The Complete Lighting Glossary

Directional light that emphasizes a specific object, architectural feature, or display surface. Typically 3–5 times brighter than the surrounding ambient light to create visual hierarchy and focal points.

General, uniform background illumination that fills a space at a comfortable baseline level. One of the three layers of lighting (alongside task and accent). Ambient light ensures safe navigation and overall visual comfort.

The SI unit of electric current. Measures the rate of electron flow through a conductor. In lighting, amperage determines wire gauge, driver capacity, and circuit breaker sizing.

The amount of electrical current flowing through a circuit, measured in amperes. Total fixture amperage on a circuit must not exceed 80% of the breaker rating per NEC guidelines.

A device that regulates the current in discharge lamps such as fluorescent and HID. LED fixtures use drivers (constant-current or constant-voltage) instead of traditional ballasts.

The angle at which luminous intensity drops to 50% of the maximum (peak) value. Narrow beam angles (15–30°) produce focused spot lighting; wide angles (60–120°) produce flood lighting.

The connecting interface between a lamp and its fixture. Common types include E26/E27 (Edison screw), GU10 (twist-lock), MR16 (bi-pin), and T8 (bipin linear).

The subjective visual perception of luminance. Unlike luminance (a measured quantity), brightness is influenced by adaptation level, surrounding contrast, and individual vision.

The SI unit of luminous intensity in a given direction. One candela equals one lumen per steradian. Used to specify the directional output of spotlights, traffic signals, and other focused sources.

The appearance of white light measured in Kelvin (K). 2700K–3000K is warm white (residential), 4000K–5000K is neutral/cool white (commercial), and 5700K+ is daylight (industrial/outdoor).

A 0–100 scale measuring how accurately a light source renders colors compared to a reference (sunlight or incandescent). CRI 80+ is acceptable for general use; CRI 90+ is preferred for retail, medical, and color-critical applications. See also R9.

The difference in luminance between an object and its background. Sufficient contrast is essential for visual task performance; too little causes eye strain, while too much creates glare.

See CCT (Correlated Color Temperature). The term refers to the color appearance of light emitted by a blackbody radiator at a given temperature, measured in Kelvin.

The angle measured from vertical (nadir) at which a fixture’s bare light source is no longer directly visible. Fixtures with a cut-off angle of 65° or less reduce glare and light trespass in outdoor applications.

An optical component (frosted lens, opal cover, or micro-prismatic sheet) that scatters light for uniform distribution. Diffusers reduce harsh shadows and visible LED pinpoint images but lower fixture efficacy by 5–15%.

Reducing a fixture’s light output via electronic control. Common protocols include TRIAC/phase-cut (residential), 0–10V (commercial), DALI (networked), and PWM (LED-specific).

A non-profit that maintains qualified product lists for commercial LED fixtures. DLC Premium certification is often required for utility rebate eligibility in North America.

The electronic power supply that converts AC mains to the constant current or constant voltage LEDs require. Driver quality directly affects flicker, dimming performance, and fixture lifespan.

The distance from the Planckian locus on the CIE 1960 UCS chromaticity diagram. Positive Duv indicates a greenish tint; negative indicates pinkish. ANSI White tolerances specify Duv within ±0.006.

The ratio of luminous flux (lumens) to electrical power consumed (watts), expressed as lm/W. Commercial LED efficacy ranges from 130–180 lm/W; this metric is central to energy savings calculations.

Battery-backed lighting that activates automatically during a power failure to illuminate egress paths. NFPA 101 and IBC require minimum 1 lux at floor level along exit routes for at least 90 minutes.

Electrical noise emitted by LED drivers that can disrupt nearby electronic equipment. FCC Part 15 Class A (commercial) and Class B (residential) set emission limits; poor-quality drivers are a common source.

The incandescent wattage an LED lamp is designed to replace. A 12W LED producing 1,100 lm is typically labeled “75W equivalent,” though the actual comparison should be based on lumens, not wattage.

A complete lighting unit consisting of a housing, light source, driver, optical components (reflector, lens, diffuser), and electrical connections. Also called a luminaire.

A wide-beam luminaire designed for area illumination. Beam angles typically range from 60° to 120°. Used for building facades, sports fields, parking lots, and security lighting.

