1. School of Physical and Geographical Sciences
PHY-30007 Physics Project 2015-2016
Multicolour Photometry of LEDs
Jacob Elliot Senior
Supervisor: Dr. Barry Smalley
27/4/2016
Word count approx: 4,100
3. Abstract
Presented in this paper is an investigation into the properties of LEDs; more specifically, if
they can operate in reverse to their traditional implementation, and act as light detectors. The
few studies in this area generally test whether if LEDs can be used as Sun Photometers, and
don’t explain the why it actually occurs. Some have suggested that there would be a peak
absorption wavelength, just as there is a peak emission wavelength, and the absorption peak
would be roughly 30nm shorter than the emission peak.
The data gathered during the experiments generally agreed with these findings, but not
all LEDs responded as expected. The absorption responses were then compared to that of a
Photodiode, although the LEDs were worse at detecting white light, they have potential uses
as spectrally sensitive light detectors, as different LEDs would absorb different amounts of
different coloured light contained in different light sources. Further work in the area could look
at how the efficiency changes over time and create the aforementioned light detector.
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6. Acknowledgement
I would like to take this opportunity to mention my greatest thanks to Dr Barry Smalley, for
his encouragement, guidance and helpful advice throughout this project. My sincere gratitude
is also due to Singham and Steve for running the labs, the module, and always being on hand
for technical and apparatus support.
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7. Chapter 1
Introduction
1.1 Background
1.1.1 Creation and Development
The initial Light Emitting Diodes (LED) were created as a consequence of competing semi-
conductor companies in the 1960s. The first was sold for 130 dollars, and initial uses were very
limited mostly replacing tungsten bulbs in punch card readers[1]. Since then they have become
an incredibly versatile piece of technology.
The invention of the Blue LED led to the creation of white LEDs, this caused an ’Illu-
mination Breakthrough’ which has led to a observation made by Dr Ronald Haitz in Nature
Photonics[2], named Haitzs law and states that: The cost per lumen will decrease by a factor
of 10 and amount of light generated per LED package increases by a factor of 20, for a given
wavelength, every decade.
1.1.2 Uses of LEDs
There has been a very noticeable increase in the number of LEDs in our everyday life,
especially over the last 20 years. A cultural change to try to be more energy efficient and
environmentally friendly has led to every new house being build with LED light bulbs installed,
while the venerable and inefficient filament bulbs are planned to be phased out completely.
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8. Introduction
The first LED backlit LCD TV was introduced by Sony in 2005. This brought about
huge advancements in not only the size and thickness of televisions, but also revolutions in
the weight and image resolution. The same technology was then applied to phones, and it
completely changed not only how we interact with them, but also what we use them for. Audi
introduced their now signature LED strip beneath the main lights of their new flagship super
car in 2008, now almost every new car’s lighting system is LED based (although advanced laser
light systems are around the corner). In the UK we are beginning to replace the orange sodium
street lights with white light LEDs which are more environmentally friendly, could potentially
make it safer at night, and produce less light pollution.
These are just a few of the new ways LEDs are being applied in 2016, which brings us to the
focal point of the project; if LEDs can emit light, can they absorb it? If they can, it presents
some interesting questions. Firstly, and most interestingly, why does this happen? How does
the absorption compare to the emission of light by the LED? Finally, how do they compare to
a Photodiode, the standard device used for detecting light in electronics.
1.2 The Physics and comparisons
1.2.1 How LEDs operate
To begin to answer these questions, we must begin by understanding the underlying me-
chanics of the LEDs themselves. They consist of a diode made up of two semi conductors.
One is doped with electrons and has a negative charge (N-Type), and the other has a positive
charge with ’holes’ due to a lack of electrons (P-Type). When a voltage is applied across the
LED, the electrons begin to move between the P and N semiconductors, across an area known
as a junction or band gap. When an electron and a hole rejoin, energy is released in the form of
a photon at a certain wavelength of light depending on the material of the semiconductor ele-
ment. This is a process known as Electroluminescence, essentially creating light with electricity.
