Wavelength-tunable eye protection

An eye protection arrangement for protection against multispectral laser threats includes goggles or spectacles with a tunable etalon optical filter. In one embodiment, the optical filter is fixed-tuned to a safe frequency. In another embodiment, light from the laser being used is sensed, and used to set the protective goggles to a different wavelength than the laser. In yet another embodiment of the invention, the protective optical filters have a comb response, and additional protection is provided by optically cascading two filters, each having a somewhat different comb response, so as to reduce the number of transmission spectra.

FIELD OF THE INVENTION 
This invention relates to eyewear for eye protection against strong lights, 
such as lasers, and more particularly to such eyewear which includes 
tunable etalons. 
BACKGROUND OF THE INVENTION 
Lasers are becoming more common in industrial, communications, and military 
applications. A laser generates a beam of collimated light, which tends to 
remain in a tight beam over long distances. Many lasers produce sufficient 
total power to damage eyes from distances of a thousand feet or more. Even 
relatively low-power lasers, which might be used, for example, for driving 
fiber-optic cables in a communication system, produce sufficient power 
density so that, if inadvertently directed in to an eye, damage may occur. 
Consequently, a need exists for eyeware for protection against eye damage 
when working around lasers. In the past, most lasers were 
fixed-wavelength, and suitable eyeware might consist of a notch filter at 
the known wavelength of the laser being used. With the advent of systems 
using tunable lasers or multiple lasers, and with the possibility of 
lasers being intentionally used in military situations to cause eye 
damage, the eye protection problem is exacerbated. It is not practical to 
provide a distinct optical filter which covers all possible optical 
wavelengths in a tunable laser or multispectral laser threat environment, 
because such an eye protector would not pass any light, thereby protecting 
the eyes of the user by rendering him sightless for the duration of the 
period of the protection. Improved eye protection is desired. 
SUMMARY OF THE INVENTION 
An eye protection arrangement for protection against multispectral laser 
threats includes goggles or spectacles with a tunable etalon optical 
filter. In one embodiment, the optical filter is fixed-tuned to a safe 
frequency. In another embodiment, light from the laser being used is 
sensed, and used to set the protective goggles to a different wavelength 
than the laser. In yet another embodiment of the invention, the protective 
optical filters have a comb response, and additional protection is 
provided by optically cascading two filters, each having a somewhat 
different comb response, so as to reduce the number of transmission 
spectra. 
An eye protection device according to the invention includes a 
wavelength-tunable filter for passing at least one wavelength selected in 
response to a control signal. A mounting arrangement is provided for 
mounting the filter before at least one eye of a user. A controller is 
coupled to the filter for setting the passing wavelength of the filter to 
a wavelength other than a wavelength at which a high-power-density light 
source emits. In a particular embodiment of the invention, the controller 
produces a signal which is constant, and which may be selected by the 
user. In another embodiment, the controller includes a second light filter 
with characteristics similar to or preferably identical to those of the 
filter located before the eye, and a light detector coupled to the output 
of the second filter. The controller also includes logic for adjusting the 
control signal so that both the first-mentioned and the second filters 
attenuate the light from the source more than adjoining wavelengths. 
The light filter, in a preferred embodiment of the invention, is a comb 
filter. 
In one embodiment, the filter is a tunable Fabry-Perot etalon. The tuning 
may be provided by liquid crystal material within the etalon cavity. In 
another embodiment of the invention, at least one of the filters is a 
cascade of two Fabry-Perot etalons, either dimensioned or controlled to 
have somewhat different transmission comb spectra.

DESCRIPTION OF THE INVENTION 
FIG. 1a is a simplified representation of a laser 10 generating a light 
beam 11, which is directed toward an item under test, represented by a 
block 12. The person operating the laser system is protected from 
inadvertent exposure of the eyes by a system designated generally as 14, 
which includes protective spectacles or goggles 16. Spectacles 16 include 
right and left earpieces 18a and 18b, respectively, and a bridge 20. 
Bridge 20 supports right and left controlled optical filters 22a and 22b, 
respectively. Right and left side shields 26a and 26b protect the eyes of 
the wearer from laser light impinging from the sides. 
Controllable filters 22a and 22b of FIG. 1a are controlled by signals 
applied over a signal path 28 from a control circuit 30. A half-silvered 
light splitter 40 splits off a portion of light beam 11, and directs it 
toward a further controllable filter 32, which preferably identical to 
controllable filters 22a and 22b. Controllable filter 32 is controlled by 
way of a signal path 34 from controller 30, and filters the light in the 
same manner as either of filters 22a or 22b. The filtered light from 
filter 32 is applied to a light detector 36, for generating an electrical 
signal in response to the received light. The electrical signal from 
detector 36 is applied to control circuit 30, to provide it. with the 
sensed signal required for control operation. 
