August 31. 1987 

FINAL REPORT 

Contract No: NAS5-30061 

SBlR 86.1 08.22-7211 

BY 

C. W. Pender. Ph. 0. 

-, W. 1. Clark 

Submitted by: 

Tennessee Space Laboratories, Inc. 

P. 0. Box 728 

Manchester, TN 37355 

Submitted to: 

NASAIGoddard Space Flight Center 

Engineering Procurement Office, Code 287 

Greenbelt Rd. 

Greenbelt MD 20771 

[NASA-CR-180772) INVESTIGATION INTO THE N88 -2 25 52 

DEVELOPMENT OF COMPUTER AIDED DESIGN 

SOFTWARE FOE SPACE BASZD SENSORS Final 

Report (Tennessee Space Labs.) 79 p Unclas 

CSCL 09B G3/61 0097764 
t 

TSL. lac . 

TABLE OF CONTENTS 

August 31. 1987 

SECT I ON PAGE 

1.0 

2.0 

3.0 

3.1 

3.1.1 

3.1.2 

3.2 

3.2.1 

3.2.2 

3.2.2.1 

3.2.2.2 

3.2.3 

3.2.4 

3.2.5 

4.0 

4.1 

4.2 

4.3 

4.4 

4.5 

4.6 

4.7 

4.8 

5.0 

6.0 

7.0 

8.0 

9.0 

10.0 

PROJECT SUMMARY ................................... 1 

INTRODUCTION ...................................... 2 

HARDWARE .......................................... 5 

BACKGROUND ........................................ 7 

Sources ....................................... 7 

Source Nomenclature ................... 8 

Typical Sources ....................... 15 

Instrumentation .............................. 18 

Optical System ........................ 20 

Radiation Transducer .................. ~1 

Detector Nomenclature ................. 3~ 

Detectors ............................. 33 

Signal Conditioning ................... 38 

Data Acquisition ...................... 43 

Data Processing ....................... 49 

SCAD MODULES ..................................... 51 

GeometrylTime ................................ 52 

Sources ...................................... 55 

Atmospheric Effects .......................... 60 

Collection Optics ............................ 64 

Optics ....................................... 64 

Detector/Preamp .............................. 66 

Electronics/Signal Conditioning .............. 67 

Data Storage/Data Transmission ............... 67 

MERGING .......................................... 68 

GRAPHICS ......................................... 70 

EXPANSION ........................................ 71 

DISCUSSION ....................................... 71 

BIBLIOGRAPHY ..................................... 72 

LIST OF SYMBOLS .................................. 75 

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LIST OF TABLES 

August 31, 1987 

TABLE PAGE 

1. Source Nomenclature ............................- 9 

-., 2 - Radiometric' Units ............................... 10 

3. Typical Radiation Sources, Types, Spectral 

Range and Appropriate Detector .................. 16 

4. Transmission of Some Optical Materials .......... 26 

5. Target Sources Modeled in SPIRITS Code .......... 54 

6. Summary of Aerie Characteristics ..............--. 63 
TSL . Inc . 

LIST OF FIGURES 

August 31 . 1987 

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14; 

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20 . 21 . ~. 2 2 . 23 . 24 . 25 . 26 . 27 . 

F 1 GURE PAGE 

Sensor/Source ....................................... 3 

SCAD Overview ...................................... 4 

Hardware Required for SCAd ......................... 6 

1500K Blackbody Radiance ........................... 13 

Blackbody Radiances for a Number of Temperatures ... 17 

Elements of an Optical Measurement System .......... 19 

The Basic Optical System ........................... 21 

The Effect of Aperture and Field Stops ............. 22 

Entrance and Exit Pupil Locations .................. 24 

IR Camera Schematic ................................ 29 

Interferometer/Spectrometer Schematic .............. 30 

Pyroelectric Preampllfier Configuration ............ 40 

Lockin Amplifier and Chopped Radiometer 42 

HgCdTe Detector and Preamp ........................ 44 

Digital Data Acquisition System .................... 47 

Sample and Hold Amplifier Signals .................. 48 

Data Processing Required for the FTS Signal ........ 50 

Geometry/Time ...................................... 53 

Example of Users Selection of Background 

in SPIRITS ......................................... 50 

Source Deflnition .................................. 57 

Source Spreadsheet Example ......................... 58 

Source Plot Exa~ple ................................ 59 

Background Scene Modeling Techniques ............... 61 

Scene Description Matrices ......................... 62 

Sample Facet Grid .................................. 62 

............ .. 

LOWTRAN Transmission Curve ......................... 65 

Matrix ............................................. 69 

... 
TSL, Inc. 

J. 

Pro jec t Summary 

August 31, 1987 

The described effort is phase one of the development of 

a Computer Aided Design (CADI software to be used to perform 

radiometric sensor design. The software package will be 

referred to as SCAD and is directed toward the preliminary 

phase of the design of space based sensor systems. The 

approach being followed is to develop a modern, graphic 

intensive, user friendly software package using existing 

software as building blocks. The emphasis will be directed 

toward.,the development of a shell containing menus, smart 

defaults, and interfaces, which can accommodate a wide 

var.iety of existing application software packages. The 

shell will offer expected utilities such as graphics, 

tailored menus, and a variety of drivers for 1/0 devices. 

Following the development of the shell, the development of 

SCAD is planned as chiefly selection and integration of 

appropriate building blocks. 

The phase one development activities have included: 

the selection of hardware which will be used with SCAD: the 

determination of the scope of SCAD: the preliminary 

evaluation of a number of software packages for 

applicability to SCAD; determining a method for achieving 

required capabilities where voids exist: and then 

establishing a strategy for binding the software modules 

into an easy to use "tool kit". 

During phase two the SCAD software will be implemented. 

Phase two activities will consist of: continuing the 

evaluation efforts of pertinent software: selecting the 

software packages which will be used1 developing the shell 

which will be used to tie the modules together: and 

providing the required graphics, device drivers, menus, and 

the like. 

Phase three activities will consist of: marketing the 

SCAD Software, extending the SCAD capabilities, and using 

the same techniques to address other problems which can be 

solved with established, but unwieldy software. 

Page I 
TSL, Inc. August 31, 1987 

1.0 INTRODUCTION 

The described effort is phase one of the development of 

a Computer Aided Design (CADI software to be used to perform 

radiometric 3en3or design. The software package will be 

referred to as SCAD and will be directed toward preliminary 

design of space based sensor systems (Fig-11. The approach 

being followed is to develop a modern, graphic intensive, 

user friendly software package using existing software as 

building blocks. The emphasis will be directed toward the -.' 

development of a shell, containing menus, smart defaults, 

and interfaces, ,which can accomodate a wide variety of 

existing application software packages. The shell will 

offer expected utilities such as graphics, tailored menus, 

and a variety of drivers for I10 devices. The goal of the 

shell development will be twofold. First the shell will 

produce a SCAD software package meeting the immediate 

requirements. Second, the shell software methodology will 

be established to implement other software tools using many 

>' other existing, hard to use software packages. The 

development of the shell is anticipated to be the most 

substantial effort in producing SCAD: in fact, the SCAD is 

viewed as an example of how the shell may be used. 

Following the development of the shell, the development 

is planned as chiefly selection and integration of the 

building blocks. This first phase of the development 

consisted primarily of determining the scope of the software 

required to implement SCAD (see Fig. 2 for overivew of SCAD 

scope) and the hardware required to implement it. The 

secondary objective of this investigation phase of the 

development was to begin, where possible, evaluating 

available software packages for applicability to the problem 

at hand, determining a method for achieving required 

capabilities where voids exist, and then establishing a 

strategy for binding the software modules into an easy to 

use "tool kit". 

The software will be developed to answer the need 

specified in the SBlR request 86-1. 

"NASA would like to develop the capability to model instruments, 

proposed for observations of the Earth's land surface, using CAD 

techniques, a task which is expected to involve a great deal of 

creativity and innovation. These models need to specifically possess functionality to simulate the effect of varying aperture size, 

focal length, detector material sensitivity and efficiency, 

electronics amplifications, etc., in order to conduct sensitivity 

analyses and trade studies. Methods must be developed for 

interfacing the CAD software for instrument design to existing NASA 

models of radiation reflectance distribution expected from land 

surface features, as modulated by atmospheric effects on the 

radiation. Methods must also be developed for graphically 

displaying the results of simulation runs utilizing CAD software." 

Page 2 

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sENSOR/SOURCE 

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..... 

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...... 

. . . . . . . . . . . . . . . . . . . . . . . . . . . . 

..... 

....... 

. . . . . . . . . . . . . . .... 

..... . . . . . . . ...... 

. . . . . . . .. ...... ....... 

.... ...'...'.' 

.. ....... ...'..*-. .......... ..-.....*... 

...... .... ..... .... . . . . . . . . . . . . . . ..... .... ..... 

FIG, 1 

PAGE 3 . 
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rSCAD OVERVIEW 

Fig.2 

Page 4 
. TSL, Inc. August 31. 1987 

A primary objective, following performance, is to build 

a usable ensemble of modules which can perform the total 

design. Good examples of what SCAD is not meant to be can be 

found in many widely advertised and used softwares which 

require weeks of training for modest returns in benefit. 

The best examples can be found in optical design, CAD, and 

elaborate project management packages. Most of these kinds 

of software packages require substantial training to be 

used. SCAD is intended to be quickly usable with minimal 

training. It will lead a user through the design, offering 

rational defaults and graphically illustrated choices, so 

that a first time user can profitably use it. SCAD is to be 

developed using many applications packages as building 

blocks: however, the user interface provided by SCAD should 

make the interaction with each block simple and 

understandable. Individual building blocks and their 

manuals will be included with SCAD so the dedicated user may 

delve more deeply into individual blocks if he so desires, 

but individual familiarity will not be required. 

This description of the first phase of the development 

is broken into five parts. In section 2 the hardware 

described.. The next section provides the background, 

including nomenclature, describing the sensor scenario. 

This section addresses the major issues facing a sensor 

designer. The following section defines the scope of SCAD 

and some of the application software packages which are 

currently available that may be used to meet portions of the 

SCAD requirements. In section 5 the approach to merging the 

component softwares into a working SCAD is discussed. 

Section 6 deals with the graphics capabilities planned for 

-. SCAD. Section 7 ,deals with the graphics capabilities 

planned for SCAD. Section 7 deals with the potential of 

expanding the SCAD to encompass both more detailed design of 

space sensors and attacking other similar problems. 

selected for the initial implementation of SCAD is -., 

2.0 HARDWARE 

The SCAD is being tailored to the personal computer 

domain since it was felt that it would find greatest utility 

there. Also knowing'that PC's are rapidly becoming more 

Page 5 
HARD WARE REQUIRED 

FUk 3 

FOR SCSD 
TSL, Inc. August 31, 1987 

powerful with each upwardly compatible component being 

developed, SCAD can grow and run more rapidly in the future. 

The decision was mainly whether to use the Apple Maclntosh 

or the IBM PC as the host machine. Due to the 

expandability, the industry acceptance, and the large number 

of IBM PCs (or clones) in use, the PC was selected. The 

minimal machine is a IBM PC with 640 kbytes of RAM, a 20 

Mbyte hard disk, a mouse, a floppy disc and a moderate to 

high resolution monitor. Some form of output will be 

necessary for hardcopies. Drivers will be in place for a -.- 

graphics printer, a color printer, and a plotter. 

The preliminary SCAD development used an IBM PC and an 

IBM AT, the PC had a NEC Multisync monitor with Video-7 Vega 

Deluxe interface, 640 kbytes of RAM, 20 Mbyte hard disk and 

360 kbyte floppy, an Epson RX80 printer and a Mouse systems 

PC Mouse. The IBM ATs had a CGA monitor, 640 kbytes of RAM, 

20 Mbyte hard disk and hi and lo density floppy disk drives 

(360 Kbyte and 1.2 Mbyte), math coprocessor, IBM 5182 color 

>' printer, Hitachi T'iger Pad digitizer, HP model 7475 and 

Houston model DMP-52 plotters. 

The PC configuration is recommended as the minimal 

configuration to use with SCAD while a 80386 based PC should 

perform much faster. Since graphics is viewed as a very 

important part of this software, an additional graphics 

component may be added during the second phase of the 

development; however, the desire to make the software 

useable on the most basic PC will also be a driving factor. 

