Light blockers give flexibility to the lighting TD. They can be used as an artificial method of masking light in a scene without the overhead of adding additional geometry. Used carefully, they provide a degree of artistic freedom, allowing you to define the light boundaries in non-physical ways.
The Light Blocker is a light filter that specifies a primitive volume defined in the Geometry Type (box, sphere, cylinder, or plane) which when connected to a light node will block or modify the light as it passes through that volume. The blocker will only influence a light it is connected to, just like other filters (i.e. the geometry affects nothing else, including GI, and is invisible). One, simplistic use would be to mimic the effect of a complex lamp housing, (possibly in combination with the barndoor filter) without modeling the lamp geometry. Another use would be to create shadows in your scene that look different to how they normally would.
A video showing the Ai LightBlocker in use can be found here.
In the following examples, an Ai LightBlocker is connected to an Ai AreaLight. The translation, rotation, and scale of the light blocker are determined by a simple wireframe shape node. By default, the position of the light blocker will appear at 0,0,0.
Determines the shape of the blocked light. A light blocker can be a box, cylinder, sphere, or plane.
Box. Rollover image for GUI (Maya).
Cylinder. Rollover image for GUI (Maya).
Sphere. Rollover image for GUI (Maya).
Plane. Rollover image for GUI (Maya).
This value is the strength of the light blocker effect. The light blocker will not be apparent unless the density value is above 0.
Plug the output of the shader you wish to use as a masking effect here. For example, a texture map used to represent the shadow of some tree leaves. Unlike gobos you can position a shadow independently of the light transform. It works with all types of lights instead of just spot lights (unlike the gobo which also contributes illumination to the scene). It only works when 'Geometry Type' is set to Box.
Tree mask connected to Shader. Mask is static when light position changes.
An example scene file can be found here.
Attenuates the ramp based on the direction set.
This is the magnitude of the ramp multiplier, applying along the Ramp Axis direction. Negative values flip the Ramp Axis direction.
Attenuates the edge of the height of the light blocker.
Attenuates the edge of the width of the light blocker.
Increases the circular shape of plane light blockers.