 
an  exact  science,  and  the  reasons  thereiore,  our  effort  could  be  better  - 
concentrated  into  doing  what  we  can  instead  of  being  too  concerned  with 
■what  we  cannot.  . In  the  following  section  are  analytical  data  on  which 
such  comprehension  oust  be  based.  i  . 
.  /. 
V~*v?..-.^>, 
■VTS.fM-a.fy.,.^
'■  o 
SECTION  It 
BASIC  IMAGERY  RESEARCH 
In  the  transmission  of  an  object's  appearance  on  the  ground  to  Its 
description  in  a  photo  analyst's  report,  the  descriptive  data  ere  forwarded 
through  a  series  of  media,  none  of  which  functions  perfectly  and  each  of 
which  degrades  the  image  or  robs  It  of  certain  detail  which  is  not  recoverable. 
The  media  in  turn  are  the  reflected  light  carrying  the  image  details,  the 
atmosphere  which  attenuates  the  light  rays  enroute  to  the  camera,  the  lens 
which  focuses  light  on  the  film,  the  roll  of  film  on  which  the  latent  image 
ts  recorded,  the  negative  or  processed  film  and  the  duplicate  positive  or 
reproduced  positive  image,  the  mechanical  equipment  used  to  light,  magnify,  and 
measure  the  Image,  the  human  eye,  and  the  analyst's  mind  wherein  the  imagery  is 
translated  to  a  facsimile  of  the  object  on  the  earth's  surface. 
..Between  the  object  and  ihe   report  the  image  can  be  said  to  have "one  stable 
or  real  stage  and  one  instable  or  imaginative  stage.  The  stable  stage  is  that 
recorded  on  the  original  negative  which,  once  processed,  remains  the  same  until 
or  unless  emulsion  deterioration  sets  in  for  organic  reasons.  Reproductions 
from  that  negative  are  essentially  also  parts,  of  the  stable  stage  although  one 
may  differ  In  quality  from  another.  The  instable  stage  is  that  recapitulated 
In  the  mind  of  the  analyst;  It  occurs  f leetingly  in  single  recognition  fragments 
the  total  and  composition  of  which  becomes  not  so  much  an  image  as  a  reasoned 
conclusion,  whose  recording  depends  on  pre-knowledge  and  descriptive  ability^ 
It  is  assumed  that  we  are  sufficiently  versed  in  the  art  and  purpose  of  under-r 
standing  to  seek  explanation  for  whatever  knowledge  is  necessary  for  insight 
to  occur.  The  physical  media  and  means  which  control  the  transmission  and 
reproduction  are  discussed  as  they  pertain  to  the  present  state-of-the-art 
of  Imagery  analysis.
r\ 
THE  FILM 
Composition  and  Characteristics 
The  requirements  specified  for  films  to  be  used  In  the  procurement  of 
aerial  photography  are  necessarily  very  stringent.  The  reliability  of 
^i^v^**"" : "'  'Information  extracted  from  the  photography  Is  dependent  upon 
the'ti Im  meeting  these  requirements. 
A  film  is  composed  of  two  basic  parts — the  film  base  and  the  photographic 
emulsion.  There  are  other  components  required  but  .Jhey  are  directly  related   _ 
to  one  of  the  above  or  to  their  combination.     *  - 
The  film  bases  used  predominantly  are  polyesters,  or  "safety"  film,  so 
cal  led  because  they  do  not  support  combustion.  The  two  most  important  charac- 
^terlstjcs,  other  than  stability,  are  greater  tear  strength  V .""... .'."J /~~.'^'^^ZZ^\ 
\&fM:x:\Z'j ■  ^fe"v:'VT?  These  films  are  much  more  expensive  than  the  cellulose 
acetates  available,  but  are  necessary  for;  .  J  cartographic  purposes 
because  of  their  high  dimensional  stability. 
.  r     The  emulsion  Is  a  suspension  of  silver  halide  crystals  within  a  gelatin 
mass.  The  gelatin  serves  as  a  protective  colloid.  Without  this  protective 
colloid,  the  developing  of  an  image  would  be  impossible  because  "the. si  I ver 
halide,  whether  11"  is  exposed  or  not,  would  be  reduced  to  silver  on  contact 
with  the  developer.  The  gelatin  is  also  essential  for  uniform  dispersal  of  — 
the  crystals  of  silver  halide.  Strict  quality  controls  must  be  exercised      . 
when  the  emulsion  is  applied  to  the  film  base.   It  must  be  smoothly  and  uniformly 
distributed  over  the  base;  otherwise  a  lens  effect  will  probably  result.  A  lens 
•  effect .produces  a  spreading  of  light  within  the  image,  solely  as  a  result  of  a   • 
non-uniformity  of  eeulsion  application.         •      ^ 
the  emulsion  and!  the  film  base  are  joined  by  an  adhesive.  -Additionally  . 
.an  anti-halation  coating  is  most  often  applied  to  the  non-emulsion  side  of.-the  .  . 
film  base..  This  coating  is  water-soluble  and  is  removed  during  processing  of 
the  film.  Some  file  bases  because  of  their  color  qualities,  do  not  require 
anti-halation  coatings.  \  ""     ■-•>-■  \ 
#*?W^:  ;  r     -       "  •  .-        J 
•  there  are  many  factors  which  must  be  considered  in  the  ultimate  selection   
txtl  a  particular  film  type  in  order  to  obtain  the  most -desirable  image  for     
maximum  interpretability.  These  include  film  speed,  granularity,  contrast- 
and  definition  characteristics,  spectral  response^-et^ .
;   Speed,  as  used  when  speaking  of  film.  Is  merely  an  expression  of  the 
sensitivity  of  the  emulsion.  This  must  not  be  confused  with  spectral  response 
which  Indicates  the  range  of  the  visible  portion  of  the  electromagnetic 
spectrum  that  a  film  emulsion  will  record.  Although  comronly  referred  to 
as  film  speed.  It  might  more  likely  be  called  emulsion  speed.  It  Is  a 
relative  quantity,  used  to  Indicate  emulsion  sensitivity  to  light.  The  speed 
depends  In  part  upon  the' size,  quantity,  and  dispersal  of  the  silver  halide 
crystals  In  the  colloidal  suspension.  These  factors  equate  to  the  granularity 
and  gralniness  of  the  film. N  Film  speed  may  be  thought  of  more  simply  as  the 
time  required  for  Incident  light  to. produce  the  desired  chemical  reaction 
within  the  emulsion,  thus  creating  a  condition  of  exposure.  Speeds  are 
expressed  In  general  terms;  I.e.  fast,'  medium,  slow,,  however  they  do  have 
definite  numerical  values  which  are  more  often  used  by  the  photogrammetrlst. 
The  granularity  of  an  emulsion  Is  an  expression  of  the  grain  size  and 
.the  density  of  their  dispersal  throughout  the  solution.  Gralniness  is  a 
psychophysical  quantity  referring  to  the  visual  appearance  of  the  granular 
structure  In  a  developed  photographic  Image.  Gralniness  is  evident  as- a  result 
of  an  agglomeration  of  developed  grains  or  an  overlapping  pattern  of  grains. 
Slow  emulsions  In  general  have  finer  granularity  and  better  resolution 
•than  the  faster  types,  but  they  are  not  necessarily  the  best  for  aerial 
photography  on  that  account.  In  past  years  fine-grained  emulsions  were 
excluded  from  use  In  aerial  photography  by  the  need  for  high  contrast  and 
full  emulsion  speed. 
of  a  film  for  certain  contrast  character- te  tics-does,  re  I  ate_d  Irect  I  y  to 
granularity  and  film  speed.  Contrast  of  images  is  a  function  of  maximum  and 
minimum* density  levels  af.  granularity  and  exposure.    
Definition  may  be  stated  as  the  degree  of  clarity  and  sharpness  apparent 
In  a  photographic  image.  Restricting  discussion  of  this  subject  to  film 
characteristics  only,  definition  is  more  dependent  upon  granularity  of  the 
emulsion  than  any.  other  factor.  Although- a  certain  level  of  sharpness  Is 
necessary,  it  should  never  be  forgotten  that  sharpness,  as  such,  without 
any  specification  of  focal  length,  conveys  no  guarante^of  good  resolu-fdn 
of  ground  details.
t 
Spectral  response  Indicates  the  range  and  peak  of  that  portion  of  the 
visible  spectrum  to  which  a  given  emulsion  Is  'sensitive. 
Many  other  experimental  film  types  are  available  and  being  tested, 
si)
Processing  Techniques  and  Procedures 
But  first,  reference  to  the  characteristic  curve  must  precede  detailed 
discussion  of  variable  factors  in  development.  It  is  the  most  common 
reference  point  utilized  in  describing  variations  In  density  scale  (or 
contrast),  and  gamma  rating  (or  relative  contrast).  The  characteristic  curve 
(Figure  8)  is  used  in  this  text  merely *as^  graphic  reference  for:  clarification 
of  statements  written  in  the  following  paragraph's. 
Density,  by  definition  in  the  photogrammetric  sense,  is  a  measure  of  the 
.degree  of  blackening  of  an  exposed  film  or  of  the  direct  image  in  a  positive 
print-out  material.  Density  scale  indicates  the  range  between  the  highest 
and  lowest  densities  in  negative  images.  The  term  density  scale  or  range  is 
"often  referred  to  as  the  contrast  capability  of  the  negative  or  positive  material-. 
This  must  not  be  confused  wfth  the  usage  of  contrast  as  it  Is  applied  to  imagery 
analysis..  In. this  latter  instance,  contrast  is  intended  to  indicate  the  relative 
physical  appearance  of  an  object  against  its  background.  ,  There  +h"e.term  has  a 
subjective  application. 
—  A  graphic  representation  of  density  range  as  applied  to  the  characteristic-  - 
carve  Is  shown  in  Figure  9.   It  is  shown  here  as  a  function  of  the  relationship - 
between  exposure. and  the  gec*r.a  va!ue  of.  a  given  emulsion.  „    ..    
"  The  dens Tty  scale  of  the  negative  is  most  Influenced  by  three  variable  " 
conditions:  the  brightness  scale  of  the  imaged  object,  the  exposure  level  of 
-the  camera,  and  the  degree  of  developmer.7.  After  the  imagery  has  been  obtained, 
onPy  one  of  the  three  variable  conditions  listed  can  be  altered— the  degree  of 
development.  It  also  is  probably  the  most  important  single  factor  which  can 
affect  the  density  scale. 
'•f^'r^-"/.-.'''-  •--••*;♦  r».-.  -"■*■  -  •    *   ■  ••*  •*-.--.  .*■  ^",A?t-*.Y'j*w>**»*-'*»  *•;••"■  y-fv*'  "\:?»****<^3r.'X<'^^*'.**".-v**»'f>c*.***,r'*  .*;*"" :
Mi»T    M*T    «* 
Bj—OMlfcy 
IMMMIIUHCt  J         T  **• 
♦- 
nnuiv«iogvi 
•  .Mi 
(VC* 
The  CtwMKilMk  Cum.  ' 
•.i 
'Figure  8.  lypical  characteristic  curve. 
1.0 
IOC  EX?OSU2c 
figure  9.  Gessa-density  relationship
y>». 
*  Of.  the  many  development  changes  possible,  variations  fn  development 
time  apparently  offers  the  most  satisfactory  method  of  controlling  the 
negative  density  scale.  The  range  may  be  regulated  toward  either  maximum 
or  minimum  density  as  shown  on  Figure  9. 
Gamma  Is  «  term  used  to  denote  Inherent  relative  contrast  of  a  given 
emulsion  and  also  the  degree  of  development.  Itvmay  be  expressed  more 
clearly  as  the  relation  of  a  negative  contrast  (or  density  scale)  to  subject 
contrast  or  brightness  range.  Gamma  is  synonymous  with  the  angular  measure- 
ment formed  by  the  slope  of  the  straight  line  portion  of  the  characteristic 
curve  (B-C,  Figure  8).  As  long  as  most  of  the  negative  density  range  falls 
within  the  bounds  of  this  straight  line,  gamma  can  be  a  reliable  guide  to 
contrast.  If  most  of  the  density  range  registers  on  the  curved  toe  portion 
of  the  curve  (A-B,  Figure  8)  it  then  becomes  an  unreliable  guide  to  contrast. 
The  preceding  description  of  density,  density  scale,  and  gamma  is,  by 
necessity,  an  oversimplification  of  each.  In  reality,  the  terms  are 
Interrelated  to  such  an  extent  that  each  Is  dependent  upon  the  variable 
conditions  created  by  the  others.     " 
Ci-"*" 
£$&&; 
'•■•*:'*■:£-  i
Resolving  Power  and  Ground  Resolution 
There-  Is  a  general  consensus  that  the  primary  limiting  factor  In 
Identification. and  mensuration  of  small  objects  is  resolution.  This  being 
the  case*  It  is  well  to  take  a  critical  look  at  resolution,  for  the 
photoanalyst  Is  concerned  with  photography  mainly  as  a  working  tool  In 
order  to  identify,  describe,  and  measure  an  object- through  the  medium  of 
the  photographic  image.  Again,  as  in  the  problem  of  optical  acuity,  we 
have  to  consider  the  camera,  the  film,  and  the  eye.  For  any  optical 
Instrument  a  theoretical  limit  of  resolution  may  be  calculated  on  the  basis 
of  the  wavelength  of  light  and  the  diameter  of  the  lens.  It  is  said  that, 
for  the  eye,  the  Image  of  an  object  must  have  a  diameter  .of  at  least  0.002 
mm  on  ths  retina  (46).  A  circle  0.1  nin  in  diameter  at  a  distance  of  12 
Inches  from  the  eye  should  be  distinguishable.  For  the  eye  we  have,  whenever 
necessary,  additional  magnifying  devices  beyond  the  present  capability  of 
any  film.  • 
If  we  are  to  work  up  to  that  limit  we  should  have  a  clear  understanding 
of  what  Is  meant  by  resolution  on  photography,  how  resolving  power  is  derived 
and:  measured,  and  just  what  values  and  problems  it  contributes  end  leaves 
unsolved  In  the  analyst's  quest  for  knowing  what  he  sees  a.-.d  his  attempt  to 
mean  what  he  says.  How  Is  a  certain  resolving  power  applied  to  a  particular  .. 
film,  and  of  what  practical  use  is  such  knowledge  to  the  analyst?  Is  there 
possibly  a  better  or  simpler  way  to  define  that  quality  of  the  film  which  - .  . 
Is  paramount  In  Its  use? 
