Autodesk 12812-051462-9011 User Guide 3 - Page 1503

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 scene. Changing the value of Physical Scale is an optional step. Use it as a last resort when the materials or maps are not rendering correctly. Changing this setting will not affect anything in the scene unless your scene has an Ambient Color different than black. If you do adjust it, set the Physical Scale value to the equivalent of the brightest light source in the scene. The software multiplies each standard light's Multiplier on page 8055 by the Physical Scale value to produce a light intensity value in candelas. For example, with the default Physical Scale of 1500, a standard omni light is treated by the renderer and radiosity as a photometric isotropic light of 1500 candelas. Physical Scale is also factored into reflections, refractions, and self-illumination. TIP When you use ray-tracing with self illumination, set Physical Scale to the equivalent of the brightest light source in the scene. This sets the appropriate conversion scale for reflections, self-illumination, and all other non-physically based elements a material offers. In some cases, an object might reflect or emit more light than the brightest light 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. A single candle is approximately 1 candela (the unit can also be called a "candle"). A 100-Watt (W) incandescent light bulb is approximately 139 candelas (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. Adjusting the range to analyze: 6758 | Chapter 21 Effects and Environments