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Welcome to Axiom's INSIDE Engineering Newsletter. ISSUE 2: Special Issue THE
CONCEPT OF LIGHTING
There is much confusion between the concepts of quantity and quality in the field of lighting design. Although interrelated, these two items are distinct. The quantity of light can be physically measured using a light meter, giving a specific illuminance value in units of foot-candles (imperial) or LUX (metric). However, a light meter cannot duplicate the sensitivity of the human eye - there can be a significant difference between what a light meter reads and what the eye perceives. It is perception that relates the quality of light to quantity. With current technology, there is no way to physically measure the quality of light - it is a perception that physically changes as a result of personal experience and age. Although the quality of light cannot be measured, it is possible to identify the causes of poor light quality in an application. Direct glare from light fixtures is one of the most common sources of low-quality lighting. This condition is easily identified by complaints of "too much light" by those affected. Often, the level of lighting may be irrelevant to the problem. This can be illustrated by the effect of daylight in a fenestrated office. The level of light from an overcast day is hundreds of times higher than the levels provided by a lighting system. Since overcast conditions provide very diffuse sunlight, there are no complaints of glare - the diffuse nature of the light works to cancel or fill-in the effects of fixture glare. However at night, the lighting system may be blinding. The quantity of light from the fixtures has not changed, but their output relative to the ambient source (the sun) has. This problem can be improved by raising the fixture height above the line of sight, or by using fixtures with parabolic louvres. Another frequent cause of discomfort is indirect glare. Light from the source reflects from the task surface into the viewer's eyes, creating a veiling reflection that reduces the contrast between task details and their surroundings. This condition typically occurs when the light fixture is directly above and in front of the viewer. The problem can be reduced easily by relocating the work surface to a position between fixtures or by increasing the level of lighting. The quantity of light is
as important as quality in any application. Providing the proper
level of illumination for a particular task or space will increase comfort
and productivity within that space.[1]
In technical terms, light is a form of radiant energy that is evaluated by its capacity for producing the sensation of illumination. Visible energy is a very narrow band within the electromagnetic spectrum - which is radiant energy that travels through all space as electromagnetic waves. It should be understood that colours and frequency wavelengths are equivalent in the electromagnetic spectrum. That is, the colour BLUE is represented as radiant energy with a wavelength of approximately 400 nanometres(nm), and the colour RED is associated with a wavelength of approximately 650nm. The human eye is not equally sensitive to energy of all wavelengths. A Spectral Luminous Efficiency Curve (or Eye Sensitivity Curve) has been established to depict the response of the human eye to different wavelengths of visible light at a constant energy. This is a very important consideration when choosing light sources and colours for objects and spaces. Maximum eye response is achieved at a wavelength of approximately 550nm, corresponding to the yellow-green portion of the spectrum. The minimum response is experienced at the blue (ultraviolet) and red (infrared) ends of the visible spectrum. This means that to achieve a perceived equal luminance of blue or red light, approximately ten times the energy is required for illumination than if using yellow-green light. In very dim light, where vision is almost colourless, the human eye is much more sensitive to blue energy than to red energy. This phenomenon is known as the Purkinje Effect. [2] The process of seeing is dependent on the visual object's size, the level of illumination the object receives, and the contrast in luminance colour between the visual object and its background or surroundings. The concept of Colour Temperature (or correlated colour temperature for fluorescent sources), is a means of describing the colour output of a light source in comparison to the theoretical Black Body Radiator. As its temperature is raised, a Black Body will change in colour from red to orange, yellow, white, blue-white, and finally, blue. Using this analogy, the colour temperature of a candle is approximately 1800 Kelvin. The colour temperature of a cool-white fluorescent tube is approximately 4100 Kelvin. Thus, as colour temperature increases, the appearance of the source becomes cooler. Colour temperature defines the colour of a source and has nothing to