The reader is referred to the specialist literature. Most objects are lit from more than one light source, or from extended sources such as diffusing panels. At 3 metres away from the lamp the flux on a square metre has fallen to one ninth of 300 lumens = 33. To deduce the value for any other distance, just use the inverse square law. If our object were at this distance it would receive 300 lumens per square metre. One steradian covers one square metre on the surface of a globe of 1 metre radius. The candela value given for 60 degrees, 300, corresponds to 300 lumens streaming out into a cone of one steradian, according to the definition given above. This value can easily be calculated from the diagram for a point source. The energy density striking the object is given in lumens per square metre, generally known as lux.
It is this energy that makes the object visible and that fades the dye. The exhibition designer needs to translate this into light energy falling on an object at any distance from the lamp. The diagram gives just the candela values emitted from the lamp. The low luminous efficiency of tungsten lights, much loved by exhibition designers, forces the installation of air conditioning in climates where it would not otherwise be necessary. This value is more important than conservators generally realise. A practical lamp of many wavelengths has the lumen output calculated from the wattage emitted as radiation multiplied by the luminous efficiency at each wavelength, as described in the section on the candela. The lumen is formally derived from the candela, which is based on light of a single wavelength. The output from a lamp is usually quoted in lumens, summed over all directions, together with the distribution diagram in candela, shown above.Īnother quantity that is often quoted in catalogues is lumens per watt. The intensity is the same but the total energy flux from the lamp, in lumens, is not the same. The candela is a unit of intensity: a light source can be emitting with an intensity of one candela in all directions, or one candela in just a narrow beam. In plain English: The flux from a light source is equal to the intensity in candela multiplied by the solid angle over which the light is emitted, taking account of the varying intensity in different directions. Luminous intensity I (cd) in an element of solid angle dR is given by dF = IdR The official definition of the lumen, the unit of luminous flux, is: This unit of invisible light in transit is the lumen. We need a new unit for the light energy moving through space in the direction of our object. One can think of it as the emission from the lamp, which then loses interest in what happens to the photons it has ejected. The candela value is independent of distance. If there is a museum object in the path of this beam we still know nothing about how much light it is receiving. The luminous intensity directly forward is 460 cd. The pale green ray shows that this particular wide angle spot light emits 300 cd in a direction 30 degrees from its axis. Manufacturers of lamps and lamp fittings issue diagrams that show the distribution of light intensity in all directions. A light source emits with an intensity in a given direction that is measured in candela.
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