Brightness
Brightness is an attribute of visual perception according to which a visual stimulus is judged to be more or less intense or to emit more or less light.[1] This perceptual quality, defined by the International Commission on Illumination (CIE) in 1970, primarily applies to self-luminous sources such as lights or emitting surfaces, distinguishing it from lightness, which describes the perceived reflectance of non-emissive surfaces like matte objects.[2] In psychophysics, brightness perception is influenced by factors including adaptation to ambient light, spatial context, and chromatic content, often leading to illusions where perceived intensity deviates from physical measurements.[3] Physically, brightness correlates most closely with luminance, a photometric quantity defined as the luminous intensity per unit projected area of a light source in a given direction, with units of candela per square meter (cd/m²).[4] Luminance accounts for the human eye's spectral sensitivity, weighting visible wavelengths according to the CIE photopic luminosity function, which peaks at approximately 555 nm for green light.[5] In radiometry, the analogous quantity is radiance, measured in watts per square meter per steradian (W/m²·sr), which quantifies radiant flux independent of human vision but is sometimes colloquially termed brightness in laser and photonics contexts to denote beam quality and power density.[6] The concept of brightness extends across disciplines, informing standards in color science, display technology, and astronomy. In colorimetry, models like CIECAM02 incorporate brightness as a perceptual correlate for image rendering and quality assessment.[7] For electronic displays, brightness settings adjust luminance output to optimize visibility under varying ambient conditions, typically ranging from 100 to 1000 cd/m² for modern LCD and OLED screens.[5] In astronomy, apparent brightness refers to the flux received from celestial objects, quantified in magnitudes, enabling comparisons of stellar luminosities despite vast distances.Definition and Perception
Perceptual Attributes
Brightness is defined as the attribute of a visual perception according to which an area appears to emit, transmit or reflect more or less light, emphasizing its inherently subjective nature as a perceptual phenomenon rather than a direct physical property.[8] This perception arises from the human visual system's interpretation of luminous stimuli, where brightness describes how intensely a source seems to radiate or reflect light, independent of its actual photometric intensity.[9] Perceived brightness does not scale linearly with physical light intensity, following instead a nonlinear relationship that compresses higher luminances and expands lower ones, as described by psychophysical laws such as Stevens' power law.[10] A classic demonstration is simultaneous contrast illusions, such as White's illusion, where two identical gray patches appear to have markedly different brightness levels solely due to their surrounding luminance patterns—one embedded in dark stripes looks brighter than one in light stripes.[11] In color appearance models like CIECAM02, brightness is quantified as the correlate Q, which represents the perceptual scale of light emission from a stimulus, integrating luminance with contextual viewing conditions to predict subjective appearance.[12] Several factors modulate brightness perception, including adaptation levels, where the visual system dynamically adjusts sensitivity to the ambient luminance over time, enhancing contrast in varying lighting environments.[9] Surround luminance further influences brightness through lateral interactions in the retina and cortex, causing a target to appear brighter against a darker background and dimmer against a lighter one.[13] Individual variations in visual sensitivity, arising from differences in retinal photoreceptor density or neural processing efficiency, also contribute, leading to inter-observer discrepancies in brightness judgments.[9] Brightness stands as the polar opposite to darkness in perceptual terms, denoting the presence of apparent light versus its absence, yet it is not synonymous with physical intensity, as contextual and adaptive effects dominate the subjective experience.[14]Distinctions from Related Concepts
Brightness is often distinguished from lightness in perceptual psychology, where brightness refers to the perceived emission of light from a self-luminous source, such as a glowing object or light bulb, while lightness pertains to the perceived reflectance of a surface relative to a white or highly transmitting reference under similar illumination.[15] For instance, a star appears bright due to its inherent light emission, whereas a white wall appears light because it reflects a high proportion of incident light compared to its surroundings.[16] This distinction aligns with CIE definitions, which describe brightness as the attribute of a visual perception according to which an area appears to emit, transmit or reflect more or less light, in contrast to lightness as the brightness of an area judged relative to the brightness of a similarly illuminated area that appears to be white or highly transmitting.[8][17] In contrast to luminance, which is an objective photometric quantity measuring the amount of light emitted or reflected per unit area in a given direction (typically in candela per square meter), brightness represents the subjective perceptual impression of that light intensity.[6] Historically, the term "brightness" was misused as a synonym for luminance in older scientific texts and even for the radiometric term radiance, leading to confusion between perceptual experience and measurable properties.[18] Within color theory, brightness is independent of hue, which identifies the dominant wavelength or color type (e.g., red versus blue), and saturation, which denotes the vividness or purity of that hue relative to a neutral gray.[19] A highly saturated red can appear equally bright as a desaturated gray if both emit similar light intensities, emphasizing that brightness concerns overall perceived light emission rather than chromatic qualities.[19] In psychophysics, the perception of brightness adheres to Stevens' power law, where the subjective magnitude of brightness scales as a power function of the physical stimulus intensity, with exponents typically ranging from 0.33 for extended sources in darkness to around 0.5 for point sources, reflecting nonlinear sensory scaling without implying a direct proportionality.[20] This relationship underscores brightness as a compressive perceptual transform of intensity. The Federal Standard 1037C (1996) explicitly restricts "brightness" to non-quantitative descriptions of physiological sensations and perceptions of light, prohibiting its use as a synonym for luminance or radiance in technical contexts to maintain terminological precision.[18]Physical and Quantitative Aspects
Photometric Quantities
Photometry is the branch of optics concerned with the measurement of visible light in terms of its perception by the human eye.[21] It quantifies light properties weighted by the spectral sensitivity of human vision, as opposed to radiometry, which measures electromagnetic radiation without regard to visual perception.[22] Central to photometry is the luminous efficiency function, denoted V(λ), which describes the average sensitivity of the human eye to different wavelengths of light under photopic (daylight) conditions.[23] This function peaks at approximately 555 nm in the green-yellow region of the spectrum, reflecting the eye's maximum responsiveness there.[22] The core photometric quantities derive from radiant analogs but are adjusted by V(λ) to account for visual efficacy. Luminous flux (Φ_v) represents the total amount of visible light emitted by a source, measured in lumens (lm).[24] Luminous intensity (I_v) quantifies the flux per unit solid angle in a given direction, in candelas (cd = lm/sr).[25] Illuminance (E_v) measures the flux incident on a surface per unit area, in lux (lx = lm/m²).[26] Luminance (L_v), often most directly linked to perceived brightness of emitting surfaces, is the flux per unit solid angle per unit projected area, in candelas per square meter (cd/m²).[27] The following table compares key radiant and luminous quantities:| Radiant Quantity | Symbol | Unit | Luminous Quantity | Symbol | Unit |
|---|---|---|---|---|---|
| Radiant flux | Φ_e | Watt (W) | Luminous flux | Φ_v | Lumen (lm) |
| Radiant intensity | I_e | W/sr | Luminous intensity | I_v | Candela (cd) |
| Irradiance | E_e | W/m² | Illuminance | E_v | Lux (lx) |
| Radiance | L_e | W/m²/sr | Luminance | L_v | cd/m² |