Laser damage control for optical assembly

An optical assembly and technique for magnified viewing which includes innal damaging laser energy protection. An objective lens subassembly is positioned on the focal axis which focuses incoming light energy over an entire field of view. An optical prism accepts the focused light energy and reorients the focused light energy which is split by a beamsplitter onto a power limiter at the intermediate focal plane so that only energy under an approximate damage threshold is allowed to pass onward. The optical assembly can provide from 4.times. to a 10.times. power magnification with up to a 60 millimeter entrance pupil diameter with internal, multi-spectral damaging laser energy protection.

CROSS-REFERENCE TO RELATED APPLICATIONS 
This application is a substitute for application Ser. No. 08/551,053 filed 
Oct. 31, 1995, now abandoned. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to optical assemblies, and more 
specifically, to optical assemblies and the techniques that offer 
magnified viewing while simultaneously accommodating optical power limiter 
devices for laser damage protection. 
2. Description of Prior Art 
Protection methods and apparatus for optical equipment have been attempted 
for providing protection from laser energy that could otherwise damage 
optical radiation detectors, including the human eye. The most common 
technique of providing protection involves optical filtering elements, 
which offer substantial protection but only over a limited, fixed spectral 
color range. Standard dielectric coatings are the most common form of 
filters, and flat plates with these "notch" coatings can be easily 
inserted into or outside many common optical assemblies. As noted above, 
however, these filters are useful only over a limited range of 
wavelengths, and also have the added disadvantage of blocking even 
non-harmful radiation within the designed spectral region. 
Typical military magnifying optical assemblies such as telescopes, 
periscopes, and binoculars vary widely, and typically have magnifying 
powers ranging from 4.times. to 10.times., with entrance aperture 
diameters going from 20 mm to 60 mm or more. As the magnifying power 
increases, the angular resolution increases, and thus the farther away a 
given target can be recognized. The larger apertures are required to 
gather sufficient light energy to allow good contrast for far-away 
targets. These magnifying optical systems are commonly designed for use 
with the human eye, but can also easily perform similar tasks when 
connected to standard television camera equipment. Given the harsh nature 
of military environments, these optical systems do not lend themselves 
easily to the use of attachments to perform laser protection functions. 
All magnifying optical assemblies of the kind found in telescopes, 
periscopes, and binoculars can be characterized as consisting of an 
objective lens set, followed by an eyepiece assembly, with either a real 
or virtual focal plane between, as well as a variety of intervening prism 
assemblies (almost always porro prisms) to keep the image orientation 
proper. The magnifying power is defined as the ratio of the objective 
focal length divided by the eyepiece focal length. Typical fields of view 
for these systems range from 2.degree. to 10.degree., depending upon the 
magnification. In the prior art for all these systems, the focal planes 
between the objective and eyepiece sections, or between any intervening 
relay optics, is not well corrected for aberrations. This does not affect 
the overall system performance, because the aberrations of the objective 
can be compensated by those of the eyepiece. It is much more difficult to 
design both objective and eyepiece optics to each have diffraction limited 
focal planes, and therefore this feature is not normally embraced by the 
current art. Additionally, since the magnifying power is the ratio of the 
objective and eyepiece focal lengths, it is desirable to have a relatively 
short focal length eyepiece to minimize the objective focal length for a 
given magnification. This reduces the overall size of the system, but does 
not offer much room between the eyepiece assembly and the intermediate 
focal plane. Because of this, prior art designs do not usually allow 
elements other than thin transmissive reticle plates to occupy the space 
in or near the intermediate focal plane. 
The prior art in developing laser protective devices offers many 
techniques, including sacrificial mirrors, transmissive optical power 
limiters, liquid cells, etc. These devices are generally designed to 
operate passively within an optical system until indicent optical 
radiation is of sufficiently high energy to activate the protective 
mechanism. In order to set the activation threshold below the damage 
threshold of the detector (human eye, TV camera, etc.), it is desirable to 
place the power limiter in or near a well corrected, diffraction limited 
focal plane. Additionally, the optical system must be able to accommodate 
the volume of the power limiter device, and be able to provide proper 
image orientation should the device create an image translation. 
