In production, it often becomes necessary to create non-physical effects and art-directed changes to your shot. Light Path Expressions (LPE) are a powerful way to collect and output specific light paths for alteration later.

Some examples of the power of LPE are the ability to:

• Allow for additive compositing, preserving the physical nature of the image and avoiding artifacts through multiplication or division in post-processing.
• Output a specific bounce of light (2nd, 3rd, etc.) or even a range of bounces (4th to 8th bounce of light). You can use this to isolate effects or even noisy bounces for subtraction.
• Output a specific object to an LPE. While you can use Cryptomatte to isolate objects with an ID, you can also output the object or collection of objects and their contribution separately.
• Isolate a particular noisy light path for subtraction in comp or an LPE without these light paths to composite.
• Create a non-physical effect like differently colored caustics from their source, change the color cast of an object or scale lighting.
• Output per-light AOVs for manipulation later.

In this example, we're going to create a LPE from an individual light in our studio scene.

First, select the light you want to create a Light and under the RenderMan tab → Light Group, create a name for your light groups.  Note, you can add more than one light into a light group to create various isolated lighting setups.

Next, we want to create the actual LPE expression to be able to isolate this light in our AOV channels.  




At the very bottom, under the Extra Render Vars section, you will there be able to enter in the desired information to create the LPE.  As you can see, the LPE will be called SIdeRoom_LPE and the source name defines the structure of the LPE and also included in the expression is the SideRoomLight, which refers to our previously created Light Group.

More information on LPE expressions can be found here

The image below demonstrates the SideRoomLight LPE contribution.