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Encyclopedia of Laser Physics and Technology

© Dr. Rüdiger Paschotta

Last update: 2008-07-21

Definition: source of red, green and blue light, which is usually provided in the form laser beams

An RGB source is a light source which emits at the same time red, green and blue light. Such sources are required mainly for color display applications. A wide range of colors can be obtained by mixing different amounts of red, green and blue light (additive color mixing). A possible combination of wavelengths is 630 nm for red, 532 nm for green, and 465 nm for blue light.

Many currently used projection displays (“beamers”) are based on an arc lamp, combined with various color filters. While an arc lamp is much cheaper than a laser source of comparable output power, it does not allow for a high wall-plug efficiency, it has a limited lifetime, and the poor spatial coherence of the output imposes restrictions on the achievable image quality. Also, the available color space is not very large. Therefore, laser sources are under investigation that could offer a wider color space, much better spatial coherence (beam quality), and higher power efficiency. Such RGB laser sources emit light in the form of laser beams – either one beam for each color, or all colors combined in one beam.

Color Perception and the Chromaticity Diagram

Color perception is not just an issue of physics, but depends on the workings of our eyes. The retina of the human eye contains just three different receptors with different sensitivity to light at different wavelengths. (There is actually a fourth kind of receptor with yet another spectral response curve, but this is used only for vision at very low light levels.) The perception of colors is based on registering the relative strength of excitation of the three types of receptors. Therefore, the perceived colors are projected to an only three-dimensional space. (Most mammals actually use only two different color receptors, shrinking the color perception even to two dimensions.) As a result, all perceived colors can be located in a two-dimensional chromaticity diagram as shown in Figure 1, if the possibility of varying the optical intensity of each color is ignored.

Figure 1: Chromaticity diagram according to CIE 1931. Monochromatic light with different wavelengths corresponds to the edge of the colored area (wavelength values in nanometers are indicated). Of course, the colors can not be correctly reproduced, because no screen and no printer can reproduce the whole range of colors.

In the case of laser-based RGB sources, the red, green and blue beams usually have a relatively narrow optical bandwidth, i.e. they are close to monochromatic. In the chromaticity diagram, such beams are associated with points at the border of the colored region. (The indicated numbers correspond to the wavelengths in nanometers.) By mixing, e.g., three monochromatic beams, all those colors can be reproduced which lie in the triangle spanned by the corresponding points. For comparison, the accessible color range for a cathode tube display, a lamp-based beamer or a color printer is a triangle with the edges lying well inside the diagram, so that the spanned area is usually much smaller. A significant further improvement of a laser-based projection system would be to add a fourth wavelength.

Color Rendering and Luminous Efficiency

The choice of the three wavelengths of an RGB source determines the so-called color rendering index (CRI), which is a quantitative measure of how well it is possible to judge the color of objects when these are illuminated with white light consisting (in our case) of the three spectral components. Further, the choice of wavelengths influences the possible luminous efficiency (also called efficacy) of the source, and the two aspects can be in conflict with each other. For example, a long wavelength of the red beam is favorable in terms of color rendering, but tends to diminish the luminous efficiency because the human eye's sensitivity drops rapidly with increasing wavelength in that region.

The reason why good color rendering is more difficult with pigments (e.g. of paints, dyes, or inks) than with laser beams is essentially that pigments work on the basis of subtractive color mixing, and this approach does not allow the combination of relatively bright colors with narrow optical bandwidths: if a pigment were to generate a very “pure” color (close to monochromatic light), it would have to absorb all light outside a narrow wavelength band, and leave only a tiny portion of the incident optical power. Conversely, a bright yellow pigment, for example, has to transmit not only yellow, but also a lot of red, orange and green light, as it could otherwise not look bright. Such a mixture of different wavelengths, however, cannot correspond to a point in the color diagram which is close to the boundary, where the monochromatic beams are located.

Note that the availability of a practical laser-based RGB source would not automatically make it possible to realize projection displays for entertainment with a faithful color rendering in a very wide color space. Video material covering that wide color space would also be required. Currently used videos are based on a smaller color gamut, so that color information is lost. The small color gamut of the video may be transformed into a larger gamut for the display, but in that case the color rendering will not be faithful.

Required Power Level and Beam Quality

Optical powers below 1 W per color are sufficient for indoor digital projection displays (with not too much ambient light) with a projection area of up to a few square meters. Cinema projectors require 10 W per color or more. Suitable projector technology exists, but the RGB sources have not yet reached sufficient power, maturity and cost effectiveness, e.g. for application in cinemas. A smaller market with lower sensitivity to cost is the area of flight simulators.

In most cases, RGB sources need to have a reasonable or even good beam quality, e.g. if the display contains a projector which scans the displayed pixels point by point; here, a sufficiently small pixel size (for high resolution) requires a high beam quality. For other projector types, the beam quality demands may be lower.

Types of Laser-based RGB Sources

Different approaches can be taken to construct RGB sources. One possibility is to use three lasers, each emitting light with the wanted color. The problem is, however, that visible lasers are severely limited in performance compared with their near-infrared counterparts. Therefore, it can be better to use near-infrared lasers combined with nonlinear crystals for nonlinear frequency conversion. For example, green light can be generated by frequency doubling the output of a Nd:YAG or Nd:YVO₄ laser emitting at 1064 nm. For moderate power levels of the order of 1 W per color, intracavity frequency-doubled VECSELs are attracting increasing interest, because they have the potential for cost-effective mass production.

It is also possible to start with a single near-infrared laser and generate all three colors through a combination of nonlinear frequency conversion stages. For example, it is possible to start at 1064 nm, generate the 532-nm green light by frequency doubling, use part of it to pump an optical parametric oscillator emitting a signal e.g. at 631 nm (red output) and an idler at 920 nm. The latter can be frequency doubled to obtain blue light at 460 nm. There are, however, many other schemes, involving different combinations of parametric oscillators, frequency doublers, sum frequency mixers, and the like. The evaluation and comparison of such conversion schemes are a complex task, since the performance of the different nonlinear conversion stages depends not only on the crystal materials but also on optical (peak) powers, pulse durations, and phase-matching issues.

Bibliography

See also: nonlinear frequency conversion, red lasers, green lasers, blue lasers, visible lasers

Categories: lasers, nonlinear optics