[at-l] So, if...

[kent_gardam at yahoo.com (Kent Gardam)]

Well, since you axed:

In general, ?brightness? is an expression of the
amount of light emitted from a surface per unit of
area.
?Brightness? is not an official term of the lighting
trade,
and lighting designers may become huffy when you use
it. However, the concept is essential for
understanding
visual quality, especially in relation to contrast and
glare.
Brightness does not inherently relate to lamps, or
even to light sources. The light could be reflected or
transmitted. For example, the bright surface could be
the surface of a fluorescent tube, a page of a book, a
window with a view of the sky, or a store window with
reflections.
The closest official term is ?luminance,? which is
expressed as candelas per square meter of light
emitting
surface. (Luminance used to be measured in
?footlamberts,? which is now an obsolete term.) For
example, the luminance of a heavily overcast sky is
about
1,000 candelas per square meter, and the luminance of
a typical frosted light bulb is about 100,000 candelas
per square meter.
Luminance is defined in terms of the direction of
light emission. The details get technical, and you
probably will not need to deal with them. In brief,
the
brightness of an object usually depends on the
direction
from which you look at it.
Note that luminance has nothing to do with size of
the light emitting surface. The light source could be
as
small as a lamp filament, or it could be as large as
the
whole sky, or it could be a task area, such as a desk
top.
Measuring brightness (?luminance?) is tricky and
requires specialized equipment. For practical work,
learn how to avoid excessive brightness, so you won?t
need to measure it. If you do a good job of laying out
lighting, people within the space will not be
subjected
to brightness that is severe enough to cause glare.
Luminance is the converse of illuminance. The
former describes the intensity of light that is
leaving a
surface, whereas the latter describes the intensity of
light
that is falling on a surface. For light reflected from
a
surface, luminance equals illuminance multiplied by
the
percentage of reflectance.
?Brightness? also is used to describe the subjective
sensation of light intensity. This sensation largely
depends on the overall layout of the scene surrounding
the viewer. An uncomfortable level of brightness is
described as ?glare.? (The term ?glare? is used in
several
ways. It is an important concept, but is not precisely
defined by the lighting trade. Various types of glare
are
explained in Reference Note 51, Factors in Lighting
Quality.)
In Summary ...
So, here is the overall picture. A lamp produces a
certain amount of light, measured in lumens. This
light
falls on surfaces with a density that is measured in
footcandles or lux. A person looking at the scene sees
different areas of his visual field in terms of levels
of
brightness, or luminance, measured in candelas per
square meter.

Now aren't you sorry that you ever brought it up?

--- Felix <athiker@smithville.net> wrote:

> So, if something is '10 Times Brighter' than
> something else...could it also be 
> said that it is 1/10th as dim?


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