Greater radiometric accuracy, decreased size and costs, plus the need for multidisciplinary research are all technology-forcing challenges to be addressed by the next generation of optical instruments. To develop a commercially successful marine spectroradiometer, the device must also appeal to the majority of investigators working in this field. This requirement led to the development of a so-called smart photodetector or microradiometer, plus its ancillary interface modules. When research objectives require clusters of microradiometers, the final instrument can be assembled easily by selecting the desired filter assemblies and optical front ends. It is not necessary to modify the sensitivity of each channel for optimal dynamic range because the sensitivity is dynamically configured. Furthermore, the design eliminates many of the electronic interconnections required in existing radiometers, which increases redundancy and improves reliability. The principal benefits of the microradiometer are as follows:

  1. Increased radiometric accuracy in the field;
  2. Decreased instrument size and weight;
  3. Lower power consumption;
  4. Enhanced flexibility in the configuration of above- and in-water instruments; and
  5. Reduced costs associated with production, calibration, maintenance, reconfiguration or modification, and field research.

The culmination of these benefits is the entire analog signal path is of minimum length, free of connectors or cables, and is totally contained in a shielded metal sleeve.

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, all of which resides on one small, thin, circuit-board assembly that is sleeved inside a metal cylinder. With the addition of the front-end optics (collector, window, and filter stack), the basic form factor resembles a shortened pencil (Fig. 1 top panel). Because each microradiometer channel has an individual ADC, no multiplexer is required, and no cabling is needed, thereby eliminating a source of electronic leakage and improving reliability. The metal cylinder provides additional isolation from electromagnetic interference sources (e.g., radio frequencies). The photodiode current is converted to voltage with an electrometer amplifier with three gain settings, and 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 in X $3.0 in. Each microradiometer is also equipped with a temperature sensor located close to the photodetector. Clusters of microradiometers can be matched with front-end optics to form small, fast, less expensive, multiwavelength radiometers (Fig. 1 bottom panel) for a variety of measurements. Each cluster is managed by an aggregator that allows the array of individual radiometers, plus any ancillary sensors, to function as a solitary device.

The Compact-Optical Profiling System (C-OPS) successfully integrates microradiometers with a number of new technologies, each focused on different aspects of the practical problem of resolving the optical complexity of the near-shore water column to improve measurements in shallow coastal waters and from a wide variety of deployment platforms. In terms of the mechanics of operating the instrumentation and its behavior during descent, the most significant improvement was to change the basic design for mounting the light sensors from a rocket-shaped deployment system, used in legacy profilers, to a so-called kite-shaped backplane. This change allowed the flotation to be distributed as a primary hydrobaric buoyancy chamber along the top of the profiler, plus an adjustable secondary set of one or more movable floats immediately below. The primary set provides the upward buoyant thrust to keep the profiler vertically oriented. The secondary set, coupled with an adjustment mechanism perpendicular to the flotation adjustment axis is used to ensure the two light sensors descend with vertical tilts less than 2.5° (nominal).

The hydrobaric buoyancy chamber can contain one to three air-filled bladders, which compress slowly and allow the profiler to loiter close to the sea surface, thereby significantly improving the vertical sampling resolution in near-surface waters. Electronically, the system is self-organizing; when initially powered, the aggregator queries each sensor to determine optimal power required for operation over the existing length of the cables and the population of detectors available to the configuration. Typically, each sensor geometry (made up of combinations of irradiance and radiance) is composed of 19 microradiometer detectors, clustered and controlled through a master aggregator. Although the use of microradiometers provides improvements in a variety of operational specifications compared to legacy sensors (e.g., reduced electronic noise and slightly faster data acquisition rates), most notable is the reduction in instrument diameter and the increase in vertical sampling resolution. C-OPS light sensors use a 2.75 in (7 cm) outside diameter housing, which is 27% smaller in diameter than many legacy sensors which used a 3.50 in (or larger) housing. In addition, C-OPS has a vertical sampling resolution of less than 1 cm in the upper 5 m of the water column, whereas legacy sensors have approximately a 10 cm resolution (or worse) and fewer wavelengths.

Although a standard C-OPS profiler can be configured to measure any two-sensor combination of the in-water light field, a specialized backplane providing simultaneous profiling for all three sensor types is being tested. The new design permits the acquisition of all three principal light field components (Fig. 2), so the Q-factor (the ratio of the upward irradiance to upwelling radiance) can be measured simultaneously with the downward irradiance. The Q-factor is an important parameter for understanding the bidirectional aspects of the underwater light field. Although Q is well understood for open ocean waters and can be computed using look-up tables based on the solar geometry and the chlorophyll a concentration, no such capability exists for optically complex (coastal) waters. The latter are a primary focus of next-generation mission planning, e.g., PACE, GEO-CAPE, HyspIRI, and ACE.

Fig. 1.

A COTS radiance radiometer (bottom panel) built with 19 microradiometers (lower top panel), which are sleeved in brass with fore-optics attached (upper top panel). The metal cylinder provides mechanical support, as well as, isolation from electronic noise. The clear acrylic housing is for demonstration purposes only. The diameter of a microradiometer is set primarily by the diameter of the photo detector (0.370 in).

Fig. 2

A schematic of the C-OPS deployment system modified for the simultaneous deployment of three sensors: downward irradiance (left), upwelling radiance (center), and upward irradiance (right). The top-most red floatation block is hollow to permit the insertion of 1–3 compressible air bladders. As the bladders compress, the package descends more quickly, and the number of bladders is used to set the terminal velocity and depth. The initial slow descent rate ensures high vertical resolution sampling near the sea surface. The adjustable red floats below the top-most block allow fine-scale tuning of the roll axis, and the sensors can be pivoted to provide adjustment of the pitch axis. In most circumstances, the package descends with an average vertical tilt of less than 2.5°.