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Enabled – Turns the light on and off.

Color Mode –  – Determines how the color of the light is controlled, either Color or Temperature.

Color – Determines the color of the current light.

Temperature –  – When the Color Mode is set to Temperature, this parameter controls the color of the light by degrees Kelvin.

Intensity – Multiplier for the light strength. The units of intensity are determined by the Units option.

Units – Specifies the light units. Using the correct units is essential when using the V-Ray Physical Camera. The light automatically takes the scene's unit scale into consideration to produce the correct result for the scale you are working with. The possible values are:

Default – The color and multiplier directly determine the visible color of the light without any conversion. The light surface appears with the given color in the final image when seen directly by the camera (assuming there is no color mapping involved).
Lumens – Total emitted visible light power measured in lumens. When this setting is used, the intensity of the light does not depend on its size. A typical 100W electric bulb emits about 1500 lumens of light.
Lm/m/m/sr – Visible light surface power measured in lumens per square meter per steradian. When this setting is used, the intensity of the light depends on its size.
Watts – Total emitted visible light power measured in watts. When this setting is used, the intensity of the light does not depend on its size. Keep in mind that this is not the same as the electric power consumed by a light bulb for example. A typical 100W light bulb only emits between 2 and 3 watts as visible light.
W/m/m/sr – Visible light surface power measured in watts per square meter per steradian. When this setting is used, the intensity of the light depends on its size.

Intensity – Multiplier for the light Color.

Falloff Type – Determines how the light transitions from full strength to no lighting inside the light cone.

Linear – The light does not have any falloff.
Smooth Cubiccubic – The light fades in a realistic way.

Cone Angle ConeAngle – Specifies the angle of the light cone formed by the V-Ray Spot Light.

Penumbra Angle PenumbraAngle – Specifies the angle within the light cone at which the light begins to transition from full strength to no lighting. When set to 0, there is no transition, and the light produces a harsh edge. 

Dropoff Drop Off – Controls the drop-off rate of the light intensity.

Show cone – Cone – When enabled, shows the light cone.

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Affect Diffuse – Determines whether the light affects the diffuse properties of the materials.

Affect Specular – Determines whether light affects the specular of the materials. This means glossy reflections.

Affect Atmospherics – Determines whether the light influences the atmospheric effects in the scene.

Diffuse Contribution – When Affect Diffuse is enabled, specifies a multiplier for the effect of the light on the diffuse component.

Specular Contribution – When Affect Specular is enabled, specifies a multiplier for the effect of the light on the specular component.

Atmospheric Contribution – When Affect Atmospherics is enabled, this value determines the amount of involvement.

Area Speculars AreaSpeculars – When enabled, the highlights match the shape of the light. When disabled, highlights are always calculated as if affected by a point light.

Cut-off Threshold – Specifies a threshold for the light intensity below which the light is not computed. This can be useful in scenes with many lights, where you want to limit the effect of the lights to some distance around them. Larger values cut away more of the light; lower values make the light range larger. When this value is 0.0, the light is calculated for all surfaces.

LPE Label – Allows a label to be assigned, which can be used to reference the light in a Light Path Expression. This is especially useful when working with the Light Select render element to evaluate custom light contribution in the scene.

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Shadows – Toggles the shadows of the current light. Disable this to turn off shadows.

Shadow Bias – Moves the shadow toward or away from the shadow-casting object (or objects). Higher values move the shadow toward the object(s) while lower values move it away. If this value is too extreme, shadows can "leak" through places they shouldn't or "detach" from an object. Other effects from extreme values include moire patterns, out-of-place dark areas on surfaces, and shadows not appearing at all in the render.

Shadow Color ShadowColor – Controls the color of shadows for this light. Note that anything different from black is not physically correct.

Shadow Radius ShadowRadius – Specifies the size of the light. A value of 0.0 makes the light a point light. Larger values produce soft (area) shadows.

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Barn Door BarnDoor – Enables or disables the barn door effect. Barn doors restrict the light cone on the four sides of the light to produce a rectangular light shape. In the real world, barn doors are four planes attached in a square around the light. Use the Left, Right, Top, and Bottom angles to control the four sides.

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The Decay parameters determine how the light fades in and out. The Near Decay determines how light fades in. The light isn't at its maximum value at its source, but instead gradually increases until it reaches the Near Decay End. The Far Decay determines how light fades out. The light isn't at its maximum value at its end, but instead gradually decreases after the Far Decay Start.

The Decay options are useful for creating hotspots or controlling the length of a "God Rays" effect created with Environment Fog.

Use Decay Regions UseDecayRegions – Toggles Decay Regions on and off. Allows fading the light into three section.

Decay – Controls how quickly the light fades.

sections - Distance 1, 2, and 3.

Near Decay On – Toggles near decay on and off.

Near Decay Start – Determines where the fade in starts. Anything before this point is rendered dark.

Near Decay End – Determines where the fade in ends. After this threshold, the light is at its full value.

Far Decay On – Toggles far decay on and off.

Far Decay Start – Determines where the fade off starts.

Far Decay End – Determines where the light reaches a value of 0, i.e. completely fades off.

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