Illumination terminologies.

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Light : 
light is defined as radiant energy from a hot body which produces the visual sensation upon human eye. 

Luminous flux (Φ) : 
It s defined as the total quantity of light energy emmited (radiated) per second from a luminous body like a lamp.

Colour vision : 
The sensation of the color is due to the difference in the wave length of the light radiations. The visible light can have wave length between 4000 A° and 7500 A°.

Luminous intensity (I) : 
The luminous intensity in the direction of the cone is the ratio of flux to the solid angle or it may be defined as the flux emitted by the source per unit solid angle. Its unit is candela or lumens per steradian.

Lumen : 
It is defined as the amount of luminous flux given out in a space represented by one unit of 'solid angle' by a source having an intensity of one candle power in all directions. Lumen is unit of luminous flux.

Candle power : 
candle power of source is defined as the number of lumens emitted by that source per unit solid angle in a given direction. The term 'candle power' is used interchangeably with luminous intensity.

Plane angle : 
It is subtended at a point and is enclosed by two straight lines laying in the same plane. It is expressed in degrees or radians, angle in degrees or radians.

Illumination [or degree or illumination or illuminance] (E) : 
It is defined as the luminous flux (number of lumens) falling on the surface per unit area.
      Illumination = flux (lumens)/area 
                           E = Φ/A

Lux or meter candle :
It is defined as luminous flux falling per unit area.
1 lux = 1 lumen per square - meter. It is the unit of illumination.

Mean horizontal candle power (MHCP) :
It is defined as the mean of the candle powers in all directions in the horizontal plane containing the source of the light.

Mean spherical candle power (MSCP) : 
It is defined as the mean of the candle power of the source of light in all the directions and in all the planes.
                            MSCP = Φ/4π

Mean hemispherical candle power (MHSCP) :
It is defined as mean of the candle powers in all directions within the hemisphere either above or below the horizontal plane.

Reduction factor :
Reduction factor of a source of light is the ratio of its mean spherical candle power to its mean horizontal candle power.
                  Reduction factor = MSCP/ MHCP

Lamp efficiency (luminous efficiency) :
It is the ratio of luminous flux  to the power input to the lamp.
      Luminous efficiency = lumens / watts

Specific consumption :
It is defined as the ratio of the power input to the average candle power.
It is expressed in watts per candela.
  Specific consumption = power input / avg. Candle power.

Brightness or luminance (L) :
It is defined as the luminous intensity per unit projected area of either a surface source of light or a reflected surface.
It is in candela/m^2.

Glare :
It can be defined as the brightness within the field of vision of such a character as to cause annoyance discomfort interference with vision or fatigue.

Space to height ratio :
It ia defined as the horizontal distance between adjacent lamps to the height of their mountings.

Utilization factor :
It is a ratio of total lumens reaching the working plane to the total lumens given out by the lamp.
This ratio is less tham unity.( Between 0.25 to 0.5 for direct lighting and between 0.1 to 0.3 for indirect lighting).

Maintenance factor :
Maintenance factor is defined as the ratio of illumination under normal working condition to the illumination when everything is perfectly clean.

Depreciation factor  :
It ia defined as the ratio of illumination when everything is perfectly clean to the illumination in Normal working condition. 
It is inverse of maintenance factor and it is more than unity.

Waste light factor  :
When so many lights sources give overlapping lights, there oughts to be wastage of light.

Absorption factor  :
It is defined as the ratio of total lumens available after absorption to the total lumens emitted by light source.
This factor may be 0.5 to 0.6 and for Clean atmosphere it is 'unity'.

Beam factor  :
This is to be considered in case of projectors where some lumens are absorbed by reflector and front glass of projector lamp. This factor ia varies between 0.3 to 0.6.

Reflection factor  :
It is defined as the ratio of reflected light to the incident light. This ratio is less than 'unity'.
      Reflection factor = reflected light /incident light 

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