Lesson 21: The Quantity and Quality of

Luminaire Efficacy Ratio Just as we can measure the They require placement in a Luminaire ( fixture) having the of a lamp necessary optics to deliver the lumens. However, luminaires are not (LPW), so too can we measure 100% efficient and lumens are lost within the lighting fixture. To the efficacy of the luminaire. determine the efficacy of the luminaire a formula was developed, which numerically expresses the total lumen output from the sources are known in luminaire compared to the watts required to operate the lighting the industry as Lamps. Some system. It is called theLuminaire EfficacyRatio (LER). lamps (such as linear fluorescent) have no reflector or other integrated means of controlling their light.

Inverse Square Law We know from Lesson One, that the basic unit of As the distance between the surface and the the measurement of light is a Lumen. When these (a point source) increases, the Intensity (I) lumens arrive on a surface they are then measured or Candelas (candlepower) reaching that surface at in footcandles. A Footcandle (fc) is the amount of a given point decrease according to the Inverse illumination from one standard candle falling on a Square Law. This can be expressed accordingly; the surface one foot away. In the international metric illumination (E) equals the luminous intensity (I) of system, LUX is the counterpart of footcandles. The the light source divided by the distance ...from the conversion from footcandles to lux can be made by light source to the surface...squared. multiplying footcandles by ten. The IES recommendedillumination values are published in both footcandles and lux.

Candlepower Distribution Candlepower distribution curves show graphically the intensity of luminous flux (candelas) in a given direction. For reflector lamps….R, BR, ER, PAR, and MR….lamp manufacturers will show, in their publications, the Center Beam Candlepower (CBCP) for these lamp types. The beam spread for these lamps is measured to 50% of CBCP.

Quality of Light: Glare Glare is uncomfortable brightness in the visual field, which is annoying and uncomfortable, causing fatigue and loss of productivity. Direct glare results from seeing high luminaire brightness in the normal field of view

Quality of Light: Visual Comfort Probability Visual Comfort Probability (VCP) evaluates the probability in a given situation that a person seated in the worst position for glare in a room (usually at the center rear) will find the degree of glare from the lighting system just acceptable.

For example; an installation of 2 x 4 lens fixtures with a VCP of 75 would signify that 75% of the people would find it visually acceptable and 25% would not.

Quality of Light: Reflected Glare Reflected glare causes discomfort and occurs when luminaire brightness is reflected from shiny (specular) surfaces in the field of view. This can include computer screens.

Quality of Light: Veiling Reflections Regular reflections superimposed Veiling reflections are more subtle reflections of the light source in upon diffuse reflections from an the task. They reduce the contrast between detail and background; object that partially or totally thus, reducing visibility and obliterating detail altogether. Some obscure the details to be seen by solutions for eliminating the veiling reflections are to alter the reducing the contrast. This is position of the viewer, alter the position of the task, or rearrange sometimes called reflected glare. the luminaires.

As a “rule of thumb” for each 1% loss of contrast, 10% to 15% more light is needed to achieve equal visibility.