I got a laser thermometer to measure the temperature of a wire for my science fair. However, when I attempted to measure the wire, it was too thin and I just measured whatever was behind it. How can I measure the temperature of only the wire?
Judging from the other post, you are designing an experiment, where you want to demonstrate the resistivity at different temperatures. To do that, you need to know the temperature of the wire. You don't necessarily need to self-heat the wire with electricity.
You've chosen a comfortable range of temperatures: between +10°C and +40°C. You can touch this kind of temperature and it shouldn't hurt. However, the room temperature is usually +18°C to +25°C, so you would need some method of chilling to get +10°C.
I would suspend the wire in a container filled with non-conductive liquid**. The container can be large enough to measure with an IR thermometer. You can also have a thermometer immersed in the liquid. Have another container with warm liquid, and another one with chilled liquid. You can move the wire from one container to another and measure the effect. The temperature of the wire will equilibrate with the temperature of the liquid very quickly.
It could make sense to wind the wire on a plastic spool, being careful that the wire doesn't cross itself. Perhaps, you could lay the wire into the thread of a large plastic screw.
** Silicone oil from the drug store, perhaps. You could add some coloring to the liquid to make it look unusual. Deionized water could work too, it has high resistivity.
The only way you would be able to measure that with an infrared (the laser just shows where you are pointing it) thermometer would be to attach a copper plate to the wire and measure the temperature of that. Because copper is such a great conductor of heat, that would give you near real-time results. Why do you have to use that thermometer, by the way? Couldn't you just attach a temperature sensor to it and save the bother? That would be cheaper and easier.
There are things called pyrometers and at the heart of the device is a photodiode. They can measure the spot temperature of anything they are pointed at but they rely on optical lenses to give a spot focus point.
Maybe, if you got a photodiode with known sensitivity (amps per incident light power) you could work it out by taking a couple of measurements at slightly different short distances. Other than that I'd be tempted to solder a thermocouple to the wire, minimizing the heat taken by the thermocouple. I've soldered them to electronic components in the past and got reasonable results.
Add a reflector behind the wire, preferably bent to a curve to focus the IR light emitted from your wire. Polished copper should work well as it reflects IR nicely but if you can’t get a suitable piece some household alumina foil might work.
If this is not enough, add some shielding that will shield of other potential heat radiating bodies, like humans, from your reflector.