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Optics and Lasers in Engineering 46 (2008) 217–221

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Testing of holographic optical elements for holographic gun-sight

Rahul Bhatt�, Lata Mainali, Gyanendra Bhatnagar, Asha Sharma, Arun K. Gupta

Photonics Division, Instruments Research and Development Establishment, Raipur, Dehradun, Uttaranchal 248008, India

Received 31 July 2007; received in revised form 17 October 2007; accepted 19 October 2007

Available online 28 November 2007

Abstract

Conventional methods of measuring the various parameters of holographic optical elements are tedious for mass production. A novel

approach is described for the holographic elements used in the holographic sight, in which the parameters are defined and measured as

per their intended final application. Since the holographic sight is used for accurate target acquisition along with the other features,

parallax in the sight becomes a critical parameter. Besides, the maximum brightness of the reticle is another parameter of the device,

which is important for the use of sight in the strong sunlight in summers. There are two holographic elements, namely holographic reticle

and holographic lens in the sight. Both can be tested in a simple set-up in terms of the parallax of the sight and the brightness of the

reticle. The masters for both elements are required to be benchmarked once and rest of the elements in a mass production can be tested

with the reference of masters.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Holographic sight; Holographic optical elements; Diffraction efficiency; Parallax

1. Introduction

Holographic sight is an electro-optic sighting system,which uses holographic technology for small arms [1–3]. Ithas the advantage of shooting with both eyes open alongwith fast and accurate target acquisition. Holographicreticle pattern is embedded in the see-through window ofthis device. Basically it is a transmission hologram thatgenerates the virtual image of reticle upon illumination bythe laser diode (LD). The shooter looks through thetransparent window in which the projected holographicreticle acts as an aim point superimposed on the targetscene.

The emission wavelength of a LD increases withtemperature as much as 0.25 nm/1C. This has the effectof shifting the position of virtual image of the reticle as thediffraction angle of the hologram increases with thewavelength. To compensate this effect, a holographicgrating or lens was used [4,5]. Using the holographic lens(HL) has the advantage that a single element combines the

e front matter r 2007 Elsevier Ltd. All rights reserved.

tlaseng.2007.10.006

ing author. Tel.: +91135 2787089; fax: +91 135 2787128.

ess: rahulholo@yahoo.com (R. Bhatt).

action of collimation and passive compensation of reticle-shift due to temperature change.There are many parameters of a holographic element,

which define its quality. These include the diffractionefficiency, angle (for off-axis recording), focal length (for alens), etc. Because of the variables of recording andprocessing conditions, the measured values of reconstruc-tion parameters differ from their required value. Based onthe tolerances, defined by the specifications, the individualelement is accepted or rejected. Many testing methodshave been reported in the literature [6–9]. Almost all themethods are based on measuring the individual parametersof a single element separately. Thus, for testing theholographic elements of holographic sight, the processbecomes tedious for mass production. It is noted thatholographic sight has a huge market potential andhence the testing of its holographic components is veryimportant.In this paper, we discuss about a novel approach of

testing the holographic elements for holographic sight, inwhich these elements are tested as per their intendedapplication. There are two holographic elements in thisdevice. One is a reticle hologram (RH) and another is aHL. The parallax and the maximum brightness of reticle

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Fig. 2. (a) Holographic sight. (b) Projected holographic reticle as observed

through the window of holographic sight.

Lens

RecordingPlate

BE

Object(Reticlemask) Collimated

referencebeam

f

Fig. 3. Recording geometry of reticle hologram (RH). f is the focal length

of the lens.

R. Bhatt et al. / Optics and Lasers in Engineering 46 (2008) 217–221218

are two critical parameters of holographic sight. A test set-up is described in which both can be tested in terms of theparallax of the sight and the brightness of the reticle. Themasters for both elements are required to be benchmarkedonce and rest of the elements in a mass production can betested with the reference of masters.

2. Holographic optical elements (HOEs) in the holographic

sight

There are various schemes for the geometry of holo-graphic sight [1–5]. For the sake of simplicity for testing,we use the equivalent geometry as shown in Fig. 1. Here,the HL acts as an off-axis collimator for the diverging laserbeam from a red LD. The collimated beam acts asreference beam for RH and the red virtual image of thereticle is seen by the observer’s eye. For target scene, theRH acts as transparent glass window. Therefore, unitmagnification target scene is observed with the super-position of the red-reticle through the window of holo-graphic sight, as shown in Fig. 2.

