9

Logarithmic Exposure Control 297
The exposure value can be thought of as an
exposure compensation setting in cameras with
automatic exposure con trol.
This parameter is animatable.
Physical Scale—Sets a physical scale for exposure
control to use with lights that are not physically
based. The result is an adjustment of the rendering
that approximates the eye’s response to the s cene.
Each standard light’s Multiplier (page 3–977) is
multipliedbythePhysicalScalevaluetogivea
light intensit y value in candelas. For example,
w ith the default Physical Scale of 1500, a standard
omni light is treated by the renderer and radiosity
as a photometric isotropic lig ht of 1500 candelas.
Physical Scale is also factored into reflections,
refractions, and self-illumination.
Tip: You need to set the Physical Scale when you use
ray-tracing with self illumination. Set t his value
to the equivalent of the brightest light source in
thescene.Thiswillsettheappropriateconversion
scale for reflections, self-illumination, and all
other non-physically based elements a material
offers. In some cases, an object mig ht reflect or
emit more light than the brig htest l ight object in
the scene; in this case, use the object’s Luminance
value as the Physical Scale.
Range=0.0 to 200,000.0 candelas. Default=1500.0.
Asinglecandleisapproximately1candela(the
unit can also b e cal led a "candle"). A 100-Watt
(W) incandescent light bulb is approximately
139 c andelas (cd). A 60W bulb emitting in all
directions is about 70 cd, while the same bulb with
a reflector is about 4500 cd because the light flux is
concentrated into a narrow angle.
Photometric lights are unaffected by the Physical
Scale value.
This parameter is animatable.
Color Correction check box and color swatch—When
the check box is turned on, color correction shifts
all colors so the color displayed in the color swatch
appearsaswhite.Default=off.
Clicking the color swat ch displays a Color Selector
(page 1–161) so you can choose the color to adapt
to.
You can use this control to simulate how the
eye adjusts to lighting. For example, even when
the light in a room has a yellow hue from an
incandescent light bulb, we will continue to
perceive objects that we know to be white, such as
printed pages, as white.
Tip: For t he best results, use a very pale color
correction color, such as a pale blue or pale yellow.
Desaturate Low Levels—When on, renders dimly
lit colors as if the light were too dim for the eye
to distinguish between colors. When on, renders
even dimly lit colors. Default=off.
Desaturate Low Levels simulates the eye’s response
to dim lighting. In dim lighting, the eye does not
perceive colors and sees tones of gra y instead.
Theeffectofthissettingisnotapparentexcept
at very low light levels, below 5.62 footcandles
(lumens per square foot). When the illuminance
is less than 0.00562 footcandles, the scene is
completely gray.
Note: 1 footcandle (fc) equals 10.76 lux (lumens
per square meter).
Logari thmi c E x posu re Control
Rendering menu > Environment > Environment and
Effects dialog > Environment panel > Exposure Control
rollout > Choose Logarithmic Exposure Control from the
list. > Logarithmic Exposure Control rollout
Logarithmic Exposure Control uses brightness,
contrast, and whether the scene is outdoors in
daylight to map physical values to RGB values. You
can use it with either the default scanline renderer
(page 3–38) and the mental ray renderer (page