Introduction to Human Factors and Ergonomics, Fourth Edition by Bridger Robert

Author:Bridger, Robert.
Language: eng
Format: epub
Publisher: CRC Press LLC
Published: 2017-09-25T04:00:00+00:00

In practical situations, for example, in offices where the lighting is fairly uniform throughout, contrast percentages can also be calculated using data on the reflectances of adjacent surfaces. In other words, it is not even necessary to measure the illuminance falling onto the surfaces—all that is required to get a fairly good estimate of contrast is the reflectance data of the materials used in adjacent surfaces. If the room has spotlights or the light is patchy, then this procedure is invalid and surface luminance will have to be measured.

The luminance ratio is the ratio of the luminance of a work area to that of its surroundings. Recommended maximum luminance ratios have been proposed—3:1 between a task and its immediate surroundings to 10:1 between the task and the walls, floors, etc. It is usually suggested that the task should be the brightest area in the visual field to take advantage of the phototropic response. This precludes the use of materials such as white Formica for desktop materials since the luminance ratio between white paper and the desktop will be too low. Wood or pastel colored finishes are preferable from this point of view.

The computer that I am using to write this chapter has a flat screen with a luminance of approximately 250 cd/m2. The screen is housed in a matte black plastic casing, which includes the stand. The desktop has a light gray matte plastic surface that has a reflectance of approximately 60%. Assuming that the black plastic casing has a reflectance of 10%, are the luminance ratios in accord with the guidelines given above?

Assuming that the illumination in the office is fairly uniform at 300 lx, the luminance of the screen casing and desktop are as follows:

1.Casing = 0.1 × 300/π = 9.6 cd/m2

2.Desktop = 0.6 × 300/π = 57.3 cd/m2

3.Luminance ratio screen/casing = 250/9.6 = 26

4.Luminance ratio screen/desk = 250/57.3 = 4.4

The luminance ratio between the screen and the casing is clearly far too large and it would probably have been better to use a material of a lighter color for the casing. Interestingly, the luminance ratio between the desktop and the casing is also large (6.0). The result is that the casing is extremely intrusive visually and stands out both against the screen and against the desktop.

Interior decoration should be chosen to achieve a balance of surface luminances gradually diminishing from the task to the surroundings. Brightly colored walls, carpets, furniture, and fittings are contraindicated for this reason.

Glare occurs when there is an imbalance of surface or object luminances in the visual field—the brighter sources exceeding the level to which the eye is adapted. Sources of glare include the sun, bright or naked lamps, or reflections of shiny objects. Although the retina is able to adapt to different levels of luminance so as to operate over a wide range of conditions, it is not able to adapt selectively to large, simultaneous discontinuities in luminance in the visual field. For example, if the ambient luminance is high compared to

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