Glossary
Accent lighting
Accent lighting is focused light used to draw attention to an object, surface, or architectural feature. Common sources include track heads, spotlights, wall washers, and adjustable downlights.
Ambient lighting
Ambient lighting is the general, overall illumination of a space for basic visibility and comfort. It typically comes from ceiling fixtures, indirect lighting, or large diffused sources.
Angle of incidence
Angle of incidence is the angle between an incoming light ray and a surface’s normal (perpendicular). It affects reflection, glare, and how light is absorbed or transmitted by materials.
Backlighting
Backlighting places light behind a subject or object relative to the viewer to create separation, depth, or a halo effect. In architecture, it often refers to lighting behind panels, signage, mirrors, or translucent materials.
Beam angle
Beam angle is the spread of a light beam, expressed in degrees, often defined between points where intensity falls to 50% of peak. Narrow beams concentrate light; wide beams provide broader coverage.
Blue light
Blue light is short-wavelength visible light, roughly 450–495 nm. Because human circadian photoreception is sensitive in this range, blue-rich light can increase alertness and influence circadian timing, especially at night.
Bright light therapy
Bright light therapy is a clinical approach that uses timed exposure to bright light (often via a light box) to influence circadian rhythms. It’s commonly discussed for circadian-related sleep and mood disorders and should follow medical guidance.
Building envelope
The building envelope is the physical barrier between inside and outside: walls, roof, glazing, and insulation. It strongly affects daylight availability, glare risk, solar heat gain, and overall energy performance.
Candela (cd)
Candela is the SI unit of luminous intensity in a specific direction. It’s used in photometry to describe how concentrated a source’s light output is within a given angle.
Clerestory window
A clerestory window is high-level glazing near the top of a wall, above eye level, used to bring daylight deeper into a space. It can improve daylight distribution while helping preserve privacy and wall space below.
Circadian rhythm
Circadian rhythm is the body’s roughly 24-hour internal timing system that regulates sleep-wake patterns and other biological processes. Light exposure, especially its timing, intensity, and spectrum, is a primary cue that shifts this rhythm.
Color mixing
Color mixing is combining multiple LED channels (for example RGB, RGBA, or tunable white) to achieve a target color or white point. It’s used for dynamic scenes, color effects, and CCT tuning.
Lighting control station
A lighting control station is a wall-mounted keypad or touchscreen used to recall scenes and adjust lighting levels in a space. It typically connects to a centralized lighting control system.
Lighting control console
A lighting control console is a controller used to program and run lighting scenes, fades, and effects, often in large venues or complex installations. In architectural lighting, similar functions may be handled by a networked lighting control system or software.
Cool white light
Cool white light generally refers to white light around 4000K and above. It can appear crisper and more alerting than warm white, depending on intensity and spectrum.
CRI (Color Rendering Index)
CRI is a 0–100 score estimating how accurately a light source renders colors compared to a reference source at the same CCT. It’s a useful baseline metric but can miss color shifts that newer methods like TM-30 capture.
Daylight
Daylight is natural light from the sun and sky reaching the earth’s surface. It varies by time, weather, and orientation and is a key driver of visual comfort and circadian timing.
Daylight factor (DF)
Daylight factor is the ratio of indoor illuminance at a point to the simultaneous outdoor horizontal illuminance under overcast sky conditions, expressed as a percentage. It’s used to compare daylight availability independent of time of day.
Daylight harvesting
Daylight harvesting uses sensors and controls to reduce electric light when sufficient daylight is available. Done well, it saves energy while maintaining target light levels.
Daylighting
Daylighting is the intentional use of windows, skylights, and reflective surfaces to deliver useful daylight to interior spaces. Good daylighting balances illumination, glare control, and heat gain.
Decorative lighting
Decorative lighting prioritizes appearance and atmosphere in addition to illumination. Examples include pendants, chandeliers, sconces, and luminous architectural features.
Dichroic filter
A dichroic filter selectively transmits some wavelengths while reflecting others. It’s used for color separation, optical mixing, and filtering in certain luminaires and systems.
Diffusion
Diffusion scatters light to reduce hotspots and soften shadows, producing more uniform luminance. Common diffusers include opal lenses, microprismatic films, and fabric or acrylic panels.