Photographic  resolving' power,  as  an  Industrial  concept,  is  the  capability 
of  the  camera  system,  tens  plus  emulsion,  to  resolve  and  reproduce  fine  black 
and  whit**  lines.  The  finer  the  lines  resolved  the  higher  the  resolving  power 
of  the  photographic  system  in  use.  Resolving  power  Is  expressed  in  lines  per 
ml  1 1 Imeter.  % 
These  three  statements  generally  constitute  a  photoanalyst's  introduction 
to  the  capability  of  the  photographic  materials  which  are  to  be  the  tools  of 
his;  trade.  If  he  Is  Inquisitive  he  may  attempt  to  learn  more,  and  what  he 
usually  finds  out  before  ceasing  to  be  concerned  is  that  the -Lines/mm -is-.-—  . 
convertible  to  the  ability  of  the  photography  to  reveal  the  image  of  an  object 
of  certain  dimensions  at  a  certain  scale.  He  discovers  ther,  when  working^ 
with  a  certain  photographic  system,*  he  should  be  able  to  see,  under  magnifi- 
cation, objects  that  are,  say,  ten  or  five  feet  in  width,  whereas  with  anoth 
sysltam  he  should  see  something  only  one  foot  wide.  -  As 
another 
he  becomes  familiar 
*f  TUf        "  -    »» 
—  *•*  1«%»-*****>.*-),*Jli>-*^"' 
*   •  ■r>";  _
c  ■      ■  o       . 
with  more  and  more  photography  he  realizes  that  In  many  Instances  he  sees  * 
perimeter' fences  around  secured  sites,  that  such  small-scaled  objects  as 
electric  power  lines  and  guy  wires  of  towers  are  clearly  discernible  on 
coverage  which  Is  not  black  and  white  but  actually  a  motley  assortment  of 
grays.  He  discovers  that,  stereoscopical ly,  he  may  see  through  haze  or 
overcast  sky,  and  that  lines/mm  is  most  often  a  completely  disregarded 
term.  He  Is  not  necessarily  disturbed  by  the  increase  of  knowledge,  but 
he  Is  by  the  lack  of  any  clear  explanation  of  why  and  how  there  seem  to  be 
only  vague  connections  between  theory  and  practice*  Let  us  investigate- the 
former  and  attempt  to* put  the  latter  on  a  reliable  base  of  reasonable 
terminology.  .  <   .      .    
The  concept  of  resolving  power  and  the  tests  devised  to  determine  the 
resolving  power  of  an  optical  system  originated  In  astronomy.  There,  where 
the  photographic  telescope  Is  turned  to  the  night  sky.  It  is  always  impossible 
to  see  the  Individual  smal ler  stars,  although  it  can  be  known  beforehand,  that. — 
a  certain  instrument  is  capable  of  detecting  stars  of  certain  magnitude.  It 
Is  sometimes  difficult  to  separate  the  Images  of  two  distant  points  of  light, 
such  as  double  stars.  A  standard  test  foe  determining  the  qualities  of  a 
iens  capable  of  resolving  such  stars  was  developed  by  G.  B.  Airy  In  the  19th 
century.  ■  '.          .  ...   . 
A  lens,  whether  single  or  complex,  does  not  produce  a  perfect  image. 
This  means  that  a  point  object  will  not  produce  a  point  image  but  a  central  • 
prominence  surrounded  by  a  .blur  or  diffraction  pattern.  A  single  point 
Image,  if  large  enough,  is  distinguishable  as  a  blurred  entity.  In  his   --—-.•• 
studies  of  the  size  and  relative  fuminar.ee  of  the  .image  center  and  the   .  .  ..    
surrounding  zones  formed  at  a  focal  point  by  a  beam  of  lignt  passing  through 
a  lens,  Airy  proved  that  the  pattern  was  the  function  of  the  wavelength  of 
light  and  properties  of  the  lens  in  use.  Because  the  central  point  in  such 
a  pattern  Is  so  great  in  comparison  to  the  diffraction  pattern  surrounding 
It,  the  fatter  was  disregarded  end  the  central  disk  is  said  to  constitute-   
the  image  of  a  point  object.  This  is  known  as  the  Airy  disc  (Figure  10a).     - 
Unless  centers  of  two  such  points  are  separated  by  a  distance  equal  to  the. 
radius  of  a  single  point  Image  the  two  points  are  not  separately  distinguish- 
able by  that  particular  lens  system,  in  which  case  they  are  said  to  be  not -■- ----- 
resolved  (Figure  10b).  When  a  series  of  discs  of  known  size  is  rated  against  - 
a  certain  lens  the  "reso I v i ng  power  of  that  lens  or  its  amp I itude- reduction     
curve  Is  determined  along  with  its  optimum  focal  length.   _   — — 
•  * 
As  the  science  of  photography  developed,  involving  lens  capabilities   . 
and  object  sizes,  the  concept  of  resolving  power  was  applied  to  it  and  a 
similar  test  was  devised  that  is  used  to  measure  the  resolving  power  of
iW     5  
•• 
•w     "  
• 
-• 
/•\  * 
/ 1  \ 
■'Aft 
••  •      .      •  *  * 
/♦I 
V    '' 
.....•- 
r  •   i 
fr» 
■  « 
i    i 
^ 
i-  I 
f 
i    1 
1; 
"  '■-   / 
i.  i 
. 
.**"•     "  / 
i     \ 
•*       m 
1      \ 
A 
M 
•      * 
.  •  t. 
.-  -'-  —  *■—  "1 
* 
Figure  10.  a-  Diffraction  pattern  produced  by  perfect  lens. 
b-  absolution  betveen  two  close  images.  . 
■»  •  »  - 
t    :••. 
"  *    •  *
the  system  used  for  aerial  photography.  Lenses  used  for  aerial  photography 
are  subject  to  tests,  both  visual  and  photographic.  The  test  with  which  we 
are  concerned  here  is  the  resolution  test,  in  which  standardized  charts  are 
photographed  at  different  focal  settings  for  the  lens.  When  images  on  the 
finished  negative  are  found  to  have  the  best  overall  definition  of  the 
smallest  possible  test  pattern,  the  equivalent  focal  length  of  the  lens  is 
determined. 
The  resolving  power  of  the  lens  Is  determined  by  the  rating  of  lines 
per  mil  I Imeter  of  the'finest  line  pattern  which  is  fully  resolved  into 
separate  lines  on  the  negative.  The  exposure  and  development  of  the  film 
must  be  adequately  controlled  to  conform  to  the  test  situation.  If  the 
width  of  a  line  plus  an  adjoining  space  in  tha  pattern  is  0.5  mm  wide,  and 
the  rate  of  reduction  from  the  photographed  chart  to  its  image  Is  40  to  one, 
the  actual  width  of  the  finest  line  and  space  on  the  chart  is  an  eightieth 
of  a  millimeter,  and  the  lens  is  said  to.be  capable  of  resolving  80  lines/mm. 
This  Is  lens  resolving  power  rather  tfcan  photographic  resolving "power  which 
has  a  test  of  Its  own.  For  resolution  of  two  image  points  from  a  good  camera 
lens,  the  necessary  separation  is  such  a  small  quantity  that  it  causes  little 
difficulty  In  the  production  of  good  aerial  phOTography.  This  Is  because 
the  size  of  silver  halide  grains,  the  active  Ingredient  in  the  photographic 
emulsion.  Is  so  iwch  larger  that  granularity  of  the  emulsion  is  the  present 
limiting  factor./ 
I 
By  Its  very  nature,  camera  film,  which  is  made  up  of  a  more  or  less 
thin  coating  of  eeulsloci  on  a  fixed  base,  cannot  capture  the  full  Integrity 
of  an  Image  which  cooes  to  it  from  the  camera  lens.  When  the  image  falls  on 
the  photographic  oauteioci  it  suffers  a  certain  diffusion  which  is  relatively 
more  serious  the  smaKer  the  detail.         , 
- —  v 
In  order  that  an  inage  be  visible  when  a  photograph  is  enlarged  or 
magnified  there  must  be  a  higher  potential  of  visibility  than  that  limited 
by  the  scale  of  the  original  negative.  For  the  film,  the  upper  limit  of 
effective  magnificat  tor.  usually  occurs  at  the  point  when  the  diffraction 
.blurring  of  the  image  on  the  film  becomes  a  distraction  sufficient  to  offset  . 
the  gain  in  visual  acuity.   In  other  words,  the  photoanalyst  is  accustomed 
to  using  film  under  higher  and  higher  magnif icavio'n  until  if  "falls  apart," 
which  Is  when  the  grateness  of  the  photograph  interferes  with  furtner 
'analysis  of  the  image.  The  present  limiT  of  visibility  under- magnif icat ion 
Is  thus  seen  to  be  seT  by  the  grain  structure,  or  granularity,  of  the 
emulsion,  the  individual  silver  halide  grains.  The  film  therefore  has  I  inured 
Resolving  power  which  is  a  limiting  factor  in  photoana lysis.  Typical  values 
of  film  resolving  power  are  supplied  by  the  manufacturer.
For  typical  aerial  camera  film  tjhere  are  two  defined  sets  of  resolving 
power  presented  tn  the  lines/mm  framework.  One  Is  the  total  overall 
resolution  a'nd  the  other  a  contrast,  factor  relating  to  brightness  end  tone. 
These  lines/mm  Units  are  Innate  to  the  f  i  lm_emulsion,  3$$T3Fe??>?v ■        W 
rtJl^s-^^i. 
tnd 
mean  simply  that  a  standard  processed  negative  Image  can  he  magnified  only 
within  the  limits  set  by  the  resolving  power  before  undue  deterioration  of 
Image  quality  sets  In.  It  Is  not  essential  to  think  of  the  two  emulsion 
factors;  gawna  and  contrast,  separately  In  order  to  see  the  relation  between 
lens  and  film  In  resolving  power.  There  are  extractab ! e^ qua  I ities  of  the 
film  emu  Is  ton  that  can  be  evaluated  against"  the  focal  length-resolving  power 
capability  of  a  camera  lens  which  permit  most  effective  combinations  of  the 
two.  .   •»:* ';  .-  ■  :.'..'■      '..;  V ;; 
•  The  terms  resolving  power  and  ground  resolution  differ  In  meaning"  but  In 
discussion  of  photo  Interpretation  analysts,  often  unknowingly,  use  the  former 
when  they  should*  be  using  the  latter.  The  resolving  power  of  a  system,  or  any 
of  its  component  parts.  Is"  known  or  can  be  determined  before  the. system. .Is 
activated  in-f light.  Its  meaning  is  of  real  significance  to  photogrammetrlc 
and  optical  design  engineers  because  it  relates  to  the  objective  capabilities 
of  the  overall  photographic  system.  On  the  other  hand,  ground  resolution  Is 
a  measure  of  the  capability  of  the  system  as  expressed  by  results  obtained 
on  the  Imagery,  which  are  affected  both  by  in-flight-conditions  such  as 
light  attenuation,  contrast  ratio,  and  image  motion  compensation,  and  by 
reproduction  and  processing  of  the  film  In  the  laboratory. 
the 
The  photo  analyst  Is  most  concerned  with  the  ground  resolution  of  a 
system  and  what  Its  Implications  are  tn  regard  to  fulfilling  .his  specific 
^^^^^•.rscutrements.     For  this  reason  it  is  felt  that  ground  resolution, 
"  "  ^Sf^^-^rT^^fr^j  warrants  more  detai  led  discussion. 
When  the  photo  analyst  is  informed  that  the  ground  resolution. obtained* 
f^^^^^^l!kr^^  is  onQ  foo+»  ,+  ls  Ssnera,,y  his  conclusion  that  one-foot 
o^^^o^rW^Sn  the  imagery,  are  identifiable  and  measurably  In 
practice  however,  this  does  not  hold  true  for  many  of  the  J^:"^^*;^  terns 
observ.sd.  One  must  understand  ™eore£lsa""".ean"ng  °f  the  statement  ..•;;.;;y,>;>f 
^^i^i^^^^^^^^^^This  is  a  measure  of  ^ the  abi  I , ity 
to  resolve  a  resolution  target  of  uniform  size  and  separation  with  an  object- 
to-back^round  contrast  ratio  of  five-to-one  (Figure  6).  Most  §,.;.  .^, >..;", ;:1 
Items  t'rr  a  natural  setting  have,  at  besv,  a  two-to-ona  contrast  ratio.  This  . 
would  reduce  one's  ability  to  resolve  a  Target  wiThin  the  stated  one-foot 
figure.  This  does  not  mean,  however,  that  one  cannot  detect  and  identify 
ohiae+«  much  «;mai  Jar  thanl^SM  but  only  that  they  cannot  be  measured  to 
objects  much  smaller  thanBw£j^3|  but  only  that  They 
a  meaningful  degree  of  accuracy.  A  perfect  example  of  this  Is  the  ability  to 
see  and  identify  power  lines  when  the  cbject-to-background  contrast  is  favorable 
and/or  light  is  reflected  from  the  object  In  a  specular  manner  (Figure  4). 
t  •**„*;  +n*£ZW-;t*i 
i  *(,  .{*  J/??*  to";*  *" 
■•»  *.  *>  ^ 
H-*-^^.-^^^.
o 
Resolution,  by  definition,  indicates  the  ability  of  a  system  to  render 
a  sharp ty  defined  image.  Many  Items,  smaller  than  the  stated  ground 
resolution,  are  detected  and  Identified  even  though  they  are  definitely  not 
sharply  Imaged.  The  reason  for  this  Is  most  often  the  result  of  identifica- 
tion by  association;  aided  by  an  analyst's  ski  I J  and  experience  in  a  particular 
■  specialty.  Object  pattern  and  the  knowledge  that,  In  a  given  set  of  circum- 
stances, a  particular  Item  could  be  presenT,.  Is  also  an  Immeasurable  aid  In 
detection  and  identification.  •.■>.i*v::.        ::        «="•  -'-*••;  '. 
•■■.'-.„—."«r  ■•*  •  ■  "."."  •  •-■-         ^r--;  ,"".-"■ 
In  further  assigning  the  proper  perspective  to  a  statement  of  ground 
resolution  It  must  be  understood  that,  two  adjacent  targets,  one  oriented  in 
en  X-directlon,  the  other  In  a  Y-directlon,  will  have  different  resolution^ 
values.  This  occurs  because  all  Image  motion  Is  not  compensated 
■»^r»y?n.'-r?-'-"-r»'--  • :-  •  •■-;••  ••'^VfcV^V**7«.v'«'^j*^»»-wwn.'i—^^  '"       ..'   This 
"Is ~noftr  necessarf fy VltetVirentai  factor" Xnteri£non''an6'XieniU]cz7i6n,  but 
It  most  be  considered  In  mensuration.     Rather  than  applying  a  resolution  . 
value  to  an  entire  mission,  it  should  be  used  as  a  comparative  figure,^   
Indicative  of  relative  resolution  between  two  or  more  missions. 
■*.<r- 
It  may  eventually  be  possible  to  derive  factors  that  can  be  applied  to  • 
aerial  image  characteristics  to  determine  photographic  resolution  of  chosen 
emulsions.  But  because  resolving  power  depends  on  contrast  the  resolWng 
power  Is  lowered  under  field  conditions  such  as  haze  and  other  factors"  that 
lower  the  tone  of  an  object.  Resolving  power  actually  performs  no  function 
In  definition  of  an  object  by  virtue  of  distinguishing  its  contrasting  parts. 
