901.15 Glossary

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AASHTO Guide – Latest edition of "An Informational Guide for Roadway Lighting."

Adaption (Transition) Lighting – Provisions for gradual or incremental changes in the intensity of illumination to allow the eye to adapt from light to dark conditions.

Average Maintained Intensity – The average level of horizontal illuminance on the roadway pavement when the output of the lamp and luminaire is diminished by the maintenance factors; expressed in average footcandles (lux) for the pavement area.

Average Initial Illumination (Intensity) – The average footcandle (lux) level of horizontal illumination on the pavement area, at the time of installation, with new lamps and clean luminaires.

Candela – The unit of luminous intensity; one lumen per unit solid angle (steradian) (1 candela per square inch = 452 footlamberts). Formerly, the term "candle" was used.

Candela per Square Meter – The International System (SI) unit of luminance (photometric brightness) equal to the uniform luminance of a perfectly diffusing surface emitting or reflecting light at the rate of one lumen per square meter, or the average luminance of any surface emitting or reflecting light at that rate. One candela per square meter equals 0.2919 footlambert.

Candlepower – Luminous intensity in a specified direction, expressed in candelas.

Coefficient of Utilization – The percentage of the lumens emitted by the lamp received on the surface of the pavement.

Complete Interchange Lighting – The lighting of the freeway through traffic lanes through the interchange, the traffic lanes of all ramps, the acceleration and deceleration lanes, all ramp terminals and the crossroad between the outermost ramp terminals.

Control Cabinet – A steel or aluminum cabinet that houses secondary magnetic breakers, the “manual-auto” switch and the fuse which protects the operating coil of the primary oil switch or the contactor, along with necessary interconnecting wiring and terminal strips. In some areas, the power companies furnish secondary voltage. In such cases all that is needed is a control cabinet similar to that described above, the main difference being addition of a manually operated disconnect switch and a high amperage contactor. The disconnect switch replaces the previously explained fuse cutout and the contactor replaces the primary oil switch. The contactor controls power flow to the lighting system as directed by the photoelectric control. It may also be operated manually by the “manual-auto” switch.

Direct Reflectance – Lighting that provides direct illumination of the side of the object toward the observer. The object is seen by variations in brightness or color over its surface without general contrast with a background.

Equipment Factor – A factor used in the illuminance or luminance calculations that compensates for light losses due to normal production tolerances of commercially available luminaries when compared with laboratory photometric test models. It is common practice to approximate these losses using a 5 to 10% loss factor (EF = 0.95 = 0.90).

Expressway – A divided major roadway for through traffic with partial control of access and generally with interchanges at major crossroads.

Footcandle – The illuminance on a surface one square foot in area on which there is uniformly distributed a light flux of one lumen. One footcandle equals 10.76 lux.

Footlambert – The unit of photometric brightness (luminance). It is equal to 1 /p candela per square foot, or the uniform luminance of a perfectly diffusing surface emitting or reflecting light at the rate of one lumen per square foot. One footlambert equals 3.426 candela per square meter.

Freeway – The highest type principal arterial highway, a divided facility with full control of access. Access is restricted to and from the freeway at interchanges which are usually constructed at major cross streets. Grade separations are provided for other major streets which are needed for local community circulation.

I.E.S. – Illuminating Engineering Society of North America. This organization develops and publishes standards for roadway lighting practices as well as other types of lighting.

Illuminance – The density of the luminous flux incident on a surface; it is the quotient of the luminous flux by the area of the surface when the latter is uniformly illuminated.

Light Loss Factor – A depreciation factor which is applied to the calculated initial average luminance or illuminance to determine the value of depreciated average luminance or illuminance at a predetermined time in the operating cycle, usually just prior to relamping, and which reflect the decrease in effective light output of a lamp and luminaire during its life. It is comprised of several variables and judgment must be exercised in arriving at a suitable factor. The more important variables that should be considered when determining the light loss factor are:

1. Decrease of lamp lumen output with burning hours (LDD-Lamp Lumen Depreciation);
2. Some electric discharge lamps, when operated in the horizontal position, exhibit lower light output than when operated in the vertical position;
3. Schedule of lamp replacement;
4. Frequency and effectiveness of luminaire cleaning (LLD-Luminaire Dirt Depreciation);
5. Equipment Factor; and
6. Operation of light sources at other rated current or voltage.

