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.