Basics of Electrical Lighting Design I

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As you may already know, Light is that part of the electromagnetic spectrum that is perceived by our eyes. There are some basic parameters that are used in the design of lighting systems that must be understood before they are used. They are:
  • Luminous Flux (Փ)

  • Luminous Efficiency

  • Luminous Intensity (I)

  • Illuminance (E)

  • Luminance (L)
Luminous Flux (Փ)
The luminous flux describes the quantity of light emitted by a light source.
It is commonly represented by the symbol, Փ. Its unit of measurement is the lumen (lm)

Luminous Efficiency
The luminous efficiency of an electrical lamp is the ratio of the luminous
flux emitted by the lamp to the electrical power consumed by the lamp. Its unit is (lm/W).
It is a measure of a lamp’s economic efficiency.

Luminous Intensity (I)
The luminous intensity describes the quantity of light that is radiated in a particular
Direction from a light source such as a lamp. This is a useful measurement for directive lighting elements such as reflectors. In lighting design, it is represented by the symbol, I. Its unit of measurement is the candela (cd).
Luminous Intensity is given by:
I = Փ/W
W = Electrical power consumed by lamp


Illuminance (E)
Illuminance describes the quantity of luminous flux falling on a surface. It decreases by the square of the distance (inverse square law). Lighting standards usually specify the required illuminance for indoor work places and industrial areas. It is represent by the symbol, E. Its unit of measurement is the lux or lx.

Illuminance is given by:
E (lx) = luminous flux(lm)/area (m2) = Փ/A . Note that lx = lm/m2

Luminance (L)
Luminance specifies the brightness of a surface and is essentially dependent on its reflectance (finish and colour). It is the only basic lighting parameter that is perceived by the eye. It is represented by the symbol, L. Its unit of measurement is cd/m2.

Luminance is given by:
L = I/A or L = E/W.
W = Electrical power consumed by lamp