Comparison of the Electromagnetic Spectra of Common Light Sources

Laboratory Experiment pubs.acs.org/jchemeduc

Comparison of the Electromagnetic Spectra of Common Light Sources: A General Chemistry Laboratory Exercise Edward Maslowsky, Jr.* Department of Molecular and Life Sciences, Loras College, Dubuque, Iowa 52001, United States S Supporting Information *

ABSTRACT: Emission spectra were recorded for several commercially available light sources using a small, portable UV−vis spectrophotometer equipped with a UV−vis fiber optic probe. These spectra help students gain a clearer understanding of electromagnetic spectra by illustrating the relationship between the spectral patterns and appropriate uses of these light sources. Calculations allow students to compare the carbon dioxide emissions, energy requirements and cost of using different light sources, and the impact of more efficient lighting on global warming.

KEYWORDS: High School/Introductory Chemistry, First-Year Undergraduate/General, Interdisciplinary/Multidisciplinary, Laboratory Instruction, Public Understanding/Outreach, Inquiry-Based/Discovery Learning, Applications of Chemistry, IR Spectroscopy, Nonmajor Courses, UV−Vis Spectroscopy

S

Independence and Security Act mandates that all light bulbs sold in the United States use 25−30% less energy by 2012− 2014 and have 70% greater efficiency by 2020.6

unlight reaching the surface of earth is a mix of UV, visible, and IR radiation (Figure 1), with the intensity at each

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BACKGROUND

Definitions of Common Package Label Terms

A requirement that packaging contain information allowing consumers to choose the most efficient lighting was Table 1. The Color Temperature and Common Associated Designations for White Lamps

Figure 1. Spectrum of sunlight (300−850 nm) recorded at noon (burgundy spectrum) and 1 h after sunrise (brown spectrum) on a clear June day. a

wavelength determined by factors that include the time of day and sky conditions.1 As the rising and setting of the sun controlled the rhythms of life until our ancestors learned to control fire,2 the current dominance of incandescent lamps over other lighting options3 is changing as new lighting choices emerge.4 Reportedly, 32% of global fossil fuel use in 2005 was for electricity production, and 19% of this electricity went to generating light.5 The voluntary United States ENERGY STAR program identifies and promotes energy-efficient lighting and estimates that replacing one light bulb in every American home with an ENERGY STAR version would annually prevent 9 billion pounds of greenhouse gas emissions, which equals those emissions from ca. 800,000 cars. 6 The 2007 Energy © XXXX American Chemical Society and Division of Chemical Education, Inc.

Color Temperature/K

Color Designationa

2700−3000 3100−3600 3700−4500 4600−6500

Soft or Warm White Bright or Pure White Cool White Natural Daylight

These terms do not describe light bulbs.

Table 2. Efficacy, Color Rendition Index (CRI), and Lifetimes of Common White Lamps Lamp Typea Incandescent Halogen Fluorescent (straight tube) CFL LED a

A

Efficacy (/lm/W)

CRI

Lifetime/h

10−17 12−22 30−110

98−100 98−100 50−90

750−2500 2000−4000 7000−24,000

50−70 27−92

65−88 70−90

10,000 100,000

Compact fluorescent light (CFL); light emitting diode (LED).

dx.doi.org/10.1021/ed200658y | J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

