Autodesk LIGHTSCAPE, Manual

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Constraints of Output Devices
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between the value of multiplying two spectra and the
value of the spectra-color equivalent. In practice,
most materials and most lights, with the notable
exception of fluorescent lights, have values that are
easy to obtain.
Color shifts occur if computations are done with
color rather than spectra (as they are in Lightscape),
but in general they are not all bad. The color shift is
minimal with white lights and few interreflections,
and it is more severe with colored lights and many
interreflections. With lights, accurate colors cause
large color shifts and give less than pleasing results.
In Lightscape, the colors of the lights are desaturated
to make the results appear better.
Constraints of Output Devices
This section describes some of the constraints
current display devices place on the accurate display
of a simulated model:
• White point
• Monitor gamma
• Dynamic range mapping
• Whiteness constancy, adaptation, and surround-
ings.
White Point
All monitors have a maximum intensity color they
can produce with the maximum intensities for the
red, green, and blue electron guns. This is called the
white point of the monitor. This white point varies
for different monitors.
Usually the white points are defined in terms of color
temperature. Color temperature represents the color
of a glowing object heated to the specified tempera-
ture. Most white points lie between 5000
°
K, an
orangish white, and 9300
°
K, a bluish white. Most
televisions are set to 6500
°
K, a white that is near the
color of daylight. This variation in white is another
reason why images on one monitor look different
from images on another monitor.
Monitor Gamma
The light from the monitor comes from electron
guns exciting the phosphors on the screen. This
process is not linear. To get light that is halfway
between zero intensity and full intensity, it is neces-
sary to have the guns fire at above half strength. This
nonlinearity is called the
gamma of the monitor.
Gamma is also used for similar nonlinearities of
other display and recording devices.
This is a problem for Lightscape because when you
compute a particular intensity, you want to display
that intensity, not the intensity produced by distor-
tions of the system displaying it.
Many display programs allow an image to be
displayed at a particular gamma. You are strongly
encouraged to display images at the correct gamma.
Dynamic Range Mapping
Perhaps the greatest constraint of the monitor is its
limited dynamic range.
Dynamic range is the ratio of
the highest intensity the monitor can produce to the
lowest intensity.
In a dark room this ratio is around 100 to 1. In a
bright room the ratio drops to around 30 to 1. Real
environments have dynamic ranges around 10,000
to 1 or larger. There is currently no good way to
compress the dynamic range of a real environment
to that of a monitor.