OCO-2 will not be measuring CO2 directly; but actually, the intensity of the sunlight reflected from the presence of CO2 in a column of air. This measurement is unique like a fingerprint and can be used for identification. The OCO-2 instrument will use a diffraction grating (like the back of a compact disk) to separate the incoming sunlight into a spectrum of multiple component colors.
The instrument measures the intensity of three relatively small wavelength bands (Weak CO2, Strong CO2 and Oxygen O2) from the spectrum, each specific to one of the three spectrometers. The absorption levels will indicate the presence of the different gases. By simultaneously measuring the gases over the same location and over time, OCO-2 will be able to track the changes over the surface over time.
The OCO-2 spectrometers will measure sunlight reflected off the Earth's surface. The sunlight rays entering the spectrometers will pass through the atmosphere twice - once as they travel from the Sun to the Earth, and then again as they bounce off from the Earth's surface to the OCO-2 instrument. Carbon dioxide and molecular oxygen molecules in the atmosphere absorb light energy at very specific colors or wavelengths.
The OCO-2 instrument uses diffraction grating to separate the inbound light energy into a spectrum of multiple component colors. The reflection gratings used in the OCO-2 spectrometers will consist of a regularly-spaced series of grooves that lie on a very flat surface.
The characteristic spectral pattern for CO2 can alternate from transparent to opaque over very small variations in wavelength. The OCO-2 instrument must be able to detect these dramatic changes and specify the wavelengths where these variations take place. So the grooves in the instrument diffraction grating will be very finely tuned to spread the light spectrum into a large number of very narrow wavelength bands or colors. In fact, the OCO-2 instrument design incorporates 17,500 different colors to cover the entire wavelength range that can be seen by the human eye. A digital camera covers the same wavelength range using just three colors.
OCO-2 measurements must be very accurate. To eliminate energy from other sources that would generate measurement errors, the light detectors for each camera must remain very cold. To ensure that the detectors remain sufficiently cold, the OCO-2 instrument design will include a cryocooler, which is a refrigeration device. The cryocooler keeps the detector temperature at or near -120° C (-184° F).