recycling workers to these releases. Therefore, fluorescent lamp recycling is encouraged.13,14 Mercury emissions can, however, be higher with mercury-free incandescent lamps than with mercury-containing CFLs when generating electricity from coal15,16 because more mercury-containing coal is needed to produce the electricity used by incandescent lamps than by more energy-efficient CFL replacements. Relatively indestructible solid-state lighting (SSL) uses solid semiconductor materials rather than the gas or vacuum tubes found in incandescent and fluorescent lighting. The development of stable, white, inorganic light-emitting diodes (ILEDs) has allowed SSL to emerge as an incandescent lamp alternative. White LED light can be produced by mixing light from red, green, and blue LEDs.17 Alternatively, blue light from a semiconducting AlInGaN LED is absorbed by a cerium-doped yttrium−aluminum−garnet (Ce:YAG) phosphor coating that emits yellow and green light to produce a color mix that the eye perceives as white.17,18 LEDs produce the same quantity of light as incandescent lamps using 20−25% of the energy,11 and are ENERGY STAR rated. With longer lifetimes than incandescent and CFL lamps, they can outlive many devices in which they are used. LED uses, in single or multiple arrays, include flashlights, traffic signals, streetlights, holiday light strings, automobile tail lamps, backlighting in cell phone displays, and residential lighting.19 Organic LEDs (OLEDs), made from small, organic or polymeric semiconductors, have efficacies comparable to CFLs and ILEDs.20 Although ILEDs are point-light sources, OLEDs can be formed into sheets as “flat” light bulbs. Emerging uses include consumer electronics,21 windows that transmit sunlight by day and emit artificial light at night,22 and room-panels that permit controlled color temperature changes.23

implemented in the United States in 2012.7 The correlated color temperature (also known as the color temperature) indicates the shade or hue of white light in Kelvin (K) units.8 Lower color temperature red and orange shades are described as “warm”, and higher color temperature violet and blue shades are described as “cool”. The color temperature of sunlight varies from dawn (2500 K) to noon (5500 K or, at high latitudes, 8000 K) to dusk (3250 K).9 Figure 1 shows the relatively higher sunlight intensity from 300 to 500 nm at noon that gives a higher color temperature than after sunrise. The color temperature of light can possibly1 influence mood and psychology and, in turn, behavior that includes store purchases and workplace productivity.9 Table 1 compares color temperature ranges and some common qualitative terms frequently used with commercial white lamps. The color rendition index (CRI) that represents how accurately white lamps show the apparent color of an object as it would appear in sunlight10 is sometimes listed. It ranges from 1 to 100, with values of 80 or higher appropriate for most indoor lighting uses.8 The number of watts (W) gives the rate of energy or electric power used, but lamps with the same wattage are not necessarily equally bright. The lumen (lm) is an SI unit for the brightness or quantity of light emitted by a lamp.8 Lumen ratings allow consumers to compare the relative brightness of different lamps. Dividing the lumen rating by the wattage gives the energy-efficiency or ef f icacy of a lamp. Reducing the quantity of radiation emitted in the UV and IR regions and increasing it in the visible region improves efficacy. Table 2 compares the efficacies, CRI values, and lifetimes of common white lamps.8

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Characteristics of Common Lamps

Incandescent lamps contain a tungsten filament that is sealed in a bulb together with unreactive gases such as nitrogen or argon. They are not ENERGY STAR rated because 90% of the electricity passing through the filament is emitted in the IR region as heat rather than as visible radiation.11 The color temperature of clear incandescent lamps (ca. 2700−2800 K) can be changed with different inner-bulb coatings. Halogen lamps are modified incandescent lamps with the tungsten filament encased in a quartz or hard glass envelope that is surrounded by the halogens iodine or bromine. The envelope permits the higher temperatures and pressures that, though absent in incandescent lamps, are required for proper halogen lamp operation. It is treated or enclosed to filter the less than 1% of UV radiation emitted at these higher temperatures.12 Halogen lamps have the same approximate CRI as incandescent lamps, but higher color temperatures (ca. 2900−3200 K). Although not ENERGY STAR rated, the 25% higher energy-efficiency relative to incandescent lamps reduces operation cost.11 Electricity in tube-shaped and compact fluorescent lamps (CFLs) excites mercury vapor that then emits UV radiation. The UV radiation is, in turn, absorbed by the inner white phosphor tube coating that fluoresces to emit visible radiation. Fluorescent lamps produce the same quantity of light as incandescent lamps using 25−35% of the energy and already meet the 70% efficiency standard set for United States lamps by 2020.11 The 3−5 mg of mercury in each CFL is only released if the lamp is broken, but there are concerns over mercury releases on disposing CFCs in landfills, and with exposing landfill and

