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Photometric light parameters

Several of the light rollouts for photometric lights are the same as those for the standard lights, but several key parameters are unique for photometric lights, such as the ability to choose a light distribution model and a shape type.

Distribution options

The Distribution options are listed in a drop-down list in the General rollout. Both Free and Target photometric lights can be set to one of four distribution types. Each of these types appears as a different icon in the viewports:

• Uniform Spherical: This distribution type emanates light equally in all directions from a central point, like the standard Omni light.

• Uniform Diffuse: This distribution type spreads light equally in all directions for only one hemisphere, such as when a light is positioned against a wall.

• Spotlight: This distribution type spreads the light in a cone shape, like a flashlight or a car's headlight.

• Photometric Web: This distribution type can be any arbitrary 3D representation and is defined in a separate file that can be obtained from the light manufacturer and loaded into the light object. Once loaded, the distribution graph is visible in the Distribution rollout.

The Uniform Spherical option distributes light equally in all directions. The Uniform Diffuse option has its greatest distribution at right angles to the surface it is emitted from and gradually decreases in intensity at increasing angles from the normal. For both options, the light gradually becomes weaker as the distance from the light increases.

The Spotlight option concentrates the light energy into a cone that emits from the light. This cone of light energy is directional and can be controlled with the Hotspot and Falloff values.

The Photometric Web option is a custom option that lets you open a separate file describing the light's emission pattern. These files have the .ies, .cibse, or .ltli extensions. Light manufacturers have this data for the various real-world lights that they sell. You load these files using the Choose Photometric File button found in the Distribution (Photometric Web) rollout. You can also specify the X-, Y-, and Z-axis rotation values.

Color options

The Color section of the Intensity/Color/Attenuation rollout, shown in Figure 20.10, includes two ways to specify a light's color. The first is a drop-down list of options. The options found in the list include standard real-world light types such as Cool White, Mercury, and Halogen. Table 20.2 lists each of these types and its approximate color.

FIGURE 20.10

The Intensity/Color/Attenuation rollout for photometric lights uses real-world intensity values.

In addition to a list of available light types, you can specify a color based on temperature expressed in degrees Kelvin. Temperature-based colors run from a cool 1,000 degrees, which is a mauve-pink color, through light yellow and white (at 6,000 degrees Kelvin) to a hot light blue at 20,000 degrees Kelvin. Typical indoor lighting is fairly low on the Kelvin scale at around 3,300 degrees K. Direct sunlight is around 5,500 degrees K. Thunderbolts, arc welders, and electric bolts run much hotter, from 10,000 to 20,000 degrees Kelvin.

You also can set a Filter Color using the color swatch found in this section. The Filter Color simulates the color caused by colored cellophane placed in front of the light.

Intensity and Attenuation options

The Intensity options can be specified in Lumens, Candelas, or Lux at a given distance. Light manufacturers have this information available. You also can specify a Multiplier value, which determines how effective the light is. There is also a setting for specifying the intensity due to a dimming effect, and the Incandescent lamp color shift when dimming option causes the light from an incandescent light to turn more yellow as it is dimmed. This effect is common as you get farther from a light bulb.

All real-world lights have attenuation, and Far attenuation