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 2012. 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.

1. Characterization of Light Emitting Diodes
(LEDs) and Compact Fluorescent Lamps
(CFLs) by UV-Visible Spectrophotometry
C. Mark Talbott, PhD, Robert H. Clifford, PhD
Shimadzu Scientific
Instruments, Columbia, MD, USA, 800-477-
1227, www.ssi.shimadzu.com

2. Introduction
ABSTRACT: 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.

3.  Compact Fluorescent Lighting technology is the current main alternative
to incandescent lighting. Compact Fluorescent Lamps (CFL) reportedly
consume 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.
Figure 1: UV-Vis emission spectra
of a typical compact fluorescent
lamp (CFL) (violet) and a Mercury
lamp (green).

4.  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 output3.
 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.

5. Figure 2: Typical LED Architecture Figure 3: White LED Architecture
Figure 4: UV-Vis Emission
Spectrum of a White LED

6.  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 5: UV-1800 Spectrophotometer with
modified mercury lamp calibration post
including the LED clamp and mirror.

7. LED Bandwidths
Figure 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.

8. 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 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:
Peak Wavelength - Wavelength at the maximum spectral band energy
Center Wavelength - Wavelength at the center of the Full Width Half Max boundary
Forward Voltage (Vf) - Voltage forward biased across the LED
Power - Measured voltage times current

9. 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.

10. Reported Measured Center Measured Voltage versus
LED Wavelength Volts mWatts Wavelength Bandwidth
UV361 361.000 4.54 87.1 362.915 13.13 Center Wavelength
UV375 375.000 3.422 65.6 374.45 9.84 700 y = -112.79x + 829.57
Center Wavelength (nm)
R² = 0.8511
UV400 400.000 3.364 64.5 391.455 10.95
600
Blue 470.000 3.037 58.2 460.48 18.86
Teal 490.000 3.059 58.7 490.23 25.42 500
Aqua 505.000 3.41 65.4 505.61 28.78 400
Green 525.000 2.818 54.0 518.41 27.22
300
Yellow 590.000 1.972 37.8 593.435 13.77
Orange 610.000 1.965 37.7 607.595 14.95 200
Red 630.000 1.919 36.8 633.245 16.37 1.5 2.5 3.5 4.5 5.5
DeepRed 660.000 1.831 35.1 653.075 21.27 Measured forward voltage (volts)
Table 1: Measured and calculated values for the Spectral
acquisitions of commercial LEDs to include colors of: Figure 8: Measured LED voltage versus measured
UV361, UV375, UV400, Blue, Teal, Aqua, Green, Yellow, Orang center wavelength.
e, Red, and Deep Red.
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.

11. CIE Chromaticity Values
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.
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.

12. 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 calculated 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 internal to the coordinate system. As such, the dominant wavelengths for
these higher energy LEDs will be shifted from the measured center wavelength. 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.

13. Figure 11: Calculated CIE x,y chromaticity and dominant
wavelength values for the LED series.
Figure 12: Plot of the calculated LED
chromaticity coordinates.

14. LED Equilibrium Measurements
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.

15. Figure 13: Equilibrium scans for the Blue LED Figure 14: Equilibrium scans for the Red LED.
LED Initial WL Final WL Initial Vf Final Vf
Blue 460.5 460.5 3.054 3.021
Red 651.0 652.0 1.844 1.834
Table 2: Dominant Wavelengths and Voltages Table 3: CIE chromaticity values

16. LED Temperature Measurements
Unlike 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.

17. Figure 15: Spectral emission of a yellow LED with increasing temperatures (24, 42, 51, 69, and 87 Deg C).

18. LED Output vs Temperature
602 8
y = 0.1438x + 587.91
R² = 0.9917 7
600
y = -0.0612x + 8.5964
Peak Maximum (nm)
Band Area (Energy^2)
6
R² = 0.9809
598
5
596 4
3
594
Peak 2
592
1
590 0
0 20 40 60 80 100
Temperature Deg C
Figure 16: Graph showing the temperature dependence of the dominant peak wavelength
(red) and overall peak area (blue).

19. 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 17: CIE plot showing the
change in x,y coordinates of
the yellow LED as the
temperature was increased.

20. LED Pulsed Supply
In 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
typical5. 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.

21. 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%.

22. Yellow LED pulsed at 500 Hz
90
80
Percent Duty Cycle
70 y = 11.659x + 1.7436
60 R² = 1
50
40
30
20
10
0
2 3 4 5 6 7
Band Area (energy^2) 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

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

24. Summary
As 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 Instrument, Maryland, USA

25. References
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