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Home , Luminous intensity

ELECTRICAL AND TRAFFIC ENGINEERING MANUAL

Appendix 300.1

Glossary of Terms

APPENDIX 300.1 – GLOSSARY OF TERMS

Accommodation: The process by which Up: Entire upper half of the sphere. the eye adapts itself to varying quantities Front: The front quarter of the of . bottom half of the sphere. Back: The back quarter of the bottom Adaptation: The process by which the half of the sphere. eye adapts itself to varying quantities of The up zone is divided into low and high light. and the lower zones are divided into low, medium, high and very high. Arrangement: The repeating pattern of The amount of light entering each zone luminaires on a roadway. Usually defines the BUG (Backlight – Uplight – described as staggered, one-sided, Glare) rating for the fixture. This opposite or medium mounted. method has superseded the method of describing fixtures by “cutoff”. Average (Lav): The average photometric of a (cd): The SI unit of luminous surface expressed in terms of the total , equal to one per (lm) actually leaving the (lm/sr). surface per unit area (cd/m2). Candela Per Square Meter: The SI Ballast: A device used with an electric unit of luminance equal to the uniform discharge lamp to obtain the necessary luminance of a perfectly diffusing circuit conditions (voltage, current and surface emitting or reflecting light at the waveform) for starting and operation. rate of one lumen per square meter or the average luminance of any surface Bikeway: Any road, street, path, or way emitting or reflecting light at that rate. that in some manner is specifically The unit is sometimes called a nit. designated as being open to bicycle travel, regardless of whether such Cutoff: A means of defining the facilities are designated for the exclusive distribution of a luminaire based on use of bicycles or are to be shared with candela per 1000 lamp lumens. other transportation modes. Luminaires are rated as Full Cutoff, Cutoff, Semi-cutoff, or Non-cutoff. Brightness: The luminance in terms of subjective sensation as experienced by Full Cutoff -A luminaire light an individual person. Brightness is a distribution is designated as full psychological response to luminous and cutoff where zero candela intensity not synchronous with it. occurs at an angle of 90o above nadir, and all greater angles from BUG Rating (Backlight-Uplight- nadir. Additionally the candela per Glare): A method of evaluating 1000 lamp lumens does not luminaire optical performance by numerically exceed 100 (10%) at an partitioning the spherical distribution of angle 80o above nadir. This applies light produced by a fixture into 3 to all lateral angles around the specific zones, and quantifying the luminaire. amount of light incident to each zone. The zones are:

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Cutoff -A luminaire light distribution sources in the field of view or from is designated as cutoff when the reflecting areas of high luminance. It per 1000 lamp lumens usually is associated with bright areas, does not numerically exceed 25 (2.5 such as luminaires, that are outside the percent) at an angle of 90o above visual task or region being viewed. A nadir, and 100 (10%) at a vertical direct glare source may also affect angle of 80o above nadir. This performance by distracting attention. applies to all lateral angles around the luminaire. Discomfort Glare - The sensation experienced by an observer when Semi-cutoff -A luminaire light brightness relationships in the field or distribution is designated as semi- view cause discomfort but do not cutoff when the candlepower per necessarily interfere with visual 1000 lamp lumens does not performance. numerically exceed 50 (5%) at an angle of 90o above nadir, and 200 Disability Glare - The effect of stray (20%) at a vertical angle of 80o light on the eye whereby visibility and above nadir. This applies to all visual performance are reduced. A direct lateral angles around the luminaire. glare source that produces discomfort may also produce disability glare by Non-cutoff – A luminaire is introducing a measurable amount of designated as non-cutoff where there stray light in the eye. is no intensity (candela) limitation in the zone above maximum intensity. High Intensity Discharge (HID) Lamp: An electric discharge lamp in Efficacy (lm/w): The ratio of the total which the light producing arc is luminous flux emitted by a lamp to the stabilized by wall and the total lamp expressed in lumens arc tube has a bulb wall loading in per . excess of three per square centimeter. HID lamps include groups Efficiency (%): The ratio of the total of lamps known as mercury vapor, metal luminous flux emitted by a luminaire to halide and high-pressure sodium. that emitted by a bare lamp. High Pressure Sodium (HPS) Lamp: Footcandle (fc): The unit of High intensity discharge (HID) lamp in equal to 1 lm/ft2 or 10.76 . which light is produced by radiation from sodium vapor operating at a partial Glare: The sensation produced by pressure of about 1.333104 Pa (100 torr). within the visual field that are sufficiently greater than the Illuminance (E): The density of the luminance to which the eyes are adapted. luminous flux incident on a surface Which cause annoyance, discomfort, or measured in lux or fc. loss in visual performance and visibility. Horizontal illuminance – Luminous flux Direct Glare - Glare resulting from high incident on a horizontal surface. luminance or insufficiently shielded light

