Light and Illumination

The strength of a lamp or other source of light is specified by a quantity called its luminous intensity. This was measured in standard candles-a unit that goes back to the use of the ordinary wax candle as a source of light. These days the unit of luminous intensity is the candela the definition which can be found in more advanced texts. However the candela approximates to a standard candle and since modern units are quite complicated to use, then this text will stick to illustrate the principles involved. In rating lamps at present the actual comparison is made with standardized filament lamps kept in testing laboratories such as the Bureau of Standards. A filament lamp of moderate size has an intensity of about one candle for each watt rating. For example the intensity of a 60-watt lamp is very nearly 60 candles.

The practical question facing the lighting engineer is how to determine the strengths and positions of lamps so that an adequate amount of light energy will fall on each unit area of the surfaces to be illuminated. The light from a small unshaded source may be thought of as spreading out on the surface of a constantly expanding sphere, much like the spreading of sound under similar conditions. A given amount of light energy will spread over a larger and larger area as it moves away from the source. This area increases as the square of the distance,so that a given amount of radiant energy will be spread over 4 times the area at 2 times the distance,25 times the area at 5 times the distance, etc. As a result the illumination or the energy falling on each unit area will vary inversely as the square of the distance from the source. The illumination of any surface that is held perpendicular to the incoming rays will also depend directly on the strength of the source, so that the complete relation is given by

E=C/d²

Key: E is the illumination due to a small source of intensity C placed at a distance d from the surface in question

Note: If C is measured in candles and d in feet, E is expressed in foot-candle

The corresponding metric unit is called a metre candle.The eye is so sensitive that it can be stimulated by as little as a ten-millionth of a foot candle,equivalent to the illumination produced by a single candle nearly 20 miles away.