IMSA Roadway Lighting I key terms

The amount of light reflected or emitted from a surface in a given direction, divided by the projected area, expressed in candelas per square meter. Roadway design uses it to describe how bright the pavement appears to a driver.

The density of luminous flux striking a surface, measured in lux or footcandles. It describes how much light arrives at a point regardless of what the surface does with it.

The total quantity of light energy emitted by a source per unit time, weighted by the eye's sensitivity, measured in lumens. Lamp packages are rated in this quantity before any optical losses.

The luminous flux emitted by a source into a unit solid angle in a specified direction, measured in candelas. It forms the basis of photometric distribution files used in lighting design software.

The SI unit of luminous intensity, equal to one lumen per steradian. It expresses how much light a luminaire sends in a particular direction, independent of distance.

The SI unit of luminous flux, describing the total visible light output of a source. Luminaire delivered output is lower than bare lamp output because of optical and thermal losses.

The SI unit of illuminance, equal to one lumen per square meter. Metric roadway lighting criteria for major roads are commonly stated in these units on the pavement.

The customary unit of illuminance, equal to one lumen per square foot, or about 10.76 lux. Older roadway plans and many North American specifications still express design criteria in this unit.

The brightness of the road surface as seen by a driver, produced by light reflected from the pavement toward the observer. It depends on both illuminance and the reflective properties of the surface.

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

A numeric comparison of the brightest and darkest, or brightest and average, values of luminance or illuminance in a defined area. Lower ratios indicate a more evenly lit roadway.

A uniform haze of scattered light in the eye caused by stray light from luminaires, which reduces contrast and washes out the driver's view. It is a key glare metric in roadway lighting design.

Glare that reduces a person's ability to see detail by scattering light within the eye and lowering retinal contrast. In roadway lighting it is quantified through veiling luminance relative to pavement luminance.

Glare that causes visual discomfort or annoyance without necessarily reducing the ability to see detail. It is often assessed subjectively and can cause driver fatigue on long night trips.

Excessive brightness within the field of view that causes annoyance, discomfort, or loss of visibility. Roadway luminaires are shielded and aimed to limit it for drivers and nearby property.

A classification of how much light a luminaire emits at high angles near and above horizontal. Full cutoff designs limit upward and near-horizontal light, reducing glare and skyglow.

A modern rating system that describes light emitted behind the luminaire, above horizontal, and at high angles that cause glare. Ratings help specifiers control trespass and obtrusive light.

Unwanted light that spills beyond the boundary of the property being illuminated, often onto neighboring homes or natural areas. It is managed by shielding, aiming, and luminaire optics.

The brightening of the night sky over populated areas from artificial light scattered in the atmosphere. Upward-directed light from poorly shielded luminaires is a major contributor.

The design illuminance level that must remain after accounting for lamp lumen depreciation, dirt on luminaire surfaces, and other losses over the maintenance cycle. It is lower than initial values.

A multiplier applied to initial lamp output to estimate the reduced light available at the end of a maintenance interval. It combines lamp depreciation, luminaire dirt, and other degradation factors.

The gradual reduction in light output a lamp produces as it ages, expressed as a fraction of initial lumens. It is a major component of the light loss factor used in design.

The ability of a light source to retain its output over time, often reported as the percentage of initial lumens remaining at a stated operating hour. LED systems are rated using standard maintenance curves.

The loss of light output caused by dust, dirt, and films accumulating on lamps, lenses, and reflectors. It depends on the environment and on how well the luminaire is sealed.

A complete lighting unit consisting of a light source, optical elements, housing, and the parts needed to position and connect it. Roadway types include cobra head, shoebox, and high-mast units.

The ratio of delivered light output to electrical power input, expressed in lumens per watt. Higher values indicate more energy-efficient conversion of electricity into useful roadway light.

A description, often tabular or polar, of how a luminaire's intensity varies with angle in each plane. Design software uses it to compute illuminance and luminance on the roadway.

A laboratory document presenting measured light output and intensity distributions for a luminaire. It supplies the data needed to model the product in roadway lighting calculations.

The property of a road surface that determines how much and in what pattern light is reflected toward the driver. Surface texture, color, and wetness all change the reflection.

Reflection in which light bounces off a surface at a predictable angle, like a mirror. Wet pavement produces more of this, which can create glare and reduce luminance uniformity.

Reflection that scatters light in many directions rather than at a single angle. Dry, rough pavement reflects diffusely, producing relatively even luminance across the road.

The distance between successive lighting poles along a roadway. It interacts with mounting height and luminaire optics to determine uniformity and the presence of dark gaps.

The vertical distance from the road surface to the light-emitting portion of a luminaire. Higher mounting generally improves uniformity and reduces glare but requires more powerful sources.

The ratio of pole spacing to mounting height, used as a quick check on layout uniformity. Higher ratios tend to produce darker gaps between poles for a given luminaire.

The process of selecting luminaires, heights, spacing, and layouts to meet visibility, uniformity, and glare criteria for a road classification. It balances safety, energy, and cost.

A categorization of roads by function and traffic volume, such as freeway, major arterial, collector, or local. It sets the lighting criteria applied in design.

A lighting system using very tall poles, often 30 meters or more, with grouped luminaires to illuminate large areas such as interchanges. It reduces glare and pole clutter but complicates maintenance.

A control strategy that adjusts roadway light levels based on traffic volume, time of night, or weather while maintaining required visibility. It saves energy without sacrificing safety at busy times.

A specialized application that manages the transition between bright daylight and darker tunnel interiors using threshold, transition, and interior zones. It prevents the black-hole effect at entrances.

The eye's adjustment to changes in brightness, which can take many minutes when moving from daylight into a tunnel or from a lit road into darkness. Lighting design must account for it.