An understanding of some of the fundamental terms in lighting calculations is a good principle to good design practice.

Intensity: The candela is the unit of intensity. It is sometimes called "candlepower" and describes the amount of light in a unit of solid angle; assuming a point source of light.

I= light energy/ solid angle

Illumination: is the intensity, in candelas, divided by the square of the distance of the source from the surface. It is measured in footcandles,

E = (I/d^2)* cos0, where 0 = angle of incidence

Light Quantity is expressed in lumens and is measure of the total light emitted by a source or light falling on a surface.

Example
A 40 watt fluorescent lamp emits 3000 lumens; only 5 watts as light, the balance as heat. Therefore,

Flourescent efficacy = 5w/40w = 12.5 %

E= LxCxd/A (fc)

where
(fc) = footcandles
L= lumens generated
A= area lighted
C= coefficient of utilization, d= Depreciation for the lamp.

Some useful links about lighting calculation