Notes based on doc PR-SpectralDM-2.0-20160928 and UCD v1.3
S. Erard 9/2/2017
Solar System measurements of light are performed either in reflection (with Sun as a light source) or in emission (usually in the mid-IR and above); some measurements can be provided in transmission (from occultations, etc).
Many spectral data related to observations of the Solar System are not calibrated / distributed in flux, but as different quantities adapted to resolved/extended sources:
- radiance: power from the source, per unit area (for reflected light, depends on light source)
- reflectance: generalization of “albedo”, i. e., measured radiance relative to incoming power (intrinsic quantity). Several variations exist depending on context/configuration.
The radiance of a source can be derived from a measured flux but this has to be calibrated (depends if the field of view is filled, and on observer-source distance), therefore no simple conversion can be performed on the fly.
Table 1 is a proposal for new quantities to support this field of studies, with description in the frame of SDM2. These UCDs are required to handle spectral data properly, i. e., using the correct scale. They belong to phys.*, because they describe intrinsic properties of the source.
These quantities can be provided per unit wavelength (more frequent in the visible and IR) or per unit frequency, consistently with the spectral axis; the unit of power is usually W but can vary.
πF is the “solar flux” and depends on Sun-source distance.
Except for radiance these quantities are dimensionless, but use different scales that must be identified in order to compare different data. They are proposed as sub strings of phys, but this may not be practical. Grouping them in phys.spectr. may be more clear.
Data.FluxAxis.Value / name | Data.FluxAxis.ucd | Data.FluxAxis.unit | Possible equivalent UCD |
Radiance | phot.radiance | W * m**(-2) * sr**(-1) * mu**(-1) (and variations) | phys.luminosity;phys.angArea;em.wl |
“Reflectance“ = “I/F ratio” (I/F may include thermally emitted light) | phys.reflectance? phys.I_over_F? | dimensionless | phys.albedo;em.wl arith.ratio;phot.flux.density |
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Derived quantities |
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Bidirectional reflectance | phys.bidir? | sr**(-1) |
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bidirectional reflectance distribution function
| phys.brdf? | sr**(-1) | (mostly used in terrestrial remote sensing) |
Reflectance factor (REFF) = | phys.reff? | dimensionless | (used with laboratory samples) |
Normalized reflectance | arith.ratio;phys.reflectance M4ast currently uses phys.reflectance;em.wl;arith.ratio | dimensionless | Normalized to 1 at some wavelength |
+ thermal / emission quantities | Same as I/F for spectral measurements |
| I/F also apply to observations in the thermal range. What about lab measurements? |
+ Hemispherical quantities |
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| TBC |
Table 1: Quantities in use (µ0 stands for cosine of incidence angle; acronyms in parenthesis are from Hapke 1981 and are commonly used)
• About the current UCD 1.3 list:
phot.radiance was proposed in v 1.3 but may be inconsistent in the general case: although it reflects an intrinsic property of the source (not a measured flux) it still depends on Sun distance for Solar System objects in reflectance. This may be different in the thermal range - in which case phys.radiance would be more adapted.
Other quantities are intrinsic and belong to phys, as indicated above (like luminosity, magAbs, and polarization).
phys.albedo is OK but not very specific; variations may have to be included (e.g., for normal albedo, Bond albedo, etc + depends on wvl). Same applies to phys.emissivity
• Usage
The above quantities are intended to be used in the spectral data model, and used by VO tools to handle data properly.
They would also be available as values of the measurement_type parameter in EPN-TAP
• Since reflectance and radiance are functions of illuminated conditions, describing the illumination angles is also required.
Currently, only the phase angle has a defined UCD. This is also required for incidence and emergence; azimuth can be useful also (the 4 angles are related):
pos.incidenceAng
pos.emergenceAng
pos.azimuthAng