Beam Lumens

The lumens contained within the beam spread of a floodlight.

Beam Spread

The angle between the two directions in the plane in which the intensity is equal to a stated percentage of the maximum beam intensity. The percentage typically is 10 percent for floodlights and 50 percent for photographic lights.

Brightness

As commonly applied, brightness (or luminance) is the light intensity from a surface which directs light into the eyes.

Geometric proportions of the ceiling, floor and  room cavities.

Coefficient of Beam Utilization (CBU)

The percentage of light from a floodlight which reaches the seeing task relative to total beam lumens.

Coefficient of Utilization (CU)

The percent of initial generated lamp lumens that reaches the work plane as determined by the surface reflectances, room shape (RCR), and fixture distribution.

Color Rendering

General expression for the effect of a light source on the color appearance of objects when compared with their color appearance under a reference light source.

Color Rendering Index (CRI)

A measurement of the color shift an object undergoes when illuminated by the light source, as compared to a reference source at the same color temperature. Color rendering is measured on an index from 0-100, with natural daylight and incandescent lamps both equal to 100. Objects and people viewed under lamps with a high color rendering index (CRI) generally appear more true to life.

Color Temperature (Chromaticity)

A scientific measurement of the balance of wavelengths making up any "white" light. The unit of measurement is the Kelvin, abbreviated K. Although it may not seem sensible, a higher color temperature means a cooler (bluer) light source. Typical color temperatures are 2800K (incandescent), 3000K (halogen), 4100K (white fluorescent.)

Cutoff and Shielding Angle

The lamp cutoff angle of a luminaire is the angle between the vertical axis (nadir) and the first line of sight when the bare source is no longer visible. Lamp image cutoff is measured the same was a lamp cutoff, but it occurs when the image of the lamp is no longer visible in the reflector. The shielding angle is the compliment of the cutoff angle.

Fire resistance ratings apply only to ceiling assemblies in their entirety. Individual components are not assigned a fire resistance rating, and are not intended to be interchanged between assemblies. Rather they are designated for use in a specific design in order that its rating may be achieved.

Generally, fluorescent fixtures bearing the UL listing mark can be utilized in most floor-ceiling designs as long as the utilized fixture area does not exceed the specified ceiling/fixture area ratio, and the fixtures are installed as required by the specific construction details of the UL Design No. being considered. Specific construction details will vary but, if considering an exposed "T" ceiling grid system, most designs require additional hanger wires at the midpoint of ceiling grid tees along each 4' side of nominal 2' x 4' and 1' x 4' luminaires as well as hanger wires at each of the four corners of the fixtures.

In addition to hanger wire requirements, most designs also require the use of fixture protection which is usually fabricated from available ceiling acoustical material bearing the UL Classification Marking with application details as specified by the specific UL Design No. The extent of fixture protection may range from as little as a single piece of material over the back of the luminaire, with air gap between the fixture and material, to the use of a complete five-sided box enclosing the fixture. Care should be observed in the application of luminaires in fire rated assemblies to ensure that ballasts do not overheat, thereby resulting in shortened ballast life and/or nuisance tripping of the ballast thermal protector. The use of low loss ballasts and reduced wattage lamps will reduce potential thermal problems in most applications involving elevated plenum temperatures. Consult factory with specific details for questionable applications.

The final approving authority is the local code enforcement official and as such should be consulted for local requirements and code interpretation. These requirements should be determined before construction, preferably during the specification period. Check the latest edition of the UL "Fire Resistance Directory" for specific assembly hour ratings and design constraints.

Fluorescent Dimming

Single-lamp and two-lamp universal dimming ballasts are available for varying light output of standard 40-watt rapid start lamps, straight or U-shaped. No special auxiliaries are needed. The dimming of reduced wattage fluorescent lamps is not recommended. Dimming ballast compatibility should be verified from the specific dimming control manufacturer.

Fluorescent Lamp

A lamp in which electric discharge of ultraviolet energy excites a fluorescing coating (phosphor) and transforms some of that energy to visible light.

Flux (Luminous Flux)

See lumen.

Footcandle (fc)

The unit used to measure how much total light is reaching a surface, such as a wall or table. One lumen falling on one square foot of surface produces one footcandle. One footcandle is equal to 10.76 lux. (see Lux)

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Metal Halide - Metal halide lamps are similar in design and operation to mercury. These lamps have additives in the arc tube which result in better color rendering. Metal halide lamps also have an efficacy approximately 100% higher than mercury vapor lamps. Metal halide lamps are used where efficiency and color are important. Hubbell Electro-Reg® Metal Halide System - A ballast circuit approach to electrically support the metal halide discharge lamp. It processes energy to the lamp in a much more lamp-compatible manner than the constant wattage autoregulator CWA/PLA circuit. The Electro-Reg ballast is a three coil magnetic structure in which the line energy is processed through one magnetic field to an energy transfer capacitor, through a second magnetic field to the lamp arc. This multi-stage energy transfer system forces a very high degree of electrical dynamic isolation between the arc discharge and the very low impedance power source. It provides excellent operating wave form control with significant lamp performance improvements.