Dimming
Dimming is controlling light output level, typically by reducing LED drive current or modulating the driver signal. Dimming behavior depends on the driver and control method (for example 0–10V, DALI, PWM).
DMX (DMX512)
DMX512 is a digital control protocol used to control lighting intensity, color, and effects, common in dynamic and entertainment lighting. It can also be used in architectural applications that need real-time scene control.
Downlight
A downlight is a ceiling-mounted luminaire that directs most of its light downward. It’s used for ambient lighting, task lighting, or accenting depending on optics and placement.
Electric skylight
An electric skylight is a ceiling-mounted lighting system designed to create the appearance of a skylight using electric light rather than roof glazing. Also called an artificial skylight, virtual skylight, or “fake skylight.”
Fenestration
Fenestration refers to the design, size, placement, and performance of windows, skylights, and other openings in a building envelope. It strongly affects daylight levels, glare risk, and energy use.
Flood (flood beam)
A flood beam is a wide light distribution intended to cover a larger area more evenly than a spot beam. Used for general coverage, wall lighting, and broad accenting.
Footcandle (fc)
A footcandle is a unit of illuminance equal to one lumen per square foot. 1 fc is approximately 10.764 lux.
Fresnel lens
A Fresnel lens is a stepped lens design that reduces thickness while maintaining focusing power. It’s used in optics to shape or concentrate light with less material than a conventional lens.
Glare
Glare is visual discomfort or reduced visibility caused by high luminance in the field of view or excessive luminance contrast. Common causes include bright sources, reflections, and poor shielding or placement.
Suspended ceiling grid (T-bar grid)
A suspended ceiling grid is a metal framing system (often a T-bar grid) that supports ceiling tiles and compatible fixtures. Many luminaires are designed for standard grid sizes (for example 2×2 or 2×4).
Halogen lamp
A halogen lamp is a type of incandescent lamp that uses halogen gas to extend filament life and maintain higher output. It produces warm light and is commonly used for accent and beam control, but is less efficient than LED.
Hard light
Hard light is directional light that creates sharp-edged shadows and high contrast. It typically comes from small or tightly collimated sources.
Incandescent lamp
An incandescent lamp produces light by heating a filament until it glows. It has excellent color rendering but low efficiency and short life compared to LED.
Inverse square law
Inverse square law states that illuminance decreases with the square of distance from a point-like light source. Doubling distance reduces illuminance to about one quarter.
Kelvin (K)
Kelvin (K) is the unit used to express correlated color temperature (CCT), which describes how warm or cool a white light appears. Lower K looks warmer; higher K looks cooler.
Illuminance
Illuminance is the amount of light falling on a surface, measured in lux (lm/m²) or footcandles (lm/ft²). It’s the main metric used to specify target light levels for tasks.
Spectral Power Distribution (SPD)
Spectral Power Distribution (SPD) describes a light source’s output across wavelengths. SPD affects perceived color, color rendering, and circadian-relevant metrics.
Light timing
Light timing is when and how long light exposure occurs across the day. Timing, intensity, and spectrum together influence alertness and circadian phase, especially in the morning and evening.
Light tube (tubular daylight device)
A light tube, also called a tubular daylight device, uses a reflective tube to bring sunlight from the roof to interior spaces. Performance depends on tube length, bends, and diffuser design.
Lighting control system
A lighting control system coordinates dimming, scheduling, sensors, and scenes across one or more spaces. Common functions include occupancy control, daylight harvesting, time schedules, and demand response.
Networked lighting controls (NLC)
Networked lighting controls (NLC) connect luminaires, sensors, and controllers over a wired or wireless network for centralized programming, monitoring, and analytics. NLC enables zone-level control, data logging, and remote changes.
Luminaire
A luminaire is a complete lighting unit, including the light source, optics, housing, and electrical components. In specs, it refers to the fixture as installed.
Lux (lx)
Lux is the SI unit of illuminance equal to one lumen per square meter. It describes how much light reaches a surface, not how bright the source looks.
Modular lighting array
A modular lighting array combines multiple fixtures into a coordinated layout to create a larger luminous surface or repeated pattern. Arrays are used to scale coverage and maintain visual uniformity.
Melatonin
Melatonin is a hormone involved in sleep-wake regulation, typically rising in the evening and falling in the morning. Light exposure, especially at night, can suppress melatonin depending on intensity and spectrum.
