HANDBOOK ON IMAGE SCANNING

• Document Type: CREST
• Collection: NGA Records (Formerly NIMA)
• Document Number (FOIA) /ESDN (CREST): CIA-RDP79B00873A000100010133-6
• Release Decision: RIPPUB
• Original Classification: S
• Document Page Count: 14
• Document Creation Date: December 28, 2016
• Document Release Date: November 1, 2012
• Sequence Number: 133
• Publication Date: March 6, 1969
• Content Type: MISC

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method in which equi-densitometry reveals low contrast structure that is below the
visual threshold. With the four-color IDT, interpretation of the traces is facilitated
by the contours' being in color.
2. Grain Integration - With these scanning instruments the impulse response of
the instrument is actually the scanning aperture itself. To change the instrument' s
frequency response capabilities to maximize the signal-to-noise ratio in a given cir-
cumstance, therefore, all we need to do is change the aperture. Large apertures
integrate noise but reduce resolution. Smaller apertures have just the opposite effect.
Wiener shaded apertures have been developed to optimize this tradeoff.
3. Derivative Output - The IQ offers the capability of displaying the one-dimen-
sional derivative of a photographic scene. This can be useful for mensuration pur-
poses. The point of maximum density gradient has been shown to be a good criterion
for mensuration, and the IQ derivative printout aids in locating this point. This mode
is also useful in outlining the borders of low contrast objects.
4. Use of Paraxial Optics - These scanning systems all utilize imaging optics
only in the paraxial region. As the transfer function of imaging optics is always better
on-axis than off-axis, the overall quality of images transferred through these scanning
instruments is improved.
THE ISODENSITRACER?
The main advantage of the IDT lies in its four-color printout, which greatly facili-
tates identifying the equidensity contours. The IDT is particularly well suited for
analyzing imagery for which it is important to follow subtle changes in density. An
example of this might be a case where small density changes in the image of a body of
water might be indicative of depth fluctuations. Another type of situation where the IDT
has proved useful is in assisting in the analysis of low-contrast imagery. We stress
that in such cases the IDT trace should be viewed in conjunction with an enlarged
image of the' scanned area. The IDT trace may indicate structure of interest that
could be overlooked in simply viewing the image or, conversely, it may give information
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about regions of interest in the image that cannot be obtained by simply viewing the
image. An IDT trace alone can give misleading information. For example, shadows
may be interpreted as discrete objects. It is important to use the IDT trace and a real.
image to.complement each other.
THE IMAGE QUANTIZER
The IQ used in the density quantizing mode is suitable for the same general applica-
tions as the IDT. Although it lacks the four-color printout, the IQ has the advantage
of much higher speed. The operator can run several traces in succession, and can
quickly produce a trace that best enhances the particular information of interest. As
different parts of a scene may require different density contour intervals to best faci-
litate interpretation, the ability to quickly produce such traces can be useful.
The IQ used in the derivative mode can also be useful when an analysis of the struc-
ture of low contrast imagery is desired. When viewed in conjunction with an enlarged
image, and perhaps also a density-quantized trace, a derivative trace might indicate
structure of interest that has been overlooked or provide additional information about
a particular part of the image.
The IQ used in the derivative mode has also proved to be a fairly accurate mensura-
tion tool. The density inflection point of an edge can be accurately and repeatably
located on a derivative trace, and measurements of small objects made by measuring
between the two inflection points on both sides of the object tend to be quite accurate.
Figures 1 through 3 illustrate how these properties of the IDT and IO can be applied.
Figure 1 shows photomicrographs of very low contrast three-bar groups of various
frequencies. Figure 2 is a trace made with the IO in the density quantizing mode; here
we see some density structure within the bars and in the area surrounding the bars that
is not readily apparent in viewing the image in Figure 1 directly. A trace, made with
the IDT would further emphasize this structure by printing out the equidensity contours
in four colors. A comparison of Figures 1 and 2 shows that the IQ trace contains
information that cannot be readily perceived by merely examining the image. For
example, the IQ traces show that there is a density gradient in groups 3 and 4 from the
lower left to upper right. The IQ trace also shows some structure within the bars in
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Figure 1. Photomicrographs of Low-Contrast Three-Bar
Groups on Test Target
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Figure 2. Traces of Figure 1 Made with the IQ in the Density
Quantizing Mode with 0. 8 mm Aperture
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group 1 which is not readily apparent in the image. Whether or not this structure is
significant is a decision that a human observer must make after comparing the IQ trace
and the image with any a priori information he might have about the object.. This ex-
ample merely shows that low-contrast structure in a photographic image can be em-
phasized by an isodensity trace.