High-Pressure Sodium Lamp - High pressure sodium lamps are similar to mercury and metal halide lamps in operation: however, construction and physical appearance differ. HPS lamps contain xenon as a starting gas to initiate the arc which vaporizes a sodium mercury amalgam. This lamp has a high efficacy of approximately 125 lumens per watt†. Other characteristics of this lamp are a golden color, long lamp life, and an excellent lumen depreciation. Special ballasting circuitry is usually require to produce high voltage pulses to start these lamps. High pressure sodium lamps are used where efficiency is the most important factor and color is not as important.

† Based on published data of 400 watt HPS lamp. Lower wattages are rated at lower lumens per watt.

Inverse Square Law

The law stating that the illuminance, E, at a point on a surface varies directly with the intensity, I, of the light source and inversely as the square of the distance, d, between the source and the point. If the surface at the point is perpendicular to the direction of the incident light, the law is expressed by E=I/d 2 .

Isocandela Line

A line plotted on any appropriate set of coordinates to show directions in space, about a source of light, in which the intensity is the same. A series of such curves, usually for equal increments of intensity, is called an isocandela diagram.

Isolux (Isofootcandle) Line

A line plotted on any appropriate set of coordinates to show all points on a surface where the illuminance is the same. A series of such lines for various illuminance values is called an isolux (isofootcandle) diagram.

A factor used in lighting calculations to account for the light loss that takes place in a lamp due to the gradual decay in lumen output over a designated period of burning time. The LLD is contingent upon relamping schedules and the specific lamp involved.

Not all factors are applicable for all luminaire types and/or applications. However each lighting design should be evaluated to determine appropriate light loss considerations.

For example:

Temperature Factor has an insignificant effect on H.I.D. and incandescent lamps; however, it is a major factor affecting fluorescent lamp lumen output.

Lamp Position (Tilt) Factor is not a consideration for incandescent or fluorescent. It is however, significant for Mercury Vapor and Metal Halide HID lamps. A typical lumen output characteristic curve for a Metal Halide lamp as a function of tilt from its vertical position is shown here. Minor variations exist between manufacturers.

Glossary — Lighting Terminology

Room Surface Dirt Depreciation Factor is not applicable in outdoor lighting calculations. This should be evaluated for interior calculations and is a significant recoverable loss for "indirect" lighting systems. Ballast Factor is a loss to be considered for both  H.I.D. and Fluorescent. Ballast factor is the difference of lamp lumen output produced from typical commercial ballasts versus the lumen output of lamps operated on reference ballasts. More definitive data is available for fluorescent systems and variations exist depending upon types of ballast-lamp combinations. Some Fluorescent "low power factor" ballasts may yield initial light output as low as 50% of rated lamp lumen output.

Luminaire Efficacy Rating (LER)

The Energy Policy Act of 1992 requires that the lighting industry develop a rating for luminaires that allows designers to compare the energy efficiency of lighting products. Luminaire Efficacy Rating (LER) has been developed by the National Manufacturer’s Association (NEMA) and approved by the National Lighting Collaborative to fulfill the requirements for the Energy Policy Act. The Collaborative represents a broad spectrum of industry professionals including manufacturers, industry associations, government, designers and energy conservation groups. This rating has already begun to be incorporated on Hubbell specification sheets and catalog information. NEMA document LE5 describes the calculation of LER as:

where:

EFF = luminaire efficiency
TLL = # lamps per luminaire rated lumens per lamp
BF = ballast factor
Input Watts = total system watts of the luminaire

This results in a lumens per watt rating that can be used to compare the energy efficiency of various products. The initial implementation of luminaire labeling covers 10 categories of common fluorescent 4' and 8' luminaires. In addition to the LER value, a  prefix indicating the type of source and general category of luminaire (such as FL for Fluorescent Lensed or FP for Fluorescent Parabolic) ensures that comparisons can be made among similar products.

This rating also recommends the use of photometric data from a National Voluntary Laboratory Accreditation Program (NVLAP) photometric lab. The NVLAP accreditation is administered by the National Institute for Standards and Technology (NIST) and includes on-site assessments as well as proficiency testing.

Light Trespass

A situation which occurs when light from a source is distributed onto areas where the illumination is not wanted.

Louver

A series of baffles used to shield a source from view at certain angles or to absorb unwanted light.

