These settings provide some general control over how lights are evaluated in Arnold. A low light threshold can be set, which can speed up rendering by allowing Arnold to skip the evaluation of light samples when the amount of light is below a certain value.
Normally, all lights in the scene can illuminate all objects in the scene. However, Maya allows you to override this by 'linking' lights with specific objects so that a light will only illuminate an object that it is linked to. By default, MtoA is set to follow whatever light linking is defined in the Maya scene, but you can override this. MtoA also lets you separately control whether shadows behave the same way.
Raising this value can speed up rendering by allowing Arnold to ignore tracing a shadow ray for light samples whose light contribution is below a certain value. The point of Low Light Threshold is to save casting shadow rays when Arnold knows that the error from not casting that ray is below a certain amount. This makes sense because below a certain threshold there will be no perceptible difference between shadowed and unshadowed areas.
Roughly, what is taken into account to discard a shadow ray is its maximum possible incoming irradiance, which is linearly related to the light's intensity and decreases with distance. The specific number will vary depending on your exposure and light intensity values, but the default value is reasonable for the default exposure/light intensity values. This will typically help more in scenes with lots of lights with a finite area of influence, like a cityscape for example. It will help much less with a single strong source of light.
You should never be able to see the boundary when this optimization kicks in. If you can see it, you need to make the threshold lower. The aim is to prune shadow rays that don't contribute to the final image
In the example below, increasing the Low Light Threshold to 0.1 shows a good speedup in render time (in this case, more than twice as fast) with little noticeable difference in the image.
A slightly higher threshold can offer significant rendering speed increases without noticeably darkening the image (rollover image).
However, increasing the Low Light Threshold value too much can have an adverse effect on the lighting:
The Utility shader's nlights color mode can be used to see how many lights are influencing a shading point when adjusting the Low Light Threshold.
Low Light Threshold: 0 to 1. Red = all lights, blue = low values, black = no lights.
Turn off light linking ('None') or use Maya's light links ('Maya Light Links').
Ensure that Light Linking is set to none when instancing lights. Otherwise, the instanced light will not render.
Shadow linking can be set to be the same as the setting for light linking ('Follows Light Linking') or you can specify explicitly that shadow linking should be turned off ('None') or use Maya's shadow linking ('Maya Shadow Links').