Seeing the Heat
The rods and cones at the back of our eyes are designed to see visible light. When we hold up our hand to a flame, we are sensing the infrared light. But how can we "see" this light? It is possible to design detectors that sense infrared energy and convert it to electrical energy. These are similar to solar photovoltaic cells that convert visible light to electricity. Once infrared light is converted to electricity, we can easily measure the variations in electrical voltage that represent the different wavelengths. In this way, we can measure the colors that our eyes can't see.
Making a Thermal IR Rainbow
Our eyes are not able to see the full spectrum of colors in sunlight. So much of the sun's energy peaks in the yellow wavelengths that we need a prism or a rainbow to see the rest of the colors. Similarly, in order to see the full spectrum of thermal IR colors, we have to change how we view them. One of the ways to do this is with filters. Just like a red filter held up to the sun only passes red light, filters that pass different thermal IR colors can be created. Another way is to use a device called an interferometer. Somewhat like a prism, an interferometer can create a full spectrum of infrared light all at once. The TES instrument uses an interferometer to create a spectrum and an infrared detector to measure it. This combination is called an infrared spectrometer.