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3 1822 04429 7174
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MAY 1986
ATA Offsite IC VISIBILITY
TEC (Annex-Jo NOTE NO. 2017
rnals)
QC
974.5
T45
no. 201
A MULTI-SPECTRAL IMAGER FOR THE AUTOMATI
DETERMINATION OF SECTOR VISIBILITIES
AND THE CONCURRENT OPTICAL STATE
OF THE ATMOSPHERE
*
R.W. Johnson
'W.S. Hering
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UNIVERSITY
The material contained in this note is to be considered
proprietary in nature and is not authorized for distribution
without the prior written consent of the Visibility Laboratory
and Air Force Geophysics Laboratory..
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CALIFORNIA
SAN DIEGO
Contract Monitor, Donald D. Grantham
Atmospheric Sciences Division
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3 1822 04429 7174
A MULTI-SPECTRAL IMAGER FOR THE AUTOMATIC
DETERMINATION OF SECTOR VISIBILITIES
AND THE CONCURRENT OPTICAL STATE
OF THE ATMOSPHERE
Richard W. Johnson and Wayne S. Hering
Visibility Laboratory
University of California, San Diego
Scripps Institution of Oceanography
La Jolla, California 92093
INTRODUCTION
The proliferation of advanced military electro-optical (E/O) systems that are designed for visible spectrum
operations within the troposphere has greatly increased the need for a compact automatic system designed to assess
and predict the optical and meteorological properties of this operational medium
The Visibility Laboratory, under the sponsorship of the Air Force Geophysics Laboratory, is developing a
family of small, solid state imaging systems to address this need. Several existing and developing versions of these
devices and their proposed uses are described briefly in the following paragraphs and illustrations.
ed
In its most basic form, each of these devices consists of a computer controlled solid state video system that
provides calibrated multi-spectral imagery suitable for the automatic extraction of local image transmission and cloud
cover information. Imbedded within the control computer are prototype and proprietary extraction algorithms
necessary to provide these numerical products. In addition to radiometrically calibrated imagery, advanced algorithm
development is currently underway to provide near real-time products of the data acquisition, processing, and display
system in the form of continuously updated digital presentations of selected operational quantities. The quantities
most desired for describing the optical state of the atmosphere in which this sensor system operates are of course task
dependent, but current algorithm development is slanted toward cloud detection, cloud free arc determinations, sector
visibilities and total cloud cover. The multi-spectral imagery is equally applicable in more generalized search, detect
and identify scenarios.
FIRST GENERATION INSTRUMENTATION
The system block diagram shown in Fig. A is intended to illustrate configurational relationships common to all
systems described herein, and is not necessarily an as-built drawing. For example, the "select" function linking the
Z-100 computer and the Time/Date Generator is conceptual only. Both the video and digital data are routed through
the system simultaneously and independently. The parallelism is an operator convenience, not a necessity. Most of
the operational characteristics illustrated in Fig. A are included in each of the currently evolving hardware
configurations.
Several versions of the basic computer controlled system designed for the semi-automatic acquisition and
archival of calibrated imagery are illustrated in Figs. B thru E. Their component sub-assemblies are listed in Table
A.
The schematic drawing of the multi-spectral imager shown in Fig. B illustrates two important features of the
camera system that contribute to its general utility.
First, the Filter Changer Assembly is shown to contain two independently controlled filter locations on the
optical axis. In actuality, this assembly contains two filter wheels, each holding four separate filters in addition to
the five lens optical relay. Each wheel can position any one of its four filters into the optical path under either
manual or computer control. In its present configuration, the forward wheel contains four glass absorption neutral
density filters, while the rear wheel contains four spectrally different interference filters. Thus, both spectral band and
flux level can be controlled either manually or by predetermined computer program.
Second, the multi-lens Turret Assembly (shown pictorially, not as built), provides an efficient method of
modifying the overall optical path to meet task specific requirements. In the prototype systems currently in use, the
two lens assemblies require manual substitution into a fixed adapter. The remotely controlled turret assembly is still
under development, as are the remote control of the Iris and Occultor sub-assemblies. However, it is important to
note that the system lends itself well to task specific changes in turret design.
