IMSA Roadway Lighting II key terms

The amount of light leaving a surface in a given direction per unit area, expressed in candelas per square meter. It describes how bright a road surface or object appears to a driver's eye.

The density of light falling onto a surface, measured in lux or footcandles. Roadway design criteria often specify maintained horizontal illuminance on the pavement.

The total light output emitted by a lamp or luminaire in all directions, measured in lumens. It is the starting point for calculating how much light reaches the roadway.

The light output emitted by a source in a particular direction per solid angle, measured in candelas. Intensity distributions define a luminaire's photometric performance and spacing.

The SI unit of luminous intensity, describing light emitted in a specific direction. Luminaire photometric files report candela values across vertical and horizontal angles.

The SI unit of illuminance, equal to one lumen per square meter. Metric roadway lighting criteria are typically written in lux on the pavement.

The imperial unit of illuminance, equal to one lumen per square foot. One footcandle equals approximately 10.76 lux; US roadway criteria often use this unit.

A ratio describing how evenly luminance is distributed across the roadway, such as maximum-to-minimum or maximum-to-average. Poor uniformity creates bright and dark patches that impair visibility.

A measure of how evenly illuminance is spread over the pavement, usually expressed as average-to-minimum or maximum-to-minimum ratios. It supports design checks independent of surface reflectance.

Stray light scattered in the eye and atmosphere that reduces contrast and washes out the driver's view. It is a key criterion for evaluating disability glare from roadway luminaires.

Glare that reduces a driver's ability to see detail by scattering light within the eye. High-angle intensity from luminaires and poor aiming are common causes.

Glare that causes visual discomfort or annoyance without necessarily reducing visibility of details. It is assessed using glare-control metrics and luminaire shielding.

Excessive brightness in the field of view that causes discomfort, reduces contrast, or impairs vision. Roadway designs control it through luminaire optics, shielding, mounting height, and aiming.

A luminaire category based on how much light is emitted at high angles above nadir, such as full cutoff, cutoff, semi-cutoff, or non-cutoff. It relates to glare control and skyglow reduction.

A classification system that rates luminaires by unwanted light behind the pole, light directed upward, and high-angle glare. It is used in dark-sky and roadway lighting specifications.

Unwanted light spilling beyond the intended roadway area onto adjacent property. Designers limit it with proper optics, shielding, aiming, and luminaire selection criteria.

The average illuminance on the roadway after accounting for lamp lumen depreciation and dirt on the luminaire. It is the value compared against design criteria over the maintenance cycle.

A multiplier applied to initial light output to predict performance before maintenance. It combines lamp lumen depreciation, luminaire dirt depreciation, and other degradation factors.

The gradual reduction in lamp light output over operating hours. It is a major component of the light loss factor used in roadway lighting design.

The loss of light output caused by dirt, dust, and pollutants accumulating on lamps and luminaire surfaces. It depends on environment, luminaire design, and cleaning schedule.

The operating time at which an LED source's light output has declined to 70 percent of its initial value. It is commonly used for LED roadway luminaire lifetime and depreciation estimates.

A system that categorizes roads by function, traffic volume, speed, and pedestrian activity, such as freeway, arterial, collector, or local. Classification drives the lighting criteria applied.

The distance between adjacent lighting poles along a roadway. It is set by luminaire photometrics, mounting height, required uniformity, and the design criteria for the road class.

The vertical distance from the roadway surface to the luminaire's light center. Higher mounting generally improves uniformity and reduces glare but requires more powerful luminaires.

The ratio of pole spacing to luminaire mounting height. It is a quick indicator of expected uniformity and is compared against luminaire-specific limits.

A complete lighting unit consisting of a lamp or LED module, optical system, housing, and electrical components. Roadway luminaires shape and distribute light onto the pavement.

The three-dimensional pattern of light intensity emitted by a luminaire, documented in photometric files. It determines spacing, uniformity, and glare characteristics for a roadway design.

The percentage of lamp or LED light output that exits the luminaire rather than being absorbed or lost internally. It is one input to overall lighting system performance.

A power supply that converts incoming AC power to the regulated current an LED module needs. Its quality, dimming capability, and surge protection affect reliability and controls compatibility.

An electrical device that regulates current and voltage for discharge lamps such as high-pressure sodium or metal halide. Ballast type affects starting, power factor, and dimming options.

The ratio of real power to apparent power in an electrical circuit, indicating how efficiently current is used. Utilities and specifications may require correction or minimum values for lighting circuits.

The reduction in voltage along conductors caused by current flowing through their resistance. Excessive drop at the far end of a lighting circuit can cause dimming, flicker, or driver failure.

Selecting wire gauge based on ampacity, voltage drop, temperature rating, and code requirements. Undersized conductors overheat and cause poor performance on long lighting runs.

A fuse or circuit breaker that interrupts current when it exceeds a safe level. It protects lighting circuits and conductors from overloads and short circuits.

Connecting metal poles, equipment, and circuit references to earth to limit dangerous voltages. Proper bonding and grounding protect workers and the public from electric shock.

A component that diverts transient voltage spikes from lightning or switching away from sensitive equipment. It is often installed at the service or within LED roadway luminaires.

The concrete structure that transfers pole loads into the soil. Its design depends on pole height, wind loading, soil conditions, and anchor bolt configuration.

Threaded steel rods embedded in a foundation that secure the pole base. Their size, embedment, and tightening torque are critical to structural integrity.

A pole or base designed to yield or separate on impact, reducing crash severity. It is used along high-speed roadways where fixed objects pose a hazard.

The force exerted on poles, luminaires, and foundations by wind. Designers use code-based wind speeds and exposure categories to size structures safely.

A system that reduces luminaire light output during low-traffic periods to save energy. It may use analog protocols, digital networks, or photocell-based schedules.

A sensor that detects ambient light level and switches or dims luminaires at dusk and dawn. Proper placement and settings prevent false switching and energy waste.

A complete path for electric current from the source through luminaires and back. Lighting circuits are designed for load, voltage drop, protection, and control zoning.

A replaceable light source such as high-pressure sodium, metal halide, or an LED module. Lamp characteristics drive luminaire selection, maintenance, and disposal requirements.