3.0 BACKGROUND 

3.1 Source 

SCAD deals with the development of near UV, visible and 

infrared radiation measurement systems giving spatial, 

spectral, and/or temporal information. The objective of 

these systems is to determine the nature of the radiation 

being generated by a source (or target). The types of 

targets to be detected, located or identified by the system 

are those encountered during the observation of the Earth's 

land surface from space. The radiation collected by the 

sensor may be emitted, reflected or scattered from an object 

located at ground level or by the intervening atmosphere. A 

typical example would be farmland covered by partial clouds. 

Emphasis will be placed with infrared sources and sensors 

during the preliminary stages of development, but the 

visible and UV will be addressed thoroughly in SCAD. 

Page 7 
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This section contains a discussion of sources. It 

includes a brief review of radiation theory and the 

nomenclature of radiation sources is presented. Discussions 

wi I I i nc I ude i t ems wh i ch mus t be considered in 

instrumentation development such as back grounds and 

atmospheric phenomena, as well as target peculiarities. 

3.1.1 Source Nomenclature 

Table 1 presents the names, symbols and units of the 

source related quantities of interest within this volume. 

Table 1 is a subset of the symbols and abbreviations 

included in Appendix I which should summarize all used in 

this document. The most often used spectroradiometric 

quantities are spectral radiance, spectral radiant 

intensity, spectral radiant emittance and spectral 

irradiance. The concept of spectral irradiance is associated 

with the sensor while the other quantities are associated 

with the source of radiation. Table 2 summarizes these 

spectroradiometric quantities, their symbols, as used in 

SCAD, and includes an illustration for further clarity. 

A source is said to produce a spectral radiant energy, 

W, , at a rate called the spectral radiant power or flux, 0A 

. A practical unit of spectral radiant energy is the 

watt-second per micron, therefore the unit for the spectral 

radiant flux is the watt per micron. The manner in which the 

flux leaves the source can be further qualified by 

specifying the quotient of the radiant flux, d0~, leaving in 

a given direction divided by the solid angle element, dA, 

covered by the radiation. This quotient is called the __ radiant intensity, l~, of the source. Since the unit of 

solid angle is steradian3 the unit of radiant intensity is 

the watt per steradian-micron. This quantity is valid only 

for point sources. It is presumed applicable to extended 

sources if the entire source can be included within the 

instantaneous field of view of the viewing instrument. To 

further clarify the manner in which the source produces the 

radiation two other quantities, the spectral rad i ant 

emittance M, and the spectral radiance L, are also used. The 

spectral radiant em'ittance is the ratio of the radiant flux 

emitted from the su'rface element, d0~, divided by the area 

of the surface element, dA. Therefore, the spectral radiant 

emittance is the density of the radiant flux per unit area 

with units watts per square centimeter-micron. Spectral 

radiance is a source function relating the flux radiated 

from a unit area into a unit solid angle. The radiance is 

the quotient of the radiant intensity which passes through a 

surface element, dA, in a given direction divided by the 

projection, dAcos(8), of the surface element, where 8 is 

Page 8 
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August 31, 1987 

TABLE 1 

SOURCE NOMENCLATURE 

QUANT I TY SYMBOL UN I TS 

Area A cm2 

Solid Angle R steradiancsr 1 

Power 

Irradiance E 

Radiant Intensity I 

Rad i ance L wa t t -crn’Ls r 

Emittance M wat t-cm-2 

Spectral Power QA watt-um 

Spectral Power Qv wa t t - ( cm- ’ I 

watt-cm-L-um 

Spectral Irradiance watt-cm 2-(cm 1 

Spectral Radiflnt Intensity IA watt-sr-,-um 

Spectral Radiant Intensity Iv watt-sr -(cm 1 

Spectra; Radiance 

Spectral Radiance L1) Spectral Emittance Ma 

Spectral Emittance MV watt-cm-2-ccm 1 

Transmission r % 

Reflectivity r % 

Absorptivity a % 

Emissivity E % 

Q watt 

wa t t -cm* 

wa t t -s r 

-1 

-1 

-1 

-1 

Spectral Irradiance EA 

E\) -1 

Lit waii-sr -1 -cm”-um - 

-1 

watt-sr -cm-’-(cm 

wat t-cm’*-pm -1 

ll 

Page 9 

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e II 

3 

4 

4 

4 II 

RAD I OMETRIC UNITS 

TABLE 2 

Page 10 - .. 
TSL, Inc. August 31, 1987 

the angle between the direction of radiation and the normal 

to the surface. The spectral radiance has units watts per 

steradian-square centimenter-micron. Both the spectral 

radiant emittance and spectral radiance are applicable to 

extended sources. 

The quantities transmission, r, reflectivity, r, 

absorptivity, a, and emissivity, E. are ratios. 

Transmissivity is the ratio of transmitted radiation to 

incident radiation. For example transmissivity is the ratio 

of spectral radiant intensity transmitted through a medium 

to the spectral radiant intensity incident upon that media. 

(The example could have been with either spectral 

radiances or spectral irradiances instead of intensity.) The 

reflectivity is the ratio of reflected radiation from an 

object to the radiation incident upon the object. The 

absorptivity is the ratio of radiation absorbed by a medium 

to the radiation incident upon it. Emissivity is the ratio 

of emitted radiation from an object to the radiation emitted 

from a blackbody at the same temperature. 

Several observations can be made concerning these 

quantities. First since incident radiation must be either 

reflected, transmitted or absorbed it follows that 

a+r+r =1. 

Kirchoff's law states that with certain restrictions -.. [see Siege1 and Howell's Thermal Radiation Heat Transfer 

(Ref. 4)) 

€=a so that 

Therefore, for opaque objects with r =O 

and for a gaseous m e d i u m w h e r e r '"0 

E+r =1 

and for a blackbody where a = E = 1 

r=r=O. 

One other issue must be considered in this nomenclature 

section. At times the point of view will switch between 

wavelength and wavenumbers. The wavenumber is defined as the 

% 

.. I 

Page I1 
TSL, Inc. August 31, 1987 

n 

number of periods of an electromagnetic wave occurring in 

one centimeter. The wavenumber, u, is related to wavelength, 

J,RS 

u = 10,000 /A 

where u has units cm-1 and i\ has units microns. It is 

important to realize that the functional forms of Xa and Xu 

are different. This is due to the fact that the ratio of the 

size of a unit wavenumber interval to a unit wavelength 

interval is a function of wavelength. This is apparent since 

the derivative of Ais 

-2 

dA /du = 10,000 u 

“This fact must be remembered when converting or 

examining spectral density as functions of wavelength of 

wavenumber. Fig. 4a and b are blackbody radiances as 

functions of wavelength and wavenumber. The differences in 

shapes of the two curves results from this wavelength 

dependence. It should be noted that the shape change occurs 

only with spectral density functions such as spectral 

radiance or radiant intensity but not with the unitless 

ratios such as r, a, E, or r. Several references which 

include more general information regarding IR radiation 

theory review are included in the Bibliography (Refs. 1, 2, 

3, 4, 5, and 61. 

Page 12 

i. 
1500K BLACKBODY RADIANCE 

Fig. 4 a 

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6' E z 1 0 

(NOLI3 I IWJ3 OS/U31S/SIlWM 1 33NW I UWU 

1500K BLACKBODY RADIANCE 

Fig. 4b 

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3.1.2 Typical Sources 

By virtue of the molecular and atomic agitation 

associated with their internal energies these sources emit 

rndiation in the form of electromagnetic waves. From Table 3 

it can be seen that the emitted energy or radiation from hot 

gases (clouds) and thermal emitters (source of interest) 

will be in the infrared, visible and ultraviolet regions of 

the electromagnetic spectrum. Selected radiation production 

-\ mechanisms for these sources are mentioned in the table. 

The typical source will fall into the thermal emitter 

category. Thermal emitters produce radiation which can be 

partially described by Planck's spectral distribution of 

emissive power equation 

LA = 2hc*E, /(A5 fexp[hc/kT*I - 1 1 1 

where La is the spectral radiance of the source; Ea is 

the spectral emissivity or measure of how well a body can 

radiate energy as compared to the blackbody! ii is the 

waveiength in microns; h is Planck's constant8 k is 

Boltzmen's constant: T is the temperature in Kelvins: and c 

is the speed of light. Fig. 5 shows the spectral radiance, 

as a,function of wavelength, as calculated from the equation 

for a series of blackbodies (Ea =1) at different 

temperatures. As the figure and equation show, considerable 

energy is produced in the infrared. 

The other major source of interest in SCAD is the sun. 

Although the sun will not be viewed directly, sunlight will 

be reflected from particles in the atmosphere, clouds, and 

the objects of interest on the ground. The sun resembles a 

5900K blackbody with a radius of 695,000 km. 

Transmission is a consideration for all the 

transmitting components within any optical path. This 

includes windows, lenses, and the like, in addition to the 

atmosphere. The optical properties of materials will be 

treated in more depth later. Presently our attention will 

remain with the atmospheric distortion which can alter the 

apparent character of a source. 

The locations of the sources which will be treated in 

this study are almost without exception ground based with 

major atmospheric paths to the space based sensors. The 

atmospheric losses can be calculated with commonly available 

atmospheric models and a measured spectrum can be 

"corrected" to give, with a high degree of certainty, the 

true source spectrum. 

Page I5 
m 

0 

V 

3 

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TYPICAL RkDIATION SOURCES, TYPES, SPECTRAL RANGE, AND APPROPRIATE DETECTOR 

TABLE 3 

Page 16 

I 
0'9 0's 0' r; 0% 02 0' I 0' 

6- ~~Ol~ (NQU3IW/U3lS/SlLWM) AlISN3lNI LNWIClWtl 

BLACKBODY RADIANCE AT SEVERAL TEMPERATURES 

Fig. 5 Ds-a 17 
TSL, Inc. August 31, 1987 

Another area which must be remembered and considered is 

the target background. The background is any radiation 

additional to the source radiation which can confuse the 

source measurement. The background is therefore just 

another, albeit unwanted, source within the field of view. 

For imaging systems, background problems are reduced over 

nonimaging systems. This is due to the fact that an imaging 

system provides spatial discrimination for objects, such as 

source and background within its field of view. There are, 

however, additional background problems which even imaging 

systems do not resolve. One example is the reflection of 

radiation from something located behind a semitransparent -.- 

object. The importance of considering the background in any 

measurement cannot be overstated. 

The variation of a source as a function of time is 

another critically important factor. With space based 

sensors operating in a swept field mode, the target being 

viewed at one instant in time may be quite different than 

that viewed in the next successive instant of t ime. 

Therefore, the radiation can vary dramatically with time and 

it is important tha't a measurement system have an adequate 

response time to deal with the fluctuations of the source. 

>< 

From the preceeding it should be clear that the source 

radiance, L, can be expressed as a function of spatial 

position (x,yl, time (tl, as well as spectral frequency (01 

or wavelength ( I. With each of the independent variables 

there are associated problems or considerations which can 

not be overlooked in the development of a measurement 

system. After the list of previously mentioned Items has 

been resolved, the more general considerations such as the 

dimensions of the source and the dynamic range of the 

radiances at different positions and wavelengths must then 

be considered. It is interesting to note that most 

applications will offer new and difficult problems which 

musttbe overcome for a successful measurement. 

3.2 Instrumentation 

After having surveyed the source nomenclature and the 

kinds of sources of current interest it is important to 

review the typical infrared measurement systems in order to 

illustrate both the current state of the technology and to 

demonstrate the value of the subject system. The infrared 

instrumentation system can assume numerous forms depending 

u p o n t h e m e a s u r e d quantity. Fig. 6 illustrates the basic 

measurement system. In the basic system, a sensor comprised 

of optical components, detector (or radiation transducer) 

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ELEMENT OF AN OPTECAL MEASUREMENT SYSTEM 

Pig. 6 

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and associated signal and control electronics generate a 

signal which is passed to a data handling system for 

analysis. Several of the components of the system will be 

individually discussed, with the more popular systems 

currently in use being used as examples. 

3.2.1 Opt ica'l System 

The first subsystem, following the source and any 

intervening transmitting media, is the optical system. The 

optical system includes the collection optic, transfer 

optics, optical modulator (optional) and perhaps additional 

apertures or stops. Although at times the optical system 

contains many elements its function can be depicted in the 

simple form shown in Fig. 7a. The purpose of the optical 

system, shown schematically in Fig. 7b, is to collect and 

reshape wavefronts to irradiate the image plane (detector). 

