Can an infrared thermometer (IR gun) be used to measure the ambient room temperature?

Question

I know the principle of an IR thermometer, but have one question regarding that are as below.

1. As an IR thermometer senses the radiated IR waves from external bodies coming in their field of view and gives surface temperature reading, can we point in air or random far space and measure the temperature of the surroundings/room?

2. Will the reading, whatever comes, be near room temperature, or will it just be a garbage value?

One example of an IR thermal sensor: https://www.sparkfun.com/products/9570

Answer 1

In the relevant part of the infrared spectrum for this application\$^1\$, air has a high transmittance:

(taken from here licensed under Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License)

So wherever you point an IR temperature sensor, the output will be dominated by the surface you are pointing at and not the air in between.

If you try and point an IR sensor in the sky (no clouds) you will get a a very low reading (mine showed −40 °C but that is the lower range end), and not the air temperature.

So you need some surface at room temperature to measure the room temperature. Do not use shiny metals, as those reflect IR radiation. Normal glass can be used, so you could hang a decorative looking thing in the room and measure the surface of that – provided you have a small enough measuring cone (so it only hits your wanted surface and not something behind it).

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\$^1\$: For −40 °C the peak in IR is around 15 µm, for 50 °C the peak is around 8 µm.

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The sensor you linked uses a thermopile (a stack of thermocouples). These sensors require knowledge of the ambient temperature to calculate the targets temperature. So integrated into the IR sensor is an ambient temperature sensor, and the measured value can actually be read out, so you get the ambient temperature of your sensing element. Depending on the location where you place it, that might be close enough to room temperature already. But in that case the IR part looses its use and you could just use a normal temperature probe.

Answer 2

You could hang a sheet of (non-gloss) printer paper from a string in the middle of the room. The paper will respond quickly to changes in room temperature, due to its low thermal mass and because it will allow air to circulate around it. Plus it provides a target surface which will be less likely to reflect IR from elsewhere in the room. Compare to a "normal" thermometer nearby and apply any compensation to make your readings match, then just use the IR gun from that point onwards.

Answer 3

As I interpret your statements, you are making 3 questions.

1. yes, you can use an IR gun to measure ambient room temperature. Whether the measurement is any good, that is a different question.
2. yes, you can point the IR gun anywhere and to/at anything. Again, whether the readings you get are "usable/reasonable," is a different question.
3. the reading might or might not be "near" room temperature. It depends on how representative, the temperature of the surface hit by the beam (if any), is of the "room" temperature in question.