LED Glossary
21 essential lighting terms explained in plain language. Your quick reference for every LED spec and acronym.
A
A19
The most common household light bulb shape in North America. The 'A' stands for 'Arbitrary' (the standard pear shape), and '19' indicates the diameter in eighths of an inch (19/8 = 2.375 inches). When someone says 'light bulb,' they're almost always picturing an A19. Most A19 LEDs use an E26 base and are available from 40W-equivalent (450 lm) to 100W-equivalent (1,600 lm).
Read the full guide →B
Beam Angle
The angle between the two directions where light intensity drops to 50% of its peak value. A narrow beam angle (15–25°) creates focused spotlight effects ideal for accent lighting and artwork. A wide beam angle (60–90°+) spreads light broadly, suitable for general room illumination. PAR and BR bulbs are specifically designed to control beam angle, while A19 bulbs emit light in nearly all directions (~270°).
Read the full guide →BR (Bulged Reflector)
A bulb shape with a bulged (rounded) reflector, commonly used in recessed ceiling downlights. BR30 (3.75” diameter) is the most common residential size, fitting standard 6-inch recessed cans. BR40 (5” diameter) fits 6–8 inch cans. Unlike PAR bulbs, BR bulbs have a softer, more diffuse beam edge, making them better for general room lighting rather than accent or task lighting.
Read the full guide →C
CCT (Correlated Color Temperature)
The numerical measurement of a light source's color appearance, expressed in Kelvin (K). CCT describes where a light falls on the warm-to-cool spectrum by comparing it to the color of a heated black body radiator at a specific temperature. CCT and 'color temperature' are used interchangeably in consumer lighting. Common CCT values: 2700K (warm white), 3000K (soft white), 4000K (cool white), 5000K (daylight).
Read the full guide →Color Temperature
The perceived warmth or coolness of white light, measured in Kelvin (K). Lower values (2200–3000K) appear warm and yellowish, resembling incandescent or candlelight. Higher values (4000–6500K) appear cool and bluish, resembling daylight. Color temperature profoundly affects mood, perception, and even circadian rhythm. Most residential lighting uses 2700–3000K; offices and workspaces typically use 3500–5000K.
Read the full guide →Cool White
A color temperature range of approximately 3500–4500K, producing a neutral to slightly blue-tinted white light. Cool white is often preferred for kitchens, bathrooms, garages, and commercial spaces where a clean, alert atmosphere is desired. It provides good visual clarity and contrast but can feel clinical in living spaces designed for relaxation.
Read the full guide →CRI (Color Rendering Index)
A scale from 0 to 100 measuring how accurately a light source reveals the true colors of objects compared to a reference illuminant (natural daylight or incandescent light). CRI 100 means perfect color rendering. CRI 90+ is considered excellent for residential and retail use. The standard CRI score (Ra) is an average of eight test color samples (R1–R8); for complete color quality assessment, also check the R9 (deep red) value separately, as it is not included in the average.
Read the full guide →D
Daylight
In LED labeling, 'Daylight' refers to a color temperature of approximately 5000–6500K, producing a bright, bluish-white light that simulates midday sunlight. Despite its name, daylight-temperature LEDs are not appropriate for all rooms—they can feel harsh in living and dining areas. Best suited for task lighting, workshops, art studios, and spaces where alertness and color accuracy are priorities.
Read the full guide →Dimmable
An LED bulb designed to work with dimmer switches, allowing brightness adjustment from 100% down to a minimum level (typically 5–20%). Not all LEDs are dimmable—non-dimmable LEDs on a dimmer circuit may flicker, buzz, or fail prematurely. Dimmable LEDs require compatible dimmer switches; older leading-edge (TRIAC) dimmers designed for incandescent loads may not work correctly. Look for dimmers specifically rated for LED loads (trailing-edge or universal).
Read the full guide →DLC (DesignLights Consortium)
A nonprofit organization that maintains a Qualified Products List (QPL) of high-performance commercial LED products. DLC certification verifies that a product meets minimum requirements for efficacy, light output, color quality, and warranty. DLC-listed products are often required for utility rebate programs. DLC Premium certification indicates even higher efficiency standards. Primarily relevant for commercial and industrial lighting, not residential bulbs.
E
E26
The standard medium screw base used for household light bulbs in North America. 'E' stands for Edison (the screw-type base invented by Thomas Edison), and '26' indicates the base diameter in millimeters. E26 is the base you'll find on virtually all standard A19, BR30, and PAR38 residential bulbs in the US and Canada. The European equivalent is E27 (27mm)—they are mechanically similar but not identical.
