Luminous intensity is the amount of light emitted by a light source in a specific direction. The unit of luminous intensity is candela. Candela indicates, how much bright is a light source.
Candela indicated by cd and written after numeric value. Such as 100 cd, 2.000 cd, 8.000 mcd.
Light is measured by different techniques. That is why there are different measurement units. The terms such as lumen, lux, and candela can be confusing sometimes.
The difference between lumen and candela is; lumen indicates total amount of light emitted from every direction, candela indicates luminous intensity from one direction at any point. Lux indicates, the amount of light reaching the surface which means light.
Candela is basic measurement to identify the luminous intensity. It would be a correct expression to use candlepower for candela. If we do a simple comparison, it can be said that one candela is equal to the light emitted from a single candle. The origin of candela is coming from candle anyway.
Floodlight and laser pointers are focusing the light to one point. Therefore, these have high candela value. Assume that, two different light sources with same lumen value which mean two different light sources with same total light output. One has 30 degrees lens, other has 90 degrees lens. 30 degrees which collect the light with acute angle has higher candela value. Total light output will remain same if the losses on lens accepted as equal.
The distance to the light source has no effect on candela value. So, 1 meter away candela value is equal to 2 meters away of candela value.
In general lighting applications, lumen value which referred as total light output (lighting power) is taken into consideration instead of candela. In special applications such as traffic signaling, the Candela value, which expresses the intensity of light in certain directions, is important.
If we summarized the differences between terms; lumen indicates how much light the light source gives, lux indicates how bright is the illuminated surface, candela indicates the intensity of light in a certain direction.