Definition related to Lighting
(a) Radiant energy : The body which is at a higher temperature than its surrounding medium radiates out energy in to the medium. At low temperatures the radiation is in the form of heat waves, but when it becomes red-hot, it emits light waves in addition to heat waves. At about 3000°C (working temperature of filament lamp) the wavelength of radiation is 0.4 x 10-6m to 0.7 x 10-6m. The wavelengths between 4000 x 10-8cm to 7500 x 10-8cm can produce the sensation of sight.
The wavelength is usually represented in the following results :
1 micron = 10-6m metre
1 A = 10-8cm
Colour : When a body heated and if it radiates out only one wavelength, then the colour of radiation will be as below for different wavelengths.
|
Wavelength |
Colour |
|
4000 |
Violet |
|
4750 |
Blue |
|
5500 |
Green |
|
6000 |
Yellow |
|
97000 |
Red |
(b) Radiant efficiency : A body when heated to a temperature, of incandescent, radiates out light energy as well as heat energy. Hence, radiant efficiency is defined as
Radiant efficiency = Energy radiated in the form of light / Total energy radiated by body
(c) Light energy : It is the radiant energy in the form of waves which produces a sensation of vision up on the human eye. It is expressed as “Lumen-Seconds” or “Lumen-hours”.
(d) Luminous flux (Φ) : It is defined as the total quantity of light energy in the form of light waves radiated per second from the luminous body. Its unit is “Lumens”.
(e) Lumen : It is a unit of flux and may be defined as the luminous flux per unit solid angle from a source of one candle power. or Lumens = Candle power x Solid angle or Lumens = C.P. x ω. The total flux emitted by a sou source of 1 C.P.is 4π lumens.
 |
| Figure A |
(f) Plane angle : When two straight lines lying in the same plane meet at a point they make an angle at the meeting point. This angle (θ) is termed as plane angle. This angle in radians is given by
Radians (θ) =ARC / Radius
 |
| Figure B |
(g) Solid angle : The angle subtended at a point ‘O’ in space by any area say A is termed as solid angle. In case of solid angle, the volume is enclosed by numerous lines lying on the surface and meeting at a point. It is represented by letter ω and is measured in streadian, where solid angle ω is equal =Area subtended/(Radius)2 = A / R2. In case of sphere, ω =4πR2 / R2 = 4 π
∴ Solid angle of a sphere = 4π steradian.
 |
| Figure C |
(h) Candle-power : It can be defined as the number of lumens emitted in a unit solid angle in a given director or Candle power = Lumens / Solid angle.
(j) Illuminance or Illumination or Degree of illumination : When the light falls on a surface it is said to illuminated. The illuminance is defined as the luminous flux received per unit area. If a light flux d, f falls on area dA, then the illuminance = dF / dA = Lumens / Area or illuminance from a source placed at the centre of a sphere =C.P. x W / Area. where, W represents solid angle subtended by the area illuminated and C.P. the candle power of source.
 |
| Figure D |
(k) Lux or Metre candle : It can be defined as the “Luminous flux falling/sq.metre area from the source of one candle power and placed one metre away from the surface. The rays being perpendicular to the surface. It can also be defined as the illumination of the inside of a sphere of radius one metre at the centre of which is placed a source of one candle-power .
(l) Candela : It is the unit of Luminous intensity. It is equal to 1th / 60 of the luminous intensity per Cm2, radiated by a black body radiator kept at the temperature of solidification of platinum i.e. 2043 K.
(m) Brightness : It is defined as the luminous intensity per unit projected area of the surface in the given Luminous intensity direction. Brightness or illuminance = Luminous intensity / Area. It is measured in candela/sq.metre.
(n) Nit : It is unit of illuminance, and can be defined as illumination of one candle per square metre.
(o) Stilb : It is a bigger MKS unit of illuminance, and is equal to illumination of one candle/sq.cm /.
(P) Polar Curves : In most sources of light (lamps), the luminous intensity is not same in all directions. If we measure the candle power in horizontal plane about a vertical axis and plot a curve between candle power and the angular position a horizontal polar curve is obtained. If the candle power is measured at angular positions in a vertical plane a polar curve in the vertical plane is obtained. The two types of polar curves for a filament lamp are shown in Figure E.
 |
| Figure E |
(q) Mean Horizontal Candle Power : The mean of candle powers in all directions in a horizontal plane passing through the source of light is called as “Mean Horizontal Candle Power (M.H.C.P.)” of the source.
(r) Mean Spherical Candle Power : If the mean of candle powers is taken in all planes in all directions it will give the Mean Spherical Candle Power (M.S.C.P.) of the source.
M.S.C.P. = Luminous flux(Φ) / Solid angle (4π) in lumens.
(s) Mean Hemi-Spherical Candle Power : It is the mean of the candle powers below a horizontal plane passing through the light source.
(t) Reduction Factor : It is the ratio of M.S.C.P. to M.H.C.P.i.e. Reduction factor M.S.C.P./M.H.C.P.