The flux received on a surface depends on its orientation with respect to the light source. If the surface is perpendicular to the light rays, the flux is maximum, whereas if it is parallel, it becomes zero. Reflectance is a property of the surface and does not depend on the instrument observing it or on the irradiance received. This reflectance, however, depends on the angle at which the surface is illuminated and observed and also, of course, on the wavelength. We sometimes use the spectral albedo which is the mean of the reflectance in all directions, or of the albedo, which is the average of the spectral albedo over the whole spectrum :.
This time we are at the end of our troubles, and we have demonstrated that the measurements of our satellites are proportional to a property of the surface, the reflectance. In fact, reflectance has many advantages :. Site web. Enregistrer mon nom, mon e-mail et mon site dans le navigateur pour mon prochain commentaire. Aller au contenu Radiometric quantities : irradiance, radiance, reflectance. Radiometry The detectors of optical instruments are sensitive to the light energy they receive during an observation period.
It got darker, of course, which means that there is now less radiance coming from it. And this happens although the wall itself did not change at all! In the remote sensing context, radiance measurements are therefore rarely used directly, but are typically used as a step toward a radiometric quantity that is more closely linked to the surface it is coming from: reflectance.
For isotropic radiation, the reflectance R of a given area is the ratio of outgoing vs. In the wall example, when you turn off the light both the incoming and the outgoing radiance is reduced, and the reflectance of the wall is unchanged. This makes it uniquely suitable for use in remote sensing, because multiple observations with varying illumination can be compared, for uses such as tracking change of an area through time, or for comparing different areas on Earth. To think about what that means, imagine that you are an astronaut and you put a hula-hoop ring horizontally right at the level of the top of the atmosphere if the top of the atmosphere were possible to define and find!
Dividing one by the other gives you TOA reflectance. Calculating TOA reflectance is done a bit differently for different data types. For Landsat 8, there is another set of calibration coefficients that you can apply exactly like you can apply those for TOA radiance Figure Note that values for bands 10 and 11 are not provided, because these bands do not measure reflected solar radiation except to a negligible degree. Also note that coefficients are the same for all bands — for Landsat 8 the DN values have been specifically generated for each scene to ensure constant TOA reflectance calibration coefficients.
Rrs is the ratio between water-leaving radiance Lw, above the sea surface and downwelling irradiance Ed, above the sea surface. Lw can be estimated from above-water radiometric measurements, in this case reflected skylight must be removed using a "surface reflectance factor" rho. I have written a detailed description of these quantities in my blog, trying also to explain how they are used the remote sensing reflectance is not described, but it is explained in Whyzar's answer.
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Asked 4 years, 10 months ago. Active 2 years, 4 months ago. Viewed 23k times. Improve this question. Figure 1. Illustration of radiance, reflectance, and TOA.
For more information: The tutorial developed by USGS shows more information about atmospheric physics and corrections.
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