OBS sensors detect light with silicon PNN+ photodiodes. Our diodes have: 1) excellent linearity and low noise, 2) nearly flat spectral response in the source spectral band, 3) extremely low, < 0.01% per °C, temperature coefficients, and 4) low time drift (~ 1% per year, per tests by NIST). The responsivity, S(λ), of the photodetector is a determining factor of the operating spectrum. Responsivity (S) is the ratio of photocurrent in Amperes (A) from a detector divided by the light power in Watts (W) incident on it, e.g. S = A W^-1. Optical filters are used to reject visible light so that it will not saturate (swamp) detectors. The transmittance spectra of these filters must be factored into the sensor design because the product of emitter power, transmittance, and responsivity give the relative spectral value for each wavelength common to the sensor components. The resulting operating spectrum for our laser-based instruments is shown on the chart labeled Laser Diode and the spectrum for IRED-based instruments looks like the one shown on the one labeled IRED. Detectors and daylight-rejection filters are selected to minimize their effects on the emitter spectra, that is their spectra are nearly flat in the operating band. Even with these filters, NIR radiation in sunlight can saturate a sensor when it is exposed to direct sunlight or reflective surfaces illuminated by the sun. Maintaining consistent operating spectra is essential to obtaining high measurement precision.