When a body is gradually heated above room temperature, it begins to radiate energy in the form of electromagnetic waves of various wavelengths into the surrounding medium. The type of this radiant energy is determined by the hot body’s temperature.

The most common method for making artificial light involves heating a solid mass or vapour to incandescence. It has been discovered that once a body warms above room temperature, it begins to radiate energy in the form of electromagnetic waves of various wavelengths into the surrounding medium. The type of this radiant energy is determined by the hot body’s temperature.

Light is thus a component of radiant energy that travels as a wave motion through ether at a rate of approximately 3×10^8 m/sec. The wavelengths that can create a visual sense range from 4×10-5 cm to  7.5×10-5  cm. Another metric for describing light wavelength is the Angstrom Unit (1 A.U. = 10^ -8 cm= 10^ -10   m) . As a result, the visible radiation ranges from 4000 to 7500 AU. A wavelength of 6000 AU typically produces  the  combination of yellow and 4000 AU produces the colour violet

Important Definitions

Plane Angle
  • An angle is contained by two straight lines in the same plane and subtended at a point is called Plane Angle.
  • It is commonly measured in degrees or radians and is symbolised by the Greek character ‘θ’ (theta).
Solid Angle
  • An area subtends a solid angle (ω), which is the angle enclosed in the volume generated by an infinite number of lines lying on the volume’s surface and meeting at the point, is called Solid Angle.
  • It is measured in steradian and is commonly indicated by the symbol ‘ω’.

Relationship between plane angle (θ) and solid angle (ω) is –  ω=2π (1-cosθ/2 )

Luminous Flux
  • The light energy radiated out every second from the body in the form of luminous light waves is known as luminous flux.As a result, it is the rate at which energy is radiated in the form of light. It’s a unit of energy per second (therefore it’s comparable to Power).
  • UnitLumen
  • 1 lumen=0.0016 watt (approx) or 1 watt=625 lumen (approx)


  • The light flux emitted in a unit solid angle by a source of one candle power is defined as a lumen.
  • Lumen=candle power x solid angle= cp x ω
Luminous Intensity
  • The light flux emitted by the source per unit solid angle, measured in the direction in which the intensity is required, is the luminous intensity in any given direction.
  • Unit – Candela (cd) or lumens per steradian
  • if, F = luminous flux, ω = solid angle, I = luminous intensity  then,    I = F/ ω 
  •  A surface is said to be illuminated when luminous flux falls on it. This phenomenon is called Illumination.
  • The luminous flux received by a surface per unit area is known as illumination.
  • UnitLux or metre-candle or
    lumens / m^2
Mean horizontal candle power: (M.H.C.P)

It’s the average of candle powers in all directions in the horizontal plane that contains the light source.

Mean spherical candle power: ( M.S.C.P)

It is defined as the average of candle powers in all directions and planes from the light source.

Mean hemi-spherical candle power: (M.H.S.C.P)

It’s the average of candle powers in all directions above and below the horizontal plane flowing through the light source.

Reduction factor

The ratio of its mean spherical candle power to its mean horizontal candle power is called reduction factor.

  •     Reduction factor = M.S.C.P./M.H.C.P.
  • The luminous intensity per unit projected area of the surface in the given direction is defined as the brightness of a surface.
  • Unit – Lambert
  • The iris of the human eye regulates the size of the pupil’s opening. When the eye is exposed to a particularly strong source of light, the iris contacts to produce the amount of light admitted and prevent retinal damage. This minimises the risk of retinal damage.
  • Other things in the area of vision can only be seen imperfectly due to the sensitivity.
  • In other terms, glare might be defined as a bright area within a person’s range of vision character as a factor irritation discomfort eyesight obstruction
Space height ratio

The ratio of the distance between adjacent lamps to the height of their mountains is called Space Height ratio.

Maintenance factor

The maintenance factor is the ratio of illumination under typical working conditions to illumination when everything is totally clean.

Utilization factor or co-efficient of utilization

The ratio of total lumens reaching the working plane to total lumens given out by the lamp is called Utilization factor.

Depreciation factor

The ratio of the starting meter-candles to the ultimate maintained meter-candles on the working plane is defined as the reverse of the maintenance factor, is called Depriciation Factor.

Illumination Chart

Residential Commercial
Bathroom300Jewellery Work700-800
Drawing Hall300Laboratories300-400
Living300Reading table300-350

Laws of Illumination

Mainly there are two laws of illumination-

The illumination of a surface is inversely proportional to the square of the distance of the surface from the source of light, this is called Inverse square law.

S – a point source of luminous intensity

I – luminous flux

r – distance of parallel plates

the luminous flux emitting from source crossing the three parallel plates which are separated by a distances of r, 2r, and 3r from the point source respectively as shown in Fig.

according to the law –

  E 1/r^2 

or, E = I/r^2

This law asserts that, the illumination at any point on a surface is directly proportional to the cosine of the angle between the line of flux and the normal at that point.

E – illumination

θ – incident angle

according to the law –

E ∝ cosθ

so, merging two laws –

E =( I/r^2 )cosθ
Light source and their Luminous efficiency
                 Type of light source       Typical Luminous Efficiency (lm/W)
Incandescent bulb8-18 
Fluorescent Lamp46-60
Mercury Vapour Lamp44-57
Sodium Vapour Lamp (Low
Pressure or LPSV)
Sodium Vapour Lamp (High
Pressure or HPSV)
Metal Hallide 60-80 
LED 30-50
Best LED105
night lamp5-7

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