hmm... I've used a thermocouple and solid state sensor combination to detect heat at my house's home made bbq. But the response time for my system wasn't imperative.
Thermocouples can be pretty slow, but they have a linear relationship and they have a big temperature range. They are super expensive as well. Only reason I used it, was I got mine for free cause I managed to salvage some that were being thrown out at where I work.
Do you know the range of temp you will be trying to measure?