AOP measurements
Rrs can be estimated from field measurements of upwelling radiance and downwelling irradiance made at a sampling location using field radiometric instruments (see table below). Radiometric measurements are collected through manually or automatically operated sensors. Measurement approaches can be broadly divided into two categories: 1) above- and in-water radiometry (Zibordi et al., 2019, Ruddick et al., 2019), where upwelling radiance and downwelling irradiance are measured above or in the water; and 2) using skylight-blocking techniques for shielding the sensor from reflected sky radiance (Lee et al., 2010) to measure Lw near the water surface. Sensors can be hand-held and pointed in the appropriate directions to measure the radiometric variables of interest or installed on a floating frame, boat, fixed platform, or profiler. Radiances are expressed in watts per square meter per steradian (W m-2 sr-1), whereas irradiances are expressed in watts per square meter (W m-2). Rrs is, therefore, reported in units of per steradian (sr-1).
Kd can be estimated from with field radiometers field measurements of downwelling irradiance made field radiometers, whereas KPAR is commonly measured in the field using quantum sensors, removing the need for unit conversions. However, KPAR is not a depth independent parameter and can lead to erroneous estimates of PAR propagated to depth (Lee et al., 2009). Measurement approaches are limited to in-water radiometry, and irradiance measurements are often taken at the same depths as other water column indicators. Sensors can be added to any profiler to accommodate their size and type. Diffuse attenuation coefficients are reported in per meter (m-1).
Table. Commonly used sensors, including pros and cons, for acquiring radiometric data. Note, this is not an exhaustive list and we do not endorse any particular sensors or companies. Specific sensors named here are simply to provide examples.
|
Sensor types |
Example Sensor |
Pros |
Cons |
|
Reflectances (e.g., Rrs) |
Multispectral radiance and irradiance sensors (e.g., BSI C-OPS and XRR (for profiling), CIMEL CE318-TV12-OC and LC (for automated above-water measuring)) |
Sensors have consolidated technology, are generally little affected by stray light and polarization, and use reliable optical-filter technology Sensor bands are distributed across a spectral range C-OPS is a radiometer system providing simultaneous in-water radiance and irradiance in 19 spectral bands and can be used for shallow-water profiling CE318-TV12-OC and LC (SeaPRISMs for OceanColor and LakeColor) are automated photometers providing sequential above-water radiance and irradiance using 12-filter optical-filter wheels and can be used for fixed station measuring |
Number of spectral bands is limited Sensor bands can need to be shifted to satellite sensor central bands in the data processing Proprietary data acquisition and processing software Sensors are typically serviced at the manufacturer |
|
Hyperspectral radiance and irradiance sensors (e.g., Sea-Bird Scientific HyperOCR, TriOS RAMSES, IMO USSIMO(for all types of measuring), ASD FieldSpec 4, SVC HR and XHR, Spectral Evolution PSR+, PSR1100f and SR-3500, Ocean Insight SR4, AvaSpec CompactLine (for above-water and near-surface in-water measuring (via fiber optics)), Water Insight WISP-3 (for above-water measuring) and WISPstation (for automated above-water measuring)) and IMO DALEC and SBE HyperSAS (for above-water) |
Sensors have consolidated technology, are generally little affected by stray light and polarization, and use reliable optical-filter technology Sensor bands are distributed across a spectral range Number of spectral bands is expansive Sensor bands can be matched to satellite sensor central bands in the data processing HyperOCR and RAMSES are radiometers with a fully characterized cosine response providing accurate radiance or irradiance for all types of measuring whereas most others are associated with specific types of measuring DALEC and HyperSAS have pointing capabilities to ensure optimal geometry is maintained. |
Sensors can lack a full optical characterization including straylight analysis and characterization Proprietary data acquisition and processing software Sensors are typically serviced at the manufacturer Deployment strategies must avoid self-shading. Single spectrometer measurements require a spectralon reflective plaque which must be kept clean. |
|
| Diffuse attenuation coefficients (K- functions, e.g., Kd) | Multi- or hyperspectral radiance and irradiance, or quantum sensors (e.g., BSI C-OPS, XRR, and QCP, Sea-Bird Scientific HyperOCR and ECO PAR, TriOS RAMSES, LI-COR Biosciences LI-192, YSI EXO PAR System with LI-192 sensors, IMO USSIMO, IMO MS9) | Sensors used for radiance and irradiance measuring in profiling applications can provide the corresponding diffuse attenuation coefficients from the same data
Underwater quantum sensors are specialized sensors and can provide PAR and its diffuse attenuation coefficients using silicon photodiodes and optical-filter technology to create a uniform sensitivity to light over wavelengths from 400 to 700 nm Underwater quantum sensors are routinely used in water quality monitoring programs and can provide a simple measurement of underwater light availability |
Sensors are typically serviced at the manufacturer |
References
Zibordi, G., Voss, K.J., Johnson, B.C., Mueller, J.L., 2019. Ocean Optics and Biogeochemistry Protocols for Satellite Ocean Colour Sensor Validation, Volume 3.0: Protocols for Satellite Ocean Colour Data Validation: In Situ Optical Radiometry. International Ocean Colour Coordinating Group (IOCCG). https://doi.org/10.25607/OBP-691
Ruddick, Voss, Boss, Castagna, Frouin, Gilerson, Hieronymi, Johnson, Kuusk, Lee, Ondrusek, Vabson, Vendt, 2019. A Review of Protocols for Fiducial Reference Measurements of WaterLeaving Radiance for Validation of Satellite Remote-Sensing Data over Water. Remote Sensing 11, 2198. https://doi.org/10.3390/rs11192198