Multi-way switching (3-way/4-way)
Multi-way switching allows a single light or circuit to be controlled from multiple locations, commonly using 3-way and 4-way switch configurations. It’s typical in corridors, stairwells, and large rooms.
Narrow beam
A narrow beam is a tight light distribution designed for focused accenting and longer throw. It increases intensity on target areas but covers less area than a flood beam.
Natural light
Natural light is daylight entering a space through windows, skylights, and other openings. Its variability affects visual comfort, glare, and lighting control strategy.
Nadir
Nadir is the direction straight down from a luminaire (0°). Photometric distributions often reference nadir as the primary axis for measuring intensity.
Non-visual effects of light
Non-visual effects of light are biological responses that are not primarily about vision, such as circadian phase shifting, alertness, and hormonal regulation. These effects depend on timing, intensity at the eye, duration, and spectrum.
Occupancy sensor
An occupancy sensor detects people in a space and can turn lights on, dim, or switch them off based on activity. Common sensing methods include PIR (motion) and ultrasonic or dual-technology sensors.
Passive solar design
Passive solar design uses building orientation, glazing, shading, and thermal mass to manage sunlight for comfort and energy performance. It influences daylight availability, glare, and cooling loads.
Photocell (daylight sensor)
A photocell, or daylight sensor, measures light levels and triggers switching or dimming based on available daylight. It’s commonly used for daylight harvesting and exterior lighting control.
Photoreceptors
Photoreceptors are light-sensitive cells in the retina: rods and cones support vision, and intrinsically photosensitive retinal ganglion cells (ipRGCs) contribute strongly to circadian and alerting responses. Together they determine how light affects both seeing and biology.
Reflector
A reflector redirects light to shape a beam or improve efficiency. Reflector geometry and finish (specular vs diffuse) affect beam spread, punch, and uniformity.
Remote lighting control
Remote lighting control allows switching, dimming, and scene selection without using a wall switch, via handheld remotes, apps, or networked systems. Capabilities depend on the underlying control protocol and driver compatibility.
RGB
RGB is a color-mixing method using red, green, and blue LED channels to create colored light. RGB can create many colors but typically needs additional channels to produce high-quality white light.
Skylight
A skylight is a roof or ceiling opening with glazing that admits daylight from above. Skylight design affects daylight distribution, glare, and heat gain.
Skylight shade
A skylight shade is a shading device that reduces daylight and glare from a skylight. Shades may be fixed, operable, manual, or motorized and can also help manage heat gain.
Smart lighting system
A smart lighting system provides automated or connected control such as schedules, scenes, occupancy response, and app or voice control. It typically combines controllable luminaires, sensors, and a hub or network platform.
Soft light
Soft light is diffuse illumination that produces gentle shadow transitions and lower contrast. It usually comes from large luminous surfaces, diffusers, or indirect lighting.
Solar access
Solar access is a site’s ability to receive direct sun without being blocked by terrain or nearby structures. It influences daylighting potential and passive solar strategies.
Solar orientation
Solar orientation is how a building and its glazing are positioned relative to the sun’s path. It affects daylight availability, glare risk, and seasonal heat gain.
Solar radiation
Solar radiation is electromagnetic energy from the sun, including visible, infrared, and ultraviolet components. In buildings it drives daylight, glare, and heat gain.
Solar tube (tubular daylight device)
A solar tube, also called a tubular daylight device, channels sunlight from the roof to interior spaces using a reflective tube and diffuser. It’s often used where conventional skylights are impractical.
Solar-powered skylight
A solar-powered skylight uses a rooftop solar panel to power an integrated motor, shade, or ventilation feature, depending on the product. Some products also use solar power for lighting or controls when grid power is unavailable.
Spill light
Spill light is unwanted light that falls outside the intended target area. Controlling spill improves visual comfort and reduces wasted light and glare.
Strip light
A strip light is a linear luminaire used for continuous illumination along a surface or edge. It’s common for coves, under-cabinet lighting, toe-kicks, and architectural details.
Sun tunnel
A sun tunnel, also called a tubular daylight device, uses a reflective tube and ceiling diffuser to bring daylight from the roof into interior spaces. Performance depends on tube length, bends, and roof capture.
Task lighting
Task lighting provides higher, focused illumination where work is performed, such as desks, counters, exam areas, and workstations. It’s often layered on top of ambient lighting to reach target illuminance.
