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.

Electrical current, measured in amperes. Circuit loading and protection depend on the equipment, load duration, installation, and applicable electrical rules.

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 color-rendering metric comparing a source with a reference. General CRI, Ra, averages eight test-color results; R9 is a separate red sample. Higher values indicate closer agreement for those samples, not a complete description of color appearance.

The difference in luminance between an object and its background. Sufficient contrast directly affects 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 that scatters light, such as opal glass or a microprismatic sheet. It changes the distribution and appearance of the source; transmission losses depend on the material and design.

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.

Lighting provided when the normal supply fails so occupants can leave safely. Required light levels, duration, testing, and exit-sign arrangements depend on the building and applicable code.

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 replaces. 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 that carries heat away from the LED junction. Its performance depends on material, geometry, contact surfaces, airflow, and operating conditions.

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.

An ingress-protection code under IEC 60529. The first digit describes protection from solids and the second from water under specified tests. Check the test conditions and installation instructions; IP does not rate corrosion resistance.

A fixture with LEDs built into its assembly rather than a standard replaceable bulb. Some designs allow the LED module or driver to be replaced; check the service instructions.

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 current flows through it. In a lighting product, the driver, optics, and thermal design affect efficiency, light quality, and dimming capability.

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 factor used to allow for light output lost during operation, including lumen depreciation and dirt. Choose its components to match the product, environment, and maintenance schedule.

A device that switches or dims lighting in response to occupancy or movement. Performance depends on detection technology, coverage, mounting, and settings; savings depend on the operating schedule.

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 in watts to apparent power in volt-amperes. It affects current for a given real-power load. Required limits and billing consequences depend on the equipment and tariff.

The science of measuring visible light in terms of human visual response. Photometric data (intensity distribution curves, zonal lumens, BUG ratings) guides 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 surge protective device limits specified voltage transients. Select its rating and coordination for the supply, installation, and equipment; a stated kV value alone does not describe the complete protection scheme.

Localized, directional light for a specific visual task, like 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.

Total harmonic distortion describes harmonic content relative to the fundamental waveform. Check the measurement, load conditions, and applicable limits when comparing drivers.

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.

UL is a testing and certification organization. A listing and a component recognition have different scopes. Verify the product record and its suitability for the installation and jurisdiction.

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 where illuminance is 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.