Eye-safety analysis of current laser-based scanned-beam projection systems

Edward Buckley (SID Member) Abstract — Scanned-beam projection systems have attracted much interest recently, with claimedadvantages including power efficiency and potential miniaturization consistent with embedding inmobile devices. However, the laser-safety classification and concomitant performance implications,which are arguably the most important issues pertaining to this technology, remain widely misunder-stood. In this paper, Class 1 and 2 laser-safety radiometric image power limits for scanned-beam sys-tems are derived with reference to the IEC 60825-1 standard. By calculating the equivalentphotometric measure of luminous flux, it is possible to show that the brightness limits for scanned-beam projection systems using current technology are approximately 1 and 17 lm for Class 1 and 2safety classifications, respectively.

Keywords — Laser, safety scanned beam, projection.

DOI # 10.1889/JSID18.11.944

1 IntroductionLaser-based light engines are beginning to emerge as a can-didate technology for powering so-called “pico-projector”products, which are characterized by battery operation andoutput luminous flux values of less than 50 lm. In theory,laser-based light engines offer some attractive features andbenefits, since they can exhibit a small factor, long depth offield, polarization independence, and potentially higherefficiencies.

To date, three candidate light-engine architectureshave been proposed and demonstrated. Lasers can be usedas light sources for conventional imaging architectures, illu-minating a small amplitude-modulating liquid-crys-tal–on–silicon (LCOS) panel and typically employing f/2relay optics to magnify the resultant field.1,2 It is also possi-ble to use an LCOS panel in phase-modulating mode, wherea fast panel is used to show sets of diffraction patterns whichare subsequently demagnified by projection optics.3 Finally,scanned-beam architectures employ a rather different approach,using a rapidly moving silicon micromirror to mechanicallydeflect a laser spot across the image in a manner somewhatanalogous to a cathode-ray-tube (CRT) system.

A great deal of interest has surrounded scanned-beamsystems, in particular, due in part to the potential for extrememiniaturization and high optical efficiency. A number oflimitations of such systems have recently become apparent,however, with laser speckle,4 limited brightness, and highcost5 being principal objections. In addition, it is clear thatconsiderable confusion remains regarding the laser safetyclassification of scanned-beam projection displays and theimplications for the achievable brightness roadmap of suchsystems.

In this paper, a model consistent with the IEC 60825-1laser safety standard is used to derive the output powerrestrictions imposed by Class 1 and Class 2 laser safety

classes upon scanned-beam projection display systems andthe D65 white-balanced luminous flux values that would result.

2 Analysis methodologyAs stated by IEC 60825-1,6 the acceptable exposure limitfor visible wavelength laser safety classification is deter-mined by measuring in a limiting aperture of radius d = 7mm, representative of the maximum dilation of the humaneye, at a distance of r = 100 mm from the projection aper-ture. The measurement geometry is shown in Fig. 1.

Assuming that the power delivered to this aperture ismeasured over a classification period T2, the maximum per-missible optical power in the measurement aperture that agiven laser safety standard allows is determined. By theninferring the total projected image power Pimage, whichwould result in this condition and imposing a white-pointcondition, a photometric measure of maximum luminousflux Lmax can be derived.

Received 06/04/10; accepted 09/07/10.The author is located at 310 E. San Rafael St., Colorado Springs, Springs, CO 80903, USA; telephone 719/434-1147, e-mail: ebuckley@ieee.org.© Copyright 2010 Society for Information Display 1071-0922/10/1811-0944$1.00.

FIGURE 1 — Measurement geometry for determination of the laser safetyclassification.

944 Journal of the SID 18/11, 2010

2.1 Projection geometryIn this study, both the projection and measurement geome-tries are independent of projection technology. A projectionsystem with horizontal and vertical projection angles of θhand θv radians respectively gives rise to a rectilinear imageat distance r containing the measurement aperture of diameterd, which is defined to have acceptance angle γ radians. Theside and top views of the projection geometry are illustratedin Fig. 2. Without loss of generality, it is assumed that thehorizontal and vertical projection angles contain M and Npixels, respectively.

