Electric Lamps and Lighting Calculations, Definitions and Units

Impulses of radiant energy are emitted from various sources at various wavelength, some of which affect the eye, causing the sensation known as light. Light can thus be expressed in terms of radiant energy of wavelength between about 4000 and 7000 Angstrom. A given amount of such energy may cause different persons to experience light in different degrees.

The following are the principal terms and units involved in lighting applications and measurements:


Definition and units:
Luminous flux(F) : This is the rate of passage of radiant energy measured in relation to the sensation of light received by the eye.

Lumen: this is the unit of luminous flux and is equal to the light flux emits in a element solid angle by a uniform point source of light of one candle strength, i.e. the quantity of light falling on a surface of 1 sq.ft. of a sphere of radius/ ft. from a uniform point source of one candle. It is a measure of lamp output. Since the area of a sphere is 4πR2 where R is radius, it follows that a consistent tip supply of one candle power emit 4pi lumens.

Luminous Intensity(l): The luminous flux per unit solid angle emitted by appoint source of light in a given direction.

Candle : The unit of power or intensity of a source of light. There are various standards such as the standard sperm candle, Vernon Harcourt Tentate Lamp. International Candle(Bougie Decimale).

Men Spherical Candle, power(M.S.C.P): The intensity of illumination from a lamp varies in different directions. M.S.C.P. is the average value in all directions.
Since 1 c.p emits 4p lumens M.S.C.P. lumens/ 4π

Mean Hemispherical Candle-power, M.H.C.P: upper or lower. The average candle-power of a light source in all direction in a plane through centre of the source and perpendicular to its axis.

Reduction Factor(r): The ratio of M.H.C.P/ M.S.C.P

Illumination (E): The luminous flux per unit area of surface measured in foot-candles or lumens.

Foot candle: A unit of illumination; the illumination produced on the surface of a sphere at 1ft, radius from a point source of light of one candle. 1 foot candle = 1 lumen per sq.ft. or 10.764 lux(metrecandles).

Consumption of lamps: The electrical energy taken by a lamp (measured in watts).

Utilization Factor: An allowance which depends on the distribution of light from the fitting, the colour of the walls, and the proportions of the room.Depreciation Factor: The Proportion of the initial light which can be assumed after lamps have deteriorated and fittings becomes dirty. May be about 1.3 for clean situation, and 1.63 for bad conditions.

Reflection Factor of a surface = Reflected light flux / Incident light flux
The illumination on a surface varies inversely as the square of the distance from the light source.

Illumination in foot-candles on plane normal to axis of light = Candle-power/ (distance)2

Radiation of different wavelength produces eye sensation of different natures which are termed colour. Radiation of wavelength exceeding, 7000 Angstrom units are termed as infra-red and are invisible, though perceptible as heat rays. Radiations shorter than about 4000 Angstrom units in the ultra-violet region and are also invisible, although they will affect a photo graphic plate and fluorescent powders.

Light output from electric lamps:

Type Wattage Efficiency (Lumens/watt) Initial Output Flux (Lumens) Average flux output through life of lamp (Lumens)
Mercury 80 38 3040 2240
Vapour 125 42 5250 3750
250 36 9000 7250
400 45 18000 13600
Sodium 45 55.5 2500 2000
60 65 3900 3120
85 71.5 6080 4850
250 100 25000 NA
400 117.5 47000 NA
Flouroscent 80 38 3040 2240
125 42 5250 3750
400 38 15200 12400
Approximate light output of electric discharge lamps