Two critical requirements of any calibration source are short and long-term operational stability and repeatability. Source monitoring is necessary in quantifying overall source performance including stability and repeatability. The NASA GSFC Code 920.1 Radiance Calibration Facility (RCF) developed a Filter Radiometer Monitoring System (FRMS) to continuously monitor the performance of its integrating sphere calibration sources. FRMS bands are in the 0.4 -2.4 µm region, with several bands selected to coincide with common remote sensing bands. The FRMS was designed and fabricated in the year 2000. Early in 2001, the FRMS was reconfigured prior to being deployed on the RCF 180cm integrating sphere. This paper describes the instrument modifications resulting from the FRMS reconfiguration and presents FRMS monitor data for three RCF integrating sphere sources.
INSTRUMENT RECONFIGURATIONThe FRMS (Figure 1) has been described in a previous publication [1]. The FRMS was designed to accommodate DC or AC signal acquisition. Originally the FRMS was configured for DC signal acquisition using a Hamamatsu C2719 preamplifier and a Hewlett-Packard 3458 Digital Multi-Meter (DMM). This configuration worked well at the 410, 440, and 460 nm bands with lower signal levels, but the pre-amp saturated with the higher signal levels in the 640, 745, 840, and 1050 nm bands. Bench tests revealed that directly driving the DMM with the detector (i.e. photo-voltaic mode) provided adequate signal levels with long settling times. To improve signal-to-noise levels and reduce the settling time, the FRMS was reconfigured for AC signal acquisition. The DMM was replaced with a Stanford Research SR830 Lock-In Amplifier (LIA), with the detector directly driving the LIA input. To modulate the detector signal, a chopper blade replaced one of the two filter wheels installed in the FRMS. The 745 nm filter was removed, and the remaining nine filters centered at 410, 440, 460, 640, 840, 1050, 1240, 1380, and 1640 nm were consolidated onto the remaining filter wheel. Only the bands between 410 and 1050 nm bands have been used to date since the mounted detector, a Hamamatsu S2592 TEstabilized Si detector, is sensitive over this range. The detector temperature was set to 0°C. The two-slot chopper blade is set to generate a 42.08 Hz chop frequency, which is dictated by the maximum pulse speed of the stepper motor driving the blade. Under this configuration, the 410 nm band exhibits high noise levels due to electrical coupling, and results from that band are not presented here.
MONITORED SOURCESSources used for FRMS monitoring runs are briefly described below.
180cm Diameter Integrating Sphere (a.k.a. Hardy)Hardy ( Figure 2) is a fiberglass shell sphere with a barium sulfate (BaSO 4 ) interior coating, and a 25.4cm exit aperture. The sphere is equipped with 16 baffled 200W quartz tungsten halogen lamps. Eight computer controlled power supplies drive pairs of lamps in a constant current mode. A black-anodized aluminum plate is used as an aperture cover. During 200...