[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/

[Manish Jain]

Luminous flux 
Luminous flux is a measure of the total amount of visible light that is emitted by a light source in all directions. It is the measure of the perceived power of light, weighted by the sensitivity of the human eye to different wavelengths of light. Luminous flux is measured in lumens (lm) and is a useful metric for comparing the brightness of different light sources.

For example, a 60-watt incandescent light bulb typically emits around 800 lumens of light, while a 10-watt LED light bulb can emit the same amount of light or more, depending on its efficiency. However, the quality of the light emitted by different light sources can also affect their perceived brightness, even if they have the same luminous flux.


Luminous intensity
Luminous intensity is the amount of light that is emitted in a particular direction from a light source. It is measured in candelas (cd), which is the unit of luminous intensity in the International System of Units (SI).

To understand this better, imagine a light source that emits light in all directions equally. The luminous intensity of the source would be the same in every direction, and would be equal to the total luminous flux (measured in lumens) divided by the solid angle (measured in steradians) over which the light is emitted.

For example, a 1000-lumen light source that emits light uniformly over a solid angle of 1 steradian would have a luminous intensity of 1000 candelas (cd). Similarly, a flashlight that produces a highly directional beam of light would have a higher luminous intensity than a light bulb that emits light in all directions.


Illuminance 
Illuminance is the measure of the amount of light that falls onto a surface per unit area. It is measured in lux (lx), which is the SI unit of illuminance.

Illuminance depends on the distance between the light source and the surface, as well as the intensity and directionality of the light. For example, a light source that is closer to a surface will produce a higher illuminance than the same light source that is farther away.

To calculate illuminance, the luminous flux (measured in lumens) of the light source is divided by the surface area (measured in square meters) that the light falls onto. This gives the illuminance in lux.

For example, a 1000-lumen light source that illuminates an area of 10 square meters would produce an illuminance of 100 lux. Illuminance is an important parameter for designing lighting systems, as it determines the amount of light that is available for a given task or activity.


Luminance
Luminance is a measure of the brightness of a surface that is illuminated by a light source. It is the amount of light that is emitted, reflected or transmitted by a surface per unit area, in a given direction. Luminance is typically measured in candelas per square meter (cd/m²) and is an important parameter for assessing the visibility and quality of images and displays.

Luminance is influenced by a variety of factors, including the intensity and color of the light source, the reflectivity of the surface, and the angle at which the surface is viewed. For example, a surface that reflects more light will have a higher luminance than a surface that reflects less light. Similarly, a surface that is viewed from an oblique angle will appear less bright than when viewed from directly in front of it.

Luminance is used in many applications, including the design of displays, computer graphics, and video production. It is also an important parameter for assessing the safety of roadways and outdoor lighting, as high luminance levels can cause glare and reduce visibility.