IR fast low light level lens system

A compound lens for use in a low light level system operating in the near-infrared region. The lens system includes a high light-gathering ability requiring both a large aperture and good transmissivity over the wavelengths of interest. A wide field of view (e.g., 50.degree.) without excessive chromatic and geometric aberrations that generally accompanies systems with a large aperture is provided. Space is provided in the lens system for a variable iris and variable density spot subassembly. Desired optical quantities are obtained through the use of zinc selenide as the material for an aspheric lens. The use of this diamond-turnable infrared material as a field lens yields exceptional performance for a fast (e.g., F/1.25) lens with a wide field of view.

FIELD OF THE INVENTION 
This invention relates to an optical system designed for low light level 
(LLL) systems. 
DESCRIPTION OF RELATED ART 
Previous low light level lens designs are fast in that they gather 
sufficient quantities of light for detection at extremely low light 
levels. These lens designs, however, cover a modest field of view of 
40.degree. or less. A wide field of view is preferred for certain 
applications such as ground or airborne navigation. 
It is the balancing of the higher order off-axis aberrations (coma, 
astigmatism, lateral color, and distortion) that are most troublesome in 
the design of a fast wide angle lens system. Additionally, sufficient back 
focus must be provided so that the lens may be adjusted in the factory to 
compensate for fabrication and assembly tolerances. 
SUMMARY OF THE INVENTION 
One object of the present invention is to provide an objective lens for a 
low light level system which uses fast optics to collect as much light as 
possible and produce a high quality image of a scene. 
Another object of the invention is to provide a low light level lens system 
with an extended field of view. 
Yet another object of the invention is to provide a low light level lens 
system with sufficient back focus capabilities to permit factory focus 
adjustment. 
The present invention meets the above criteria by providing a nine element 
lens system that will fit a typical low light level system package while 
fulfilling the requirements of a low light level system. 
Off-axis performance is greatly enhanced by employing a high refractive 
index material for the field lenses. Zinc selenide, which is typically 
reserved for use in system designed to operate in the far infrared (i.e., 
3 to 12 microns) range is used for purposes of the present invention in 
the near infrared or visual range. 
Off-axis performance may be further improved by employing an aspheric 
surface on one of the zinc selenide field lenses. The aspheric surface 
constants provide additional variables for the optimization process. The 
resultant design has exceptional performance throughout the field of view. 
Most of the field operates at full aperture. The system is permitted to 
vignette, starting at approximately .+-.20.degree. off axis and up to 50% 
of the aperture at full field (.+-.25% off axis). 
The aspheric surfaces may be advantageously produced by a diamond turning 
process. A diamond turned aspheric surface is suitable in the present 
application because the surface quality (departure from the ideal surface) 
of the field lens is not as critical as it is for the components used in 
the aperture. The surface tolerance is therefore comparable to that used 
for infrared systems with the exception that a post polish is required to 
remove the diamond turning marks and make the surface smooth. 
In addition to the basic optical requirements of the lens system, space may 
be provided for an auto-iris. The auto-iris will limit the amount of light 
permitted to fall on the photocathode. This will protect the tube from 
damage if a bright target were to enter the field of view. Additionally, 
the auto-iris may be accompanied by a variable density spot in the center 
of the lens element near the aperture stop. This spot on the center will 
cause a small transmission loss when the aperture is wide open, but will 
permit the system to operate at a reasonable f/number when bright sources 
enter the field of view.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates a presently preferred lens layout of the new lens form. 
It is a nine element lens system, including a dome shaped window, that 
will fit in a typical low light level system package. Specifically, the 
lens system comprises a concentric window (dome) 111, a negative meniscus 
lens 112, a positive meniscus lens 113, a null planar window 114, a 
positive double convex lens 115 in contact with a negative double concave 
lens 116, a positive double convex lens 117 followed by a positive 
meniscus lens 118, followed by a positive meniscus lens 119 and a negative 
meniscus lens 120. This lens system focuses infrared radiation on an image 
intensifier 122. 
The specific optical characteristics of the lens system of the presently 
preferred embodiment appears below in Table I. Table I is an ACCOSV (an 
optical design computer program sold by Scientific Calculations, Inc., a 
division of the Harris Corporation, 7796 Victor Mendon Road, P.O. Box H, 
Fishers, N.Y. 14453) listing of the lens system construction parameters 
and identifies the aspheric deformations. 