Lighting Unit – Assembly of lighting pole, bracket arm, and luminaire with lamp and ballast.

Lumen – A unit of measure of the quantity of light. One lumen is the amount of light that falls on an area of one square foot every point of which is one foot from the source of one candela (candle). A light source of one candela emits a total of 12.57 lumens solid angle (steradian) by a uniform point source.

Luminaire – A complete lighting device consisting of a light source and appurtenances such as globe, reflector, refractor, ballast, housing, those parts to position and protect the lamps, and connect the lamps to the power supply and such support as is integral with the housing.

Luminaire Maintenance Factor – An empirical depreciation factor of 70 percent to be applied to the calculated initial average footcandles (lux) to determine the value of average illumination at time of lowest output, and that represents the combined effects of reduced lamp lumen output and luminaire dirt accumulation.

Luminance – The luminous intensity of any surface in a given direction per unit of projected area of the surface as viewed from that direction.

Lux – The International System (SI) unit of illuminance. It is defined as the amount of light on a surface of one square meter all points of which are one meter from a uniform source of one candela. One lux equals 0.0929 footcandle.

Luminous Efficacy of a Source of Light – The quotient of the luminous flux emitted by the total lamp power input. It is expressed in lumens per watt.

Magnetic Breakers – These isolate one lighting circuit from another and thus help in circuit testing and trouble-shooting. These breakers perform the same function as a fuse. Their operation is dependent upon an overload current flow energizing a trip coil rather than heating a low-melting-temperature wire to its melting point.

“Manual-Auto” Switch – This switch is used to bypass the control of the photoelectric cell over the primary oil switch. This makes it possible to operate lights manually for testing and trouble-shooting purposes. This switch also serves as a manual control if the photoelectric control fails.

Partial Interchange Lighting – Lighting that consists of a few luminaires located in the vicinity of some or all ramp terminals. The usual practice is to light those general areas where the exit and entrance ramps connect with the through traffic lanes of the freeway and generally those areas where the ramps intersect the crossroad.

Photoelectric Control – A light-sensitive cell which actuates the solenoid-controlled contacts of the primary oil switch. This starts power flow to luminaries when daylight intensity falls to a pre-determined level (usually 1.0 footcandle). When natural light intensity raises to a predetermined level (usually 2.0 footcandles), the cell operates the oil switch solenoid or contactor and power flow is shut off.

Silhouette Discernment – Lighting that provides a contrast between an object and its background so it becomes readily recognizable to the motorist that the darker object is separate and different from its light background.

Uniformity of Illuminance – See Uniformity Ratio.

Uniformity of Luminance – The Average Level-To-Minimum Point method uses the average luminance of the roadway design area between two adjacent luminaries, divided by the lowest value at any point in the area. Maximum-to-Minimum Point Method uses the maximum and minimum values between the same adjacent luminaries. The luminance uniformity (avg./min. and max./min.) considers traveled portion of the roadway, except for divided highways having different designs on each side.

Uniformity Ratio – The ratio of the average footcandles (lux) of illumination on the pavement area to the footcandles (lux) at the point of minimum illuminance on the pavement. It is commonly called the uniformity ratio. A uniformity ratio of 3:1 means the average footcandles (lux) value on the pavement is three times the footcandles (lux) value at the point of least illuminance on the pavement. A perfect uniformity ratio is 1:1.

Veiling Luminance – A luminance superimposed on the retinal image that reduces its contrast. It is the veiling effect produced by bright sources or areas in the visual field that results in decreased visual performance and visibility.