NCQLP LC key terms

The total amount of visible light a lamp or luminaire emits in all directions, measured in lumens. It describes output quantity without regard to where the light goes.

The amount of light emitted by a source in a particular direction per solid angle, measured in candelas. It describes directional strength and underlies photometric distribution curves.

The density of luminous flux arriving on a surface, measured in lux or footcandles. It is the quantity most lighting recommendations specify for tasks and circulation areas.

The light reflected or emitted from a surface in a given direction per unit area, measured in candelas per square meter. It corresponds to brightness as the eye perceives it.

The metric unit of illuminance, equal to one lumen per square meter. Metric lighting recommendations and most international standards express design targets in lux.

The customary unit of illuminance, equal to one lumen per square foot. One footcandle equals about 10.76 lux, a conversion candidates should be able to apply quickly.

The SI base unit of luminous intensity, describing light emitted in a specific direction. Luminaire photometric data are typically reported in candelas at many angles.

The gradual loss of light output a lamp or LED module experiences over operating hours. It is one factor in the light loss factor used to size initial versus maintained illuminance.

The ability of a light source to retain its output over time, often stated as the percentage of initial lumens remaining at a given operating hour. Good maintenance reduces overdesign.

The operating hours at which an LED source's light output has declined to 70 percent of its initial value. It is a common lifetime rating for LED products.

A multiplier applied to initial illuminance to predict maintained levels, accounting for dirt on luminaires and surfaces, lamp aging, and burnout. It keeps designs realistic over the maintenance cycle.

The fraction of lumens leaving a luminaire that reach the work plane, given room proportions and surface reflectances. It is central to the lumen method of average illuminance calculation.

A calculation technique that divides total delivered lamp lumens by the area served to estimate average maintained illuminance. It assumes uniform layout and is not suited to accent or task lighting.

A dimensionless index describing a room's proportions, derived from its length, width, and mounting height above the work plane. It is used to look up coefficient of utilization values.

A test document reporting a luminaire's intensity distribution, efficacy, and often glare and color data. It is the basis for lighting calculations and product comparison.

The ratio of light output to power consumed, expressed in lumens per watt. Higher efficacy means more light for the same energy, though color and distribution quality also matter.

A number in kelvin describing whether white light appears warm, neutral, or cool, based on the blackbody radiator it resembles. It does not by itself describe color fidelity.

A scale, usually to 100, rating how faithfully a light source shows object colors compared with a reference source. Higher values generally indicate more natural appearance.

The color quality of light independent of its brightness, often plotted on a diagram to compare sources. It underpins specifications for color temperature and tint consistency.

Visual discomfort or loss of visibility caused by luminance in the field of view that is much brighter than its surroundings. It is managed through shielding, brightness limits, and placement.

Glare that physically reduces the ability to see detail, for example by scattering light within the eye. It impairs task performance rather than merely causing annoyance.

Glare that produces annoyance or a sensation of visual discomfort without necessarily reducing task visibility. It is often predicted with rating systems based on source luminance and position.

A method that computes a glare index from luminaire luminance, background luminance, and viewing geometry. Lower values indicate less glare, and limits are often set by application.

Reflected glare that reduces contrast on a task surface, such as a screen or glossy paper, by superimposing a bright image. It is controlled by geometry, matte finishes, and diffuse lighting.

The proportion of light striking a surface that is reflected rather than absorbed or transmitted. Surface reflectances strongly affect how much interreflected light reaches the work plane.

The share of a luminaire's total lumens emitted within defined angular zones, such as downward cones. It helps evaluate distribution and suitability for a space.

Installed lighting power divided by floor area, usually in watts per square meter or square foot. It is a common energy code metric and is compared against allowed limits.

The maximum lighting power permitted for a space or building under an energy code, often calculated by area method or space-by-space method. It drives design choices toward efficient equipment and controls.

Devices and strategies that adjust light output or operation, including dimming, occupancy sensing, daylight harvesting, and scheduling. They reduce energy use and support visual needs over time.

Automatically dimming or switching electric light in response to available daylight, typically using photosensors. It saves energy while preserving required illuminance.

A control device that detects whether a space is occupied and switches or dims lighting accordingly. It is distinct from a vacancy sensor, which requires manual turn-on.

How evenly illuminance is distributed across an area, commonly expressed as a ratio of minimum to average or maximum to minimum. Poor uniformity creates bright and dim patches that hinder tasks.

A design approach combining lower general ambient light with higher localized light where tasks occur. It can improve efficiency and give occupants control over their own work area.

A complete lighting unit comprising a lamp or LED module with the parts that distribute light, position it, protect it, and connect it to power. Its optics and shielding shape the delivered light.

A shielding element, often a series of vanes, that blocks direct view of a lamp or LED at certain angles. It reduces glare while allowing controlled light distribution.

A change in a light source's chromaticity over time or across units of the same product. It matters in installations where consistent appearance between fixtures is expected.

A classification of how dirty an environment is expected to be, used to select light loss factors for cleaning schedules. Clean offices and dirty industrial spaces fall in different categories.