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9. Introduction
Figure 1.1: Diagram showing how a red LED emits light
The colour of the photon produced is determined by the energy required to get an electron
and a hole to rejoin and is primarily governed by the equation:
E =
hc
λ
(1.1)
When the voltage applied to the semi conductor is increased, more electrons have the energy
to make the transit to the P-Type semi conductor, as a result, more of the P-Type’s holes are
filled by electrons, which increases the number of photons produced and is translated by our
eyes into a more luminous light.
1.2.2 Basis for the Project
It was expected that like other diodes, LEDs should be able to operate in a reverse bias.
Given incoming photons at a certain wavelength of light, electrons should be induced to move
back to the other semi conductor. This is known as a photocurrent, as photons are creating a
movement of electrons. it was also expected that there would be a point at which a maximum
photocurrent occurred, indicating the point at which the most amount of photons are being
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10. Introduction
absorbed.
There are very few items of literature around this topic. The main study [3] states that
the peak absorption wavelength of LEDs, should be 25-35 nanometres shorter than the peak
emission wavelength of the LED. The two processes described above can be compared to the
absorption and emission of photons from electrons in atoms, as when a photon is incident on
the atom, it excites an electron into a state where it can then produce a photon .When the
electron then moves back to its ground state, it releases the energy difference between the two
states in the form of a photon.
There have been a few papers using LEDs as light detectors [4,5,6]. However, these tend
to focus on their applications, with most focusing on using them as a sun or atmospheric
photometer. It seems that there has been very little study into the underlying mechanics and
properties of this behaviour, which is what this project was investigating.
1.2.3 Comparison to Photodiodes
Photodiodes are the current standard instrument used to as a light detector [4]. They
operate in a similar way to LEDs in that they also utilise semiconductors, but they use a
different diode type, using a P-I-N diode, rather than a P-N Type to increase the speed of the
response. As they generally are used to detect any light, they have a much broader range of
spectral operation compared to LEDs, and these differences should be evident in the results.
The other major difference between the two is the shape of the plastic cap. LEDs have a
rounded end to the cap as this helps to concentrate some of the emitted light when operating
in a forward bias. The end of photodiodes, on the other hand, are generally flat, as they do
not have any light to focus.
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11. Introduction
1.3 Research Questions
With this taken into account, aims and research questions could be made, and for this
project, were the following:
1. Can LEDs act as Light Detectors?
2. If so, how is this related to the LEDs emission response?
3. How does the absorption response compare to that of a Photodiode?
4. What possible uses could this be suitable for?
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12. Chapter 2
Experimental
2.1 Equipment and operation
2.1.1 Separating light
To determine whether LEDs actually absorb light, an optical spectrometer with a prism
was initially used to diffract the light into its composite wavelengths, with a pure white light
source so the entire visible spectrum of light was produced. However, this did not provide us
with a great enough resolution across the spectrum to be able to discern different wavelengths
of light. The prism was replaced by different diffraction gratings, starting at approximately
100 lines/mm and going up to 600 lines/mm. Whilst they made a significant improvement in
resolution of the spectrum, giving a much wider band of wavelengths, it was still too narrow
to use to determine the individual wavelengths of light entering the LEDs. Finally, an optical
Monochromator, which not only splits the light into a spectrum at a great enough resolution,
it also allows us to control the wavelength of light that is outputted to the LED by turning a
dial on the equipment.
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13. Experimental
Figure 2.1: Experimental Setup
The Monochromator operates by taking the incoming light, reflecting it off a Collimating
mirror, which reflects onto a diffraction grating. This then splits the light into its component
visible wavelengths. The separated light is then reflected onto a Focusing Mirror, which then
reflects the light into the output, where the LED is placed. On the right of the Monochromator,
there was a rotatable dial. This changed the angle of the focusing mirror, which in turn changed
the colour of light that was incident on the LED.