EXPERIMENTAL OVERVIEW In a 2 h session, nonscience majors in a general education global warming course work in groups of 2−4 students to record the emission spectra (300−850 nm) of several commercially available light sources and the absorbance spectrum of an acetone extract of a leaf. Time also permits an explanation of the apparatus used and a discussion of the experimental results. Spectral analysis helps students develop a clearer understanding of electromagnetic spectra by showing the relationship of the spectral pattern and appropriate use of a light source. Calculations allow students to compare the carbon

Figure 2. Setup for recording the (A) emission spectra of light sources using an optical fiber probe (with a 6500 K CFL used in the figure), and (B) absorption spectrum of an acetone extract of a leaf using a cuvette holder/light source. B



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HAZARDS Touching light sources either during or immediately after use could cause burns. Acetone is flammable, can cause skin, eye, and respiratory tract irritation, and is harmful if swallowed or inhaled. It should be used in a fume hood. Appropriate eye protection should be worn during the experiment.

dioxide emissions, energy requirements and cost of using different light sources, and the impact of more efficient lighting on global warming. A General Electric Web site24 shows spectral power distribution curves for some of the lamps used, but not the relationship of the spectra to application. An Ocean Optics, Inc. (Dunedin, Fl) USB 2.0 UV/vis Spectrometer System (model no. CHEMUSB4000-UV/vis, 200−850 nm bandwidth) equipped with a cuvette holder/light source or UV/vis fiber optic probe (2 m length, 400 μm diameter, 300−1100 nm wavelength range) is connected to a laptop through the USB hub to obtain spectra. Emission spectra (300−850 nm) are recorded by directing the fiber optic probe at sunlight and, in an otherwise darkened room, at each light source that is clamped to a ring stand (Figure 2A). Spectral peak intensities are adjusted by changing the fiber optic probe orientation relative to the light source. Each group of students cuts one-quarter of a leaf into roughly 5 mm squares that are placed in a 100 mL beaker with acetone (10 mL) and crushed with a metal spatula. The resulting clear green solution is poured into a 10 mm quartz cuvette, and its absorbance spectrum (300−850 nm) is recorded using the cuvette holder/light source (Figure 2B).

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RESULTS AND DISCUSSION

Standard and Halogen Incandescent Lamps

Although the emission spectra of incandescent lamps with color temperatures of 2900 K (Figure 3A) and 2850 K (Figure 3B) are similar, students see that the relatively lower radiation output from ca. 500 to 600 nm produces a lower color temperature for the latter, and that both lamps waste much energy as heat in the IR region at wavelengths greater than 700 nm. Students can also compare the emission spectra of the two incandescent lamps to that of a halogen lamp (Figure 3C). CFL Lamps

Emission spectra of CFLs with color temperatures of 3000 K (Figure 4A) and 6500 K (Figure 4B) illustrate that CFLs, in contrast to incandescent and halogen lamps, emit little radiation in the IR region and produce noncontinuous spectra. Comparing the spectra of the two CFLs shows students the

Figure 4. Emission spectra of 13 W CFL lamps with color temperatures of (A) 3000 K and (B) 6500 K.

relation between different color temperatures and the different relative intensities from ca. 425 to 525 nm. LED Lamp

Students observe that the white light from a six LED flashlight array (Figure 5) arises from two major peaks and that the spectrum is less continuous than those of incandescent bulbs, but more continuous than those of CFLs. They also see that most radiation is emitted in the visible region.

Figure 3. Emission spectra of a 60 W (A) 2900 K incandescent, (B) 2850 K incandescent, and (C) clear halogen lamp. C




Figure 5. Emission spectrum of a six white LED flashlight array.

Figure 6. Emission spectrum a 250 W red-colored infrared heat lamp.

