Simulates light from an area source (a quadrilateral, specified by 4 vertices). Can be used to model light from an extended source (flourescent strip lights) or in some circumstances from a window.
As well as the settings that are common to all lights, the Quad Light also has these attributes:
When a shader is connected to a quad_light, Arnold will automatically construct importance sampling tables similar to those used in the skydome_light. This permits efficient sampling according to the luminance of the texture, which can greatly reduce sampling noise, especially when using HDRI textures. Like the skydome_light, the resolution of the table is controlled by the resolution parameter with a default value of 512. If you are using a color image as an input then you will not need to set this value higher than the resolution of the image passed to the color parameter.
Emits light focused in the direction along the normal. The default spread value of 1 gives diffuse emission, while lower values focus the light more, until it becomes a laser like beam at value 0. Low spread values can be noisier than the default high spread, so be careful when using them.
The animation below shows the effect on an interior scene when decreasing the 'Spread' value.
Spread: 1 to 0.25
Cars lit with large 'quad' area lights simulating soft box studio lighting
Quad light positioned in front of monitor (rollover image)