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Q & A: infrared thermometers

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Most recent answer: 05/15/2011
Q:
Does food release infrared waves when it is heated? My partner and I are trying to think of ways to make a microwave a more practical sort of tool. Instead of having to remember a certain time to key into the microwave, you can heat something up based on the temperature you want it to be! So: 1. You put the food into the microwave 2. You select a certain temperature you want (from warm to very hot; these are ranges for specific degrees of temperature) 3. The microwave will start heating up the food 4. The food will start releasing infrared waves as it is being heated up 5. Once the infrared waves reach that certain temperature you selected before, the microwave's infrared sensor will sense the heat the food reached. If it is the perfect heat that matches exactly what you selected, the microwave will stop heating the food up. 6. Your food is ready! Please tell me what you think of this idea. If infrared waves do not work, is there a different method we can use?
- Alice (age Mackenzie)
MV, CA, USA
A:
Your basic idea is perfectly reasonable. You can measure temperatures via remote infrared sensors. However, there's a complication that might get in the way of this particular practical application. The rate of emission at some wavelength is the product of two factors:
1. A universal black-body radiation factor, with a precise value dependent only on temperature
2. An emissivity-absorptivity factor depending on the surface properties.

It's that latter factor, which varies between materials, that might complicate the measurement. In the visible you can see that that factor varies a great deal, from matte-black surfaces with almost 100% absorptivity to shiny surfaces with low absorptivity. Perhaps, however, there's some particular IR wavelength range where most water-rich foods have about the same absorptivity.

Mike W.

(published on 05/15/2011)

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