Imperial unit of illuminance: one lumen per square foot. 1 fc = 10.764 lux. Still widely used in North American standards; IES recommended practice documents specify target levels in both fc and lux.

Visual discomfort caused by excessive brightness in the field of view. Disability glare impairs visibility; discomfort glare causes annoyance without reducing visibility. Quantified by UGR in interior spaces.

The geometric arrangement of fixtures across a space. Grid spacing is determined by ceiling height, beam angle, and target uniformity ratio. Uniform grids are common in offices; irregular grids suit retail and hospitality.

A thermally conductive component (usually aluminum) that draws heat away from the LED junction. Proper thermal management is critical: every 10°C reduction in junction temperature roughly doubles LED lifetime.

A fixture designed for mounting heights above 6 meters (20 ft), common in warehouses, factories, and gymnasiums. Typically 100W–500W with narrow beam angles for long throw distances.

The professional body that publishes the most widely adopted lighting standards in North America. IES Recommended Practice documents (e.g., RP-1 for offices, RP-7 for industrial, RP-8 for roadways) define target illuminance, uniformity, and glare limits.

The luminous flux incident on a surface per unit area, measured in lux (lm/m²) or foot-candles (lm/ft²). This is the primary metric in lighting design — all calculations target a specific maintained illuminance at the working plane.

A two-digit code (IEC 60529) defining dust and water resistance. First digit (0–6) = solid particle protection; second digit (0–9) = liquid protection. IP65 = dust-tight + water-jet resistant; IP67 = submersible.

A fixture with a non-replaceable LED light source built directly into the housing. Offers superior thermal management and optical efficiency but requires full fixture replacement at end of life.

The internal temperature at the LED semiconductor junction. Excessive junction temperature (typically above 125°C) accelerates lumen depreciation and shifts color. Proper heat sink design keeps Tj well below this limit.

The SI unit of thermodynamic temperature, used in lighting to express CCT. Lower values (2700K–3000K) appear warm/yellow; higher values (5000K–6500K) appear cool/blue. The scale does not use the degree symbol.

The operating hours at which LED output drops to 70% of initial luminous flux — the industry-standard threshold for “useful life.” A typical L70 rating is 50,000–100,000 hours. See also TM-21.

The fraction of initial lumen output remaining at a given point in a lamp’s life. For LEDs, LLD at 50,000 hours is typically 0.70–0.85. LLD is a key component of the Maintenance Factor.

Light loss caused by accumulated dirt on the fixture lens and reflector. LDD values range from 0.70 (dirty industrial) to 0.94 (clean office) depending on environment and cleaning schedule.

A semiconductor device that emits light when forward current passes through the p-n junction. LEDs offer high efficacy (130–180 lm/W), long life (50,000+ hours), instant on/off, and full dimmability.

The IES test method for measuring the electrical and photometric performance of LED luminaires. LM-79 reports provide total flux, efficacy, CCT, CRI, and angular distribution data from an integrating sphere or goniophotometer.

The IES test method for measuring lumen maintenance of LED packages over a minimum of 6,000 hours. LM-80 data is the input for TM-21 lifetime projections.

The SI unit of luminous flux — the total quantity of visible light emitted by a source in all directions. A 100W incandescent produces ~1,600 lm; a 100W LED can produce 16,000+ lm.

The gradual reduction in light output over a source’s operating life. For LEDs, this is slow and continuous; for incandescents, it is rapid. See LLD and L70.

The SI unit of illuminance: one lumen per square meter. 1 lux = 0.0929 fc. Typical targets: 300–500 lux for offices, 150 lux for corridors, 20–50 lux for parking lots.

A combined light-loss factor used in lighting calculations to ensure a space meets target illuminance throughout the fixture’s life. MF = LLD × LDD × BF × LF. Typical MF values: 0.6–0.8. Without MF, a space will be under-lit within months.

A device that switches or dims lighting based on detected movement, using PIR (passive infrared), microwave, or dual technology. Reduces energy consumption by 20–60% in intermittent-use spaces like corridors and restrooms.

A NEMA classification system (Types 1–7) for floodlight beam spread based on the horizontal and vertical beam angles at 10% of peak intensity. Type 3 (narrow) suits long-throw applications; Type 6 (wide) suits broad area lighting.

A control device that turns lights on when a space is occupied and off (or dims) when vacant. Differs from a vacancy sensor, which requires manual on but auto off. Required by ASHRAE 90.1 in many space types.

