what is the sensor I should use for digital lux meter

 

what is the sensor I should use for digital lux meter

photodiode or LDR.. these directly work on light interaction. you will have to use ADC for conversion. a small and powerful controller would be ATTiny13

 

If you want to measure light, it must be calibrated with a known accurate light meter.

 

If you want to measure light, it must be calibrated with a known accurate light meter.

You can measure light without calibration, but of course you're right that the result won't be in lux units without calibration.