Specularity

Specularity is the property of a surface to reflect light in a restricted, mirror-like direction, producing a specular reflection or highlight. It contrasts with diffuse reflection, where light is scattered in many directions. The presence and sharpness of specular highlights depend on the surface roughness at scales comparable to the wavelength of light and on the angle of incidence.

Optically smooth or polished surfaces display strong specular reflection, while rough or granular textures yield weaker or broader highlights. At the microscopic level, each surface feature acts as a tiny facet that reflects light according to the law of reflection. The aggregate result is governed by the distribution of facet orientations and by the material’s refractive index, which determines Fresnel reflectance and the color of the highlight.

Mathematically, specularity is described within the Bidirectional Reflectance Distribution Function (BRDF) as a specular term, often modeled as a sharp peak or delta function in idealized materials. Real materials use microfacet models (for example, Beckmann or GGX distributions) that combine a Fresnel term with a roughness function to generate a realistic glossy appearance. Gloss or specular reflectance is sometimes quantified with measurement scales or gloss units.

Applications span computer graphics, photography and cinematography, material science, and vision research. Understanding specularity is crucial for realistic rendering, coating design, and algorithms that separate or remove specular highlights in images. The concept also has analogs in acoustics, where sound can reflect specularly from smooth surfaces.