Afocal lens system

An afocal lens assembly which includes a first and second lens group used r testing an infrared detector assembly. The first lens group includes three lens elements and the second lens group includes four lens elements. The output of the lens assembly includes up to a 60 degree collimated field-of-view. The lens elements are infrared transmissive.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The field of the invention disclosed herein is directed to afocal lens 
assemblies, and more specifically to afocal lens assemblies used with a 
collimator having up to a 15 degree field-of-view output for testing an 
infrared detector assembly. 
2. Description of Related Prior Art 
In the prior art, thermal imaging systems were developed to overcome 
distinct disadvantages of other detection methods such as radar or visible 
optics. These other methods generally operate in an "active" mode in which 
electromagnetic radiation is emitted and a sensor detects that return 
signature but allows for detection and countermeasures to render such 
systems ineffective. Also such active systems are limited due to such 
performance characteristics as range and day/night operation. First 
generation Forward Looking Infrared (FLIR) sensors were developed to 
extend vision beyond the visible light spectrum and are capable of 
"passive" operation. Such systems allow for superb action since most 
objects radiate in the IR region. The testing of such systems typically 
employs collimating optics (usually mirrors) which provide a plane-wave 
input, and a detector which is positioned in the focal plane of the test 
system. A more thorough description of system performance testing for 
first generation FLIRs may be found in Chapter 11 of "Thermal Imaging 
Systems" by J. M. Lloyd. 
Deficiencies in the first generation FLIRs have led to the development of a 
second generation of FLIRs. First generation FLIRs included deficiencies 
such as cryogenic cooling and scanning. The second generation of FLIRs 
were developed to include some of the following characteristics: 
1. detector material responsive to ambient temp; 
2. the elimination or minimization of cooling "noise"; 
3. more efficient cooling technique; 
4. staring focal plane arrays which eliminate scanning; 
5. simplified power supply 
6. up to a 60 degree field-of-view. 
With the development of second generation FLIRs there is now a need to 
measure and test the performance of these systems. 
SUMMARY OF THE INVENTION 
The invention disclosed herein is directed to an afocal lens assembly used 
with a collimator having up to a 15 degree field-of-view output for 
testing an infrared detector assembly. The assembly includes a first and 
second lens group. The first lens group which includes three lens elements 
focuses incoming collimated light. A second lens group accepts the focused 
light and collimates a lens assembly output which is extended up to a 60 
degree field-of-view. The lens groups are infrared transmissive and the 
assembly output performance includes an on-axis Modulation Transfer 
Function of 50% at 3 cycles per milliradian. 
The primary objective of this invention is to provide an afocal lens 
assembly to allow for the testing and measurement of thermal viewers.

PREFERRED EMBODIMENTS 
The afocal lens assembly of the present invention is preferably utilized in 
the simplified test system shown in FIG. 1. In FIG. 1 there is shown an 
infrared collimator 10 which outputs infrared collimated light with an 
output field-of-view (FOV) of 15 degrees. That output is received as input 
to a magnifying telescope 11 which contains the afocal lens assembly of 
the present invention. Magnifying telescope 11 will output via the afocal 
lens assembly of the present invention a FOV of 60 degrees. Sensor 12 is a 
second generation Forward Looking Infrared (2nd Gen FLIR) camera sensor 
which has an input FOV of 60 degrees. Collimator 10, telescope 11, and 
sensor 12 are all in the same focal plane of the test system. It is 
understood that the measurement to be taken by use of the lens assembly of 
the present invention may be a system level measurement or an optical 
component measurement. 
It is also understood that the invention is not limited to a specific 
collimator or sensor but may be any type of infrared collimator with a FOV 
of up to 15 degrees and infrared sensor with a FOV of up to 60 degrees. 
FIG. 2 depicts the lens assembly of the present invention. A first and 
second lens group is shown positioned along a common focal axis 20. The 
first lens group comprises of three lens elements 21, 22, and 23 which 
focus incoming collimated light. The second lens group comprises four lens 
elements 24, 25, 26 and 27 which accepts the focused light and collimates 
a lens assembly output. Each lens element is described in Table 1 below 
where all dimensions are given in inches and a positive and negative 
radius indicates the center of curvature in to the right and left 
respectively according to what is shown in FIG. 2. 