Since the nomenclature related to optics is so broad and 

diverse a complete listing of the jargon of this field will 

not be attempted. The nomenclature required to understand 

the optical systems which are to be discussed will be 

presented within the description of a basic optical system. 

The 'iext Optics by Hecht and Zajac (Ref. 7) is suggested as 

a reasonable source of the nomenclature of this field. A 

number of texts and articles are mentioned in the 

Bibliography pertaining to optical systems (Refs. 8, 9, 10, 

11, 12, 13, and 14). 

For the radiometric sensors under discussion it is 

important to have a measure of how effective the optics are 

in collecting the' radiation and to have an accurate 

knowledge of the size and location of the radiation source 

which the optics are viewing. These light gathering and 

field of view properties are defined by the effective stops 

and pupils and the spectral transmissivity of the swbsystem. 

The quality of the image can also be degraded from an ideal 

scaled reproduction of the source by aberrations introduced 

by the nonideal character of the lenses and mirrors used in 

the sensor's optics. 

In all optical systems there are apertures which limit 

the passage of radiation through the system. These 

apertures include the edges of lenses or mirrors, the edges 

of detectors and the clear diameters of baffles or 

diaphragms introduced specifically to limit the image 

irradiance. The aperture stop is that element in the 

optical train which limits the angular size of the cone of 

radiation accepted by the system (see Fig. 81. The image of 

the aperture stop as viewed from the source plane is called 

Page 20 
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F 

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a) BASIC SYSTEM 

b) FUNCTION OF SYSTEM 

0 

THE BASIC OPTICAL SYSTEM 

FIG, 7 

PAGE 21 
4 

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I 

FIELD STOP 

THE EFFECT OF APERTURE AND FIELD STOPS 

FIG, 8 

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the entrance pupil. The entrance pupil is located in the 

plane containing the intersection of the line coincident 

with the chief ray (a ray from an off-axis point on the 

object passing through the center of the aperture stop) as 

it enters the system and the optical axis. The image of the 

aperture stop as viewed from the image plane is called the 

exit pupil. This pupil is located in the plane containing 

the intersection of the line coincident with the chief ray 

as it passes from the system and the optical axis. Fig. 9a --' 

and 9b show examples of the aperture stop and entrance and 

exit pupils. The field stop is the element in the optical 

train which limits the size of the image which can be formed 

by the system (i.e. defines the field of view). 

Generally the light gathering power is defined not by a 

schematic giving dimensions and pupil location3 but rather 

by the f-number or its reciprocal, the relative aperture, of 

the system. The f-number is defined as the ratio of the 

effective focal Ien'gth to the diameter of the entrance pupil 

of the system. The irradiance at the image plane then 

varies inversely as the square of the f-number. 

The f-number depends upon the effective focal length of 

the optical components of the system. Further, the 

transmissivity of the optical components also limits their 

light gathering effectiveness. These properties of the 

system are not influenced by the stops within the system but 

rather by the shapes, combinations and materials within the 

optical train. These parameters also influence the image 

quality of the system. For the purposes of this study 

spherical lenses and mirrors treated with paraxial 

(Gaussian) theory will illustrate the necessary concepts. 

The term paraxial refers to a system wherein the rays 

representing the flow of the radiation make very small 

angles with the optical axis and remain quite close to the 

axis. The properties of the spherical mirrors or lens 

encountered in this study can be adequately described by the 

Gaussian lens formula (which has the identical form of the 

spherical mirror formula) 

l/so + l/Si = l/f 

and by the lens maker's formula for a thin lens 

l/f = (n-l)tl/R, + l/R, I I L 

Here 3 is the distance from the source to the optical 

is the distance from 0 

i surface (along the optical axis); s 

the optical surface to the image; and f is the focal length 

Page 23 
o) REAR APERTURE STOP a 

o) FRONT APERTURE STOP 
TSL, Inc. 

- ., 

August 31, 1987 

of the optic. For the spherical mirror the focal length is 

twice the radius of curvature of the mirror. In the lens 

formula, the curvatures of the two sides of lens have radii 

R, and RP and the lens has an index of refraction n. 

pairs using the relations 

2% Combinations of lenses and/or mirrors can be reduced in 

front focal length = f, (d-fp) 

d-( f ,+f 2l 

and 

where d is the separation of the two lenses and/or 

mirror? and f, and f are their respective focal lengths. 

The front focal focal 

point to the first surface and the back focal length is the 

distance from the last surface to the second focal point. 

Combinations can also be treated with ray tracing 

techniques. 

lengtfi is the distance from the first 

The cases of the flat lens and mirror must also be 

mentioned. In the case of reflection from a mirror the law 

of reflection is used. The law merely states that the angle 

of incidence of a ray of radiation upon a surface equals the 

angle of reflection of that ray. In the case of refraction 

Snell's law is used. This law states that 

= n sin et t n sin Bi 

where n and n are the indices of refraction of the i media of the incidenl and transmitted radiation and 8 and 

8 are the angles of incidence and transmission of the ray 01 radiation. 

i 

i 

The transmissivity of the lenses Cor reflectivity if 

using mirrors) must be such that radiation in the desired 

specular region be passed through the system. Generally a 

high transmission over the entire spectral band width of the 

detector is desirable for all of the optical components 

except possibly the modular element components. The optical 

modulator sometimes is a spectral filter which limits the 

transmission to a narrow band of frequencies of interest. A 

list of some of the most commonly used materials transparent 

in the near ultraviolet, visible, and near infrared is 

presented in Table 4. Glasses used for visible optics 

Page 25 
TSL. Inc. August 31. 1987 

Table 4. TRANSMISSION OF SOME NEAR-INFRARED OPTICAL 

MATER I ALS 

MATER I AL PEAK TRANSMISSION(%)/ 50% TRANSMISSION 

T H I CKNE S S ( mm 1 POINTS ( 1 

FUSED SILICATE(glass1 90/2 

SAPPHIRE (AI 0 1 93/2 

CALC I UM FLU06 I D8 (CaF2 1 

QUARTZ (Si0 1 95/2 

MAGNESIUM F~UORIDE~M~F~) 95/2 

IRTRAN 1 (MgF21 93/1 

IRTRAN 2 (ZnSe1 74/1 

IRTRAN 3 (CaF 1 95/ 1 

IRTRAN 4 (ZnSe1 70/ 1 

IRTRAN 5 (MgO1 88/ 1 

IRTRAN 6 (CdTe1 65/ 1 

SILICON (Si) 54/2 

GERMANIUM (Gel 47/2 

SODIUM CHLORIDE (NaCI) 92/2 

THALLIUM BROMIDE-IODIDE [KRS-51 72/5m 

POTASSIUM BROMIDE [KBRI 92/10 

POTASSIUM CHLORIDE (KC11 93/10 

ARSENIC TRISULFIDE 75/2 

95/10 

c.5-4.2 

< - 5-7 

(-5-9.0 

< -5-4. 

(-5-8.7 

-8-7.4 

-6-14.7 

< -5-1 1 - 5 

(. 5-2 1.6 

< -5-8.8 

-9-3 1.6 - _, 

1-2-16 

1.8-23 

( - 5-20 

-56-40 

( - 5-27 

< - 5-22 

.65- 12 

Page 26 
TSL, Inc. August 31, 1987 

generally are useful to only 2.7 micron due to OH or water 

band absorption at 2.8 micron. Some high silica glasses or 

fused quartz recover transmission beyond 2.8 micron and have 

reasonable transmission to beyond 4 micron. Crystals such 

as germanium and silicon are used frequently as are other 

hot pressed crystals such as the lrtrans from Kodak. The 

transmission of a given element is usually known (this 

tr&n:mission characteristic is dependent upon thickness so 

it is peculiar to a particular optical element, not to all 

elements made of t h e s a m e m a t e r i a l ) so it must be used as a 

scale factor to determine t h e a m o u n t of radiation passed by 

the given element. Combinations of elements result in an 

effective transmission which is the product of the 

transmissions of the individual elements. Optical material 

properties, in addition to spectral transmission, which must 

be considered when selecting lens materials include the 

index of refraction, hardness, solubility, thermal expansion 

and melting temperature. 

Reflective optics can sometimes be used to minimize the 

transmission and chromatic problems associated with lenses. 

Caution must be exercised however since specular reflectance 

is not necessarily uniform over a range of wavelengths. The 

most frequently used mirror is the front surface vacuum 

deposited aluminum type. It is highly reflective 

(reflectance greater than 95%) in the 2 to 10 micron range. 

Usually the aluminum surface is protected by a coating of 

silicon oxide which must be of the appropriate thickness to 

not introduce any interference losses. 

A spectral filter may be used to absorb, reflect or 

otherwise deviate radiation of all frequencies from the 

optical path except the particular band of interest. In 

most near infrared measurement systems the filter of choice 

is an interference filter of the multilayer dielectric or 

Fabry-Perot type. In both types the bandpass phenomenon is 

based on interference of multiple reflected radiation. The 

terminology of the filter includes the passband or primary 

wavelength interval of transmission, the peak transmittance 

which is the maximum transmittance in the passband and the 

halfwidth which is the width of the passband at 50% peak 

transmission. 

The phenomena of aberrations are a consequence of the -_, 

shape and composition of al I the optical elements. 

Aberrations can be lumped into two categories: chroma t i c 

and monochromatic. Chromatic aberrations cause light of 

different wavelengths to be brought to a focus at different 

locations (because of the nonuniform index of refraction as 

a function of wavelength of the lens). Monochromatic 

>. 

Page 27 
TSL, Inc. August 31, 1987 

aberrations cause the image to be blurred, smeared or 

deformed. The five primary monochromatic aberrations are 

coma, spherical, astigmatism, field curvature and 

distortion. A thorough treatment of these so called Siedel 

aberrations is beyond the scope of this document so the 

interested reader should refer to a text such as that by 

Born and Wolf (Ref. 9). 

After having briefly summarized the properties and 

nomenclature of the elements making up optical systems it is 

interesting to examine two examples. The optical train of 

one of the instruments, the Agema Thermovision, is shown 

schematically in Fig. 10. All of the transmitting elements 

in the camera are germanium except the optional spectral 

filter. The unique character of this camera is due to the 

presence of the two rotating octagonal prisms. These prisms 

are examples of the optical modulator referred to in Fig. 6. 

The optical train may be examined at several different times 

and the function of the prisms will become obvious. The 

prisms at any particular time appears to the system as 

merely two windows,, having an index of refraction of about 

4, inclined with respect to the optical axis. Consequently 

the prisms serve to effectively shift the optical axis, 

remember Snell's law, with respect to time and to position 

different portions of the image on the field stop at 

different times. By correlating the prism positions and the 

irradiance within the field stop, an effective wide field of 

view image can be obtained from this basically single 

JI element small field,of view instrument. 

The second example is the optical system of the Block 

197RS interferometer spectrometer. Fig. 11 shows the 

optical schematic of this instrument. Here a large 

reflective element is used to collect radiation which can be 

limited by the aperture stop. The system has a variable 

field of view which is selectable with the variable field 

stop. The unique character of this instrument is due to the 

presence of the Michelson type interferometer as the optical 

modulator. 

Page 28 
IR CAMERA SCHEMATIC 

ne 10 

Page 29 
I 

INTERFERONETER/SPECTROHETER SCHEMATIC 

FIG. 11 

PAGE 30 
TSL, Inc. August 31, 1987 

3.2.2 Radiation Transducer 

It has been shown how a portion of the radiant flux 

leaving a source is collected and directed to the field stop 

of the optical system. Following or coincident with the 

field stop, is the first element of the next subsystem, the 

radiation transducer system. Usually the edge of the first 

element, the detector in this system, determines the field 

stop. The function of the detector is to convert the 

irradiance incident upon it to a voltage, a current, a 

change in conductivity, etc. The radiation transducer 

system includes the detector and signal conditioning 

elements necessary to convert the output of the detector to 

voltages of appropriate levels to be handled by the next 

subsystem, the data acquisition system. 

