Minimum resolvable contrast measurement device

An apparatus for creating images of nonperiodic patterns, which patterns are variable in contrast with respect to a uniform and constant level of background luminance. Two beams of uniform luminance are projected through individual, orthogonally oriented linear polarizers, and then into a common cube beam splitter to be divided and interlaced. One combined output beam from the splitter is projected through a rotatable linear polarizer onto the image sensing optical detector undergoing test. The second beam is projected toward a detection system. The patterned region of the second beam passes through a second rotatable linear polarizer, synchronized to the first rotatable polarizer, and onto a CCD array detector system. Contrast between the pattern and the background is represented by the two electrical voltage levels in the array output, irrespective of the spatial frequencies in the pattern. Two unpatterned background regions from either of the interlaced beams are projected through individual and orthogonal linears polarizers to individual luminance detectors. Each detector, in turn, regulates its corresponding beam intensity to compensate for drift in the beam generating elements.

CROSS-REFERENCES TO RELATED APPLICATIONS 
This invention relates to copending patent application, Ser. No. 109,592, 
filed on Jan. 4, 1980, common as to the inventors and the assignee and now 
abandoned. 
BRIEF SUMMARY 
The invention disclosed herein is directed to an apparatus for detecting 
the minimum contrast resolution of a video sensor or the like, alone or in 
conjunction with a human observer, through generating optical images 
having backgrounds of uniform and constant luminance with a variable 
contrast pattern superimposed thereon. Two controllable intensity beams of 
uniform luminance are projected to a focus at the image plane. One of the 
beams contains a partly opaque pattern. Each beam passes through a linear 
polarizer, which polarizers are orthogonally oriented relative to each 
other, and then through a common beam splitter where the beams are 
interlaced. 
After beam splitting and interlacing, one of the two output beams formed is 
projected through a rotatable linear polarizer and onto the video sensor 
at the image plane. The second split and interlaced beam is projected 
through a synchronized, rotatable linear polarizer, and then onto a 
detector assembly for measuring both background intensity and pattern 
contrast levels. Variations in the background intensity are sensed in the 
detector assembly, decoupled according to polarization and apportioned 
according to polarization as feedback signals to correct for drift in each 
of the two beam sources. 
Rotation of the polarizer in the optical path leading to the video sensor 
changes the photometric luminance of the nonperiodic pattern without 
altering the background luminance level. Stated otherwise, the contrast of 
the pattern image is varied while retaining a constant and uniform 
background luminance.

