[harshita]

Basic Terminology and Definitions


In illumination, four main units are used to describe light and its effects
Luminous flux : Luminous flux (Φ) is the quantity of the energy of the light emitted per second in all directions. unit :- lumen (lm).
Luminous  intensity : Luminous intensity (I) is the ability to emit light into a given direction, or it is the luminous flux that is radiated by the light source in a given direction within the unit of the spatial angel.  unit :-candela.
Illuminance : Illuminance (E) is the total luminous flux incident on a surface, per unit area. unit :- lux (lx). 
Luminance : Luminance (L) is the luminous intensity emitted by the surface area of 1 cm² (or 1 m²) of the light source. unit :-candela /m2

E=Φ/A
A= Illuminated surface (m2)
Φ = Luminous flux
E = Illuminance

L=I/A
A = Illuminated surface (m2)
I = Luminous intensity
L = Luminance

SOURCE: ZUMTOBEL LIGHTING HANDBOOK

Ω = solid angle into which luminous flux is emitted
A = area hit by luminous flux

All four Light units can be described as psychophysical as they are combination of human response and physical units of power.

luminaire efficiency

Luminaire Efficiency are quoted in terms of light output ratio ( LOR )
Downward Light output ration (DLOR) = Downward lumen output of luminarire / Total Lamp Lumen output
Upward LIght output ratio (ULOR) = Upward lumen ouput of luminaire / Total Lamp Lumen output
Light output ratio (LOR) = DLOR + ULOR

To know more about illumination read .... http://frontdesk.co.in/building-science/illumination-in-architecture/