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Lighting Jargon

A - Ambient Lights: lighting throughout an...

Ambient Lights: lighting throughout an area that produces general illumination.

B - Ballast: a device used with an electric-discharge lamp to obtain the necessary...

Ballast: a device used with an electric-discharge lamp to obtain the necessary circuit conditions (voltage, current and wave form) for starting and operating. Ballast Factor: the fractional flux of a lamp(s) operated on a ballast compared to the flux when operated on the reference ballasting specified for rating lamp lumens.

C - Candela, CD: the SI unit of luminous intensity. One candela is...

Candela, CD: the SI unit of luminous intensity. One candela is one lumen per steradian. Formerly, candle.

Coefficient Of Utilization, CU: the ratio of the luminous flux (lumens) from a luminaire calculated as received on the work- plane to the luminous flux emitted by the luminaire’s lamps alone. Color Rendering: general expression for the effect of a light source on the color appearance of objects in conscious or subconscious comparison with their color appearance under a reference light source.

Color Rendering Index: (of a light source) (CRI): measure of the degree of color shift objects undergo when illuminated by the light source as compared with the color of those same objects when illuminated by a reference source of comparable color temperature.

Cut Off Angle: (of a luminaire): the angle, measured up from nadir, between the vertical axis and the first line of sight at which the bare source is not visible.

D - Diffuse Ratio: the ratio of the flux leaving a surface or...

Diffuse Ratio: the ratio of the flux leaving a surface or medium by diffuse reflection to the incident flux.

Diffuse Reflection: the process by which incident flux is re- directed over a range of angles.

Difuser: a device to redirect or scatter the light from a source, primarily by the process of diffuse transmission.

Direct Component: that portion of the light from a luminaire which arrives at the work-plane without being reflected by room surfaces. See indirect component.

Direct-Indirect Lights: a variant of general diffuse lighting in which the luminaires emit little or no light at angles near the horizontal.

Downlight: a small direct lighting unit which directs the light downward and can be recessed, surface mounted or suspended.

E - Efficiency: see luminaire efficiency and...

Efficiency: see luminaire efficiency and luminous efficacy of a source of light.

F - Footcandle, FC: the unit of illuminance when the foot is taken as...

Footcandle, FC: the unit of illuminance when the foot is taken as the unit of length. It is the illuminance on a surface one square foot in area on which there is uniformly distributed flux of one lumen, or the illuminance produced on a surface all points of which are at a distance of one foot from a directionally uniform point source of one candela.

H - High Intensity Discharge: (HID) LAMP: an electric discharge lamp in which the light...

High Intensity Discharge: (HID) LAMP: an electric discharge lamp in which the light producing arc is stabilized by wall temperature, and the arc tube has a bulb wall loading in excess of three watts per square centimeter. HID lamps include groups of lamps known as mercury, metal halide, and high pressure sodium.

I - Indirect Component: the portion of the luminous flux from a...

Indirect Component: the portion of the luminous flux from a luminaire arriving at the work-plane after being reflected by room surfaces. See direct component.

Indirect Lights: lighting by luminaires distributing 90 to 100 per cent of the emitted light upward.

L - Lens: a glass or plastic element used in luminaires to change...

Lens: a glass or plastic element used in luminaires to change the direction and control the distribution of light rays.

Light: radiant energy that is capable of exciting the retina and producing a visual sensation. The visible portion of the electromagnetic spectrum extends from about 380 to 770 nm. Louver: a series of baffles used to shield a source from view at certain angles or to absorb unwanted light. The baffles usually are arranged in a geometric pattern.

Lumen,LM: SI unit of luminous flux. Radiometrically, it is determined from the radiant power. Photometrically, it is the luminous flux emitted within a unit solid angle (one steradian) by a point source having a uniform luminous intensity of one candela.

Luminaire Efficiency: the ratio of luminous flux (lumens) emitted by a luminaire to that emitted by the lamp or lamps used therein. Luminous Efficacy Of a Source Of Light: the quotient of the total luminous flux emitted by the total lamp power input. It is expressed in lumens per watt.