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14. Experimental
2.1.2 LED Positioning
The next issue we needed to overcome was to find a way to hold the LED in place at the
output of the Monochromator, so it was perpendicular to the incoming light. The LED holder
would ideally also block out ambient light from potentially causing its own induced voltage
across the LED, effecting the results.
Using the open source 3D modelling software Blender, a cube was designed with two circular
holes of different sizes. After converting to different file types and using the ReplicatorG
software, we were able to 3D print the design and use it as our LED holder for the experiment.
The cube was placed over the output of the Monochromator (normally where the eye piece
would be) using the larger hole and then the LED was put into the smaller hole of the cube.
After a few tweaks, everything seemed to fit well.
Figure 2.2: Render of the LED Holder made in Blender, with a Red LED emitting.
It was noticeable from an early stage that the angle between the incoming light from the
Monochromator and the LED needed to be as perpendicular as possible. It was hypothesised
that this was due to the rounded end of the LED. This lens is used to concentrate light when
operating in a forward bias, but in a reverse it limits the amount of light that reaches the
semiconductor. This explains why the cap of photodiode is flat as it must allow a greater
amount of light to reach the semiconductor. This issue could be resolved with further refinement
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15. Experimental
of the LED Holder, as the hole to insert the LED was slightly too large, meaning it was difficult
to position the LED exactly the same every time.
2.1.3 Increasing the gain of the reading
At this point, it was thought that readings could begin to be taken, and some initial data
could be gathered to see if the predicted phenomenon occurred in LEDs; however, there was
another issue. The voltage induced across the LED was too small to take meaningful readings.
The solution to this was to build a signal amplifier.
A Trans-impedance Operational Amplifier was used as this converts currents into voltages,
with the current in this case being the photocurrent induced across the LED. Given that it is
an amplifier, it also increases the gain of the signal too, which is proportional to the resistance
of the resistor applied across the amplifier. A 56MΩ (Rf in Figure 2.3) resistor was chosen in
the end as this produced enough of a gain boost to take begin taking readings. The amplifier
produces a voltage from a current according to the following equation:
Vout = −IP Rf (2.1)
Figure 2.3: Diagram of a Trans-Impedance Op-Amp[7]
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16. Experimental
2.2 Method
2.2.1 LEDs and the Monochromator
Initial tests began using 5 LEDs:
1. Red (λ=630nm, SC08051)
2. Orange (λ=601nm, SC08052)
3. Yellow (λ=590nm, SC08054)
4. Green (λ=525nm, SC08057)
5. Blue (λ=460nm, SC08061)
Three of the LED’s had a voltage induced across them when exposed to the light from the
monochromator. Red with a peak wavelength absorption at 610nm, Orange at 590nm and
Yellow at 570nm. The Green and Blue LEDs induced a voltage when exposed to the pure
white light but unfortunately, nothing happened when used with the monochromator. It is
hypothesised that this is simply due to the way the LEDs were manufactured, as when a
Halogen bulb was used with the monochromator, with 100 times the counts for all wavelengths
of light, still no photocurrent occurred.
As each of the LEDs peak absorption wavelengths contained a multiple of 10, for the data
gathering, it was decided that a 700-440nm range would be used decreasing in increments of
10, as this will incorporate one reading inside each of the LEDs peak absorption wavelength.
It was found that the voltmeter reading tended to fluctuate often and was quite sensitive to
external influences such as movement around the instruments, believed to be due to the high
gain of the amplifier. Therefore,the voltage induced was regarded to be stable when it did not
fluctuate for 5 seconds. Taking this all into consideration, the data in the table in the appendix
was attained for voltage induced across each of the three LEDs, at different wavelengths.
LEDs with different coloured plastic caps were tested briefly however none responded to
incoming light, our theory for this is that we don’t know the actual colour of the light that their
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17. Experimental
semi conductors produce, but it’s likely to not be the colour of the lens because otherwise there
wouldn’t be much point to using the coloured lens. All the light that reaches the photodiode
will, of course, be the colour of the plastic around it. Therefore, the majority of light that is
incident on the Semi-conductor wouldn’t necessarily have photons with the correct amount of
energy to induce a photocurrent. These graphs from this data is also available in the appendix.
