Editing Candela (section)

== SI photometric light units ==
{{SI light units}}

=== Relationships between luminous intensity, luminous flux, and illuminance ===
If a source emits a known luminous intensity {{math|''I''<sub>v</sub>}} (in candelas) in a well-defined cone, the total [[luminous flux]] {{math|''Φ''<sub>v</sub>}} in [[lumen (unit)|lumen]]s is given by
<math display="block">\Phi_\mathrm{v} = I_\mathrm{v} 2 \pi [1 - \cos(A/2)]</math>
where {{math|''A''}} is the ''radiation angle'' of the lamp—the full vertex angle of the emission cone. For example, a lamp that emits 590&nbsp;cd with a radiation angle of 40° emits about 224&nbsp;lumens. See [[MR16]] for emission angles of some common lamps.

If the source emits light uniformly in all directions, the flux can be found by multiplying the intensity by 4{{pi}}: a uniform 1&nbsp;candela source emits 4{{pi}} lumens (approximately 12.566&nbsp;lumens).

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/m<sup>2</sup> (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 {{math|'''r'''<sub>''i''</sub>}} is the position of the ''i''th source of uniform intensity {{math|''I<sub>i</sub>''}}, and {{math|'''â'''}} is the unit vector [[normal (geometry)|normal]] to the illuminated elemental opaque area {{math|''dA''}} being measured, and provided that all light sources lie in the same half-space divided by the plane of this area,
<math display="block"> \text{illuminance at point } \mathbf{r}\text{ on } dA\text{, } E_\mathrm v(\mathbf{r}) = \sum _{i}{ \frac{|\mathbf{\hat{a}}\cdot(\mathbf{r}-\mathbf{r}_i)|}{|\mathbf{r}-\mathbf{r}_i|^3} I_i }.</math>
In the case of a single point light source of intensity ''I<sub>v</sub>'', at a distance ''r'' and normally incident, this reduces to
<math display="block"> E_\mathrm{v} (r) = \frac{I_\mathrm v}{r^2}.</math>