The color rendering index (CRI) is an international measurement scale that describes how colors are rendered under an artificial source of light. The standard against which artificial lighting is compared is daylight, because daylight renders the widest variety of colors. Artificial lighting, by contrast, can render very many or very few colors, depending on the nature of the light source. The color rendering index has many applications, especially in art and photography.
The appearance of color varies under different light sources because every light source emits different wavelengths of light. The visible spectrum of light that can be perceived by the human eye consists of wavelengths ranging from about 400-750 nanometers. Low-pressure sodium lamps, which are often used as streetlights, emit a monochromatic light that has a wavelength of about 589 nanometers. An ordinary incandescent light bulb, on the other hand, emits many wavelengths of light.
An object appears a certain color because it reflects certain light wavelengths, which are then perceived by the eye. A low-pressure sodium lamp emits only in a narrow range at about 589 nanometers, so only that one color can be reflected. All objects, regardless of their color in daylight, will appear yellowish under the light of the sodium lamp. Alternatively, many colors can be perceived in the light of an incandescent bulb, because the incandescent bulb emits a wide spectrum of wavelengths.
The color rendering index quantifies the ability of artificial light sources to show color. The scale ranges from 0-100, with 0 being a poor ability to show color and 100 being the best ability. A low-pressure sodium lamp has a color rendering index of 0-18, and an incandescent bulb might have a CRI of 99 or higher.
There is a difference between the color rendering index and a related measure known as color temperature. CRI describes how colors appear under a given light. Color temperature, on the other hand, describes the actual color of the light source and what kind of light it emits. Higher color temperatures emit bluer light, and lower color temperatures appear reddish. The color rendering index can be used as a basis of comparison only if the two light sources in question also have the same color temperature.
Although incandescent bulbs have a high CRI, they have a low color temperature of about 4,400 degrees Fahrenheit (2,427 degrees Celsius), as opposed to normal daylight, which has a color temperature of 7,640-8,540 degrees Fahrenheit (4,227-4,727 degrees Celsius). It therefore is more difficult to distinguish shades of blue under a light bulb than it would be under daylight. So, although an incandescent bulb has an excellent ability to render color when compared with other light sources of the same color temperature, it is not the most ideal light source overall in terms of color rendering. A better light source would have both a high CRI and a color temperature that is closer to natural sunlight.