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A candela is the SI base unit of luminous intensity. It measures how bright something is. It is one of the basic units of science. A light bulb is bright, but the Sun is much brighter, so the Sun has more candelas than the light bulb.

The frequency chosen for measuring candelas is in the visible spectrum near green. This is a wavelength of about 555 nanometres. The human eye is most sensitive to this frequency, when adapted for bright conditions.