candela metre^2 (cd m^2) (1 footlambert = 3.43 candela m^2).Luminance (brightness) - the luminous flux per unit projected area of a surface in a given direction (luminous flux/unit solid angle/unit area) measured in.units Illuminance is measured in footcandles (fc) in U.S. Illuminance is measured in lux (lx) in S.I. I lluminance - the luminous flux falling on a unit area of a surface at a point.Formally the candela is defined as "the luminous intensity in a given direction of a source emitting monochromatic radiation at 555 nm of which the radiant intensity is 1/683 watts steradian-1."īoth these have AREA measures associated with them. candela (cd) which is equivalent to the lumens per steradian (lumens steradian -1).Luminous intensity - the luminous flux emitted per unit solid angle in a specified direction.However, to describe the distribution of light from a source, Luminous flux is useful for describing the total light output of light sources. In SI units, luminous flux is measured in Luminous flux = radiant flux x relevant spectral sensitivity of visual system. Luminous flux - the fundamental quantity measuring the rate of flow of radiant energy modified for its effectiveness in creating the sensation of seeing i.e.Radiant flux is usually measured in watts.In the following section we will look at some of the theory behind photometry. Lamp manufacturers often aim to make fluorescent lamps which reproduce this distribution in the energy they emit. The light emitted by a more typicalįluorescent tube consists of several prominent spectral lines.ĭaylight consists of a much more even spread of wavelengths. Some fluorescent lamps are monochromatic: they emit light at just one wavelength or spectral line. They have a long life compared to incandescent lamps but are more expensive and more complicated electronically. This gives fluorescent lamps a relatively high efficacy and good color rendering properties. Most of the energy radiated by a fluorescent lamp, on the other hand, is emitted as visible light. Incandescent lamps are, however, cheap and easy to work with. This is obviously inefficient in terms of the conversion of electrical energy into light. A tungsten-filament (incandescent) lamp, for example, emits most of its radiant energy in the infrared region of the electromagnetic spectrum. The radiant-flux or electromagnetic-power spectra of different light sources varies considerably. The reason that our eyes are so easily damaged by looking at the sun is explained when we see that its luminance is 1000 times greater than this maximum level. The upper limit is determined by the luminance required to damage the retina. The eye can detect luminances from as little as one millionth of a cd/m2 up to a maximum of one million cd/m2. A typical luminance for a piece of white paper under an illuminance of 500 lux is thus 130 cd/m2. If the surface is diffuse then § can be replaced with "p", the diffuse reflection coefficient for the material. Iu is the luminous intensity in that direction.Īlternatively, the luminance of a surface can be calculated from the formula L = E x § / ¹ where § is the luminance factor of the surface material and is read from a table of values. Thus, L = Iu / A' where A' tends to 0.įor a plane surface the apparent area can be found from the equation: A' = A x cos u, where a is the actual area of the source, and u the angle between the normal to the surface and the direction of observation. The apparent area, A', is the area the source seems to have as seen by the observer. The brightness observed is called the luminance, L, and is defined as intensity per unit apparent area of light source. When a part of the incident light striking a surface is reflected the human eye will observe that surface as a light source. 7.5R/4/12 will be a strong red, 5B/9/1 will be a pale blue. Chroma - 16 steps (how saturated) (See Figure)Īny particular color is then given a Munsell reference for Hue/Value/Chroma e.g.Hue - this circle is divided into 5 main colors and 5 intermediate colors with 10 steps between each pair of colors.This consists of 1200 small plates of different colors classified along 3 dimensions.Įach of these scales is constructed as follows: There are various systems for classifying colors but the 2 most commonly used are: Munsell Book of Color DEA3500: Ambient Environment: Lighting and ColorĬolor classification (of surfaces).
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