O
The composite, as-built system shown in Fig. C is the currently operating Z-100 system. Both the video and
digital options previously illustrated are contained in the two instrument racks. The camera, filter changer, and
fisheye lens assembly, are supported on the temporary camera pedestal. This successfully deployed operational
system is designed primarily for image acquisition and archival only. The computer does not contain the
programmed algorithms necessary for sophisticated image manipulation and analysis, although relatively simple data
extractions and numerical computations are readily available. The system can reliably acquire an image every two
seconds and store it on the internal hard disk. Identifying time/date and filter information is imbedded within each
image data array for subsequent processing and analysis control. At the present time 360 images are stored on disc
prior to their batch mode downloading to magnetic tape. A simplified version of this system, similar to that in Fig.
C only without the VCR mode, is illustrated in Fig. D.
The fundamental shortcoming of this computer system is its limitation to eight bit I/O, and lack of image-
oriented, third party software/firmware support. The first generation systems shown in Figs. C and D both use the
GE2505A2 solid-state CID video camera as the primary detector. This camera outputs a standard RS170 composite
video signal which is grabbed and digitized by the Poynting 505 frame grabber. The resultant 8-bit 256x256 data
array is subsequently operated upon to yield an image that is radiometrically calibrated in absolute radiance units
traceable to the National Bureau of Standards. Geometric calibrations are performed on the array such that with the
174(degree) Fisheye adapter lens in place the system yields an angular resolution of approximately 0.7 deg/pixel, and
with the 30(degree) small FOV lens in place yields approximately 0.1 deg/pixel resolution.
primeru *
SYSTEM CALIBRATION
Whereas many useful algorithms for the determination of atmospheric properties can be devised to require only
the input of the relative values of radiant flux fields, it is generally true that far more redundant and reliable
methodologies are available when absolute values of radiance are available. Thus, to enable an optimum selection of
techniques for analytic applications, the camera systems described in this note are all calibrated against standards of
radiant intensity traceable to N.B.S. The general calibration procedures are outlined in Fig. F.
The step-by-step sequences required to accomplish the procedures outlined in Fig. F, are discussed in detail
within several separate notes and will not be addressed herein. The key feature underlying all of the calibration
sequences is, of course, the stability and reproducibility of the system radiometric linearity. Two examples of the
Linearity Calibration conducted on the systems illustrated in Fig. C are shown in Fig. G.
SECOND GENERATION INSTRUMENTATION
Having satisfactorily demonstrated the ability to rapidly and reliably acquire and archive calibrated whole sky
imagery with the Z-100 based systems described above, there was an immediate recognition that an even more
powerful tool was clearly within our grasp. By increasing the image manipulation capabilities within the host
computer, and by further customizing the optical collection optics, it seemed possible to create an acquisition and
computational environment suitable for advanced, automatic determinations of the optical state of the atmosphere.
The following paragraphs describe the cogent electro-optical and computational characteristics of this emerging
system.
The evolution of the second generation system is defined by two basic changes. First to an enhanced host
computer, and second to a more exotic optical "turret". Other than that the system concept remains the same,
calibrated imagery is manipulated to mimic the observational skills of a human observer.
With regard to the first change, it was felt that a true 16-bit machine was necessary to handle the data flows and
computational techniques anticipated. Since the early Z-100 systems had performed well, it was decided to stay
within the Zenith environment, but to upgrade to their IBM clone, the Z-200 series, in order to avail ourselves of the
rapidly expanding IBM support within the third party vendor market. Following this plan, the second generation
system outlined in Table A has been partially acquired and exercised. Most of the algorithm development and
preliminary computations discussed later in this note have been performed in the Z-200 environment. Other than
some minor inconveniences in fully exploiting the computer's enhanced RAM (1.2 MB) via the LOTUS 1-2-3 code,
the computer system has performed well. It is anticipated that both the second and third generation systems will
continue to use the Z-200 family computer. Only the tenant P systems will change, in association with future task
specific I/O and display requirements.
The revision of the system to incorporate a second generation "turret" has required several iterations of optical
mock-up and evaluation. The sensor assembly currently proposed for use is illustrated in Fig. E. In this semi-
exploded pictorial, the essential features of this emerging system are clearly illustrated.
