mental ray attributes are available only for point, spot, directional and area lights. Find these attributes on the LightShape node in the Attribute Editor.
mental ray area lights can be created using spot lights or area lights. For more information on mental ray area light attributes, see mental ray Area Light Attributes (created using area light) and mental ray Area Light Attributes (created using spot light).
Caustic and Global Illumination
- Emit Photons
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Turns on photon emission for the light source.
- Energy
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The amount of light distributed by the light source. Each photon carries a fraction of the light source energy.
8000 is the default. 0 means no photons are emitted.
If caustic effects are not bright enough, try increasing these values equally, in small increments (to 12000, to start) until you have better results.
If ‘hot spots’ (blown out) areas occur, try decreasing these values equally to around 4000 or 5000.
- Exponent
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This is similar to decay—the intensity increases as the value decreases. To increase the chances that indirect light will reach a greater distance, decrease the value.
Visible noise can occur with values less than 1. The default (2) simulates natural (quadratic) decay, but violates the conservation-of-energy law (that happens in the natural world), so bright spots from distant light sources could occur in unexpected locations.
- Caustic Photons
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The number of photons to be generated by the light source and therefore stored in the photon map. The default, 10000, is suitable for quick, low-quality caustics. To improve the quality of caustic effects, increase this number, incrementally, to 20000 to start (render time will increase as a result). Generally, 100000 produces medium quality and 1000000 produces highly accurate effects.
Shadow Map Attributes
- Shadow Map Format
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Select between Regular Shadow Map and Detail Shadow Map. For more information, see mental ray shadow maps.
- Use mental ray shadow map
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If on, shadow maps are generated by mental ray. If off, shadow maps are generated by Maya. You can apply render layer overrides to this attribute.
- Take Settings from Maya
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Click this button to load settings from Maya.
- Resolution
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Sets the resolution for the shadow map
- Samples
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Specifies the number of samples per pixel.
- Softness
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Determines the softness of the shadow.
- Bias
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The Bias attribute applies the specified Bias value on a per-light basis, resulting in a slight offset to the shadow depths, and a slightly shifted shadow. This option is useful in tuning shadows in specific cases, such as when rendering Maya® Fur™.
When the per-light Bias attribute is set to 0, the general Shadow Map Bias setting in the Render Settings (Render Settings window, Options tab, mental ray Overrides section, Shadow Map sub-section) is used. When the per-light Bias is a value greater than 0, the specified per-light Bias value is used.
The bias value should typically be smaller than the smallest distance between a shadow caster and a shadow receiver. However, bias values that are too small may cause self-shadowing artifacts.
- Shadow Map File Name
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Enter a file name into the text field for the resulting shadow map.
- Add Light Name
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Turn on if you want the name of the light included in the shadow map.
- Add Frame Ext
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Turn on if you want the frame extension to be included in the shadow map.
- Add Scene Name
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Turn on if you want the name of your scene to be included in the shadow map.
Use Shadow Map Camera (directional lights only)
Use the shadow map camera to control the area of focus for directional lights.
- Camera Resolution
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Resolution of the shadow map camera. A Maya shadow map camera is square shaped.
- Camera Aperture
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The width of the camera’s aperture.
- Camera Aspect
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Shadow map X resolution to Y resolution. Since Maya uses a square shadow map, this value is always 1.
- Camera Name
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Name for your camera.
- Camera Focal
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The focal length of the camera, in inches. For a directional light, it is “infinity”.
Detail Shadow Map Attributes
- Detail Shadow Map
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Turn on this attribute to use a mental ray detail shadow map for the light.
- Samples
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Specifies the number of samples per pixel. If this is set to 0, a mental ray for Maya internal default value is used.
- Accuracy
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Determines the distance required between two depth values in a sample, in order for the values to be considered distinct. Setting a very low accuracy value results in larger memory and resource requirements, but increases quality. Setting a very high accuracy value leads to visible artifacts. If this is set to 0, mental ray for Maya determines a reasonable value based on the scene.
- Alpha
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Turn on this attribute to use only the scalar intensity of the transmission color of the object for the detail shadow map creation. This results in grayscale shadows.
Custom Shaders
If you connect a mia_photometric_light to either the Light Shader or Photon Emitter attribute, then duplicate the spotlight using Edit > Duplicate Special > 
with the Duplicate input connections option enabled, the duplicated light does not drive the cone angle. Only the original light drives the cone angle for the duplicated light. To workaround this issue, use the Duplicate input graph option instead when selecting Edit > Duplicate Special > .
- Light Shader
- Connect a light shader to compute the amount of light contributed to by the light source to a previous intersection point, as well as to perform computations for shadow casting.
- Photon Emitter
- Connect a photon emitter shader for use in the photon tracing phase to control the emission of photons from the light sources.