The radiation transducer technology is especially 

interesting at this point in time due to the rapid 

developments going on in both the detector and electronics 

areas. The past decade has seen the acceptance of 

integrated analog and digital electronics into the 

transducer systems. During the same period the detector 

industry has progressed in capabilities to the point that 

mosaics of literally hundreds of thousands of individual 

detectors can be manufactured in a single package. The 

hybrid and integrated systems incorporating detector, 

preamplifier and signal processing in a single package are 

rapidly becoming commercially available. 

In discussing the radiation transducer it is convenient 

to separately treat the detector and electfoniCs. It is -.’ 

useful to first mention the unique nomenclature of the 

detector: to then discuss the different types of detectors 

in use, with specific examples of detectors used in this 

study and then to separately discuss the electronics 

required to interface the detectors with a data handling 

system. The Bibliography includes numerous references which 

discuss the various detectors and systems mentioned in the 

following sections (Refs. 15 through 291. 

3.2.2.1 Detector Nomenclature 

Numerous figures of merit, operating characteristics 

and other constraints must be considered when matching 

detectors and applications. The most often used figures of 

merit for infrared detectors are the responsivity, the noise 

equivalent power, and the detectivity. Also of extreme 

importance in many applications is the operating temperature 

of the detector since the requirement for cryogenic cooling 

CR~ sometimes not be met. Additional factors which also must 

be considered are spectral response, size, and cost. 
TSL, Inc. August 31, 1987 

The responsivity,R, is the ratio between the root mean 

squaredCrms1 signal voltage Cor current) and the rms - .. 

incident signal power. Spectral responsivity, R or Ra , 

refers to a monochromatic input signal while blackbody 

responsivity, RBB, refers to an input signal having a 

blackbody spectrum. Typical units for responsivity are volts 

or amperes per watt. 

The noise equivalent power, NEP, is the value of 

incident rms signal power for a given radiation source, 

bandwidth, and chopping frequency required to produce an rms 

JL signal to noise ratio of unity. The NEP is a measure of the 

minimum power which can be detected. The NEP is given by 

where H is the irradiance on the detector, A is the 

detector surface area, S, is the voltage developed, N is the 

noise voltage developed by the detector, and f is the noise 

bandwidth used in measuring N. 

Since most detectors are spectrally selective the 

radiation source must be specified. Usually a 500K blackbody 

or a particular wavelength source is specified. Further, 

s i n c e m a n y detectors are used with a mechanical chopper to 

permit the use of phase sensitive detection, for extraction 

of a signal from noise, it is customary to specify the 

chopping frequency used for the NEP measurement. For 

uniformity, NEP values are specified for an equivalent 

noise bandwidth of 1Hz. 

The detectivity, D, is simply the reciprocal of NEP. A 

more commonly used expression is the dee-star, D*, of the 

detector. D* is the S/N ratio at a given source temperature 

and chopping frequency with an amplifier bandwidth of 1Hz 

for a lcm2 detector receiving one watt of radiant power. It 

is therefore a normalization of the reciprocal of NEP to 

take into account the area and electrical bandwidth of the 

detector. 

The detector time constant, r, is a measure of the 

speed of response of a detector. This time constant 

indicates the responsivity of the detector as a function of 

modulation frequency. Knowledge of this parameter is 

necessary to chose an appropriate chopping rate or the 

applicability of a detector for a rapidly transient source. 

The time constant is generally defined as 

Page 32 
TSL, Inc. August 31, 1987 

where f is the signal frequency at which the responsivity 

has dropped to 70.7% of its maximum value. The detector 

responsivity is generally 

C 

where R(f1 is the detector responsivity at f Hz, Ro is the 

responsivity at low frequency and f is the modulation 

frequency. 

3.2.2.2 Detectors 

A wide variety of infrared detectors is in use. At this 

point in the preliminary stages of the development of SCAD, 

the emphasis is being directed toward the infrared and 

consequently this discussion will be limited to infrared 

detectors. The most common are photoelectric detectors and 

thermal detectors. The thermal detectors include bolometric, 

thermovoltaic, pyroelectric and thermopneumatic types. The 

photoelectric detectors i nc I ude photovoltaic, 

photoconductive, photomagnetic and photoemissive types. 

Of the different types the most unusual is probably the 

thermopneumatic Golay detector. The Golay detector consists 

of a gas cell into which radiation is allowed to enter. The 

radiation heats the gas, deforms the cell and causes motion 

of an attached mirror or alters the character of an 

integrated capacitor. The detectors used in this study are 

the pyroelectric thermal type, the photovoltaic Indium 

Antimonide(lnSb1 and the photoconductlve mercury cadmium 

telluride (MCT or HgCdTel types. 

- _r 

In thermal getectors the responsive element is 

sensitive to changes in its temperature, which are caused by 

fluctuations in the irradiance. In the bolometric type (e.g. 

the thermistor is an example), the electrical conductivity 

is the temperature related variable. In the thermovoltaic 

detector (for example the thermocouple), a temperature 

fluctuation of a junction of dissimilar metals results in a 

generated voltage. With the pyroelectric detector, a change 

in the temperature of the sensitive element results in the 

generation of a current proportional to the rate of change 

of the temperature. 

The pyroelectric detector response is based on the 

property of some noncentrosymmetrical crystals (sometimes 

c~lled ferroclectric crystals) of exhibiting an internal 

electric field along a certain crystal axis. A number of 

materials such as triglycene sulfate (TGS1 and lithium 

Page 33 
TSL, Inc. a. 

Augu8t 31, 1987 

tnntalate (LiTaO31 display this response. In these crystals 

the electric field is produced by the polarization of 

electric dipole moments. The polarization of the dipoles is 

dependent upon the lattice spacing within the crystal 

which is in turn proportional to the crystal temperature. 

These crystals are insulating materials and electrodes may 

be attached to the crystal surfaces normal to the axis of 

polarization so that an electric charge will result from the 

electric field. The electrodes make the detector essentially 

a capacitor. Then if the electric field is perturbed by a 

crystal temperature fluctuation an observable current can be 

generated in an external circuit. The current is given by 

l(t1 = F(t1 A dT/dt 

where F(T) is the pyroelectric coefficient at temperature T, A is the electrode surface area and dT/dt is the 

rate of temperature change. Since the current varies with 

changes in temperature the detector is insensitive to steady 

irradiance and responds only if the irradiance fluctuates. 

Since the detector response is based on its thermal 

condition a judicious choice of detector size, material and 

substrate is required. As with any thermal detector, the 

response time depends in part on the heat transfer rate. The 

heat transfer balance is of the form 

0 = Ao (TD4-Tw41+x CTD-TSI+cAh= 

h dt 

where the expression on the left is the power being 

absorbed by the detector, the first term on the right 

represents heat losses due to radiation, the second term is 

the conduction loss and the last term reflects the ability 

of the detector to store heat according to its heat 

capacity. In the equation 0 is radiation power (watts), A is 

ar",,,("T21, u is the Stefan-Bo1 tzman constant (5.67*10-4watt 

cm 1. K is the thermal conductance (watt/tcm°KII,h is 

the detector thickness in cm, c is the specific heat of the 

detector (joules* K-l*crn-31 and t is time (sec). Generally 

it is assumed that the radiation loss term is insignificant 

and the expression is simplified. Then the first order 

differential equation is solved to obtain the expression for 

the temperature of the detector 

-K t T(t1 = Constant + (constantI*e - 

cAh 

so that the thermal time constant, r, is cAh/K. This -. thermal behavior will influence the frequency response of 

Page 34 
TSL, Inc. August 31, 1987 

the detector unless the irradiance fluctuations ere 

significantly faster than the time constant. For the 

pyroelectric detector the detector current response at 

frequencies below Wr (= l/rl is 

a WAF(T1 

'?iLo K 

while at frequencies above 

a F(T) 

ch '?iHl 

The high frequency response will be valid throughout 

this study. 

The preceding treatment assumed the detector to be a 

current source with a current responsivity expressed as 

amperes per watt. In the context of how this mode of 

operation is achieved, an operational amplifier is used with 

I' the detector across its differential inputs. This 

con!lguration, see Fig. 12a, holds the voltage across the 

detector near zero while causing a current to flow through a 

feedback network. This current through the feedback 

generates a voltage at the amplifier output. In this case 

the voltage responsivity has the form 

F(T1 

EcA RVHl a W 

when W is above W . Here e is the dielectric constant of the 

detector. These c6rrent and voltage responsivities suggest a 

superiority of the current mode of operation. The current 

response is constant at frequencies somewhat higher than 

frequency. Therefore the pyroelectric detector is generally 

used in the current mode. 

wr 

while the voltage response is inversely related to 

The photoelectric detectors (sometimes called quantum 

detectors) are based on the i n t erna I or ex ter na I 

photoelectric effect. The internal photoelectric effect 

involves the excitation of electrons to higher energy levels 

in the conduction band by the irradiance on the detector. 

Electron-hole pairs are generated, either near a 

semiconductor p-n junction or inside a homogenous 

semiconductor, which establish a voltage across the junction 

or change the conductivity of the semiconductor. The 

external photoelectric effect is more dramatic and results 

in the escape of "free" electrons from the irradiated 

substance. The irradiated substance absorbs sufficient 

Page 35 
TSL, Inc. August 31, 1987 

energy to excite electrons on the photoemissive material 

sufficiently to liberate them from the material. Most 

quantum detectors used in the infrared are based on the 

internal effect due to the low energies associated with the 

low frequency photons. As mentioned earlier the two quantum 

detectors used in this study are the photovoltaic indium 

antimonide type and the photoconductive mercury cadmium 

telluride type. 

The indium antimonide detector operating at 77K acts as 

a diode that generates a current proportional to the number 

of photons falling on it. A general expression for the 

conversion of arriving photons to a photo current, I, in the 

photovoltaic detector is 

where Q is the fraction of photons absorbed (quantum 

efficiency), q is the electron charge and N is the average 

number of incident photons arriving per unit time. This 

current occurs when photons with sufficient energy to 

photoexcite electrans from the valence band to the 

conductive band in the detector material fall on it. The 

photoexcitation can occur only when the energy associated 

with the incident photon equals or exceeds the band gap of 

the detector material. If the photoexcitation process takes 

place, electron-hole pairs are generated either near the 

p-n junction or in the bulk region where they diffuse to the 

junction. Then the electrons are attracted to the positive 

charge in the n-type material while the holes are attracted 

to the negative charge in the p-type material. There changes 

modify the equilibrium charge concentration in the junction 

and if sufficient electron-hole pairs are formed, cause the 

photocurrent to f’low in an external circuit. 

In the case of the lnSb detector which has a band gap 

of about 0.23 electron volts at 77K, incident radiation must 

be of wavelengths less than or equal to about 5.3 microns to 

be absorbed. lnSb at 77K has a quantum efficiency greater 

than 50 percent from 3.5 microns to greater than 5 microns 

making it an excellent detector for 4.3 micron carbon 

dioxide band investigations. In the short circuit mode a 

typical lnSb photoygltaic detector-!as a peak dee-star of 

approximately 8*10 cm JHZ watt . The short circuit 

current developed is linearly related to the irradiance on 

the detector over five or six orders of magnitude. 

Page 36 
TSL, Inc. August 31. 1987 -.' 

The frequency response of the photovoltaic detector is 

shown by the expression for photocurrent 

I = NQ Nm fl-exp(-iwT 11 

jwTr 

where m is the modulation index of the irradiance and 

T is the drift transit time. Therefore from the standpoint 

of the photoelectric phenomena alone the transit time has 

the dominant influence on the response of the detector. 

Therefore the dimension of the detector through which the 

carriers must travel should be as small as possible to 

minimize the transit time. Additionally, the associated 

capacitances, inductances and resistances associated with a 

practical diode and circuit have a strong influence on 

the frequency response. For the typical lnSb detector 

mentioned earlier the dee-star is effectively constant at 

modulation frequencies from about 10 Hz to about 100 KHz. 

The HgCdTe detector operating at 77K acts as a resistor 

whose value varies according to the number of photons 

fa!! Ing on it. In this detector electrons in the valence 

band absorb the energy of the incident photons and are 

excited into free states in the conduction band. The 

electrons remain in the conduction band for some 

characteristic lifetime. Electrical conduction then takes 

place by the electrons in the conduction band or by the 

holes vacated in the valence band. The resistance varies 

inversely with irradiance. HgCdTe is a pseudo-binary alloy 

composed of the semimental HgTe and the semiconductor CdTe. 

