Redness

Redness is the perceptual attribute of objects that appear red. In human vision, red corresponds to wavelengths roughly 620 to 750 nanometers. Surfaces or light sources that reflect or emit mainly in this range are perceived as red. In color theory, red is a primary color in additive RGB, and a component in subtractive models such as CMYK (often via magenta and yellow).

In biology and medicine, redness is called erythema. It results from vasodilation and increased blood flow in the skin or mucous membranes. Common causes include inflammation, infection, sun exposure, heat, irritation, and allergic reactions. Blushing is a transient facial redness triggered by emotion. Persistent or unexplained redness may signal rosacea, dermatitis, or vascular disorders and may require medical evaluation.

Redness also appears in nature and food when pigments accumulate, such as lycopene or anthocyanins in ripe fruit. In imaging and display, accurate rendering of red depends on light source quality and sensor characteristics. Color measurements use standardized color spaces (for example CIE L*a*b*) to quantify redness, often expressed as chromaticity coordinates or a redness index.

Redness carries cultural associations ranging from warmth to danger, depending on context. Scientifically, it is a property of light interaction and a physiological response in tissue. Because redness can reflect normal variation or pathology, interpretation relies on context and, when appropriate, clinical assessment.