Lighting Design Basics and Terminology – Part One

The electrical designer usually designs the lighting loads firstly, in this article and the next articles I will explain lighting design for different areas and activities and by different methods indicated in the contents of this course " Advanced Course for Lighting Design - Level I "

You can review the previous articles that had been included in our course " Light-1: An Introduction to Lighting Design ".

First: Lighting design basics and terminology

In this division, I will show the definitions for all lighting terms under the following sub-divisions:

  1. Lighting design – main terms, 
  2. Lighting design - Units of measurements, 
  3. Methods of lighting definitions, 
  4. Lighting design curves/charts, 
  5. Lighting design coefficients / factors, 
  6. Lighting Luminaire types definitions, 
  7. Lighting Luminaire components definitions, 
  8. Lamp types definitions, 
  9. Lighting design distances definitions, 
  10. Lighting design laws, 
  11. Lighting design methods definitions, 
  12. Lighting design Abbreviations. 

1- Lighting design – main terms

  • Absorption: The dissipation of light within a surface or medium. 

  • Brightness: An attribute of visual perception in which a source appears to emit a given amount of light. “Brightness” should be used only for non quantitiatve references to physiological sensations and perceptions of light. Brightness was formerly used as a synonym for the photometric term “luminance” and incorrectly for the radiometric term “radiance”.

  • 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. A typical 100 watt incandescent lamp has about 1700 lamp lumens. A 400W HPS lamp has 50,000 lumens. The luminous flux rating is provided by the lamp manufacturers and a listing of common lamp and lumen values can be found in the lamp matrix. 

  • Luminous Intensity: The luminous intensity (candelas) is the strength (intensity) 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. 

  • “Batwing” Distribution: Candlepower distribution which serves to reduce glare and veiling reflections by having its maximum output in the 30 degree to 60 degree zone. 

  • Cutoff Angle: The angle at which a lamp is not directly visible in a reflector.

Cutoff Angle

  • Glare: The sensation produced by luminance within the visual field that is sufficiently greater than the luminance to which the eyes are adapted to cause annoyance, discomfort, or loss in visual performance and visibility. 

Direct Glare

  • Direct Glare: Glare resulting from high luminances or insufficiently shielded light sources in the field of view. It usually is associated with bright areas, such as luminaries, ceilings and windows which are outside the visual task or region being viewed. 

  • Discomfort Glare: Glare producing discomfort. It does not necessarily interfere with visual performance of visibility. 

  • Luminance: The amount of light reflected or transmitted by an object. 

  • Radiance and Spectral Radiance: Radiometric measures that describe the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle in a specified direction. They are used to characterize both emission from diffuse sources and reflection from diffuse surfaces. The SI unit of radiance is watts per steradian per square meter. 

  • Reflected Glare: Glare resulting from specular reflections of high luminances in polished or glossy surfaces in the field of view. It usually is associated with reflections from within a visual task or areas in close proximity to the region being viewed. 

  • Radiance: characterizes total emission or reflection, while spectral radiance characterizes the light at a single wavelength or frequency. 

  • Reflection: Light bouncing off a surface. In specular reflection the light strikes and leaves a surface at the same angle. Diffuse reflected light leaves a surface in all directions.

  • Refraction: The process by which the direction of a ray of light changes as it passes obliquely from one medium to another in which its speed is different. 

  • Sphere Illumination: The illumination on a task from a source providing equal luminance in all directions about that task, such as an illuminated sphere with the task located at the center. 

  • Candlepower: Luminous intensity expressed in candelas. 

2- Lighting design - Units of measurements:

  • Candela: The unit of measurement of luminous intensity of a light source in a given direction. 

  • 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 a uniformly distributed flux of one lumen. 

  • Lumen: The unit of luminous flux. 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. 

  • Lux: The metric unit of illuminance. One lux is one lumen per square meter (lm/m ). 