This composition permits a continuously adjustable band gap 

of -0.3ev to 1.6ev depending upon the ratio of CdTe to HgTe. 

The cutoff wavelength can therefore be tailored in the near 

infrared range according to the equation (a rewrite of E=h 1 

Ac = 1.241 Eg 

whereitc is the threshold wavelength in microns and Eg 

is the band gap in electron volts. 

These photoconductive detectors are generally used with 

circuits which produce varying currents with fluctuating 

irradiance. In these detectors the free electron 

concentration, Ne, is related to the rate at which photons 

arrive at the detector, N, by 

Ne = Nnre 

where fl is the fraction of photons absorbed (quantum 

efficiency) and e is the lifetime of the resulting excited 

Page 37 
TSL, Inc. August 31. 1987 

electrons. Although a more complex situation occurs 

(involving the change in hole concentration, differing 

lifetimes of holes and electrons, and surface versus bulk 

behavior), it will be omitted for brevity. The change in 

conductivity, 6c, is then 

6 c = Neau 

AL 

where c is the electron mobility, q is the electron 

charge, L is the electrode separation and A is the detector 

area. Then any current change, 61, for a dc biased 

photoconductor will be 

where V is the bias voltage. 

The photoconductive HgCdTe detectors are capable of 

covering a wide portion of the near infrared spectrum in its 

various alloy compositions. A typical alloy has a cutoff 

w~velentjhh in the vici ity of 14 micron and a peak dee-star 

of 2-10 cm J Ht W . Generally as the cutoff wavelength -.* 

is increased the dee-star is decreased. 

-P 

The frequency response of the photoconductor is shown 

by the more complete representation of the optically induced 

current 

where m is the modulation index of the irradiance and 

the other parameters are as previously mentioned. As this 

expression shows a large t is desirable for large current 

responsivity but a small t is desirable for rapid frequency 

response. To obtain a flat frequency respo se this means the 

T should be approximately equal to C2nfI where f is the 

bandwidth of operation. In the typical HgCdTe detector 

referred to earlier, the dee-star is effectively constant at 

modulation frequencies from about 10 KHz to about 300 KHz. 

-9 

3.2.3 Signal Conditioning 

The signal conditioning required in sensors can be 

very diverse. It is important that the appropriate 

conditioning be used or the potential of the detection 

system may not be fully realized. As with the field of 

optics this area will only be discussed cursorily. A 

Page 38 
TSL, Inc. August 31, 1987 

reasonably thorough familiarity with the nomenclature of 

analog and digital electronics is assumed. Texts such as 

Strobel's Chemical Instrumentation or Diefenderfer's 

Principles of Electronic Instrumentation (Refs. 5 and 291 

are suggested as excellent introductions to the nomenclature 

of electronic instrumentation. These types of detectors 

were described in some detail: therefore, the signal 

conditioning required in a typical application of these 

detectors will be discussed along with as much nomenclature 

as reasonable. A brief discussion will be given for arrays 

of these detectors as they offer great promise in most 

sensor concepts. 

The first detector mentioned earlier was the 

pyroelectric detector. As that discussion pointed out the 

usual application finds the detector operating in the 

current mode. Fig. 12a or b shows the first stage of the 

signal conditioning circuit. This circuit serves to convert 

the change in capacitance of the detector to a fluctuating 

voltage which can be dealt with by later stages of signal 

conditioning. As is typical with most electronic systems the 

preamplifier provides some gain and is !ocated near the 

detector so that signals which must be transmitted 

appreciable distances will be much larger than any noise 

which might be introduced into the cabling. Manufacturers of 

pyroelectric detectors currently include either all or a 

portion of the components required for the preamplifier 

within the detector package. Figs. 12a and b show two 

typical detector/preamplifier circuits with the common 

detector configurations. 

Electrically one of the most important parameters 

associated with the detector/preamplifier combination is the 

electrical band width. Here it must be clearly understood 

that no longer is the concern with the spectral bandwidth 

which was critically important in the selection of windows, 

lenses, and detector. This electrical bandwidth is simply 

the range of the frequencies of the alternating current 

which can flow through detector and electronics. The 

electrical bandwidth is defined as the range of frequencies 

within which the amplifier will respond. The frequency range 

is often measured between the half power (3-dB) points on 

the output response versus frequency curve for a constant 

input. The band width of the signal electronics is often 

tailored for various reasons through the use of electronic -, 

filters. High and low pass filters are often employed to 

remove the ever present 60 Hz noise and noise of frequencies 

outside the frequency band of interest. A high pass filter 

passes signals with frequencies higher than a selected 

value. The low pass filter passes frequencies below a 

selected cutoff value. 

Page 39 
v* v- 

- - 

- 

a) INTERNAL FET IIP AMP 

PY ROELECTRIC/PREAMPLIFIER CONFIGURATIONS 

FIG. 12 

Page 40 
TSL, Inc. August 31, 1987 

It is also important that the gain and noise figure, 

NF, of the preamplifier be adequate to assure that the S/N 

of the detector is not seriously degraded by the remainder 

of the system. Here the noise figure is defined as 

S /No 

0 

where the numerator is the signal to noise ratio of the 

input to an amplifier and the denominator is the SIN at the 

output. The NF of a detector, preamplifier, and amplifier 

combination is 

G~~~ G G~~~ p 

where G is the sensitivity of the detector, and G 

is the gain '51 the preamplifier. From this expression i! 

can be seen that noise figures of near unity are desirable 

especially in the detector and preamplifier and !hat high 

sensitivities or gains are also important. It is also 

apparent from the equation that noise becomes progressively 

less critical in those amplifier stages with gains greater 

than unity added beyond the preamplifier. 

Another signal conditioning procedure often encountered 

in applications of the pyroelectric detector is the use of a 

type of correlation analysis known as synchronous 

demodulation or phase sensitive detection. This procedure 

which involves the use of modulated signal is.well suited to -.. 

the pyroelectric detector. As pointed out this detector 

responds to changes in irradiance and is therefore 

frequently used with a mechanically chopped source. The 

technique involves the use of a stage called a lock in 

amplifier. The lock in amplifier is depicted in use in an ac 

radiometer in Fig. 13. The lock in amplifier serves two 

primary roles in this application. First it serves to 

subtract any offset or drift associated with detector, 

amplifiers, and filters from the final result. Second it 

2L serves to improve- the SIN of the detection system. 

Basically the lock-in amplifier mixes the reference signal 

derived from the chopper position with the detector signal. 

Since the signal corresponds alternately to that due to the 

source and that due to the back side of the chopper, the two 

signals are correlated. As can be seen from Fig. 13 

essentially a difference in signals, which are 180 deg out 

of phase, is found at the output of the summing amplifier. 

The noise is very effectively discriminated from the output 

of the amplifier. 

Page 41 
--c 

LOCK IN 

AMPURER 

+ 

Np- ,ti 

H 

I 

O 

- 

LOCK IN AMPUFER AND. CHOPPED RADIOMETER 

fW. 13 

Page 42 
TSL, Inc. August 31, 1987 

The second detector discussed was the photovoltic lnSb 

type. In the short circuit mode the circuit is essentially 

that shown in Fig. 12a with the pyroelectric detector 

replaced by the photo voltice diode. It is important to 

remember that this detector responds to the irradiance 

falling on it rather than to changes in irradiance like the 

pyroelectric. Therefore the lnSb preamplifier may need a 

bandwidth down to dc. With this exception the signal 

conditioning for this detector is very similar to the 

previous one. 

The HgCdTe photoconductive detector requires a 

different input stage to the signal conditioning circuit. 

This detector’s response is a small change in resistance. 

To sense this change a bias voltage and a load resistor are 

needed. A circuit such as shown in Fig. 14 may be used. 

From the output of this stage, signal conditioning is also 

very similar to that described for the pyroelectric --. 

detector. 

3.2.4 Data Acquisition 

The fluctuating voltage occurring at the final stage of 

the signal conditioning module in the infrared measurement 

system is recorded. The act of recording the signal in a 

form which is amenable to analysis is called data 

acquisition. One common form of data acquisition involves 

-‘ manually recording ‘a series of voltages taken from meters or 

oscilloscopes during the course of an experiment. Other 

forms of data acquisition involve the use of x-y plotters, 

photographs, and microdensitometersr strip charts and Gerber 

scales; analog tape recorders and digital data acquisition 

systems. With the exception of the digital data acquisition 

system the other devices mentioned are only parts of a total 

acquisition system. For example the tape recorder is only a 

part of a total acquisition system because little can be 

done with data in the form of magnetized strips of tape. 

The analog recorder and several of the other devices 

mentioned serve only to allow limitless delays in or 

modifications in the timing of the acquisition process. 

Since the analog tape recorder and digital systems are used 

in much of the current infrared instrumentation, a summary 

of the important considerations will be presented. 

The analog tape recorder operation consists of moving a 

magnetizable tape at constant speed across a recording head 

which induces a magnetization proportional to the current in 

a coil in the recording head. The signal can then be 

retrieved from the tape by passing the tape over a 

reproducing head where flux from the moving tape passed 

Page 43 
c 

Hg Cd Te DETECTOR 

Fig. 14 

Page 44 

~ 
TSL, Inc. August 31, 1987 

through a coil in the recording head in proportion to the 

tape magnetization. Important considerations for the use of 

an instrumentation tape recorder include the type recording 

(for example direct or FM), the tape speed, the number of 

channels available, the signal to noise ratio, the recorder 

head type and cost. 

Analog.tape recorders can be operated in either the 

direct recording mode or the frequency modulation, FM, mode. 

In the direct mode the voltage from the sensor is converted 

to a current in the record head coil so that the magnitude 

of input voltage corresponds to the degrees of magnetization 

of the tape. This mode of operation depends on the 

>L induction of a magnetization which is caused by variations 

of coil current and therefore can not be used for dc 

signals. With the FM mode a voltage controlled oscillator 

(VCO) biased to a frequency near the center of the bandwidth 

of the recorder provides the input to the record head. A 

large current can be used to assure a high degree of 

magnetization. The VCO can then be driven to produce 

frequencies throughout the recorder bandwidth. The FM mode 

is aseful for frequency ranges of dc to considerably less 

than the highest frequency possible in the direct mode. The 

signal to noise ratio possible with the FM technique is 

approximately a factor of 5 higher than with the direct 

technique. The tradeoffs between the two modes basically 

concerns factors such as: what is an acceptable S/N, is the 

recording of dc necessary, what is the upper frequency limit 

and cost (direct electronics are cheaper). 

Tape speed is also an important consideration. As the 

tape speed is increased the recorder bandwidth and S/N 

increases. The factors which must be considered with 

choosing a tape speed include: recorder cost increases with 

higher tape speed capability, the volume of tape used In a 

recorder operating at high speed: and of course are higher 

bandwidth and S/N required? 

The analog recorder serves to i I lustrate the 

considerations not to be overlooked in any data recording 

scheme. The record must be such that the stored data can be 

used in the data processing stage (currently this suggests a 

record which can be read by a computer). The bandwidth of 

the recorder must be adequate to ensure acquisition of the 

desired data. Bandwidths much greater than necessary should 

be avoided since excessive volumes of data will be recorded, 

as well as spurious noise. 

With the previous considerations in mind digital data 

acquisition can be examined. Digital data acquisition can 

Page 45 
TSL, Inc. August 31, 1987 

take numerous forms: however there are two basic systems: 

one which includes a computer and one which does not. The 

system that does not include a computer is generally just a 

data acquisition system while the other generally has 

additional capabilities. The limited system shown in Fig. 

14 consists of a multiplexer, a sample and hold amplifier, -, 

an analog to digital converter, an interface (which includes 

a formatter and controller), and some form of digital 

storage media (for example, a digital tape recorder). The 

more sophisticated system contains a multiplexer, the sample 

and hold amplifier, the analog to digital converter, an 

interface to a central processing unit, high speed and bulk 

memory, and input/output (I/O) devices. Figure 15a is a 

block diagram of the limited system, and Fig. 15b is a block 

diagram of the system which includes a computer. Although 

J, included in the transducer subsystem, part of the signal 

conditioning is also shown in the acquisition system since 

it may be under the control of the acquisition system 

controller. 

The signals enter the data acquisition system through 

the analog multiplexer. If only one signal is to be 

acquired this stage is omitted. The multiplexer functions 

as a bank of switches which allow the signals to be 

alternately coupled to the sample and hold amplifier. 