Figure 3, which is a derivative trace made with the IQ, shows how this type of
trace can be used to locate edges. For group 4, for example, the positions of maxi-
mum density gradient in the edges of the bars cannot be accurately found from either
the image or the IQ isodensity trace. These positions are emphasized by the derivative
trace and can be much more reliably located. Again, the knowledge that these are
three-bar groups, which is obtained from the original image, is necessary to allow
the derivative traces to be interpreted properly.
OPERATING PARAMETERS
CHOICE OF A SCANNING APERTURE
In all instruments used for scanning photographic images, the scanning aperture is
chosen so that it will provide the needed resolution but keep grain noise to a minimum.
In a microdensitometer, slit apertures are often used when scanning objects con-
taining long edges in a direction normal to the edges. The advantage of this type of
aperture is that high resolution can be obtained by making the slit narrow in the scan-
ning direction, and grain noise, which decreases as the total area, of the scanning
aperture increases, can be minimized by using a slit long compared to its width.
In two-dimensional scanning instruments we generally want to have the same reso-
lution both parallel and normal to the direction of scan. This requires the use of
symmetrical (square or circular) apertures. Slit apertures may be used in particular
cases where it is desired to investigate objects with some degree of one-dimensional
symmetry. Care must be taken in interpreting such traces since all shapes except
edges parallel to the slit will be distorted.
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I
Figure 3. Traces of Figure 1 Made with the IQ in the Derivative Mode
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When symmetrical clear apertures are used, it is important that resolution is not
"wasted" by using.too small an aperture. Generally, the smallest aperture that should
be used is one whose total width is half the smallest detail of interest. I
Shaded apertures are advantageous whenever the use of a clear aperture that is by
necessity made small to maintain the required resolution leads to an. amount of grain
noise that hinders interpretation. This is always the case when grain-limited imagery
is to be analyzed. The optimum shaded aperture for use in these cases can be derived
by computer. Although no shaded aperture is optimum in a mathematical sense when
lens-limited imagery is to be traced, it might still be desirable to reduce grain noise
so that a smaller density contour interval can be.used. Shaded apertures can be tried
in such cases, but there is no assurance that they will produce an improvement; here
selection of the best aperture is somewhat of a trial and error proposition. A reason-
able starting point is to use a shaded aperture having a half-power (in transmission)
width that is equal to the total width of the clear aperture whose size has been deter-
mined by resolution requirements. If grain noise is still excessive, larger shaded
apertures can be tried.
SPECIAL PROPERTIES OF THE IQ
When too large a scanning aperture is used in making isodensity traces in the IQ
and IDT, the result is a spreading of the contours in areas of high frequency informa-
tion such as edges or single bar images. When an edge is traced with the IQ in the
derivative mode, however, spreading of the edge is noticeable as the aperture size is
increased, but the position of the density inflection point does not change (see Figure
4). This effect can be useful when grain noise hinders mensuration based on the den-
sity inflection point.method. The scanning aperture can be made larger than is neces-
sary to "resolve" the object, and grain noise can thus be reduced without affecting
the measurement accuracy.
Figure 5 shows'Ahe effect of changing the sensitivity setting of the IQ when it is
used in the derivative mode. Higher sensitivity settings lead to an increase in the
number of quantized derivative contours and, up to a point, allow a more precise
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Figure 4. Traces of Single-Bars on
Target Test Made With
the IQ in the Derivative
Mode Using Apertures of
Different Sizes
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2 .4 .8 1.2 1.6
APERTURE SIZE
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Figure 5. Traces of Three-Bar
Groups Made with the
IQ in the Derivative
Mode and with a Time
Constant of 0. 01
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determination of the inflection point. If the sensitivity is made too high, however,
random fluctuations can lead to some ambiguity in the inflection point location.
THE PROJECTION PRINTER
The degree to which photographs should be enlarged in the Projection Printer for use
in the IQ will depend on several considerations. For most work, a magnification that
enlarges one resolution element on the original to 2 mm on the photomicrograph will
Abe satisfactory. The photomicrograph can then be scanned with the smaller apertures
in the IQ with the assurance that the original details will be well resolved, or with the
larger apertures to provide minimum grain noise while still resolving the original
details. Larger magnifications can be used when resolution is a critical factor, and
smaller magnifications can be used when the main requirement is scanning large
areas in a short time.
The film used to make photomicrographs should normally be of low contrast
(y = 1. 0 or less) and high speed. Tri-X film with Microdol-X developer is recom-
mended. The exposure should be made so that the original scene is exposed as much
as possible on the linear part of the H and D curve of the recording material. Higher
contrast films can be used if structure of very low contrast is to be analyzed.
Vibrational stability of the instrument is critical at very high magnifications (250X
and up). Placing the unit on a stable surface such as a granite slab will minimize
this problem.
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