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DEA3500: Ambient Environment: Lighting and Color
Color classification (of surfaces). There are various systems for classifying colors but the 2 most commonly used are:
Munsell Book of Color
This consists of 1200 small plates of different colors classified along 3 dimensions.
- Hue = hue
- Value = lightness
- Chroma = perceived chroma
Each of these scales is constructed as follows:
- Hue - this circle is divided into 5 main colors and 5 intermediate colors with 10 steps between each pair of colors.
- Value - 10 steps from black to white
- Chroma - 16 steps (how saturated) (See Figure)
Any particular color is then given a Munsell reference for Hue/Value/Chroma e.g. 7.5R/4/12 will be a strong red, 5B/9/1 will be a pale blue.
Luminance
When a part of the incident light striking a surface is reflected the human eye will observe that surface as a light source. The brightness observed is called the luminance, L, and is defined as intensity per unit apparent area of light source. The apparent area, A', is the area the source seems to have as seen by the observer. Thus, L = Iu / A' where A' tends to 0.
For 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. Iu is the luminous intensity in that direction.
Alternatively, 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. If the surface is diffuse then § can be replaced with "p", the diffuse reflection coefficient for the material. A typical luminance for a piece of white paper under an illuminance of 500 lux is thus 130 cd/m2.
The eye can detect luminances from as little as one millionth of a cd/m2 up to a maximum of one million cd/m2. The upper limit is determined by the luminance required to damage the retina. 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.
Spectra of Light Sources
The radiant-flux or electromagnetic-power spectra of different light sources varies considerably. A tungsten-filament (incandescent) lamp, for example, emits most of its radiant energy in the infrared region of the electromagnetic spectrum. This is obviously inefficient in terms of the conversion of electrical energy into light. Incandescent lamps are, however, cheap and easy to work with.
Most of the energy radiated by a fluorescent lamp, on the other hand, is emitted as visible light. This gives fluorescent lamps a relatively high efficacy and good color rendering properties. They have a long life compared to incandescent lamps but are more expensive and more complicated electronically.
Some fluorescent lamps are monochromatic: they emit light at just one wavelength or spectral line. The light emitted by a more typical
fluorescent tube consists of several prominent spectral lines.
Daylight consists of a much more even spread of wavelengths. Lamp manufacturers often aim to make fluorescent lamps which reproduce this distribution in the energy they emit.
In the following section we will look at some of the theory behind photometry.
Photometric Quantities:
- Radiant flux is usually measured in watts.
- 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.
Luminous flux = radiant flux x relevant spectral sensitivity of visual system.
In SI units, luminous flux is measured in
- lumens (lm). Luminous flux is useful for describing the total light output of light sources. However, to describe the distribution of light from a source,
luminous intensity is used.
- Luminous intensity - the luminous flux emitted per unit solid angle in a specified direction. The measure is the
- candela (cd) which is equivalent to the lumens per steradian (lumens steradian -1). 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."
Both these have AREA measures associated with them.
- Illuminance - the luminous flux falling on a unit area of a surface at a point. Illuminance is measured in lux (lx) in S.I. units Illuminance is measured in footcandles (fc) in U.S. units
- 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
- candela metre^2 (cd m^2) (1 footlambert = 3.43 candela m^2)
Cosine Law (Lambert cosine law)
Illuminance of any surface varies as cosine of angle of incidence.
E = 1/d^2 cos theta where d = distance of source
angle of incidence
(cos theta = 1, cos 90 = 0)
Cosine cubed law - E = I cos3theta / h^2 distance of source 'd' can be replaced by h/cos theta where h is perpendicular distance of source from plane in which measurement point is located.
Inverse square law and cosine law can be combined.
E = (I cos theta ) / d^2
Most meters are cosine corrected.
Luminance/Reflectance/Apostilbs
With a non-luminous surface e.g. a wall, what the eye sees - the brightness or luminance of the surface - depends on the
reflectance i.e. the ratio of reflected light to incident light.
With an illuminance of 500 lux and a reflectance of 0.4 the luminance of the surface will be 200
apostilbs.
- Illuminance (lux) x reflectance = luminance (apostilbs).
- Apostilbs is not an SI unit. To convert this to SI (candelas m-2) divide by pi (or multiply by 0.318).
- If the Munsell "Value" reference of a color is known the reflectance can be approximately calculated using Reflectance = V(V - 1) where V = value.
- If Munsell "value" is 6, reflectance = 6 x 5 = 30% = .3.
- Since in most rooms the different surfaces will have different colors, they will reflect differing amounts of light and this will affect the distribution of light in the room.
Reflectance
- For a perfectly diffusely reflecting surface, the ratio of the reflected luminous flux to the incident luminous flux is the reflectance.
- Luminance = illuminance x reflectance / pi
- Reflectance = illuminance / luminance
- When the surface is not perfectly diffusely reflecting, reflectance is replaced by a
luminance factor
Luminance factor is the ratio of the luminance of a surface viewed from a particular position and lit in a specified way to the luminance of a diffusely reflecting white surface viewed from the same direction and lit in the same way. Here Luminance = illuminance x luminance factor / pi
In lighting practice
illuminance
and luminance
are most frequently used to characterise the effect of lighting.
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Luminance factor
Luminance factor is the ratio of the luminance of a surface viewed from a particular position and lit in a specified way to the luminance of a diffusely reflecting white surface viewed from the same direction and lit in the same way. Here Luminance = illuminance x luminance factor / pi
In lighting practice illuminance and luminance are most frequently used to
characterize the effect of lighting.
Go to the next lecture