Glossary of Terms and Formulae


The following terms and formulae appear in this manual and in discussions of highway lighting. For electrical terms not listed here, consult the National Electrical Code (NEC).


American Association of State Highway Transportation Officials.

alternating current (AC)

Current and voltage alternates from maximum positive to maximum negative in a sinusoidal pattern.

ampere (I)

The unit of current strength (flow of electrons).


A device that includes a transformer that modifies incoming voltage and current to provide the circuit conditions necessary to operate electric discharge lamps.

blanket agreement

Agreement identifying lighting projects authorized under the agreement by making the plans an attachment to the agreement. The agreement must have a statement that appears on the title sheet of the plans identifying the plans as part of the agreement. This statement must be signed by the appropriate city official.

breakaway support

A lighting pole support designed to sheer easily under vehicular impact. The breakaway feature can be an aluminum transformer base, a frangible insert between pole base foundation, a slip base, or other device. The breakaway support must meet current AASHTO and FHWA requirements.

candela or candlepower (cd)

The unit of luminous intensity (I) emitted by a light source in a given direction.

capacitance (C)

Ability to store energy in an electrostatic field. Measured in farads or microfarads.

clear zone (also called the clear recovery area)

The area provided along highways to allow vehicles veering off the travel lane opportunity for safe recovery or stopping. The clear zone width (always measured from the edge of the travel lane) depends on several roadway factors. The Roadway Design Manual contains a full discussion of the clear zone (“Horizontal Clearance to Obstructions”) and provides the minimum and desirable widths for various roadways.

complete interchange lighting

The lighting, within the limits of the interchange, of the main lanes, direct connections, ramp terminals, and frontage road-crossroad intersections. (See partial interchange lighting.)

concrete median/traffic barrier (CTB)

Concrete traffic barrier.


Electrical wire, bare or insulated.

continuous lighting

Roadway lighting providing uniform illumination on all main lanes and direct connections and complete lighting for all interchanges.

conventional lighting

A highway lighting system in which the luminaires are typically mounted no higher than 50 feet. (See also high mast lighting.)

cosine law

The mathematical expression of the principle that illumination of a surface is proportional to the cosine of the angle (A) of incidence of the light beam.


A luminaire light distribution is designated as cutoff when the candlepower per 1,000 lamp lumens does not numerically exceed 25 (2.5 percent) at an angle of 90 degrees above nadir (horizontal), and 100 (10 percent) at a vertical angle of 80 degrees above nadir. Cutoff type luminaires usually have flat glass lenses.

direct current (DC)

One-direction flow of electricity.

electrical details (ED)

TxDOT’s standard sheets showing specifications for electrical specifications. Contained in the Traffic Operations Standard Plans.


Electrical metal tubing.

electrical service

Point of receiving power from utility company. Typical service voltages used on highway electrical systems are 120/240 VAC, 240/480 VAC, and 480 VAC.


Federal Highway Administration.


An aimable luminaire generally employed for spot or wide-angle lighting.

footcandle (FC)

English unit of measurement for the illumi-nation (E) on a surface. (See "lux" for metric.) One footcandle is the illumination on a surface that is one foot from and perpendicular to a uniform point source of one candela. Combining the inverse square law and the cosine law, the formula for footcandles (FC) is

where CD is the candlepower, A is the angle of incidence of the light beam (see diagram under “lux,”) and D is the distance of the surface from the light source.


Readily or easily broken.

High Mast Illumination Details (HMID)

The TxDOT’s standard sheets showing specifications for high mast illumination. Contained in the Traffic Operations Standard Plans.

Illuminating Engineering Society of North America (IESNA also IES)

The IESNA writes many of the industry standard specifications for lighting fixtures.

high mast lighting

Lighting units mounted at heights of 100 feet or more.


(See " starter.")

inductance (L)

Ability to store energy in electromagnetic field. Measured in henrys or millihenrys.

inverse square law

Expresses mathematically the relationship between luminous intensity (CD) and illumination (FC). It states that illumination at a point on a surface is directly proportional to the luminous intensity of the light in that direction and inversely proportional to the square of its distance (D) from the source


iso-lux (or iso-footcandle) curves

Curves plotted from metered photometric readings for a specific lighting unit, of a particular design and rated wattage, when the unit is mounted at a certain height. These readings are taken on a rectangular grid that is oriented from the center of the light source. From such readings, contour lines are then platted for identical values in lux (footcandles); and when contours are platted for equal increments of increase in illumination intensity, the resulting contour lines produce a series of generally concentric semiparabolic curves, described as iso-lux (iso-footcandle) curves.

kilowatt (KW)

A measure of real power (generators, lamps, and heating elements are rated in watts or kilowatts).

