Characterization of Light Emitting Diodes (LEDs) and Compact Fluorescent Lamps (CFLs) by UV-Visible SpectrophotometryC. Mark Talbott, Ph.D., Product Manager – Spectroscopy, Robert H. Clifford, Ph.D., Industrial Business Unit Manager, Shimadzu Scientific Instruments, Columbia, MD, USA

IntroductionABSTRACT: The acceptance and commercial utilization of Compact Fluorescent Lamps (CFLs) and, more recently, Light Emitting Diodes (LEDs) have grown significantly in the past five years. This change was fueled in part by the United States Congress 2007 legislative mandate to eliminate sales, and phase out the use, of conventional incandescent lighting beginning in 20121. This mandate has ultimately increased research into CFL and LED technologies. This poster demonstrates the use of a typical laboratory UV-Vis spectrophotometer, with no modification to the bench or software, to measure and characterize CFL and LED lamps. Using a customized accessory, spectral characteristics such as peak wavelength (λp), Full Width at Half Maximum (FWHM), centroid wavelength (λc), dominant wavelength, color, and color purity were readily determined for a variety of commercial CFLs and LEDs.

Figure 1: UV-Vis emission spectra of a typical compact fluorescent

lamp (CFL) (violet) and a Mercury lamp (green).

LED BandwidthsFigure 6 shows the spectral emission of a red LED emitting at 634 nm and a red diode laser emitting at 653 nm. The bandwidth of the LED (16 nm FWHM) is not as narrow as that of the laser (1.5 nm FWHM). However, even with the large bandwidths LEDs are considered to be monochromatic sources. This can be evidenced by examining the position of the CIE x,y color coordinates for the individual LEDs. Figure 7 shows the spectral emission of a series of LEDs. Accurate spectral acquisition and measurement of LEDs require a spectrophotometer with a monochromator capable of resolving these narrow bandwidth sources. The UV-1800 spectrophotometer’s 1nm bandwidth is well within the resolution requirement for accurate LED spectral acquisitions.

Figure 6: UV-Vis spectral emission

scans of a red LED at 634 nm

and a red diode laser at 653 nm.

LED Equilibrium Measurements

• Compact Fluorescent Lighting technology is the current main alternative to incandescent lighting. Compact Fluorescent Lamps (CFL) reportedly con-sume 75% less energy than incandescent counterparts.2 CFL lamps do contain a small quantity of mercury which may offer a potential future safety risk both for homes and in landfills. A typical UV-Visible spectrum from a CFL is shown in Figure 1.

• Light Emitting Diodes (LEDs) are a viable alternative to CFL technology and manufacturers are beginning to offer LED options as alternatives to CFL lighting. LEDs are semi-conductor devices with a p-n junction. When a forward voltage is biased across the p-n halves of the junction, photons of light are emitted. Because of the large refractive indices of the junction materials the photons tend to leave the LED die at narrow angles between the junction. Typical LED design technology places the LED die on an anode aluminum reflector. The cathode is bonded to the top of the die with a gold wire and the complete package is encapsulated in a polymer that has appropriate optical properties. The end of the plastic encapsulation can be shaped to form a lens, adding directionality to the LED’s output.3

• A typical yellow LED design is shown in Figure 2. The reflector, gold wire, and LED die can readily be seen. In addition, the LED was biased with a low forward voltage to demonstrate that the photon emission is from the area between the p-n junction. Another LED design that is typical for white LEDs is to imbed a blue LED die in a luminescent gel (Figure 3). When the blue LED die is forward biased, the blue photons stimulate the gel to fluoresce, providing broad spectrum white light. A UV-Vis scan of such an LED (Figure 4) clearly shows the 455 nm blue excitation peak and the broader white spectrum.

• This poster will demonstrate the use of the Shimadzu UV-1800 scanning spectrophotometer to characterize and measure CFL and LED properties. A mercury lamp calibration post is offered as an optional accessory for the UV-1800 spectrophotometer. An arm was attached to this post that allowed vertical height adjustment, Figure 5. To the arm was attached a clamping assembly for LEDs and a mirror to direct light from CFLs housed outside the lamp compartment into the entrance slit of the monochromator. No modifications or changes were made to the spectrophotometer or its lamps to perform the tests. In addition, Shimadzu’s UVProbe software and UVPC Color Analysis software provided the functionality required to acquire all spectra collected and presented in this poster.

Figure 2: Typical LED Architecture Figure 3: White LED Architecture

Figure 7: Spectral acquisitions of commercial LEDs to include colors of:

UV361, UV375, UV400, Blue, Teal, Aqua, Green, Yellow, Orange, Red,

Deep Red, and IR.

Figure 5: UV-1800

Spectrophotometer with

modified mercury lamp

calibration post including the

LED clamp and mirror.

