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.
The candela, used for measuring illumination, isn't practical because it only applies to idealized point light sources, which are small compared to the distance from which their light is measured. It also assumes no other light sources are present. A light meter measures the actual incident light on a sensor with finite area.