Lighting
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Luminance Balance
Luminance balance recommendations first appeared in illuminating engineering
material in the mid 1940s. These recommendations came initially from research
on street lighting effects, and then from indoor
lighting research. In the body of original
research about active illumination
levels, a just-noticeable difference in human
comfort reports was found when differences in environmental illumination
levels within a space exceeded 100:1. In other words, this body of
research found a robust human visual system, even in environments where
there were great differences in lighting conditions.
Lighting in the peripheral field of view may affect the visibility of a
target within the central field of view (the eye's focus) in two ways:
1) There is some adaptation spread over the retina of the eye, and
2) there is some scatter of light within the entire eye itself, caused by lighting
outside the central field of view. These potentially confounding effects increase
as the light source in the peripheral field of view grows brighter. Consider
the following:
- A retinal receptor need not be stimulated by light to have its adaptation
level changed. When one part of the retina is stimulated by light, receptors
near that part also will adapt to that light level. This effect extends
to distances of up to 1 degree. If there is a bright source in the field
of view within this distance from the target of focus (very near), that
source may change the adaptation level of the area of the retina used to
view the target and may make the target more difficult to see. To put this
into context, note that the central field of view (the eye's focus) is
considered to be about 2 degrees. Therefore, this potential confound mainly
applies to retinal receptors within the central 4 degrees (a small area).
Figure 60 shows the relative relationship of these degrees within the visual
landscape.
- There are many imperfections in the media of the eye. Most of the light
is focused on the retina, but some of the light is scattered within the
eye. This scattered light forms a veiling luminance over the retina, reducing
the contrasts of the retinal image. A bright light
in the peripheral field of view may
cause enough scattering of light
over the retina to reduce noticeably
the visibility in the area of focus. This potential confound could
be created by very bright lighting anywhere within the field of
view, not just within the very small central field of view.
On the other hand, going down in illumination level within the periphery
of vision does not cause competing spread or light scatter. Using a brightly-lit
computer screen within a dark room, however, is not recommended, just as
watching television in a totally darkened room is not. This high contrast
in illumination level within the environment is not the same as looking at
a
light bulb, but may make the computer or television
screen seem too bright. These same phenomena do not occur from the
presence of very dark or very light-colored surfaces in the peripheral
field of view; surfaces in the peripheral field of view themselves
have no effect on the perceptibility of
the target.
Figure 60: Relative relations of display, bezel and room surround subtenses
(in degrees).
The research on active illumination differences and effects was not
originally intended to apply to color or contrast differences among matte
and diffusely-reflecting surfaces and objects within a field of view. Good
theories were developed, however, applying the results of these studies to
diffuse reflectance properties of matte surfaces. These early theories led
to hypotheses and ecommendations to evaluate and refine, as is consistent
with the scientific method. As subsequent events showed, however, these theories
quickly became quoted as conclusions and found their way directly into text
books and standards. In some cases, the recommendations were even more stringent
than those originally proposed by the theories.
At the time this trend began, the "recommendations" that set particular
contrast ratios were severely criticized by Chapanis (1949) as not supported
by the data used to justify them and, in some cases, the recommendations
themselves were contradicted by the data. His concern was that people might
think there was some validity to those recommendations, and therefore that
they could be misapplied by governments and/or other regulating bodies. As
events have shown, his concern was valid.
In some guidelines today, it is recommended that there should be no more
than a 3:1 difference in luminance between the task area and its immediate
surrounds and no more than a 10:1 difference between the task area and more
distant surrounds. On the other hand, the International Organization for
Standardization (ISO) specifications refer to a 10:1 and 100:1 difference.
ISO 9241, part 3, states: "The difference in area average luminance
between frequently sequentially viewed task areas (e.g., screen, document,
etc.) should be held within a ratio of 10:1. For a stationary visual field,
a significantly higher ratio of space average luminance between the task
area and its surrounds (e.g., display housing, room walls, etc.) should
not have any adverse effect. A luminance ratio of 100:1 between those two
areas, however, would be expected to produce a small but significant drop
in performance." *
The language in these guidelines for office lighting is not problematic
in itself, and ensures a relatively even lighting level within a typical
office space. For example, it is prudent to minimize the distraction that
could be created by a too-bright or too direct angle of lighting-source from
within the periphery of vision, while trying to focus instead upon a task
area. And the contrast in amount of light falling upon sequentially-viewed
work surfaces within a field of view built with these ratios will certainly
be uniform, which is not a bad thing.
However, this language does not directly state (nor, as we have learned,
is
it indicated by the research) that it is similarly prudent to minimize edge
contrasts among matte surfaces within the periphery of vision, or that it
is wise to minimize contrasts within the items of direct focus. As we know,
legibility itself requires high contrast between object letters and their
surrounding area and this high contrast is recommended by typography and
reading researchers.
Recent studies have shown that a diffusely reflectedluminance balance
above a ratio of 10:1 does not impact users' comfort nor do they experience
a drop in performance (Hunter, Joyce, Watt, 2003 and Soderston, etal. 2003),
even when the eye must move across high contrasts in sequentially-viewed
task areas.
These studies find that users' performance and comfort levels are equal
between darker and lighter bezel frames and keyboard colors, while these
particular samples of people preferred the darkest color after using dark,
midrange, and very light alternatives. These were the results, despite the
fact that both the display screen and paper materials were dark text on a
white background. The contrast between these target-work background surfaces
and the dark bezel and keyboard were found to be irrelevant
—
this contrast did not lead to comfort, performance
or preference disadvantages — and, in fact, the highest contrast
condition was preferred.
The results of these studies support a clarification of theory, that there
is no ergonomic reason to eliminate boundary-edge contrasts within a working
environment: for example, between the display screen area and a monitor bezel
frame or keyboard. As with picture frames, and door and window frames, some
people prefer framing contrasts, and these design elements are preference
attributes rather than items for ergonomic proscriptions.
It is clear that research studies such as those cited above are needed to
formulate ergonomic guidelines that are rooted in empirical evidence based
on the full range of human capacities.
* Note: The term "significant" is
used to report a statistical difference between experimental conditions;
the actual performance difference was, in itself, pragmatically small.
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