The visible light sensor in the Go Direct® Light and Color Sensor (GDX-LC) uses a photodiode to measure illuminance, which is a photometric measurement. It measures the intensity of incident light weighted toward the response of the human eye. The unit of measure of illuminance, and thus the sensor, is lux.

The visible light sensor spectral response is 320 to 730 nm.

The most recent version of the UV sensor has a half-sensitivity range from 300 nm to 425 nm. It is important to note that this range extends into the blue end of the visible light spectrum. This sensor will provide a non-zero response to some deep-blue but visible light sources, such as fluorescent light bulbs. The label indicating the location of the sensor for this version reads “UV-BLUE.”

If the label for the UV sensor is not “UV-BLUE” then the sensor has a half-sensitivity range from 320 nm to 375 nm — spanning most of the region considered to be UVA. This sensor does not respond significantly to the visible light spectrum.

The UV sensor measures irradiance, which is a measurement of the amount of light energy incident on a unit of surface area. The units for irradiance are mW/cm^2. However, due to the sensor range and half-sensitivity, this value should be treated as a relative value.

The RGB sensors also measure irradiance. However, since the sensors have a range of response around the peak values(465 nm, 525 nm, and 615 nm) we determined that it was most appropriate to use the values as relative intensity, rather than report units of mW/m^2. The relative intensity has a range of 0 to 1000 in relative units.

For additional information refer to the user manual.

Go Direct Light and Color Sensor Troubleshooting and FAQs
What is the conversion from lux to W/cm2 ?