In photometry 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 commonly averaged over the reflected hemisphere to give the hemispherical spectral reflectivity:
where and are the reflected and incident spectral (per wavelength) intensity, respectively.
This can be further averaged over all wavelengths to give the total hemispherical reflectivity,
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.
The reflectance spectrum or spectral reflectance curve is the plot of the reflectivity as a function of wavelength.
Most real objects have some mixture of diffuse and specular reflective properties.
Reflection occurs when light moves from a medium with one index of refraction into a second medium with a different index of refraction.
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.