V-Ray IES Overview
VRayLightIES is a special kind of VRay light that uses an .ies file to specify the shape of the light. An .ies file contains complete specifications of a real world light bulb or tube. They are usually provided by the manufacturer and the information in those files is gathered through lab experiments. V-Ray can use these files to create light sources that will act the same way as the real bulb.
Basic Parameters
Enabled - turns the light on and off.
IES File - allows you to choose an .ies file to use for the current light.
Mode - allows you to chose the mode in which the color of the light will be specified.
Light Color - when the Mode is set to Color this parameter controls the color of the light. When using photometric units, this color is normalized so that only the color hue is used, whereas the light intensity is determined by the Intensity multiplier.
Temperature - when the Mode is set to Temperature this parameter controls the color of the light.
Filter color - this is a color multiplier for the light.
Intensity - this is the intensity of the light.
Shadows
Shadows - when on (the default), the light casts shadows. Turn this option off to disable shadow casting for the light.
Soft Shadows - sometimes the .ies file contains information about the physical shape and size of the bulb. When this option is on the light will cast shadows according to this information. Otherwise the light will cast shadows as if it was a Point Light.
Shadow bias - the Shadow bias moves the shadow toward or away from the shadow-casting object (or objects). If the value is too low, shadows can "leak" through places they shouldn't, produce moire patterns or create out-of-place dark areas on meshes. If the Shadow bias is too high, shadows can "detach" from an object. If the value is too extreme in either direction, shadows might not be rendered at all.
Shadow Color - this parameter controls the color of shadows for this light. Note that anything different from black is not physically correct.
Sampling
Subdivs - this value controls the number of samples V-Ray takes to compute lighting. Lower values mean more noisy results, but will render faster. Higher values produce smoother results but take more time. Note that the actual number of samples also depends on the DMC Sampler settings.
Light cutoff threshold - this parameter specifies a threshold for the light intensity, below which the light will not be computed. This can be useful in scenes with many lights, where you want to limit the effect of the lights to some distance around them. Larger values cut away more from the light; lower values make the light range larger. If you specify 0.0, the light will be calculated for all surfaces.
Options
Affect Diffuse - this determines whether the light is affecting the diffuse properties of the materials.
Affect Specular - this determines whether the light is affecting the specular of the materials. This means glossy reflections.
Area Speculars - when this parameter is enabled, the specular highlights will be computed with the real light shape as defined in the .ies files. For this to work you also have to enable Soft Shadows. When this parameter is disabled, the specular highlights will be computed using a point light approximation instead of the real shape.
Diffuse contribution - a multiplier for the effect of the light on the diffuse.
Specular contribution - a multiplier for the effect of the light on the specular.
Photon Emission
Photons subdivs - this value is used by V-Ray when calculating the Global Photon Map. Lower values mean more noisy results, but will render faster. Higher values produce smoother results but take more time.
Diffuse multiplier - a multiplier for the diffuse photons.
Caustics subdivs - this value is used by V-Ray when calculating Caustics. Lower values mean more noisy results, but will render faster. Higher values produce smoother results but take more time.
Caustics multiplier -this value is a multiplier for the generated caustics by the selected light. Note that this multiplier is cumulative - it does not override the multiplier in the Caustics render roll-out section.
Light Shape
The Light shape parameters allow you to control the shape of the light when its being sampled for soft shadows. Some .ies files have information about the light shape and you can use that. Otherwise you can choose from a list of predefined shapes and adjust their size.