Lighting

	Overview   |   Measurement of light   |   Luminance balance   |   Illuminance   |   Workstation light level   |   Transient Adaptation Factor (TAF)   |   Disability glare   |   Other considerations

Illuminance
Illuminance is defined below. Many readers, however, may find it helpful to understand the concept from some everyday examples.

Illumination follows a square law. For example, for any given reading, if the light meter is held twice as far away from the light, the meter will read only onefourth as much; if the light meter is held half as far away from the light, the meter will read four times as much.

Luminance

This is a measure of light coming from a surface (in contrast to illuminance, or light falling on a surface). If the surface is similar to a sheet of bond typewriter paper, a good mental image is that in an ordinary office, the luminance of the sheet is about 100 cd/m². Luminance does not follow a square law, but the measurement area must be defined. The luminance of a wall, for example, is the same whether measured two meters away or four. Similarly, moving closer to or further away from a source document does not change its apparent brightness. The particular luminance of a bright surface is usually referred to as its "brightness" because it is the quality of brightness that we perceive.

Luminance Level and Acuity

Over the years, acuity has been measured in several ways. One of the first widely accepted methods was the Snellen chart, made up of several rows of letters, with the letters on each row smaller than those on the row directly above. To measure acuity, the person is asked to read the letters on the top row, then the next and so on until the letters can no longer be correctly discerned. Although this method gives a rapid estimate of acuity, it is
seldom used for scientific purposes.

Visual resolution, or acuity, is often stated as a spatial modulation transfer function. This can be shown by viewing equally spaced parallel line patterns. The density, or closeness, of the lines is stated in terms of cycles per degree. A line pair, a dark line and an adjacent light line represent one cycle.

Figure 52 shows a set of modulation curves of contrast sensitivity for the human eye. The separate curves are for different light densities and are labeled in terms of average retinal illumination, or trolands. The 90troland curve would be about equivalent to viewing a surface with an average luminance of 10 cd/m2, (DeGroot and Gebhard, 1952). Acuity, or contrast sensitivity, is measured by first establishing the angular size of the pattern, stated in cycles per degree. Then, the minimum modulation required to detect the pattern is determined.

There are two conclusions that may be drawn from the curves in Figure 52. First, there is no significant improvement in acuity or contrast sensitivity for average luminances above about 10 cd/m2. Contrast is the important factor after that point. In fact, there is little practical difference between the 9 and 90troland curves. Improvements in acuity produced by light levels higher than those may be attributed to a reduction in refractive error caused by using a smaller pupil. Second, for the luminances of interest in an office environment, contrast sensitivity is best for a spatial frequency of about 5 to 7 cycles per degree. That would be a bar pattern, with each bar being about 4 to 6 minutes of arc in width. That corresponds to letter heights of about 12 to 18 minutes of arc.

The percent of light beam reflected back from a smooth, untreated glass surface as a function of the angle of incidence. This is for an air-to-glass surface (Sears, 1958).

There are other ways to determine contrast sensitivity. The thresholds for a brief flash of light over a small area may be determined for various background luminances. That determination, however, is used to detect the brief flash of light rather than to detect a pattern. Pattern detection seems to be more directly related to visual display use than is the simple detection of the presence or absence of light.

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