Higher  resolving  power  in  a  system  is  a  laboratory  or  comparative  factor  only. 
In  review^  it  should  be  emphasized  that  ground  resolution  in  aerial 
photography  is  not  to  be  regarded  as  a  characteristic  of  Jha  system  itself. 
It  Is  a  threefold  combination  depending  or.  the  light  source  and  Its  energy;, 
the  Instrument  and  Its  capabilities;  and  the  receiver  and  its  sensitivity. 
It  becomes  rather  meaningless  to  speak  of  resolving  power  unoer  These  -_ 
circumstances,  but  the  practice  is  conrinued.  Conventional  resolving  power 
concerns  only  calculated  images  and  has  very  little  meaning  compared  with 
the  ground  resolution  to  be  applied  to  detected  images,  which  is  {the  value 
of  practical  interest  for  tfrose  who  work  with  aeriai  photography.    ;_._  ._-.-. 
.'I-'  ».:•..-     -         i*' .  ■;-  . 
Photoanalysts  are  really  Interested  In  "a  scalar  value  that  can  be' 
appt«*d  directly  to  the  film  and  used  to  derive  actual  dimensions  of  tne 
object  Image.  At" present  +h«*  va,ue  ls  besf  expressed  In i  terms  of. around 
resolution,  which  is  minimum  object  size  measurable,  Ci:.V.^..^  ..     ...   J. 
« ••- ■• -.  .,-.,■..-,   .,.,.--.-• — ~ —   -~-\.  Byt  even  grounc  .-ssolution 
i's~nbt  constant  because  landscapes'  are "hoY  plane  surfaces,  photography  is  _._ 
most  often  not  vertical,  and  the  Image  may  not  be  at^the  nadir  of  the ^      . 
photograph.  -.  -  ;.";.  .y*2   yjTV       .£&-/£-.•..- 
&&&:V^>r;  "•'.■''  ■\=^;:^^:^>}i^if-yl>y:.  :!^\^:-::^:r.
FIELD  CONDITIONS  *.  ••— .  '.'.  '  » 
■    -'-  ."■"  '■■..-     .."/•■•' 
The  Nature  of  LIcht  ff-J  - 
^W—   r  ■*—!  Illl    I    I  *  • 
"  *  *'**%*%*■  • 
•  •••,-■*' 
Light  Is  radiant  energy  evaluated  in  proportion' to  Its  ability  to 
stimulate  our  sense  of  sight.  Through  space  It  travels  In  straight  lines, 
commonly  referred  to  as  rays.  There  Is  a  repeated  pattern  of  electric  and 
magnetic  forces  along  the  path  of  such  a  ray.  The  unit  of  the  pattern  in 
this  repeated  sequence  is  the  wavelength  of  light."  Light  waves  approximately 
400  to  700  Mi  1 1  itr.icror.s  long  are  normally  sensed  by  the  human  eye  (Figure  il), 
and  are  said  to  be  the  link  between  the  object  which  is'  seen  and  the  viewing 
eye  which  sees  the  object.  For  a  given  amount  of  light  intensity  the  eye  is 
relatively  insensitive  to  the  red  end  of  the  spectrum.  Sensitivity  increases 
through  the  spectrum  from  red  to  yellow-green  (about  554  millimicrons)  and 
Beyond  that  point  diminishes  to  reach  inser.sltivity  in  the  violet  region 
(Figure  12).  The  exact. wavelength  of  light  which  Is  most  readily  visible 
changes  with  the  intensity  of  light  as  is  shown  In  Figure  13.  Light  perception 
and  color  sensitivity  vary  from  one  person  to  another;  the  variations  in  these, 
however,  are  due  to  human  factors  and  are  not  phys lea I  attributes  of  the  light, 
as  such.  * 
The  scattering  of  sunlight  in  the  atmosphere  results  In-the  phenomenon 
we  know  as  daylight.  When  the  air  is  sufficiently  clear  we  recognize  direct 
sunlight  and  extended  visibility.  When  air  contains  appreciable  amount  of 
water  vapor,  dust,  smog,  or  other  particles,  differentiated  as  Rayleigh  or 
Mees  atmospheres  depending  on  elevation,  known  collectively  as  haze,  there 
Is  weakening  of  sunlight  as  a  result  of  loss  of  light  scattered  skyward  again,. . 
as  well  as  limitation  of  distant  visibility  which  results  from  acruel  inter-  - 
ference  of  particles  and  from  diffusion  of  light  in  the  surface  atmosphere. 
Light  rays  that  undergo  either  refraction  or  reflection  in  the  course  of 
passing  through  the  atmosphere  experience  seme. polarization  before  reaching 
an  object  on  the  earth's  surface.  Because  blue  light  survives  the  journey 
better  than  the  other  wavelengths  we  have  the  impression  that  the  sky  is 
blue.  The  depth  of  blueness  is  a  measure  of  the  amount  of  light  "broken.. up   __•■ 
chromatically.   In  the  various  shades  of  blue  the  discerning  individual 
recognizes  certain  meteorological  or  atmospheric  conditions.  To  such  an 
individual  even  the  elevation  of  the  sky  is  not  a  constant.  "How  high  is 
the  sky?"  is  not  ar.  idle  question.
.•   Visible  spectrum 
Wbv«- 
la^th(X) 
8    -.      |       § 
.     I    I Tl'llllllMllll 
fttqucncyC^SO  20 
'•  I.  I     I'l'I'I'i'MI1 
M, 
»t87  6  5  -4-  3XK>  Cycfes 
Figure  U.  Wavelength-frequency  relationship  for  the  visible  and 
.  .  "  adjacent  spectrums  for  light  traveling  in  free  space. 
*    t  :. 
•      it-'; 
-•-  •  — «i 
400 
SN  tCO  »wv 
..  • 
.Figure  12.  Spectral  sensitivity  of  the  normal  human  eye.  ^ 
Si"?1  '* 
3&  * 
  
Visibility  curve  mt   $^*2|    /fisiw/ityeurv*  for 
vtty  low  tight  h^rth,  *    »    ■•     •  rcrmolltght testis 
LO 
|  
0.4 
"S  ■    • 
"™*^ 
/•A   \! 
1    '  1   \     \ 
•i 
/ 1/  K  K 
! 
/       /I 
\!\ 
^v nek 
350     400     <S0     500      S50      630      fe»      700      759 
VfovttengHt  in  Millimicrons  •. .  . 
"•■5?" 
Figure  13.  Relative  visibility  of  the  normal  human  eye  at  normal  and 
.-       greatly  reduced  light  levels,  showing  the  Purkinje  shift v 
•  <46
Light  in  the  Atmosphere 
* 
The  usefulness  of  aerial   photography  always  depends  on  atmospheric  and 
iitions  at  the  time  and  place,  v 
lighting  cond 
V"     ^^^^^^         ..„.,'JThe"  stato  of  fho  atmosphoro  may  allow  photo- 
graphy of  great  clarity,  of  complete  lack  of  any  imagery  frcrr.  the  earth's 
surface,  or  of  some  Intermediate  state.  To  the  photoanalyst  the  exact  cause 
of  the  obscuration  is  of  interest  mainly  as  he  seeks  to  fully  understand  the 
whole  condition  under-which  the  achieving  of  photography  is  limited. 
In  photographing  :""    J      ' '""■7T^^'V^  tne  light 
reaching  the  camera  from  the  ground  through  clean  air  is  appreciably 
attenuated.  The  long  paths  of  light  waves  suffer  more  from  attenuation. 
Obviously,  the  greater  the  distance  the  light  must  travel  through  the  • 
atmosphere,  the  greater  the  variety  and  amount  of  attenuation  produced. 
t 
All  particles  in  the  atmosphere  contribute  to  some  degree  to  the  scatter-' 
Ing  of  light.  Scattering  occurs  as  either  reflection  or  refraction  and  is  one 
of  the  major  processes  Involved  in  the  attenuation  of  light  waves,  and,  conse- 
quently, in  the  procurement  of  aerial  photography.  It  Is  manifested  by  the 
scattering  of  different  wavelengths  of  light  Fn  direct  proportion  to  the 
size  of  the  atmospheric  particles  which  they  may  strike.  Therefore,  the     . 
denser  the  atmosphere  and  the  more  diverse  its  molecular  composition,  the 
smaller  the  number  of  light  waves  properly  reaching  the  desired  object  on   _ 
the  ground. 
Playing  a  less  dominant  role  in  the  attenuation  of  light  waves  Is 
absorption.  In  absorption,  light  waves  are  completely  assimilated  by       
atmospheric  particles  equal  to  or  greater  in  size  than  the  wavelengths  of 
light  attempting  to  penetrate.  A  high  content  of  moisture  in  the  atmosphere" 
Is  the  most  absorptive  condition  usually  encountered  and  the  most  damaging  . 
in  the  procurement  of  usable  photographic  images.  The  common  relegation 
of  absorption  to  a  less  dominant  role  in  attenuation  Is  due  to  the  fact     — 
that  It  is  significantly  minimized  on  a  seemingly  clear  day,  whereas 
scattering  always  occurs,  regardless  of  atmospheric  conditions.   .    ..   
The  majority  of  the  light  waves  which  "ultimately  reach  the  object  on 
the  ground  is  reflected  back  into  the  atmosphere,  either  in  a  diffuse  or 
..  specular  manner,  and  is  subject  to  further  atmospheric  attenuation  along 
Its  path  to  the  camera.  In  addition,  the  effect  of  light  radiated  upward  _ 
from  the  atmospheric  impurities  may  be  even  more  important.  This  light 
plays  no  part  in  the  Image  formation  but,  falling  upon  the  camera  lens
■  ■•   '"  -•■    -'  "    .  ••■  .r.O 
along  with  the  image-forming  light,  •*  weakens  the  Image  and  further  lowers 
the  contrast  of  the  scene.  The  term  haze  is  used  for  both  the  minute 
particles  In  the  atmosphere  which  Impede  light  and  vision  and  for  the  lack 
of  transparency  "of  the  atmosphere.  Its  effects  are  important.  Haze  modifies 
Imagery  both  before  and  after  the  Image-creating  light  fails  on  the  object  of 
-Interest.  As  light  ts  filtered  earthward  through  haze  It  becomes  diffused 
and  scattered  instead  of  direct,  with  an  apparent  tendency  toward  scatter  In 
a  forward  direction.  Fully  diffused  light  Is  Incapable  of  creating  shadows; 
It  limits  variations  in  reflectivity  from  unlike  surfaces,  and  is  devoid  of 
strong  contrasts.    '  '/'■.  ■:  :  \ ■••-.."  -^  .     :\.  .  •:■■'.'  ■•.  ;'  "' 
••.•-*•  -v  -.-.v.  •.  •••■  ...   •  '••••.  •  , 
The  scatter  of  blue  light  by  thin  mist  or  haze  Is  "sixteen  times  the 
scatter  of  Infrared  light.  By  applying  an  Infrared  filter  to  screen  out 
the  scattered  blue  and  other  visible  light  waves,  a  clear  picture  may  be 
obtained  through  mist  or  haze.  However,  the  desirability  of  this  procedure 
•Is  dependent  upon  the  sensitivity  of  the  recording  film  to  Insure  that  there 
is  n©  appreciable  loss  of  resolution.  .  Ultra-violet  light,  which  is  said  to 
be  scattered  by  haze  In  the  atmosphere  to  a  much  greater  extent  than  is  light 
In  the  visible  spectrum,* Is  particularly  effective  In  causing  deterioration 
of  the  photo  negative,  which  degrades  the  picture  contrast. 
There  appears  to  be  no  way  to  measure* the  amount  of  atmospheric 
attenuation  encountered  on  photo  imagery.  At  present  the  only  way  to  quantify 
atmospheric  conditions  is  to  use  verba?  description.  Such  terms  as  clouds, 
haze,  fog,  and  poor  image  quality  are  the  most  comronly  used.  If  It  has  been 
Impossible  to  circumvent  undesirable  atmospheric  effects  in  the  camera  and 
flftn  systems,  there  is  nothing  the   analyst  can  do  to  increase  the  information 
transmission  capacity  of  the  imagery. 
•.■•-•. . — .. 
&/<»•.  ;• 
-.•-,
Reflection  and  Imagery 
Light,  reaching  through  the  atmosphere  to  an  object  ts  absorbed,- 
transmitted,  or  reflected  according  to  physical  laws  governing  light 
propagation  (Figure  5).  The- absorbed  portion  turns  Into  heat  energy  which 
may  be  detected  by  special  non-visual  sensors.  If  soma  of  the  light  is 
transmitted  It  passes  through  the  object  In  obeyar.ee  with  laws  concerning 
the  density  of  the  materials  through  which  it  passes,  The  reflected  light, 
the  portion  In  which  we  are  Interested  in  aerial  -photography.  Is  reflected 
Into  the  atmosphere,  carrying  potential  Images  of  the  objects  from  which 
It  ts  reflected.  ,         , ,  ...,.- 
Only  by  being  reflected  from  the .object  does  light  convey  the  image 
to  the  camera  where  it  is  captured  by  the  film,  or  to  the  eye  whence  It  Is 
conveyed  to  the  brain  and  translated  to  a  mental  image.  As  the  amount  of 
light  or  energy  Increases,  th'e  ability  to  capture  an  Image  or  stimulate     •- 
the  sense' of  sight  varies  from  zero  to  a  certain  maximum  and  back  to  zero   ~   - 
again.  With  complete  lack  of  radiant  energy,  image  projection  Is  Impossible. 
Opttmum  light  falling  In  clean  straight  lines  on  common  surfaces  creates   -•   - 
sharp  contrasts,  strong  shadows,  and  clear  visual  images.  Greater  absorption, 
of  light  by  an  object  results  in  darker  Imagery;  greater  reflection  causes-   ~- 
lighter  values.  Overabundance  of  light  reflected  from  a  surface,  whether  ... 
diffuse  or  specular  reflection,  obstructs  the  Image  and  produces  in  its 
place  a  formless  blob  of  light  (Figure  7).  Within  the  Image  producing  range   .. 
the  actual  amount  of  light  reaching  the  camera  film  or  the  retina  of  the  - 
eye  Is  less  Important  than  the  clarity  of  the  Image  which  it  transmits,  the 
sharpness  of  the  Image  boundaries,  the  strength  of  its  contrasts,  the  perfection 
of  Image  transmission. 
The  reflecting  capability  of  an  object  or  surface  varies  with  the  nature 
of* the* materia I  from  which  the  object  is  made,  with  its  surface,  and  with  the 
angles  that  its  various  portions,  large  and  small,  subter.c  to  the  direction 
of  the  sun,  the  eye  and  camera.  Where  the  surface  of  an  object  absorbs  and 
transmits  much  of  the  incident  light  a  lesser  amount  of  light  will  be  reflected 
(Figure  5a).  If  the  object  surface  is  rough  in  comparison  to  the  wavelength 
of  light.  Its  eletaentary  sections  lie  at  various  angles  to  the  direction  of  . 