Important considerations involving this block are the 

programmability of the sequencer, the range of voltages 

permissible, switching speed [multiplexer settling time), 

and switching transients. 

The sample and hold amplifier is the next device in the 

digital acquisition system. The sample and hold (S/H) 

cleans up and buffers the signal for the next stage which is 

the noise sensitive low impedance converter. ,Important 

considerations for the S/H include the acquisition and 

operative times along with their settling times. Fig, 16 

illustrates the function of the sample and hold. In this 

figure, the top trace is the control signal to the S/H, the 

middle trace is an input to the S/H, and the bottom trace to 

the corresponding output of the S/H. As shown the 

acquisition time is the time required for the output to 

settle within the rated accuracy after a sample input 

control signal has been applied. The operative t.ime is the 

delay between the time the hold control signal is applied 

and the actual time the circuit enters the hold mode. 

Following the sample and hold amplifier, the analog to 

digital converter [ADC) accomplishes the translation from 

the analog to the digital world. The ADC may take many 

forms. The most popular types are the integration, 

Page 46 
1 I I 

DIGITAL DATA ACQUISITION SYSTEM 

Fig. 15 

, 

Page 47 
'1 

SAMPLE AND HOLD AMPLIFIER SIGNALS 

FIG, 16 

PAGE 48 
TSL, Inc. August 31, 1987 

successive approximation, tracking, multicomparator ladder 

and voltage to frequency convertors. The questions arising 

when applying ADC's are usually how rapidly and precisely -. 

accurately are the conversions mode. Information theory (by 

Shannon or Nyguistl suggests that the conversions should 

occur at a rate at least twice the maximum frequency of 

interest. The required accuracy precision of conversion is 

very situation dependent. For some applications, very low 

resolution may be adequate while for others one part in a 

million is insufficient. 

The ADC has as its input an analog voltage while as an 

>\ output it has e'ither a single line for a serial 

representation of the input or parallel lines for a parallel 

representation of the input. In either case the 

representation is usually a fixed point binary number which 

reflects the input magnitude. From this point, this 

discussion, if allowed to continue, could fill many volumes; 

therefore, the interested reader should examine one of the 

current texts dealing with small digital computer system. 

3.2.5 Data Processing 

The signal recorded by the data acquisition system is 

finally retrieved and processed in some fashion to yield 

that final information desired from the measurement system. 

The data processing step might be as simple as the addition 

of units of measure on a strip chart or as complex as the 

digitization, scaling, and Fourier processing required for a 

FTS. The latter is typical of data processing in today's 

infrared radiation measurement systems and will be discussed 

briefly in this section to provide an example of the kind of 

data processing which is typical for a moderately 

sophisticated sensor. 

Currently data processing is synonymous with the 

digital data acquisition process followed by computer aided 

data manipulation and graphic presentation of results. 

Subsequently hard ware is inclusive of that shown previously 

in Fig 15b together with output devices such as graphics 

terminals, digital plotters, or video copiers,. 

As an example of the digital data processing, the 

treatment of data acquired from a FTS will be explained. 

The flow diagram in Fig. 17 schematically presents the steps 

involved. As explained previously the first step in the 

digital processing process is the conditioning of the analog 

signal to provide the appropriate levels and bandwidths of 

signals the the digitizer. The second step is the actual 

digitization of the data and transfer to computer memory. 

Page 49 

. 
CONDITION 

ANALOG DATA 

b 

DI GlTl ZE I ANALOG DATA 

*. 

CALCULATE PHASE 

ERROR IN DATA 

r-5 RETRIVE I DIGITAL TEST DATA I 

I 1 1 

RETRIVE DIGITAL 

BACKGROUND DATA 

SUBTRACT 

1 

BACKGROUND DATA 

APODIZE 

DATA 

FOURIER 

TRANSFORM DATA 

PHASE 

1 

CORRECT DATA 

MU LTlP LY DATA 

BY RESPONSIYM 

I 

SCALE DATA BY 

I 

GEOMETRIC FACTOR 

GRAPH DATA 

HARDCOPY RESULTS 

DATA PROCESSIN0 REOUIRED FOR THE FTS SIONAL 

Fig. 17 

Page 50 
TSL, Inc. 

*. August 31, 1987 

The usual next step is the storage of that data on bulk 

memory for later processing or reprocessing if required. 

Therefore the next step in the process is the retrieval and 

probably the reformatting of the data for subsequent file 

manipulations. Usually at that time a previously created 

instrument responsivity is also retrieved as the storage is 

probably via some bulk media such as magnetic tape. 

Assuming the data collection included the acquisition of 

background data, the sixth step is the subtraction of the 

background file from the data file. To this point steps are 

essentially the same regardless of instrument type: 

however, the next step is peculiar to the FTS. The 

difference file is processed to fine phase error in the 

data, and that information is stored for later use. The 

data is then processed with an apodization function (this 

function will be defined in a later section) and then 

Fourier transformed. The phase error is then retrieved and 

used to phase correct the data. The instrument response is 

m u l t i p l i e d w i t h the data and finally the data is scaled to 

remove any geometrical factors. The final steps are data 

storage and presentation. 

4 .o SCAD Modules 

The goal of the SCAD development is a user friendly 

software to be used during the design of a space based 

sensor. The SCAD software will address the areas shown in 

Fig. 2. The software will be used to design a sensor system 

with components as shown in Fig. 6. There is not a one to 

one correspondence between sensor components and SCAD 

software components. Several additional software components 

are required as there are more considerations which must be 

treated than there are sensor components. 

The first component of SCAD addresses the problem 

geometry, that is, the location of the target and the 

sensor. The second component defines the source radiant 

output. The third component determines atmospheric effects 

and the fourth allows selection of sensor collection optic 

area and the irradiance upon that optic. The fifth block of 

SCAD is used to define the optical train of the sensor: the 

sixth contains the detector and preamplier characteristics: 

and the seventh deals with the electronics associated with 

the sensor. The eighth will address data storage and data 

transmission. 

The SCAD modules will be designed to be used on an as 

needed basis. Individual modules will be invoked during a 

design session. Where practical, inputs will be permitted to 

negate using modules which are of little value to a 

Page 51 
TSL, Inc. August 31, 1987 

particular problem. For examp 1 e, if atmospheric 

transmission is of little interest, then that module which 

calculates it may be skipped and a default or a constant 

transmission loss may be specified in later modules 

requiring minimal transmission information. 

4.1 GeometryITime 

The geometry issues are depicted in Fig. 18. The 

inputs must contain sufficient information to define the 

relative positions of the source, the sensor, and the sun. 

The atmospheric conditions must be specified to the degree 

that the atmospheric transmission losses and scattered solar 

light can be calculated. The objective of the first SCAD 

module is to either default to an appropriate set of 

geometry and time parameters or to offer the user an __ 

opportunity to input or modify them. The intent of the 

module is to provide input to the remainder of the SCAD 

modules concerning the location of the target, sensor and 

sun and the time of the year. The kinds of parameters 

required in later modules are the sensor to target range, 

the angle of the sun, and the like. A rninirnal set of inputs 

providing adequate information for standard parameters to be 

selected consists of: the target altitude, longitude, and 

latitude: the sensor altitude, longitude, and latitude: and 

JI the time of year (which includes day, hour, minute). 

The module will have graphics capabilities which will 

include a topographical map of the world on which sensor and 

source locations can be chosen by a mouse driven cursor. The 

module will permit zooming for accurate positioning of the 

cursor and will provide output of the chosen geometry. Time 

will be entered in terms of Zulu or from any selected time 

zone. 

The algorithms for extracting the appropriate 

information from these inputs are straight forward and can 

be implemented with high level languages such as FORTRAN, C, 

or BASIC. The choice of an appropriate software to provide 

the desired graphics is the most significant issue for this 

module. At this point another major decision remains to be 

made of whether to implement this and the next several 

modules using components or to adapt an existing software 

which contains a significant number of required features to 

address this problem. A software package developed for DOD 

by Aerodyne Research, Inc. named SPIRITS includes a number 

of features (see Table 5 for a summary of sources modeled by 

Spirits) which are desirable for SCAD. The SPIRITS software 

was developed for larger computers, but is currently being 

imported to the PC. The SPIRITS software is presently 

Page 52 
GEOM ETR Y /TI M E 

SENSOR ALTITUDE 

TARGET ALTITUDE 

SENSOR LATITUDE 

SENSOR LATITUDE 

TARGET LONGITUDE 

TARGET LATITUDE 

TIME 

COMPATIBLE WITH ACAD GRAPHICS 

- . 

Pig. 18 

Page 53 
Table 5 

Phenomena 

Exhaust plume edgeion and radiance 

~~ ~ 

Engine hot parte 

I'atget emission 

R~?fLc.cted Run, earth, atid 

sky shine 

Internal heat eourcee 

Atmospheric transmission and- radiance 

U bs cu ta t i on 

Noiiaxisymmetric flowfield 

arid radiation 

Airframe aerodynamic flow 

Ln te rior thermal conductance 

Comment e 

Precomputed; currently ueing SPFl 

and NEXTARC; tranemieeion and 

obecuration included 

Tranemitted through plume 

Aerodynamic and eolar heating; 

directional emitance; engine 

heating 

Bidirectional reflectance 

Semi-empirical 

LOWTRAN6; pathe connecting 

vehicle, ground, Bun, and obeerver 

From both eensor and eun (for 

ehadowe ) 

~- 

Combine SPFl correlatione with 3-0 

flowfield etudiee. 3-U plume 

radiation model ie applied to' thie 

f lowf ield ~ ~~~ 

Spatial variation in the adiabatic 

wall temperature 

Account for traneient temperature 

variation 

HARDBODY module may be executed without executing the firet five modules if no 

plume is desired in the ecetiario. 

Target Sources Modeled in SPIRITS Code 
TSL, Inc. August 31, 1987 

configured to allow input through menus, as shorn in Fig. 

19, and this feature will be enhanced in the PC version. 

Since there are advantages, economic most,ly, to using 

government owned software, this avenue is under strong 

consideration. If the SPIRITS code were modified for this 

application, it would do the job of the first three planned 

modules. 

The work during phase one in this area has been done with 

the AutoCAD software from Autodesk. This CAD software is 

very versatile and can do the required tasks for this 

module. However, the AutoCAD requires a trained operator 

for sophisticated functions. If AutoCAD is the final 

selection talso under consideration are packages such as 

that by Intergraph. The basic functions required for this 

module will be programmed tin LISP) for easy use by the 

user. The advantage of using a software package such as 

this is that the serious mechanical design user will have, 

in addition to SCAD, the versatility of a sophisticated PC 

CAD software package. If the SPIRITS approach is used, it 

does not necessarily preclude the use of a CAD package. 

-, 

4.2 Sources 

The source issues are partially described by Fig. 20. 

The source must be adequately defined to allow calculations 

of radiant power levels expected to be generated. The 

inputs which must be made to SCAD to account for the source 

related factors concern the source/sensor geometry and the 

source area, emissivity, and temperature. Several of the 

geometry parameters will be provided by the previous module 

d. if it has been in>voked. The approach to this and other 

modules is planned to be responsive to a variety of users. 

For the most casual user a simple level of treatment will be 

possible while for a user more interested in this phase of 

the problem, more in depth treatments will be offered. 

The simple treatment will be by a module which will 

permit selecting a simple source shape, dividing that shape 

into portions with individually selected temperatures and 

emissivities. The module output will consist of the 

spectrally resolved or the inband radiant power generated by 

the source. 

A prototype source module has been implemented using 

the LOTUS spreadsheet. Fig. 21 shows the input section of 

the spreadsheet and Fig. 22 shows the spectrally resolved 

radiant intensity calculated from those inputs. The inputs 

refer to a source which is subdivided into five portions. 

