Examples of the channel-to-channel ratios between SIM channels (recorded on the same calendar days) as a function of solar exposure time. Because the different channels are measured nearly simultaneously, the spectral ratios are free of solar activity and reflect the optical throughput degradation due to solar exposure. The degradation is fit with a two-term exponential decay as a function of the cumulative solar exposure time. The SIM B irradiance used in the SIM A/SIM B ratio is the corrected SIM B irradiance determined from the SIM B/SIM C ratio fits. These ratios are generated for each wavelength across the spectrum so degradation fits provide a spectrally dependent decay constant for use in data processing. The standard deviations of the fits are used to derive the spectrally dependent correction rate uncertainties (ppm/yr) over the current mission. As of January 2023, these plots show that for 220 nm the degradation is 10% for SIM A and 3% for SIM B, at 430 nm 3% for A and 0.8% for B, and at 681 nm 0.4% for A and ~0.1% for B.