Figure 7 shows the spectral acquisitions for each of these commercial LEDs exhibiting a given dominant color. Table 1 shows reported and measured values for each LED. Characteristic LED information obtained from these scans were:

Figure 8 demonstrates the linear relationship expected between the center wavelength (color) observed and the forward voltage. The spectral scans and measurements demonstrate the value of being able to monitor this relationship when comparing LEDs, in design, and in manufacture.

With the variety of CFL coating phosphors available, CFL technology offers many color options that may be more suitable for various environments and work spaces. The ability to measure and quantify color values is paramount to the successful design and testing of new CFL phosphors. Figures 9 and 10 show in tabular and graphical format, the calculated x,y chromaticity values for spectra acquired from various commercial compact fluorescent lamps.

Two important abilities for successful manufacturing, quality control, and comparison of LEDs is the measurement of dominant wavelength and purity. The dominant wavelength is defined4 as the point on the International Commission on Illumination (CIE) 1931 coordinates that is intersected by a line that is drawn from a theoretical illuminant “E” which has (CIE) coordinates located at the center (x=1/3, y=1/3) through the x,y coordinate values calcu-lated from the LED spectra. Purity is the ratio of the distance from the illuminant “E” chromaticity coordinates to the LED calculated coordinates over the distance from the illuminant “E” chromaticity coordinates to the coordinates of the dominant wavelength. Illuminant “D65” was used in the Color Analysis software as it has CIE x,y coordinates of (0.31271,0.32902) and most closely matches those of illuminate “E”.

Figure 11 shows the calculated CIE chromaticity and dominant wavelength values for the LED series tested. Figure 12 shows the plot of those values on the CIE chromaticity coordinate system. The plot shows that the LEDs on the red end of the spectrum lie on the border of the chromaticity plot and by definition would be expected to have a high purity value. LED colors starting with green, however, begin to fall away from the border and lie more in-ternal to the coordinate system. As such, the dominant wavelengths for these higher energy LEDs will be shifted from the measured center wave-length. This can be readily seen by comparing the calculated dominant wavelengths in Figure 11 with the dominant wavelengths reported in Table 1 and contrasting those to the measured center wavelengths in Table 1.

Peak Wavelength - Wavelength at the maximum spectral band energyCenter Wavelength - Wavelength at the center of the Full Width Half Max boundaryForward Voltage (Vf) - Voltage forward biased across the LEDPower - Measured voltage times current

Table 1: Measured and calculated values for the Spectral acquisitions of commercial LEDs to include colors of: UV361, UV375, UV400, Blue, Teal, Aqua, Green, Yellow, Orange, Red, and Deep Red.

Figure 8: Measured LED voltage versus measured

center wavelength.

Figure 9: Calculated CIE x,y chromaticity values for various

commercial compact fluorescent lamps.Figure 10: Plot of the calculated

commercial CFL on the x,y

chromaticity coordinates system.

CIE Chromaticity ValuesFigure 4: UV-Vis Emission

Spectrum of a White LED

LEDs can be designed to emit a specific narrow-band wavelength or dominant color. The wavelength that an LED emits is related to the bandgap energy of the semiconductor materials used in manufacturing the p-n junction. The equation, � = 1.24 / eV, relates the LED emission wavelength to the bandgap energy for a specific LED. For visible LEDs, this limits the bandgap energy to between 3.10 to 1.55 eV. An array of LED colors can be readily purchased from the commercial market. Emission spectra for these LEDs were acquired with the UV-1800 using a constant current power source to power the LEDs under test so that the constant forward current (If) was limited to 19.18 milliamps. The forward voltage (Vf) was allowed to vary as needed by the LED. The spectral acquisitions were acquired using a fast scan speed and a fixed sampling interval of 0.5. UVProbe software was operated in the energy mode and the silicon photodiode gain was set at 3.

Figure 11: Calculated CIE x,y chromaticity and dominant

wavelength values for the LED series.

Figure 12: Plot of the calculated LED chromaticity coordinates.

LEDs reach an equilibrium operating temperature only after a period of time. The equilibrium temperature is a function of the ambient temperature and the ability of the LED to lose heat through the package leads or attached heat sink. The ability to monitor an LED’s output to steady-state operation is important to evaluate spectral output changes and to assure that measurements taken coincide with normal operating conditions. The UV-1800 spectrophotometer was used to acquire spectra of selected LEDs from initial startup to steady-state operation. UVProbe software was set to acquire a series of repeat spectra with the wavelength range centered around the emission peak of the LED. Delay time between acquisitions was set to zero. In addition, forward bias voltage across the LEDs was also measured during this period with a current limit set to 19.18 milliamps. The peak pick function of UVProbe was used to determine the dominant wavelengths of the spectral scans. Time between scans was on the order of three seconds.