The "turret" has become a rotating stage assembly upon which are mounted a telephoto lens for scanning the
local horizon, a prismatic beam splitter, and a mechanical two aperture shutter assembly. Rotation of the shutter
about the optical axis of the beam splitter controls which of two image sets are transmitted through the filter changer
to the fixed camera. In one position the optical path of sight is through the fisheye lens, and whole sky images are
acquired for the determination of local cloud cover, cloud-free paths of sight, etc. In the second position, the optical
path of sight is through the telephoto lens, and small field of view images containing the local horizon are acquired
for the determination of sector visibilities.
The entire electro-optical assembly is contained within an insulated weatheproof housing. The remaining design
problems relate to the inevitable problems associated with maintaining optically clean windows through which the
lenses must collect their imagery. While the solution of these problems will involve some severe aggrevation, the
fundamental concept of the device should remain uncompromised.
The third generation system summarized in Table A represents two refinements to the basic Z-200 system. The
first is a change to the more powerful Image Technology FG-100 Frame Grabber, and the second is a change to the
2710 camera. Both changes are directed toward improving the precision and accuracy of the radiometric calibrations
and do not imply a change in the basic philosophy underlying the systems performance.
SYSTEM PERFORMANCE CAPABILITIES
The dedicated system which combines solid-state image sensing capability with microcomputer technology is
ideal for the acquisition, processing, and analysis of background sky, cloud and terrain radiance fields. Measurement
accuracy and operational efficiency are achieved through:
(a) excellent control over both the spatial resolution and the relative brightness accuracy. Since each pixel element
has exact placement in the solid-state sensor array, the geometry is fixed. Thus, any imperfection in discrete element
performance can be diagnosed and specific corrections applied. The responses tend to be highly linear and stable so
that the corrections for individual pixels are easily made and tend to be valid for extended time periods.
(b) the rapid response of the solid state detectors with no memory problems. Thus, once the brightness field is read
out, it is immediately set for renewed data acquisition and provides excellent temporal resolution.
(c) the digital data format, which allows quick and easy computer conversion of the relative brightness field into the
absolute radiance field for direct determination of the optical properties of the atmosphere.
BASIC EXPRESSIONS FOR VISIBILITY DETERMINATION
Each pixel value in the resultant computer processed image is the calibrated apparent radiance of the
corresponding line of sight. Let us now consider the application of these data for quantitative determination of
atmospheric clarity as represented by specific estimates of the prevailing visibility or the visual range for designated
lines of sight
Neglecting turbulence effects, the apparent spectral radiance of a distant object t at range r along a path of sight
specified by zenith angle 8 and azimuthal angle o, can be written (see Duntley et al., 1957)
where
L (2, 0, 0 ) = T; (2,0 ) Lo (Zt, 0, 0 ) + L* (z, 0, 0 )
.
.
(1)
Lo
is the inherent target radiance at altitude ze corresponding to a measurement at zero range on
the path of sight.
т.
is the radiance transmittance or the fraction of the inherent radiance remaining after traversing the
optical path of length 1.
L*
is the path radiance generated by the light reaching the path of sight from the sun and the
surrounding sky and terrain and in turn is scattered by the air molecules and aerosol particles along
the path in the direction of the sensor located at altitude z.
Similarly, the apparent radiance of the background b, immediately adjacent to the target is given by
bL (2,0,0) = bLo (Zt, 0, 0 ) T: (2, 8 ) +L* (z, 0, 0)
where
bho
is the inherent radiance of the background at range zero (target position).
The radiance transmittance is given by
Ty = exp = 1 a(z) dr
where
is the volume attenuation coefficient and is equal to the sum of the scattering (as) and absorption
(aq) extinction coefficients.
The inherent spectral contrast is defined
Co (zt, 0,0 ) = [ Lo (Zt,0,0)-blo (zt, 0, 0 )] /blo (zt, 0, 0 )
MA
and the corresponding definition for apparent spectral contrast at range r is
4-
Ç(z, 0, 0 ) = (x4, (2, 0, 0 )-64, (2, 0, 0)] / 6L (2, 0, 0).
wa
Subtracting Eqs. (1) and (2) and combining with Eqs. (4) and (5), it follows that
ç (3, 0, 0 ) / Co (z , 0, 0 ) = T; (2, 0, 0 ) blo ( 26 0,0 ) / 647 (2, 0, 0).
