Lunar colorimeter image processing aboard Apollo 

Goetz, A. F. H.

English

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BELL-COMM, INC.
955 l ENFANT PLAZA NORTH, S.W WASHINGTON 0 C 20024
COVER SHEET FOR TECHNICAL MEMORANDUM
TITLE- Apollo 8 Colorimetry	 TM- 69- 2015 -3
DATE- March 11 , 1969
FILING CASE NO(S)- 340	 AUTHOR(S)- A. F. H. Goetz
FILING SUBJECT(S)-
(ASSIGNED BY AUTHOR(S)-
Apollo 8 Sclence
Lunar Colorimetry
Image Processing
ABSTRACT
The results of the two color CSM photographic
experiment aboard Apollo 8 are discussed. Operational problems
precluded obtaining data useful for data analysis by image
processing.
The image processing technique applied to earth
based lunar photography is discussed. A computer produced,
color difference photograph is shown.
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955 L'ENFANT PLAZA NORTH, S.W.	 WASHINGTON, D. C. 2DO24
SUBJECT: Apollo 8 uolorimetry	 DATE: March 11, 1969
Case 340
FROM: A. F. H. Goetz
TM-69-2015 -3
TECHNICAL MEMORANDUM
I. INTRODUCTION
Apollo 8 presented a unique platform from which to
perform high resolution lunar colorimetric photography. All
previous spectral reflectivity or color measurements, with tree
exception of Surveyor, 1
 have been made from the earth where
even good atmospheric seeing limits resolution to approximately
1 km at the lunar sub-earth point. No backside color measure-
ments had been undertaken previously.
Lunar color has been a long standing interest of
planetary astronomers. Recently, a number of investigators
have attempted to interpret color differences as due to
differences in particle size, 2,3 age, 4,5 or composition. 6'7
McCord 7
 makes a strong argument in favor of compositional
differences being, the cause of color differences based on the
results of an exhaustive series of differential photometric
measurements.
If subsequent evidence proves the correlation between
spectral reflectivity and composition, and sufficient "ground
truth" can be obtained, accurate high resolution color measure-
ment will be an extremely useful tool in geologic mapping of
the lunar surface.
Photoelectric photometry is the most accurate means
with which to make spectral reflectivity measurements.
Accuracies of 0.1% are obtainable. 7 However, at the present
time, photometry is a slow, point for point method, not suitable
for large areal coverage or convenient data display. With some
sacrifice in accuracy, photographic methods can be employed.
Using microdensitometry, photographic plates can be analyzed
for quantitative color differences, but again data display is
difficult and the method is time consuming. Telescopic color
difference pictures have been produced by Whitaker 8 which give
qualitative color information in image form. In this method
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BELLCOMM. INC.	 - 2 -
a print is produced from a sandwich of a blue negative and a
red positive plate. Color boundaries, particularly in mare
regions, are easily detected. Errors are introduced because
of the difficulty in accurately controlling the photographic
development and reproduction processes over a wide dynamic
range.
An image processing method has been developed to
utilize the high information content and good display character-
istics of photography and to circumvent the difficulties of
standard microdensitometry, to obtain quantitative color
information. Details of this method will be discussed below.
Study of differential color photometry 7 reveals
that a large number of spectral curves, particularly in the
mare regions, exhibit a nearly linear behavior in the wavelength
0
region 4000-7000 A. Therefore, the greater the wavelength
separation of the points in the spectrum observed, the greater
the difference signal obtained. Typical differences obtained
across color boundaries in the maria, after albedo normalization
are 4 to 77o. Such color differences are not detectable by the eye
or on color film without special enhancement techniques.
II. APOLLO 8 COLOR EXPERIMENT
The design of the Apollo 8 two-color experiment was
largely dictated by hardware and mission constraints. Color
separation photography on black and white film was the only
feasible method for obtaining quantitative color information
from orbit. Two filters were chosen to give the greatest
wavelength separation commensurate with spacecraft window
transmission and film sensitivity ccnstraints. The filters,
0
47B (dominant wavelength 4530 A) and 29 to .6ND (dominant
0
wavelength (6330 A) were mounted in a slide device which attached
to the lens in place of a filter. Exposures were made alternately
through each filter. The time between exposures was less than
5 seconds. Using the Hasselblad camera and 80 mm lens, lunar
surface resolution from orbit was expected to be 50 m or
approximately 20 times greater than the best earth-based
telescope photography.
III. DATA RETURN
For operational reasons, no scheduled target of
opportunity photography, including red-blue filter photography,
was carried out in the 9th orbit. At this time the filter
BELLCOMM, INC.	 - 3 -
0
holder was used to reduce the light level in the TV camera.
After transearth injection (TEI), a number of red-blue photo-
graphs were taken. All were taken with the 250 mm lens.
Accidently, 33 F^xposures were made on SO 368 (color) film
and were, therefore, not usable for analysis. Unfortunately,
these were the best exposures made, as -,;he images filled almost
the entire frame. Shortly after TEI, 8 pairs of red-blue photo-
graphs were exposed on 2485 high speed film. This film had
been intended for dim light photography but was exposed according
to the plan for 3400 film. Thirteen exposures were also made
using the polarizing filters. No exposure calibration was
made before processing the 2485 magazine.
Therefore, no quantitative color data can be retrieved
from these images. Further attempts will be made to recover
some qualitative color information since backside areas are
included in the images. Magazine E of 3400 film contains 10
pairs of red-blue exposures. However, the full moon, quarter
phase images measure only 8 mm in diameter or less, making
them unsuitable for analysis.
IV. IMAGE PROCESSING
The requirement for reducing a large number of color
separation photographs to obtain high resolution quantitative
color data precluded the use of standard microdensitometry
methods.
The Jet Propulsion Laboratory Image Processing
Laboratory, under the direction of F. C. B'llingsley, offered
to assist in the analysis of Apollo 8 two-color photography.
The process is a variation of routines used in the analysis
of other types of photography. 9 , 10 ,11
In order to develop and test the method, telescope
photographs of several lunar areas were taken through a
rendezvous heat shield window and the appropriate filters.
Film and processing were identical to that used on the flight.
Sensiometric calibration strips were pre-exposed through the
same filters.
Both the film and the calibration strips were
digitized with a scanning spot size of 251j. A 10,000 pixel
(picture element) sample of each calibration step was scanned.
Plots of the DN (digital number) versus density of the initial
exposing wedges produced overall system calibration curves
(DN vs. log E) for each of the filters.
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IBELLCOMM. INC.	 - 4 -
Each picture was scanned several times and the
multiple scans averaged to minimize noise. Subsequently, a
geometric correction program was used to register the red and
blue pictures to each other. These became the master input
pictures for the processing.
Each picture was then converted to the log exposure
domain through application of thE- appropriate calibration
curve to each pixel, after which the relative exposure between
pictures was adjusted to give un equal DN in each picture in
an area defined to be gray by observation. Subtraction of one
picture from the other is the equivalent of taking the ratio
of the exposures, resulting in a "picture" independent of
overall albedo differences, whose negative values represent
varying shades of ore color, zero represents gray, and positive
values represent the other color. The colors are progressively
more intense as the DN departs from zero.
Figures 1 and 2 are the original scanned 47B and
092+0.6 ND (dominant wavelength 6460 A) photographs respectively.
Plato, Sinus Iridum and a portion of Mare Imbrium are visible.
Figure 3 is the difference picture in which the light areas
are blue and the dark areas are red. The differences nave
been expanded to nearly fill the dynamic range of the processor.
A point known as Plato E 7
 was chosen to be gray.
The major color boundary in Mare Imbrium has been
measured photoelectrically. 7
 A color difference of 5.5% ± 0.5%
was measured across the boundary at the points Mare Imbrium 3
and 4. The corresponding difference found by image processing
is 5% ± 2.5%.
Other methods of data display such as two color
printing and color-difference contouring are planned. A
complete discussion of this method will be published elsewhere.
V. SUMMARY
A two-colon photographic experiment was flown on
Apollo 8. A number of exposures were made but operational
requirements did not allow for the planned target-of-opportunity
photography in lunar orbit. Exposures made after transearth
injection were not suitable for analysis except possibly for
frames on high speed 2485 film. Computer image processing
techniquEs have been developed for obtaining quantitative color
imagery from color separation photographs such as those taken
17	 --
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BELLCOMM. INC.	 - 5 -
on Apollo 8. The two color experiment should be flown on
subsequent lunar flights to obtain high resolution imagery for
use in geologic mapping.
ACKNOWLEDGEMENTS
I wish to thank Dr. Bruce Murray for suggesting this
type of expPri men', and in particular, Mr. Sol Giles of the
Cal y fernia Institute of Technology for aid in the design of
the filter holder and for the manufacture of the devices
on very short notice. Mr. Jurrie AA.F.Woude ga excellent
assistance in the photographic tesgram.
	
