The VRayLightRect, also known as the V-Ray Rectangle light or an "area light," is a planar light source. Its shape can be set to a rectangle or a circular disc.

^{Image © Yuki Sugiyama}

Certain parameters are only available as node inputs for the Light Rectangle. For optimal editing, open the light in the V-Ray Node Editor. Follow these steps to see all the parameter locations for this node:

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This section describes the input sockets of the node. Some sockets have dedicated parameters, which are further described below, and some sockets are just inputs.

Transform – Allows a V-Ray Matrix node to be connected to determine the rotation and scale of the light.

Pivot Offset – Allows a vector-outputting node to be connected to control the pivot of the light. Alternatively, they can be set manually.

Color (input socket) – Overrides the color of the light, regardless if Color Mode is set to Color or Temperature.

Shadow Color (input socket) – Overrides the Shadow Color and allows a texture to be connected to control that property.

Intensity Tex (input socket) – Allows an integer-outputting node to be connected to control the Intensity.

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

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

Color – Controls the color of the light when Color Mode is set to Color. 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.

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

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. 

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

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.

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.

DoubleSided – 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.

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 channel to evaluate custom light contribution in the scene.

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. 

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 settings.