Color rendering index

Color rendering index (CRI) is a number rating given to light bulbs or lamps for how "right" they make colors look. This number is usually written on the side of the light bulb's box. The CRI number can be as high as 100, or as low as zero; and sometimes it can go under zero. A high CRI number is good; it means colors will look right and real. A low CRI number is bad; it means colors will look wrong and weird.

You can see CRI number printed on light bulb's box (under "Color Accuracy").

If we used prism with the lamp and smooth rainbow appears, CRI number of the lamp is probably high.

However, there is a limit to what a CRI number can say. A good CRI number can guarantee correct colors only when the light itself is white. This is because of limits on how scientists test them. Candles and incandescent light bulbs have CRI 100 but they are not white, so we cannot rely on them to make stuff show real colors.

The CRI number of daylight at noon is 100, because this is the natural way people see colors. Good fluorescent tubes or LED lamps have a CRI 80 or more. Anything white with a CRI 80 or more is good enough for homes. The orange sodium vapor lamp has CRI lower than 0, so they are only used as streetlamps.

Related pages

• Color temperature
• Spectrum