It all boils down to the concept of color temperature. Color temperature is a way of measuring the quality of a light source. It is based on the ratio of the amount of blue light to the amount of red light, and the green light is ignored. The unit for measuring this ratio is in degree Kelvin (K). A light with higher color temperature (i.e., larger Kelvin value) has "more" blue lights than a light with lower color temperature (smaller Kelvin value). Thus, a cooler (warmer) light has a higher (lower) color temperature. The following table shows the color temperature of some light sources.