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.
Figure 1: A side view of a microradiometer (top) showing the two-sided circuit board design, and a sleeved version with fore-optics attached (bottom). The ruler is marked in centimeters.
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.
Microradiometer Electronic Specifications
- Detectors: Si (13 mm2) or InGaAs (3 mm2) (specifications in this document refer to Si; InGaAs detector-based channels exhibit higher noise)
- Photocurrent-to-voltage conversion: Electrometer amplifier with three gain stages. Gain factors are 1, 200, and 40,000.
- ADC: 24 bit bipolar: 4–125 Hz data rates.
- Dynamic range: 9+ decades
- Linearity: Measured on all microradiometers over a signal current range of 1 x 10-12 to 1 x 10-5 A using a programmable light source. Errors are typically <1% compared to a reference system electrometer. Gain ratios are individually measured and programmed into each microradiometer.
- Speed: ADC sample rate is programmable from 4–125 Hz, and is normally set to 125 Hz with averaging performed internally by the microradiometer.
- Response time: Exponential change with time constant of <0.01 s. Time required for gain change is <0.1 s.
- Electronic Sensitivity and Dynamic Range: The ADC resolution is 0.5 μV with a current resolution of <10-15 A. The saturation current is 160 μA. The dynamic range is 3.3 x 1011, defined as the saturation signal divided by the minimum resolvable signal. Radiance sensors may be pointed directly at the solar disk without saturating.
- Noise: 15–20 fA noise when ADC is sampling at 125 Hz with internal microradiometer averaging of 25 samples, resulting in a data rate of 5 Hz.
- Optical Sensitivity: Sensitivity depends on the spectral region and entrance optics (irradiance or radiance). Expressed as Noise Equivalent Signals at 5 Hz for radiance (NER: μW /(cm2 nm sr)) and irradiance (NEI: μW/(cm2 nm)):
||2.9 x 10-6
||9.0 x 10-5
||5.0 x 10-6
||6.9 x 10-5
||1.8 x 10-6
||2.3 x 10-5
||9.9 x 10-7
||1.1 x 10-5
||6.8 x 10-7
||8.0 x 10-6
- Dark Offsets: Dark offsets are measured and set at time of calibration for each gain level. Offsets can also be automatically measured and applied in the field to accommodate different temperature regimes.
- Microradiometer power: ±5 VDC at 3 mA.
- Spectral Range: 250–1650 nm. (Range of 1100–1650 nm requires InGaAs detectors).
- Cluster Sizes: Microradiometers are assembled into collections of 1, 7, 19 wavebands in a single housing. The diameter of a 19 channel sensors is 2.75 in (7 cm).
- Speed: 19-waveband sensor heads can be operated at rates greater than 30 Hz. Complete systems composed of three 19-waveband radiometers can operate at rates greater than 20 Hz depending on computer speed.
- Data rate: Sensor heads communicate at 115,200 baud, using RS-232 or RS-485 (full or half duplex). The control unit ("deckbox") communicates at 115,200 baud using RS-232.
- System Power requirements: Sensor head with 19 channels: 7.5 V at 90 mA. Deckbox: 10–14.5 VDC at 6 A maximum. Some system configuration require less power.