TABLE I 
__________________________________________________________________________ 
BASIC LENS DATA 
SURFACE REFRACTIVE 
DISPERSION 
ELEMENT 
NUMBER RADIUS THICKNESS MEDIUM INDEX FACTOR 
__________________________________________________________________________ 
0 0.00000000 
1.00000000E+10 
AIR 
1 0.00000000 
-1.33962709 AIR 
111 2 3.02950000 
0.16000000 MATL SILICA 
1.453322 39.466 
3 2.86950000 
0.28000000 AIR 
112 4 1.38248000 
0.15748000 SCHOTT LAK9 
1.681792 0.001 
5 0.70122000 
0.46961500 AIR 
113 6 1.25596000 
0.15527559 SCHOTT SF56 
1.764913 1.061 
7 3.68400000 
0.23918300 AIR 
114 8 0.00000000 
0.08000000 SCHOTT BK7 
1.510780 0.384 
9 0.00000000 
0.12085600 AIR 
10 0.00000000 
0.11600000 AIR 
115 11 0.81102000 
0.27559055 SCHOTT LAK9 
1.681792 0.001 
116 12 -0.81103000 
0.08415354 SCHOTT SF56 
1.764913 1.061 
13 0.64814000 
0.10478000 AIR 
117 14 0.81103000 
0.21653543 SCHOTT LAK9 
1.681792 0.001 
15 -1.37150000 
0.00500000 AIR 
118 16 0.45857000 
0.14025590 SCHOTT LAK9 
1.681792 0.001 
17 0.40457000 
0.10000000 AIR 
119 18 0.77580000 
0.09842520 MATL RZNSE 
2.524186 2.427 
19 1.08952000 
0.03937008 AIR 
120 20 1.38248000 
0.06889764 MATL RZNSE 
2.524186 2.427 
21 0.70122000 
0.11811023 AIR 
22 0.00000000 
0.22047244 MATL SILICA 
1.453322 39.466 
23 0.00000000 
-0.88487939 AIR 
24 0.00000000 
0.88487939 AIR 
25 0.00000000 
0.00000000 AIR 
__________________________________________________________________________ 
CONIC CONSTANT AND ASPHERIC DATA 
Conic 
Surface Constant AD AE AF AG 
__________________________________________________________________________ 
19 8.96959E+00 1.88519E-01 
-1.54350E+00 -3.47649E-01 
1.87313E+02 
Wherein AD, AE, AF, and AG represent the 4th, 6th, 8th and 10th order 
deformation coeffecients, respectively. 
__________________________________________________________________________ 
OPERATING CONDITIONS 
Gauss 
Effective Back Image 
Focal Focus 
F/Number Length 
Height 
__________________________________________________________________________ 
0.7874 0.0000 
1.25 3.2500 
0.3670 
__________________________________________________________________________ 
Reference Reference 
Object Reference 
Image 
Object Height Aperture Height 
Surface Surface 
Surface 
__________________________________________________________________________ 
-0.466308e+10 
(25.0000 DG) 0.34914 0 10 25 
__________________________________________________________________________ 
DESIGN WAVELENGTHS 
WAVL NBR 1 2 3 4 5 
WAVELENGTH 0.80000 0.70000 0.75000 0.85000 0.90000 
SPECTRAL WT 1.0000 1.0000 1.0000 1.0000 1.0000 
__________________________________________________________________________ 
______________________________________ 
APERTURES 
SURFACE TYPE SEMI CLEAR APERTURES 
______________________________________ 
2 CIRCLE 0.7940 
3 CIRCLE 0.7350 
4 CIRCLE 0.5900 
5 CIRCLE 0.4850 
6 CIRCLE 0.4450 
7 CIRCLE 0.4350 
8 CIRCLE 0.4050 
9 CIRCLE 0.3950 
10 CIRCLE 0.3491 
11 CIRCLE 0.3481 
12 CIRCLE 0.3400 
13 CIRCLE 0.3200 
14 CIRCLE 0.3550 
15 CIRCLE 0.3550 
16 CIRCLE 0.3500 
17 CIRCLE 0.3050 
18 CIRCLE 0.3050 
19 CIRCLE 0.2850 
20 CIRCLE 0.2900 
21 CIRCLE 0.2800 
22 CIRCLE 0.3150 
23 CIRCLE 0.3900 
24 CIRCLE 0.6791 
______________________________________ 
APERTURE STOP AT SURF 10 (EN AND EX ADJUSTMENTS) 
Table I lists the basic lens data such as the actual elements by element 
number, the actual and imaginary surfaces by a surface number, the radius 
of the various surfaces wherein a listing of zero corresponds to a flat 
surface a listing of the thickness or spacing between the surfaces, the 
medium into which the light rays are passing such as air, silica, Schott 
LAK9, SF56, BK7 (supplier's material designations), and Zinc Selenide 
RZNSE, the refractive indices, and dispersion factors. After the basic 
lens data, Table I lists the conic constants and aspheric data as well as 
the clear apertures, operating conditions and design wavelengths. In the 
optical system specified in Table I, the aperture stop is positioned at 
surface 10 and the lens units are in inches. 