2.2.2 Comparisons
To compare these results to the emission of light from the LEDs, the emission response curve
for each LED needed to be produced. This data was collected using a Princeton Instrument
Acton SP2150 Monochromator with a computer running Spectrasense software. The software
plots a graph with the wavelength of light against count rate and shows all of the wavelengths
of light emitted by the light source, and how much light is emitted at each wavelength. This
method provided us with the peak wavelength of emission, for each LED, which was needed to
confirm the results found in literature that the peak absorption wavelength of light is 25-35nm
shorter than the emission wavelength. The emission responses of the light sources used were
also calculated using this technique.
To find the absorption response for the photodiode, the same method was used when the
absorption response for the LEDs were being found. Theoretically, this graph should be very
similar or even identical to the graph of the emission response for the light source, as the
photodiode should simply detect all of the wavelengths of light produced by the light source,
when they are split into their component wavelengths.
During the taking of readings for the absorption response of each LED it was noticed that
with each repeat, the photocurrent across the LED decreased, compared to the previous reading
of the same wavelength of incident light. This was an unexpected result, so to investigate, a
National Instruments NI USB-6008 data logger was connected to a computer running, Signal
Express software. This then measured the voltage outputted from the Op-Amp for extended
periods of time.
The final area that was to be investigated was to see how the LEDs compared to photodiodes
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18. Experimental
at detecting light over different distances. A Red LED and the tungsten bulb were used as
the light sources, and the photocurrents induced were measured across a Red LED, and a
photodiode over 10, 20 and 30 cm distances. The detector was exposed to the light source for
3 seconds, and then left until the reading was close to 0, which was then repeated four times.
For the LEDs to be considered to be useful light detectors, they need to be as good as or better
than photodiodes under the same conditions.
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19. Chapter 3
Results and Discussion
The following graphs were produced from the data gathered for the absorption and emission
response curves for each LED
Figure 3.1:
Red LED Absorption, Peak: 620nm
Figure 3.2:
Red LED Emission, Peak: 650nm
Figure 3.3:
Orange LED Absorption, Peak: 600nm
Figure 3.4:
Orange LED Emission, Peak: 630nm
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20. Results and Discussion
Figure 3.5:
Yellow LED Absorption, Peak: 580nm
Figure 3.6:
Yellow LED Emission, Peak: 600m
When comparing the emission and absorption responses, a decline in the voltage induced
from wavelengths greater than or equal to the peak wavelength is seen. It is roughly the same
for both the emission and absorption graphs, the latter is possibly slightly steeper. The earlier
part of the curves are completely different. The absorption responses have long tails increasing
to the peak, with voltages being induced across all the LEDs with incoming light of greater
than 480nm. This can be explain as some the incoming photons at the shorter wavelengths of
light still have enough energy to cause an electron to move to the other semiconductor. The
number of photons with the correct energy gradually increases as the energy of the incoming
light is lowered, and the wavelength increases towards the peak absorption wavelength.
The emission spectrum of the Tungsten bulb, measured using the monochromator and a pho-
todiode was recorded as the following:
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21. Results and Discussion
Figure 3.7: Absorption response of a Photodiode
using separated light of a Tungsten Bulb
Figure 3.8: Emission response of a Tungsten
Bulb
From this figure it can be seen that there was very little light absorbed at the lower wave-
lengths of light. It was thought that this was the reason that the Blue and Green LEDs did
not respond to the separated light source, however, as previously mentioned it is now thought
to be due to the manufacturing.
