Video Displays, Work, and Vision (1983) / Chapter Skim
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5. Lighting and Reflections
5. Lighting and Reflections
Pages 111-128
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From page 111... ...
The basic principles and most of the details of how lighting and reflections affect visual performance and comfort are known. Lighting specification systems based on principles of geometry, the physics of light and reflectance, and characteristics of the human visual system are routinely used by illuminating engineers and other lighting specialists to design appropriate lighting for workplaces.
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From page 112... ...
ILLUMINATION Illumination in offices and other workplaces comes from light sources, windows, and reflections from a variety of objects and surfaces. VDTs differ from most other task objects or surfaces in that they emit light, and they usually have a highly specular curved glass surface in a more vertical plane.
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From page 113... ...
Visibility losses may also occur when secondary task lighting is used on the source document. In this situation, a VDT operator using a positive-contrast display may be particularly prone to the effects of transient adaptation, especially when a negativecontrast source document is used.
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From page 114... ...
by Barlow and Andrews (1973) , and suggests that the visual system's level of adaptation is determined by the luminance of the light symbols and not by an integrated luminance level or background luminance level.
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From page 115... ...
For example, if an operator at 70 cm from the screen sees his or her face reflected from the screen, the image will appear to be located about 22 cm behind the screen. Reflected images of luminaires or windows can produce a veil of light (reflected glare)
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From page 116... ...
, which could cause transient adaptation problems (DeBoer, 1977~. Reflected images could also induce binocular rivalry, which might cause discomfort or affect task performance (Reitmaier, 1979)
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From page 117... ...
L = Luminance of a glare source, in cd/m2 w = Solid angle of source, in steradian P = Guth position index Ed = Direct vertical illuminance at eye due to all sources, in lux Ee = Vertical illuminance at eye, in lux. Ee includes the indirect illuminance: Ee = Ed + Ei The position index P is based on Luckiesh-Guth's research.
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From page 118... ...
A lighted environment that is properly designed and therefore comfortable for workers performing traditional desk-top tasks may not be comfortable for workers performing tasks involving VDTs for two reasons. First, the design of general office lighting assumes a depressed line of sight; however, when a VDT screen is viewed, the line of sight is at or near horizontal.
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From page 119... ...
119 In In CD 3 a: .E o A: ._ Cm: $E An a o 3 m o m LO m o Cal o _ 00 ~ U
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From page 120... ...
This model is ~ n - S VL = Cref X - of -- X CRF X DGF X TAF 0.0923 where (as defined in Commission Internationale de l'Eclairage, 1981 ) : VL = Visibility level, a measure of the extent to which the equivalent contrast of a task visual display exceeds the visibility threshold of an observer for the same display at the same level of task background luminance, measured in units of the observer's threshold contrast.
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From page 121... ...
In summary, VDTs differ from objects and surfaces in non-VDT workplaces because they usually have a highly specular, curved glass surface in a more vertical plane. Consequently, workplaces in which VDTs are used require lighting designs that differ from those required in non-VDT workplaces.
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From page 122... ...
REVIEW OF VDT STUDIES Field Surveys of VDT Workers Several field surveys have attempted to determine the opinions of VDT operators regarding problems caused by lighting and reflections in the workplace. In some of these surveys, measurements of various aspects of the lighting conditions have also been made, and some investigators have attempted to relate those measurements to visual symptoms and complaints reported by VDT operators.
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From page 123... ...
, and 31 data entry workers, who reported that they worked with VDTs 75 percent of the time. Complaints about lighting related specifically to the VDT were significantly higher in the data entry group; complaints about general workplace lighting, however, were made by the same percentage (37 percent)
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From page 124... ...
Luminaires of different construction elicited significant response differences to a question concerning direct glare. Bare fluorescent lamps were complained about more frequently as a source of direct glare than were luminaires constructed with some type of diffusing cover.
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From page 125... ...
Of the workplaces sampled, approximately one-third exceeded a 3:1 ratio of luminances between hard copy and screen, and more than half exceeded that ratio between the immediate background of the screen and the screen itself. For those VDT tasks requiring hard copy, 54 percent of the operators viewing hard copy illuminated with less than 250 lux reported asthenopia (sore eyes and visual discomfort)
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From page 126... ...
At 90 percent of the VDT workstations, the illumination levels of source documents were between 100 and 1,900 lux; at workstations used for traditional clerical work or typing, the levels were between 100 and 3,200 lux. The luminance ratios between source document and screen background ranged from 7:1 to 87:1 at VDT workstations.
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From page 127... ...
Because of employee anonymity involving the use of the questionnaires, specific complaints cannot be related to specific VDTs and therefore to specific design features. This study did, however, determine that a significant relationship exists between complaints regarding visual function and employee rating of workplace design parameters, including glare, screen angle, noise from the VDT, and screen flicker.
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From page 128... ...
compared the speed and accuracy of subjects using positive- or negativecontrast displays in detecting discrepancies between a VDT presentation and a typewritten presentation. The two conditions compared were positive Contrast display (symbol luminance 40~5 cd/m2, screen background luminance < 10 cd/m2, ambient illumination 270 lux)
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