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TL;DR:

Light linking determines which objects a light illuminates. Shadow linking determines which objects cast shadows for that light. In Solaris, the LOP Light Linker lets you create and manage both relationships.

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What Is Light Linking?

By default, lights in a 3D scene illuminate every eligible object within their effective range. Light linking overrides this behavior by specifying which objects each light can illuminate.

Why is it useful?

Light linking gives lighting artists more control over the final image without restructuring the entire scene.

Common uses include:

Art direction: light a character without unintentionally illuminating nearby objects.
Look development: control how individual props respond to each light.
Lighting separation: apply distinct lighting treatments to different elements.
Shot refinement: adjust part of an image while limiting unintended changes elsewhere.

lightlinking_hos_re1mon_gerardojustel_red.mp4

Light Linking in Solaris

Solaris uses USD primitives (prims), along with their properties and relationships.

The Light Linker LOP creates USD light-link properties that define which scene objects each light affects.

These properties act as a set of rules:

Element	Role
Light	The source of illumination
Target prim	An object that may receive illumination
Include rule	Specifies objects the light should affect
Exclude rule	Specifies objects the light should not affect

USD also supports collections, allowing rules to target groups of prims instead of individual objects. The Light Linker interface lets you manage these relationships without manually authoring USD properties.

Creating Light Links with LOP Light Linker

The Light Linker LOP is the dedicated node for managing these relationships.

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In Solaris, the Light Linker LOP uses rules to define these relationships for individual prims or collections, giving you precise control over lighting without duplicating the scene or modifying its geometry.

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https://www.sidefx.com/docs/houdini/nodes/lop/lightlinker.html

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Light 1 Affecting Only the Skull

Default No Light Mix

With Light Mix

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Light 2 Affecting Only the Ground

Default No Light Mix

With Light Mix

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Step 1 — Open your scene

Step 2 — Add the LOP Light Linker

The interface is divided into three areas:

Left: the lights in the scene.
Center: the light-linking rules.
Right: the model prims in the scene graph.

LOP Light Linker interface

Step 3 — Create a light-linking rule

Let's say you want the Key Light to illuminate only the skull.

Find the Key Light in the left-hand tree.
Drag it into the central rules area to create a rule card.
Find the skull in the right-hand tree.
Drag the skull into the rule card's Include list.
If you want to explicitly prevent the light from affecting them, add the crown and grid to the Exclude list.

The rule now defines which objects the Key Light affects.

Create a light-linking rule