Figure 8. Emission spectra of (A) 15 W tube-shaped fluorescent and (B) 60 W incandescent grow lights, and (C) absorbance spectrum of an acetone extract of a sugar maple leaf.

into the IR region relative to those of white incandescent and halogen lamps, which explains the use of this lamp as a heat source. Violet-colored black lights are often used to illuminate posters containing fluorescent pigments. Ideally, their maximum spectral intensity lies at the opposite end of the visible spectrum from that of infrared heat lamps. Students observe that the incandescent version (Figure 7A) emits radiation in both the IR and longer wavelength visible regions and the desired shorter wavelength visible region to make the fluorescent version (Figure 7B) more energy-efficient. Grow lights promote plant growth, and the spectra of both fluorescent (Figure 8A) and incandescent (Figure 8B) types are shown. Comparing the grow light emission spectra with the absorbance spectra of plant leaf pigment solutions allows students to explore the effectiveness of both lamp types in maximizing energy emissions in the wavelength ranges absorbed by photosynthetic plants. The absorbance spectrum of an acetone extract of a sugar maple leaf is shown (Figure 8C).

Figure 7. Emission spectra of a (A) 60 W incandescent black light (recorded at two different relative intensities by pointing the fiber optic probe at the light source more directly to produce the upper spectrum and less directly to produce the lower spectrum) and (B) 15 W tube-shaped fluorescent black light.

Specialized Lamps for Specific Applications

Specialized lamps emit specific wavelengths of radiation using different phosphor coatings, colored filters, or tinted glass. The red-colored infrared lamp emission spectrum (Figure 6) is seen as shifted more to the longer wavelength end of the visible and

Calculations

To determine the CO2 emissions for different light sources that use fossil fuel generated electricity, the bulb wattage is first D




Table 3. Comparison of Annual Energy Use, CO2 Emissions from Bituminous Coal-Generated Electricity, and Electricity Cost for a 60 W Incandescent, and 60 W Equivalent 13 W CFL, and 10 W LED Light Sources Used 6 h a Day Lamp Type

Wattage/W

Energy Used?(kW h)

