In photometry "Photometry (optics)") and in heat transfer, reflectivity is the fraction of incident radiation reflected "Reflection (physics)") by a surface. In general it should be treated as a directional property, depending on the reflected direction, the incident direction, and the incident wavelength. However, it is also commonly averaged over the reflected hemisphere to give the hemispheric spectral reflectivity:.

where and are the reflected and incident spectral intensities (per wavelength) respectively.

Thus it can be averaged with all wavelengths, giving the total hemispheric reflectivities.

Reflectivity is an important concept in the fields of solar thermal energy, telecommunications and radar.

Reflectivity measures the relationship between the amplitude of the reflected electromagnetic field with respect to the amplitude of the incident field, while reflectance refers to the relationship between the incident electromagnetic power with respect to the power that is reflected at an interface. Therefore the magnitude of the reflectance is the square of the magnitude of the reflectivity. Reflectivity can be expressed as a complex number as demonstrated by the Fresnel equations for a simple layer, while reflectance is always a positive real number.

In certain fields, reflectivity is distinguished from reflectance by the fact that reflectivity is a value that refers to thick reflective objects. When reflection is produced by the intervention of thin layers of material, the effects of internal reflections can cause the reflectance to vary with the thickness of the surface. Reflectivity is the limiting value of reflectance as the thickness of the surface increases; It is the intrinsic reflectance of the surface, so its value does not depend on other parameters such as the reflectance of the deep layers of the material.

The reflectance spectrum or spectral reflectance curve is the graph of reflectivity versus wavelength.

Considering that reflectivity is a directional property, many surfaces can be divided into those that produce a specular reflection and those that reflect light diffusely.