The acceptance angle γ is related to the measurementaperture diameter d and measurement distance r by

(1)

2.2 General methodologyA given laser safety classification is defined by an acceptableexposure limit (AEL), which is dependent upon a numberof physiological and technology-specific factors. The AEL isstrongly influenced by the apparent source size, subtendingan angle between αmin and αmax radians, which is particu-larly relevant to scanned-beam projectors in which thesource size dspot is effectively the diffraction-limited laserspot with size of the order of a millimeter. The AEL alsodepends upon an energy scaling factor pertaining to thermalretina damage, which, in turn, is weighted by the numberand duration of modulated optical pulses delivered to themeasurement aperture, and an exposure time t or classifica-tion time T2. The generic eye-safety parameters defined byIEC 60825-1 are given in Table 1.

In general, there may be several AELs covering singlepulse or pulse train conditions and photothermal or photo-

chemical effects, so the limiting AEL is defined as the mostrestrictive of AELn, where n = 1 ... N, so that

(2)

The maximum power that can be delivered to the pro-jected image Pimage is then determined such that the energyat the measurement aperture Eaperture is less than the AEL,thereby satisfying

(3)

To calculate the maximum optical image power Pimagethat can be delivered while maintaining the appropriateAEL at the measurement aperture, the proportion of energyη delivered to the aperture in T2 sec is calculated so that

(4)

where, neglecting any distortion in the image, the fractionof power delivered into the measurement aperture is givento a first-order approximation by

(5)

As previously shown,4 a scanned-beam system is notable to direct the entire laser-beam power into just one pixeldue to limitations imposed by the maximum laser modulationfrequency and the resonant nature of the MEMS mirrorsemployed. Hence, the worst-case situation for eye safety isderived from the full-white-screen condition; using this factand combining Eqs. (3) and (4) gives the result that theradiometric power Pimage exiting a scanned beam systemshould not exceed a limit defined by

(6)

in order to satisfy a given laser safety classification.

2.3 Radiometric-to-photometric conversionSince the aim of the paper is obtain a maximum luminousflux value for the given laser safety classification, it is neces-sary to convert the radiometric figure Pimage [in for the totaloutput of Eq. (11)] into photometric quantities for the red,green, and blue sources. Although the luminous-efficacyfunction ν(λ)7 can be used to convert the output power intoluminous-flux values LR,G,B, the process becomes compli-

tan .g2 2

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AEL AEL AEL AELN= min , , , .1 2 Kl q

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FIGURE 2 — Projection geometry for an image of extent θh × θv radians,containing the measurement aperture of diameter d and acceptanceangle γ.

TABLE 1 — Eye-safety parameters defined by IEC 60825-1.

Buckley et al. / Eye-safety analysis of currect laser-based scanned-beam projection systems 945

cated by the need to ensure the laser-power balance is con-sistent with a sensible white-point value.

To determine the respective laser powers which give apre-determined white point, it is necessary to start from thedefinition of the CIE XYZ tristimulus functions, which aredefined in terms of the color-matching functions x(λ), y(λ),z(λ), and a spectral power distribution I(λ).8

(7)

In this analysis, it is assumed that a D65 white point ata color temperature of 6500K is to be obtained from RGBlaser sources of wavelengths λb = 445 nm, λg = 532 nm, and

λr = 642 nm such as those manufactured by Nichia,9 Corning,10

and Opnext,11 respectively. The CIE standard observercolor-matching functions are plotted in Fig. 3(b), and thecolor-matching function values (x, y, z) for the chosen laserprimaries are shown in Table 2.

The tristimulus values of Eq. (7) can be converted tothe (x, y, z) coordinates of the CIE color-space chromaticitydiagram using the following equations:

(8)

and from the resultant CIE 1931 color-space diagram shownin Fig. 3(a), it is possible to determine the coordinates of theappropriate color point. For D65 white, the (xw, yw) whitepoint chromaticity coordinates are (0.31271, 0.32902), giv-ing (X, Y, Z) tristimulus values of (95.04, 100.00, 108.88).

Finally, the luminous flux for each color LR, LG, andLB can then be calculated from the color-matching functionvalues at each wavelength and color-matching functions atthe prescribed white point using the following matrix opera-tion12

(9)

and the ideal laser output powers PR, PG, and PB can becalculated from the luminous-flux values using

(10)

3 Scanned-beam projector analysisHaving outlined the general measurement philosophy, theanalysis proceeds by making some assumptions about thenature and performance of the projection system. A scannedbeam system operates by spatially modulating red, green,and blue laser sources using one or two micromirror devices,with the spot resulting from the combination of the three

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FIGURE 3 — (a) CIE 1931 color-space chromaticity diagram, showingwhite points and laser primary wavelengths. (b) The CIE standardobserver color-matching functions x(λ), y(λ), z(λ).