  • Foot Lambert (fl): A unit of luminance of a perfectly diffusing surface emitting or reflecting light at the rate of one lumen per square foot. 

3- Lighting function definitions:

Lighting functions

  • Accent Lighting: Directional lighting to emphasize a particular object or to draw attention to a part of the field of view. 

  • Floodlighting: A system designed for lighting a scene or object to a luminance greater than its surroundings. It may be for utility advertising or decorative purposes. 

  • Ambient Lighting: Background or fill light in a space. 

  • Local Lighting: Lighting designed to provide illuminance over a relatively small area or confined space without providing any significant general surrounding lighting. 

  • Task Lighting: Lighting directed to a specific surface or area that provides illumination for visual tasks. 

  • Wall Wash Lighting: A smooth even distribution of light over a wall. 

4- Lighting design curves/charts:

  • Candlepower Distribution Curve: A curve, generally polar, represents the variation of luminous intensity of a lamp or luminaire in a plane through the light center. 

  • Isolux Chart: A series of lines plotted on any appropriate set of coordinates, each line connecting all the points on a surface having the same illumination. 

Isolux Chart

  • Visual Edge: The line on an isolux chart that has a value equal to 10% of the maximum illumination. 

5- Lighting design coefficients / factors

  • 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 Index (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. 

Color Rendering Index (CRI)

  • Color Temperature: The absolute temperature of a blackbody radiator having a chromaticity equal to that of the light source. 

Color Temperature

  • Contrast: The difference in brightness (luminance) of an object and its background. 

  • Lamp Efficacy: The ratio of lumens produced by a lamp to the watts consumed. Expressed as lumens per watt (LPW). 

  • Lamp Lumen Depreciation (LLD): Multiplier factor in illumination calculations for reduction in the light output of a lamp over a period of time. 

  • Luminaire Dirt Depreciation (LDD): The multiplier to be used in illuminance calculations to relate the initial illuminance provided by clean, new luminaries to the reduced illuminance that they will provide due to dirt collection on the luminaries at the time at which it is anticipated that cleaning procedures will be instituted. 

  • Luminaire Efficiency: The ratio of luminous flux (lumens) emitted by a luminaire to that emitted by the lamp or lamps used therein. 

  • Maintenance Factor (MF): A technique of calculating illuminance after a measured period of time, under specific conditions. It takes into account temperature and voltage variations, dirt accumulation on luminaire and room surfaces, lamp depreciation, maintenance procedures and atmosphere conditions. 

  • Reflectance: Sometimes called reflectance factor. The ratio of reflected light to incident light (light falling on a surface). Reflectance is generally expressed in percent. 

  • Cavity Ratio: A number indicating cavity proportions calculated from length, width and height. 

  • Room Cavity Ratio (RCR): A number indicating room cavity proportions calculated from length, width and height. 

  • Spacing Ratio: Ratio of the distance between luminaire centers to the mounting height above the work-plane for uniform illumination. 

  • Shielding Angle: Determining factor of the quality of a reflector, it is the angle at which lamp and lamp image in a reflector are the same. 

  • Coefficient of Beam Utilization (CBU): The percentage of light from a floodlight which reaches the illuminated task relative to total beam lumens. 

  • Light Loss Factor (LLF): A factor used in calculating illuminance after a given period of time and under given conditions. It takes into account temperature and voltage variations, dirt accumulation on luminaire and room surfaces, lamp depreciation, maintenance procedures and atmosphere conditions. Formerly called maintenance factor. Generally light loss factors are divided into two groups, classed as “recoverable” (with cleaning and relamping) and “nonrecoverable.” 

  • 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. Minor variations exist between manufacturers. 

  • Ballast Factor: is a consideration 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 and some electronic ballasts as high as 120% of rated lamp lumen output. 

  • Spacing Criterion (SC): The ratio of the distance between luminaire centers to the mounting height above a reference plane, usually the work plane. 

in the next Article, I will explain the definitions of lighting design terms under other sub-divisions. please, keep following.

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