title: Filter Transmittance Measurements in the Infrared.
abstract: We have set up a novel direct detection system to measure filter transmittances over an attenuation range of at least 5 decades, with relative combined standard uncertainties as low as 0.5% (1σ) per decade, in the 9μm to 11μm spectral region. This system, using an apparatus originally designed for a heterodyne measurement of transmittance, achieves higher accuracy at the expense of a reduced dynamic range. This independent measurement of transmittance allows verification of the heterodyne technique. Our system uses a source modulated at 30 MHz and 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 beam caused by the nonparallel surfaces of the filter. We discuss the sources of systematic errors and the methodology to reduce their contribution.
authors.affiliation: National Institute of Standards and Technology, Gaithersburg, MD 20899-0001.
authors.forename: A L
authors.initials: AL
authors.lastname: Migdall
authors.affiliation: National Institute of Standards and Technology, Gaithersburg, MD 20899-0001.
authors.forename: A
authors.initials: A
authors.lastname: Frenkel
authors.affiliation: National Institute of Standards and Technology, Gaithersburg, MD 20899-0001.
authors.forename: D E
authors.initials: DE
authors.lastname: Kelleher
keywords: attenuators; direct detection; filter transmittance; heterodyne detection; infrared