In FIG. 1a, controllable filters 22a, 22b, and 32 are wavelength-tunable 
Fabry-Perot transmission-type etalons. Such etalons are described, for 
example, in U.S. Patent application Ser. No. 08/234,771, filed Apr. 28, 
1994 in the name of Wagner. FIG. 2 is a more detailed diagram of a filter 
of FIG. 1a. For definiteness, filter 22a is represented. In FIG. 2, filter 
22a is a tunable liquid-crystal Fabry-Perot etalon, also known generally 
as a Fabry-Perot interferometer, tunable etalon, resonant cavity 
interferometer, and the like. Filter 22a of FIG. 2 includes transparent 
silica or quartz left and right substrates 210l and 210r, respectively, 
which preferably have mutually parallel interior surfaces. The inside 
surfaces of substrates 210l and 210r are each coated with a layer 212l, 
212r, respectively, of transparent electrical conductor material, which 
may be, for example, tin oxide or indium-tin oxide. Conductive layer 212r 
is connected to ground, and conductive layer 212l is connected to 
conductor 37 for receiving voltages which are selected by controller 30, 
as described above in conjunction with FIG. 1a. A partially transparent or 
semitransparent reflector 214l overlies electrically conductive layer 
212l, and a similar semitransparent reflector 214r overlies conductor 
212r. Such a semitransparent reflector layer corresponds conceptually to a 
"half-silvered" or "one-way" mirror, but such half-silvered mirrors tend 
to have high attenuation or loss. Instead, semitransparent reflector 
layers 214l and 214r are layered dielectrics, known in the art for low 
loss, selected to produce the desired semi-transparency and reflectivity. 
A cavity 26 lying between semitransparent reflectors 214l and 214r is 
filled with liquid crystal material. A further pair of layers 216l and 
216r of buffed polyimide may be placed on reflector layers 214l and 214r, 
respectively, for aiding in aligning the molecules of the liquid crystal. 
The liquid crystal material filling cavity 26 of the etalon of FIG. 2 
exhibits birefringence, which is a difference in the index of refraction, 
depending upon the polarization of the light which passes therethrough 
relative to the orientation of the liquid crystals. This may be explained 
by noting that under normal, unenergized conditions, the liquid crystal 
material in cavity 26 tends to assume a "crystalline" form, with the 
molecules aligned in a particular direction, illustrated in FIG. 2 as 
being the vertical direction. The direction of the preferred orientation 
may be controlled by forming mutually parallel grooves in the polyimide 
interior surfaces facing the cavity, which orient the molecules adjacent 
the surface parallel to the grooves, and thereby establish the "crystal" 
orientation. Under the condition of vertical molecular alignment, light 
which is principally polarized vertically will experience a particular 
propagation delay, which relates to the index of refraction. When a 
voltage is applied by way of conductor 37 to generate an electric field 
across the liquid crystal material in cavity 26, the molecules of the 
liquid crystal material tend to rotate approximately 90.degree. to become 
parallel with the field, whereupon they are no longer parallel to the 
electric field component of the incident light, and the propagation delay, 
and consequently the index of refraction, changes. The change in delay 
corresponds to changing the effective length of cavity 20 as a function of 
the applied voltage. Whatever the actual mechanism by which the result is 
accomplished, an etalon such as that described in conjunction with FIG. 2 
has the property of filtering light at frequencies which depend upon the 
applied voltage. Some embodiments of an etalon filter may exhibit 
preferred axes of polarization, i.e. the performance is best for a 
particular polarization of light passing therethrough. A polarizing filter 
(not illustrated) may be inserted into the light path for best 
performance. 
As known to those skilled in the art, Fabry-Perot etalons typically have a 
periodic filter function given by 
EQU 2ndCOS.theta.=m.lambda. (1) 
where 
n is the refractive index of the medium; 
d is the mirror spacing; 
.theta. is the inclination of the normal of the mirrors; 
m is the order of the interference; and 
.lambda. is the wavelength. 
For the case of mutually parallel mirrors, cos .theta. is unity. In 
general, the etalon passes or transmits light at a wavelength at which the 
cavity length is a multiple of fractional submultiples of a wavelength. 
This may be understood by considering that, in order to transmit light, 
the multiple internal reflections must constructively add at the output 
semitransparent layer, and that an even number of reflections must occur 
for light to exit. The comb or multispectral response of one of the 
etalons, such as etalon 20 of FIG. 1a, is illustrated by plot 310 of FIG. 
3. As illustrated, the transmission peaks occur periodically at 
wavelengths of 564, 580, 597, 616, 635, and 655 nm. 