TABLE 1 
__________________________________________________________________________ 
ELEMENT 
RADIUS OF CURVATURE APERATURE DIAMETER 
NUMBER FRONT BACK THICKNESS FRONT BACK GLASS 
__________________________________________________________________________ 
OBJECT INF INFINITY 3.0018 
0.0050 
3.0018 
0.0000 
21 A(1) -9.4789 CX 
0.5044 3.0018 3.1199 
GERMLW 
0.0050 
22 9.4817 CX 9.0051 CC 0.5012 3.1107 3.0189 
ZNSE 
7.0880 
23 -7.4609 CC -7.3264 CX 
0.5115 4.0070 4.2318 
GERMLW 
14.3043 
5.7346 
9.0704 
6.6671 
11.0853 
24 A(1) -106.9158 CX 
0.7317 9.5539 9.5239 
GERMLW 
0.0050 
25 3.8791 CX 3.2738 CC 0.4981 7.1012 6.1808 
ZNSE 
0.0050 
26 3.1578 CX 2.7553 CC 0.4969 6.0976 5.3563 
ZNSE 
0.3467 
27 2.8454 CX A(3) 0.4973 5.3270 4.6651 
GERMLW 
5.0000 
APERTURE STOP 0.7486 
1.50000 
2.4581 
IMAGE 3.000 
IMAGE DISTANCE = NF 3.1597 
__________________________________________________________________________ 
Each lens element is described in terms of a radius of curvature for front 
and back of each lens where CC denotes concave and CX denotes convex. The 
values A(1), A(2), A(3), denote aspheric surfaces which are described in 
terms of the constants listed in Table 2 below: 
TABLE 2 
__________________________________________________________________________ 
ASPHERIC 
CURV K A B C D 
__________________________________________________________________________ 
A(1) -0.10448640 
10.649340 
1.46156E-03 
9.76460E-05 
1.21621E-05 
2.70689E-07 
A(2) 0.04839538 
0.000000 
-4.74179E-04 
1.86815E-05 
-4.75946E-07 
4.17332E-09 
A(3) 0.41878728 
-0.165358 
-1.18140E-03 
1.33017E-04 
5.96033E-06 
1.03506E-07 
__________________________________________________________________________ 
which were derived using the following equation: 
##EQU1## 
Referring again to Table 1, the lens assembly also is described in terms 
of thickness defined as axial distance to the next surface. Thickness 
between lens elements denote air gaps. Front and back aperture diameters 
are also disclosed for each lens element with equivalent diameters shown 
for air gaps. Image diameter shown in Table 1 is a paraxial value, not a 
ray traced value. The reference wavelength of the above values is 10 
microns for the spectral region of 8 to 12 microns. Entrance and Exit 
pupil dimensions and overall length (OAL) are given below: 
______________________________________ 
ENTR PUPIL 
DIAMETER = 3.0000 
DISTANCE = -0.5674 
EXIT PUPIL 
DIAMETER = 1.5000 
DISTANCE = -3.0000 
OAL = 52.1558 
______________________________________ 
The lens elements are made of infrared transmissive materials with minimal 
vignetting across entire FOV. Along with a low infrared absorbance, these 
materials should also have ideally a zero coefficient of thermal 
expansion, high surface hardness and high mechanical strength. The lens 
materials used are Germanium and Zinc Selenide and are indicated in Table 
1. It is understood that the invention is not limited in the preferred 
materials indicated above but may utilize equivalent materials which also 
include the above properties. 
Operational performance of the lens assembly of the present invention will 
next be disclosed with reference to FIGS. 3, 4, and 5. 
FIG. 3 discloses the diffraction Modulation Transfer Function (MTF) 
performance of the lens assembly. One of the primary performance 
measurements of an optical system is its resolution, represented as the 
on-axis and off-axis MTF. In FIG. 3, line 30 is the diffraction limit and 
line 31 is the on-axis (0.degree. FOV) MTF. Line 32 is the tangential 
component of the off-axis (7.5 degree FOV) MTF and line 33 is the saggital 
component of the off-axis (7.5 degree FOV) MTF. As shown in FIG. 3, the 
afocal lens assembly is a nearly diffraction limited MTF response of 50% 
at 3 cy/mrad. 
FIG. 4 discloses a field curvature plot in the lens assembly. A sharp focus 
is desired in an optical system but especially in complex optical system 
there is a problem with astigmatism which gives rise to blurred images at 
the margins. Line 40 is the tangential astigmatic field curve while line 
41 is the saggital component of the astimatic field curve. As shown in 
FIG. 4, astigmatism does approach a "greater than minimum" out of focus 
range beyond +/- 3.77 degrees FOV. 
FIG. 5 discloses a distortion plot for the lens assembly performance. An 
approximately uniform lateral magnification over its entire FOV is 
required, and is measured in terms of distortion. Types of distortion 
include pin cushion and barrel distortion. Line 50 is the percent 
distortion for the lens assembly performance. As seen in FIG. 5, 
distortion does approach a "greater than minimal" 10% at +/- 7.5 degrees 
FOV. 
Any thermal imaging system which uses a lens assembly may exhibit image 
defects which result from internal retroreflection of cold surfaces onto 
the sensor. The phenomenon called the narcissus effect occurs when a 
sensor detects its own cold surface relative to its warm surroundings. 
Table 3 shows the parameters for the narcissus analysis of the lens 
assembly which resulted in "minimal" Narcissus effects with a 1.30208 
Narcissus Intensity Ratio with a reflectivity per surface of 0.010 and a 
Narcissus induced temperature difference of 0.772 degrees C. 
TABLE 3 
__________________________________________________________________________ 
INTEGRATED BLACK- 
DIFFERENTIAL 
BODY RADIANT RADIANT 
TEMPERATURE 
EMITTANCE EMITTANCE 
Deg. C. 
(Deg. K.) 
mW/cm**2 Mw/cm**2/deg C. 
__________________________________________________________________________ 
DETECTOR: -195.9 
(77.3) 
0.000 
HOUSING: 20.0 (293.2) 
8.992 
SCENE BACKGROUND: 
20.0 (293.2) 0.1517 
COLD STOP SURFACE: 20 
COLD STOP DIAMETER: 2.4581 IN 
SCENE FULL ANGULAR 0.07478 Radians 
SUBTENSE: (from detector) 
__________________________________________________________________________ 
The lens assembly of the present invention of which the design is disclosed 
herein therefore has the following performance characteristics: 
Max 15 degree Input and max 60 degree Output FOV 
Overall length&lt;53" 
8-12 um Wavelength Band 
5"Flange Relief from last element to exit pupil 
"Minimal" Distortion and Narcissus 
Nearly Diffraction Limited MTF Response of 50% @ 3 cy/mrad 
Minimal Vignetting across entire FOV 
3" Entrance diameter 
Transmission&gt;75% 
These performance characteristics allows for the testing and evaluation of 
second generation FLIR sensors. 
Industrial applicability of this invention includes but is not limited to: 
military, optics, photographic and IR technology. 
This preferred embodiment is not intended to restrict the invention to the 
precise embodiment or embodiments described.