do with its physical temperature. TABLE 1 provides a partial numerical listing of colour temperatures for various common light sources. While it is important to stress the effects of light source colour, the colour and the contrast between the visual object and its surroundings has an even greater impact on human perception. Typically, if a space lacks adequate colour contrast, brightness contrast will be required to create an interesting space. At low illumination levels - below 25 foot-candles (250 LUX) - object colours will appear normal if the illumination is tinted with yellow, pink, or orange. At higher levels, normal appearance comes with cooler light colours (higher colour temperature). This quality effect is particularly important in merchandise lighting, where ambient levels are typically above the 30-50 foot-candle (300-500 LUX) range. It has also been found that under cool lighting conditions, the sensory perception of physical temperature and noise are reduced, while under warm light they are increased. [3] Along with a feeling of coolness, blue and green lighting tends to create an illusion of space and distance. In this way, colour can be used in subtle ways to create a desired atmosphere within a space. [2] These perceived effects of light indicate that lighting has a much more important role in human behaviour than providing illumination to allow us to see with. TABLE 1: COLOUR TEMPERATURES (In Kelvin)
Several studies are being carried out which are attempting to discover how light affects the biological rhythms, hormones, and brains of humans. One such study that has been carried out by Dr. George Brainard, Ph.D., for the Department of Neurology, Jefferson Medical College in Philadelphia, may lead to the determination of the optimum brightness levels for human physiology. Since the beginning of time, mankind was forced to evolve under a 24 hour cycle of light and darkness. Early humans had to adapt to the extreme differences in temperature caused by the presence and absence of light. The physiology of humans (and animals) adapted to the light cycle on an hourly and daily basis, generating a natural rhythm known as the Biological Clock. Much more recently, however,
man has had to adapt to the development of artificial illumination, which
is very different from sunlight. In Dr. Brainard's study, four parameters
were measured to determine the effect of artificial light on humans; intensity;
wavelength (colour); time of exposure; and duration of exposure.
All of these parameters affect the biological rhythms of humans and animals.
Dr. Russel J. Reiter, Ph.D., and Dr. Brainard concluded that environmental light (the light that reaches the biological clock) inhibits the production of melatonin. Generally, it was found that the higher the light intensity, the greater the suppression of melatonin. In humans, it was found that bright white incandescent light and bright green light dramatically suppresses the production of melatonin. The consequences of this reduced hormone production are net yet clear in human studies. In laboratory hamsters, it was found that a reduction in melatonin leads to a suppression of breeding patterns - particularly with yellow and red light. Blue, and green light acted to stimulate breeding in the hamsters. It is reasonable to assume that similar effects would be expected in humans. Currently, neonatal jaundice clears up when infants are exposed to light in the blue and ultraviolet wavelengths. Also, bright white light has been used to reverse the effects of seasonal depression (Seasonal Affective Disorder). Dr. Brainard's work may one day provide additional information on how light can be used to help affect humans in positive ways. [4][5] INCANDESCENT: Incandescent light sources that utilize a burning filament to produce incandescence via an electric current. Although this source has the lowest initial cost of all commonly used light sources, it has the shortest life span (750 to 1500 hours), and the lowest light output per watt (efficacy). Parabolic incandescent lamps have a slightly higher nominal life of 2000 hours and are typically used for highlighting displays. The life span of an incandescent lamp can be increased by utilizing a lamp with a rated operating voltage of 130 volts, compared to the standard 115 volt or 125 volt ratings typically available. A 130 volt rated lamp will increase the lamp life span by approximately 50%. Inherently, the light output of incandescent lamps decreases over time - making relamping very important where maintained illumination levels are required. Over-voltage operation increases light output and efficiency, but dramatically decreases lamp life. Vibration and shock also reduces the life of burning filament lamps. FLUORESCENT: A fluorescent lamp produces
light by the photofluorescence of phosphor coatings on the inside of the
tube wall. The normal rated life of fluorescent lamps is 20,000
hours, however, frequent ON/OFF switching reduces lamp life.
