Radiometry and photometry

Radiometry means the measurement of radiation. Photometry is a modified radiometry when wavelength is weighted by how sensitive the human eye responses.

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.