Timers
A lighting timer switches or schedules lighting automatically at preset times. Timers are used for energy savings, security, and consistent daily lighting patterns.
Tubular skylight
A tubular skylight is a roof-to-ceiling daylighting system that channels sunlight through a reflective tube to a diffuser. It’s a common solution when a conventional skylight shaft is impractical.
Warm white
Warm white describes white light with a lower CCT, typically about 2700–3000K, which appears more amber or yellow. It’s commonly used in residential and hospitality settings.
Wall wash (wash lighting)
Wall washing is a lighting technique that provides a smooth, even distribution of light across a vertical surface. It’s used to make spaces feel brighter and to highlight textures and finishes.
Zone control
Zone control groups lights so each area can be switched, dimmed, or scheduled independently. Zoning supports energy savings, scene setting, and occupant comfort.
Zoom (adjustable beam)
Zoom is the ability to vary beam angle from one luminaire, typically from spot to flood. It’s used to fine-tune coverage as layouts or focal points change.
Lumen (lm)
A lumen is a unit of luminous flux, representing total visible light output from a source. Lumens describe quantity of light, not its distribution.
Luminous efficacy (lm/W)
Luminous efficacy is the ratio of light output (lumens) to electrical power (watts). Higher lm/W generally indicates better energy efficiency for the same light output.
Luminance (cd/m²)
Luminance describes how bright a surface appears in a given direction, measured in candela per square meter. It’s closely tied to glare and perceived brightness.
Chromaticity
Chromaticity is the specification of a light’s color independent of brightness, often expressed as CIE x,y coordinates. It’s used to define and match white points and colors.
Duv
Duv indicates how far a light’s chromaticity is from the blackbody locus at a given CCT. Positive Duv tends to look slightly green; negative Duv tends to look slightly pink.
TM-30
TM-30 is an IES method for evaluating color rendition using fidelity (Rf) and gamut (Rg), offering more detail than CRI. It helps compare how different sources shift color appearance.
Rf (TM-30 Fidelity)
Rf is the TM-30 fidelity index describing how closely colors appear compared to a reference source at the same CCT. Higher Rf generally means smaller color shifts.
Rg (TM-30 Gamut)
Rg is the TM-30 gamut index describing average saturation shift relative to a reference. Values above 100 indicate increased saturation; below 100 indicate reduced saturation.
SDCM (MacAdam steps)
SDCM describes allowable color variation between LEDs or luminaires. Lower SDCM means tighter color consistency and fewer visible differences between fixtures.
LED binning
LED binning sorts LEDs by color (chromaticity/CCT) and output so fixtures can be matched more consistently. Better binning reduces noticeable variation across an installation.
Flicker
Flicker is rapid modulation of light output caused by the driver or control method. It can contribute to discomfort, headaches, and camera artifacts depending on frequency and modulation depth.
PstLM
PstLM is an IEC metric estimating the likelihood of visible short-term flicker from a light source. Lower values generally indicate less perceptible flicker.
SVM
SVM is an IEC metric estimating the likelihood of stroboscopic effects under flickering light. Lower values generally indicate reduced stroboscopic risk.
Unified Glare Rating (UGR)
UGR is a method for estimating discomfort glare in indoor environments based on luminaire luminance, size, and position. Lower UGR generally indicates less glare.
Veiling reflections
Veiling reflections are reflections that reduce contrast on a task surface, such as a computer monitor or glossy desk. They can make reading harder even when illuminance is adequate.
Beam spread
Beam spread describes how a luminaire’s light output is distributed across angles and space. It’s typically communicated via beam angle, photometric curves, or an IES file.
IES file
An IES file is a standardized photometric data file that describes a luminaire’s light distribution. It’s used in lighting calculation software and renderings to simulate performance.
Light distribution
Light distribution describes the direction and proportion of light a luminaire emits, such as direct, indirect, direct/indirect, asymmetric, or wall wash. Distribution affects uniformity, glare, and visual comfort.
ipRGCs (intrinsically photosensitive retinal ganglion cells)
ipRGCs are retinal cells containing melanopsin that contribute strongly to non-visual responses to light, including circadian timing and alertness. They respond to overall light at the eye and are especially sensitive to short-wavelength content.
