Visible light refers to the visibility of the rays of light between the light source and the object(s) it illuminates. A couple of examples would be a car’s headlights on a foggy road; or spot lights in a smoke-filled discoteque.
This type of effect is only available for Spot and Omni lights. Area, Infinite and Parallel lights cannot emit visible light. The types of Visible Light from which you can choose are: Visible, Volumetric and Inverse Volumetric. Examples of each type is shown in the image below in which a Spot light is used to illuminate a group of cubes. At the far left is the basic setup itself; from left to right, the types of Visible Light are: Visible, Volumetric, Inverse Volumetric.
The simple visible light option creates a foggy effect within the area of illumination. Objects illuminated by this type of visible light will not cast shadows. The Volumetric light on the other hand will cast shadows and will take correspondingly longer to render. A good example of volumetric visible light is sunlight passing through the leaves of a tree, which will create a "streaking" of light rays. As the name already suggests, the Inverse Volumetric light type inverts this effect. The visible light will only be present where objects within the visible light’s area of illumination cast shadows. This effect is commonly used for logo animations to give the impression of the objects being illuminated from behind. The light’s visibility can be defined independently of the range of the light. The visible light’s range and intensity can be adjusted via the Inner Distance and Outer Distance settings in the Visibility tab. The visible light will remain constant until the Inner Distance value has been reached. Note that the visible light is affected directly by the Color and Intensity of the light source. A light source with an Intensity value of 10% will only have an effect equal to 10% on the visible light even if the Brightness value is set to 100% in the Visibility tab.
Rendering volumetric light takes longer since shadows cast from objects in this foggy environment must be calculated as well. The number of steps that will be used are defined via the Sample Distance value. On the right of the image below you can clearly see what happens to the quality of a volumetric shadow if the Sample Distance is set too high. The shadows will fray and fragment. If this occurs, reduce the Sample Distance until these unwanted effects subside.
The visible light’s brightness can be inverted when Dust isapplied (see example below). The Brightness value was reduced to 100% and the Dust value raised to 100%. This will cause the visible light’s bright fog to turn dark.
Furthermore, a smoke pattern can be added, which will cause the fog pattern to become less uniform. This effect can be added in the light’s Noise tab. The turbulence can be applied to both the illumination itself and the visible light. This Noise effect’s pattern can be varied and can also be moved via a virtual wind, which makes it ideal for simulating passing wafts of fog.