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Light Sources for Architectural LightingEdition 1.1 - (2000)Copyright (c) 1999-2000 by Bill Williams1.0) INTRODUCTION The modern lighting specifier has
many different choices regarding the selection of light sources for architectural
lighting applications. This was not always the case. One hundred years
ago, practical electric lighting was largely limited to the incandescent
(electric filament) lamp. Early incandescent lighting was quite inefficient
by today's standards, however it offered a much better alternative to both
oil and gas lighting of previous days. During the 20th Century, there was
a explosion in the development of new light sources, starting with the
introduction of the fluorescent lamp in the 1930's. The latter half of
the Century resulted in many new light sources being either developed or
further refined.
Today, the lighting designer can chose from; incandescent, tungsten halogen, fluorescent, mercury, metal halide, high pressure sodium, low pressure sodium, cold cathode, neon, light emitting diode, induction lamps, sulfur lamps, and other new light sources. Light sources for the human environment are evaluated and selected in respect to their light or lumen output, their lumen efficacy, color temperature, color rendering index and life. Other considerations include initial cost, operating cost and maintenance cost. A brief description of the more important properties, follows:
2.0) LUMINOUS EFFICACYLamp 'efficacy' refers to the 'lumens per watt' produced by different types of light sources. The 'lumen' is the name of the unit used in measuring the amount of light given off by a light source. The higher the lumen efficacy, the more efficient the source is at producing light.Edison's first electric filament lamp produced only 1.4 lumens per watt. Research over the years slowly brought improvements in lamp efficiency. The common incandescent lamp of today, produces approximately 15-25 lumens per watt. The balance of the electrical energy is transformed into heat. It is estimated that 40 lumens per watt is about as high as can be obtained by a tungsten filament, just before it reaches the melting point (3370 degrees C.) About 1938 the lighting industry was revolutionized with the introduction of the fluorescent lamp. Lumen efficacies were suddenly raised and today the modern fluorescent lamp has an efficacy of between 65 and 100 lumens per watt. Luminous efficacy increased again with the development of the Metal Halide and High Pressure Sodium lamps, to up to 140 lumens per watt. The Low Pressure Sodium lamp pushed efficacy up to about 200 lumens per watt. Scientists have determined that if all of the electrical energy put into a lamp could be converted into white light, a lamp would give out 220 lumens for each watt of energy put in. Today the lighting designer should endeavor to use the most efficient light source possible while still maintaining proper color rendering qualities required by the specific lighting application. The following table shows how lumen efficacies have gradually increased over the years. SOURCE LUMEN EFFICACY (lumens/watts) ============================================================ Candle (equivalent) 0.01 Oil Lamp (equivalent) 0.03 ------------------------------------------------------------ Edison lamp (1879) 1.4 Carbonized bamboo (1879) 2.0 Carbonized cellulose (1891) 3.0 Metalized (Gem) (1905) 4.0 Drawn Tungsten (1911) 10.0 60W Tungsten C.C. (1968) 14.7 Filament Lamp (1970) 10 - 18 Tungsten Halogen (1980) 17 - 22 Stage/Studio Lamps (1980) 20 - 40 ------------------------------------------------------------ Cooper-Hewitt Lamp (1901) 13.0 Mercury Lamp (modern) 55 - 60 Fluorescent Lamp (1938) 65 -100 Metal Halide Lamp 85 -120 HPS Lamp 80 -140 LPS Lamp 120 -200 ============================================================ 3.0) COLOR TEMPERATUREAs most objects are heated they give off light. As the temperature is raised the object first glows red, then yellow, white, pale blue and then finally brilliant blue. Color temperature is a measure of the 'whiteness' of a source and is expressed in degrees K. (Kelvin).Tungsten filament lamps used for general lighting applications have color temperatures of between 2600 and 3000 degrees Kelvin. Stage and studio lamps used for theatre and television applications generally have a color temperature of between 3000 and 3400 degrees Kelvin. The specification of color temperature technically applies only to incandescent lamps. The term 'apparent color temperature' (or correlated color temperature) is often used however to describe the degree of 'whiteness' for fluorescent and H.I.D. (high intensity discharge) lamps. Color temperatures (and apparent color temperatures) of common sources are as follows: SOURCE COLOR TEMPERATURE (degrees K.) ============================================================ Incandescent filament lamp 2600-3000 Tungsten Halogen lamp 3000-3400 Metal Halide 3300-5700 Mercury lamp 55-60 High Pressure Sodium 2000-3200 Low Pressure Sodium 1600 Warm White Fluorescent (WW) 3000 Deluxe Warm White Fluorescent 2900 White Fluorescent (W) 3500 