THE OPTICAL SPECIFICATION OF PHOTOGRAPHIC VIEWERS

• Document Type: CREST
• Collection: NGA Records (Formerly NIMA)
• Document Number (FOIA) /ESDN (CREST): CIA-RDP78B04747A002500050023-5
• Release Decision: RIPPUB
• Original Classification: K
• Document Page Count: 15
• Document Creation Date: December 28, 2016
• Document Release Date: November 16, 2001
• Sequence Number: 23
• Content Type: REPORT

ase 2002/01/02 : CIA-RDP7 B 478002500050023-5
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1HE OPTICAL. SPECIFICATION
OF PHOTOGRAPHIC VIEWERS
STATINTL
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 THE OPTICA . SPECIFICATION OF PHOTOGRAPHIC VIEWERS
STATINTL
ABSTRACT
STATINTL
The modulation `t runs fear Qnct ion of optical i nst rumo;nt s to
view or enlarge' photoraphir rranspare"cir's ire d.?t.?rmi,nabli from
a knowledge of the e?y e': modulation requirement and the amount of
modulation and granul8rity or the ttanspare-ncv Two hypothetical
cases illustrate the` Specification of the required performances of
fer furtt t Lou o vie~,te 5 antd t ~?largvr; .shneld be vnrv high at all
a viewer or enlarger! It is concluded that the modulation trans-
~, -' "~kl z E ,
the spatial ,1rt*gt.ri up to - t t~c limit Ing rt?soIur. ion contained i n
the transparency.
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For photography which is viewed by means of an optical sys-
tem or is optically enlarged, it is desirable to have a rational
approach to specify the required optical performance.. Any text on
optics gives the basis on which mag..i f icat ion and field of view can
be specified? but the resolution performance is not covered, In
this discussion, it is shown how the modulation transfer f.,ncti.oti
can be extended to viewt'rs, l`3 The benefit of this approach is that
it L-s based on the physical processes. Involved, that ' it has been
proves: accurate for cameras, aitd that it is easy to apply co the
analysis commonly apply cd to the photographic acgt.isit iot.
viewing of any particular photography
In what followq, the method will be explained and illustrated
with hypothetical examples, While these are me'a it to be physically
reasonable cases, other workers are cauti.one.d to extrapolate cau-
tiously, ~r preferably, to apply the method exactly to a!.v photog-
raphy for which they wish to specify the performance of viewers
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borne operat tot,, have modl.rlat ion transfer f+rrrct for=s, T(k), similar
Aerial photogra,phs arre obtai?red by camera', which, frr air-
APPROACH TO PROBLEM
1 ; these functions decrease stead-
ily as the spatial frt?irier,cv, k, itic reaIcs Such cameras will pho-
tograph objects, for wfiHi the Fourier components will have all
possible modulations (contrast ;), Mo, from high to very low, Con-
sequo-itly, the correspondir:g modr,tat ion in the expose re (aerial)
imago which impi,vges or the eme:lssio' , MA, will rar'ge from 'ri2h to
very low since
t1A
T(k) Mo
Even for high coritrist objects, for which M. = I, MA, decr,asos to
very low vali.rs at high spatial freruerrcir?s because T(k), do reascs
For the image t>f the Fourier component of an objt-et to b"
resolvable on the film, MA must Feriua.l or exceed a modulation de-
tectability limit, M &'b T?ti general, MD ia fe:rrct iorr of the-
shape of the object, rhp granr,larity of the film, and the spat.i.+l
frequency? Selwvt,7 has shown the evt' to re.lui ree ML ' 0,012 for
long litres an gr.aiaile?:ss film; for USAF 1951 target thu relation
is more complicated;" and for other
MD are required?
