distinguishing daylight from artificial light

Is there a characteristic of daylight (Sunny or cloudy condition) that artificial light such as from a street doesn't have? I would like to design a sensor that would turn a light on or off depending on the natural lighting but ignores artificial light.

david90 said:

Is there a characteristic of daylight (Sunny or cloudy condition) that artificial light such as from a street doesn't have? I would like to design a sensor that would turn a light on or off depending on the natural lighting but ignores artificial light.

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That sensor would have to recognise so called "colour temperature" which is measured in Kelvin degrees. You have setting for it for instance in every CRT monitor, so that you can adjust colour temperature that you like the most.

How about Ultraviolet or Ozone?

This can be a difficult problem to solve.
The problem with Color Temperature is that light from the sun is filtered differently depending on the time of day. At sunrise or sunset the color is around 1600K (red sun sunsets) and at noon 5200K (bright blue sky).
The problem with ultraviolet is that many street lamps produce significant UV bands.
Ozone is a gas and doesn't apply to solving this problem.

I think it will be difficult to build a 'general' system that works everywhere. However, if you just want to build something that works at you location, you can probably do this easily. You need to identify the sources of light that you may receive and characterize their spectra. This is actually pretty easy - just find out what kind of lamps are being used and lookup their spectra.

Astronomers are constantly analyzing light from the street because it affects their 'seeing' and there is a lot of info on the internet about this. EG - http://www.astrosurf.com/buil/us/spe2/hresol4.htm shows the spectra for the standard US street lamps.

If your artificial light source is an incadenscent bulb, it's a little harder since its spectrum is fairly continuous. Also you may want to research light detection for autos - they have a similar problem with automatically controlling headlights.
You will also need a delaying filter so your device doesn't trigger on passing car headlights, etc.

A microprocessor could help by scheduling times that the device is enabled.
Sorry I don't have a specific solution to give you, but I hope this helps some.

Any recommendation for a light sensor that sense ambient light in gerneral? I don't care about color sensing.

I'd pick an LDR (Light Dependent Resistor) for your application because it has broad spectral response. It's slow but you don't care about that since you'll need to filter the heck out of it anyway.
Photodiodes and phototransistors usually have a single sharp response peak at some wavelength and wouldn't work well if your artificial light happened to produce one of these wavelengths.
As for part numbers, you should look in your local electonics shop (Radio Shack, etc). Or you could search digikey.com or mouser.com if you're in their shipping area.