A candela is a measurement of the intensity of light, used in the International Standard (SI) measurement system. In the modern age, it is technically defined as the intensity in a given direction of a source that emits monochromatic radiation of a frequency of 540 x 10^12 hertz and which has a radiant intensity in the same direction of 1/683 watts per steradian.

The candela as a unit of measurement grew out of the desire to adopt a standard method to measure light. In the 19th century, each country had its own measure of light, most extremely difficult to reproduce with accuracy. At the beginning of the 20th century, groups of nations began adopting new standards, such as the international candle, which were based on a single type of lamp and much more consistent. Eventually, in the 1930s, it was decided that a more precise definition was needed.

Initially, the candela was defined as the luminous emission of a Planckian radiator, a type of blackbody, at the temperature of freezing platinum (2045 K). This correlated roughly to the light emitted by a typical candle, making it a suitable measure. In the late 1970s, it was determined that the experimental difficulties in creating a Planckian radiator at such high temperatures made the existing definition of candela less than desirable. The new breakthroughs in radiometry allowed scientists to have a more specific definition, and so the current hertz/watts definition of candela was adopted.

One reason the watt was not initially tied to the candela is the eye's differences in seeing various wavelengths of light. Since the human eye is less observant of blue and red light, more wattage is needed to produce the same result in the brain as with yellow or green light. This is why the definition of candela is defined as a very specific hertz, unachievable in actual common lighting. There are those who critique the modern definition of the candela for this reason, arguing that it has removed any common-sense understanding of the candela as an actual emission of light.

The reason the modern definition of candela includes the somewhat bulky 1/683 watts per steradian is to make it conform to the previous definition, in order to avoid any confusion when comparing the modern unit to the historical candela. In this context, the steradian is the cone of light as it emerges from the source, such that it would light up one meter square of the inside of a sphere with a one meter radius.

Since the precise definition is so unwieldy, it is not uncommon in standard usage to see the candela referred to as "roughly" the amount of light generated by a single candle. This definition is perfectly suitable for common usage, as is the translation of 120 candela to roughly the light emitted by a 100 watt light bulb.