Lux,LX: the SI unit of illuminance. One lux is one lumen per square meter (lm/m2 ).

N - Nadir: when used in lighting, the point...

Nadir: when used in lighting, the point directly below the center of the luminaire.

Nanometer,NM: unit of wavelength equal to 10-9 meter.

P - Photometer: an instrument for measuring photometric quantities such...

Photometer: an instrument for measuring photometric quantities such as luminance, luminous intensity, luminous flux, and illuminance.

Photometry: the measurement of quantities associated with light. Point To Point Method: a method of lighting calculation now called point method.

Point Method: a lighting design procedure for predetermining the illuminance at various locations in lighting installations, by use of luminaire photometric data.

Q - Quality Of Lights: pertains to the distribution of luminance in...

Quality Of Lights: pertains to the distribution of luminance in a visual environment. The term is used in a positive sense and implies that all luminances contribute favorably to visual performance, visual comfort, ease of seeing, safety, and esthetics for the specific visual tasks involved.

R - Rapid Start Fluorescent Lamp: a fluorescent lamp designed for...

Rapid Start Fluorescent Lamp: a fluorescent lamp designed for operation with a ballast that provides a low-voltage winding for preheating the electrodes and initiating the arc without a starting switch or the application of high voltage.

Reflection: characteristic of a surface to return (bounce back) light or energy. Various surfaces will reflect light in different ways, i.e.: specular or diffuse surfaces.

Reflectometer: a photometer for measuring reflectance.

Reflector: a device used to redirect the luminous flux from a source by the process of reflection. See retro-reflector.

Regular (Specular) Reflectance: the ratio of the flux leaving a surface or medium by regular (specular) reflection to the incident flux. See regular (specular) reflection.

Regular(Specular) Reflection: that process by which incident flux is re-directed at the specular angle. See specular angle.

Retro-Meflector (Reflex-Reflctor): a device designed to reflect light in a direction close to that at which it is incident, whatever the angle of the incidence.

Room Cavity Ratio, RCR: a number indicating room cavity proportions calculated from length, width, and height.

S - Semi-Direct Lighting: lighting by luminaire distributing 60 to 90 per cent...

Semi-Direct Lighting: lighting by luminaire distributing 60 to 90 per cent of their emitted light downward and the balance upward.

Semi-Indirect Lighting: lighting by luminaire distributing 60 to 90 per cent of their emitted light upward and the balance downward. Spectophotometer: an instrument for measuring the transmittance and reflectance of surfaces and media as a function of wavelength.

Specular Angle: the angle between the perpendicular to the surface and the reflected ray that is numerically equal to the angle of incidence and that lies in the same plane as the incident ray and the perpendicular but on the opposite side of the perpendicular to the surface.

T - Task Lighting: lighting directed to a specific surface or area that...

Task Lighting: lighting directed to a specific surface or area that provides illumination for visual tasks. Translucent: allows light to pass through without being transparent, scattering or diffusing light so that objects cannot be seen through the material. Troffer: a recessed lighting unit, usually long and installed with the opening flush with the ceiling. The term is derived from “trough” and “coffer”.

V - Visual Angle: the angle subtended by an object or...

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

Visual Comport Probability, VCP: the rating of a lighting system expressed as a per cent of people who, when viewing from a specified location and in a specified direction, will be expected to find it acceptable in terms of discomfort glare. Visual comfort probability is related to discomfort glare rating (DGR).

W - Watt: unit of measure of electrical power consumption...

Watt: unit of measure of electrical power consumption (called “load”). The total load of luminaire will be expressed in the total watts consumed by the lamps and/or any auxiliary equipment.

Work-Plane: the plane at which work usually is done, and on which the illuminance is specified and measured. Unless otherwise indicated, this is assumed to be a horizontal plane 0.76 meters (30 inches) above the floor.

end faq

All definitions taken from “IES Lighting Ready Reference”
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