Measuring Light

As we've said, a photocell's resistance changes as the face is exposed to more light. When its dark, the sensor looks like an large resistor up to 10MΩ, as the light level increases, the resistance goes down. This graph indicates approximately the resistance of the sensor at different light levels. Remember each photocell will be a little different so use this as a guide only!

Note that the graph is not linear, its a log-log graph!

Photocells, particularly the common CdS cells that you're likely to find, are not sensitive to all light. In particular they tend to be sensitive to light between 700nm (red) and 500nm (green) light.

Basically, blue light wont be nearly as effective at triggering the sensor as green/yellow light!

What the Heck is Lux?

Most datasheets use lux to indicate the resistance at certain light levels. But what is lux ? Its not a method we tend to use to describe brightness so its tough to gauge. Here is a table adapted from a Wikipedia article on the topic!
Illuminance Example
0.002 lux Moonless clear night sky
0.2 lux Design minimum for emergency lighting (AS2293).
0.27 - 1 lux Full moon on a clear night
3.4 lux Dark limit of civil twilight under a clear sky
50 lux Family living room
80 lux Hallway/toilet
100 lux Very dark overcast day
300 - 500 lux Sunrise or sunset on a clear day. Well-lit office area.
1,000 lux Overcast day; typical TV studio lighting
10,000 - 25,000 lux Full daylight (not direct sun)
32,000 - 130,000 lux Direct sunlight
Last updated on Nov 19, 2015 Published on Jul 29, 2012