Remote sensing FAQs and links
A spectrometer is an instrument that measures the amount of electromagnetic (light) energy present across certain wavelength ranges. The incoming light is focussed through a diffraction grating which splits the light field into is component wavelengths - which in turn are focussed onto a change coupled device (CCD) which registers the intensity as voltages whose magnitude is stored as digital numbers for each wavelength. https://sensing.konicaminolta.us/blog/what-is-the-difference-between-radiometers-spectrometers-and-spectroradiometers/
A digital number is a measurement of the intensity of the radiance for the pixel/measurement. It is the intensity as measured by the sensor eg. For an 8-bit RGB camera (255,255,255)=white, (0,0,0)=black . Instrument calibration creates a direct relationship between the instrument voltages, digital numbers and an actual physical energy unit. Surface radiance is typically measured in watts per unit area (ie. per cm2 )
https://crisp.nus.edu.sg/~research/tutorial/image.htm https://www.rapidtables.com/web/color/RGB_Color.html
Radiance = how much light the instrument “sees” from the object being observed, including atmospheric scattering and absorption, the intensity and direction of illumination, the orientation and position of the target. Reflectance= the ratio of the amount of light leaving the target to the amount of light striking the target. This is a ratio and has no units. https://www.harrisgeospatial.com/Support/Self-Help-Tools/Help-Articles/Help-Articles-Detail/ArtMID/10220/ArticleID/19247/3377
“the amount of light reflected by the surface of the earth” https://www.usgs.gov/land-resources/nli/landsat/landsat-surface-reflectance?qt-science_support_page_related_con=0#qt-science_support_page_related_con
Digital Earth Australia data is processed to Nadir-corrected BRDF Adjusted Reflectance, to account for the effects of topography and the variation in reflectance angles off the planet’s surface https://docs.dea.ga.gov.au/data/data.html Nadir – from the point of view of the satellite, down toward the planet. http://wiki.gis.com/wiki/index.php/Nadir What is BRDF? Bidirectional reflectance distribution function: used to adjust all satellite images to the equivalent at 45 degrees incidence https://en.wikipedia.org/wiki/Bidirectional_reflectance_distribution_function
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