ENVIRONMENTAL STUDIES

Environmental studies

Environmental studies

ARTIFICIAL LIGHTING

Luminous intensity (l)

The concept of luminous intensity is used to compare different light sources and measure their 'strength'.

Luminous intensity (I) is the power of a light source, or illuminated surface, to emit light in a particular direction. (CIP 2000)

Unit: candela (cd) 

The effect of one candela is still approximately the same as the original idea of one candlepower, and the mean spherical intensity (MSI) of a 100 W traditional 'light bulb', for example, is about 100 cd.

Typical lumen values:

candle: 1 cd

electric lamp:  100 cd

lighthouse: 100 000 cd

Luminous flux (F)

The rate of flow of any electromagnetic energy can be expressed in terms of power, but light energy is also measured by luminous flux.

Luminous flux (F) is the rate of flow of light energy.

Unit: lumen (lm)

Illuminance (E)

When luminous flux falls on a surface it illuminates that surface. The lighting effect is termed illuminance.

Illuminance (E) is the density of luminous flux reaching a surface.

Unit: lux (lx) where 1 lux = 1 lumen/(metre)2

 

Typical lighting values in lux

moonlight: 0.1 lx

street lighting:           10 lx

office lighting:           300 lx

overcast day:             1000 lx

direct sunlight:          100,000 lx

Foot-candle: A non-SI (metric) unit still in use. 1fc = 10.76 lx

 

Diagram: Summary of lighting measurements

Luminance

The amount of light emitted per unit area of an illuminated surface.

Luminance depends on reflection coefficient on the ALBEDO OF THE SURFACE (fraction of the incident light diffusely reflected by the surface)

UNITS:

LAMBERT- light emission/ reflection of 1 lumen/cm2 by the surface

MILLILAMBERT-brightness of one thousandth lambert

FOOT LAMBERT- equivalent FOOT-CANDLE; brightness of the surface emitting a flux of one lumen/ft2

Inverse square law

As the luminous flux emitted by a point source of light travels away from the source, the area over which the flux can spread increases. Therefore, the luminous flux per unit area (i.e. the illuminance) must decrease. This relationship is expressed by the inverse square law:

The illuminance produced by a point source of light decreases in inverse proportion to the square of the distance from the source. (CIP 2000)

Examples of distance on illumination

2 x distance gives 1/4 lux

3 x distance gives 1/9 lux

4 x distance gives 1/16 lux

Cosine law of illumination

When the luminous flux from a point source reaches the surface of a surrounding sphere, the direction of the light is always at right angles to that surface. However, light strikes many surfaces at an inclined angle and therefore illuminates larger areas than when it strikes at a right angle. The geometrical effect is shown in the diagram. ...