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Lighting Answers: Multilayer Polarizing Panels Multilayer Polarizer Panels . Volume 1. Number 2 August 1993 Background used with ceiling-mounted fluorescent lamp luminaires. Light emitted from luminaires Fluorescent lamp luminaires with multilayer that are equipped with these panels is partial- polarizer panels produce partially polarized ly polarized and can improve the contrast of light*, which can increase the contrast of a the visual task to a greater or lesser degree visual task under some viewing conditions. depending upon several factors that are dis- In principle, a decrease in illuminance can cussed on p. 4. accompany an increase in task contrast with- The Illuminating Engineering Society of out affecting visual performance. Because a North America (IESNA) recommends illumi- reduction in illuminance implies a reduction nance levels for different visual tasks in an in the electric power used for lighting, multi- effort to ensure acceptable levels of visual layer polarizer panels have been promoted as performance in typical applications such as energy-saving devices. This issue of Lighting offices, schools, and libraries (IESNA 1993). In Answers discusses the effectiveness of multi- general, recommended illuminance levels are layer polarizer panels as a means of maintain- relatively high where the visual task is difficult ing visual performance at reduced illuminanc- to see (for example, of low contrast) or where es and, thus, as energy-saving devices in the speed and accuracy of the task are critical. typical commercial spaces. There is no question that polarized light can improve contrast. However, the important practical question that faced the National Light- Introduction ing Product Information Program (NLPIP) was, Visual performance is affected by the amount “For typical applications, is the contrast im- of illuminance on the task and the contrast of provement from multilayer polarizer panels of the objects, or targets, being illuminated. sufficient magnitude to justify a reduction in Contrast is a quantitative expression of the recommended illuminances and, thus, to visual distinctness of a target against its back- justify lower lighting power densities?” To ground. A typewritten letter on white paper is answer this question, NLPIP addressed sever- an example of a high-contrast target; the al other questions regarding polarized light same letter typed on dark gray paper is a low- and the use of multilayer polarizer panels. contrast target. In principle, contrast and illuminance can be traded off against each What is polarized light? other while maintaining a constant level of visual performance; a low-contrast task will Like waves in the ocean, light travels as a require a higher illuminance than a high- transverse wave. This means that as light contrast task, and vice versa. moves in a particular direction the frequency Light distribution also affects the contrast (and wavelength) of the light vibrations (or of a task. Mirror-like, specular reflections from waves) is measured in a given plane centered a task surface will reduce contrast. For exam- along the light path (Figure I). ple, reading a glossy magazine often requires that the magazine be repositioned to avoid Figure 1 specular reflections from the page. Lighting Transverse Wave systems that limit the amount of specular x reflections will increase task contrast and, therefore, allow lower illuminances to main- tain the same level of visual performance. Under certain conditions, polarized light will reduce specular reflections. Multilayer polarizer panels are commercially available, flat lenses that are specifically designed to be A transverse light wave travelling along the z axis. The frequency of the wave can be measured along any plane perpendicular to the z axis; two such planes are * All terms in italics are defined in the glossary on p. 5. shown above. If the light is completely polarized, all a horizontally positioned luminaire with a . vibrations occur in just one plane along the multilayer polarizer will be characterized as Multilayer direction of travel; for unpolarized light (see partially vertically polarized. polarizer panels / Figure 2a) these vibration planes will occur Whether the transmitted light is referred to and video display randomly at every angle perpendicular to the as horizontally or vertically polarized light terminals (VDTs) direction of travel. The vibration planes of depends upon the relative magnitudes of the Light reflected from a partially polarized light will be more concen- x and y vectors. As light travels through more vertical surface becomes trated about a particular direction. Figures 2b and more layers, the transmitted light be- vertically polarized, and and 2c illustrate completely and partially comes more and more polarized. Thus, the vertically polarized light polarized light, respectively. The vibration incident on a vertical degree of polarization of light emitted from a surface is reflected with planes are denoted by double-ended vectors, luminaire equipped with a multilayer polariz- little or no attenuation. which represent the light oscillations. er panel will depend upon the number of Therefore, ceiling- All of the vibration planes in any beam of layers in the panel material. mounted multilayer light can be resolved into x and y component The degree of polarization also depends polarizer panels that emit vectors, as shown in Figure 2d. The average upon the incident angle of the light reaching partially vertically polar- of the x component vectors and the average the layer interface. Light striking the material ized light provide little or of the y component vectors yield overall normal to the plane of the layer interface will no reduction in reflected component vectors, x and y in both direc- be transmitted without being polarized. As glare from vertical tions (Figure 2e). These values are then the incident angle increases, the degree of surfaces such as VDT screens. used to calculate the degree of polarization polarization increases until an angle of maxi- expressed as a percentage. The formula is: mum polarization, known as Brewster's angle is reached; larger incident angles produce lower degrees of polarization. The exact value of Brewster’s angle depends upon the refrac- where y is the larger of the two vectors. tive index of the materials. When resolving the vectors (vibration planes) of polarized light, horizontal and vertical directions are used most often because these represent planes of interest within architectural spaces like desk tops, . ..* . -. video display terminal (VDT) screens, and walls. Thus, one often refers to “horizontally” Figure 2 and “vertically” polarized light. Describing Polarized Light with Vectors What are multilayer polarizer panels? Multilayer polarizer panels are commercially available products consisting of several lami- nated plastic layers that partially polarize the light following the principle described below. They typically are manufactured in panels that replace the plastic prismatic or diffusing panels in standard 2-foot by 4-foot (0.6-meter by 1.2-meter fluorescent lamp luminaires; however, they may assume other sizes or be used in a luminous ceiling. How do multilayer polarizers work? There are several ways to polarize light, including the method of reflection/transmis- sion shown in Figure 3. By this method, as light passes through several layers of a mate- The center dot in each of these figures represents a beam of light travelling out of the rial, some of the light is reflected at each page toward the reader. layer interface, and some is transmitted. The (a) Unpolarized light vibrates randomly at every angle perpendicular to the direction of travel. reflected light is more polarized in the plane (b) Completely polarized light vibrates in a single plane. of the layer interface; the transmitted light is (c) Partially polarized light consists of many waves clustered about a particular more polarized perpendicular, or normal, to orientation. the layer interface. Multilayer polarizers are (d) Any double-ended vector can be described by two perpendicular, double-ended almost always installed horizontally in ceiling- component vectors, called here x and y. (e) The x and y component vectors in (d) for each wave are averaged to produce mounted luminaires. Thus, light emitted from these overall component vectors, x and y. 2 Figure 3 How efficient are luminaires equipped How Multilayer Polarizers Produce Partially Polarized Light with multilayer polarizer panels? w It is difficult to accurately measure the lumi- naire efficiency of polarizing lenses with mir- ror photometers, the most common type available for making luminaire efficiency and luminous intensity measurements, because the reflecting mirror interacts with the inci- dent polarized light and affects the amount of light reaching the detector. NLPIP estimated the efficiency of a 2-foot by 4-foot two-lamp luminaire equipped with a multilayer polarizer. Luminance measure- ments were obtained for luminaire angles between 0˚ and 85˚ for a fluorescent lamp luminaire fitted with a multilayer polarizer The circle size represents the magnitude of the horizontal panel. These measurements were roughly component; the length of the bars equivalent to those obtained at the same 1 represents the magnitude of the angles from the same luminaire equipped vertical component. with a 1/8-inch translucent, flat, white plastic diffuser instead of the multilayer polarizer panel. These data suggest that the efficiency of the luminaire equipped with the multilayer polarizer
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