As emphasized by Duntley et al., (1957) the above expression for the contrast transmittance of the path of sight
is the law of contrast reduction in its most general form. It applies to any path of sight regardless of the extent that
the ambient light or the optical scattering properties of the atmosphere vary from point to point.
DETERMINATION OF DAYTIME VISIBILITY
The process of daytime visibility determination either instrumentally or by a human observer ultimately
involves the resolution of the distance from pre-selected targets that the apparent contrast, Cr, reduces to some
minimum (threshold) value needed for detection. The threshold contrast is dependent on a number of factors
including the visual acuity of the observer as well as the angular subtense of the target, its shape and its location
with respect to background features. Human estimates of visibility result from adaptive integration by the eye in
time and space of all scene features. However, for official observations the World Meteorological Organization
(WMO) and the Federal Meteorological Handbook (FMH) recommend that daytime visibility targets be black or very
dark targets viewed against the background sky. However, in order to achieve adequate areal coverage, objects pre-
selected for visibility markers for routine meteorological observations often are less than ideal both in angular size
and inherent optical properties.
With these problems in mind, let us consider the general approach involved in the application of the solid-state,
E/O camera system to visibility measurement. The goal is to develop a completely automated system to provide
objective quantitative estimates that are consistent regardless of time and location. The degree of visual modeling
required to provide, for example, the visual range along specific pre-selected horizontal or slant paths depends upon
the accuracy needed to satisfy the individual user requirements. In the following discussion, we limit attention to the
more general problem of determining prevailing visibility, which is defined by Douglas and Booker (1977) as the
greatest visibility equal or excluded through at least half of the horizon circle which need not be continuous, i.e. the
median visibility around the horizon circle. The E/O camera system provides a continuously updated representation
of the detailed radiance scene for the discrimination of prevailing visibility and its sector variations. The objective is
to use effectively the measured apparent radiance contrast for all suitable objects in the scene, adjusting the resultant
visibility estimates as necessary to compensate for the non-standard nature of individual targets.
We note that for the common situation where an object is viewed against the clear horizon sky, the inherent
background radiance, blo, as measured at the target position and the apparent background radiance at the measuring
point, bLq, tend to be equal. To a good first approximation, the horizon sky brightness does not change appreciably
as one approaches or backs away from the target. Thus, Eq. (6) for the contrast transmittance reduces to
(Note: henceforth for convenience, the directional notation will be omitted in most cases)
G/Co = Ty = erão
where
ā
is the average attenuation coefficient for path of sight of length r.
If the object is at the maximum range of detection
E = Co e a Vic
where
E
is the threshold contrast. A value of .05 is recommended by the WMO.
.
is the corresponding object range and is the visibility as determined from the measured
apparent contrast of the object.
From Eq. (4) we see that the for the special case of a black target, blo=0, the inherent contrast, C, is always
-1. Since, the contrast cannot change sign with distance along the path of sight, the negative sign can be ignored.
So, for the black target viewed against the horizon sky Eq. (7) may be written simply as
ç = TG
and if the object is just visible
E = e-avy
(10)
where
VN
is the visibility as determined for a black target with horizon sky background
which we define as standard conditions or the "normalized visibility".
If we solve for Vc and VN and divide Eq. (10) by Eq. (8) we have
VN = Vc In € / (In ε - In Co).
(11)
Thus, for objects with horizon sky background, the normalization adjustment of the visibility as determined for
objects other than black is independent of the transmittance and the directional scattering properties of the
atmosphere, but is a function of the inherent contrast, Co.
Additional modeling procedures are required to obtain reliable estimates of prevailing visibility from the
measured apparent contrast of objects viewed against a terrestrial background. For the essentially horizontal paths of
sight involved in visibility determinations, Eq. (2) for the apparent background radiance may be written with good
approximation (Duntley, 1948)
bha = blo Ty + Lg (1 - Tp)
(12)
and
19 (3, 0, 0 ) -S_L (2, 6", "") P (2, B) aN
411
where
is the point source function of the path radiance and is the radiance that must exist at each point
such that the loss of radiance within that path segment is balanced by the gain
is the single scattering phase function
is the scattering angle between ambient source light direction and path of sight.