Read the full guide →F
Flicker
Rapid, repeated changes in light output that can be visible (perceptible flicker) or invisible to the naked eye (imperceptible flicker). Both types can cause headaches, eye strain, and fatigue. LED flicker is typically caused by poor driver design, incompatible dimmer switches, or voltage fluctuations. IEEE standard 1789 recommends flicker below 8% at frequencies under 80 Hz. Quality LEDs achieve <1% flicker. Always check independent flicker measurements when buying for long-use spaces like offices.
Read the full guide →G
GU10
A bi-pin twist-and-lock base commonly used for directional spotlights and track lighting. The two pins are spaced 10mm apart and lock into the socket with a quarter-turn. GU10 bulbs run directly on mains voltage (120V in the US, 240V in Europe)—no transformer required. GU10 LEDs are typically available in PAR16 shape with beam angles from 15° to 60°, making them popular for accent lighting, display lighting, and kitchen task lighting.
Read the full guide →I
IP Rating (Ingress Protection)
A two-digit code defined by IEC standard 60529 that classifies a fixture's protection against solid particles (first digit, 0–6) and water (second digit, 0–9). IP20 means indoor-only with no water protection. IP44 protects against splashes. IP65 withstands water jets from any direction. IP67 survives temporary submersion (1m for 30 minutes). IP68 handles continuous submersion at manufacturer-specified depth. Always choose the appropriate IP rating for your installation environment.
Read the full guide →K
Kelvin (K)
The unit of measurement for color temperature in lighting. Named after physicist Lord Kelvin, it is based on the color of light emitted by a theoretical black body heated to a specific temperature. In practical lighting: 2700K = warm/yellowish, 3500K = neutral, 5000K = cool/bluish. The Kelvin scale is absolute—it does not measure physical temperature of the bulb, only the appearance of the light it produces. See also: CCT, Color Temperature.
Read the full guide →L
LED (Light-Emitting Diode)
A semiconductor device that emits light when an electric current passes through it. Unlike incandescent bulbs (which heat a filament) or fluorescents (which excite gas), LEDs produce light through electroluminescence—electrons recombining with electron holes in the semiconductor release energy as photons. This process is inherently more efficient, producing less waste heat. Modern white LEDs typically use a blue LED chip coated with a yellow phosphor to create broad-spectrum white light. LEDs last 15,000–50,000+ hours and use 75–90% less energy than incandescent equivalents.
Read the full guide →lm/W (Lumens per Watt / Efficacy)
The measure of how efficiently a light source converts electrical power into visible light. Calculated by dividing light output (lumens) by power consumption (watts). Higher lm/W means better efficiency. Incandescent bulbs achieve ~15 lm/W. Current consumer LEDs range from 80–150 lm/W. The theoretical maximum for white light is approximately 300 lm/W. When comparing LED products, lm/W is the best efficiency metric—more useful than wattage alone.
Read the full guide →Lumens (lm)
The SI unit measuring the total quantity of visible light emitted by a source, weighted by the sensitivity of the human eye. Lumens measure brightness—the higher the number, the brighter the light. This is the correct metric for comparing LED brightness (not watts, which measure power consumption). Reference points: a standard 60W-equivalent LED produces ~800 lumens; a 100W-equivalent produces ~1,600 lumens. Always compare lumen ratings when shopping for LEDs.
Read the full guide →P
PAR (Parabolic Aluminized Reflector)
A bulb shape with a parabolic reflector that produces a controlled, directional beam. The number after 'PAR' indicates the diameter in eighths of an inch: PAR20 = 2.5”, PAR30 = 3.75”, PAR38 = 4.75”. PAR bulbs have a sharper beam cutoff than BR bulbs, making them ideal for accent lighting, artwork, and outdoor security lights. PAR38 is the standard for outdoor flood/spot fixtures.
Read the full guide →W
Warm White
A color temperature range of approximately 2700–3000K, producing a yellowish-white light similar to traditional incandescent bulbs. Warm white is the most popular choice for residential living spaces (living rooms, bedrooms, dining rooms) because it creates a cozy, inviting atmosphere. It flatters skin tones and complements warm-colored interiors. The 2700K standard has become the default for most consumer LED bulbs labeled 'Soft White' or 'Warm White.'
Read the full guide →Watts (W)
The unit measuring electrical power consumption—the rate at which energy is used. In the incandescent era, watts served as a rough brightness indicator because all incandescent bulbs had similar efficiency (~15 lm/W). With LEDs, this relationship is broken: a 10W LED can produce the same brightness as a 60W incandescent. Watts now indicate how much electricity a bulb consumes (affecting your energy bill), not how bright it is. For brightness, compare lumens instead.
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