While the prior art has reported using magnifying optical apparatus and 
methods of providing laser energy damage protection none have established 
a basis for a specific apparatus that is dedicated to the task of 
resolving the particular problem at hand. What is needed in this instance 
is an approach for an optical system that performs the tasks of prior art 
magnifying optics but also includes provisions for incorporating an 
optical power limiter device by providing a near-diffraction limited 
intermediate focal plane for the entire field of view, room for the power 
limiter, and optics to maintain the proper final image orientation and 
image quality. 
SUMMARY OF THE INVENTION 
It is therefore one object of the invention to provide an optical system 
that performs the tasks of prior art magnifying optics but also includes 
provisions for incorporating an optical power limiter device by providing 
a near-diffraction limited intermediate focal plane for the entire field 
of view, room for the power limiter, and optics to maintain the proper 
final image orientation and image quality. 
According to the invention, there is disclosed a magnifying optical 
assembly and technique which includes internal, multi-spectral, damaging 
laser energy protection. The invention allows for protection not only 
against fixed wavelength threats (as a filter can do), but also against 
"wavelength agile" threats that may cover the spectrum. An objective lens 
assembly is centered on the optical axis which focuses light from the 
scenery into a well corrected, near diffraction-limited focal plane over 
the entire field of view. A non-powered subassembly positioned between the 
objective and its focal plane reorients the light to maintain image 
orientation at the final output of the system, and also includes a 50%/50% 
beamsplitter to allow a fiat mirror perpendicular to the optical axis. The 
mirror may be a simple "sacrificial" optical power limiter, or it may work 
in conjunction with other transmissive power limiters such as non-linear 
crystals or liquid cells, etc. The high quality focal plane is designed to 
optimize the damage protection threshold of the chosen device(s), and 
provides this capability over the visible spectral range and over the full 
field of view. When the light rebounds off of the mirror and back through 
the beamsplitter, it enters an eyepiece assembly which is specially 
designed to take the high quality focal plane and re-collimate the energy 
such that it is suitable for viewing with the detector (human eye, TV 
camera, etc.) The eyepiece is designed with an extra long back focal 
length to fit both the power limiter(s) and the 50%/50% beamsplitter. Once 
the power limiter has activated, a portion of the harmful radiation will 
be prevented from continuing through the eyepiece and reaching the 
detector (human eye, TV camera, etc.) The methods and system of the 
invention apply to the design any magnifying system with powers ranging 
from 4.times. to 10.times., fields of view from 2.degree. to 10.degree., 
and entrance apertures ranging from roughly 20mm to 60 mm.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION 
Referring now to the drawings, and more particularly to FIG. 1, there is 
shown the optical assembly superimposed on an optical ray-trace diagram. 
The optical assembly includes objective lens subassembly 10, which 
includes lens elements 11, 12 and 13; an eyepiece lens subassembly 14 
which includes lens elements 15, 16, 17, 18, 19; and optical prism 
subassembly 100 to be described herewithin. Objective lens assembly 10 
focuses light energy over the field of view through the optical prism 
assembly. The focal plane of objective lens subassembly 10 is very well 
corrected with a spot size of approximately 25 microns. 
The optical roof prism subassembly 100 is positioned between objective lens 
subassembly 10 and eyepiece subassembly 14. Objective lens assembly output 
101 impinges on orientation means 102 within the optical prism subassembly 
100 which redirects objective lens assembly output 101 substantially 90 
degrees in orientation while maintaining image orientation. Optical prism 
subassembly 100 flips the imagery horizontally (left to right) to provide 
the proper image orientation when viewed through eyepiece lens subassembly 
14. It is understood that other types of orientation means may be used by 
different combinations of mirrors and prisms. Redirected objective lens 
assembly output 103 impinges upon beamsplitter means 104 also within the 
intermediate optical subassembly, where beamsplitter means 104 in the 
present embodiment is a 50/50 glass cube beamsplitter. Fifty percent of 
output 103 will be reflected from beamsplitter means 104 onto an optical 
power limiter 105 which in the present embodiment is a sacrificial mirror 
located directly on an intermediate focal plane. 