hapo,, still other values of
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44.1444:14,111 f"I r"! j '':j
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0.5
N02v1ALIZED SPATIAL FREQUENCY
Figure 1. Typical
1.0
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After the latent- image is developEd, the transilliuni!lated
negative will have a modulation, MT The useful portion of this
modulation is that in which the signal modt:lat tor. MA, exceeds the
noise modulation MD. This useful port ions of mod,tat ion is then
reduced by the transfer function of the viewer (or enlarger), and
only those objects for which the resulting modulation still ex-
ceeds the eye's detectability limit will be resolved As has heen
shown previously,9 the relation of MT and MA is complicated, and
M.r is sometimes great e than MA, sometimes equal to MA, and some-
times less than MA. MT and MA wi11 be assumed here to be -umer-
ically equal, which'Is approximately correct if a wide range of ex-
posures is probable. Tice analysis for cases when 41, is ,,o' equal
to MA is analogous to what follows.
Further, in what- follows, we assume the object shapes are
such that MD 0.04, which is probably rather reasonable for many
cultural shapes (llowev.-r. the e e''s modulat ion limit is not yet
well established and Is certainly variable.) Also, for illustra-
tive purposes. 'i(ko~aeri,al photography is assumed to be gaussian,
which is a reasonable- approximation to actualiiv
In Figure the modulation requirement of the eyf', assumed
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curvfe is the exposure modulation avail-
"h M 1 where T(k) Is assumed gaisssian The
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1.0
z
0
I-
-J 0.5
D
0
0
M
MODULATION AVAILABLE
TO FORM IMAGE IN CAMERA
MODULATION REQUIRED
FOR RESOLUTION
01 I
0 0.5
NORMALIZED SPATIAL FREQUENCY
Figure 2u Available Modulation vs. Resolution Requirement
for Eye Limit of 4 Percent
1.0
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spatial frequency is normalized -: ch that it has i of 1 0
when MA 0,.04 for M0 T" Thr s . t hi t would bye the ;pat ial fry -
quency cited as the aerial camera's I imit ing resol. t tor, :.;rtdvr
best conditions, 100 cycle,;"miIIimeter for ir,sta'rr, Similar ct,rves
have been drawn for MA wht?ii M0 0 72 (medinrm con{t rasr) , a:id M,
0,023 (low contrast;).
One well-known fact is immediately obvioi.;s -- tht lok' con-
trast resolution 1 imi t is lowtt t ha,r t hi, For t t i s i I lust r tive case, it is 75 perct'rit of the hi4h contrast spat i it fre.ja.,n v
limit, that is 75 cycl-,t:/millimeter if the hit_tcr is 100 c,c Iis/
millimeter.
Now, rPr l i i ng that t he r ;?r needs MD? 0:,04 r
that the viewer's mod ulat Corr transfer fernct io' T,(k), mc: , bt
Tv (k) 7, 0.04
M1
where M1
MA
s assumed T!,is is ill?strated i,r Figure 1, wher.-
the required modulation t rarrsfer f;.:-rrct ions of viewei s, arc ;how?}
for the low, medium and high cor,tr3st imac10s co;nsidcr,'d in Figure
Also shown is the regrnir..d modular ion trsr,sft?r fo,lct ion: for a very
low contrast (M0 0.06) object , i i li nt rat iI;% t hat the viewer re-
quires a high modulation tr,irsfer fr .-ncr ion at alI epat ial fre-.7 en-
cies; On thf. same graph. threi, theottet[cal (diffraction;) Iimirs
of incohereritly illuminated cir?ular aperture viewers at re shown.
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REQUIRED MODULATION TRANSFER FUNCTION
TO VIEW PHOTOGRAPHS OF
OBJECTS OF STATED CONTRAST
z
0
ATTAINABLE MODULATION
TRANSFER FUNCTION OF
VIEWER WITH STATED HIGH
CONTRAST RESOLUTION LIMIT
(IN UNITS OF NORMALIZED
SPATIAL FREQUENCY)
01 I i
0 0.5 1.0
NORMALIZED SPATIAL FREQUENCY
Figure 3. Required Viewer Performance Compared With
Attainable Performance for Eye Limit of 4 Percent
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For examplet lie up pe - cc:rve is that modulat ioi: t ra :a fer f u