The LDR (light-diode resistor) is perhaps one of the very first component that an electronics engineer will be using, the functioning principle being very simple to explain and the component being one of the most cliche sensors out there that one can use to detect light intensity.

The major shortcoming of the LDR is that, even though the sensor module has an analog output, an output that is proportional to the resistence of the diode, that is, in turn, relative to the ambient amount of light, is that the LDR is not meant to perform actual measurements.

In other words, the sensor and the module will output a value that is just a relative measurement of the illumination level, but the value cannot be converted into, say "lux" or any other standard physical measurement unit for a proper measurement without having the necessary equipment to calibrate the LDR. Calibration would involve shining a light onto the LDR and then using the potentiometer on the module to somehow correlate the analog value output by the sensor to the actual reading in lux or lumens of the light source.

Having said that, many applications are still valid and it is clear that a layman's calibration is still possible to make the light sensor distinguish between pitch dark (by covering the LDR with a finger, for example) and ambient light.