Incident  light,  and  the  reflected  rays  are  scattered  In  many  directions.  "  If 
the  surface  is  minctety  rough  so  light  is  uniformly  scattered  In  all  directions 
the  reflection  is  said  to  be  diffuse  (Figure  5b).  A  matte  surface  is  diffusely 
reflective.  Diffuse  scarterSr.g  of  light  seriously  affects  object  definition  on 
a  photograph.  On  the  other  hand,  a  smooth  metal  object  or  water  surface 
reflects  light  from  a  single  plane,  the  surface,  and  mainly  In  a  single  direc-  - 
Tlon.  This  Is  called  specular  or  regular  reflection  (Figure  5c). 
*'*.*'•*•  •irff'S'i' ••'.*'■-*  .-    -  *  "* 
&;^^&^^
It  Is  Important  to  note  that  for  each  ray  of  light  the  angle  at  which 
It  strikes  the  surface,  the  Incident  angle,  determines  the  angle  at  which 
It  Is  reflected  (Figure  14).  If  the  sun  Is  high  In  the  sky  it  will  be 
necessary  to  be  closely  above  an  object  to  catch  strongly  reflected  light. 
We  earth-bound  creatures  have  more  occasion  to  experience  light  reflected-   
from  low  objects,  such  as  house  windows  facing  a  sunset,  or  headlights 
from  oncoming  cars  shining  on  rainy  pavements. 
Very  little  effort  has  been  made  to  clarify  what  actually  happens  when 
specular  reflection- strikes  camera  film.  To  fill  the.  photoanalyst's  needs 
the  explanation  is  simple.  In  aerial  photography  of  highly  polished  or 
smooth  surfaces,  the  amount  of  direct  reflection  may  be  so  great  as  to 
completely  destroy  the  Image  of  the  area  from  which  the  reflection  comes. 
.Destruction  of  such  an  image  results  not  from  the  actual  failure  of  the  light 
to  carry  details  or  outline  of  the  Image,  but  from  the  incapability  of •  the 
film  coating  to  absorb  immediately  the  overabundant  light,  The  resulting 
Irradiation  within  the  emulsion  has  the  same  net  effect  as  the  inability  of 
the  Human  eye  to  see  through  a  blinding  light'. 
■   *  * 
The  quality  of  an  Isage  Is  determined -cy  what  happens,  to  the  light 
reflected  from  an  object  as  a  whole  and  from  Its  component  parts.  Image 
sharpness  and  object  definition  on  a  single  photographic  mission  with  a 
given  quality  of  file  may  vary  from  excellent  to  useless,  ceper.clng  upon, 
the  clarity  of  the  light  which  reaches  the  object,  the  reflecting  capability 
of  Its  surface,  and  the  perfection  with  which  the  image  is  transmitted 
through  the  atmosphere  to  the  camera  and  film.  If  the  surface  which  faces 
the  camera  has  variable  IFght  absorbing  capacity,  this  should  be  revealed 
by  variations  ih.gray  shades,  from  black  to  white.  The  shades  end  variable 
qualities  of  images  are.  referred  to  as  tone.  Distinction  between  tonal 
values,  their  contrast,  Is  as  essential  to  photographic  Imagery  analysis  as 
light  is  to  vision.
V 
f 
t 
m    •     ■ 
•  v.  : 
»"*-. 
» 
..-','»  ' 
Figure.  1%.  Relationship  between  a  ray  Incident  on  a  plane  surface  . 
and  the  reflected  and  refracted  rays  vhich  result. 
-  •■ 
V  -. 
•■'"%.
visroN 
Its  Relation  to  Light 
  
i     -^— —   »*i  ii  
i 
Astronomers  have  found  that  If  an  observer  is  placed  In  a  dark  room 
having  only  a  very  small  opening  towards  the  night  sky,  he  can  see  stars" 
of  magnitude  7,  8,  and  even  8.5.  This  is  so  because  the  sTars  are  then 
seen  against  a  dark  background.  It  is  the  brightness  of  th«s  sky  on  which 
the  stars  appear  which  prevents  us  from  seeing  these  dim  stars  under 
ordinary  conditions  of  observation.  The  illumination  produced  by  the  faint- 
est star  we  can  see  with  the  naked  eye  is  about  I0~*4  times  that  produced 
by  the  sun.  This  means  that  the  earth,  receives  from  the  faintest  star,  per 
unit  area,  an  amount  of  light  which  is  one  hundred  million  million  times 
less  than  that  received  from  the  sun.  In  tests  determining  the  threshold 
of  vision  the  amount  of  radiant  energy  or  light  required  is  taken  as  the 
energy  content  of  the  flashes  which  are  seen  In  a  proportion  of  the  Trials 
equal  to  55  percent  frequency  of  seeing  range  from  two  to  5.6  times  10"*° 
erg.  The  mechanics!  energy  of  a  pea  falling  from  a  height  of  one  Inch  would, 
If.  transformed  Into  luainous  energy,  be  sufficient  to  give  a  faint  impression 
of  light.  These  figures  tell  us  something  of  the  high  quality  of  our  visual 
sensitivity. 
• 
The  deciding  factor  In  vision  is  not  the  amount  of  light  but  the  f 
wavelength.  The  Magnitude  of  the  energy  quantum  is  inversely  proportional 
to  the  wavelength  of  the  radiation,  as  is- shown  in  the  following  example. 
The  energy  of  a  quantum  of  violet  light  of  400-mi 1 1 imicron  wavelength  Is 
twice  as  large  as  that  of  a  quantum  of  red  light  of  800-mil  I imicron  wave- 
length. A  flipc  of  light  does  not  beh2ve  as  a  stream  of  Individual  quantum 
particles,  like  a  series  of  bullets.  It  is  only  when  light  interacts  with 
matter,  for  instance  when  It  Is  absorbed  by  the  retina,  that  its  quantum 
properties  come  into  evidence.  » 
In  the  case  of  a  human  eye,  vision  Is  achieved  by  its  refractive  system.  " 
Light  coming  from  all  parts  of  an  object  falls  or.  el  i  the  external  parts  of 
the  eye.  As  a  result  of  the  passage  of  light  through  the  transparent  media 
of  the  eye,  the  rays  originating  from  the  same  point  of  The  object  are,  .Idea I  ly, 
reunited  at  one  point  of  the  retina  so  that  to  each  point  of  thd- object -which 
emits  light  there  corresponds  a  point  on  the  retina  which  receives  seme  of 
this  light.  These  corresponding  points  are  arranged  in  the  same  order  on 
the  object  and  on  the  retina.  But  no  optical  image  Is  ever  completely  sharp 
and  clear.  Because  of  light  diffraction  and  optical  aberration  the  average 
eye  does  not  reach  the  theoretical  limit  in  Its  functional  activity.  In
seeing,  the  sharpness  of  the  image  Is  determined  to  some  d££re£  by  the  accuracy 
with  which  convergence  of  light  onto  the  retina  takes  place.  Normal  visual 
acuity  approaches  the  limit  imposed  by  size  and  spacing  of  receprors  in  the 
fovea.  The  sensitivity  of  the  eye  is  so  high  that  it  comes  vary  close  to 
the  absolute  limit  set  by  tho  quantum  properties  o'f  light.   Its  maximum   
accuracy  In  the  discrimination  of  form  Is  high  enough  for  the  wave  properties 
of  light  to  have  a  strong. bearing  on  the  subject.  It  Is  expectable  therefore 
that  the  theory  of  the  natural  limit  of  sensitivity  and  accuracy  of  physical 
Instruments  be  applied  to  the  study  of  the  physiology  of  vision.  But  it  is 
equally  expectable  that  not  all  attributes  of  vision  are  directly  comparable 
to  those  derived  as  either  attributesor  I  Imitations* of. such  physical 
Instruments.  ;"         '■',;•■•• 
=  '(£  >s 
P&&JJ:
o 
Visual  Acuity  > 
Acuity, .a  dictionary  synonym  of  sharpness,  is  also  defined  as  the  ability 
to  see  flue  details  of  an  object,  or  as  keenness  of  sense  perception.  To  the 
degree  that  It  is  mechanical,  visual  acuity  is  comparable  to  the  resolving 
power  of  a  camera  lens.  As  a  physical  phenomenon,  visual  acuity  Is  measured 
In  terms  of  a  critical  dimension  (diameter,  width,  or  other  scalar  value)  of 
a  black  test  object  just  barely  detected  on  a  white  background.   It  Is  defined 
as  the  reciprocal  of  the  visual  angle  subtended  by  the  smallest  details  which 
the  eye  can  distinguish,  in  this  case  the  thickness  of  the  black  bars,  similar 
to  those  In  the  camera  and  film  tests.  This  angle  is  expressed  in  mi  mires  of 
arc,  so  that  If  the  thickness  of  the  bars  which  are  distinguished  corresponds 
to  an  angle  of  one  minute,  the  visual  acuity  is  unity*;  if  it  corresponds  to 
60  minutes,  the  visual  acuity  is  1/60.  .Standard  visual  acuity  is  defined  as 
the  ability  to  see  an  object  so  small  that  its  image  subtends  an  angle"  of  only 
one  minute  of  arc  at  the  eye*.  This  one-minute  standard  is  said  to  be  used  in 
optomarrlc  tests  Involving  letters  on  wall  charts. 
Tests  of  visual  acuity  ere  commonly  made  with  black  or  colored  test  forms 
on  a  white  or  luminous  background.  Recognition  of  such  objects  depends  on 
other  variables  besides  size  and  shape,  and  on  the  whole  is  thought  to  depend 
on  distinction  between  object  and  background  over  a  sufficient  distance  to 
permHt  discrimination.  Mhen  bCack  test  objects  are  used  with  a  sufficiently 
wide,  luminous  background;  or  surround.  It  is  found  that  visual  acuity  increases 
with  Increasing  baeicjround  EcrJnance  until  it  reaches  a  maximum  value  after 
which  It. rises  no  farther-  This  is  in  keeping  with  the  everyday  observation 
that  in  dim  light  we  are  unabte  to  read  fine  print  which  is  easily  legible  at 
higher  Illuminations,  white  on:  the  other  hand  there  is  a  limit  to  the  fineness 
of  detail  which  can  be  seen  wrsth  the  naked  eye  even  in  sunlight.  Objects  of^ 
lower  contrast  t\e&3   to  be  larger  than  a  black-and-white  test  object  in  order' 
to  be  detected.  » 
In  one  field  experiimsnt  it  was  foj.nd  that  a  wire  w?s  seen  against  a  sky 
of  high  brightness  when  J-trs  diameter  subtended  only  half  a  second  of  visual 
angle.  For  this  it  was  foumd  tfcat  the  wire  had  to  be  at  least  one  degree  long. 
Shorter  wires  had  to  b>e  thicker.  Silhouetted  squares  were  seen  at  angles  of 
only  18  seconds.  Coast Bering  length  and  width,  squares  were  more  efficient 
visual  targets  than  fine  J!rses  by  about  a  factor  of  three.  In  another  test 
It  was*  demonstrated  rr.a?  tta  seeing  of  fine  wires  in  horizontal  or  vertical 
meridians  was  supar'Cor  to  such  seeing  in  obi ique  orientations.  This  may  be 
of  Interest  to  anyone  ccrcerned  with  imagery  because  there  are,  apparently, 
no  dioptric  factors  to  account  for  such  selectivity.   In  another  study  of 
visual  texture  discrimination  and  depth  perception,  results  are  believed  to
show  that  connectivity  detection  is  basic  to  both  of  these  visual  tasks  ar.d 
that  It  Is  a  more  primitive  process  than  form  recognition.  From  such  tests 
as  these  one  may  anticipate' further  understanding  of  the  visual  process 
which  may  be  of  service  to. the  imagery  analyst.  •.-•••' 
In  actual  practice,  the  ability  to  see  and  the  objects  that  one  sees  do 
not  necessarily  conform  to  theoreTical  findings.  This  Is  particularly  evident 
In  photoanalysis  where  much  of  what  Is  written  on  the  subjecT  is  reported  as 
the  results  of  tests  Involving  either  pure! y  mechanical  bases  or  human  vision 
under  prescribed  conditions.  There  is  no  doubt  that..nwch  of  the  testing  and 
reporting  is  both  valid  and  valuable,  and  that  the  better  read  analyst  is  the 
most  versatile  in  appreciation  of  his  own  visual  processes.  However,  it  must 
be  realized  that  the  experience  of  photoana'ysts  does  not  occur  under  control  Jed 
conditions  but  rather  over  the  full  range  of  light,  contrast,  object  variety, 
and  scale  of  necessary  definition*.  The  purpose  In  cartography  is  to  reproduce 
as' faithful  ly  as  possible  exa'ctly  what  is  seen  on  the  photography  used  for 
that  purpose.  That  cartography  and  imagery  analysis  are  two  different  fields 
of  competence  Is  strongly  avowed  by  anyone  who  has  changed  frcn  the  forr.er  to 
the  latter.  There  seems  to  have  been  little  investigation  of  what  an  analyst 
really  sees  en  photo  imagery  and  the  mental  comparisons  and  processes  by  which 
he  produces  hfs  findings.  It  is  not  surprising  that  the  mature  analyst  has 
.developed  reflective  and  reasoning  abilities  that  are  not  verified. 
r} 
-•V.*,7"V"i'.*^"*.    *■-  -  —--——a^^^^^^m  ,  .  .  . „.  ->,...  ..„.**.,  ^i  -^  ^.«t**i-«»— *.+ 
I
Stereoscopic  Vision 
IV 
There  appears  to  be  sorr.e  confusion  as  to  Just  how  stereoscopic  vision 
takes  place,  both  as  a  visual  activity  and  with  respecT  to  the  attributes 
of  the  two  images  that  enable  It  to  result  In  three-dimensional  perception. 
The  exact  nature  of  the  seeing  process  Is  obscure  er.d   Is  not  a  part  of  the 
current  discussion.  But  the  attributes  of  the  two  images  That  are  detected 
and  analyzed  In  the  visual  process  which  results  In  stereoscopy  are  the 
subject  of  some  concern  and  difference  of  opinion.  On  the  one  hand  it  is 
said  that  stereopsls  wilt  occur  when  correctable  stimuli  are  out  of  phase 
with  respect  to  some  reference  system  such  as  another  set  of  correlated  objects. 
Any  mechanism  which  can  detect  the  correlation  between' the  binocular  stimuli 
and  also  detect  a  difference  in  their  phases  can  yield  representation  of  depth. 
Fusion  is  not  necessary.  A  slightly  different  dimension  is  given  to  the  problem 
In  considering  disparity,  probably  the  most  responsible  factor  in  stereo  acuity. 
It  Is  said  that  disparity  exfsts  between  two  points  when  they  occupy  different 
visual  direction.  Disparity  is  an  adequate  stimulus  for  stereopsis  when 
there  are  other  points  of  different  disparities  in  the  field;  only  similar 
objects  can  fuse  and  therefore  only  similar  objects  can  be  seen  in  depth 
when  thay  are  disparate.  The  fusion  concept  fails  to  predict  outcome  where 
deptb  occurs  without  disparity  except  on  the  outer  edge  of  the  patterns. 
