The VRayLightMesh allows you to create direct mesh based lighting in your scene. Using this feature allows you to create light sources that have volume and shape without the need to use self illuminated objects and global illumination. To convert a mesh to a light source you need to add the VRayLightMeshProperties node to it. This is done by selecting the object(s) and executing the Turn selection into lights command from the Create > V-Ray menu.
If the VRayLightMesh is close to other surfaces in the scene, it is best to use it with GI enabled. This will allow V-Ray to use combined direct and GI sampling of the mesh light for best results. Without GI, the light may produce noisy results for surfaces that are very close to it.
Enabled - turns the VRayLightMesh on and off.
Color Mode - allows you to chose the mode in which the color of the light will be specified.
Light Color - the color of the light. When using photometric units, this color is normalized so that only the color hue is used, whereas the light intensity is determined by the Intensity Multiplier.
Temperature - when the Color Mode is set to Temperature this parameter controls the color of the light.
Intensity Multiplier - multiplier for the light color; this is also the light intensity in the units chosen by the intensity units parameter.
Units - allows you to choose the light units. Using correct units is essential when you are working with the VRayPhysicalCamera. The light will automatically take the scene units scale into consideration to produce the correct result for the scale you are working with. The possible values are:
Default (image) - the color and multiplier directly determine the visible color of the light without any conversion. The light surface will appear with the given color in the final image when seen directly by the camera (assuming there is no color mapping involved).
Lumious power (lm) - total emitted visible light power measured in lumens. When this setting is used, the intensity of the light will not depend on its size. A typical 100W electric bulb emits about 1500 lumens of light.
Luminance (lm/m²/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.
Radiant power (W) - total emitted visible light power measured in watts. When using 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 as visible light.
Radiance (W/m²/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.
Subdivs - this value controls the number of samples V-Ray takes to compute lighting. Lower values mean more noisy results, but will render faster. Higher values produce smoother results but take more time. Note that the actual number of samples also depends on the DMC Sampler settings.
Light cutoff threshold - this parameter specifies a threshold for the light intensity, below which the light will not be 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 from the light; lower values make the light range larger. If you specify 0.0, the light will be calculated for all surfaces.
Override motion blur samples - allows you to overide the default number of samples that will be used to sample the current light for motion blur.
Motion blur samples - when the Override motion blur samples is enabled this value will be used when sampling the motion blur created by the current light.
Shadows - when on (the default), the light casts shadows. Turn this option off to disable shadow casting for the light.
Shadow bias - the Shadow bias moves the shadow toward or away from the shadow-casting object (or objects). If the value is too low, shadows can "leak" through places they shouldn't, produce moire patterns or create out-of-place dark areas on meshes. If the Shadow bias is too high, shadows can "detach" from an object. If the value is too extreme in either direction, shadows might not be rendered at all.
Shadow Color - this parameter controls the color of shadows for this light. Note that anything different from black is not physically correct.
Use Texture - this tells the light to use a texture for the light surface. If there are surfaces which are close to a texture-mapped light, it is best to have GI enabled. This allows V-Ray to use combined direct and indirect sampling for the light, reducing the noise for surfaces close to the light.
Texture - specifies the texture to use.
Cache Texture - when this option is checked V-Ray will sample the texture and produce a lower resolution one to be used for the lighting. This greatly speeds up the rendering of mesh lights with textures.
Texture Cache resolution - specifies the resolution at which the texture is cached.
Ignore light normals - normally, the surface of the source emits light equally in all directions. When this option is off, more light is emitted in the direction of the source surface normal.
No decay - normally the light intensity is inversely proportional to the square of the distance from the light (surfaces that are farther from the light are darker than surfaces which are closer to the light). When this option is on the intensity will not decay with distance.
Double Sided - this option controls whether light is beamed from both sides of each face.
Invisible - this setting controls whether the shape of the VRayLightMesh source is visible in the render result. When this option is turned off the source is rendered in the current light color. Otherwise it is not visible in the scene. Note that this option only affects the visibility of the light when seen directly by the camera or through refractions. The visibility of the light with respect to reflections is controlled by the Affect specular and Affect Reflections options.
Skylight Portal - when this option is on, the Color and Multiplier parameters are ignored; instead the light will take its intensity from the environment behind it.
Simple Skylight Portal - this option is only enabled if the Skylight Portal option is on. It tells the VRayLightMesh that there is nothing of interest behind the light itself, and so the environment color can be used directly. Normally, the portal light takes its color from whatever objects are behind it. In order to do this, the light traces additional rays, which may slow down the rendering. Turning this option on makes the rendering of portal lights faster.
Store with irradiance map - when this option is on and GI calculation is set to Irradiance map V-Ray will calculate the effects of the VRayLightMesh and store them in the irradiance map. The result is that the irradiance map is computed more slowly but the rendering takes less time. You can also save the irradiance map and reuse it later.
Affect Diffuse - this determines whether the light is affecting the diffuse properties of the materials.
Affect Specular - this determines whether the light is affecting the specular of the materials. This means glossy reflections.
Affect Reflections - this determines whether the light will appear in reflections of materials. This means both perfect and glossy reflections.
Diffuse contribution - a multiplier for the effect of the light on the diffuse.
Specular contribution - a multiplier for the effect of the light on the specular.
Photons subdivs - this value is used by V-Ray when calculating the Global Photon Map. Lower values mean more noisy results, but will render faster. Higher values produce smoother results but take more time.
Diffuse multiplier - a multiplier for the diffuse photons
Caustics subdivs - this value is used by V-Ray when calculating Caustics. Lower values mean more noisy results, but will 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 light. Note that this multiplier is cumulative - it does not override the multiplier in the Caustics render roll-out section.