TABLE 2 — Color-matching function values forthe chosen laser primaries.

946 Journal of the SID 18/11, 2010

wavelengths being scanned across the image. To achievepixel definition, the lasers are pulsed rapidly. To providegray scale, the red and blue semiconductor lasers are pulse-width modulated while limitations in the green laser-modu-lation bandwidth may necessitate analog modulation aboveand below threshold.

Due to the extremely high modulation frequency ofthe laser sources and the small beam diameters employed inthe optical system, scanned-beam systems are not able toemploy a diffuser. Although this makes speckle reduction inthe system very difficult, the simplicity of the optical archi-tecture eases the safety analysis. In such a system, the angu-lar subtense of the apparent source α is simply given by

(11)

where r is the distance to the measurement aperture and,due to the very high f-numbers inherent in scanned-beamarchitectures, the beam divergence can assumed to be zero.

For a given projection system, the image size in theplane of the measurement aperture, determined by thehorizontal and vertical throw angles θh, and θv, is an impor-tant parameter in determining Pimage. It is straightforwardto show using a one-dimensional geometrical argument thatthe throw angle θ is related to the image size h and theprojection distance x by

(12)

and when the projection distance x is approximately equalto the image diagonal size d, it follows that the throw anglesand the aspect ratio ra are related by

(13)

For the purposes of this analysis, a single-mirrorscanned-beam projection system operating at WVGA reso-lution (850 × 480 pixels) and a frame rate of fr = 60 Hz is

assumed. The system parameters, assumed and calculated,are summarized in Table 3.

3.1 Class 2 operation analysisThe IEC 60825-1 standard defines the Class 2 AEL for anexposure time t, where 1.8 × 10–5 s � t � 10 sec and wave-length λ, where 400 nm � λ � 700 nm, as

(14)

where C6 is the effective source size correction factor givenby

(15)

and α is the angular subtense of the source defined in Eq.(11). The beam-correction factor for the scanned-beam sys-tem under analysis is found to be C6 = 4.

3.1.1 Single-pulse analysisAccording to Freeman,14 a typical pulse width in a WVGAscanned-beam system is of the order of τ = 20 nsec, whichIEC 90825-1 permits to be treated as a pulse of duration t =Ti = 18 µsec in the visible region. For a single pulse, themaximum permissible power is equal to the AEL of Eq. (14)divided by Ti, and hence

(16)

which results in Pimage = 3.4 W, using Eq. (5) to account forthe fractional aperture area. The relatively large value is dueto the fact that Eq. (16) represents a thermal AEL and theradiometric power figure obtained corresponds to the smallthermal hazard posed by one short pulse.

3.1.2 Pulse-train analysisIn a single-mirror scanned-beam projection system, thepulse width is determined by the behavior of the fast scan

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TABLE 3 — Summary of the system parameters for a WVGA scanned-beam projection system.

Buckley et al. / Eye-safety analysis of currect laser-based scanned-beam projection systems 947

axis which, in turn, corresponds to the horizontal resolution.Given pulses of approximately τ = 20 nsec duration, the totalpulse on time τtot is determined by the number of horizontalpixels, throw angle, and aperture size so that

(17)

neglecting mirror fly-back time between rows. Figure 4depicts the relationship between the pulses of duration τincident upon the measurement aperture of acceptanceangle γ radians, and the horizontal and vertical resolution Nand M, respectively.

Since ttot is of the order of 1 µsec, all pulses in a hori-zontal line can therefore be grouped into a single pulse ofduration Ti = 18 µsec according to IEC 60825-1. This dra-matically simplifies the analysis since the duration Ti is nowindependent of the gray-scale modulation in each pulse, andall that remains is to determine how many times the scannedbeam intercepts the measurement aperture in the verticaldirection per one frame period.