Elsewhere  it  has  been  said  that  ssonocular  acuity  depends  on  a  process  of 
boundary  recognition,  whilst  stereoscopic  acuity  relies  on  pattern  recognition 
over  large  areas.  It  Is  not  expected  here  to  explore  the  subject  further 
except  to  observe  that  the  exact  nature  of  the  process  is  important  to  the 
photoanalyst  because  of  its  bearing  on  the  processing  of  film  to  be  used  in 
stereoscopic  study.   If  it  is  not  known  whether  the  effect  of  fusion  or 
disparity,  or  the  recognition  of  edges  of  areas,  Is  the  more  important,  there 
Is  a  question  as/ro  the  desired  processing  of  an  individual  negative  or  positive 
for  special  study  purposes.  Solution  of  such  problems  as  these  should  be  sought 
from  actual  operational  material  and  varied  processing  rathar  than  from 
laboratory  test  objects  because  the  latter  do  not  include  the  natural  variables 
which  are  found  in  the  former. 
A  clearer,  more  meaningful  concept  of  stereoscopy  as  the  photoanalyst 
makes  use  of  It  is  contair.ad  In  recocr.Ition  that  the  absolute  convergence  of 
a  stng,le  point  never  determines  its  apparent  depth  5n  a  stereoscopic  Image. 
Its  relative  convergence  with  other  points  Is  used  by  the  brain  tovlocate'it 
with  regard  to  those  other  points  after  the  general  position  of  the  Image  is 
fixed.  In  binocular  vision  there  is  a  fixed  tie  between  accommodation  and 
convergence.  Thus  in  looking  at  a  point  two  feet  away  the  eyes  are  focused 
for  a  two-foot  distance  and  the  axes  of  the  eyeballs  intersect  at  two  feet.
Regardless  of  the  distance  of  the  fixation  point,  nearly  The  same  angle  of 
convergence  always  goes  with  a  given  focus,  only  varying  slightly  for  objects 
not  drrectiy  in  front  of  the  observer.  Soth  change  together  as  objects  of 
different  depth  are  viewed.   In  stereoscopic  vision  this  established  tie  is 
broken.  The  two  views  of  the  point  on  the  stereograph  determine  the 
convergence.  This  convergence  changes  with  changes  of  separation  between 
different  points  on  the  two  views.  However,  the  accommodation  remains 
constant.  I.e.,  the  focus  of  the  eyes  is  either  fixed  for  the  distance  from 
the  eyes  to  the  stereograph  or  by  a  distance  dictated  by  the  lenses  of  the 
stereoscope— usually  Infinity.  This  spilt  of  the  accommodation-convergence 
habit  is  the  outstanding  difference  between  binocular  and  stereoscopic  vision. 
Since  the  eyes  are  presented  with  a  situation  foreign  to  binocular  vision, 
the  psychological  Interpretation  tsnds  to  differ  frop»that  of  binocular 
vision.  v  . 
In  very  simple  stereographs  containing  only  separate  points  and  lines 
and  viewed  without  a  stereoscope,  the  resultant  interpretation  tends  to 
place  one  prominent  point  or  plane  of  the  picture— ca I  led  the  "fixation" 
point  or  plane— In  or  near  the  plane  o*f  the  plate.  Thus  or.e  point  is 
approximately  determined  from  the  acco.T.Todation.  The  depth  of  tne  rest  of 
the  points  of  the  picture  are  interpreted  from  this  point  in  accordance  with 
both  the  relative.converger.ee  with  this  poifft  and  the  other  <te?.n   factors 
relating  the  points  and  objects  to  each  other.
CHARACTERISTICS  OF  THE  IMAGERY 
Tone 
The  total  tmage  on  overhead  photography  exhibits  a  variety  of  related 
conditions:  tone,  brightness,  surrounds,  sharpness,  end  centres?;  whose 
effects  must  be  evaluated  In  order  that  object  recognition  end  description 
may  be  accorr.pl t shed.  Of  these,  tone  and  brightness^ are  direct  aspects  of 
object  appearance  and  result  from  light  modifications  as  the  object  modulates 
or  attenuates  the  Incident  radiation  both  in  Intensity  and  color.  The 
quantitative  relationship  between  the  light  or  photometric  Input  to  a 
photographic  system  and  the  output  of  that  system  is  expressed  i.i  terms  of 
tone  of  the  developed  photograph* image. 
Tone,  generally  defined  as  color  quality  or  value,  or  a  distinguishable 
shade  variation.  Is  detectibfe  as  visible  differences  or  texturai  values  in 
the  gray  scale  or  density  level  or.  black-and-white  photography.   It  is  the 
overall  tonal  quality  that  makes  one  photograph  immediately  more  atTractive 
than  another,  and  the  textura!  quality  of  iodividual  portions  cf  the  photograph 
that  give  It  character.  A  very  small  inequality  in  the  tonal  level  of  a 
positive  may  play  a  significant  part  in  the  information  quality  of  an  Image 
reproduction,  and  .careful  cor.troi  in  the  photographic  laboratory  is  essentjal 
to  first  class  Imagery.  For  specific  purposes,  such  as  Image  enhancement, 
the  tone  quality  of  adjacent  portions  of  the  film  may  be  varied  by  controls 
In  the  reproduction  process. 
Tone  reproduction  is  concerned  with  the  relationship  that  exists  between 
-the  object  and  its  .Image.  There  are  two  distinct  phases  in  the  conventional 
approach  to  the  solution  of  tone  reproduction  problems.  The  first  phase  deals 
specifically  with  the  objective  or  physical  relationships  of  the  tone  repro- 
duction cycle.  .The  second  deais  with  the  subjective  or  psychophysical  factors 
which  relate  to  the  human  observer.  It  is  in  connection  with  the  first,  or 
objective,  phase  that  analytical  methods  of  particular  interest  to  engineers 
have  been  developed.  For  the  photoanalyst  the  main  concern  is  the  relartonships 
between  tone  differences  on  the  photographic  copy  in  use  and  the  assurance  with 
which  he  can  derive  the  image  they  represent  in  order  to  identify  and  describe 
the  original  represented  by  that  irn^ge.  By  tonal  and  texturai  evaluation,  he 
distinguishes  between  a  flat,  smooth,  hard  surface  and  one  w:th  noTable  deprh 
or  minor  Irregularities.  A  functioning  railway  (Figure  15)  appears  different 
from  a  rail  line  under  construction  where  the  rails  are  lacking.  Hard  topped 
roads  differ  from  dirt  surfaces,  grasslands  from  shrubbery  as  a  ground  cover, 
a  corn  field  from  a  meadow.
r   •. 
¥   * 
FIGURE  15 
Photograph  shoving  rails  and  ties  on  railroad  bed. 
At 
;  
s9  
■
p 
Brightness  ..'.""■•"'■■ 
As  changes  in  wavelength  of  light  causa  changes  In  tone,  so  changes  In 
Its  amplitude  result  In  luminosity  or  brightness  changes.  Brightness  of 
Individual  colors  Increases  with  intensity  or  amplitude  of  iignT  and  is 
highest  in  yellow  light.  But  In  this  sonso  it  must  bd  distinguished  from 
the  curve  of  energy  of  the  spectrum  which  rises,  with  waveier.grr.,  from  the 
violet  to  the  red   end.  Brightness  is  the  afrribute  of  a  light  source  by 
which  It  appears  to  emit  light  from  a" minimum  for  very  dim  to  a  maximum  for 
very  bright,  the  measurement  being  in  lamberts  or  candle  power  per  square 
centimeter.  With  the  use  of  a  photometer  it  Is  possible  to  measure  differences 
In  brightness  varying  on  the  order  of  one  to  several  million.  Vision  is  said 
to  be  at  Its  best  when  brightness  contrasts  are  about  1:20,  end  without  visible 
discomfort  If  less  than  1:100.  Much  iower  contrasts  are  discernible.  Flash' 
glare  or  spot  glare  causes  strained  brightness  accommodation.  The  optimum 
level  of  brightness  varies  with  the  image. 
Actual  levels  of  brightness  vary  with  clarity  of  the  armosphere,  the 
time  of  day,  angle  of  The  sun,  and  the  season.  Variation  with  clarity  of 
the  atmosphere  is  well  illustrated  In  differences  in  photography  on  clear 
and  hazy  days.  Objects  of  different  brightness  In  clear  lighT  are  said 'to 
suffer  the  same  proportional  reduction  In  brightness  by  haze.  With  increasing 
light  scatter  the  difference  tends  toward  unity  of  brightness,  and  mereby  to 
decreased,  contrast „ 
Differences  resulting  from  time  of  day  are  well  recognized  by  aircraft 
crews  who  fly  sapping  coverage,  their  flights  generally  be.'r.g  restricted  to 
the  time  of  day  tn  n   desert  region  when  the  air  is  calm,  or  in  a  humid 
region  before  air  humidity  reaches  undesirable  concentration  iater  in  the 
day.  The  relation. of  photographic  brightness  to  angle  of  7ne  sun  is  illus- 
trated by  the  desirability  of  shadows  sufficient  to  show  some  aetail  but  not 
to  obscure  brightness  and  outline  of  related  images.  Seasonal  differences 
cut  across  el  I  of  these.  One  investigator  found,  for  example,  that  in 
Washington  June  morning  light  is  brigivrer  and  June  afternoon  light  is  duller 
than  December's.  Strictly  seascr.al  differences  include  the  dar*  quality  of 
burgeoring  vegetation  or  the  aioedo  of  snow  cover. 
The  range  of  brightness  which  can  be  acceptably  recorded  represents 
the  useful  range  of  a  photographic  materiel,  ir.visibi  I  iTy  is  experienced  when 
emulsion  sensitivity  is  insufflcienT  to  record  an  overweak  light  source. 
Irradiation  results  when  the  upper   iimit  of  brightnesses  overreacr.ed.  When' 
specular  reflection  or  overbrighTness  occurs  the  emuision  suffers  excessive 
Co 
TV 
AV,\
particle  activation  and  the  resulting  Image  Is  only  a  nominal  representation 
of  the  object.  There  are  Instances  when  specular  reflection  from  small 
objects  such  as  wires  or  other  smooth  metal  objects  thus  bring  Into  the 
range  of  detection  Images  that  could  not  otherwise  register  within  the  limits 
of  the  resolving  power  of  the  film.  The  useful  range  of  negative  film  is 
greater  than  the  range  of  brightness  In  the  vast  majority  of  subjects   — — _ 
photographed.  The  exposure  scale  of  positive  materials  Is  much  less,  and 
In  general  the  higher  the  gamma  to  which  It  Is  developed  the  lower  Is  the 
material's  latitude  of  capability.  .     ;.'. 
*  Ideally,  a  photographic  objective' should  be  capable  of  reproducing 
accurately  the  full  range  of  brightness  in  the  object  -photographed.  |n      • 
order  to  do  so,  It  Is  necessary  that  each  area  In  the  Image  receive  light"   •""- 
only  from  the  corresponding  area  In  the  object.  When  other  than  true  image-  . 
forming  light  falls  upon  the  image  area,  an  adulteration  of  brightness  occurs 
and  *»e. photographic  reproduction  tacks  the  brightness  quality  possessed  by  ....   
the  original  subject.  If  the  negative  linage  were  perfect,  and  the -brightness  '.'* , 
of  the  photo  copy,  translucent  or  opaque,  varied  exactly  with  the  brlght.ness__.__ 
of  the  negative,  the  photographic  image  brightness  should  be  comparable  to  .  .  „. 
that  of  the  object  as  seen  by  the  human  eye.  Because. exact  rendition  of    .... 
brightness  falls  to  occur  at  each  stage  tn  the  process  ,~the  I  deal -Image.  J  s   
never  fully  realized.  *
(  o  
■ 
Surround 
The  surroundings,  generally  called  the  surround,  of  an  object  are 
Influential  .In  Its  detection,  enhancing  the  possibility  when  differences 
are  notable,'  minimizing  It  when  they  are  minimal.  Probably  the  most  notable 
difference,  and  one  often  overlooked.  Is  the  general  site  location.  A  man-' 
made  site  Is  most  apparent  In  an  otherwise  natural  setting  (Ftgure  16).  A 
missile  or  Industrial  site  In  the  wilderness,  a  frequented  waterholej  ;, 
ESS8S&  8  +ral1  across  the  desert:  these  are  most  obvious  not  because  "Crf .... 
size',  luminance,  or  tone;  but  because  of  differences  In  context.  While  ft 
Is  true  that  what  the  analyst  sees  depends  on  the  perception  of  differences 
In  luminance  and  tone,  and  that  these  are  used  to  distinguish  an  object 
from  Its  surround.  It  Is  Important  to  bear  In  mind  that  the  term  surround 
encompasses  an  overall  view  of  the  site  In  which  an*object  occurs.  In  this 
respect  we  distinguish  between  surround  and  contrast. 
•  An  object  anywhere 'Is  recognized  because.  It  has  different  tone  or 
brightness  from  .Its  surroundings.  Brightness  difference  allows  a  distinction  -. 
•  of  the  border  between  an  object  and  Its  background,  which,  according  to  tests, 
can  be  rated  as  a  product  of  the  area  and  the  luminance,  one  decreasing  as  the 
-  other  Increases.  With  low  contrast  object  and  background  the  size  of  the 
object  must  be  greater  In  order  that  the  brightness  differences  can  be  detected;  - 
It  has  been  said  that  larger  targets  require  less  contrast  but  more  total  flux  ■  — 
as  the  area  Increases  until  a  certain  stage  Is  passed  when  the  required  contrast 
becomes  Independent  of  area.  Although  It  Is  also  reported  that  the  greater  the 
ratio  of  length  to  width  the  greater  contrast  required,  It  must  be  realized 
that  under  field  conditions  this  Is  not  necessarily  true.  The  relationship 
also  vartes  notably  with  shape  and  fine  detail  of  a  complex  object. 
• 
It  Is  dlf*lcult  to  compare  profitably  actual  photographic  analysis  problems 
with  such  laboratory  test  series  on  object  and  surround.  In  test  studies  the 
objects  are  most  often  simple  geometric  shapes  of  different  shades  from  black  to 
white  viewed  under  different  brightness  conditions.  Under  field  conditions  It 
Is  only  rarely  that  the  analyst  seeks  a  simple  geometric  object  or  a  uniform 
tone.  The  surround  of  an  object  is  stable  on  any  single  or  stereo  view,  but 
the  contrast  between  the  two  parts  of  a  stereo  pair  may  be  revealing.  This  Is 
mainly  as  a  result  of  the  luminance  and  tone  and  not  because  of  the  change  In 
the  surround  as  such.  The  effects  of  variations  In  luminance  on  recognition 
-•of  an  object  on  a  dark  or  light  ground  Is  difficult  to  equate  with  the  effect 
of  object  size,  shape,  and  composition  as  It  actually  occurs  on  one  set  of 
coverage  In  comparison  to  another.  It  Is  the  analyses  feeling  .that  there  needs 
to  be  a  clarification  between  the  objective  concept  of  surround  and  the  analyst's 
Interpretation  of  the  word. 
