The imaging spectrometers of the second orbiting carbon observatory were radiometrically calibrated before launch during instrumentlevel ground testing. The gain and dark responses were characterized for each focal plane array detector element. An integrating sphere source with an integrated monitoring spectroradiometer illuminated the OCO-2 spectrometers at many light levels. Instrument output was compared with the calibrated output of the source to derive gain coefficients. This source was calibrated in situ with respect to the National Institute of Standards and Technology reference standards, and the instrument met its absolute performance requirement of 5%. Matching fields of view for the internal monitor detectors and the external instrument under test was found to be particularly important, as observed in the results and supported by modeling. Temperature-dependent dark offsets were corrected in a separate process. Solar spectra with varying neutral density filters were used to validate the linearity of the spectrometers.