Page 55 
AVAlLhnLE TERRAINS MENU IWI 

AV A ILAII LE SCENES 

FILE NAME NENU 

EIT/FUREST 1 - BAND 1 

EtT/FOREST 2 - BAND 1 

b 

0 

e 

MT/FOREST 1 - BANI) 2 

Efr/FOREST 2 - BANI) 2 

b 

b 

DISPLAY SCENE 

PWIET E RS 

RETURN TO PREVIOUS 

MENU 

El 

STOP 

I 

RETURN TO CONTINUE PL==l 

.f 

HELP MENU 

(SELECT FILE FOR SCENE 

PARAMETERS) c 

m/FuREs*r 1 - BAND 1 

MT/FOREST 1 - BANI) 1 

- TERRAIN! MT/FOREST 

MT/FOREST 2 - BANI) 1 

0 CLOUDS ABOVE: NONE 

e CLOUDS BELOWt THLN HAZE 

b 

EIT/FURGST 1 - BAND 2 

M'P/FOREST 2 - BAND 2 SEASON: SPRING/ 

0 SUMMER 

0 

RETURN TO PREVIOUS 

0 TIME OF DAYr 3 porne 

MENU BAND: 3-5 pm 

LOOK ANGLE:' ~WJ~ZENITH 

ALT1TUUE t 10,uuo ft 

COMMENT: NONE - . 

LOOK DOWN 

Examples of User Selection of Background Within SPIRITS 

Fig. 19 

Page 56 
- .I 

SOURCE 

AREA 

A% Ak Ae 

B% Bk Be 

C% Ck Ce 

D% Dk De 

E% Ek Ee 

I =JzLk(Ao k) dAdX . .. 

FORTRAN, BASIC, LOTUS 123 

I 

Fig. 20 

Page 57 
A B C D E F G H 

1 750 Temp (K) 1 Emlslvty 0.2 Portlon of wen e 11 

2 775 Tamp (K) 1 Emlslvty 0.2 Portion of areQ e 12 

3 800 Temp (K) 1 Emlslvty 0,2 Portlon of QreQ e T3 

-4 825 Temp (K) 1 Emlslvty 0,2 Portlon of hrea e 14 

5 850 Temp (K) 1 Emlslvty 012 Portlon of QreQ e T5 

6 

7 0.1 Flrst wave\cngth 

8 0,1 Wavelength Increment 

9 Last wavetength Flrst+lncrenent~lOO= 10.1 

10 

11 1000 SQ M TOTAL AREA 

12 

13 Integra\=0,477439 

14 In tegral=0,551732 

15 Integra\=O1634096 

16 Integral=0.725070 

17 Inte~ra\=0,825206 

18 

19 

20 

3-AUQ-87 041.41 PM 

TUTAL RADIANT INTENSITY = 6427,090 

.. .. 

- 4, 

SPREAD SHEET EXAMPLE 

,8 Fig. 21 
ic 

-f-J 

-0 

U 

73 

U 

fY 

.- 

.- 

c 

U 

x 

0 

Iz 

CY 

bI I I I I I I II I I I I I I I I I 

Fig. 22 

Page 59 
TSL, Inc. August 31, 1987 

In this example, source is assumed to be a greybody with 

individual emissivities assigned to the different portions. 

The individual portions of the source have selectable 

temperatures, which are entered in kelvins. The 

calculations will be typically accomplished using 

wavenumbers for compatibility with atmospheric codes. 

Display of results will be optionally in wavenumbers or 

wavelengths. 

The more in depth treatment will include a treatment of 

reflected sunlight, which can also take several forms. The 

simplest approach is to use a selected reflectance (a 

function of angle, etc.), the solar position and assume a 

5900 blackbody viewed through the appropriate atmosphere. A 

more sophisticated approach to these scene generation issues 

is provided in other Aerodyne codes named SEGlR and AERIE. 

These codes treat the problem as depicted in Fig. 23. 

The first code (SEGIR) treats terrain using a segmented 

scene (derived for LANDSAT data) on which is superimposed a 

textured variation of both reflected and emitted light 

derived from ERlM data (Figs. 24 and 25). Strzng solar 

scattering from water surfaces is included with allowance 

for wind driven surface roughness. Clouds are superimposed 

on the scene using physical cloud models and an analytical 

representation of the reflectivity obtained from scattering 

fro spherical particles. 

The second code (AERIE) adds topographical features to 

model terrain, as described in Table 6, typically from DMA 

data, but with a statistical overlay of the critical 

underlying surface properties (reflectance, emittance, and 

thermal factors) to simulate the resulting texture in the 

scene. Reflectance and emittance from broken cloud decks 

are also included, as are effects of clouds and terrain 

shadows, and atmospheric transmittance and radiance. 

4.3 Atmospher'ic Effects 

Atmospheric effects will be treated with the LOWTRAN 6 

software developed by the Air Force Geophysics Laboratory. 

Optionally the FASCODE software might be used as an extended 

option. The inputs to the LOWTRAN will be generally derived 

from the geometryltime module with the provision to input 

desired modifications. The climatic conditions will be 

entered when this module is invoked. 

A FORTRAN version of LOWTRAN 5 was converted to the PC 

during this study with no significant problem, however, at 

least one vendor is marketing a version of LOWTRAN 6. It is 

Page 60 
CLOUD /9 

FiELD 

CLCUD REFLECT 1 ON COHPONENTS EARTH REFLECT1 ON COHPOtlEflTf 

ANALYTIC SIDIRECTIONAL SUN 0 AVERAGE COMPONENT DETERMINED ay GREY aooy 

e-, ~~ATTER I NG MODEL 

T40 WAY ATHOSPHER I C ATTENUAT I ON 

(LOWTRAN 6) 8 ATMOSPHERIC EHISSION, SOLAR SCATTERING, AN0 

iTMOSPHE3IC EHlSSlON (LOWTRAN 6) 

MISSION (ASSUMED TEHP) AND SOLAR REFLECTANCE 

USING EHlSSlVlTY AND REFLECTANCE OF TERRAIN 7:: 

ATMOSPHERIC ATENUATION FROM LOWTRAN 6 

SSLAR SiATTE3 I NG (LOURAN 6) 

BACKGROUND SCENE MODELING TECHNIQUE 

I Fig. 23 

Page 61 
CLOUD HATERIAL 

ALTITUDES FACTORS FACTORS ALTITUDES INDICES 

TERRAIN MIl-rANCE THEIUlAL 

Page 62 

SCENE DESCRIPTION MATRICES 

Fig. 24 

v-axis 4 

(line-of lonuitude) 

SAMPLE FACET GRID 

Fig. 25 

A86-543 

x-axis 

(1 tnc of latitude) 
Table 6 

Component 

errain: Geometry 

Thermal Emiseion 

Solo r Ref lec tioti 

Emi ttance/Ref lec tnnce 

: ... .. 

Tempera tutee 

Thermal Constants 

:loud: Georne try 

Thermal Emiseion 

Solar Ref lec tion 

\ 

\ thoephere: Geometry 

Radiation Traneport 

Commen te 

'ace ted topography; varying ma terials 

'arying tempera turee, emi ttancee 

'arying reflectancee; cloud elmdowe 

!nndom wi tli inprr t epec trnl eliepe, 

:orrela tiori length, vnriattce, and PSD 

itape for each material type 

\ir tempera ture ve. a1 ti tude plue 

aolet heating 

{andom, wi ti1 input correla tion length, 

!wri,srice, mid PSD e!!spe for e~ch 

naterial type 

Face Led topography 

rempera ture varfee with a1 ti tude 

Diffuee (water cloud) and epecular 

(ice cloud) 

Pa the be tween Bun, ground/cloud, and 

eeneor 

LOWTRAN6 

.C 

SUMMARY OF AERIE CHARACTERISTICS 

Page 63 

- 
TSL, Inc. August 31, 1987 

therefore not resolved whether the atmospheric codes used in 

SCAD will be from those supplied by the Bureau of Standards 

or the commercial packages supplied of ONTAR or by a SPIRITS 

or AERIE invoked LOWTRAN 6. 

The LOWTRAN provides 20 cm-1 resolution over the -25 to 

20 micron range (an example transmission curve derived from 

LOWTRAN 6 is shown in Fig. 261 and is deemed adequate for 

any foreseen sensor design problems. For the user who has a 

deeper interest in atmospheric effects, the FASCODE software 

mRy be optionally available. FASCODE does line calculations 

an offers much higher spectral resolution than LOWTRAN. 

4.4 Collection Optics 

The product of the geometry, source, and atmospheric 

effects is a resultant irradiance at the sensor collection 

optic. This module will be a very simple block which brings 

together the results determined by the previous modules. 

The most significant effort in this area will be the 

graphics development to innovatively display the product of 

the previous results. 

The results from the section can permit sensitivity 

studies to be performed for different sources and 

atmospheric conditions. The disign of the optical train 

will take place in the folowing module, but will be designed 

to start with an entrance aperture determined in this step. 

4.5 Optics 

The greatest opportunity for innovation is in the area -I 

of the optical design module. Several commercial products 

designed for the PC have been evaluated and found to be 

~lmost useless to the typical engineer. The typical 

software presumes a great deal of knowledge of optics, is 

poorly documented, and under the best of circumstances, 

requires a lengthy training time for marginal proficiency. 

It is in this area that it is most tempting to develop a 

modern, effective optics design package. 

Since most opt'ical designs will be adaptations of a few 

basic designs, the approach planned is to offer the most 

common designs with provision for easy modifications. The 

optical package used will be invoked to permit either 

transparent use for the standard designs or access to the 

sophisticated optics package for the dedicated optical 

des igner . 
n 

. : 

2 

.. . 

0 

h. 

>-.-.-.-.- 

------I___-. "..* I -.-.. 

t- I I 1 I I I I I 1 

0 

k 

Fig. 26 

Page 65 
TSL, Inc. August 31, 1987 

An important aspect of the design of an optical system 

is the inclusion of the optical properties of the windows, 

lenses, mirrors, filters and the like which are included 

within the optical train. 

The optics packages which have been reviewed for 

incorporation into SCAD have most of the features which are 

desirable for optics design except they are much too 

difficult to master. The Genesee Computer Center GENII-PC 

software appears to be typical of the optics packages 

currently available. It was designed in the 1970's for use 

on a large computer operating in batch mode and has been 

modified somewhat to operate on the IBM-PC. If this -.. particular package is used, a substantial effort will be 

required to permit rational inputs, modifications and a 

reasonable output. There is some hope that the optics 

software manufacturers will improve their product during the 

near future and a more useful package will become available 

for SCAD. 

4.6 Detector/Preamp 

n The detector/preamplifier is the transition stage 

between the optics and the electronics. From the optics 

standpoint the detector is the last stop in the system and 

may contain baffles, apertures, or the like which limit the 

potential field of view of the optical system. From the 

electronics point of view the detector is the first 

electronic element and has electrical characteristics which 

are of paramoqnt importance to the performance of the 

system. 

From the optics perspective issues of interest include 

the detector solid angle, the detector area and shape, the 

detector temperature, the detector time constant, the 

spectral detectivity, the detector linearity and the dynamic 

range of the detector. From the standpoint of the 

electronics the issues of interest include the impedance of 

the detector, the rms noise voltage, the time constant of 

the detectorlpreamplifier, and the like. The software 

treating the detector/preamplifier will be consequently be 

divided into two parts, one dealing with optics and one with 

the electronics. 

The optics portion of this module will include the 

common detectors and their common configurations. The 

detector characteristics of common interest will be 

tabulated. The spectral dee-star and other figures of merit 

will be provided. The inputs to this module will include 

detector type, size, etc. 

Page 66 
TSL, Inc. August 31, 1987 

The spectral response of the detector will be convolved 

with the spectral irradiance on the detector to provide the 

output voltage expected from a given source. 

The preamplifier portion of this module will include 

the common preamplifier configurations and their important 

figures of merit. 

The desired output from the module is the detector/ 

preamplifier signal. 

4.7 Electronics/Signal Conditioning 

Following the detector preamplifier, the electronics 

and signal conditioning elements'which may be included in a 

s y s t e m a r e m a n y and varied. In a simple noh-scanning -' 

radiometric system the analog electronics may include 

amplifiers, integrators, summing amplifiers, high and low 

pass filters, isolation amplifiers, and others. In a 

scanning system the electronics are generally even more 

complicated. 

A s w a s the case in several of the preceding optics 

modules, the first level of this module will be aimed toward 

the preliminary design of the sensor system. The first 

level will include the basic building blocks which will be 

assembled in several typical configurations. The blocks 

will be easily modified and rearranged. The product of this 

level design will be a detailed block diagram of the desired 

electronic and signal conditioning system. The basic gain 

requirements, the noise figures, the response time, the 

expected signal level for a given source, and the like will 

be determined. 

