Both radiometry and photometry have their own tricky/bizarre names for basic quantities:
1. Radiometry quantities:
| Name | Other name | Units | Symbol |
|---|---|---|---|
| Energy | joule (J) | Q | |
| Flux | Radiant power | watt (W) | Φ |
| Irradiance | Flux Density | W/m2 | |
| Radiant exitance | W/m2 | M | |
| Radiant incidance | W/m2 | E | |
| Radiant intensity | W/Sr | I | |
| Radiance | W/(m2Sr) | L |
Descriptions:
Flux (radiant power): time rate of change of energy.
Irradiance: areal density of power. I noticed more and more scientific articles have used this term to replace "intensity", which is good.
Radiant intensity: power per unit solid angle.
Radiance: power per unit projected area per unit solid angle. (This is equivalent to brightness often used by laser people).
2. Photometry quantities:
Same as radiometric quantities but units have different names:
| Radiometric name | Photometric name | Radiometric units | Photometric units |
|---|---|---|---|
| Energy | joule (J) | ||
| Radiant power (flux) | Luminous power (flux) | watt (W) | lumen (lm) |
| Irradiance | Illuminance | W/m2 | lm/m2 = lux (lx) |
| Radiant intensity | Luminous intensity | W/Sr | lm/sr = candela (cd) |
| Radiance | Luminance | W/(m2Sr) | lm/(m2Sr) = cd/m2 = nit |
Descriptions:
Candela: (unit of luminous intensity) one of the seven base units of the SI system. If a monochromatic 555nm source emits 1 W per steradian at a given direction, then at that direction the luminous intensity is 683 candelas (or 683 lm/sr). 555 nm is the wavelength that human eye has the max spectral responsivity.
Lumen: (unit of luminous power) For an isotropic source having 1 candela luminous intensity, the total luminous power emitted is 4π. If a source is not isotropic, one needs to measure the luminous intensity in many directions using a goniophotometer, and then numerically integrate over the entire sphere.
Lux: (unit of illuminance) = lm/m2. Most light meter measures this quantity.
3. Conversion between Watt and Lumen
The simplest thing to remember is:
1 Watt = 683 Lumens @555 nm.
At other wavelengths, this value is smaller and we need to multiply the eye's spectral response curve V(λ).
References:
[1] There is an excellent article on this subject by late Professor James M. Palmer:
http://www.optics.arizona.edu/Palmer/rpfaq/rpfaq.pdf
[2] The book Introduction to Radiometry (SPIE Optical Engineering Press 1998) by William L. Wolfe is a full-range detailed reference. I found some slight differences between the two references on quantity naming. This means that this subject is still somehow not coordinated.