Light that penetrates beamsplitter means 104, shown in FIG. 1 as light rays 
106, will fall harmlessly into a light baffle. The light baffle simply 
absorbs the radiation and does not permit stray light rays from 
progressing further through the system. While a glass cube beamsplitter is 
the preferred choice for military usage, other means such as thin 
pellicles could be used. It is understood that the invention is not 
limited to the specific beamsplitter means disclosed herein. The 
intermediate focal plane array is located far enough away from either the 
objective or eyepiece lenses to allow insertion of both the mirror and the 
beamsplitter. The need to insert beamsplitter means 104 places a critical 
design constraint upon eyepiece subassembly's 14 back focal distance, 
which must be longer than typical binocular eyepiece. 
If light incident upon power limiter means 105 is harmful to the user's 
eye, then power limiter means 105 becomes absorbent and does not reflect 
the light onward through eyepiece lens subassembly 14. In the preferred 
embodiment, power limiter means 105 is a sacrificial mirror having a 
mirrored film coating with a damage threshold of 1.6 to 2.0 Joules/square 
centimeters. Before reaching damage levels, the mirror is greater than 90% 
reflective. After damage by a sufficiently powerful laser, the "blown out" 
spot on the mirror is only less than 2% reflective. It is understood that 
the invention is not limited to the specific power limiter of the 
preferred embodiment. The present invention will function with other 
optical power-limiting materials without significant modification. 
Examples of other power limiters may include non-linear crystals or 
gas/liquid cells. These devices may have the property of being optically 
transparent during normal, unthreated operation. They respond to high 
energy incoming light by either becoming opaque (thus blocking the 
damaging energy) or by undergoing a change in its refractive index (thus 
defocusing the incoming light). These types of power limiters can also be 
utilized to work in conjunction with a sacrificial mirror, thereby 
providing extra measures of protection capability in tandem. 
Eyepiece lens subassembly 14 corrects light rays 106 for astigmatism, field 
curvature, and distortion for larger field angles which are residual 
aberrations of objective lens subassembly 10. Eyepiece lens subassembly 14 
is also designed with a long focal length to accommodate the beamsplitter 
means. Dummy focal plane 107 illustrates that light passing all the way 
through the binocular optical assembly will enter the eye pupil as 
collimated light, and thus focus upon the human eye retina, which may be 
located at dummy focal plane 107. 
Each element is described in Table 1 below where all dimensions are given 
in millimeters and a positive and negative radius indicates the center of 
curvature is to the right and left respectively according to what is shown 
in FIG. 1. 