Equilibrium spectra for blue and red LEDs are shown in Figures 13 and 14. Table 2 gives the measured dominant wavelength and forward voltage for the initial and final scan for each LED. As steady-state operation was achieved, the red LED exhibited more change in dominant wavelength, intensity and voltage than did the blue LED. However, the calculated CIE x.y chromaticity values show no change between initial and final scans for both LEDs, Table 3. This data demonstrates that the Spectrophotometer was capable of recording LED output variances that would not be visually observable.

LED Pulsed SupplyIn some applications LEDs are not driven by a constant DC voltage but rather by a pulsed square wave with variable duty cycle. Frequencies of 100 Hz to 1 kiloHertz are typical.5 LEDs operated at these frequencies would show no visible flutter to the human eye. By using a pulsed power supply, a smaller duty cycle can be used which allows for current pulses that would normally be above the upper operating limit for the LED. In this way higher output can be achieved from a given LED with power conservation.

The spectrophotometer was used to characterize a yellow LED driven by a pulsed voltage source with duty cycle control. Pulse frequencies tested were 100 HZ, 500 Hz, 1 kHz, and 3.3 kHz. Pulsed duty cycles tested were 30%, 50% and 78%.

Figure 18 shows the spectral acquisitions from this testing. For all frequencies, as the duty cycle was reduced, the emission energy decreased linearly. Figure 19 shows this for the 500 Hz tests. Although there was a noticeable power decrease with the reduced duty cycle, Figure 20 demonstrates that there was no observable change in the calculated CIE x,y chromaticity coordinates or dominant wavelength.

Figure 21 shows the spectra graphed in groups of duty cycle. The graphs show for any given duty cycle, there was very little change in spectra emission power and/or dominant wavelength.

SummaryAs the move to replace incandescent lamps with the more energy efficient CFL and LED technologies continues, research will continue to be focused on the development of future LED systems. Much of this research will be conducted in academic and private laboratories that may not have access to the specialized equipment that is normally used to characterize LED systems.

This poster has demonstrated that the Shimadzu scanning UV-1800 spectrophotometer with no modifications to the bench or software, and only a minor modification to a readily available mercury lamp maintenance stand, can readily be used to measure and evaluate both CFL and LED characteristics that are of significant importance to their design, comparison, and manufacture.

Acknowledgements:

Suja Sukumaran, Ph.D, Jeff Head, MSc, Shimadzu Scientific Instruments, Maryland, USA

LED Temperature MeasurementsUnlike the minor spectral changes observed in LED equilibrium temperature measurements above, LEDs can exhibit significant spectral emission with larger changes in temperature. The spectral output temperature dependence of a yellow LED was measured using the UV-1800 spectrophotometer. Temperatures were measured at the LED with a thermocouple. Temperatures at the LED were altered using a heat gun on various settings and distances from the LED.

Figure 15 shows that significant changes in the LED’s spectral emission curves were observed with increasing temperatures. Temperatures tested were 24, 42, 51, 69, and 87 degrees C. Also observed in Figure 15 was not only a change in the intensity of the dominant spectral emission, but the appearance of a lower energy peak that was approximately 12 nm higher in wavelength and became more prominent as the temperature was increased.

Figure 16 shows numerically the temperature dependence of the dominant wavelength and overall peak area (integrated from 530 to 630 nm). The graph shows an increase in dominant wavelength of approximately 0.14 nm/C. The corresponding decrease in overall band area would represent a decrease in LED output intensity with increasing temperature. The measured forward bias voltage across the diode did not change with increasing temperature and remained constant at 1.057 volts.

The calculated CIE x,y chromaticity coordinates for these temperature-dependent spectral scans, Figure 17, shows the center wavelength changing as the temperature increases. The change in temperature would result in a visibly noticeable change in LED color.

Figure 13: Equilibrium

scans for the Blue LED

Figure 14: Equilibrium

scans for the Red LED

Table 2: Dominant Wavelengths and Voltages Table 3: CIE chromaticity values

Figure 15: Spectral emission of a yellow LED with increasing

temperatures (24, 42, 51, 69, and 87 Deg C).

Figure 16: Graph showing the temperature dependence of

the dominant peak wavelength (red) and overall peak area

(blue).

Figure 17: CIE plot showing the

change in x,y coordinates of the

yellow LED as the temperature

was increased.

Figure 18: Spectral emission curves for a yellow LED pulsed at frequencies (clockwise from top left) 100 Hz, 500 Hz, 1 KHz, and 3.3 KHz. In each graph the

spectra represent duty cycles of 34 %, 50% and 70%.

Figure 19: Spectral emission curves for a yellow LED pulsed at 500 Hz.

Figure 20: Calculated x,y chromaticity coordinates for the 500 Hz

pulsed LED spectra

Figure 21: Spectral emission curves for a yellow LED for various duty cycles (clockwise from top left) 70%, 50%, and 30%

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