42 Glossary: Displays
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Electronic displays recreate spatial, spectral, and temporal light distributions for human observers. This glossary defines key photometric units, colorimetric metrics, and engineering parameters used to characterize display systems.
42.1 1. Photometric and radiometric metrics
| Term | Symbol | Primary Concept | Units | Key Definition |
|---|---|---|---|---|
| Luminance | \(L_v\) | Photometrically weighted luminous intensity per unit projected area | \(\text{cd/m}^2\) (or nits) | Integrates spectral radiance weighted by the human photopic luminous efficiency function \(V(\lambda)\) |
| Nit | — | Practical industry name for one candela per square meter | \(\text{cd/m}^2\) | Standard unit of display brightness (\(1\text{ nit} = 1\text{ cd/m}^2\)) |
| Illuminance | \(E_v\) | Total luminous flux incident on a display surface per unit area | Lux (\(\text{lx} = \text{lm/m}^2\)) | Ambient room light falling onto the display screen |
| Contrast Ratio | \(\text{CR}\) | Ratio of peak white luminance to deepest black luminance | Dimensionless | \(\text{CR} = \frac{L_{\text{white}}}{L_{\text{black}}}\) (often \(1{,}000:1\) for LCDs; \(> 1{,}000{,}000:1\) for OLEDs) |
42.1.1 Luminance and the nit
Luminance (\(L_v\)) quantifies the perceived brightness of an emitting or reflecting surface. The standard SI unit is candela per square meter (\(\text{cd/m}^2\)), universally called a nit in display engineering. Typical consumer display targets: * Standard office monitor: \(200\text{--}350\text{ nits}\) * Smartphone outdoors (direct sunlight): \(1{,}000\text{--}2{,}000+\text{ nits}\) * High Dynamic Range (HDR) peak highlights: \(1{,}000\text{--}4{,}000\text{ nits}\)
42.1.2 Contrast ratio
The contrast ratio dictates the dynamic range of the display. Emissive displays (OLED, microLED) can turn individual subpixels completely off (\(L_{\text{black}} \approx 0\)), achieving nearly infinite static contrast ratios. Transmissive displays (LCDs) rely on a backlight modulated by liquid crystal shutters, where small light leakage through closed crystals limits native contrast to roughly \(1{,}000:1\) (improved by local-dimming mini-LED backlights).
42.2 2. Color reproduction and gamuts
| Term | Primary Concept | Standard Examples |
|---|---|---|
| Display Primaries | The fixed spectral power distributions (Red, Green, Blue) emitted or transmitted by display subpixels | LED phosphors, OLED emitters, quantum dots |
| Color Gamut | The triangular region in chromaticity space encompassing all colors achievable by linear combinations of the three primaries | sRGB / Rec. 709, DCI-P3, Rec. 2020 |
| White Point | The chromaticity coordinate and correlated color temperature produced when \(R=G=B=\text{max}\) | CIE Standard Illuminant D65 (\(6500\text{ K}\)) |
42.2.1 Color gamut
A display creates color by additive mixture of its three primaries. By Grassmann’s laws, the set of reproducible chromaticities forms a triangle in the CIE \(1931\) \((x, y)\) chromaticity diagram whose vertices are the primaries: * sRGB / Rec. 709: The historical baseline standard for web content and HDTV. * DCI-P3: Cinema and modern mobile/desktop standard covering roughly \(25\%\) larger gamut area than sRGB (richer reds and greens). * Rec. 2020: Wide-gamut target standard for ultra-high-definition television (UHDTV) and HDR content.
42.3 3. Electro-optical transfer and modeling
- Tone Reproduction Curve (TRC) / Gamma (\(\gamma\)): The non-linear transfer function relating digital frame buffer code values (\(V \in [0, 1]\)) to relative emitted luminance (\(L \in [0, 1]\)), historically approximated by a power law: \[ L = V^\gamma \quad (\text{typically } \gamma \approx 2.2) \]
- Standard Display Model (SDM): A matrix-vector computational model that predicts the full spectral power distribution \(S(\lambda)\) emitted by a display from RGB frame buffer values: \[ S(\lambda) = \sum_{c \in \{R,G,B\}} T_c(V_c) \, P_c(\lambda) \] where \(T_c(V_c)\) is the non-linear tone reproduction curve for channel \(c\), and \(P_c(\lambda)\) is the invariant spectral power distribution of primary \(c\).
- Subpixel: The smallest addressable physical color element on a display. A conventional pixel consists of three side-by-side subpixels (Red, Green, Blue stripe); alternative layouts include PenTile arrays and RGBW patterns.
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