Melanopic EDI
Melanopic EDI (Equivalent Daylight Illuminance) is a CIE metric estimating melanopsin-weighted, daylight-equivalent illuminance at the eye. It’s used to quantify circadian-relevant light exposure more directly than photopic lux.
Melanopic DER
Melanopic DER (Daylight Efficacy Ratio) is the ratio of melanopic to photopic output relative to a daylight reference, for the same photopic illuminance. It helps compare how “melanopically strong” different spectra are at a given lux level.
EML (Equivalent Melanopic Lux)
EML is a legacy metric used in earlier WELL guidance to approximate melanopic content at the eye. It is not equivalent to CIE melanopic EDI and should not be compared 1:1.
Circadian Stimulus (CS)
Circadian Stimulus (CS) is a model-based metric estimating circadian activation from light at the eye using the Rea model. It’s often used in research and design targets, but it is not the same as melanopic EDI/DER.
S/P ratio
The S/P ratio is the scotopic-to-photopic ratio of a light source, describing how it stimulates rod vision versus cone vision. Higher S/P often increases perceived brightness at low light levels and can affect adaptation.
0-10V dimming
0-10V dimming is an analog control method where a low-voltage signal sets driver output level. Behavior depends on the driver, including dimming range, low-end stability, and whether it dims to off.
DALI
DALI (Digital Addressable Lighting Interface) is a digital, addressable lighting control protocol for drivers and luminaires. It supports individual addressing, grouping, scenes, and status feedback in many implementations.
DMX512
DMX512 is a digital control protocol designed for real-time control of intensity, color, and effects. It’s common in entertainment lighting and used in some architectural applications needing dynamic scene playback.
BACnet
BACnet is a building automation protocol used to integrate systems like HVAC, lighting, and shading under a building management system (BMS). It enables centralized scheduling, monitoring, and alarms across trades.
PoE lighting
PoE lighting uses Power over Ethernet to deliver low-voltage DC power and data over network cabling. It can simplify controls integration and enable network-level monitoring, depending on the system architecture.
Sensor commissioning
Sensor commissioning is the setup and calibration of sensors, zones, and control logic after installation. It typically includes sensitivity, timeout, daylight setpoints, and scene behavior tuning.
LED driver
An LED driver regulates current to LEDs and interfaces with control signals for dimming and tuning. Driver design affects flicker, dimming smoothness, reliability, and compatibility with control protocols.
Power factor (PF)
Power factor is the ratio of real power used by a load to the apparent power drawn from the electrical system. Higher PF generally means more efficient use of electrical capacity and fewer utility penalties in some contexts.
THD (Total Harmonic Distortion)
Total Harmonic Distortion (THD) quantifies electrical waveform distortion caused by non-linear loads such as some drivers and power supplies. High THD can reduce power quality and contribute to heating or nuisance issues on circuits.
L70
L70 is the point in time when a light source’s lumen output has declined to 70% of its initial value under specified conditions. It’s a common benchmark for LED lumen maintenance, not a guarantee of failure at that time.
CCT tuning
CCT tuning is adjusting correlated color temperature over time or by scene, typically using multi-channel LED mixing. It’s used for preference, visual comfort, and circadian-aligned schedules.
Collimation
Collimation is optical shaping that reduces beam divergence, making rays more parallel. It’s used when a system needs a tight, directional beam, such as a sun-like effect.
Divergence angle
Divergence angle describes how quickly a light beam expands as it travels away from its source, usually expressed in degrees. Lower divergence produces a tighter, more directional beam that can appear more sun-like at distance.
Apparent sun size
Apparent sun size is the perceived angular size of a sun source as seen by an observer. It affects realism, perceived distance, and comfort because larger apparent sources can change glare perception and visual dominance.
Etendue
Etendue is an optics property that limits how tightly light can be concentrated or collimated without losses, based on source size and beam angle. It helps explain why creating a small, high-intensity, low-divergence “sun beam” is optically challenging.
WELL Building Standard
The WELL Building Standard is a certification system focused on health and well-being in buildings. It includes requirements and guidance related to lighting quality, circadian-relevant light exposure, and visual comfort.
LEED
LEED (Leadership in Energy and Environmental Design) is a green building certification program that evaluates sustainability across categories including energy and indoor environmental quality. Lighting can contribute through efficient systems, controls, and daylighting strategies.