Channel | NER | NEI |
320 nm | 2.9 x 10-6 | 9.0 x 10-5 |
395 nm | 5.0 x 10-6 | 6.9 x 10-5 |
490 nm | 1.8 x 10-6 | 2.3 x 10-5 |
683 nm | 9.9 x 10-7 | 1.1 x 10-5 |
780 nm | 6.8 x 10-7 | 8.0 x 10-6 |
The instrumentation technology used to produce the latest line of Biospherical Instruments' radiometers, including C-OPS and the OSPREy family of sensors, is based on microradiometers, which were developed by Biospherical Instruments with support from a NASA SBIR grant.
A microradiometer consists of a photodetector, preamplifier with controllable gain, high resolution (24 bit) analog-to-digital converter (ADC), microprocessor, and an addressable digital port. In other words, it is a fully functional networkable sensor residing on one small, thin, circuit-board assembly, which is sleeved inside a shielded cylinder (Figure 1). With the addition of the front-end optics (collector, window, and filter stack to set the center wavelength and bandwidth), the basic form factor resembles a shortened pencil.
The microradiometer design was developed in response to a need for smaller, faster, and potentially less expensive radiometers, which could be easily scaled to either more or fewer channels and to be more easily deployed in coastal waters where self-shading effects are frequently significant. Each microradiometer is also equipped with a temperature sensor located close to the photodetector. A list of microradiometer specifications are provided in the table below. Clusters of microradiometers can be matched with front-end optics to form small, fast, inexpensive multiwavelength radiometers for a variety of measurements. Each cluster is managed by an aggregator, which allows the array of individual radiometers, plus any ancillary sensors, to function as a solitary device.
Because each microradiometer channel has an individual ADC, no multiplexer is required and no analog cabling is needed, eliminating a source of electronic leakage and improving reliability. All channels can be sampled synchronously at rates of up to 25 Hz. The metallic cylinder provides additional isolation from electromagnetic interference sources (e.g., radio frequencies). The photodiode current is converted to voltage with an electrometer amplifier with originally two, but subsequently three, gain settings. The resulting voltage is directly fed to the ADC. The entire assembly, including the photodetector, is located on a single circuit board measuring 0.35 x 3.0 in2.
Channel | NER | NEI |
320 nm | 2.9 x 10-6 | 9.0 x 10-5 |
395 nm | 5.0 x 10-6 | 6.9 x 10-5 |
490 nm | 1.8 x 10-6 | 2.3 x 10-5 |
683 nm | 9.9 x 10-7 | 1.1 x 10-5 |
780 nm | 6.8 x 10-7 | 8.0 x 10-6 |