Cool White Fluorescent (CW) 4200 Deluxe Cool White Fluorescent 4200 Daylight Fluorescent 7000 Xenon 6000 ------------------------------------------------------------ Sunlight (at sunrise 1800 Sunlight (at noon) 5000 Sky (overcast) 6500 ============================================================ 4.0) COLOR RENDERING INDEXAlthough 'color temperature' is a measure of the visual 'whiteness' of a source, it does little to specify the source's ability to accurately show colors. This is referred to as the color rendering index (or CRI) of a light source.The modern electric filament lamp produces a continuous spectrum with all colors present. As a result this lamp has a high CRI of 100 as it renders all colors 'correctly' to the eye. Fluorescent and H.I.D. sources all have lower CRI's than 100 and are able to accurately render colors to varying degrees. A light source should be carefully selected to provide a high CRI only when needed. Specifically a high CRI should be used for color matching and other related applications. Lamps with CRI's of above 70 are typically used for the lighting of the human environment, including living and work areas. Some modern fluorescent lamps now have CRI'S of 85 or more. SOURCE COLOR RENDERING INDEX.) ============================================================ Incandescent filament lamp 100 Tungsten Halogen lamp 100 Mercury lamp 15-55 Metal Halide 65-80 High Pressure Sodium 22-75 Low Pressure Sodium 0 Warm White Fluorescent (WW) 52 Deluxe Warm White Fluorescent 73 White Fluorescent (W) 60 Cool White Fluorescent (CW) 66 Deluxe Cool White Fluorescent 89 Daylight Fluorescent 79 Xenon 95 ============================================================ 5.0) LAMP LIFEGenerally, a source type with the longest possible life should be selected for any particular lighting application. A long life lamp however should not be used at the expense of providing proper lumen efficacy, color temperature and CRI requirements.SOURCE LIFE - IN HOURS ============================================================ Incandescent filament lamp 1000-2000 Tungsten Halogen lamp 100-2000 Mercury lamp 12,000-24,000 Metal Halide 6,000-20,000 High Pressure Sodium 10,000-15,000 Low Pressure Sodium 16,000 Fluorescents 10,000-20,000 ============================================================ 6.0) SUMMARY AND CONCLUSIONSThe modern lighting specifier must be knowledgeable and aware of light source characteristics, particularly as applied to lighting for the human environment. It is important not to select a light source based on efficiency alone. Although lamp efficacy is very important, the color temperature and color rendering index must also be carefully considered. Other factors that must be considered include the initial cost of light source (and lighting system) in relationship to operating costs (and life). A light source with a low initial cost, may have a very high operating cost. Conversely a light source with a higher initial cost, may have a lower associated operating cost.The following tables provide a summary of efficacy, color temperature, color rendering index, and life - for common incandescent and discharge light sources. Table 1 - Incandescent SourcesLAMP EFFICACY CT. CRI AVG. HOURS ====================================================================== Candle (equivalent) 0.01 - - - Oil Lamp (equivalent) 0.03 - - - ---------------------------------------------------------------------- Edison lamp (1879) 1.4 - - 40 -45 Carbonized bamboo (1879) 2.0 - - 40-100 Carbonized cellulose (1891) 3.0 - - - Metalized (Gem) (1905) 4.0 - - - Drawn Tungsten (1911) 10.0 - - - 60W Tungsten C.C. (1968) 14.7 - - - ---------------------------------------------------------------------- Filament Lamp (1970) 10 - 18 2700-3000 K 100 1000-2000 Tungsten Halogen (1980) 17 - 22 2850-3050 K 100 1000-2000 Stage/Studio Lamps (1980) 20 - 40 3200-3400 K 100 100-1000 ====================================================================== Table 2 - Discharge SourcesLAMP EFFICACY CCT CRI AVG. HOURS ====================================================================== Cooper-Hewitt Lamp (1901) 13 - - - Mercury Lamp (modern) 55 - 60 3300-5700 K 15-55 12-24,000 ---------------------------------------------------------------------- Fluorescent Lamp (1938) 65 -100 3000-6500 K 52-92 10-20,000 HPS Lamp 80 -140 2000-3200 K 22-75 10-15,000 LPS Lamp 120 -200 1600 K 0 16,000 ---------------------------------------------------------------------- Metal Halide Lamp 85 -120 3000-5200 K 65-80 6-20,000 CSI = Compact Source Iodine 70 - 80 4000 K 85 500 Compact Iodine Daylight 80 - 90 5500 K 85 500 HQI (Osram) - 3000-6000 K 81-93 - XENON (Osram) - 6000 K 95 800-2,000 ====================================================================== CT = Color Temperature. CCT = Correlated Color Temperature CRI = Color Rendering Index. Bill Williams is a Canadian lighting designer and consultant, with over 30 years of experience in entertainment and architectural lighting. With over 500 project credits he has worked throughout Canada, and in the USA, Britain, Europe, South America and Asia. |
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