TABLE 1 
__________________________________________________________________________ 
RADIUS OF APERTURE 
ELEMENT 
CURVATURE DIAMETER 
NUMBER 
FRONT BACK THICKNESS 
FRONT 
BACK 
GLASS 
__________________________________________________________________________ 
OBJECT 
INF INFINITY 40.0000 
APERTURE STOP 
11 43.5423 CX 
-160.3509 CX 
20.3461 40.0000 
37.5885 
BK7 Schott 
5.0384 
12 -84.9839 CC 
34.0524 CC 
19.7433 35.1733 
30.7162 
LF4 Schott 
24.3446 
13 68.3451 CX 
-151.7158 CX 
8.1272 38.3242 
38.3790 
BKIO Schott 
20.7767 
102 R(1) 
INF -85.0000 52.7447 REFL 
104 R(2) 
INF 0.0000 39.9844 REFL 
26.7348 
15.0000 
105 INF 0.0000 25.1023 REFL 
25.1023 
-35.0000 
15 182.6146 CC 
-59.3088 CC 
-2.0000 36.8169 
38.5447 
SFL6 Schott 
16 -59.3088 CX 
46.2000 CX 
-9.7366 38.5447 
39.2793 
LAK8 Schott 
-0.1000 
17 -49.5639 CX 
301.2543 CX 
-6.5457 38.6829 
37.8821 
LAF2 Schott 
-0.1000 
18 -23.3014 CX 
-12.6245 CC 
-3.5000 31.9142 
24.1893 
SF4 Schott 
19 -12.6245 CX 
-15.3418 CC 
-10.3460 24.1893 
18.9063 
LAKN13Schott 
-20.0000 
9.6353 
IMAGE DISTANCE = -20.0000 
IMAGE INF CC 17.9142 
__________________________________________________________________________ 
Each of lens elements 11-13 and 15-19 are described in terms of radius of 
curvature for front and back of each lens where CC denotes concave and CX 
denotes convex. The thickness for all the objects described in Table 1 is 
the axial distance to the next surface, and the image diameter shown above 
is a paraxial value not a ray traced value. The reference wavelength is 
500.0 nanometers for the spectral region of 450.0 to 700.0 nanometers. 
Glass materials utilized for the lens elements are also shown in Table 1, 
but may be of other materials that are functionally equivalent. Many of 
the selected glass types listed in Table 1 are utilized in standard army 
visual optical systems, but it is understood that the present invention is 
not limited to the glass types listed. 
Dimensions for entrance pupil, exit pupil, and overall length (OAL), all in 
millimeters is given below in Table 2: 
TABLE 2 
______________________________________ 
Entr Pupil 
Diameter = 40.0000 
Distance = 0.000 
Exit Pupil 
Diameter = 6.9909 
Distance = 0.2551 
______________________________________ 
A 40 millimeter entrance pupil diameter is relatively large for typical 
6.times. binoculars, but is desirable to offset the transmission losses 
caused by the 50/50 beamsplitter. The exit pupil diameter of 7 mm allows 
the user to comfortably position his eye pupil (typically approximately 5 
mm) behind the eyepiece. The exit pupil is located roughly 15 mm from the 
last lens thus allowing the wearing of eyeglasses. Fold mirror 102 and 
beamsplitter 104 angles are described in terms of R(i) and R(2), both in 
degrees is given below in Table 3: 
TABLE 3 
______________________________________ 
R (1) = 45.0000 
R (2) = 45.0000 
______________________________________ 
With both folds at a 45 degree angles, there results a total of 90 degree 
deflection in the light path. 
FIG. 2 is a top view of military binoculars utilized as the preferred 
embodiment of the present invention with a half-cutaway view showing 
generalized internal positioning of subassemblies of the optical assembly. 
It is understood that the positioning of the three subassemblies as 
depicted in FIG. 2 are intended to show relative subassembly positions to 
each other in a binocular 20 and in no way is limited to specific 
dimensions. The non-damaging laser energy for the preferred embodiment is 
"eye safe" energy. The term "eye safe" for purposes of the preferred 
embodiment is defined (according to the Laser Institute of America) as 
that radiation below a maximum permissible exposure (MPE) for any 
particular wavelength, exposure period, or viewing condition. The term 
"eye safe" energy is also that energy which is safe under prolonged 
intrabeam viewing with or without optical instruments. 
Operational performance characteristics of the optics assembly of the 
present invention will next be described with reference to FIGS. 3 through 
8. It is understood that the graphical representations shown in FIGS. 3 
through 8 are approximate in nature and as such do not limit the present 
invention to exact data points. The choice of specific values for 
magnifying power and field of view are meant to be representative of 
typical systems which range from 4.times. to 10.times. and have field of 
views up to 10.degree.. 
