Color Rendering Index

The colors of objects can be seen only if a suitable illumination is provided. The most basic condition for color perception is that the incident light contains a substantial range of spectral components, which are associated with different colors. Usually, one will use white light, e.g. daylight, being a mixture of all visible spectrum components, or some artificial white light source. The color balance of light which is transmitted or reflected (or scattered) at colored objects is then modified according to the wavelength-dependent transmittance or reflectance. Thus, the transmitted or reflected light produces a color impression, while the incident white light did not when directing reaching the eye. For example, if an object strongly absorbs the shorter-wavelength components of the incident light (in the blue and violet region), the transmitted or reflected light will be primarily consisting of the longer-wavelength components, which create a red or orange appearance of the object. Similarly, a green appearance arises if the medium wavelength components of the white light are preserved while the longer- and shorter-wavelength components are attenuated.

Most serious problems with color perception arise if the illumination light is not white. An extreme example case is the illumination with a low-pressure sodium vapor lamps, which emits only yellow light – not far from monochromatic light (single-color light). In such a case, no illuminated object can ever appear green or blue or deep red, because light with such colors is simply not present in that situation (except if the object exhibits fluorescence).

Less severe color perception problems occur if the illumination light contains a wide, but not continuous distribution of wavelength components – for example, if the optical spectrum of the light source consists only of some limited number of spectral lines. The illumination light may then still appear white, e.g. quite similar to daylight, and illuminated objects can appear with various colors. However, the color impression may then substantially deviate from that for illumination with broadband (continuous-spectrum) light.

As an example, consider an object which efficiently scatters most wavelength components while strongly absorbing a narrow range of wavelength components e.g. in the blue spectral range. If the light source happens to emit blue light only within that strongly absorbing region, a strong color impression (e.g. deeply saturated orange) can occur. In daylight, however, that orange color would be much less saturated, since only a small part of the incident blue light can be absorbed due to the narrow absorption bandwidth.

Another object, where the absorption in the blue region is more broadband but outside the emission lines of the light source, may appear white under the artificial illumination, since it cannot absorb any of the incident light, while under daylight it appears colored. Such color deviations are what is meant with a limited color rendering capability of such light sources.

Such types of color perception problems occur for various artificial light sources, in particular for the following types:

• Fluorescent lamps exhibit some combination of spectral lines (often above some continuum), determined by the used type of phosphor. Earlier models, containing a phosphor with light emission essentially only in two different wavelength regions, had quite poor color rendering properties. Nowadays, a triphosphor mixture is common; this has much improved color rendering and also an improved luminous efficacy. There are special phosphor mixtures with even better color rendering, but a reduced efficacy.
• Some other types of gas discharge lamps also emit light with the spectrum featuring prominent peaks, but depending on the type of lamp, there can also be a broad continuum – particularly for high pressure discharge lamps. For example, high intensity discharge lamps based on xenon have a quite smooth emission spectrum, causing minimal color deviations.
• White light-emitting diodes (LEDs) also have a quite structured spectrum – often a combination of blue light, which is directly emitted by the LEDs, with fluorescence e.g. in the yellow spectral region from a phosphor which is excited by absorbing part of the blue light. Just as for fluorescent lamps, there are different LED designs, using different phosphors which can significantly vary in terms of color rendering.

How serious color perception problems are in practice, depends on three aspects:

• on the details of the optical spectrum of the light source,
• on the spectral properties of the illuminated objects (e.g. whether they exhibit narrow absorption features), and
• on how color-critical the application is.

For example, color details are often not particularly important for lighting of offices and manufacturing workplaces, while they can be critical in the context of fashion or arts. Fortunately, the spectral absorption features of most colored objects are relatively broadband, so that really severe color deviations are rare, even for illumination with light having a quite structured optical spectrum.

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