Although one of the most commonly ignored aspects of merchandising, proper lighting can be used to direct customer traffic and attract attention to certain displays to stimulate impulse buying - it serves as a silent salesman. Another consideration in providing well-lit stores is that they are more comfortable places to work in - improving employee attitude and selling enthusiasm. There are basically two approaches to merchandise lighting design. The first method involves providing general ambient lighting for the entire sales floor and adding layers of specific-task lights to highlight display areas. The second approach provides lighting specifically aimed at product displays and relies on spill-over light to illuminate the general areas. Both methods can be very effective or very detrimental to merchandising depending on how carefully they are applied. The best solution involves examining each proposed sales area layout, and determining which method best suits the owner's intentions based on the existing conditions, type of product to be displayed, and the economic budget available for the application. Certain layouts may allow a combination of the two approaches - optimizing economy and creating a stimulating atmosphere by varying the levels of illumination across the retail space. A consideration in lighting design that is often overlooked is coordination with the heating and air-conditioning systems. If the space is typically cold year-round, then utilizing incandescent lamps will provide supplementary heat and produce a warm atmosphere. If heat is excessive year-round, an increase in the use of fluorescent lamps can be used to lower heat build-up, resulting in lower air-conditioning operating costs. The choice of light source will also have an impact on the economics and atmosphere of the lighting installation. Typically, store lighting utilizes a combination of incandescent/halogen lamps and fluorescent lamps. The choice of fluorescent light source colour temperature should be determined based on the colour and material of the goods to be illuminated. High-end clothing stores will exclusively use incandescent/halogen sources whereas large chain-type stores will use fluorescent lamps only. Soft goods such as clothes or other textiles should be lit by a 3000Kelvin source to produce a warm feel. Outdoor clothing, appliances, and other similar equipment should be lit by a 4100 Kelvin source to highlight sharp detail. A general guideline is to use a source colour that will show merchandise as it would appear in the place it is to be used. Note that sunlight at noon has a colour temperature of approximately 5250 Kelvin, an overcast sky is 7000 Kelvin, and a clear blue sky has a colour temperature between 10,000 and 30,000 Kelvin. Recall that the higher the colour temperature, the cooler the atmosphere. The guiding principle in store lighting is to make the product appear to be brighter than the other areas within view. The counter display areas should be illuminated to approximately 100-200fc (1000-2000 LUX) or three to ten times the level of ambient lighting. High profit or impulse items should be located at the end of the counter or near the cash register and should have two to five times as much illumination as the rest of the counter. Incandescent spot-lights are generally very effective for this type of highlighting, as are in-counter fluorescent strip-lights. The intent of feature displays is to add sparkle and drama to the store in order to attract shoppers. Where these displays are at normal eye level or lower, they should be illuminated at two or three times higher than other displays. Elevated displays require three to five times as much illumination as non-feature displays to capture attention. Perhaps the most valuable space in a store is the display window. To be effective it must capture the attention of prospective buyers and must consist of dramatic or interesting displays in addition to a flexible lighting system. The lighting should be flexible enough to highlight a small display with spot-lights, and to flood the entire window with diffuse light. Dimmed lights and switching configurations can provide an economic means of achieving the required flexibility. The display window is even more crucial in open-front stores where the entire store interior is on display. COMMON MISTAKES IN MERCHANDISE LIGHTING: Even when a merchandising lighting design has been engineered properly, over time a system may tend to lose its ability to capture attention. This is usually a result of improper maintenance and operation of the lighting system. Typical areas of concern are listed below. (i) Spot-lamps are replaced with flood lamps, creating dull and non-vibrant presentations; (ii) Parabolic (PAR) lamps are replaced with reflector (R) lamps. Reflector lamps are not as effective or efficient as PARs; (iii) Directional spot-lighting is not aimed properly, or is aimed at angles that cause glare; (iv) High colour-rendering fluorescent lamps are replaced with less expensive cool-white lamps. Cool-white lamps tend to create a flat look to textile products and sales areas.
