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. illuminance in lm/m2 (lux). However, if designing illumination from many point light sources, like light bulbs, of known approximate omnidirectionally-uniform intensities, the contributions to illuminance from incoherent light being additive, it is mathematically estimated as follows. If r"i" is the position of the "i"-th source of uniform intensity "Ii", and â is the unit vector normal to the illuminated elemental opaque area "dA" being measured, and provided that all light sources lie in the same half-space divided by the plane of this area,