FILTER TRANSMITTANCE MEASUREMENTS IN THE INFRARED

We have set up a novel direct detection system to measure filter trans mittances over an attenuation range of at least 5 decades, with relati ve combined standard uncertainties as low as 0.5% (1sigma) per decade, in the 9 mum to 11 mum spectral region. This system, using an apparat us originally designed for a heterodyne measurement of transmittance, achieves higher accuracy at the expense of a reduced dynamic range. Th is independent measurement of transmittance allows verification of the heterodyne technique. Our system uses a source modulated at 30 MHz an d a specially constructed high dynamic range and high accuracy lock-in amplifier capable of operation at the modulation frequency. The high modulation frequency and narrow bandwidth of the system allow thermal background radiation to be suppressed and high accuracy to be achieved . We correct for the non-ideal natures of the detector and attenuators . In particular, the detector position is scanned to reduce the effect of its spatial nonuniformity and the deflection of the transmitted be am caused by the nonparallel surfaces of the filter. We discuss the so urces of systematic errors and the methodology to reduce their contrib ution.