Specular reflection is the mirror-like component of how a surface reflects light — the part of the reflection that gives you a sharp bright highlight where the light source bounces directly off the surface toward your eye. A polished marble countertop has high specular reflection; a matte plaster wall has very low specular. The amount and tightness of the specular highlight is one of the strongest visual cues for material type.
In physically-based rendering terminology, specular reflection is one half of the standard material model, paired with diffuse reflection. The split is usually expressed as a ratio: a fully specular surface (a perfect mirror) is 100% specular, 0% diffuse; a fully matte surface (idealized chalk) is 0% specular, 100% diffuse. Most real materials sit somewhere in between, with the ratio depending on the material and the angle of view.
AI rendering responds well to specular direction in prompts. 'Matte finish' or 'low specular' suppresses highlights and reads as a non-shiny surface; 'satin finish' or 'soft specular highlights' produces a subtle controlled glow; 'glossy' or 'polished' produces sharp tight highlights. Most architectural rendering wants careful control of specular: too little and surfaces look painted-on, too much and the render reads as plastic or commercial.
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