The light sensor in the Go Direct® Light and Color Sensor, order code GDX-LC, $79 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 the sensor is lux.
Its spectral response is in the range of 320 to 730 nm.
The UV sensor measures irradiance - or power per unit area - in units of mW/m^2.
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 - 1000.
For additional information refer to the user manual.
TIL 3966: Go Direct Light and Color Sensor Troubleshooting and FAQs
TIL 413: What is the conversion from lux to W/cm^2 ?