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

R_L = 500 / Lux Kohm

With the LDR connected to 5V through a 3.3K resistor, the output voltage of the LDR is

Vo = 5*R_L / (R_L+3.3)

Reworking the equation, we obtain the light intensity

Lux = (2500/Vo – 500)/3.3

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 level detection.

Table 1: Typical Low Cost LDR Specification

If a more accurate light intensity measurement is required, we recommend using the Photodiode (see Application Note on Measuring Light Intensity using Photodiode)

The LabVIEW VI that reads the voltage across the LDR and converts the voltage to lux is called EMANT300 Example LDR.VI

Front Panel

Block Diagram