Two holographic elements, namely RH and HL, are thevital components of holographic sight. Both elements areoff-axis transmission holograms. The theory for recordingand reconstruction of such elements can be found in theliterature [6,10,11]. In the RH, the mask of a reticle is takenas an object and is recorded with an off-axis collimatedreference beam. The second HOE is a hologram of a pointsource, i.e., it is the recording of an interference patternbetween a diverging beam and an off-axis collimated beam.Figs. 3 and 4 show the recording geometries of both HOEs.In Fig. 3, the reticle mask is kept at the front focal plane ofthe lens, which forms the parallax free collimated imageof the reticle towards the recording plate.

It is evident from the geometry of the device that thediffracted light from the RH, which is seen by theobserver’s eye, is governed by the product of diffractionefficiencies of both holographic elements. Similarly thereconstruction angle of the reticle also depends on both theelements. Thus, the performance of the sight is a product ofthe individual quality of both holographic elements.

We had recorded these elements on silver halideemulsion and developed with the conventional processingtechniques [7]. To increase the efficiency, other processingschemes can also be applied [12,13]. Further, otherrecording materials like DCG and photopolymer [14,15]can also be used. It is obvious that the quality of HOEs

Holographic lens HL

Hologram RH

Virtualimage

(Reticle)

Observer’s eye

LaserDiodeLD

Fig. 1. Equivalent geometry of holographic sight.

depends on the recording material. However testing, fortheir intended application in the holographic sight, is doneirrespective of the recording material used.The exact mechanical placement of these HOEs in the

holographic sight limits the flexibility, which is required forproper alignment of holographic optics in many applica-tions [7,16–18]. This is reflected in the tight tolerancesbased on the mechanical aspects. In fact, the constraintsare because of required compactness of the holographicsight. Another way, to accommodate the loose tolerances,is to cater for the flexibility in the mechanical design. Itis obvious that this approach will increase the complexityand the compactness will be compromised, which is

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Collimating lens

BE

BERecordingPlate

Fig. 4. Recording geometry of holographic lens (HL).

HL(Holographic lens)

RH(Reticle- hologram)

LaserDiode

LensDetector

HL(Holographic lens)

RH(Reticle-hologram)

Observer’s eye

LaserDiode(LD)

GP Collimator Auxiliary telescope

Fig. 5. (a) Test set-up for measurement of parallax error. (b) Test set up

for measurement of reticle brightness.

R. Bhatt et al. / Optics and Lasers in Engineering 46 (2008) 217–221 219

undesirable. Therefore, testing of HOEs becomes a crucialtool to select or reject a particular HOE for holographicsight.

3. Testing parameters

The testing of various parameters of HOEs of holo-graphic sight can be done separately. For example, thediffraction efficiency can be measured in the usual mannerby measuring the intensity of diffracted and the incidentbeams. However, the application specific scheme is alwaysmore efficient and also helpful in testing of the sight inmass production. In the present paper, we discuss thescheme based on the parameters of the sight.

There are various parameters, which affect the perfor-mance of the holographic sight. These include parallax, sizeand type of reticle, maximum brightness of reticle, the ratioof the reticle brightness between its maximum andminimum setting, total and minimum possible movementof reticle for zero-in, height of the sight from the bore ofthe weapon, etc. As these parameters, in turn, depend onthe quality of various components in the device, thecomponents should be tested in terms of these parameters.This will help in mass production, as the testing becomesapplication-oriented. We have selected two parameters,parallax and maximum brightness of reticle, to test theholographic elements of the device. These two parametersare the most critical parameters from the application pointof view.

Since the sight is required for fast and accurate targetacquisition, the parallax between the reticle and the target,as seen by the observer, becomes an important criterion.The parallax error will cause the inaccuracy in shooting.Parallax error is basically a measure of collimation error ofdiffracted wavefront coming out from the RH. If the RH isok (master) then parallax error is a measure of collimationerror of diffracted wavefront coming out from HL. This isdue to the fact that diffracted wavefront from HL acts as areference beam for RH. Further, the angles of reconstruc-tion (between diffracted beam and the incident beam) forboth elements and focal length of HL can be easilychecked, as the placement of the elements in the test set-up

is itself a measurement (Fig. 5a). No separate parameter isrequired for this testing.The second parameter, maximum brightness of reticle, is

important for the use of sight in the strong sunlight insummers. The power P of the diffracted beam after RH ismeasured for this purpose. This is a measure of thediffraction efficiency of RH and HL both. Thus, if one ofthe HOE is taken as a master, then P is a measure ofdiffraction efficiency of second HOE.

