A Light Dependent Resistor (LDR) changes its electrical resistance based on the illumination level.

LDRs can be used to detect slow changes in illumination level. If you want to use them to measure actual illumination levels you need to calibrate your circuit using a lux meter. The spectral sensitivity (sensitivity as function of the wavelength of light) of an LDR resembles that of a human eye closely. If you want to detect fast changes a photo diode or photo transistor may be a better choice.

From the datasheet of a given LDR you can obtain the actual relation between illumination level and the electrical resistance (usually in the range of kilo ohms). Increasing the level of illumination will decrease the resistance accordingly. Normal illumination levels are between 100 and 5000 lux for living and work spaces.

Most important properties

  • Dark resistance value (usually hundreds or more kohms)
  • Resistance value at illumination level of 10 lux, usually expressed as minimum, typical, and maximum value (usually in the kohm range).
  • Resistance sensitivity (resistance change as function of illumination level change) in linear approximation expressed as γ (values about 0.8 to 0.95).
    $\gamma = \frac{\log(\frac{Ra}{Rb})}{\log(\frac{b}{a})}$
    Where Ra and Rb are the resistance values at illumination levels a and b respectively.

    LDR resistance versus illumination

 

Notice:
The response time to large changes in illumination level can be several seconds.