In effect, Eq. (12) assumes that L, is spatially constant for the integration of the path function over range, r.
Notice that, for the long path length associated with the horizon sky background, T, tends to zero and the apparent
clear-sky horizon radiance, bly is equal to the source function (equilibrium radiance), Lg, for the corresponding
scattering angle with respect to the sun.
Substituting Eq. (12) in Eq. (6) and solving for the apparent contrast, Co we have,
G = Co Tpl [S + T; (1-3) ]
(14)
where
S = L / blo
is the sky/ground radiance ratio (Duntley, 1948).
If the object is just visible, C = ε and re-arranging terms we have
e-a Vc = ES I [Co - € (1 – S)] .
(15)
Dividing Eq. (10) by Eq. (15), the expression for the normalization of the visibility, Vc, as determined for the
general target to the visibility corresponding to the standard black target conditions, VN, is given by
VN = Vc In ε l{In (ES) – In (Co- € (1 – S)] }
(16)
Thus, the adjustment needed to help ensure consistent determinations of visibility from this general class
targets, involves the specification of the inherent contrast, Co, and the sky ground ratio, S, or its components L, and
blo. These quantities are subject to significant variability depending upon such factors as sun angle, cloud cover and
the relative amounts of direct sun and multiply scattered diffuse light incident on the target, the background, and the
path of sight. The quantity Lg can be determined from the measured apparent horizon sky radiance where the sun
scattering angle, ß, is the same for the path of sight to the horizon and the path of sight to the object. However, the
other parameters are not available directly from the calibrated radiance field as determined by the remote solid-state
camera system. Appropriate analytic methods must be introduced to derive the information from the measured
radiance distributions.
The analytic techniques inherent in computationally fast atmospheric radiance models such as FASCAT (Hering,
1984) can be used to extract diagnostic information from the observed radiance fields and continuously update the
adjustment factors for designated visibility targets in accordance with significant changes in background radiance.
Sensitivity calculations with the FASCAT model serve to identify the responses of the sky/ ground ratio and inherent
target contrast to key factors such as the variations in the 3-dimensional target irradiance (direct sun and diffuse) and
variations in the optical and meteorological properties of the atmosphere. The approach is to establish provisional
algorithms for real-time calculations of the adjustment factors as required for pre-selected targets and then refine these
algorithms as necessary on the basis of information acquired through operational use of the system. Ideally, the
objective estimates of normalized visibility from individual targets should correspond closely during periods when
boundary layer mixing is sufficient to result in a spatially uniform distribution of atmospheric attenuation
coefficient, Oly and source function, Lg. Careful monitoring of the system operation during periods of horizontal
uniformity provides the data needed to help establish the uncertainties associated with the available target array as
well as a basis for refinement and improvement in the daytime visibility determinations.
ACKNOWLEDGEMENTS
This research was funded by the Air Force Geophysics Laboratory under Contract F19628-84-K-0047.
REFERENCES
Douglas, C.A. and R.L. Booker, "Visual Range: Concepts, Instrumental Determination, and Aviation Applica-
tions", Final Report No. FAA-RD-77-8, U.S. Department of Transportation, Federal Aviation Agency.
Duntley, S.Q. (1948), "The Reduction of Apparent Contrast by the Atmosphere", J.Opt.Soc.Am. 38, 179-191.
Duntley, S.Q., A.R. Boileau, and R.W. Preisendorfer (1957), "Image Transmission by the Troposphere 1,
J.Opt.Soc.Am. 47, 499-506.
Hering, W.S. (1984), "Analytic Techniques for Estimating Visible Image Transmission Properties of the Atmos-
phere", University of California, San Diego, Scripps Institution of Oceanography, Visibility Laboratory, SIO
Ref. 84-6, AFGL-TR-83-0236.
Table A. Hierarchy of Prototype Imaging Systems
(Not Including Optical Accessories & Housings)
First Generation, Z-100 Based System
Item Description

Item No.
"
.
..
........
..
