The Light Analysis Render Element provides visual representation of the lighting intensity in the rendered frame. It collects Illuminance and Luminance information during rendering and maps it as color gradent or a grid of measured values onto the frame.
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Illuminance measures the amount of light on a surface per unit area in lux. It shows how bright your surface is. Luminous intensity measures the brightness of a light source in candela.
The collected lighting information is stored in internally generated Illuminance and Luminance render channels using the respective light measurement units. Then, the Lighting Analysis Render Element simply takes the information from the selected channel and applies display modifications to it. This makes it readable with a naked eye when overlayed onto the fianal render as a post-effect.
During the render, the lighting analysis may display some approximation to the false-color image, but it should not be interpreted as being the final result. Once rendered, the VRayLightingAnalysis parameters can be adjusted further and its representation can be updated without the need to re-render.
To use render elements, select from the Available render elements column and click the Add button to add the passes you wish to generate at render time. When you render the scene, the added render elements listed in the column on the right, will also render, which can be viewed from the VFB window's channel drop-down menu.
For more information on the parameters on this dialog, see the Render Elements tab page.