4. Test set-up

Test set-up for testing the holographic components forholographic sight is shown in Fig. 5a and b. This is same asFig. 1 with some additional components for measurement.General Purpose (GP) collimator is used to measurethe parallax error. Auxiliary telescope is used to read thescale of collimator for reticle movement with eye-motion(Fig. 5a).We have used one master set of RH and HL to test the

samples of 10 RH and 10 HL. For making master set, theparameters are required to be calibrated separately, butthis is a one-time job. After the master set is ready, testingof the samples is very easy and fast in the proposed set-up.Master of HL acts as a reference for testing the samples ofRH, and vice-versa.First the master set is placed in set-up. There is zero

parallax error in the master set and the position of thereticle against the scale of collimator is taken as zero. Thisacts as a reference for the samples, which are placedsequentially in the set-up. Respective position of reticleagainst the scale is a measure of angle of reconstruction ofrespective sample. The parallax error is also measured for

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Table 1

Test-results for samples of reticle holograms RH1 to RH10

Sample RH1 RH2 RH3 RH4 RH5 RH6 RH7 RH8 RH9 RH10

Parallax error P (in min) 0 0 0 0 0 0 5 15 20 35

Shift in reticle position S (in min) 0 0 0 0 5 10 15 0 0 50

Reticle brightness B (normalized value) 1 0.9 0.9 0.8 0.8 0.8 0.8 0.7 0.6 0.5

Table 2

Test-results for samples of holographic lenses HL1 to HL10

Sample HL1 HL2 HL3 HL4 HL5 HL6 HL7 HL8 HL9 HL10

Parallax error P (in min) 0 0 0 0 0 5 10 15 5 30

Shift in reticle position S (in min) 0 0 0 0 0 0 5 10 20 25

Reticle brightness B (normalized value) 1 0.9 0.9 0.8 0.8 0.5 0.7 0.7 0.6 0.5

R. Bhatt et al. / Optics and Lasers in Engineering 46 (2008) 217–221220

each sample. Basically it is the displacement of reticleagainst the scale in moving the position of observer’s eye.

After taking the readings of parallax, the lens anddetector are placed in place of auxiliary telescope to takethe readings for reticle brightness (Fig. 5b), which is ameasure of diffraction efficiencies. The incident power atthe detector for master set is noted. Samples are againplaced sequentially and the respective incident power ismeasured for each sample. Normalized value for aparticular sample is obtained by dividing the incidentpower of the sample with the value of master set.

In the holographic sight (Fig. 2a), for which the HOEtesting is required, one additional mirror is used to fold thebeam path between LD and HL [5]. It can be easilyincorporated in the test set-up to simulate the conditionsfor exact application. In fact, the set-up can be altered tosuit the kind of actual geometry of the device. The idea is tomeasure the parallax error and the reticle brightness fortesting the individual holographic components of thedevice.

5. Results and discussion

We have taken 10 samples of each HOE in theholographic sight, just to demonstrate the testing metho-dology for mass production of holographic sight.

Table 1 shows the measurements of shift in reticleposition ‘S’, parallax error ‘P’ and normalized value ofreticle brightness ‘B’ for samples of RHs (RH1 to RH10).As described above, S indicates the error in the angle ofreconstruction, and P indicates the collimation error of thediffracted wavefront of the respective sample. B indicatesthe relative value of diffraction efficiency.

For mass production, all the values (P, S and B) of eachsample are not required to be tested. Only the acceptedsamples of one criterion are required to undergo the nexttest. However, we have taken all three values of eachsample just for the sake of analysis.

To accept or reject a particular sample in the massproduction, P is the most critical parameter. Therefore,

it is measured first. For an application requiring highlyaccurate shooting, e.g., military application, P should bezero. So, samples RH7, RH8, RH9 and RH10 are rejected.However, for an amateur sports application, where P canbe taken up to 5min, sample RH7 can also be accepted.There is no need for further testing for rejected samples(RH8, RH9 and RH10).