CO2 Emissions?lbsa

Cost/US$b

Incandescent CFL LED

60 13 10

131 28.5 21.9

265 57.6 44.2

14.41 3.14 2.41

a

Ref 25 gives an emission value of 2.02 lbs CO2 per kW h of electricity generated with bituminous coal. bRef 26 gives kW h costs to generate electricity for Iowa and other states. 109, E1215−E1220. http://www.pnas.org/cgi/doi/10.1073/pnas. 1117620109 (accessed Aug 2013). (3) Johnson, J. Chem. Eng. News 2007, 85 (Dec 3), 46−51. (4) DiLaura, D. Opt. Photonics News 2008, 19 (Sep), 23−28. (5) International Energy Agency Publication. http://www.iea.org/ newsroomandevents/pressreleases/2006/june/name,20191,en.html (accessed Aug 2013). (6) ENERGY STAR Home Page. http://www.energystar.gov (accessed Aug 2013). (7) Federal Trade Commission light bulb package label requirements. http://www.ftc.gov/opa/2010/06/lightbulbs.shtm (accessed Aug 2013). (8) Lighting term definitions, and a comparison and description of different lighting. http://www.energysavers.gov/your_home/lighting_ daylighting/index.cfm/mytopic=12030 (accessed Aug 2013). (9) Jou, J.-H.; Wu, M.-H.; Shen, S.-M.; Wang, H.-C.; Chen, S.-Z.; Chen, S.-H.; Lin, C.-R.; Hsieh, Y.-L. Appl. Phys. Lett. 2009, 95, 0133071−013307-3. (10) Description of the Color Rendering Index and how it is determined. http://www.lrc.rpi.edu/programs/nlpip/lightinganswers/ lightsources/whatisColorRenderingIndex.asp (accessed Aug 2013). (11) Comparison of energy efficiencies for different lighting. http:// www.energysavers.gov/your_home/lighting_daylighting/index.cfm/ mytopic=11975 (accessed Aug 2013). (12) Publication LSD 1-2003 (R2011) describes the operation and potential risks of halogen lamps. http://www.nema.org/Standards/ Pages/All-Standards-One-Page.aspx (accessed Aug 2013). (13) National Electrical Manufacturers Association Home Page. http://www.lamprecycle.org (accessed Aug 2013). (14) EPA site on recycling mercury containing lighting. http://www. epa.gov/cfl/cflrecycling.html (accessed Aug 2013). (15) Engelhaupt, E. Environ. Sci. Technol. 2008, 42, 8176. (16) Echelman, M. J.; Anastas, P. T.; Zimmerman, J. B. Environ. Sci. Technol. 2008, 42, 8564−8570. (17) Ashdown, I. Opt. Photonics News 2007, 18 (Jan), 24−30. (18) Ball, D. W. Spectroscopy 2007, 22, 14−18. (19) Cassarly, W. J. Opt. Photonics News 2008, 19 (Jan), 19−23. (20) Tullo, A. H. Chem. Eng. News 2008, 86 (Jul 14), 20−21. (21) LEDs Magazine. http://ledsmagazine.com/features/10/3/15 (accessed Aug 2013). (22) Dezeen Magazine video about Philips OLEDs. http://www. dezeen.com/2012/12/13/oled-philips-lumiblade/ (accessed Aug 2013). (23) LEDs Magazine. http://ledsmagazine.com/news/8/2/9 (accessed Aug 2013). (24) General Electric spectral power distribution curves. http://www. gelighting.com/na/business_lighting/education_resources/learn_ about_light/distribution_curves.htm (accessed Aug 2013). (25) U.S. Energy Information Administration data on pounds of CO2/kWh emitted by fossil fuels in electricity production. http:// www.eia.gov/tools/faqs/faq.cfm?id=74&t=11 (accessed Aug 2013). (26) U.S. Energy Information Administration data on state electricity rates. http://www.eia.gov/electricity/monthly/ (accessed Aug 2013).

divided by 1000 to convert it to kilowatts. The kilowatt value is then multiplied by the hours used to give the kilowatt−hours (kW h) of electricity. Multiplying the kilowatt−hours by the amount of CO2 released per kilowatt−hour of fossil fuel generated electricity gives the amount of CO2 emitted. Annual CO2 emissions (Table 3) were calculated assuming that a 60 W incandescent and 60 W equivalent LED and CFL lamps were used 6 h per day, and that the electricity was generated from coal. Similar calculations can be performed using CO2/(kW h) emission values from electricity generated with other fossil fuels.25 Lamp operating costs were calculated using available kilowatt−hour electric rates, with those in Table 3 based on a rate of $0.11/(kW h) reported for Iowa.26 Data in Table 3 show potential reductions in global CO2 emissions and consumer operating costs associated with replacing incandescent bulbs with more energy-efficient lighting.

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CONCLUSIONS The relatively low cost and simplicity of the instrumentation employed in this experiment make practical its use in introductory and general education chemistry courses. One goal has been to stimulate instructor and student interest in using it with other lighting sources that are currently available or that will be introduced in the future. Another is to aid students to gain a clearer understanding of the relevance of electromagnetic spectra in developing more energy-efficient lighting for different applications that can reduce greenhouse gas emissions.

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ASSOCIATED CONTENT

S Supporting Information *

Procedures, questions, and data sheet for students; list of required instrumentation and supplies. This material is available via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Instrumentation used in this work was funded through a National Science Foundation CCLI grant (number 0632817). REFERENCES

(1) Potential effects of exposure to full-spectrum light sources. http://www.lrc.r pi.edu/programs/nlpip/lightinganswers/ fullspectrum/claims.asp (accessed May 2013). (2) Berna, F., Goldberg, P., Horwitz, L. K., Brink, J., Holt, S., Bamford, M., Chazan, M., Proc. Natl. Acad. Sci. U. S. A., Early Ed. 2012 E


Comparison of the Electromagnetic Spectra of Common Light Sources

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