The ratio of real power (watts) to apparent power (volt-amperes). PF ≥ 0.9 is required by most utility regulations for commercial LED drivers. Low PF causes higher current draw and utility penalties.

The science of measuring visible light in terms of human visual response. Photometric data (intensity distribution curves, zonal lumens, BUG ratings) is essential for fixture selection and layout design.

An IES-format data file (.ies) describing a luminaire’s light distribution. Contains candela values, zonal lumens, efficacy, and CRI. Imported into lighting design software (AGi32, Dialux) for point-by-point illuminance calculations.

The CRI sub-score for saturated red (test color 9). R9 is not included in the general CRI (Ra) calculation but is critical for rendering skin tones, meat, and produce. R9 > 50 is preferred for retail and medical lighting; many cheap LEDs have negative R9 values.

The ratio of light reflected by a surface to the light incident upon it. Typical reflectances: white ceiling 0.80–0.90, light walls 0.50–0.70, dark floor 0.10–0.20. Room reflectance significantly affects illuminance calculations.

An optical element that redirects light via specular or diffuse reflection. Reflectors control beam shape and distribution — from narrow spot (parabolic) to wide flood (matte white).

Replacing a legacy lamp or fixture with an LED equivalent while retaining the existing housing or socket. Common in commercial buildings where full fixture replacement is cost-prohibitive.

The scotopic-to-photopic ratio, measuring how efficiently a light source supports vision at low (mesopic) light levels. Higher S/P ratios (typical of 5000K+ LEDs) can improve perceived brightness in outdoor applications at the same lux level.

A graph showing the radiant power of a light source at each wavelength. The SPD curve determines CCT, CRI, R9, and S/P ratio. Unlike CCT/CRI (single numbers), the SPD reveals the full color rendering capability.

A narrow-beam luminaire (typically 5–25°) for accent or task lighting. Used in retail displays, museums, and architectural highlighting to create dramatic contrast and focal points.

A device (SPD) that protects LED drivers from voltage transients caused by lightning, grid switching, or inductive loads. Outdoor fixtures should include 10kV surge protection; indoor fixtures typically 4kV.

Localized, directional light for a specific visual task — reading, cooking, assembly, surgery. One of the three layers of lighting. Task lighting typically targets 300–1000 lux at the work surface.

The IES method for projecting long-term LED lumen maintenance from LM-80 test data. TM-21 extrapolates L70 lifetime (e.g., 50,000+ hours) from 6,000–10,000 hours of measured data.

A measure of waveform distortion introduced by the LED driver. THD < 20% is standard for commercial fixtures; < 10% is preferred for sensitive environments. High THD causes neutral conductor overheating.

A rectangular recessed fixture common in office T-bar ceilings. Standard sizes: 1×4 ft, 2×2 ft, 2×4 ft. LED troffers typically deliver 3,000–5,000 lm at 110–150 lm/W.

A CIE metric (0–30) for discomfort glare from luminaires in interior spaces. UGR ≤ 19 is the standard limit for offices; ≤ 16 for precise visual tasks; ≤ 22 for circulation areas. Lower values indicate less glare.

A global safety certification organization. UL Listed means the complete fixture meets safety standards; UL Recognized applies to components. Required for code compliance in most North American jurisdictions.

The ratio of minimum to average illuminance across a surface. 0.6 is typical for offices; 0.4 for warehouses; 0.3 for parking lots. Poor uniformity creates harsh bright spots and dark patches even when average lux is on target.

The overall subjective satisfaction with a lit environment, influenced by illuminance, uniformity, glare, CCT, and color rendering. No single metric captures it; design requires balancing all factors per IES recommended practice.

The electrical potential difference driving current through a circuit. Common LED driver outputs: 12V DC, 24V DC, or constant-current (e.g., 700mA, 1050mA). Mains voltage is 100–277V AC.

The measure of electrical power consumed by a fixture. Wattage alone does not indicate brightness — a 100W LED at 160 lm/W produces 16,000 lm, while a 100W incandescent produces only ~1,600 lm. Always compare by lumens, not watts.

The surface at which illuminance is calculated and measured. Standard heights: 0.75 m for seated offices, 0.85 m for standing desks, 0.0 m (floor) for corridors. All lux targets reference a specific working plane.