..•>•„.•.,.,,  a*
*  • 
*  * 
*  
r 
*•: 
f1. 
■i 
V  i 
-FIGURE  16 
Photograph: 
• ;  i
i  <» 
..■I
.  r   \ 
i    * 
i  • 
i 
[
Sharpness  of  Definition  '   *      ('/ 
•  •       ■ 
Sharpness  of  definition  relates  to  Image  clarity  rather  than  to  contrast 
as  a  term  denoting  differences  In" either  brightness  or  tone.  Thus  It  Is  not 
precisely  synonymous  with  contrast,  although  the  two  are  known  to  act  together 
to  enhance  or  degrade  the  value  of  the  photo  coverage.  For  example,  a  hazy 
atmosphere  degrades  the  definition  of  the  object  by  diffusing  the  light  before 
It  strikes  the  object,  as  Is  exemplified  by  lack  of  shadows.  At  the  same  time 
the  atmosphere  lowers  the  photographic  contrast,  reducing  both  brightness  and 
tone.  But  the  two  dl'ffer  In  their  effects  on  Image  clarity.  Although  the 
general  effect  of  flare  (non- Image-forming  light)  Is  to  Increase  Illuminance 
of  every  point  of  the  camera  Image  and  to  reduce  the  contrast,  It  Is  said 
that  over  a  limited  range  of  exposure  a  certain  amount  of  flare,  or  for  that 
natter  haze,  can  actually  Increase  contrast  In  the  negative,  it  Is  possible 
for  the  overlay  which  affects  Image  quality  to  be  either  lighter  or  darker 
than  the  object  of  attention*. 
•.  •     •     ■■'•.    •*.•..     •  •    .   '••/ 
In  addltton  to  brightness  contrast  and  tone  contrast,  linage  clarity  Is 
affected  by  the  depth  of  focus  of  the  lens,  by  the  position  of  the  object 
Image  on  the  film  frame,  and  by  the  relation  between  sun  angle  and  camera. 
In  optical  terminology  the  term  depth  of  focus  means  the  total  allowable 
variation  of  the  sensitive  surface,  from  the  position  of  best  focus,  within 
which  the  Image  Is  tolerably  sharp,  according  to  a  given  standard  of  perform- 
ance. In  aerial  photography  the  range  Is  understandably  wide  and  the  Image 
clarity  variable.  V     ......  -^ 
,£*<;;»»<**«"< 
Object  distortion  can  be  thought  of  as  variation  of  magnification  with 
field  angle.  All  other  things  being. equal,  an  object  Is  most  perfectly 
defined  at  the  nadir  of  the  frame.  It  tends  to  be  degraded  with  position 
elsewhere  by  Imperfection  of  the  camera  lens  Cnp  lens  Is  perfect),  by  the 
Increasing  distance  of  the  object,  by  attenuation  of  the  Image,  and  by  the 
look  angle  of  the  object.  The  desirable  look  angle  of  an  object  Is  not 
necessarily  the  same  as  the  desirable  mensuration  angle.  The  look  angle  on 
vertical  photoqraphy  has  been  said  to  be  best  If  shadows  fall  toward  the    ^ 
observer,/  '        '"'"  \ 
<"  ,    But  the  presence  of  shadow  often  obscures  detal  Is  which  are  more 
ynportant  than  comfortable  viewing.  *r   
j  Ciarlty,  a 
.V 
\ 
£',.•  **■'••***
subjective  sensation,  re     -s  to  the  quality  aspects  of  p*  fographs  associated 
with  which  ease  of  visibility  occurs,  while  sharpness  of  Jn  object  Image 
refers  to  "the  degree  of  resolution  of  element  detail  as  Is  determined  by.  .. 
granularity  and  resolving  power  of  the  film. 
Mechanically,  the  outline  of  a  black  object  on  a  white  background  Is.  r 
a  sharp  line,  and  the  objects  dimensions  can  be  precisely  determined. "' *   __\ 
Theoretically,  the  contrast  in  photography  of  a  black  and  whl.te  test  object   ~ 
should  be  equally  distinct.  But,  because  the  photographic  emulsion  Is  made*  "  ".: 
up.  of  myriads  of  mlnl'scule  Irregular  particles,  the  line  is  not  a  sharp  cut 
'between  the  white  area  from  which  all  color  has  been  removed  and. the  blacky- 
area  which  has  suffered  no  effect  from  exposure.-  .Instead,  there  Is  a  zone,".  .^.. 
however  narrow,  of  gradual  change  between  the  black  and  white.  The  quallty- 
of  a  picture  Is  closely  related  to  the  shape  of  the  curve  which  describes   ___ 
the  brightness  transition  from  a  light  to  a  dark  area*  the  edge  gradient..  ...  ^. 
__•  The* character  of  that  zone,"  the  steepness  gradient  of  the  curve  revealing., 
the  remaining  emulsion  density  contrast,  or  the- sharpness,  of  the  I  mage  outline, 
Is  referred  to  In  photogrammetry  as  the  edge  gradient  or  the  spatial- Inductions- 
depending  upon  whether  the  Investigator  Is  concerned  with  the  steepness  or  -— - 
declivity*  of  the  most  nearly  .vertical  portion,  the  edge  gradient,- or  the  wWthV- 
of  the  zone,  measured  horizontally.  between*the  density  plateaus-j3ijthe_-Pure^-^ 
white  and  pure  black,  which  Is  referred  to  as "spatial  Induction.   In  the 
.mechanical  derivation  of  an  object  Image  on  photography  _the_  edge,  gradient  may- 
be employed  to  distinguish  the  most  probable  locations  of  object  perimeters, .-;.. 
from  which  mensuration  Is  possible;  or  the  width  of  the  zone  of  gradual  change ,~
by  means  of  whlch.a.  longest  orjsftortest  "distance-between  object  perimetexs._:__ 
permits .a  greatest  or  least  size  to  be  attributed  to  the  object  as  a  whole. 
The  procedure,"  however.  Is  not  altogether  that  simple  because  ;  data.._ 
for  the  photographic  mission,  as  well  as  object  position  on  the  frame,  are_.__ 
-entered.  Into  the  machine  calculation  to  accounj  for  additional  mechanical  . 
variations  In  Image  response  .< And,  Just-as  the  visual  .analyst's  pbotograpMc  -_
..Image  clarity  or- sharpness  of  definition  Is  subject  to  envIronmentaLcondltions 
and  optical  limitations  In  determS  n  I  ng  the  "object's- absolute  position  and  size, 
so  the  photogrammetrlst  has  prafelems  of  equal  complexity  In  his  determlT>at Ions- 
made  from' mechanlca I  response  to  the  photography}  -  - •  -  --.  '  ;„  -,-. 
' ... , .  .^ ;. y 
.•  .-*.•-..•■•••       *»•
Contrast 
Contrast  Is'  the  expression  of  difference  In  brightness,  color,  or  tone, 
and  In  aortal  photography  analysis  It  Is  used  to  denote  recognizable  differences 
In  both  mechanical  and  visual  stimulation.  In  mechanical  considerations* 
negative  contrast  Is  defined  as  the  difference  between  maximum  and  minimum'' 
densities  of  the  Image,  in  this  context  the  contrast  of  a  duplicate  positive 
may  depend  on  the  brightness  range  of  the  subject,  the  degree  of  exposure  of 
the  film,  or  the  degree  of  development,  often  confused  with  gamma  which  Is 
relative  contrast.  The  terms  contour,  border,  and  outline  are  equated  to 
-  contrast  location.  In.  that  they  are  used  to  Indicate  /"the  narrow  region  that  ..   - 
separates  something  from  something  else  "on  '  Vphotographlc'  Image  or  t>ther  two- 
dimensional  representation.  In  photogrammetry  contrast  differences  are  measured 
In  a  variety  of  concepts  such  as  spatial  Induction,  edge  gradient,  object 
ixxtitatlon,  modulation  transfer  function,  and  others.  Each  of  these  IS  basically 
•  a«  analysts  of  the  hortzontaf  of  spatial  difference  between  two  adjacent,   
•Measurably  or  visibly  different  fractions  of -the  Image,  or  between  two  recog- 
-  nJzable  differences  In  density  of  the  emulsion.  Unfortunately,  In  the  emphasis 
'  an  mensuration 'If  "Is" frequently  the  dertvlng  of  the -exact.  I ocat ton  -of  a   --  -—-:-_ 
boundary  line  that  appears  to  be  the  most  Important  contribution  expected  of  . 
-H-he^hofo^arotVs'tV-.JRiat.anaJystv makes  a  distinction  between  .a  target  which   _  * 
can  be  discriminated  and  described  because  of  contrast  and  the  evaluation  of  -■  - 
a  minute  area  which  outlines  differences  of  brightness  or  tone,  such  as  are  __ 
"T-.the  subject  of  .laboratory  studies  of  contrast,    .  :_ .........  .  ,.•  _  ...    .   .  ^ 
  •  •  •.     •  •      ■  '      •  •  t  ..  ■  .  :..." 
—-—-—Contrast- has  been  equated. with. brightness. grange  of  a  subject^  completely    . 
disregarding  tone.  But  contrast  may  also  vary  from  weak" to  strong  according  =•-?•- 
to  the  degree,  of -differentiation  between -tones  of.  a  photo  Image.  Pattern  ..______. 
recognition  by  the  visual  analyst  depends  as  often  as  not  on  non-adjacent 
*  variations.  In. contrast,  either  luminance  or  tone.  In  this  context  contrast    
has- long  been i  used. In i  a  descriptive  sense  rather  than  as  a  mensural  quality.   
And,  as  the  Imagery  analyst  "uses  fTr"cbhrrast~eair-mearrthe  ccmpeHson-of   -   
relative  luminance  and  tone  differences  between  two  or  more  sequential  Images  --• 
of  "an  object*.   .••—-..  . ..  ..:.  ... —   —     ..  .    .   _   
Visually,  though  not  for  mensuration  purposes,  the  exact  edge  of  a  contrast 
need  not  be  precisely  located,  but  the  concept  ts  confused  by  statements  .which- 
matte  distinctions  between  surface  contrasts  ont opposite  sides  of  a  line  of 
separation,  and  the  Implication  that  contrast  occurs  directly  at  the  border. 
It  Is  true,  photogrammetrtcally,  that  a  contrast  effect  takes  place  at  a  line 
of  separation,  but  In  Imagery  analysis  the  actual  contrast  between  two  things 
Involves  two' areas  rather  than  limitations  of  the  edge  or  space  separating 
-■r**V*'-',*2!*^; 
'^^fo  ■  '  ■;'  :•  '■  .:^-?S^^^&%v^-:^i^V  ? ?.'#<'   ■- !  
?&>#&.'  £'■?£&
o 
the  two.  When  sharply  outlined  edges  or  contrast  borders  occur  In  photo 
Imagery  there  Fs  no  detectlble  Increase  In  contrast  across  the  borderline 
as  distinct-  from  the  contrast  of  adjacent  areas.  As  a  matter  of  fact,  In 
analysis  exper fence  on  aerial  photography  there  Is,  probably  more  often 
than  not,  a  natural  decrease  of  contrast  near  the  edge.  A  few  examples 
will  serve  to  Illustrate.  A  concrete  highway  does  not  Immediately  abut  the  . 
grass  or  forest  cover  of  the  area  It  transits.   Invariably  there  are 
shoulders  or  disturbed  areas  with  Intermediate  tone.  A  slanted  or  rounded 
roof  loses  contrast,  near  the  edge,  duft  either  to  differential  reflection 
of  light  o«r  to  dlffractron.  A  pond  of  water  Is  almost.  Invariably  surrounded 
by*a  moist  but  not  surficlally  wet  shore  zone  which  ^frequently  (indiscernible, 
on  black-and-white  coverage  but  nicely  detectlble  In 'certain  chromatic  _.   " 
'photography.  . •  •  ;   
The  behavior  of  object  contrast,  both  as  to  brightness  and  tone,  must 
•be  consldtered  In  the  recognition  or  detection  performance  of  a  photographic 
.system  and  the  resolutlon-of  a  standard  film  type.  According  to  a  report 
on  tests  on  object  recognition  it  has  been  found  that  If  a  lens  Is  found  to  .  ..'.._ 
have  an  angular  resolving" power  of,  say,  I s2;000 -for. detail  of  cont.-ast  0.2,  ~  . 
:  It  will  resolve  from  20,00-3  feet  details  ten  feet  apart' on  the  -ground  "If  „..;—-;  - 
their  contrast  with  their  surroundings  Is  0.2,  but  much  smal  ler  objects. wl  1 1  ....* 
be  seen  In  the  "negative  If  their  contrast  l§  high  enough.  White  objects  ....  .." 
"oft"a-b*ack  background,  for  exaople,  will  be  resolved  if  they  are  about. f|ve"   "" .. 
feet  apart.  Conversely, "objects  of  lower  contrast  than  0.2  will.be  seen  In;  -_-■_ 
the  Imagery  only  if  they  are  larger  and  more  widely  separated  than  ten  feet_...___: 
In  this  case  the  contrast  differences  are  due  to  object  and  background  tone  -  - 
and  not  to  changes  In  luminous  flux  or  brightness  from  test  to' test ._ _  _   .-•■  ^ 
It  should  also  be  remembered  that'  single  objects  wl  14  of  ten.  "be  recorded  — 
at  sizes  smaller  than  the  l.lmit  of  resolution  for  two  adjacent  objects".  -As~~  - 
the  camera  gets -farther  away  from  an  object,  its. Image  at  first  gets  smaller   - 
In  accordance  with  the  laws. of  geometrical  optics,  but  eventually  a  stage  Is:— ~- 
reached  at  which  the  patch  of  light  "is  the  smallest  that  the  lens  -will  give.  _■.- 
-If  the  camera  continues  to  recede  the  size  of  the  Image. remains  about  -the-  —   _ 
same,  but  l ts* contrast  wEth  the  background  diminishes;  since  less  flux-enters 
the  lens  to  be  .concentrated  Tn-the  luminous  patch.  The  distance  at  which  .  ._.—  - 
the  object  finally  vanishes  in  the  negative  depends  mainly  on  Its  contrast"   .:  : 
with  -fine  background,  though  the  contrast  of  the  emulsion  also  has-some   .. 
