Laser Safety Analysis Of A Scanned Light Display ((E. S.Viirre, R. S. Johnston, H. L. Pryor and S. Nagata)) Human Interface Technology Laboratory, Box 352142, University of Washington, Seattle WA 98195-2142
Purpose. The Virtual Retinal Display (VRD) is a new visual display device that uses laser light scanned directly onto the retina to create images. Unlike other scanning technologies, the VRD uses a scanner less than one cubic centimeter in volume which operates at greater than 12 kHz. This high frequency gives the ability to produce high resolution images. We expect that the VRD will become a ubiquitous display source for normally sighted and partially sighted viewers. Our research goals include understanding of spatial and temporal integation of light energy scanned into the eye and image perception. We first wanted to determine the safety limits on power output of the device. Images from the VRD are bright, high contrast and yet are produced with laser power outputs on the order of 100 to 300 nanowatts. A prototype color VRD produces VGA images (640 ¥ 480) with a 30 ¥ 40 degree field of view. The color sources are: red diode laser at 650 nm, green helium neon at 543.5 nm and blue argon at 488 nm. Methods. The American National Standards Institute (ANSI) publishes the standards (Z136.1, 1993)that govern laser safety. Because the ANSI standard does not explicitly cover intrabeam viewing of a scanned source, we performed several different methods of safety analysis. The standard approach to safety analysis is to calculate the Maximal Permissible Exposure (MPE) for the aperature of the eye. Conservative estimates are used to determine MPE, including assuming light of wavelengths 400-550 nm (the most hazardous) and assuming a continous 8 hour exposure. Results. Our analysis for a pulsed laser results in an MPE of 160 milliwatts, which is 5 to 6 orders of magnitude above the output of the VRD. Analyzing the VRD as a continuous wave laser source gives similar values to pulsed laser calculations. If the VRD is considered an extended laser source because of its scanning nature, the MPE reduces to 150 microwatts which is 2-3 orders of magnitude above typical operating power. Failure analysis indicates that the VRD is safe at the typical power levels, even if the scanners were to fail completely and the source were to remain on. Conclusion. At typical operating power outputs, the VRD is a safe laser based display device.
Supported by Microvision Corporation (C5).