This Light Dependant Resistor (LDR) can be used to easily measure ambient light with your Arduino or other board with analog input. You will need to set this up as a voltage divider with a resistor of around 10KΩ.

 

Specification:

Maximum voltage: 150V, DC
Maximum wattage: 90mW
Operating temperature:  -30°C ~ 70°C
Spectral peak: 540nm
Bright resistance (10Lux) (KΩ): 5 - 10
Dark resistance: 0.5 MΩ
100λ10: 0.5
Response time: 20ms (Rise), 30ms (Down)
Resistance illumination: 2