To find the broadness of the instrument, the white light source was changed to a Sodium
lamp, which is known to have two peak wavelengths of emission at 588nm and 589nm. A pho-
todiode (SFH203P(cpc)) was used at the output of the monochomator, to detect the broadness
of the instrument’s resolution, and any other possible errors in the equipment. What was found
when this was implemented was a single broad peak at 582nm (Figure 3.9), this was corrected
in our red, orange and yellow LED graphs by simply as an 8nm shift of the absorption graphs
towards the longer wavelengths, which also brings our results closer to results found in literature
[3].
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22. Results and Discussion
Figure 3.9:
Photodiode Absorption Response using a Sodium Lamp, Peak: 582nm
This test could be extended to using lamps with different elements, producing different
coloured light. The difference between the peak wavelength from the monochromator and the
known peak wavelengths of the elements in use would be compared, and then it could be
found whether the error is uniform for all wavelength of light outputted, or dependant on them
somehow.
Another factor that could bring error into the results is that at the output for the monochro-
mator there are two slides that change the light coming to the detection area, where the LED
is located.
Figure 3.10:
Diagram showing how the monochromator output can be changed, varying the wavelength light on the
LED
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23. Results and Discussion
It can be seen in Figure 3.10 that the actual range of wavelengths of light that is outputted
by the monochromator is still quite broad, and definitely not completely monochromatic. This
final source of error was difficult to investigate as to change the position of the slides uniformly
we needed to remove the LED, and replace it into the exact same position afterwards. Further
refinements to the LED Holder could provide the solution to this problem. The size of the
gap between the slides also effected the photocurrent induced, as having the gap too large and
the light is not monochromatic enough, however if it was too small the photocurrent induced
would be too small to take any meaningful readings. It was decided that this error would be
accounted for in the graphs with 10 nm error bars.
The graphs for the data collected from testing the colour capped LEDs can be found in
the appendix, along with the data collected for Figures 3.1, 3.3 and 3.5. Trying to find an
explanation for the decrease of voltage for each repeat was trickier than expect, an example
of the graphs produced can also be found in the appendix. The final data that was collected
was studying the responses of LED and photodiodes to different lights, over 10, 20 and 30cm
distances. Using this data, the following graphs were produced:
Figure 3.11:
White Light Source
Red LED 10cm away
Figure 3.12:
White Light Source
Photodiode 10cm away
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24. Results and Discussion
Figure 3.13:
White Light Source
Red LED 20cm away
Figure 3.14:
White Light Source
Photodiode 20cm away
Figure 3.15:
White Light Source
Red LED 30cm away
Figure 3.16:
White Light Source
Photodiode 30cm away
Figure 3.17:
Red LED Light Source
Red LED 10cm away
Figure 3.18:
Red LED Light Source
Photodiode 10cm away
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25. Results and Discussion
Figure 3.19:
Red LED Source
Red LED 20cm away
Figure 3.20:
Red LED Source
Photodiode 20cm away
Figure 3.21:
Red LED Source
Red LED 30cm away
Figure 3.22:
Red LED Source
Photodiode 30cm away
These graphs show that the photodiode was faster to detect a source of light than the LED.
This is not wholly surprising as mentioned in the introduction, photodiodes have a P-I-N diode,
which are designed specifically to respond quickly to incoming light sources. The photodiode
reached the capped voltage level (10V) very quickly where as the LED didn’t reach that amount
of voltage. The LED still produced a similar response to the photodiode, therefore, it could
still have potential as an alternative light detector.
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26. Chapter 4
Conclusions and Areas for Further
Work
4.1 Predictions
Although it was a struggle to get reliable readings at first, once the equipment was correctly
set up a good set of data was recorded. It showed that the peak of absorption tends to be
30nm shorter than the peak of emission for each LED. As the ’preferred’ state of an electron in
this situation is combined with a hole in the P-Type semiconductor, and the band gap of the
semiconductor remains the same, there is slightly more energy required to remove the electron
from the hole to the N-Type semiconductor. This additional required energy explains the 30nm
shorter peak wavelength, as shorter wavelengths of light have more energy.