S values are considered next. Actually S- and P-value ofa particular sample are measured in one go, as both arechecked by looking at the reticle against the scale ofcollimator. S ¼ 0 implies that there is no change in theangle of reconstruction of diffracted wavefront with respectto master set and therefore such samples are accepted forfurther testing. Further, there can be an upper limit, say10min, up to which the S values are accepted. This isbecause of the fact that the change of 10min will reflect inthe equal change in the reticle position, which can beaccommodated to a certain extent by the provision ofreticle movement in the device. Thus, the tolerance forS values is slightly relaxed as compared to P values, andsamples RH5 and RH6 are also accepted for furthertesting.The tolerance of B-value is again dependent on the

application. If the sight is intended to be used in anenvironment, where ambience lighting, against which thereticle is required to be seen, is not very strong, the lowervalue of B up 0.6 can also be acceptable. However, for theapplication of the device in the bright sun in the summersof equatorial region, minimum acceptable B-value can beas high as 0.9.Table 2 shows the measurements of shift in reticle

position ‘S’, parallax error ‘P’ and normalized value ofreticle brightness ‘B’ for samples of HLs (HL1 to HL10).Similar to the testing of RH, P is the most criticalparameter and hence measured first.

P is also an indicator of the error in focal length of HL.P ¼ 0 implies that the focal length of the sample lens HL isequal to the desired value [the distance between the LD andHL (Fig. 5a)]. However, P6¼0 means that there is change infocal length. It is to be noted that absolute value of focal

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length is not required in this case. However, to measure theexact value of the change, the translation mechanism in themechanical holder of LD and/or HL can be provided.Thus, by moving the position of LD or HL, P ¼ 0 isattained and the movement is a measure of change in focallength. In practice, the movement in the mechanicalholders of the test set-up (Fig. 5a) is provided within thetolerances of placement of the particular component in theactual device. Therefore, if the P-value is not within itstolerance even after moving LD and the HL (sample) up totheir extreme positions, the respective sample is rejected.Thus, for a military application (P ¼ 0), samples HL6,HL7, HL8, HL9 and HL10 are rejected, out of which HL6and HL9 can be accepted for amateur sports application(P ¼ 5).

S-values are considered next for the accepted samples inP-testing. Here HL9, which is accepted for P-value, can berejected (S ¼ 20). Finally, B values are considered. HereHL6, which is accepted for P-value, can be rejected(B ¼ 0.5).

We have excluded some parameters like transmissioncoefficient (in the visible region) and signal-to-noiseratio of RH. However, if required, these can also beeasily measured in the proposed test set-up with somemodification.

The causes of error in HOEs are mainly due to the errorsin recording and processing, besides the alignment error.Variables in recording include various parameters likeintensities of the interfering beam, exposure time, vibrationisolation of the recording platform (table), uniformity ofcoating of photosensitive recording material, etc. Variablesin processing include the purity, dilution and temperatureof various chemicals including the water, time in variousbaths (developer, fixer, bleach, etc.), humidity of the darkroom, etc. In the mass production of HOEs, slight changein these variables may affect the quality. During testing ofHOE, the holders of these elements have same tolerance, asin the holographic gun-sight. However, provision formovements (linear, tilts, etc.) in the holders of proposedtest set-up can be given to check for the alignment error. Ifan HOE shows error in the testing even after moving to theextreme positions in its holder, it can be inferred as thedefect in HOE. It is to be noted that if a particular HOE isnot found suitable in the proposed testing, the same isrejected irrespective of whether it is an alignment error ordue to the error in recording and processing. This is due tothe fact that same element cannot be used in the device(holographic gun-sight).

6. Conclusion

We have discussed a testing methodology of holographiccomponents of holographic sight, which is useful for massproduction. There are two holographic elements, namelyholographic reticle and HL in the sight. Both can be tested

in a proposed test set-up. The parallax, position, andmaximum brightness of the reticle have been taken as thetesting parameters to accept or reject a particularcomponent. The masters for both elements are requiredto be benchmarked once and rest of the elements in a massproduction can be tested with the reference of masters. Theproposed testing is application-oriented and saves the time-consuming process of conventionally measuring variousparameters of interest separately for each component.

Acknowledgment

The authors are grateful to the Director, IRDE,Dehradun for granting the permission to publish thispaper.

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