The off-axis performance of the lens system is improved by the use of high 
refractive index glass and the liberal use of advanced optical design 
optimization programs such as the ACCOSV design computer program. The 
off-axis performance is greatly enhanced by employing even higher 
refractive index material for the field lenses, i.e., positive meniscus 
lens 119 and negative meniscus lens 120. One such material is zinc 
selenide which is typically reserved for use in systems designed to 
operate in the far infrared (i.e., 3 to 12 micron) range, but is rarely 
used in the visual or near infrared range. However, this material 
transmits very well in the 700 to 900 nm spectral range and provides a 
means to improve the off-axis performance of the system. The vertices of 
the lens elements 111-120 and the principal axis of the image intensifier 
122 lie along a the optical axis (or chief ray) 124 of the system. 
The off-axis performance may be further improved by employing an aspheric 
surface on one of the zinc selenide field lenses such as at any of the 
last four surfaces of field lenses 119 and 120. The aspheric surface 
constants provide additional variables for the optimization process using 
a computer iterated design program. The resulting design has exceptional 
performance throughout the field of view. The performance is reflected in 
the modulation transfer function (MTF) shown in FIG. 2. This graph shows 
the modulation in comparison to the spatial frequency (cycles per mm). The 
diffraction modulation transfer function is represented for the 
wavelengths of 700, 750, 800, 850 and 900 nm. 
By the use of zinc selenide as the material for the aspheric lens, the 
present invention obtains the desired optical qualities along with the 
ability to use a machining method employing diamond turning for a 
controllable, non-spherical lens shape with an acceptable surface profile. 
This aspect of the invention is particularly advantageous since the cost 
of aspheric surfaces has been significantly reduced in recent years by the 
introduction of the diamond turning of optical parts. 
A diamond turned aspheric surface is suitable for the present application 
because the surface quality of the field lens is not as critical as is the 
surface quality of components used in the aperture. The surface tolerance 
is therefore comparable to that used for infrared systems with the 
exception that a post polish step is required to remove the diamond 
turning marks and make the surface finish smooth, which is important for a 
field lens. 
Zinc selenide has an index of refraction of over 2.4 as opposed to the 
index of refraction of 1.9 or less for most optical glasses as used in the 
prior art. 
In addition to the basic optical requirements of the lens system, the 
system may also provide space for a variable transmission obstruction 126. 
The variable transmission obstruction 126 will limit the amount of light 
permitted to fall on the photocathode in the image intensifier 122. The 
variable transmission obstruction 126 may be as simple as an auto-iris 
such as an radially varying transmission N.D. filter located adjacent null 
planar window 114. This will protect the tube 122 from damage if a bright 
target were to enter the field of view. The variable transmission 
obstruction 126 is not necessary to the operation of the system 110 for 
most applications. 
The sensitivity of the image intensifier 122 is such that it may drive the 
iris 126 to the size of a pin hole when a bright source enters the field 
of view. This would render the present system so slow that the diffraction 
effects would significantly reduce the systems performance. However, this 
situation may be avoided by locating a variable density spot in the center 
of the lens element near the aperture stop. The variable density spot may 
be a dot coating on a surface of the null planar window 114. The spot on 
the center will cause a small transmission loss when the aperture is wide 
open, but will permit the system to operate at a reasonable f/number when 
bright sources enter the field of view. 
As can be seen from the above, the low light level system described herein 
will have a 30.times.40.times.50 degree field of view and therefore be 
compatible with a forward looking infrared system. Assuming the above 
field of view, the lens would be required to have a 19.3 mm focal length 
assuming an 18.0 mm format for the image intensifier 22 as illustrated by 
the equation EFL=18.0 mm/(2.0.times.tan 25.degree.) where EFL=effective 
focal length. 
The 25 mm focal length commercial lens or lenses designed for ANVIS goggles 
could serve at least for a rough and ready system. This system is 
desirable because it can be quickly acquired and inexpensively 
constructed. However, there are several factors that mitigate against the 
use of such commercial lenses. These factors include the fact that 
commercial lenses are typically designed to operate in the visual spectral 
range, whereas the present invention will principally operate from the 700 
to 900 nm range. Also, commercial optics that are designed for the correct 
spectral range cover a smaller field of view. Additionally, the lens 
system would likely be located behind a dome-shaped window to make it 
compatible with a typical infrared lens system package. Although the dome 
111 has a relatively low power and has a relatively small effect on the 
aberrations of the system, these effects cannot be totally eliminated by 
refocusing and therefore it is desirable to design the lens system with 
the dome 111 as an integral part. For the foregoing reasons, a lens system 
may be designed for the low light level system herein disclosed to meet 
specific requirements of the desired system if conventional designs will 
not yield the performance needed. 
The description provided herein is intended to be illustrative only and not 
intended to be limiting. Those skilled in the art may conceive many 
modifications to the disclosed embodiments. However, any such 
modifications which fall within the purview of the description are 
included in the scope of the invention as well. The scope of this 
invention shall be determined from the scope of the following claims.