NATIONAL BUREAU OF STANDARDS REPORT

377k

THE DETECTION OF OBJECTS FROM PHOTOGRAPHIC TRANSPARENCIES

by

Deane B. Judd

To

Aerial Reconnaissance Laboratory

Wright Air Development Center

Wright-Patterson Air Force Base

Department of the Air Force

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Vi eeks, Secretary NATIONAL HI RKAU OF STANDARDS

A. \ . Astin, Director

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

0201-20-2325 November 12, 195U 377U

THE DETECTION OF OBJECTS FROM PHOTOGRAPHIC TRANSPARENCIES

by

Deane B. Judd

Photometry and Colorimetry Section

Optics and Metrology Division

To

Aerial Reconnaissance Laboratory

Wright Air Development Center

Wright-Patterson Air Force Base

Department of the Air Force

Contract No. AF 33(6l6) 32-21

E. 0. No. R677-3U( H) SA-10

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication,

unless permlssioi

25, D, C, Such f

cal ly prepared II

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015

or In part, Is prohibited

Standards, Washington

report has been speclflreport for Its own use,

PREFACE

This is one of a series of reports prepared under National

Bureau of Standards Project No. 0201-20-2323, "Color Reconnaissance

Studies". This project is financed by the Aerial Reconnaissance

Laboratory, Wright Air Development Center, Wright-Patterson Air

Force Base, Ohio (Air Force Contract No. AF 33(61 6) 32-21 )t, and it

is coordinated with Air Force Contract No. AF 33(6l6)-262 (Suppu.

Agreement S3(3U-383)) under Dr. Hugh T. O’Neill, O’Neill Associates,

Annapolis, Maryland, who supplies all requests for studies, and all

specimens and materials for spectrophotometric and colorimetric

analyses, except those directly requested or supplied by the Optics

Division and others at the Wright Air Development Center.

The present report resulted from two of Dr, 0 'Neill’s work

requests, which were assigned Oral Work Request Serial Numbers:

WADC-16/33 and WADC- 22/33.

H. J. Keegan, Project Leader

THE DETECTION OF OBJECTS

FROM

PHOTOGRAPHIC TRANSPARENCIES

byDeane B» Judd

I. Request

Information in the shape of a short report or memorandum discussing

the difference between the two following systems of Increasing the

degree of contrast between an object and its immediate surrounding on

a color -transparency:

System No* 1 FJye Colored filter Color film Source of light

System No. 2 Eye Color film Colored filter Source of light

In system No. 1, the eye adjusts itself gradually to a certain degree at

least and thus the contrast becomes less sharp. Any summary and literature pertinent to this topic will be appreciated.

The above is the statement of Problem. 22. Problem 16 is essentially

the same but couched In more general terms: "A discussion of the difference between the results obtained by studying color transparencies

(a) by means of filters attached to a frame, such as a spectacle frame,

and therefore held between the eyes and the transparency, and (b) by

rneans of a filter placed between the source of light and the transparency".

The solution of Problem 22 is thus a partial solution of Problem 16,

and perhaps is all that is required until there is supplied a more explicit statement of what, in addition to Problem 22, is meant by

"results obtained by studying color transparencies".

II. Introduction

Formulas are a-vailable for reasonably reliable prediction of the

color difference, or color contrast, between an object and its surround;

see NBS Report 3773 jjL]

.

The variables of the observing condition to

-^Figures in brackets indicate the index references at 'the end of this

report.

be specified are (l) the average luminance of the color transparency,

(2) angular size of the object or element in the picture to be discriminated, (3) angular size of the surround, and (10 character of the boundary

between object and surround. The primary data determining the degree of
(
-2 -

contrast, hence the ease or certainty of discrimination of the object

from the ground, are the tristimulus values, X,Y,Z, of the object or element, and the tristimulus values, X0 ,Y0 ,Z 0, of the immediate surround.

The formulas referred to above all assume that the average luminance is

sufficient for substantially perfect cone vision, that is, 10 millilardberts or more. Formula 1 of MBS Report 3773 further assumes that

the object subtends two degrees or more. If objects of smaller angular

size than this have to be discriminated, formula 2 must be used in which

the constants are expressed as functions of angular size of the con- trasting element. To develop whether system 1 or system 2 is superior,

we must study the basic quantities (X,Y, Z) and the four variables of

observing condition in turn.

