FOC Instrument Science Report #094 July 9 1996 "FOC Neutral Density Filter Throughputs" by W. Hack and M. Voit Abstract excerpted from the report: We analyze a series of observations taken with different neutral density filter combinations yielding new measurements of the neutral density filter throughputs. This report describes the observation set, the method of photometry, and how potential problems with the data were avoided. These new measurements provide a different result from previous internal calibrations, and various reasons for this discrepancy are discussed at the end of this report. In general, the 1ND, 2ND and 4ND filters appear to be within 3% of the expected throughputs for the F430W bandpass. However, there may be a significant color effect which will be investigated with further calibration observations.