Measure Light Intensity using Light Dependent Resistor (LDR)

The resistance of the Light Dependent Resistor (LDR) varies according to the amount of light that falls on it. The relationship between the resistance R_L and light intensity Lux for a typical LDR is

If the LDR connected to 5V through a 3.3K resistor, using the voltage divider rule, the output voltage of the LDR is

Substituting R_L from equation 1 into equation 2, we obtain the light intensity

For a low cost LDR, at the same light intensity, the part to part variation in resistance can be as high as 50%. Therefore such a low cost LDR is seldom used for measuring light intensity but more for light presence/absence detection.