There are some additional details that need to be considered before anyone can give you good advice, such as:
How is this going to be powered? Battery or Wall-wart? The power source will greatly influence the options you have to solve your issue via analog or digital means.
What is the power draw of the light (volts/amps) and do you want it simply to turn on/off once a threshold has been met?
If accuracy is not important, any manual thermostat will do what you're asking it to. Old style mercury units simply close a contact whenever a set point is reached and modern replacements don't need the mercury. You can then do something with that signal or use it to drive a low-power bulb.