If the wavelength of the laser 10 of FIG. 1a is at a fixed frequency, then 
a tunable spectacle or goggle in accordance with the invention is not 
necessary, but it can be used in that case, too. If the laser source has 
fixed wavelength, the control circuit 30 does not even need an input from 
detector 36, but instead control circuit 36 needs only to produce an 
alternating voltage of a magnitude selected to place the peaks of the 
transmission spectrum of the etalons 22a, 22b at a wavelength other than 
the wavelength of the laser. Since the laser frequency is fixed in this 
particular arrangement, no tuning is required. With the transmission peaks 
of the filters 22a and 22b at wavelengths other than the laser wavelength, 
the laser wavelength is at a null in the transmission response. Thus, 
light from laser 10 of FIG. 1 cannot pass through the spectacles, or, more 
properly, the light is attenuated. 
If, on the other hand, laser 10 of FIG. 1a is tunable, the wavelength may 
at some time reach one of the spectral transmission peaks of plot 310, at 
which time the goggles would not provide protection of the user's eyes 
against an inadvertent reflection of the laser light. In accordance with 
the invention, control circuit 30 adjusts the control signal on signal 
paths 28 and 34 so as to reset the peaks of the transmission wavelength 
spectrum of the light filters away from the transmission wavelength of 
laser 10. 
It should be noted that the case of a fixed laser frequency or wavelength, 
the voltage required to tune the filter depends upon the angle of 
incidence of the light on the filter. A fixed tuning voltage is applicable 
only to normal incidence, which will seldom be the case. Thus, even for a 
fixed-tuned laser, it is desirable to have a tunable wavelength filter in 
the spectacles. 
For some applications, such as protection of military personnel from 
intentional laser exposure, it may be advantageous to reduce the number of 
transmission peaks in the transmission spectrum of each of the protective 
filters 22a, 22b. For this purpose, each of filters 22a, 22b may be made 
as a cascade of two slightly different etalon filters. For example, filter 
22a may be made as a cascade of two etalon filters, one of which has a 
wavelength spectrum as illustrated in plot 310 of FIG. 3. FIG. 1b 
illustrates a filter, such as filter 22a of FIG. 1a, made up as an optical 
cascade of two separate tunable filters 50 and 52. The first filter 50 of 
the arrangement of FIG. 1b may have a response similar to that of plot 310 
of FIG. 3, and the second filter 52 may have a wavelength spectrum which 
is illustrated as plot 312 of FIG. 3. 
According to an aspect of the invention, filter 22a of FIG. 1b, is an 
optical cascade of two structures such as that of FIG. 2. This optical 
cascade is similar in construction to two filters 22 illustrated in FIG. 
2, each with slightly different characteristics, to produce a comb 
spectrum different from comb spectrum 310 of FIG. 3. This is most readily 
accomplished by making the width of cavity 26 of FIG. 2 of the two 
optically cascaded filters of different dimensions, which changes the 
"order number" m of equation (1), which, together with other parameters, 
determines the filter transmission spectrum. Plot 312 of FIG. 3 represents 
the transmission spectrum of a filter with different characteristics than 
the filter having plot 310. As illustrated, plot 312 has transmission 
peaks at about 551, 569, 590, 612, 635, and 661 nm. None of these 
transmission peaks, except the peak at about 635 nm, corresponds with a 
transmission peak of plot 310. Consequently, the only spectral line or 
band which is transmitted by both filters is the one which peaks at about 
635 nm, and one or the other of the two optically cascaded filters 
attenuates or rejects all other wavelengths. Thus, the optical cascade of 
two filters used as filter 22a of FIG. 1a, when set to voltages giving the 
spectral responses illustrated in FIG. 3, passes only 635 nm, 
corresponding to a red hue. A green hue at about 550 nm could be 
transmitted by the pair of filters, by leaving filter 22 of FIG. 1a with 
the response illustrated as 312 in FIG. 3, and by modifying response 310 
of filter 20 to move the set of peaks of plot 310 to the left, toward 
smaller wavelengths, until the peak illustrated at 562 nm overlies the 
peak at 550 nm. Blue at about 480 nm is not illustrated in FIG. 3, but the 
same principles apply to blue. While retuning of a single filter may 
provide filtering, optimum response may require retuning of both filters 
by the control circuit 30 of FIG. 1a. FIGS. 4a and 4b are plots of the 
spectral response of a pair of optically cascaded filters, with both 
filters retuned for optimum combined response. In FIG. 4a, the combined 
transmission response is at 634 nm, and in FIG. 4b, the combined 
transmission response is at 585 nm. Those skilled in the art know that the 
bandwidth of the etalon filter transmission peaks may be controlled by, 
for example, controlling the amount of transmission provided by 
partially-reflective surfaces 214 of FIG. 2. The bandwidth is selected in 
accordance with the severity of the threat. In a laboratory in which 
several tunable lasers are in use, the two-cascaded-filter arrangement may 
provide more protection. 