This is trivial to calculate; since the fast scan directionis horizontal, and because all pulses in the measurementaperture count as one pulse of duration Ti, the number ofpulses n′ incident upon the aperture per frame is equal tothe number of times the beam passes from the top of theaperture to bottom in one frame, or

(18)

so that the total number of pulses incident upon the meas-urement aperture during the classification period T2 is

(19)

In a scanned-beam system, the pulse duration is con-stant even though the pulse energy is not, since each pixelcan take a different gray-scale value. In this case, the IECstandard allows the average value of the exposure to be com-pared with the AEL, and the presence of n pulses in theblink response time T2 = 0.25 sec is accounted for by multi-plying Eq. (16) by n–0.25 (Ref. 6, p. 31). The final expressionfor the AEL then becomes

(20)

from which the radiometric image limit Pimage = 46 mW canbe obtained. [IEC 60825-1 also allows the use of the totalon-time-pulse (TOTP) method, in which the AEL is deter-mined by the sum of all pulse durations within the emissionduration T2; in this way, the same result for Eq. (20) can beobtained by setting the total on-time t = NTi in Eq. (16).15]

At some 74 times smaller than AEL1, this is clearly thelimiting case for Class 2 classification. Equations (7)–(10)can be used to calculate a D65 white-balanced luminous fluxof ≈11 lm corresponding to this radiometric power limit. Asummary of the relevant parameter values for the Class 2safety analysis is provided in Table 4.

3.2 Class 1 operation analysisIn certain situations, for example in the case of young chil-dren where the blink response may not be fully developedor when an observer has dark-adapted eyes, it may not bepossible to assume a blink-response-limited classificationperiod of T2 = 0.25 sec. In this case, it is necessary to considerthe requirements imposed by a Class 1 safety classificationin which the exposure time is t = 100 sec for 400 nm � λ �700 nm and a source of angular extent greater than 1.5 mrad.6

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FIGURE 4 — Pulses of width τ, corresponding to a resolution of N × Mpixels, incident upon a measurement aperture of acceptance angle γ.

TABLE 4 — Summary of relevant parameter values for the Class 2 safety analysis.

948 Journal of the SID 18/11, 2010

For a wavelength range 400 nm = λ = 1400 nm, the classifi-cation period T2 is then defined as

(21)

which, given the scanned-beam system parameters of Table4, gives T2 ≈ 11 sec.

The analysis of Class 1 laser safety is further complicatedsince, in addition to the thermal limits, photochemical-basedrestrictions apply for the green and blue wavelengths λg andλb, which are weighted by the function C3, where

(22)

for 400 nm � λ � 700 nm.To determine a radiometric power figure that satisfies

the Class 1 classification requires that the photochemicalpower at the output of the projector for blue and greenwavelengths, and the photothermal power summed acrossall wavelengths, are less than the respective limits. Expressedmathematically, if the photochemical limits at blue andgreen wavelengths are Pph(λb) and Pph(λg), respectively,and the photothermal output power limit is Pth, then toachieve a Class 1 the following must simultaneously hold:

(23)

and

(24)

3.2.1 Photochemical hazardThe AEL for the photochemical hazard for t = 100 sec anda source angular subtense of α = 11 mrad is given by

(25)

which, for a single pulse and α ≠11 mrad, gives a photo-chemical power limit of

(26)

in the 7-mm measurement aperture. For green and bluewavelengths, respectively, the Class 1 photochemical powerlimits are approximately Pg = 40 mW and Pb = 1 mW.

3.2.2 Photothermal hazardFor a source of angular subtense α > 1.5 mrad, there are twoformulae for the AEL depending upon the classification andemission duration T2 and t, respectively:

(27)

For continuous-wave (CW) emission, t = 100 sec andT2 ≈ 11 sec so the first of these conditions apply. For single-pulse and pulse-train conditions, however, the photother-mal AEL is determined by t = Ti < T2 and the secondcondition is used. The analysis proceeds in a similar fashionto Sec. 3.1, with the only difference being the new value forT2. Hence, the approximate radiometric power limits arePimage = 0.15 mW, Pimage = 10 mW, and Pimage = 120 mW forCW, pulse-train, and single-pulse conditions, respectively.

A summary of the results and relevant simulationparameters for the Class 1 analysis are summarized in Table 5.

An iterative method was used to find the radiometricimage power limit that satisfied Eqs. (23) and (24) in thepulse-train mode of operation representative of a scanned-beam system. Since the limiting AEL is determined by thephotochemical hazard in the blue wavelength, the resultantmaximum image brightness Lmax was found to be just 1 lm.

4 SummaryThis paper has shown that scanned-beam systems are funda-mentally limited in terms of maximum achievable luminous

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TABLE 5 — Summary of the relevant parameter values for the Class 2 safety analysis.

