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In optics and heat transfer, reflectivity is the fraction of incident radiation reflected by a surface. In full generality it must be treated as a directional property that is a function of the reflected direction, the incident direction, and the incident wavelength. However it is also common averaged over the reflected hemisphere as follows
where and are reflected and incident spectral intensities, respectively.
Note: The above equation is actually a ratio rather than an average. This should be fixed.
This can be further averaged over all wavelengths to give the total hemispherical reflectivity,
Going back to the fact that reflectivity is a directional property, it should be noted that most surfaces can be divided into those that are specular and those that are diffuse.
For specular surfaces, such as glass or polished metal, reflectivity will be nearly zero at all angles except at the appropriate reflected angle.
For diffuse surfaces, such as matt white paint, reflectivity is uniform; radiation is reflected in all angles equally or near-equally. Such surfaces are said to be Lambertian.
Most real objects have some mixture of diffuse and specular reflective properties.
In certain fields, reflectivity is distinguished from reflectance by the fact that reflectivity is a value that applies to thick reflecting objects. When reflection occurs from thin layers of material, internal reflection effects can cause the reflectance to vary with surface thickness. Reflectivity is the limit value of reflectance as the surface becomes thick; it is the intrinsic reflectance of the surface, hence irrespective of other parameters such as the reflectance of the rear surface.
That part of incident light that is reflected from a body of water is specular and is calculated by the Fresnel equations. Fresnel reflection is directional and therefore does not contribute significantly to albedo which is primarily diffuse reflection.
A real water surface may be wavy. Reflectivity assuming a flat surface as given by the Fresnel equations can be adjusted to account for waviness. A formula and graph for correction for waviness for two different wave energy spectrum definitions exists. 
- Photometry (optics) Main Photometry/Radiometry article
- Federal Standard 1037C (clarification required)
- Reflectance Data Painted surfaces etc.
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