FIG. 5 is a simplified block diagram illustrating details of the control 
circuit for the spectacles of FIG. 1. In FIG. 5, light detected by 
detector 32 produces signals which are coupled to control circuit 30. 
Control circuit 30 includes an amplifier 510, which amplifies the signals, 
and applies them to an analog-to-digital converter (ADC) 512. ADC 512 
converts the analog signals to digital form. The digital signals from ADC 
512 are applied to a processor 514, which processes the signals as 
described in more detail below. The processed signals from processor 514 
are filter element tuning voltage signals in digital form. The filter 
element tuning voltage signals are applied to a digital-to-analog 
converter (DAC) 516, which produces analog signals which represent the 
amplitude of the tuning voltages. The analog signals from DAC 516 are 
applied by way of a driver amplifier 518 to a DC-to-AC converter 520. 
Converter 520 "chops" or converts the unipolar signal from driver 
amplifier 518 into an alternating or square-wave signal with related peak 
amplitude. The alternating voltage from converter 520 is applied to the 
liquid-crystal etalon tuning elements 22a and 32 by way of signal paths 28 
and 34, respectively. The etalon filter elements are assumed to be 
identical for purposes of this control system, so the etalon filter 
associated with the spectacles tunes to block the same wavelength that is 
blocked by filter 32, and the eyes are protected against the particular 
laser wavelength. 
FIG. 6 is a simplified flow chart illustrating a control scheme for 
controlling spectacles 16. In FIG. 6, the logic starts at a START block 
612, and proceeds to a block 614, which represents the reading of the 
current detector signal power or amplitude P1. 
From block 614, the logic flows to a block 616, which represents comparison 
of the current amplitude P1 with the amplitude a few clock cycles 
previously, which may be ten clock cycles, for example. Thus, block 616 
represents taking the ratio R1=P.sub.10 /P.sub.1. A sudden rise in the 
ratio R1 thus gives an indication of whether a laser has directed a beam 
into the spectacles. The logic proceeds to a decision block 618, which 
compares ratio R.sub.1 with a threshold TH, chosen to indicate the 
presence of a threat to the eyes. If the current value of R.sub.1 is less 
than threshold TH, the logic flows back to block 614. As so far described, 
the logic of blocks 614, 616, and 618 represents the main logic loop, 
which monitors the incident power for sudden increases in the light power 
entering the detector. 
In the event of a sudden rise in the ratio R.sub.1, indicative of the 
appearance of a threat to the eyes, the logic leaves decision block 618 of 
FIG. 6 by the YES path, and arrives at a block 619, which represents the 
setting of a variable I to its positive value +I. From block 619, the 
logic flows to a block 620, which represents the incrementing of the 
current value of the tuning voltage Vc to a new value of Vc+I, where I is 
a small increment value, such as +0.01. From block 620, the logic flows to 
a further block 622, which represents measuring the amplitude ratio 
A.sub.N =(P.sub.N /P.sub.N-1) between one clock cycle and the next, to aid 
in indicating control direction. From block 622, the logic flows to a 
decision block 626, which compares the current value of A.sub.N with 
A.sub.N-1, to see if the detected power has increased or decreased as a 
result of the slight tuning change. If A.sub.N is less than A.sub.N-1, the 
logic leaves decision block 626 by the YES output, which indicates that 
the power transmitted through filter 32 to detector 36 has decreased. The 
YES output of decision block 626 directs the logic by a path 628 to a 
block 630, which represents setting the current value of I to +I, to 
continue the same direction of control. From block 630, the logic then 
flows by a path 632 back to block 620, where the current value of I is 
added to control voltage Vc. Since I has a positive value, the logic again 
increments the value of Vc. The logic proceeds about the loop including 
blocks 620, 622, 626, and 630 until such time as the detected power no 
longer decreases. At this time, decision block 626 directs the logic by 
its NO output to a block 628, which represents resetting of the current 
value of I to -I. From block 628, the logic flows back to block 620, where 
the value -I is added to Vc, thereby decreasing Vc. The logic then flows 
around the loop, alternating between the paths including blocks 628 and 
630. Such loops are well known in the art, and may be configured to take 
into account various contingencies. The logic flow of FIG. 6 in essence 
detects the presence of a laser threat by a rapid rise of detected power, 
and then generates a digital ramp voltage which progressively detunes the 
filter, to thereby progressively reject the increased power, until the 
amount of attenuation reaches a maximum, following which the filter 
dithers about the maximum-attenuation point. 
Other embodiments of the invention will be apparent to those skilled in the 
art. For example, the control circuit may employ analog circuitry instead 
of, or together with, digital circuitry. While ten clock cycles "delay" 
are described as having been used in the logic of FIG. 6 between 
successive sensing times, the number of clock cycles will depend upon 
system time constants and upon the processor clock rate, and may be 
greater or fewer.