...
voo AWN
ZENITH Z-100 COMPUTER, 8088/8085 up, enhancements, 40 MB hard disk
Poynting 505, Video Frame Grabber
Poynting 208, Digital Video Memory
Cipher F880640, Microstreamer Tape Drive
RCA TC1910 CCTV Monitors
GE2505A2 CID Video Camera
VisLab Dual Wheel Optical Filter Changer
VisLab Accessory Control Panel
Archival Only, No Image Processing (i.e. I.P.) Software
Second Generation, Z-200 Based System
Item Description

Item No.
....
..
..
.
Zenith Z-200 Computer, 80286 uP, enhancements, 40 MB hard disk
ITI, PFG-8-1-U-XT Video Frame Grabber (Internal Board)
Zenith ZVM-135 Hi Resolution Monitor
Cipher 540 Streaming Cartridge Tape Drive (Proposed)
Sony PVM 1271(Q) Hi. Res. Video Monitor (Proposed)
GE2505A2 CID Video Camera
VisLab Dual Wheel Optical Filter Changer
VisLab Accessory Control Panel
Okidata u83A MicroLine Printer
........
.
000 voor A WN
..
..
.
..
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Designed to run LOTUS 1-2-3 and PC Vision/Image Lab I.P. Software
Third Generation, Z-241 Based System
Item Description

Item No.
...
.......
.......
600 voor AWN
Zenith, Z-241 Computer, 80286 uP, enhancements, 40 MB hard disk
Imaging Technology FG-100 AT Frame Grabber
Zenith ZVM-133 RGB Video Display
Cipher 540 Streaming Cartridge Tape Drive
Sony PVM 1271(Q) Hi. Res. Video Monitor
GE 2710 CID Video Camera
VisLab Dual Wheel Optical Filter Changer
VisLab Accessory Control Panel
Epson FX85 Printer
Designed to run LOTUS 1-2-3 and PC Vision/Image Lab I.P. Software
.
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COMPOSITE SYSTEM PROPOSED FOR CFLOS ALGORITHM DEVELOPMENT

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DIGITAL
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IRIS
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Assembly
Multi-Lens Turret Assembly
Fig. B.
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8 bit Frame Grabber
Real Time &
Frozen Image
Display
Accessory Controls
a. Filter Changer
b. Video Switch
c. Iris Control
d. Occulter Control*
Video Cassette
Recorder for
Auxillary Data
Archive
*In Development
Z-100 Computer
W/40 Megabyte H.D.
for Control & Analysis
Multi-Spectral
E/O Camera Assy
TIVO
8 Bit Video Display
for Playback Q.C.
Nine Track Tape
for Data Archive
Composite System Proposed for CFLOS Algorithm Development
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Real Time &
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Display
Multi-Spectral
E/O Camera Assy
8 bit Frame Grabber
8 bit Video Display
for Playback Q.C.
Accessory Controls
a. Filter Changer
b. Video Switch
c. Iris Control
d. Occulter Control
"In Development
Z-100 Computer
w/40 Megabyte H.D.
for Control & Analysis
chouten
Nine Track Tape
for Data Archive
Fig. D. Z-100 System for Whole Sky Image Archival.
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have
Automatic Observing System for Whole Sky and Horizon Imagery

Occuher
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E/O CAM III: SYSTEM CALIBRATION PROCEDURES
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....
..
POLARIZER
TRANSMITTANCE
CHARACTERISTICS
SEE REF SHT 6
RADIOMETRIC
LINEARITY
CHARACTERISTICS
RADIOMETRIC
ABS. SENSITIVITY
| CHARACTERISTICS
SEE REF SHT 4
SEE REF SHT 5
INPUT FROM
HARDWARE
PROCEDURAL RELATIONSHIPS WITHIN CALIBRATION SEQUENCE
OUTPUT TO
TAPE
Fig. F.
.13-

EJO CAM III: LIN CAL
CAM 436-1854
GRABBER 84 06 02
30 SEP 85
RELATIVE FLUX
50
100
150
BYTE VALUE OUTPUT
200
250
300

EJO CAM III: LIN CAL
CAM 436-1854
GRABBER 85 09 03
7 OCT 85
RELATIVE FLUX
50
100
200
150
250
0
0
BYTE VALUE OUTPUT
Fig. G.