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Hi bashiruddins,

The ALS-PT19 ambient light sensor on the Weather Shield is designed to output an analog voltage. In order to convert that to Lux, you would need to refer to the graphs in the datasheet linked above. To "calibrate" the sensor with the ADC you have it connected to, point light with a known Lux value at the sensor, record the output and then repeat with another light with a different Lux value at it. That should give you some basic "calibration" information to create a correlation between the analog output and Lux values you could then use to convert between the two.