III. Basic quantities (X,Y,

Z

)

If H is the spectral irradiance supplied by the light source, T

the spectral transmittance of the picture element to be discriminated,

and T^ the spectral transmittance of the colored filter, then the

tristimulus values, X,Y,2, of 'the picture element are defined for

system 1 ass

X «= Z H T T

f x A X

Y = ZHTTf y A\

z - Eh T Tf I A X

where x,y, z are the tristimulus values of a spectrum of unit irradiance

for the 1931 CIE standard observer.

For system 2 the definition becomes:

X = E H T

f

T x A X

Y = EfITf Ty AX

Z = EHTf Tz AX

It will be noted that the only difference' in these two definitions

is that T and Tf are interchanged in accord with the order of placement

of film and colored filter, and since this makes no difference in the

evaluation of the indicated products, systems 1 and 2 are identical as

far as the basic quantities X,Y,Z are concerned. The same argument

applies to the tristimulus values, X0 ,Y0 ,Z 0 , of the immediate surround.

IV. Average luminance of the color transparency

The value of Y for the color transparency as a whole is its average

>

-3-

luminance, and, as just shown, this quantity remains unchanged regardless of which of systems 1 and 2 is used.

Vo Angular size of picture element

Obviously the change from system No. 1 to system No. 2 leaves the

angular size of picture element unchanged*

VI. Angular size of surround

The condition most favoring the detection of a small color difference

between two contiguous elements is that the eye be adapted to a color

half-way between the two to be discriminated (SchBnfelder, 1933) txQ.

System No, 1, placing the colored filter directly in front of the eye

as in a spectacle frame, is probably significantly better than system

No. 2 in this respect. The entire visual field is filled with light

that has passed through the filter and adaptation to nearly the most

favorable color is facilitated. System No. 2, placing the colored

filter between the light source and the transparency, effectively

reduces the size of the surround, and thus provides for less complete

adaptation to the color of the filter. For transparencies of large

angular subtense ( U0° or more) this advantage of system No. 1 is negligible -unless the element to be discriminated is near the border.

The specific question whether system No. 1 promotes adaptation and

thus reduces contrast relative to system No. 2 may be ansx^ered as follows:

Yes, system No. 1 promotes adaptation relative to system No. 2, but this

should result in a gain rather than a loss in contrast. Note that the

eye of the operator constantly changes its fixation point as a part of

the searching process, just as Hie color matcher constantly looks from

one half-field to the other in an attempt to discriminate a small difference. The best discrimination of local details by the roving eye is

obtained with the eye adapted to the average color of the transparency.

If the eye remained perfectly fixed, the details would have to be discriminated immediately if at all, because adaptation reduces almost to zero in a few minutes the perceived size of ny color differences

originally perceptible, and this decline in perceptibility takes place

very rapidly in the first few seconds of exposure. Even in this condition,

it would favor discrimination to pre-adapt the eye by system No. 1 to the

average color of the transparency, and then to present the transparency,

rather than to use system No. 2.

VII. Character of the b oundary between object and surround

If the boundary between object and surround is sharp, the detection

of an object lighter or darker than the surround is greatly facilitated}

see NBS Circular U78, Colorimetry, page 3Q U3l» If the optical quality

-liof the colored filter is perfect, systems 1 and 2 alike leave the

character of the boundary unchanged. But if, as is likely in practical

cases, the colored filter has minor imperfections (haze or striae in

the filter material; pits, scratches, dirt, grease, or dust on the

surfaces), system No. 2 is definitely advantageous. By system No. 1,

these imperfections may so degrade the sharpness of the boundary in

the retinal image between object and surround as to hide small objects

otherwise easy to detect; but by system No. 2 no such degradation can

result, since the eye may then view the transparency directly without

any intermediate optical surfaces.

T7III . Summary

The relative advantages of placing the colored filter between the

transparency and the eye on the one hand (system No. 1), or between

the transparency and the light source (system No. 2) on the other,

have been analyzed on the basis of the fundamental formulas for size

of color contrast and on the basis of the pertinent variables of the

observing condition. It is shown that system No. 1 provides somewhat

better chromatic adaptation, and that system No. 2 provides better

freedom from the bad effects of optical imperfections in the colored

filter. In other respects the two systems are identical. System No. 2

is recommended.

IX. References

D. B. Judd, Determination of Colors of Maximum Contrast,

NBS Report No. 3773, November 12, 195>U.

¥. Schbnfelder, Der Einf'luss des Umfeldes auf die Sicherheit

der Einstellung von Farbengleichungen Z. Sinnesphysiol.

,

63, 228; 1933.

D. B. Judd, Colorimetry, NBS Circular No. U78, page 30,

March 1, 195>0.

m

[J

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