It turns out that the greatest-intensity wavelength produced—the peak in the curve above—depends on the temperature of the item. As it will get hotter, the wavelength of peak emission decreases—it moves to the still left, back toward the visible spectrum.
So for some thing at room temperature (like 300 Kelvin), this peak wavelength is about 9.7 μm (micrometers). That puts most of the radiation in the infrared aspect of the spectrum. That’s why you ordinarily just cannot tell just by searching at issues how heat they are.
But if you heat it up to, say, 1,200 K (like that oven factor), the maximum-depth wavelength moves down to about 2.4 μm. That is still in the infrared area, but by shifting the curve you also get much more light-weight down in the obvious aspect of the spectrum (< 0.74 μm), so your eye can see it glowing. (Try it in the PhET simulator!)
This temperature-wavelength relationship is called Wien’s displacement law, which looks like this:
In this expression, λ is the wavelength of the light with the maximum intensity and T is the temperature (b is just a constant). This means I can get a value for the temperature of an object just by looking at the color of light it produces.
Only most of the light is invisible, so you need an infrared camera for that. It’s basically just like a normal digital camera, but instead of having a sensor that detects visible wavelengths, this one can “see” infrared wavelengths. My IR camera can even give a temperature reading right on the image. Seriously, these things are awesome.
No Reflection on You
Oh, but there’s an issue: Wien’s law only works for radiation from a “blackbody.” What’s that? A blackbody is an object that doesn’t reflect outside light all the light it gives off is produced by the object itself. An incandescent light bulb is a pretty good example — it glows because the filament gets super hot. (That’s why incandescents are sucky light sources. They waste a lot of energy in the infrared range that you can’t see.)
In reality, the light from most things is a mix of emission and reflection. So if we want to use that light to get the temperature of an object, we need to know the ratio. There’s an index, called emissivity, that captures this. It ranges from 0.0 for a completely reflective surface up to 1.0 for a perfect blackbody. There are tables where you can look up the emissivity of different materials.