For the purpose of measuring illumination, the candela is not a practical unit, as it only applies to idealized point light sources, each approximated by a source small compared to the distance from which its luminous radiation is measured, also assuming that it is done so in the absence of other light sources. What gets directly measured by a light meter is incident light on a sensor of finite area, i.e.
For measuring illumination, the candela is not a practical unit because it only applies to idealized point light sources, each approximated as small compared to the distance from which its luminous radiation is measured, also assuming that it is done so in the absence of other light sources. What is directly measured by a light meter is incident light on a sensor with finite area.