Rectangle Light

The V-Ray Rectangle Light or an "area light" is a planar light source. The light's shape can be set as a rectangle or a circular disc.

UI Path: ||V-Ray|| > V-Ray Lights > Rectangle Light

Enabled – Turns the V-Ray Rectangular Light on and off.

Color – Controls the color of the light. When using photometric units, this color is normalized so that only the color hue is used, and the light intensity is determined by the Intensity parameter.

Intensity – Multiplier for the light Color; this is also the light intensity in the units set by the Units parameter.

Units – Choose 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. Note that V-Ray uses a conversion unit of 683 lum/watt, that is, the maximum physical possible Luminous efficacy.
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. With this setting, 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 of visible light. Note that V-Ray uses a conversion unit of 683 lum/watt, that is, the maximum physical possible Luminous efficacy.
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. 

Disc Shape – When enabled, the rectangle light is shaped like a planar disc.

Width/Height – Determines the width/height of the light measured in centimeters.

Directional – When this value is 0, the light from the Rectangle Light is spread out equally in all directions. Increasing this value makes the light beam more narrow and concentrates it in one direction.

Directional Strength – Controls the blend between diffuse distribution and directional distribution. For example, setting the directional strength to 0.9 makes the light emit a portion of its energy (i.e., 10 percent) with diffuse distribution. This makes the light visible from the side in the reflections.

Use Texture – Enables the light to use a texture for the light surface. It is best to have GI enabled if there are surfaces that are close to a texture-mapped light. This allows V-Ray to use combined direct and indirect sampling for the light, reducing the noise for surfaces close to the light.

Rect Tex – Specifies the texture map when Use Texture is enabled. The light uses the texture's alpha as an opacity map.

Invisible – Controls whether the shape of the light source is visible in the render result. When this option is enabled, the light source is not visible in the rendering. When this option is disabled, if the light source is seen directly by the camera or through refractions, it appears in renderings in the current light color.

Occlude Other Lights – Controls the behavior of the light, when Invisible is enabled, in relation to other light sources. When enabled, the invisible light blocks the lighting from other lights as if it's visible. Also, in reflections the light behaves as if it's visible and occludes other lights. When disabled, the light is transparent for other lights and does not occlude them.

Double Sided – When enabled, light emits from both sides of the plane.

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

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

Affect Reflections – Determines whether the light appears in reflections of materials, both perfect and glossy reflections.

Affect Atmospherics – When enabled, the light influences the atmospheric effects in the scene. This option is not available with V-Ray GPU.

Diffuse Contribution – A multiplier for the effect of the light on the diffuse component.

Specular Contribution – A multiplier for the effect of the light on the specular component.

Atmospherics Contribution – Determines the amount of influence the light has on the atmospheric effects.

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.

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. See the Far Decay End example for more.

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.

Shadows – When enabled (the default), the light casts shadows. Disable this option to turn off shadows for the light.

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 rendering.

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

Caustics Subdivs – Only used when calculating Caustics. Lower values mean more noisy results but render faster. Higher values produce smoother results but take more time.

Caustics Multiplier – This value is a multiplier for the generated caustics by the selected object. Note that this multiplier is cumulative - it does not override the multiplier in the Caustics render settings rollout section.

Project is used to create a list of objects that can be excluded (or included) from the light's effects, such as illumination or shadow casting.

Mode – Specifies whether the objects in the list are considered Included or Excluded from the light's effects.

Objects – Creates a list of objects to be excluded or included. When an object is added to the list, the  icons appear, which determine how the light affects the object. The  icon controls whether the emitted light affects the object. The  icon controls whether the shadows caused by the light affect the object. The  determines whether the object's children are affected.