350nm is outside of the Human Observer weighting.so the zero reflectance or transmittance has no bearing on Scale or Index results. We actually have very low signal and moderate noise at 350nm which is why even when we report the results at this wavelength there is greater variation than at higher wavelengths.
Returning the sensor to a state where the 350nm result is calculated can sometimes occur with a thorough cleaning of all optics, such as is done during a Preventive Maintenance Calibration Verification service. If that does not work then during the PMCV service the technician may try to increase the systems lamp signal levels. Both of these actions are included in the PMCV service. If neither of these actions solves the issue, then that would indicate that the integrating sphere surface has dulled or been contaminated. Typically we would try to gently clean the surface using compressed air, if that did not work then we would need to replace the sphere.