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REFERENCES
1. Rennilson, J. J., H. E. Holt, and E. C. Morris, "In Situ
Measurements of the Photometric Properties of an Area on
the Lunar Surface", Jet Propulsion Laboratory, Project
Document 602-23, 1967.
2. Adams, J. B. and A. L. Felice, "Spectral Reflectance 0.4
to 2.0 Micros of Silicate Rock Powders, J. Geophys. Res.,
72 9 5705, 1967.
Salisbury, J. W. and G. R. Hunt, "Martian Surface Materials:
Effect of Particle Size on Spectral Behavior", Science, 161,
365, 1968.
Nash, D., "Proton Irradiation Darkeninf; of Rock Powders:
Contamination and Temperature Effects and Applications
to Solar-Wind Darkening of the Moon", J. Geophys. Res.,
72, 3087, 1967•
5. Hapke, B., "Lunar Surface: Composition Inferred from
Optical Properties", Science 159, 76, 1968.
6. Adams, J. B., "Lunar Surface Composition and Particle Size:
Implications from Laboratory and Lunar Spectral Reflectance
Data, J. Geophys. Res., 72, 5717, 1967.
7. McCord, T. B., "Color Differences on the Lunar Surface",
in press, J. Geophys. Res., 1969.
8. Whitaker, E. A. and G. P. Kviper, "Interpretation of
Ranger VII records", Jet Propulsion Laboratory Technical
Report 32 -700, Part II. Experimenters' Analyses and
Interpretations , 1965. 
9. Nathan, R. ; "Digital Video-Data Handling", Jet Propulsion
Laboratory, Technical Report 32-877, 1966.
10. Selzer, R. H., "Digital Computer Processing of X-Ray
Photographs", Jet Propulsion Laboratory, Technical Report
32-1028 5 1966.
11. Billingsley, F. C., "Digital Image Processing Considera-
tions", in Progress in Biological Electron Microscope
Techniques edited by D. F. Parsons, Harper and Row, in
press.


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