Buckley et al. / Eye-safety analysis of currect laser-based scanned-beam projection systems 949

flux, principally because of the very small effective sourcesize. To achieve a Class 1 classification, the D65 white bal-anced luminous flux emitted by a WVGA single-mirrorscanned beam system is limited to approximately 1 lm, whilethe corresponding Class 2 limitation is roughly 11 lm. It isclear, therefore, that the photochemical hazard at bluewavelengths prevents the realization of a practical Class 1scanned-beam system.

The achievable Class 2 D65 luminous flux can beincreased slightly in a number of ways. The higher luminousefficacy of sources at shorter red wavelengths can be exploitedto produce a higher photometric power for the same radio-metric power, so using a source with λr = 635 nm would thenresult in an upper limit of 12.7 lm for a WVGA system.Unfortunately, this strategy may be counterproductive forapplications requiring low power consumption since electri-cal-to-optical conversion efficiencies of 635-nm devicestend to be lower due to heating effects in AlGaInP layers atshorter wavelengths.16

Brightness can also be increased by increasing thenumber of vertical lines, as per Eq. (18); as Fig. 5 demon-strates, in which the maximum Class 2 brightness is evalu-ated at resolutions of similar aspect ratio, a 720p systemcould emit a maximum of 15.2 lm while remaining a Class 2system. While this appears to be an attractive way for manu-facturers of scanned-beam systems to simultaneously improvetwo aspects of system performance, this approach is funda-mentally compromised for low-power applications since theconcomitantly increased laser modulation frequency increasespower consumption in the laser itself10 as well as in theassociated analog drive electronics. Limitations in the fre-quency-doubled green laser modulation frequency alsoappear to place an upper limit on the achievable resolutionat WXGA.17 Nevertheless, a 720p system using 635-nm redlaser illumination could feasibly reach 17 lm while remain-ing Class 2 eye safe. A range of Class 2 eye-safe luminous-

flux values, plotted as a function of resolution and redsource wavelength, are provided in Fig. 5.

Finally, it may also be possible to relax the maximumbrightness limitation by increasing the horizontal and verti-cal throw angles. However, since noticeable image distor-tion is already evident in scanned-beam systems, it is clearthat projection angles achievable using commercially avail-able bulk silicon MEMS systems are likely to be limited tothe ranges considered in this study for the foreseeablefuture, although carbon-fiber-based MEMS structures18

show promise in this regard.In conclusion, it can be reasonably stated that the use-

ful range of the D65 white-balanced luminous flux obtain-able from scanned-beam systems whilst maintaining a Class2 eye-safety classification is between 10 and 17 lm, based onnominal resolutions between WVGA and 720p, and redlaser wavelengths in the range 635–642 nm. It remains to beseen whether this class of device can remain competitivewith LED-based WVGA products capable of producing15 lm,19 the expected emergence of 20-lm 720p systems inlate 2010,20 and the existence of LCOS-based laser-projec-tion products which are already demonstrating Class 1 eye-safety performance at 1521 and 20 lm.22

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12 D. Armitage et al., Introduction to Microdisplays (John Wiley andSons, West Sussex, 2006), Chap. 10: Projection Displays, p. 310.

13 Microvision Inc., “ShowWX laser pico projector specifications,” May2010. [Online]. Available: http://www.microvision.com/showwx/specs.html.

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16 Sony Corp., “The science of the laser projector,” October 2009. [On-line]. Available: http://www.sonyinsider.com/2009/10/12/the-science-of-the-laser-projector/.

17 Corning, Inc., “Corning Green Laser G-2000 Module,” 2010. [Online].Available: http://www.corning.com/WorkArea/linkit.aspx?LinkIdentifier=id&ItemID=32243.

FIGURE 5 — Maximum achievable D65 white-balanced luminous fluxfor Class 1 (dotted line) and Class 2 operation, as a function of red sourcewavelength and resolution with constant aspect ratio.

950 Journal of the SID 18/11, 2010

18 Mezmeriz, Inc., “Carbon fiber MEMS micromirrors: enabling the nextgeneration of picoprojectors,” September 2008. [Online]. Available:http://www.memsinfo.jp/whitepaper/730.pdf.

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Buckley et al. / Eye-safety analysis of currect laser-based scanned-beam projection systems 951