(1KW = 1.34 horse power)

kilovolt-amp (KVA)

A measure of apparent power. Equipment is rated in KVA when heat dissipation is a concern (transformers are rated in KVA)



(See "light source.")

light source

The device that converts electric energy to visible light (also called the “lamp”).

lumen (lm)

The unit of quantity of light flux; in other words, the total output of a light source.


A device that directs, controls, and modifies the light produced by a light source. It consists of a light source, reflector, refractor, housing, and such support as may be integral with the housing.

lux (lx)

Metric unit of measurement for the illumination (E) on a surface. (See footcandle for English unit.) One lux is the illumination on a surface one meter from and perpendicular to a uniform point source of one candela. Combining the inverse square law and the cosine law, the formula for lux is

where CD is the candlepower, A is the angle of incidence of the light beam (see Figure a-1), and D is the distance of the surface from the light source.

  Measuring the angle of incidence of a
light beam. (click in image to see full-size image) Anchor: #i999745grtop

Figure G-1. Measuring the angle of incidence of a light beam.

mast arm

An attachment to a lighting pole on the end of which a luminaire is mounted.

mounting height

Generally the vertical distance between the base of the pole and the luminaire.

National Electrical Code (NEC)

Nationally recognized rules and regulations for the installation and use of electrical power. The NEC is considered the minimum acceptable standard for a safe installation.

National Electrical Manufacturer’s Association (NEMA)

Provides specifications and industry standards.

ohm (R)

The unit of electrical resistance.

partial interchange lighting

The lighting of acceleration and deceleration lanes, ramp terminals, crossroads at frontage road or ramp intersections, and other areas of nighttime hazard. (See “complete interchange lighting.”)

photometric curves - (Also called “photometric data.”)

Derived from metered measurements of horizontal or vertical lux (or footcandles). These metered measurements are obtained by the use of a light meter usually calibrated in lux (or footcandles).


A galvanized steel or aluminum shaft to support the lighting unit (also called “lighting standard”).

power (P)

Measured in watts. Formulae as follows:

  • for DC circuits: or
  • for AC circuits: or .

For power loss due to resistance in lighting circuits, the power factor can be considered equal to one. This power can also be calculated: .

power factor (pf)

Time relationship between current wave and voltage wave in an A.C. system.


Polyvinyl chloride, the material of which a kind of tubing used for conduit is made.


Polished aluminum device used to reflect light.


Prismatic glass element used to refract light.

regulated output ballast

A form of electrical transformer that maintains the wattage of the lamp at a nearly constant value, though the line voltage may fluctuate as much as ±10 percent. Such Ballasts or transformers may be integrally mounted within the luminaire or separately mounted in a ballast enclosure.


Rigid metal conduit.

roadway illumination assembly

The luminaire and supporting members (pole, mast arm, etc.) with other related lighting equipment attached.

Roadway Illumination Details (RID)

The TxDOT’s standard sheets showing specifications for roadway illumination to be used with TxDOT standard specification Items 610, 611, 612, and 656. The details are contained in the Traffic Operations Standard Plans.

safety lighting

Roadway lighting installed at interchanges, highway intersections, and other points of nighttime hazard to the extent necessary to provide for the safe and orderly movement of traffic.


The distance between successive lighting units measured along the center line of the roadway.

starter or starting aid (also called igniter)

A device producing a high voltage pulse to begin arcing in a lamp.


An electrical device that changes one AC voltage to another. Utility companies generate and distribute power at voltages usually greater than 12,500 VAC. Transformers reduce the voltage to a safe and easily applied level, 120 VAC for typical receptacles.

transformer base

A hollow cast aluminum base, the bottom of which is bolted to a concrete foundation and to the top of which the bottom flange of the pole is bolted.


The ratio of the average level of illumination to the minimum level of illumination on the roadway

volt (V or E)

The unit of electromotive force, electrical pressure, or difference of potential. Analogous to water pressure. One volt will cause one ampere of current to flow through a resistance of one ohm.

voltage drop

A result of current flowing through a resistance.

Example: A current of 30 amperes flowing through 300 feet of No.8 conductor whose resistance loop is 0.3924 ohms will result in a voltage drop of 11.77 volts.


Warrants are applied to determine whether or not the lighting system is justifiable at a particular location on an eligible highway.

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