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Vertical illuminance – Luminous flux Light Pollution: The scattering of incident on a vertical surface. electric light onto the atmosphere, usually caused by luminous flux above Illumination: Commonly used in a the horizontal. Often referred to as sky qualitative or general sense to designate glow or sky brightness. the act of illuminating or the state of being illuminated. Light Trespass: Light that strays from Incident Light: Light falling on an object its intended purpose, causing visual or surface. annoyance. A sever form of light trespass involves glare. Intensity: A shortening of the terms luminous intensity and . Low Pressure Sodium (LPS) Lamp: Often used for level of illumination or A discharge lamp in which light is illuminance. produced by radiation from sodium vapor operating at a partial pressure of Lamp: A generic term for an artificial 0.13 to 1.3 Pa (10-3 to 10-2 torr). source of light. Lumen (lm): The SI unit of luminous flux. Photometrically, the luminous flux Lamp Life: The average life of a lamp emitted within a unit (1sr) by defined as the point in when 50 a point source having a luminous percent of a statistical sample of lamps intensity of 1cd. has failed. Luminaire: A complete lighting unit Lamp Lumen Depreciation (LLD) consisting of a lamp or lamps together Factor: The multiplier to be used in with the parts designed to distribute the illuminance calculations to relate the light, to position and protect the lamps initial rated output of light sources to the and to connect the lamps to the power anticipated minimum rated output based supply. on the relamping program to be used. Luminaire Component Depreciation Light: Radiant energy that is capable of (LCD) Factor: The multiplier to be exciting the retina and producing a used in illuminance or luminance visual sensation. The visual portion of calculations to relate the initial flux the electromagnetic extends output of a clean luminaire to the from about 380 to 780 nanometers. reduced flux output due to permanent component deterioration at a particular Light Loss Factor (LLF): A factor used time in the future chosen for desirable in calculating illuminance after a given maintained values. period of time and under given conditions. It takes into account Luminaire Dirt Depreciation (LDD) temperature and voltage variations, dirt Factor: The multiplier to be used in accumulation of luminaire, lamp luminance or illuminance calculations to depreciation, maintenance procedures reduce the initial light level provided by and atmospheric conditions. Formerly clean, new luminaires to the reduced to called maintenance factor. the light level that they will due to dirt collection of the luminaires at the time at

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which it is anticipated that cleaning procedures will be instituted. Nadir: The vertical projection beneath Luminance (L): The luminous intensity the apparent light source of a luminaire. of any surface in a given direction per Offset: The distance from the face of unit area of that surface viewed from that curb or edge of the traveled roadway to direction. the traffic hazard (lighting poles, etc.). Reflectance: The ratio of the reflected Luminous Flux (F): The time rate of light (flux) to the incident light (flux). flow of light, expressed in lumens. Reflector: A device used to redirect the Luminous Intensity (I): The luminous luminous flux from a source by the flux per unit solid angle in a specific process of reflection. direction. It is the luminous flux on a small surface normal to that direction Refractor: A device used to redirect the divided by the solid angle (in ) luminous flux from a source primarily that the surface subtends at the source, by the process of refraction. expressed in cd. Setback: The lateral offset of the pole from face of curb or edge of traveled Lux (lx): The international unit of roadway. illuminance, equal to one lumen evenly distributed over a surface of one square Small Target Visibility: A method of meter. One lux is equal to approximately design that determines the visibility level 0.093 fc. of an array of targets on the roadway. The weighted average of the visibility Maintenance Factor (MF) or Lumen level of these targets results in the STV. Maintenance: A factor formerly used to denote the ratio of the illuminance on Spacing: The distance in a given area after a period of time to the between successive lighting units initial illuminance on the same area. measured along the control line of a Light output from all lamps deteriorates street. over time at various rates. These must be addressed if the system is to be Street Side: The horizontal direction effective when “minimum” operational that is toward the roadway from the levels are attained. nadir of the luminaire.

Mean Lamp Lumens: The mean lumen Veiling Luminance: A brightness output of a lamp is calculated by superimposed on the retinal image that determining the area beneath the lumen reduces its contrast. It is this veiling maintenance characteristic curve of that effect produced by bright sources or source over a given period of time and areas in the visual field that results in dividing that area by the time period in decreased visual performance and hours. visibility.

Mounting Height (MH): The vertical Veiling Reflection: Regular reflections distance between the roadway surface superimposed upon diffuse reflections and the centre of the apparent light from an object that partially or totally source of the luminaire. obscure the details to be seen by

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reducing the contrast. This sometimes is called reflected glare.

Visibility: The quality or state of being perceivable by the eye. In many outdoor applications, visibility is defined in terms of the distance at which an object can be just perceived by the eye. In indoor and outdoor applications it usually is defined in terms of the contrast or size of a standard test object, observed under standardized view-conditions, having the same threshold as the given object.

Visibility Level (VL): A contrast multiplier to be applied to the visibility reference function or provide the luminance contrast required at different levels of task background luminance to achieve visibility for specified conditions relating to the task and observer.

Visual Angle: The angle subtended by an object or detail at the point of observation. It usually is measured in minutes of arc.

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