There are some cases in which we only want a light to cast diffuse light, or we may want a light to cast only specular light. We achieve this with a Light Gel.
A typical case of wanting to avoid specular highlights in a light would be when using a Directional light for a quick skylight – instead of a Light Dome.
The problem with this Directional Light is the extra specular highlights we get.
To avoid this bluish highlight we have the Light Gel for the light, and in the Light Gel, the Specular Reflectance parameter.
Creating a new Gel, setting the Specular Reflectance to zero and applying it to the light in question eliminates this extra highlight.
Of course, the converse – just specular highlights – can be achieved setting the Diffuse Reflectance parameter to zero.
This ability makes using Directional lights as skylights viable, saving us much rendering time for simple projects.
