Simulates light from an area source (a quadrilateral, specified by four vertices). It can be used to model light from an extended source (fluorescent strip lights) or in some circumstances from a window.
The Lights page has more detail about the controls. In addition to honoring the standard Maya light attributes, the Attribute Editor will also show the following attributes under the Arnold group:
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 almost a laser-like beam at value 0. Currently fully focused laser beams at value 0 are not supported, there is always a small minimum spread. Low spread values can be noisier than the default high spread, so be careful when using them.
The animation below shows the effect when lowering the Spread value.
Spread value lowered from 1 (Quad light)
Changes the shape of the light from a square at 0, to rounded corners, to a disk at 1.
Specifies a smooth falloff for the edges of the light. The value specifies the width of the soft edge, from no soft edge at 0, to smooth falloff all the way to the light center at 1. Works similarly to the penumbra_angle for spot lights.
Cars lit with large 'quad' area lights simulating soft box studio lighting
Quad light positioned in front of monitor (rollover image)