Definitions Related to Illumination

What is Illumination?

The luminous flux received by the surface per unit area is known as illumination. It is denoted by letter 'E' and is measured in Lux. Mathematically, illumination is given by,

$$\mathrm{\mathit{E}\:=\:\frac{\mathrm{Luminous\: flux}}{Area}}$$

In this article, let's check some of the definitions related to illumination.


The luminous intensity per unit surface are of the projected surface in the given direction is termed as brightness of that surface. It is denoted by the letter 'L' and measured in candela/m2.

Mathematically, if the luminous intensity of the source is 'I' candela on area 'A'. Then, the projected area will be Acosθ. Therefore, the brightness of the surface is given by,

$$\mathrm{\mathit{L}\:=\:\frac{\mathit{I}}{\mathit{A}\mathrm{cos\:\theta }}}$$

Mean Horizontal Candle Power

The mean (or average) of candle power of source in all directions in a horizontal plane is known as mean horizontal candle power (MHCP).

Mean Spherical Candle Power

The mean (or average) of candle power in all directions and in all planes from the source of light is termed as mean spherical candle power (MSCP).

Mean Hemi-Spherical Candle Power

The mean (or average) of candle power in all directions above or below the horizontal plane passing through the source of light is called the mean hemi-spherical candle power (MHSCP).

Reduction Factor

The ratio of the mean spherical candle power to the mean horizontal candle power of a source of light is known as reduction factor of the source of light, i.e.,

$$\mathrm{\mathrm{Reduction\: factor}\:=\:\frac{\mathrm{MSCP}}{\mathrm{ MHCP}}}$$

Lamp Efficiency

The ratio of the luminous flux to the power input is known as lamp efficiency. The lamp efficiency is expressed in lumens per watts.

$$\mathrm{\mathrm{Lamp\: Efficiency}\:=\:\frac{\mathrm{Luminous\: flux}}{\mathrm{ power\:input}}}$$

Specific Consumption

The ratio of electric power input to its average candle power is known as specific consumption, i.e.

$$\mathrm{\mathrm{Specific\: consumption}\:=\:\frac{\mathrm{Input \:electric \:power}}{\mathrm{Average\: candle\: power}}}$$

Space to Height Ratio

It is defined as the ratio of horizontal distanced between adjacent lamps to the height of their mounting.

Utilization Factor

The ratio of total number of lumens reaching the working plane to the total number of lumens emitting from the source is known as utilization factor, i.e.

$$\mathrm{\mathrm{Utilization\: factor}\:=\:\frac{\mathrm{Total\: lumens\: reaching\: the\: working\: plane}}{\mathrm{Total\: lumens\: emitting\: from\: the\: source}}}$$

Maintenance Factor

The ratio of illumination under normal working conditions to the illumination when everything is clean is known as maintenance factor, i.e.

$$\mathrm{\mathrm{Maintenance\: factor}\:=\:\frac{\mathrm{Illumination\: under\: normal\: working \:conditions}}{\mathrm{Illumination\: under\: everything\: is\: clean}}}$$

Normally, the value of maintenance factor is less than unity and in practice, it will be around 0.8. From the definition, it is clear that the frequent cleaning of the lamp improves the maintenance factor.

Depreciation Factor

The ratio of initial illumination to the ultimate maintained illumination on the working plane is known as depreciation factor. Mathematically, it is defined as the reciprocal of maintenance factor, i.e.

$$\mathrm{\mathrm{Depreciation\: factor}\:=\:\frac{\mathrm{1}}{\mathrm{Maintenance \:factor}}}$$

The value of the depreciation factor is always greater than unity.

Waste Light Factor

When a surface is illuminated by several number of sources of light, then certain amount of light is wasted due to overlapping of light waves. The measure of this wastage of light is termed as waste light factor.

Reflection Factor

When the light rays incident on a surface, some of the rays are reflected from the surface and a portion of the incident light is absorbed by the surface.

Thus, the ratio of luminous flux leaving the surface to the luminous flux incident on the surface is termed as reflection factor and is given as

$$\mathrm{\mathrm{Reflection\: factor}\:=\:\frac{\mathrm{Reflected \:light}}{\mathrm{Incident \:light}}}$$

The reflection factor is also known as coefficient of reflection and its value is always less than unity.

Beam Factor

The ratio of the lumens in a beam of a projector of the lamp to the lumens given out by the lamp is known as beam factor. The value of beam factor varies from 0.3 to 0.6.

This factor is used to determine the absorption of light by the reflector and front glass of the projector lamp.