The other prediction made about the exponential tail was also correct. These tails are ex-
plained by photons at shorter wavelengths of light have higher energies, so there are still photons
with enough energy to induce a photocurrent. As the wavelength moves further away from the
peak absorption wavelength, the number of these photons gradually decreases, producing the
absorption tails. Two slight anomalies were found in the Red and Orange absorption graphs,
both seemingly at the point where the voltage was half the value of the voltage at the peak
absorption, although due to the time constraints of this investigation, this could be a potential
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27. Conclusions and suggestions for further work
area for further study.
There were slight voltages induced at wavelengths longer than the emission wavelength, this
was unexpected as the photons at longer wavelengths of light should not have enough energy
to cause the LED to absorb them. These were explained once we found the emission responses
for each of the LEDs, which contained a similar proportion of photons emitted at wavelengths
longer than the peak emission wavelength.
The difference between the peak wavelengths for absorption and emission could also explain
why the Blue and Green LEDs didn’t induce a voltage, as the absorption peak wavelengths
would be predicted at 430nm and 495nm. At this wavelength, the voltage induced on the other
LEDs was only slightly above zero, and tending towards it at these points.
4.2 Areas for further work
An unexpected finding from the data is that the LEDs seemed to lose efficiency fairly
quickly, with each repeat generally showing a lower induced voltage at each wavelength of
incoming light, as the light source was not turned off between repeats.
Trying to find how the LEDs drop in efficiency over time and how this compares to pho-
todiodes is a potential area to explore for the rest of the project. After attempting to find an
experimental set up that would work for all of the LEDs for a couple of weeks it was decided
that this was best left for further research, as it was taking up valuable time towards the end
of the project.
Another area that would have been liked to be investigated was the dependence between
the photocurrent induced and the angle between the LED and the incident light, as from the
experiments it was seen to have an effect, however, we do not have the time to fully explore
the dependency.
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28. Conclusions and suggestions for further work
4.3 Conclusion
To conclude, this research has uncovered some interesting and relatively unknown properties
of LEDs, they can indeed operate in a reverse bias, operating as light detectors. For general light
detection, however, a photodiode would generally still be more suitable, despite the photodiode
used being quadrupole the price of the of the LEDs used. An application where LEDs would
out perform photodiodes is being used in a cost-effective spectrally sensitive light detector.
Photodiodes require an interference filter to detect specific colours of light, which adds further
cost to each photodiode. Four different coloured LEDs would have different photocurrents
induced, depending on the composite colours of the light, for the cost of one photodiode.
The final suggestion to be proposed is regarding the name, as Light Emitting and Absorbing
Diodes is clearly a truer description of their actual properties.
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29. References
[1] LED Lights - How it Works - History. 2016 http://www.edisontechcenter.org/LED.
html. [Accessed 23 April 2016].
[2] Haitz, R. Haitz’s Law, Nature Photonics 1, 23.
[3] Mims, 1992 Sun photometer with light-emitting diodes as spectrally selective detectors Appl.
Opt., 31, 965-6967.
[4] O’Toole and Diamond, 2008 Absorbance Based Light Emitting Diode Optical Sensors and
Sensing Devices. Sensors 2008, 8, 24532479.
[5] Acharya et al 1995 Compact light-emitting-diode sun photometer for atmospheric optical
depth measurements. Applied Optics 34, 1209-1214.
[6] Acharya 2005 Spectral and emission characteristics of LED and its application to LED-
based sun-photometry, Optics and Laser Technology, 37(7):547-550.
[7] Transimpedance amplifier - Wikipedia. 2016. https://en.wikipedia.org/wiki/
Transimpedance_amplifier#/media/File:TIA_simple.svg. [Accessed 24 April 2016].
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30. Appendix
A.1 Colour Capped LED data
Figure A.1: Red Capped LED Figure A.2: Orange Capped LED
Figure A.3: Yellow Capped LED Figure A.4: Green Capped LED
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31. Appendix
Figure A.5: Output of SignalExpress software, recording the output of the
Op-Amp over time.
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