Low Temperature Starting

Standard ballasts for standard bipin-base, 2-lamp slimline and high output fluorescent lamps are designed to provide reliable starting at ambient temperatures down to 50°F. All reduced wattage lamps will start reliably with standard ballasts down to 60°F. Special ballasts are available for applications where standard lamps must be started at lower temperatures. Most low temperature ballasts will start standard lamps down to 0°F, with low-temperature HO ballasts good for -20°F. Some standard 1500 ma ballasts provide for cold starting, but do not improve the reduced light output that normally results. Recommended temperature limits should be verified from the manufacturer. High pressure sodium ballasts will start lamps down to -40° F, and metal halide ballasts down to -20°F. Electro-Reg® will start metal halide lamps down to -40° C.

Edison’s first lamp 1.4 lpw

Incandescent lamp 10-40 lpw

Fluorescent lamps 35-104 lpw

Halogen lamps 20-45 lpw

Mercury lamps 37-63 lpw

Metal halide lamps 80-125 lpw

High pressure sodium lamps 54-153 lpw

Luminaire / Fixture

A complete lighting unit consisting of a lamp (or lamps), or ballasts where applicable together with the parts designed to distribute the light, position and protect lamps and connect them to the power supply.

Luminaire Dirt Depreciation (LDD)

A factor used in lighting calculations to account for the light loss due to the accumulation of dirt on the luminaire. The LDD is contingent upon environment, cleaning schedules and the type of luminaire involved.

Luminaire Efficiency Rating

Luminaire efficiency is the ratio of lumens emitted by a luminaire in relation to those produced initially by the lamps contained within.

Luminance

(Photometric brightness) The luminous intensity of any surface in a given direction per unit area of that surface as viewed from that direction. Measured in candela/m 2 . All visible objects have some luminance.

Units: Candela per unit area,Symbol L

Luminous Flux

Luminous flux (lumens) is the measure of the total light producing power of the light source. It is the amount of light leaving the source without regard to direction. An incandescent 100 watt inside frosted lamp has about 1700 lamp lumens. A 400W HPS lamp has 50,000 lumens. The luminous flux is provided by the lamp manufacturers and a listing of common lamp and lumen values can be found in the lamp matrix.

Units: Lumens (lm) Symbol:

Luminous Intensity

The luminous intensity (candelas) is the strength of the light produced in a specific direction. The luminous intensity of an optical system is compiled graphically into diagrams known as candela or candlepower distribution curves. Both polar and Cartesian graphs are used within the lighting industry for this purpose. This information is also available in numeric tabular form.

Units: Candela (cd) Symbol: l

Lux

The metric unit of measurement of illuminance. The light on a surface of one meter square on which there is a uniformly distributed flux of one lumen. 10.76 lux equal 1 footcandle. One lux is equal to.09 footcandle. Decalux = 10 lux.

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Parabolic Aluminized Reflector (PAR) Lamp

A type of sealed beam lamp that controls its light beam by an internal reflector and sometimes a prismatic lens.

Plenum

That space between the structural ceiling slab and the finished ceiling. This space may contain air ducts, electrical wiring, etc. It’s the area which conceals the housing part of a recessed fixture.

Preheat

A circuit used on fluorescent lamps wherein the electrodes are heated or warmed to a glow stage by an auxiliary switch or starter (can be a glow switch, thermal type, or a mechanical device like a push button) before the lamps are lighted. This system was used on the original fluorescent lamps and is still in use today.

Reflectance is the ratio of luminous flux (lumens)  reflected from a surface in relation to luminous flux (lumens) incident onto the surface. Types of surface reflectance range from specular (mirror like) to the diffuse (lambertian), with many objects exhibiting combinations. Typically, reflectance becomes more specular with larger incident angles. Reflectances of room surfaces are important consideration when calculating illuminances using the "Lumen Method."

Reflector

A device used to direct the light from a source by the process of reflection.

Refraction

The process by which the direction of a ray of light changes as it passes obliquely from light transmitting material to another.

Specular Surface

Shiny or glossy surfaces (including mirror and polished metal) that reflect incident light, providing a relatively narrow beam pattern.

Spill Light

Lumens distributed by the luminaire which are outside the beam spread.

Surface Mounted

Any fixture mounted directly on a ceiling or wall is surface mounted.

Suspended or Pendant Mounted

Any fixture hung by supports (chains, hangers, stems, etc.) is suspended or pendant mounted.

Specular Surface

Shiny or glossy surfaces (including mirror and polished metal) that reflect incident light, providing a relatively narrow beam pattern.

Spill Light

Lumens distributed by the luminaire which are outside the beam spread.

Surface Mounted

Any fixture mounted directly on a ceiling or wall is surface mounted.

Suspended or Pendant Mounted

Any fixture hung by supports (chains, hangers, stems, etc.) is suspended or pendant mounted.