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 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. High CRI number is good; it means colors will look right and real. Low CRI number is bad; it means colors will look wrong and weird.