•-  influence.  The  shape  of  the  Image  at  this  stage  is.  mainly  determined  by -the  .*«... 
characteristics  of  the  leas  and  bears  little  relation  to  the  shape  of  the  .-..-«  . 
object,  but  It  will  be  recorded  in  some  fashion  from  much  greater  distances  r:-;- 
than  might~be  expected  from  the  resolving  power  as  determined  by  standard  .7Vi.V
tests.  As  an  example  of  this  kind  of  effect,  negatives  taken  from  10,000 
feet  with  an  eight-Inch  lens  will  often  show  the  white  broken  lines  painted 
on  roads  for  traffic  control  at  corners;  the  Individual  white  patches  are 
of  the  order  of  three  Inches  wide,  while  the  ground. resolution  from  that 
distance  with  that  length-  lens  Is  approximately  two  feet  for  black-and-white-  • 
line  test.  The  nominal  resolving  power  of  a  lens-film  combination  has 
therefore  to  be  used  with  care  In  attempting  to  predict  the  size  of  objects 
which  can  be  recorded  from  a  given  height;  some  thought  has  first  to  be  given 
to  their  nature  and  the  contrast  with  their  background.   . 
"     ...■""•".'•.-•.■■- ;  ■  ""  ■  ;  ' '   '""'•.: 
One  of  the  greatest  problems "tn  photo  reconnaissance  Is  caused  by  tow 
levels  of  contrast.  After  Identifying  the  existence  of  a  contrast  there  is 
concern,  photogrammetrlcally  speaking,  with  the  linear  shape  of  the  line  of 
differentiation,  or  its  acuity.  In  much  of  the  test  work  reported  on  photo- 
grammetrlc  studies,  emphasis  has  been  put  on  the  premise  that  visual  acuity 
depends  In. large  measure  on  the  ability  to  perceive  contrast  at  the  border 
of  the  object.  It  has  been  suggested  that  the  critical  .region  of  a  target    -  . 
Is  a  ribbon  Just  Ins4de  Its  perimeter.    .   ^  ""*    """"!"."•  " 
-  When  mensuration  Is  attempted  the-sharpness  of  a  contrast  becomes"--'   
objectively  Important^as  does" the  abruptness  w \ th  which  .the  variation  takes 
place.  Attempts  aade  to  correlate  sharpness  rat  I  ngs"w  I  th"  physical  measurements  - 
of  some  aspect  of  the  developed  Image  have  not  been  entirely  successful.  Neither 
resolving  power  .nor  simple  density  relationships  across  an  abrupt  boundary 
between  light  end  dark  areas  result  In  satisfactory  correlations  with. sharpness 
ratings.  Because  the  existence  of  contrast-Is  the  more  clearly  discernible 
and  the~less  adapted .dto .determination  of  Its  origin,  the  greater  emphasis  In 
photoana lysis  has  been  placed  on  its  quantlf  ication.and,_as  a  result,  the 
term  contrast  is  now  most  commonly  used  to  Indicate  the  density~dTfference-or- — ^ 
edge  gradient.  Whether,  photogrammetrlcally,  a  contrast  Is  a  contour  to  be 
perceived,  a  soatTa  I  dev  I  at  i  on  to-be_r.eso  I  ved ,  ort  an  edge  gradient,  there  Is  ""■'- 
more  to  the  concept  of  visual  analysis  than  a  mere~tf1menstonal-va.lu£,  and  "     - 
more' to  Interpretation  of  photo  Imagery  than  definition  of  perimeters. 
-  The  details  of  an  object,  as  they  appear  in  duplicate*  posltlve-trans-  i  __ 
parencies,  are  often  incapable  of ..  def  J  nit  iqn._  They  may  have  suffered 
reduction  of  contrast  which  always  obscures  somebf'the  smallest  details  of  - 
any  object.  In  visual  analysisthat-obscurfjiglis/not-necessarlly  synonymous' 
with  lack  of  object  "identification.  Mensuration  of  the  minute  detail,  would  ."_   - 
truly  depend  on  sharpness  of  contrast  between  its'  area  and  Its-surround.  .....  . 
But  this  mensuration  activity  should  occur  only  after  recognition  has  taken 
place;  and,  from  the  photoana I yst*s  point  of  view,  the  need  for  mensuration 
must  depend  on  determination,  by  other  aspects  of  the  imagery,  otherwise 
derived,  that  the  minute  detail  Is  worthy  of  mensuration
Lighting  Defects 
Other  than  poor  contrast,  lighting  defects  are  usually  referred  to 
as  flare,  light  scatter,  poor  Image  quality,  or  halation,  which  plays 
little.  If  any,  part  In  modern  photography.  Any  report,  written  or  verbal,     
which  falls  to  use  these  or  other  terms  adequately  also  falls  to  convey 
to  Its  recipient  actual  conditions  which  may  be  of  significance.  Any 
explanatory  term  should  convey  a  message.  Transmission  of  Ideas  can  be 
notably  Improved,  and  even  reasonable  understanding  of  the  Imagery  may 
benefit,  by  knowing  and  using  each  term  to  mean  what  others  expect  It  to;  _.*~  /_''" 
signify.*  Poor  Image  quality,  a  catch-all  phrase.  If  used  only  for  the   ~    -^V 
briefest  reporting,  may  suffice  as  a  general  excuse  for  limited  review."   ":.:.  — 
Light  scatter,  used  to  Indicate  diffusely* reflected  light,  actually  says ;- «_  .:..  .*;
nothing  about  an  Image  nor  Its  quality.  Light  scatter  and  flare  should  -  ...  ."^J" 
be  distinguished  from. both  diffuse  and  specular  reflection.  The  latter."  :*_" 
Is  also  sometimes  referred  to  as  glare,  because  It  comes  from  particularly"^  -~ -T 
highly  reflective  surfaces  and  results  In  excessive  Irradiation.  Much  of*  " ~:3~ 
what  Is  commonly  referred  to  as  either  flare  or  halation  Is  also  Irradiation..  ---••• 
In  professional  usage  flare  refers  only  to  the  non-Image-  forming "I  fgh+  "_j 
coming  to  the  -fl!m  from  Jlght  reflection  wfthin  the  camera.  Non-Image-1. "  _*",//" 
forming  I  Fght  from  extraneous  sources  has  Identifiable  characteristics.  ari~d*':*zr*"-T* 
Should  be  Identified  by  otfcer  terns.  Light  scatter  results  from  atmos-   - 
pherlc  conditions  and  manifests  itself  In  the  reduction  of- contrasts  j.n_"~"~    ~" 
the  Image  because  of .  excessive  amounts  of  light  being  reflected  from  the."!  ...  T. 
atmosphere  ..Into  the  camera.  Tfcese  two  effects  are  clarlf  led.Jjn_  the  following^" 
explanation.   Although  the  man- Image-forming  light  must  be  consldered.as  ;r~TI*r^- 
part-of  the  Image  which  acts  opon  the  -negative  material.  It-  Is  convenient  ..  ,_. 
In  the  treatment  of  the  tone  reproduction  eye  I  e~  to  refer"'to  the  camera  Image  ---__- 
as  distinct  from  the  "flare  Image.  In  this  sense  the  camera  Image  may  be   __"::-_ 
attrl6uted  only  to  the  l?gf>1r  coming  directly  from  the  object  by  virtue. of   rLz.- 
the  refractive  characterise «cs  of  -the  .Lens  system.  *  The  flare  Image  Is  not   ~  V." 
an  Image  In  the  conventional*  sense,  since  It  conforms  to  no  geometrical."   T^ir'.".; 
pattern,  but  a  veiTbver  the  entire- Image  plane.  Its  effect  Is  to  Increase  •  -._" 
the  Illuminance  of  every  potwf  of  the  camera. Image  and  to  reduce. the  contrast. ~ 
One  of  the  simplest  explanations  of  Irradiation  Is  given  In  a  dictionary 
definition,  which  states  that  3t  is. the  spreading  of  light  by  the  grains  of  ..'...■  ~\ 
a  photographic  emulsion  causing  the  developed  image  to  be  larger  and  more_ -__-"_;. ^ 
diffuse  at  the  edges  "than  tltue  optical  Image,  /it  Is  differentiated  from   """-"-1L'.. 
halation  In  Figure  17.  To  a  noma!  degree  Irradiation  Is  caused  whenever  a......»r~". 
beam  of  light  enters  an  erraision -because  the  light  Is  always  scattered  or  _-"2^ 
diffused' to  some  extent  by  successive  reflections  from  the  sMver- hal  Ide— j .::^il"T
'  *"  *  .  *   * 
immmm*  ■  '*■ 
i?  J    -ft  k     J    u" 
.**   •    8  *>»»  1 8  •    111  iam*h».  S  <*       ' 
••«•    ■•  •t^.«^p» 
~ *       .     "»       •  '  •  ft  • 
~     -      «   -*•      „       - 
-■«-«•- i.ri*-  . 
'"  Jlgure  17.  Diagram  Illustrating  the  difference  between 
^v'..r  „  ^  _  irradiation  and  halation.  . "  :.l 
*  '     »■     •    ■    JMriMMy    *»•# 
'  *  ••^•ti 
Figure  18.  Imagery  of  the  edge  of  a  razor  blade  fros  fine  and 
coarse  grained  eaulsions.
grains  In  the  emulsion.   ,ie  greater  the  exposure  the  mo:  jpronounced  the 
spreading  of  tbe  Image  beyond  the  boundary  of  the  exposure  on  the  surface. 
Irradlatton  Is  the  term  which  fully  covers  both  the  cause  and  effect  of   ' 
such  diffusion  of-  light.  
* 
The  amount,  character,  and  dispersion  of  light  are  not  alone  responsible 
for  Irradiation  effects  In  photography.  Layer  thickness  of  the  emulsion 
and  Its  grain  size  also  affect  light  distribution  at  the  time  of  exposure.     
Results  of  one  study,' Investigating  these  factors  In  connection  with  image  —  ■"-  — 
definition*  indicate  that  the  exposing  light  diffuses  more  the  longer  it  . 
travels  through  the  emulsion  layer,  i.e.,  the  thicker*the  layer.  It-appears.^ 
that  definition  decreases  with -increasing  emulsion-thickness,  possibly 
because  of  increasing  light  diffusion  for  thicker  layers  where  the  total.— ^.  _.  ... ;
amount  of  scattered  light  is  the  same.— The  heavier -scattering  in  high.  -  :  _._ 
silver  hallde  coating  weights  offsets  the  advantages  of  increased  contrast »- M*.  ,-., 
Really  fine  grain  emulsions  show  little  Irradiation  (Figure  l8)..._The..  .  ^.--^  ^ 
process  erf  light  attenuation  in  the  development  of  duplicate  posJtLves-._^,„-i^"^ 
from  original  negatives  is  ordinarily  explained  by. absorption. and. scatter Ingu"Zn:-. ~ 
of  the  I  Ught  by  the  substance  of  the  image. 
•  *  A-pitenomenon  that-must  be-  understood  I  f.  ana  I  y  s  i  s  of  1 1  ght  I  ng  defects  ..  i ..  ,-.  ^ 
are  to  be  meaningful  is  halation,  it  was  coined  In  the  early  days  of 
photography  when  glass  plates  were  used  as  backing -for  emulsion.   It-was.. ..-— 1   L
shown  to  arise  from  light  being  reflected  from  the  far  side  of  the  glass  .J^  _- 
plate,  or  the  standard  film  backing,  and  thus  affecting  the  emulsion 
beyond  what  should  have. been  the  normal  boundary  of  the  Image  (Figure  17).  ;    
It  occurs  when  Image  forming  I  fght  passes  through  the  film.  Some  of  the  -•■—-• 
light  is  reflected  back"  into  the  emulsion  andr  because  of-  Lighted  liiuslond^^z..^ 
this  light  spreads  sidewise  beyond  the  boundaries  of  the  image  and  1hus 
reduces  the  sharpness'bf  the  latter.  If  a  small,  strong  source  of  light  is  '  -- 
_in  the  field,  and  an  especially  large  proportion  of  light  is  reflected  at  an-.-.^r 
appropriate  angle  "the  Imace  of  *ttie  point  of  light  "is  surrounded  by -a  c ire te 
 
or  halo— whence  the  name.  Halation  is  comparatively  small  when  the  thickness  _ 
tf  the  emulsion  is  so  great" that  the  emulsion  absorbs  most  of  the  image*- -^ — -— — :
forming  light, -or  when- the  <ani*tston  contains  the  diffused  light-so  well  that  -..-■ 
little  passes  through  the  file-  In  current  practice- an  opaque  backing  is    
used  for  both  glass  plates  and  camera  film.  In  aerial  photography  nonhalative  :._;.: 
film  Is  now  used  exclusively  and  with  such  success  that  film  experts  report —~-^ 
that  halation  Is  no  longer  a  problem.  However,  those  who  have  not  investigated  -= 
its  meaning  persist  fn  uslng-halation  as  an  expression  for  light,  defects  of_ 
all  sorts,  of  which  Irradiation,  flare,  light  scatter,  and  specular  reflection  -i; 
are  Important  In  present  photographic  practices.  Each  of  these  has  a  — z '-.ii-.;'^ 
particular  meaning  which  Is  as  Important  to  the  photographic  analyst  as  are  -  -  - 
the  exact  meanings  of  tone,  contrast,  and  other  terms  which  give  his  work  a  ...._,„ 
scientific  base.  „•""'   , '~"«\
i 
J 
C3J£CT  RECOGNITION  ANO  IDENTIFICATION 
*»  T    A  principal  conclusion  of  objective  Interest  Is  the  observation  that 
under  uniform  Illumination,  such  as  found  under  cloudless  daylight  conditions 
In  aerial  photography,  the. luminances  of  a  group  of. subjects  will  be  directly 
proportional  to  their  reflectances.  A  principal  aim  of  the  several  technical, 
photogrammetrfc'and  photographic  skills  is  to  provide  aerial  coverage  in 
which  the  reflectances  will  be  proportionately  imaged.  This  is  a  polnt_of 
contact  in  imagery  between  the  objective  or  engineering  aspects  and  the      ~ 
.  subjective  role  and  operations  of  the  Imagery  analyst.   ... 
3  *"   The  recording  or  definition  of  an  object  on  phofography  Is  obviously 
not  the  same  as  object  recognition  as  a.  visual  performance.  It  has  been 
said,  that  the  most  urgent  task  in  the  general  problem  of  qualitative 
.  evaluation  of  a  photographic  system  or  of  Its  separate  elements—photographic 
material,  lens,  etc. —Is  the  establishment  of  a  clear- relationship  between 
the.  results  of  objective  measurements  and  the  visual  perception  of  the"  image.  "" 
In  photography  the  capability  of  defining  the  object  depends  on  the  sufficiency 
of  film  quality  and  processing  for  adequately  registering  the  object.  "Visual " 
__ recognition  requires  an  object  size  and  clarity  that  are  within  the  range  at  "'" 
which  the  eye;  with  or  wlthout.supporti ng  optical  lens,  can  detect  the  image 
because  It  exhibits  a  sufficient  lateral  "sfze; — The-ob ject.musX.be  above  the 
limit  of  registration  of  the  photographic  system  currently  in  operaTion.   *   ^ 
Recognition  of  small  objects  is  enhanced  by  sharpness  of  outlines,  closely 
related  to  gralniness  of  the  film,  and  by  the  clarity  of  detail,  a  subjective 
term  that  signifies  overa 1 1  appearance  or_good  seeing  quality  of  a  piece  of 
_film,  which  Is  qurte  often 're I ated" to  contrast.  The  objective  basis  of  small 
c'etaTl  "read  I  bfMty -ts..  I  n_th_e_  photographic  capability  of  the*  combined  film 
and  camera  system.  The  problem "has" beer. "5eparated-4.nto._jts  component  parts 
in  the  observation  that,- with  highest, ..black-and-white  contrast,  "the  smaiiness— 
of  detectable;  objects- Is -a  measure  of  the  capabi  I  ityof  the  photographic  system 
or  the  absolute  or  "differentia  I-  sensi.tiv.Lty..cf_  the*  eye.--  These  conditions  are  " 
readily  differentiated  in  photographic  analysis.  """  """" 
 .... 
