The Sunlight Discount Factor was introduced to compensate for the fact that in most installations the sensor will measure the effect of the artificial lights differently than the effect of sunlight on the work surface. Ideally, all of the light measured by the sensor will be light that is reflected off of the work surface and up at the sensor. This would result in a constant ratio between the light measured at the work surface and the light measured at the sensor. Often the sensor also measures sunlight which does not bounce off the work surface but instead comes from bouncing off other surfaces. The result is that the ratio of the artificial light hitting the work surface to the artificial light measured at the sensor is different than the ratio of the sunlight hitting the work surface to the sunlight measured at the sensor. This ratio is always higher for sunlight in that it takes a larger change in sunlight than artificial at the sensor to result in the same change at the work surface.
To compensate for this effect the part of the light measurement which results from sunlight is divided by the Sunlight Discount Factor. This is possible since the artificial contribution is known. For On/Off Daylighting Control Products it can be shown that this is mathematically equivalent to multiplying the Set Point by the Sunlight Discount Factor. This is also true for Automatic Dimming Control products when the lights are off or at full bright. When the Automatic Dimming Control products are actively dimming this simplification is not possible and the effective threshold will vary along with the dimming level.