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Measure for safe front projection Disclosure Number: IPCOM000130180D
Publication Date: 2005-Oct-17
Document File: 3 page(s) / 63K

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Measure for safe front projection

Any front projection device emits a divergent beam of light from the projection lens. Close to this lens, the intensity of this light beam is high enough to cause eye damage. A typical front projector that generates 1000lm on the screen emits several Watts of optical power. Nowadays, UHP-lamps are most commonly used as the light source in such a projector. Recently, lasers have attracted substantial interest as the future light source for projection devices. In the case of a laser-based projector, much stricter safety regulations are applicable than for those based on an incoherent light source.

The excellent focussability of lasers, their efficiency and their monochromaticity makes lasers ideal light sources to replace UHP-lamps in projection applications. Even miniaturized devices, such as hand-held projectors (sometimes called "picobeamers") are thinkable. Lasers can be used as light sources very similar to nowadays' used UHP-lamps both in front- and rear-projection. In the case of rear -projection the laser beam is expanded to a large screen and all the beams are kept covered within the device, a rear-projector can be classified as a safe laser product (class 1).

This is different from front-projectors, where free optical beams are emitted from the projector. Even though these beams are generally diverging towards a large screen, the optical power density close to the objective is always remarkably high, making such a device an unsafe product. Besides dangers to the human eye and skin, it poses also a remarkable fire hazard. In the case of a laser-based projector, this product has to be classified as laser class 4.

For laser projection different display technologies can be applied. A popular technology is the so-called "fly ing-spot" technology, where a laser beam is scanned with the help of two mirrors over the screen, writing the information point-by-point like it is known from cathode ray tubes. This technology involves a well-collimated laser beam of high beam quality. With a different technology, a laser with low beam quality can be used, that illuminates a DLP- or LCD-display as known from lamp based projection. This technology is referred to as 2d-light- valve projection. A hybrid technology uses a line display and scans only this line over the projection screen (1d-light valve). Here we want especially consider the case of 2d-light valve projection with laser sources of low beam quality.

A laser product emitting radiation in the visible wavelength range is classified as a safe product, when no more than 1mW of radiation can enter the eye's pupil. The pupil diameter is assumed to be 7mm. From these values one can calculate a critical power density Ic above which the laser product is unsafe:

Ic = 26 W/m2 = 2.6 mW/cm2

(It should be noted here, that already the unsafe laser class 3a is defined by a critical power density of I c=25W/m2).

A typical front-projector delivers 1000lm to the scre...