12.4.2 Iray attributes: Light Portal

A typical problem with architectural scenes is that environment light can sometimes only enter through few small openings, such as small windows. In cases like this the convergence can often be improved by giving the renderer a hint where these openings are in the scene.
For Iray Photoreal this can be done by placing an arealight over the openings and marking them as light portals using the attribute Light Portal in the Iray section of the placement node of the arealight
Technically, these lights are no longer light sources (they are acting no longer as a light emitter), but merely a hint to Iray Photoreal where to expect environment light from.
Using light portals should not change the rendering result, only convergence speed. However, it should be stated that they do not always help, in fact they can cause lower performance if placed incorrectly. For an architectural scene a rule of thumb for using portals is:
A significant amount of illumination comes through the windows.
Light Portals work only with the standard area light. The don't work with light emitting geometry.
Area Light Attributes section in the Attribute Edtior of the Maya Area Light
Light Portal
Use this attribute to control the portal light functionality. Switches the area light to a light portal
 Light Portal Yes
  Activates the selected arealight as light portal. The arealight emits no light.
Light Portal No
Deactivates the selected arealight as light portal. The arealight emits light.
Light Portal Use Inherited
Applies the settings of the Visibility attribute of the parent node.