Concepts of practical light measurement

There are four basic concepts of practical light measurement

Relationship between the various lighting units :

Relation between luminous flux (f) and luminous intensity (l)

Relation between luminous intensity (l) and illuminance (E)

Cosine Law & Vertical illuminance

Light measurement uses two dissimilar parameters :

When considering light which is visible electromagnetic radiation, we are concerned on the one hand with energy and the other with a sensation obtained through the eye.These two are principally dissimilar things. That makes it difficult to talk about light in quantitative terms.To solve the problem, a convention has been adopted that fits in well with illuminating engineering practice: viz. the product of radiant energy and eye sensitivity. Thus, light is regarded as radiation measured in terms of human eye sensitivity.

The Light Watt :

The sensitivity of the eye varies with wavelength of light. Under conditions ofphotopic (Day time) vision, the maximum sensitivity lies at 555nm. Now one could choose to define one watt of power radiated at a wavelength of 555nm as being equal to one 'light-watt' . One watt of power radiated at a differentwavelength within the visible range would then have to be multiplied by the relative eye-sensitivity factors as defined by the spectral eye sensitivity curve for photopic vision, that is to say the V(l) curve. By so doing, one arrives at the light-watt value corresponding to that wavelength. For example, for radiation with a wavelength of 490nm the eye sensitivity is only 20 per cent of that for radiation with a wavelength of 555 nm. One watt of power radiated at 490 nm therefore equals 0,2 light-watts.

At low lighting levels, the sensitivity of the eye shifts left (ie., towards shorter wavelengths) with the peak sensitivity at 507nm.This is called scotopic vision.

By adopting the light-watt as the unit of visible radiation for the quantitative expression of visual perception, ambiguity would have been completely avoided. However, this concept was not adopted because, long before it was even formulated, a light unit had already been derived from one of the earliest reproducible lighting standards. This standard was the 'candle power' which became (after 1948) the 'candela'. It was in fact a unit of light intensity in a given direction. A light source radiating a light intensity of one candela in all directions produces a well-defined quantity of light per second, which has been given the name 'lumen'. This has become the principal lighting standard in use today.

Calculations have shown that one watt of radiant power with a wavelength of 555nm equals 683 lumens. This figure is known as the 'maximum spectral luminous efficacy'. Thus, one watt of power radiated at 490 nm equals0,2 x 683 = 137 lumens.

The lumen can therefore be defined as a certain quantity of radiant energy emitted per second, weighted against the spectral sensitivity of human eye.