Lux is determined in the Ambient Light Measurement Gain register as LUX/LSB.
For example, with ALSTIM set to '10' (10 bit resolution) and ALSPGA set to '01'(0.125 Lux per LSB) the device can detect up to 128 Lux full scale(1024 x 0.125). The full scale light the device can read with the settings at ALSTIM '10' and ALSPGA '00' is 32 Lux. The device shows an error message as the light seen is higher than 32 Lux. Any reading when the error message shows is incorrect.