J,-         For  the  Imagery  analyst  the  main  concern-  Is  the  assurance  with  which  he 
distinguishes  character  differences  of  the  photographic  copy  in  use  and  with 
which  he  recognizes  the  details-  of  the.  image  In  order  to  identify  the  object   
represented  by  that  image.  For  him  imagery  analysis  consists  of -two  different  •' 
processes.  Image  detection  and  recognition  and  object  identification  and 
description*.  As  tfie'new  analyst  f ami  I iarizes  himself  with  imagery  he  draws   ~'~ 
on  several  unrelated  experiences.  He  has  looked  at  ground  photography  and  so 
Us  acquainted  with  landscape  imagery  as  such.  He  has  looked  down  on  landscapes 
•  I  ■  -    •  -    •  ..-■-•■•<■•■  "'■>■:- 
J
o 
either  from  a  h?M  or  mountain  or  from  an  aircraft  and  thus  recognizes 
layouts  and  tonal  differences.  He  realizes  that  many  elements  of  a 
landscape  may  be  perfectly  familiar  and  Identifiable,  but.  If  put  to  a 
test,  he  knows  that  In  no  view  will  he  be  able  to  Identify  everything. 
Some  of  the  efements  will  be  as  unfamiliar  to  him  as  a  word  on  paper  Is 
to  the  Illiterate.  And  some  of  the  elements  will  be  too  small  scale  for.  . 
seeing. 
2 ;  —  His  greatest  help  In  early  stages  af   training  comes  .from  looking  at 
lmagery"and.  having  pointed  out  tojilm  the  Images  of  things  with  which       -.--, 
ha  Is  familiar,  roads,  but  Id  I  ngs,  "aircraft,"  trucks;  and  representative    -  V".  '  \ 
patterns.  -This  makes  him  a  photo  readers  But  he  becomes  an -Independent    ~   ■; 
analyst  only  when  he  studies  the'ooverlf led  coverage  and  discerns  something  •-*'."*• 
for  which  he  has  had  no  previous  evaluation.  Only  experience,  a  conditioning  •-  - 
•  process,  *an  make  a  dependable  analyst  of  a  photo  reader,  but  he  can  be   "•. 
amazing  J  y_  as  sJsted  along  the*  way  If  he  knows  why  he- discerns  only  so  much  :^T 
at  any  stege  lo.hls  training.  An. analyst  acquiring': ski  1 1  In  Identifying  -"r''7-Z 
and  descplblng.speclaity"dbiects, 'Industrial  sites,  military  Installations,  •-V.~I 
'K2G25MZ£3£ii^  9r  whatever,  will  learn  wore  slowly  than  the  analyst-who-  — ~L^j 
already  has  competence  In  another  specialty  field  because  the  latter  will "r-  .::~S.. 
know  from  experience  what  specially  competence  Involves.  Detection  requ Ires -=- ^ :- 
only  the  recognition,  "that  thwriemey  be  something  of  Interest  ata-partlcular_  7Z 
spot  on  a  photograph.  Recognition  requires  areal  J  Imitation;  identification   .;:;
means  establishing  the' essential  character;  and  description  Involves -recount"   
Ing  or  classifying  critical  features.-  In  reflection,  It  seems  rather 
Incongruous  that  such  competence  stems  from  such  a  simple  thing  as  .the-  -  ■:  r.  ~_  — 
apprec ration  of  tonal  variations.  '   ------ 
A.  wide  variety  of_tests.  has  been  made  to  measure  an  Individual's  ability  ^  ": 
to  see  as  a  physical  performance  arid  as  a  mental'  perception.  When  this     -  • 
ability  Is  applied  to -skirt  In  reading  photography  the  question 'becomes  -"-'-"':"" 
terrlf really  Involved  because  very  little  serious  attempt  has.  been  made' to  -  '■'••'"'- 
distinguish  Just  what  the  analyst  can  be  expected  to  see  physically  from"  ~  - 
what  hetmaybe  able  to  Interpret  and  rationalize  because  of  special  knowledge 
or  skli'l  in  a  certain  field  enrich  may  be~of^photographJc_concern."  .If.  Is  — '^— ~" 
doubtful  that  the  quantities  derived  from  any  number  of"  relatively  simple -_:__-^£ 
vision,.,  detection,  perception  and  recognition  tests  can  add  up  to  an  adequate . 
conclusion  of  the  total  quality  of  an  analyst's -per  forma  nee.  His  recognition  -  ~-~ 
or  delect  Ion  performance  way  rate  higher  or  lower  than  his  ability  to  Identify.^. 
a  site.  .  |  -  -  .-=.**-. 
•  
■    •  - 
*■  •  --  ■ .  .  •   .■■■•-  -•.—...  ..  .■.         • 
  -.--  _•-••—....  -•■•  —  -,.. 
Let,  us  take,  as  an  example,  the  recognition  of  an  Installation  made  up 
of  six  or  eight  carjionent  parts  in  more  or  less  well  defined  arrangement.  ~L~"_v*.: 
If  phota  scan  reveals  most  of  them,  the  expert  analyst  will  know  where  to
A  J  ■  
<  .' 
' 
,  'rch  for  the  missing  bits  and  recognize  Ill-defined  but  nevertheless 
-otectlble  Indicators  near  the  threshold  of  vision.  Falling  this  he  may 
*   whether  there  Is  enough  assurance  In  the  parts  he  sees,  and  the  con- 
dition of  the  photograph,  to  postulate  that  the  Installation  can  be 
virHiod  without  them.  At  this  point  the  seeing,  or  visual  acuity,  must 
te  dlstlagulshed  from  the  mental,  or  subjective,  activity.       _  ........  .  ,___,.._ 
Throughout  reports  on  vision  the  term  threshold  ts  used  In  Irrecon- 
cilable connotations.--  It  ts  commonly  used  In  a  step-sequence  sense  such 
*_  brightness  difference  or  contrast  thresholds,  which  are  only  detect! ble 
variations  of  a  quality  rather  than  the  Introduction  of-a  new  effect.  The 
tidiest  use  of  threshold,' In  Its  unique  meaning  as  e-polnt  of  beginning,.- 
Is  the  threshold  of  vision,  which  has  been  defined  as  the  minimum 
eerccptlble  luminance  after  adaptation  to  utter  darkness  in  test  situations 
has  become  complete.  Such  a  test. Involves. a  person  sitting  In  a  dark  room,   
knowing  that,  after  his  eyes* have' become  accommodated  to  the  dark,  a  faint-  •— — 
lloht  will  come  on  In  small  step  Increments  and  that  he  Is  to  Indicate'   ___ 
frnodiately  his  awareness  of  the  light  at  whatever  stage  he  f  Irsf  detects 
It   This  Is  a  measure  of  a  person's  brightness  perception,  but  as  It  Is   ,  _  . 
neither  detection  nor  recognition  of  an  object  Image,  this  threshold  has  " 
*o  real  place  In-.photo  anaJy-Si-S-..^..  ________  ___.       '._.  _"  
. 
.  Another  use  to  which  threshold,  has  been  put  Is  the  threshold  of      _■ 
recognition,  or  of  recognition  or  detection,  as  If  the  two  were  identical. 
Htrv  threshold  Is  used  In  a  meaning  of  significance  to  the  photo  analyst.  ._  _, .__ 
!-*•;«  detection  Involves  simply  the  awareness  of  a  presence,  such  as.  Is   ....  .._. 
Ir.aicated  In  the  statement  that  objects  In  tests  be  large  enough  so  that   _- 
tne  detection  threshold  Is  not  a  limiting  factor.  In  other  tests  where 
♦  _'«!  thresholds  are  to  be  compared  there  Is  not  a  real  threshold  but  a 
rc;j!f(-_nt  of  recognition,  or  Identification,  which  are  goals  of  The  photo    _ 
•r-alyst.  In  the  form  test  cited  It  was  found  that  the  form. .threshold  varied  ___ 
ilrtrtly  with  the  ratio  of  perimeter -to  eree  and  Inverse  I  y_w.Ith  the  .magnitude  ... ; 
of  critical  detail.  Pet  In  practical  terms  this  means  that  with' 4  given  area 
a  circular  form  is  more  easily  distinguished  than  a  square,  rectangle,  triangle-, 
er  Irregular  form,  and  an  lhfrl<Mte~desTgirB6TO 
«v>r  r.an  that  a  photographic  image  of  one  form  could  be  detected  more  readily  - 
**.*n  that  of  another  btrty  rather,  that  In  real  life  simple  forms  .are  resolved-  — 
•id  recognized  more  readfly-tbaiv-complex  ones.      __..  '.J ■,_. — r.— _  __i 
Image  outl Ines  Tn  a'camera  film  are  dependent  on  what  has  been  cal  Led 
♦^  emulative  effect  of  everything  affecting  light  from  the  .object  to  the 
;-..:*oHo3ical  system  of  the  observer.  The  ability  to  see  or  to  distinguish 
*n»l|  Cb tails  of  an  Image  always  remains  a  goal  of  the  photo  analyst  because   • 
._,.—-— •*"?
■-,*, 
•'"•  ■■•"•"*»*i"««.. 
-*■:'*.■•.*» 
the  more  h©  can  extract'  from  a  photo  Image  the  more  r*..  Ined  his  Interpretation 
can  become.  -The  photo  analyst  recognizes  the  small  details  both  objectively 
and  subjectively.  If  a  simple' Image  Is  large  enough  to  be  reproduced  the 
task  Is  a  matter  of  observing  shape,  area,  and"  other  attributes  that  can  also 
be  measured  and  described  equally  well  by  another  photo  analyst  or  a  photo- 
grammetrlst  working  from  the  same  piece  of  fUm.  However,  the  Imagery  - 
analyst  Is  rarely  concerned  with  Identifying  a  simple  shape  or- Image..  .He   -\. 
Is  generally- concerned  In  what  may  be  called- 1  retaliation  analysis -to  differ-.^-, 
•ntlate  It  from  component  analysis  or  "area  analysts.  True,  he  must  evaluate 
the  area  or  site  locatlonyand  atso-the  minor  components  or- sma I- 1-  -f terns  that  .-- 
are  defined  by- contrast  pert  meters.-  But  his  most "  Important  function"  Is  the  _. 
knowledge  or  Judgment  he  uses  In  evaluating  the-lmagery- to  compare  such  -  .-.-..  - 
attributes  as -site-size,  location,. and  layout  or  pattern  wlth..the  .various:;:..:  -jzi 
components  whose  relative  arrangement" and  proportions,  are  collectively   :  ..  ..:„, 
Indicative  of  the  Installation  purpose  and  capacity.  The  evidence  of  con-. 
sttuctfon  or -operational  activity  at  the  site  or  In  proximal  or.  Hke_-locatiohs__ 
Is  «lso  Important;  Only: the~wel  I  qual I f  led  analyst: canJuMy.  equatri'hese  — ~_; 
etUrl  butes  -w I  th  the  performance  such  an  object  wou !  d  have .  And  I  f.  .the  out. 1 1  ne— 
and) detail  are  vague  he  cay  discern. suff I cfent  detail  to  deduce 
he-Is  viewing  has  .In  fact  certain  features  that  he. knows  should 
be  there.  .  • 
whether-  what   
or  „  should,  not.  .-^. . 
C  cu^t  ^j^r* "  ^fi'^y
,  i*  .*  .it-*' :»*>*i»*  *♦**»»**»-"  ■ 
!  ».      r     ■:-  .-:       ■      ..  Q 
g  ".Is  •    •         •■-  * 
il 
VjMARY      '   .   ■••.   .  .-*  .    •'■■■.•:-'■.■•'■  •    ,  ' 
'     *•■•."""■   .-   "  •      *"•  .'"■'• 
In  this  study  attempt  has.  been  made  to  discuss  photo  analysis  In 
-cneral  and  Its  problem  areas  fri  particular  from  the  viewpoint  of  the  visual 
I --.alyst,  and  to  Include  basic  factual  or  experimental  data  on  which  that 
I  analyst  can  draw  to  understand  why  and  how  problems  and  limitations  in 
•'jlysls  occur.  The  scope,  depth,  and  treatment  of  the  material  Is  expected 
I  «j  serve  three  distinct  purposes.  First,  as  a  llnj^e^ejn.^the^actlve 
I  .ia5ysts  and  the  non-4magery  analystsl^^^^^^^^^l^k.  It  Is 
-pessary  that  certain  ground  rules,  so  to  speak,. be  recognized  In  order 
f»nat  both  groups  know  why  sometimes  the  Impossible  Is  fac.ed  and  why  It  Is 
.^.attainable.  Second,  as  a  link  between  the  faagery  analysts  and  the 
jHatogrammetrlc,  photographic  and  engineering  professions,  It  has  been 
{...jgested -that  such  a  discussion  of  visual  ana.'ysis  problems  may  be  useful 
in  directing  Improvement  In  both  the  Imagery  ar  d  Its  content  and  In  the 
elective  support  provided  to  the  visual  analyst*.'  *   —  •••'■  *  ««--■■•— 
[     Last  but  not  least,  as  a  fairly  complete  coverage' of  analysts1- -problems—. 
»  and  actlvlties7~lt -should  serve_as.a_usefui  reference.  It  may  be  read  simply 
•:r  Informational  purposes.  It  presents  summary  Tdeas^and-a  wide -variety —  .. 
|  of  topics  that  are  open  to  elaboration,  discussion,  and  probably  to  dlsagree- 
I  rent.  It  may  serve  as  an  Invitation  and  a  starting  point  for  airing  and 
v changing  Ideas.  11"  should  evoke  suggestions  and  means  for  Improvements 
I  in  what  the  analyst  does  as  well  as  what  he  produces,  it  should  prove 
.r.cful  In  helping  anyone  Involved  In  Imagery  analysts  or  Its  benefits  to 
face  problems  more  realistically.  It  is  meant  to  explain  the  basic 
difference  and  resulting  conflict  between  visual  analysis  and  the  subsidiary 
activities  that  go  Into  the  analysis  as  such  and  Into  Its  exploitation  and 
results.  Most  of  all,  It  Is  meant  as  a  catalyst  to  clear  up  static  concepts 
!n  order  that  the  dynaatc  functions  of  the  art  may  be  used  as  a  basis  for 
valid  procedures.  .         * 
i