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To create a mesh light in Solaris place down a PxrMeshLight LOP node and assign the mesh primitives you wish to turn into a light.
Inside the node you will find similar parameters to the normal Solaris Light LOP (exposure, intensity, color).
You may find that the mesh is illuminating the scene (and self illuminating) but does not emit light in the camera.
The mesh camera glow is controlled by the material so you may want to set something in the glow parameter of the PxrSurface material. This gives you greater control of how the object looks to the camera and how it illuminates the rest of the scene.
An additional parameter "Material Sync Mode" offers more control of the mesh visibility and how the material glow affects the rest of the scene:
materialGlowTintsLight - Mesh is visible to camera and the output is multiplied by the mesh's material glow parameter (including textures / shading networks).
independent - Mesh is visible
noMaterialResponse - Mesh is invisible
If your mesh is visible you may also need to turn off indirect visibility using a Geometry Render Settings LOP so it doesn't illuminate the scene twice.
"Material Sync Mode" does not function in Houdini 19.5. You can control visibility using a Geometry Render Settings LOP. You may also need to restart render for "Material Sync Mode" to take effect.
Currently you can only turn "Mesh" primitives into mesh lights. Other primitives such as "Sphere" will not work. In order to get mesh lights working in older versions of USD we turn the Mesh primitive into a PxrMesh primitive. Both these limitations should be resolved in a future version of Solaris.