The software packages under consideration for this 

module include MicroSim Corp.'s PSpice and Visionics Corp.'s 

EE Designer. 

4.8 Data Storage/Data Transmission 

At this time, it is planned to treat the data storage 

and transmission areas in a cursory manner. The objective 

of the sensor developer does not generally include the 

design of the data storage and telemetry system. He does 

have to address the issues related to generating reasonable 

volumes of data and the like so some consideration will be 

made for these critical areas. 

Page 67 
TSL, Inc. August 31, 1987 

5.0 MERGING 

The SCAD components have been identified or considered 

in preceding sections. The degree of utility with which 

these components may be applied depends upon several 

factors. Chief among these factors are their ease of use, 

their applicability, and their compatibility. The greatest 

effort in the design of SCAD is planned in the area of 

building the shell which provides interfaces between the 

various software components which will constitute SCAD. 

During much of the phase one effort, several software 

packages have been scrutinized for applicability to the 

chore of serving as host to the various component packages 

previously discussed. Some of the software packages which 

may be used as components have also been considered as 

potential building blocks for the shell. The AutoCAD 

software has been strongly considered since it provides some 

of the graphics capabilities required and does possess some 

interfacing capabilities. The ability to work within MS-DOS 

and its ability to use customized menus makes it a vinble 

candidate to provide the shell within which to build SCAD. 

Several other software packages, such as ASYST and WINDOWS, 

proved less suitable for this application although perhaps 

useful as tools for later expansions of SCAD. 

For a time, the LOTUS 123 software was considered as a 

candidate software for developing the shell. For a portion 

of SCAD the LOTUS 123 software could be used to build 

individual components and to provide the integration of the 

components. For example, the output from the first four 

components in the SCAD as shown in Fig. 2 can be reduced to 

a matrix of numbers, such as that shown in Fig. 27. This n 

x m matrix contains much of the information required to 

define the sensor collection area, bandpass filter, and 

detector. The matrix is very amenable to being handled by a 

spreadsheet software such as LOTUS 123. The user skills -.- 

required to use SCAD are anticipated to be those of an entry 

level 123 user, so there is impetus to use that software. 

The major deficiency with using 123 as a component is the 

difficulty with interfacing it to another program. It is 

very easy to import or create message files to and from 123, 

but at this time it appears to be difficult to have the 

interfacing be transparent to the user. Another reason it 

was attractive to consider using 123, was that it includes 

some plotting capabilities. On the other hand, a more 

A. sophisticated graphics package was desirable. 

Page 68 
a a 

C = Geometry factor from Optfcs 

R - Normalized response of rensor 

n - 1-5 

1 

c 

at 

= CONSTANT 

OR 

MATRIX 

Fig. 27 

LOWTRAN 

v, e1 

a 

at 

N 

Page 69 .. 
TSL, Inc. August 31, 1987 

After considerable attention has been addressed to the 

investigation and evaluation of the previously mentioned PC 

bRsed softwares for use in developing the SCAD shell, the 

conclusion is none of them would perform as well as desired 

for SCAD. At this time the choice appears to be to 

construct that shell software using a high level language, 

such as LISP, and an AI shell such as KES from Software A&E. 

6.0 GRAPHICS 

Excellent graphics is considered to be of paramount 

importance to the success of any future software which will 

be used by a general audience. Therefore, considerable 

attention is planned for the implementation of a easy to use 

but very graphic set of tools. 

Each component will include a graphical representation 

of the inputs made to that module and will also graphically 

display the results of that module. 

The graphics planned for !he geome?ry component of SCAD 

will include a map of Earth with zoom capabilities for 

finding topographical features, geodesic location and -, 

political boundaries. The location of the sensor and target 

will be available in the form of easy to understand monitor 

pictures and drivers will be provided for output to a 

variety of hardcopy devices. 

The graphics for the atmospheric transmission will 

include a visual presentation showing the relative position 

of sensor, sun, and source. The atmospheric conditions will 

be presented graphically and the atmospheric transmission 

"I wi I I be displayed' as a plot of transmission versus 

wavenumber or wavelength. The atmospheric emission will be 

displayed as radiance versus wavenumber or wavelength. 

Since the function SCAD serves is to provide aid in the 

graphics will be made in the area of optical design. In the 

vein of CAD, the system will offer output of the final 

design both with determinations of design parameters but 

also will permit the output of so called conceptual drawings 

as well as detailed mechanical drawings of the sensor 

system. 

design of optical sensors, the greatest attention to 

Page 70 
TSL. Inc. August 31, 1987 

7.0 EXPANSION 

The development of SCAD will provide two products which 

may be expanded to serve other and larger audiences. First 

the SCAD can be expanded in several ways to enhance its 

performance. It can also be expanded to i nc I ude 

capabilities required by other sensor designers than those 

examining earth resources. For example, the SCAD can easily 

be expanded to be more useful to DOD designers. But, 

perhaps the most opportunity for expansion is in the area of 

applying the shell structure to other problems using other 

component softwares. The SCAD shell will have addressed the 

problem of interfacing to older unwieldy softwares and the 

techniques should be in hand to expedite the handling of 

similar softwares. Also, the shell will already have built 

in utilities, sucha s a broad graphics capability and device 

handlers ~ 

The design of the SCAD will be with the aim of adding 

further capabilities as required for more sophisticated 

sens~r design as well as app!ica?!an ?o sirniter problems 

One of the first potential outgrowths of SCAD is its 

application by DOD users who are involved with SDI or space 

defense designers. 

8.0 DISCUSSION 

The work described has been directed toward 

establishing a detailed plan to assemble a set of software 

tools which can be used in the design of space based optical 

sensors. The preliminary plan was to incorporate as much 

previously developed software as possible into an easy to 

use package. That preliminary plan has been followed and 

appropriate softwares have been examined for applicability 

to this problem. The most promising softwares and areas of 

potential difficulty have been identified. 
TSL, Inc. 

\ 

August 31, 1987 

9.0 BIBLIOGRAPHY 

1. The Infrared Handbook, edited by W.L. Wolfe and G.J. 

Zissis, Office of Naval Research Department of the Navy, 

Arlington, Va., United States Government Printing Office, 

1978. 

2. Handbook of Military Infrared Technology, Editor 

William L. Wolfe, Naval Research Labratory, US Government 

Printing Office 1965. 

3. Lecture Notes, Infrared Physics and Technology, A 

Short Course presented By UTSI, Course Director A.A. Mason 

Nov. 1979. 

4. Optoelectronics Theory And Practice Edited by Alan 

Chappell, McGraw Hill Book Company, New York 1978. 

5. Chemical Instrumentation: A Systematic Approach, 

>I Howard A. Strobel Addison-Wesley Publishing Co. Reading Mass 

1973. 

6. Handbook of Infrared Radiation from Combustion 

Gases, C.B. Ludwig, W. Malkmus, J.E. Reardon, J.A.L. 

Thomson, Editors- R. Gonlard, J.A.L. Thomson, NASA SP-3080. 

7. Optics, E. Hecht and A. Zajac, Addison-Wesley 

Publishing Company, Reading, Massachusetts, 1979. 

8. Concepts of Classical Optics, John Strong, W.H. 

Freeman Co., San Francisco, 1978. 

9. Principles of Optics, Max Born and Emil Wolf, Fifth 

Edition, Pergamon Press, New York, 1975. 

10. Modern Optical Engineering, Warren J. Smith, McGraw 

Hill Book Company, New York, 1966. 

11. loop ti cs for the Electronics Eng i neer“, 

Electro-Optical Systems Design, February 1977. 

12. “The F-stops Here”, M.R. Hatch and D.E. Stolttman, 

Optical Spectra, June 1980. 1 

13. “Complete Analysis of a Two-mirror Unit 

Magnification System. Part 1, A. Suzuki, Applied Optics, 

Vol. 22, No. 24, December, 1983. 

14. “Complete Analysis of a Two-mirror Unit 

Magnification System. Part 2, A. Suzuki, Applied Optics, 

Vol. 22, No. 24, December, 1983. 

Page 72 
TSL, Inc. August 31, 1987 

15. Infrared Detectors, edited by R.D. Hudson Jr. and 

J.W. Hudson, Dowden, Hutchinson and Ross Inc. Distributed by 

Halsted Press a division of John Wiley and Sons Inc., 1975. 

16. "New Infrared Decfor~~~, E.H. Putley Part 5, 

Advances in Geophysics ,Vol. 14, edited by H.E. Landsberg 

and J. Van Mieghem. Academic Press, New York, 1970. 

17. llCadmium-Telluride Detector Has Highest Sensitivity 

Yet", Electronic Design, Vol. 12, June 1974. 

18. "HgCdTe:Number One in Night Vision" R. Martineau, 

G. Robillard and T. Wong, Optical Spectra, July 1979. 

19. "Implementation of a System of Optical Calibration 

Based on Pyroelectric Radiometry", W.M. Doyle, B.C. McIntosh 

Nov./Dec. 1976. 

and Jon Geist, Optical Engineering, Vol. 15, No. 61 

20. IIC r yo g e n i c S em i con d u c t o r N.A. 

Pankratov and V.P. Korotkov, Optical Technology, Vol. 41, 

No. 4, February 1974. 

Bo I ome t e r s , 

21. "The General Theory of Bolometer Performance", R. 

Clark Jones, Journal of the Optical Society of America, Vol. 

43, No. 1, January 1953. 

22. "Application Note AN-10", C.B. Roundy, Spiricon, 

Logan, Utah. 

23. Solid State Electronics, Shyh Wang, Mcgraw-Hill 

Book Company, New York, 1966. 

24. "Pyroel ect r i c Nul I Detector for Absol Ute 

Radiometry", Pradhan,M.M. and R.K. Garg, Applied Optics, 

Vol. 21, No. 24, Dec. 1982. 

25. "Infrared Thermal Detectors:A Comparison", Chao 

Huang, Optical Spectra, September 1978. 

26. "Pyroelectric Vidicons - The Ir Imaging Dark 

Horse@', B. Singer, Electro-Optical Systems Design, July 

1975. 

27. "The Pyroelectric/CCD Focal Plane Hybrid: Analysis 

and Design for Direct Charge Injection", R. Watton, P. 

-' Manning, D. Burgess'and J. Gooking, Infrared Physics, Vol. 

22, 1982. 

Page 73 
. TSL, Inc. August 31, 1987 

28. Solid State Electronics, Shy1 Wang, McGraw Hill 

Book Company, New York, 1966. 

29. "Principles of Electronic Instrumentationii, A. 

-I James Diefenderfer,cW. B. Saunders Co., Philadelphia, 1972. 

Page 74 

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TSL, Inc. August 31, 1987 

LIST OF SYMBOLS 

A 

B 

C 

d 

D 

D* 

e 

E 

f 

f/ 

FCTI 

h 

i 

I 

k 

K 

I 

L 

m 

M 

C 

n 

N 

NEP 

NF 

Q 

QE 

r 

R 

>. S 

t 

T 

v 

a 

E 

S 

U 

e 

.. . 

Area (cm 1 or Electric Field Magnitude 

Magnetic Field 

Specific Heat or Speed of Light 

Conductivity 

Distance 

Detectivity 

Dee-star, Normalized Detectivity 

Excited Electron Lifetime 

Irradiance 

Focal Length (cml or Frequency [Hz) 

F-number 

Pyroelectric Coefficient 

Planck's Constant or Detector Thickness 

Current (amperes) 

Radiant Intensity (watt/srl 

Boltzman's Constant 

Thermal Conductance 

Length (cm) 

Rad i ance 

Modulation Index 

Emi t tance 

Index of Refraction 

No i se 

Noise Equivalent Power 

Noise Figure 

Power 

Quantum Efficiency 

Reflection Coefficient 

Responslvity 

Distance 

SignailVol tage 

Time or Transmission Coefficient 

Temperature (Kelvins) 

Electron Mobility 

Frequency CHzI 

Absorptivity (%I ' 

Emissivity (%I 

Angle (degree1 

Wavelength (microns) 

Permeability Constant 

Phase Difference 

Reflectivity (%I 

Wavenumber (cm 1 or Stefan-Boltzman's Constant 

Transmissivity (96) or Time Constant (second) 

Angular Frequency (radians/second) 

Solid Angle Csrl 

Delta 

Page 75