FIG. 3 is a graphical plot of modulation transfer function (MTF) resolution 
performance of the binocular optics assembly. One of the performance 
measurements of an optics assembly is its resolution, represents the 
on-axis modulation transfer function (MTF) at zero degrees field of view 
(FOV). In FIG. 3, line 30 is the diffraction limit and line 31 is the 
on-axis MTF. For a 6.times. magnification, the optics assembly at a 
spatial frequency of 10.2 cycles/milliradian (for the resolution of the 
human eye) yields an MTF of 87 line pairs/millimeter. The MTF on-axis 
therefor exceeds 5% for human visual perception (where 5% is the generally 
accepted minimum). 
FIG. 4 is a graphical plot of optical spot size performance at the 
intermediate focal plane of the binocular optics assembly with optical 
spots 40, 41, and 42 shown in FIG. 4. The spots shown in FIG. 4 show that 
for all visible wavelengths and over the full + or -4 degree FOV, the spot 
size remains under 25 microns over a relatively flat field. This provides 
enough energy gain on the mirror to be more sensitive than the retina of 
the human eye depending upon the properties of the chosen optical power 
limiter. A quality spot size of approximately 23.6.times.10.sup.-6 meters 
is shown in FIG. 4 which is sufficient for presently existing sacrificial 
mirror materials. Mirror materials must damage and become absorptive when 
incident laser energy is focused to a spot size of 25 micrometers or less. 
FIG. 5 is a graphical plot of distortion performance for the binocular 
optics assembly. An approximately uniform lateral magnification over the 
entire FOV is desired, and is measured in terms of distortion. Line 50 is 
the percent distortion for the binocular optics assembly performance. As 
seen in FIG. 5, distortion does not approach a "greater than minimal" 10% 
at +/-4 degrees FOV. 
FIG. 6 is the field curvature plot for the binocular optics assembly. A 
sharp focus is desired in an optical system but especially in complex 
optical systems there is a problem with astigmatism which gives rise to 
blurred images at the margins of the FOV. Line 60 is the tangential 
astigmatic field curve while line 41 is the sagittal component of the 
astigmatic field curve. As shown in FIG. 4, astigmatism easily meets the 
standard visual requirement for less than 1/8 diopter astigmatic 
curvature. 
FIGS. 7 (a and b), 8 (a and b), and 9 (a and b) are graphical plots of X 
and Y fan meridional ray aberrations for +4.000 degree off-axis input, 
on-axis input, and -4.000 degree off axis input respectfully to the 
binocular optics assembly. Each X and Y fan is derived by taking 
measurements at only one wavelength band at a time. Meridional ray 
aberration plots represent the aberrations of the entire optics assembly 
with the different figures representing different angles of entry of the 
radiation into the refractive system. Referring to FIGS. 7 (a and b), 8 (a 
and b) and 9 (a and b), there is shown k-Band 1-3 for each X- and Y- fan 
corresponding to a chosen input. As can be seen in these meridional ray 
plots for the binocular optics assembly, all are below 0.0500 mm optical 
path differences, which is an acceptance level of the assembly. Rim ray 
fan plots illustrate focus error margins over the wavelength spectrum. 
They indicate types of aberrations present and often correlate directly 
with the MTF curve. 
During the operation of the binocular optics assembly (herein described as 
the preferred embodiment) when a laser is incident upon the device, the 
laser beam is first focused onto the intermediate focal plane which is 
co-located upon a flat sacrificial mirror. The focused energy will burn 
away the reflected mirror coating and be absorbed into the non-reflective 
mirror substrate. The energy will not reach the user's eye, but the user 
may notice a "black spot" in the binocular field of view. Hence, the 
burned off portion of the mirror is "sacrificed" to save the eye retina 
from a similar fate. Thus focused laser energy will "ablate" the mirror, 
thus preventing the energy from continuing through the eyepiece and 
harming the human eye. 
While this invention has been described in terms of preferred embodiment 
consisting of a binocular optics assembly, those skilled in the art will 
recognize that the invention can be practiced with modification within the 
spirit and scope of the appended claims.