DETAILED DESCRIPTION 
Video systems, typically a camera coupled to a display, are frequently used 
to supplement or supplant the human senses. This is particularly true in 
weapons systems, where the magnification and electronic processing speeds 
easily outperform their human counterparts. In the normal course of 
comparing such video systems, it is necessary to quantitatively evaluate 
certain performance characteristics, singly and in conjunction with human 
operators. The subject invention discloses an apparatus for quantitatively 
ascertaining the pattern detection thresholds, where the thresholds are 
functionally related to the contrast and spatial frequencies of the test 
patterns, while the background luminance is fixed at constant and uniform 
level. 
Though the above-noted copending application addresses a similar analysis 
technique, it is fundamentally constrained to symmetric image patterns. In 
contrast, the present invention fully encompasses both symmetric and 
nonsymmetric patterns. Furthermore, this invention contemplates and 
surmounts the practical problem of drift, an inherent characteristic of 
luminous energy sources. Objectionable levels of drift are detected and 
suppressed by a closed loop control, a functional element clearly absent 
from the other invention. 
Another distinct structural element lacking in the art and the above-noted 
copending application is the contrast measuring device. As embodied 
herein, the apparatus encompasses a charge coupled device (CCD) placed 
transverse to the longitudinal axis of the bar pattern coupled to level 
sensing electronic circuitry. 
Directing attention to FIG. 1 now, there appears in the figure one 
embodiment of the invention. The apparatus generally comprises an 
enclosure, 1, with internal optical elements and peripheral luminous 
energy sources and sensors. At sealed opening 2 of the enclosure is video 
sensor 3, the device undergoing evaluation, either alone or in conjunction 
with a human observer. Another opening, 4, holds an easily interchanged 
transparency type target 6 which selectively obstructs the free entry of 
light from luminance integrating sphere 7. A typical target pattern 
appears schematically in FIG. 2. This pattern is a periodic or symmetrical 
one, although nonperiodic or nonsymmetrical patterns may be used also. 
Orthogonal to the latter opening, and axially aligned with video sensor 3, 
is another access into enclosure 1, a translucent window, 8, with neutral 
density filter 9 for altering the composition of light entering from 
luminance integrating sphere 11. And finally, opposite, and at the image 
plane of target 6, is detector system 12, for both measuring the pattern 
contrast and closed loop monitoring of background luminance. 
Unpolarized luminous energy sources 13 and 14 are substantially equal as to 
intensity and spectral composition, and are located within their 
respective luminance integrating spheres so as to project uniform 
intensity beams into enclosure 1 through neutral density filter 9 and the 
non-opaque areas of target 6. 
Dashed lines 16 and 17 show that corresponding lenses 18 and 19 are 
selected and positioned to form images of target 6 and filter 9 at both 
the input to video sensor 3 and the sensing plane of detector system 12. 
In the path of the uniform intensity luminous energy beam defined by line 
17 is a horizontally oriented linear polarizer, 21. A similar polarizer, 
22, though vertically oriented to be orthogonal to polarizer 21, 
intercepts the path of the luminous energy beam containing the target 
pattern. The two beams enter cube beam splitter 23, where they are split 
into substantially equal segments, interlaced, and transmitted along two 
orthogonal axes. Ideally, the two beams leaving splitter 23 are equal in 
background and pattern luminance. Note, however, that the luminance 
content of the bar target pattern retains its vertical polarization while 
the uniform beam contribution remains horizontally polarized. 
At this point it is worthwhile to describe the operations performed in 
detector system 12 and its relationship to luminous energy sources 13 and 
14. For that purpose, mechanically linked rotatable linear polarizers 24 
and 26 are presumed absent. Referring now to FIG. 3, the sensing elements 
of the detector system portrayed in FIG. 1 are magnified to show linear 
vertical and horizontal polarizers 27 and 28 situated optically preceeding 
their corresponding background luminance sensors, 29 and 31. The image 
plane, shown from another aspect in FIG. 4, shows the presence and 
relative location of CCD array 32. The sizes and locations of sensor 29, 
sensor 31 and CCD array 32 are selected and arranged so that the paths of 
the unpatterned background luminance of the beam transmitted from splitter 
23 project through polarizers 27 and 28 to illuminate sensors 29 and 31. 
Accordingly, the opaque regions of the bar target pattern never obstruct 
those paths. CCD array 32 is otherwise, in that it lies transverse to and 
always intersects the image, 33, cast by the bar pattern. Undoubtedly one 
recognizes that the individual CCD array sensor elements must be 
measurably smaller than any single bar image. 
Sensors 29 and 31 provide feedback signals to the intensity controls 
regulating the luminous energy radiated by sources 13 and 14. Polarizers 
27 and 28 decouple the background luminance to insure that the drift 
adjustment error signals are routed to the correct source control. 
The CCD array in the detector system provides a quantifiable measure of the 
contrast between the bar target pattern and the background. When one 
recognizes that contrast is defined by a mathematical relationship between 
luminance levels in which 
##EQU1## 
and further recognizes the substantial linearity between the array signal 
amplitudes and input luminance, it becomes apparent that the electrical 
responses from the array carry the information necessary for calculating 
the contrast. In terms of the electrical signal plot in FIG. 5, contrast 
is: 
##EQU2## 
At one extreme, where both rotatable polarizers are horizontally oriented, 
the illuminance of the video sensor 3 is proportional to the transmittance 
coefficient of neutral density filter 9. No bar pattern is present. The 
other extreme occurs when both polarizers are vertically oriented. In this 
orientation, only the image from the target 6 is transmitted to the video 
sensor 3. The neutral density filter 9 is chosen to have a uniform 
transmittance coefficient equal to the average of the dark bar and bright 
bar transmittance coefficients of the target 6. With this condition, 
space-averaged illuminance at video sensor 3 and detector system 12 are 
identical at the two extremes of polarizer rotation zero contrast and full 
contrast. The terms "background luminance" and "space-averaged 
illuminance" as used herein are considered to be interchangeable. 
Upon recalling the objectives sought from the apparatus, it becomes 
apparent that intermediate orientations of rotatable polarizers 24 and 26 
must not change the background luminance as the bar pattern contrast is 
varied between the two above-noted extremes. The apparatus attains these 
objectives. Begin by considering Malus's law, a well recognized 
relationship defining the transmission of unpolarized luminous energy 
through crossed polarrizers: 
EQU L(.theta.)=L(0) cos.sup.2 .theta., 
where L(.theta.) is luminance as a function of polarization misalignment 
angle .theta., and L(0) is the luminance transmitted for a misalignment 
angle of .theta.=0.degree.. 
If .theta.=0 is defined to be the vertical axis, then the horizontal axis 
relationship, with respect to angle .theta., is: 
EQU L.sub.H =L.sub.H (0) cos.sup.2 (.theta.+90.degree.). 
Recalling from trigonometric equivalence that cos.sup.2 
(.theta.+90.degree.)=sin.sup.2 .theta., the total background luminance 
eminating from one side of beam splitter 23 is: 
EQU L.sub.B =1/2L.sub.H (0) sin.sup.2 .theta.+1/2L.sub.V (0)cos.sup.2 .theta.. 
If the two levels of background luminance are, as originally defined, 
equal, then the equation simplifies to: 
EQU L.sub.B =1/2L.sub.H/V (0)[sin.sup.2 .theta.+cos.sup.2 .theta.], L.sub.B 
=1/2L.sub.H/V (0). 
Note, the background luminance level is no longer related to misalignments 
of the polarization angle, remaining constant irrespective of the 
orientation set in rotatable polarizers 24 and 26. 
Rotatable linear polarizer 24 is sufficiently large to encompass the whole 
of the beam directed toward video sensor 3. Rotatable linear polarizer 26 
differs, in that its active area encompasses only the bar pattern region. 
The background luminance radiates without obstruction toward the 
individual polarizers 27 and 28, immediately preceeding the detector 
system. This structural distinction between rotatable polarizers avoids 
interaction between polarizers rotation and the regulation undertaken by 
the intensity control loops.