The probe measures illuminance (lux) defined as the ratio between the luminous flux (lumen) passing through a surface and the surface area (m2).
There is no need for external power supply. The output
Signal in mV or better in V is given through a resistor shunting the photodiode ends.
The photocurrent generated by the photodiode when hit by light, is
Converted to a potential difference, which is read by a voltmeter. Once the
DDP (Potential Difference) is known, the measured value can be calculated
Through the calibration factor.