The following is a recommended list of lamp types to be incorporated into merchandise lighting design or retrofit applications. The recommendations make use of the most economical lamps that give the highest performance. Fluorescent fixtures in merchandising areas for non-clothing goods are to be lamped with standard-size F32-T8 tubes with a colour temperature of 4100 Kelvin, tri-phosphor coated. In locations where clothing or textile products are to be displayed, use of F32-T8, 3000 Kelvin, tri-phosphor coated lamps is recommended. Fluorescent fixtures in all non-public spaces are to be lamped with F32-T8 cool-white tubes. All track lighting fixtures are to have PAR38 Sylvania CAPSULYTE or Philips Master-Line, 150 watt lamps, with 130 volt rating. Both these lamps have light outputs equal to 250 watt standard PAR lamps. Use 90 watt ratings of the above lamps to obtain output equal to 150 watt standard PARs. Obtain narrow flood, spot, or narrow spot configurations as required. Use spot if display is greater than 7ft from the track fixture, and use a narrow spot where a display is up to 10ft from the track fixture. Do not attempt to light objects that are greater than 10ft from the track fixture. MR-16 and similar lamps shall be limited to specialty merchandise, where the lamps can be mounted less than 5ft from the object or display. The use of these lamps is to be minimized. Lighting systems are usually noticed only when they are not working properly. The benefits of a lighting system maintenance program cannot be overstated, particularly in merchandising since it has been observed that good lighting helps to increase sales. [6] The accumulation of dust, dirt, and grease on the lamp, reflector, and lens of a fixture decreases its light output, while consuming the same amount of power. Furthermore, a decrease in light output over time is an inherent characteristic of all lamps, particularly in incandescent sources. Failing to replace lamps that have burnt-out not only leads to lower lighting levels, but to an appearance of neglect - which can inhibit a customer from making a purchase. The regularly-scheduled cleaning of a lighting system prevents the build-up of light-absorbing dirt and results in maintained visibility and a better utilization of light energy. The cleaning of a lighting system should be performed at the most economical time for the owner, once or twice a year, making use of the most efficient methods and equipment available to reduce the time and materials required. A clean lighting system improves the appearance of the entire merchandising area, increasing customer comfort and satisfaction. Part of the maintenance program should include periodic cleaning of walls, or repainting as required. Another maintenance aspect deals with the relamping of fixtures. The lamps can be replaced as they fail (spot relamping), or the entire system of lamps can be replaced prior to failing (group relamping). Although spot relamping would appear to make the most economic sense, there are several advantages to group relamping that should be seriously considered. When lamps are replaced as a group, labour costs per lamp are reduced due to set-up time required each time spot replacement is required. Group relamping also means higher maintained levels of illumination. With fluorescent lighting systems, the lamps would be replaced before they burn out, eliminating the distraction associated with blinking, flashing, colour variations, and brightness differences between adjacent old and newly-replaced lamps. Furthermore, the life of fluorescent ballasts is reduced when operated with burnt-out or improperly functioning lamps. LIGHTING SYSTEM MAINTENANCE SCHEDULE: The following is a guideline to creating a maintenance program schedule: (i) Survey the entire lighting system and determine the quantity of lamps required for each different lamp type, and record; (ii) Perform weekly inspections of the lighting system to monitor dirt accumulation and lamp failures; (iii) Bi-annually clean lamps, fixture housings, reflectors, and lenses with a damp cloth - do not use cleaning solutions. Make sure that power is OFF, and that lamps are cool to the touch; (iv) The average life of a fluorescent lamp is 20,000 hours. Perform group relamping at 15,000 hour intervals or every 3.5 years; (v) The average life of a
130 volt rated CAPSULYTE lamp is 3,000 hours. Perform group relamping at
2,500 hour intervals or every 6 months.
REFERENCES [1] "Profiting From Lighting
Modernization"; National Lighting Bureau;
[2] Philips Lighting Handbook; North American Philips Lighting Corporation; USA; 1984 [3] Erhardt, Louis; Lighting
Design + Applications; VOL 17/No.1;
[4] Brainard, George et al; "Pineal-Retinal Relationships"; Academic Press; O'Brien and Klein, USA; 1986 [5] Shankman, Sarah; "Light in the Laboratory"; Lighting Design + Applications; VOL 16/No.5; Illuminating Engineering Society of North America, USA; 1986 [6] Fink, Donald and Beaty,
Wayne; Standard Handbook for Electrical
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