Sensor BH1750

How to measure voltage BH1750 , to calibration

Calibration is simple. Known input, check output.
In this case: shine a light on it with known brightness, check the output.

want to know the output voltage with a Voltmeter and adjust it to the datasheet, but why is the output voltage only 4V and does not change with the light provided?

Because it's an I2C output, not an analog signal. Have your Arduino read it. There's quite likely a library available for the thing.

yes I am using the BH1750FVI Library so that the output is lux without the need for calculations, but in my thesis for data analysis, I need to measure the output voltage, is there a way to find out the output signal ??

It's a digital signal. Voltage switches between 0 and Vcc. Connect it to a scope or logic analyser.

Thank you very much