The Color of Light

One aspect of lighting that often gets overlooked is bulb color temperature, which describes the color appearance of the bulb, not the object being viewed. Color temperature ranges from 2600 Kelvin for "warm" sources like incandescent lamps, to approximately 6500 Kelvin for "cool" sources like daylight fluorescent. Color temperature data is readily available from lamp manufacturers. The selection of color temperature should be considered relative to the application.

Lamps that are high to very high in color temperature (5000K and above) provide improved visual acuity compared to lower-color temperature lamps (typically 2700K) at the same light level. Visual acuity is generally defined as sharpness of vision, with normal visual acuity rated at 20/20. High color temperature lamps are rich in the blue portion of the color spectrum and have a noticeably "cool" appearance.

The Color Rendering Index (CRI) of a bulb specifies on a scale of 1 to 100 how colored objects appear under that bulb's light compared to their appearance in daylight. A CRI of 100 means no difference, while a low CRI could mean a big difference. Incandescent lamps typically have CRIs above 90, but that doesn't mean that they always give suitable results. Suppose, for example, we wish to illuminate white cabinets in a kitchen or a hospital examination room. A design objective might be to enhance the impression of whiteness, cleanliness and sterility. In this case, the color temperature of incandescents would be too low to achieve the desired effect. As a general rule, one should, therefore, select color temperature first, then select the lamp giving the optimum CRI for the application.

Most fluorescent lamps operate at 3000K to 4100K, with a CRI from the low 50s to 86, but recent technology advancements can produce fluorescents with CRIs above 90.