Civil Engineering Reference
In-Depth Information
can also be covered with a neutral density film (usually a gray sheet of plastic) to
reduce the transmittance of light from the outside.
2. Place light fixtures strategically. Figure 14.4 provides a side view and a view from
behind of an operator at a workstation. In the figure we assume that the light fixtures
have a restricted light angle of about 100°. This may be typical for “egg crate” types
of luminaires. The operator in Figure 14.4(A) sits at the borderline location of
luminaires A and C where there is no direct glare from luminaire C and no reflected
glare from luminaire A. Figure 14.4(B) illustrates that luminaire B2, which is closer
than the other luminaires, will cause more veiling reflections and wash out more
contrast on the screen. Locations 131 and B3 are better. In summarizing the points
made in Figure 14.4, luminaires should be placed to the side of operators and not at
the front or the back, where they cause more direct glare, indirect glare, and veiling
luminance.
3. Use directional lighting. The examples in Figure 14.4 illustrate the use of directional
lighting or task lights.
4. Move the workstation. An operator should not face a bright window, since the large
contrast between the dark screen and the bright window may cause discomfort due to
glare. Nor should an operator work with his or her back against a window, as screen
reflections from the window are inevitable. Rather, the screen should be positioned at
90° to the window. Workstations can also be moved from a bright area to a darker area
in an office. This will reduce veiling screen luminance and wash-out of contrast.
5. Tilt the screen. The tilting mechanism, which is mandatory in many standards, makes it
possible to angle the screen so as to avoid reflections from overhead luminaires and
other light sources. Just as with a tilted mirror, one can decide what to look at and
what not to look at!
6. Screen filters or coatings. Filters, such as the neutral density (gray) filter, color filter,
and polarized filter, enhance the contrast between characters and background. The
enhancement in contrast is achieved in the following way. The incoming illumination
is filtered twice: the first time on its way to the screen, and the second time after being
reflected by the screen. However, the character luminance is filtered only once (Figure
14.5). Most of these filters were used in the past. Today, they have been replaced by
quarter-wave length filters. This is an optical coating, similar to what is used on a
camera. As the light enters the filter, there are reflections from the first and the third
surface. Since the difference in traveled distance between these two reflections is one
half wavelengths, the two reflections are in counter phase and the reflected light is
therefore extinguished. The quarter wavelength filter can be bought separately and
mounted on top of the display. This will reduce reflections considerably. However,
one must make sure that there is not already a filter attached to the display, since this
would lower the character luminance, so that the characters become difficult to read.
7. Hang or erect partitions. By hanging partitions from the ceiling or standing them on the
floor, it is possible to block off illumination from light sources in an open-plan office
or plant.
Application of an etching or frosting to the screen surface reduces specular (mirror-like)
reflections. It is no longer possible to see clearly any reflections of one's clothes or face
or overhead luminaires, since the reflections become fuzzy. Unfortunately, the screen
characters also become a bit fuzzy.
