The present work is concerned with the study and feasibility analysis of various options for generating white light for semiconductor based work light systems. Analyzed were phosphor converted (PC) systems (state of the art) and a system for RGB color mixing (generation of white light via red, green and blue light sources). Specific parameters for the evaluation are the color temperature and color rendering index (CRI). An LED and a laser diode concept for the RGB white light generation were developed. As a reference, existing systems based on phosphor conversion were evaluated. In the literature, options for optimizing white light generation via laser diodes by 4, 5 or 6 laser diodes for color rendering indices of > 80 are described. The present investigation uses only three laser diodes, which as a result produces a CRI of up to 45. Compared to phosphor- converted modules with blue laser diodes with a CRI of 71, this is an improvement worthy value. Furthermore, the problems and obstacles which can prevent a possible industrialization are analyzed in detail. For a laser-based system, these are the strong temperature dependence and the difficulty of bringing homogeneous white light onto the road. Furthermore I introduce a new method (TM30) for calculating new color reproduction criteria and I compare it with the CRI method. This new method, known from the solid state lighting, shows an appreciation of RGB white light sources. The presented RGB LED prototype achieved with the TM30 measurement method better parameters by an average of 2% than a comparable PC system.
In summary, the present work is a comparison of both technologies (PC and RGB). Generation of white light with RGB LEDs shows great potential, particularly in special applications (applications requiring variable spectra). RGB LD systems can be realized at the current state of the art well with satisfactory values. For further development there is a need of optimized LD type and number, optimized thermal management and an optimized optical system. At present, the quality parameters of RGB LD Systems are on average 20% lower than for comparable PC systems.
Index
1 Motivation, Objectives
2 Introduction to Photometric Terms and Units
2.1 Colorimetry
2.1.1 Normalized Colors X, Y, Z
2.1.2 CIE Chromaticity Diagram
2.1.3 Correlated Color Temperature (CCT)
2.1.4 Color Rendering Index (CRI)
2.1.5 Spectral Structure of Light Sources
2.2 Generation of White Light
2.2.1 Phosphor Conversion
2.2.2 Color Mixing
2.3 Physiological Aspects
3 Light Amplification by Stimulated Emission of Radiation – LASER
3.1 Possible Concepts for Worklamps
3.1.1 Phosphor Conversion
3.1.2 RGB Color Mixing
4 State of the Art Systems
4.1 LED Worklamps
4.2 Currently available colored worklamps for special applications
4.3 State of the Art LASER Applications in Lighting
4.3.1 BMW i8 High Beam Laser Light
4.3.2 RGB Laser Scanning Module
5 Simulations
5.1 Visualization of different Color Temperatures within DIALUXevo
5.2 Calculation of CRI and CCT
5.3 Influence of Colored Light on CAL simulations within HELIOS
6 Development of RGB LED/LD Worklamp Concept
6.1 Optics Development within CAD
7 Prototypes
7.1 Investigation Goal
7.2 Color Tuning with Multichip LEDs
7.3 Color Tuning with Laser Diodes
8 Characterization
8.1 Measurement RGB LED Prototype – Quality
8.2 Measurement RGB LED Prototype – Light Distribution
8.3 Measurement RGB LD Prototype – Quality
8.4 Measurement RGB LD Prototype – Light Distribution
8.5 Measurement Temperature Dependency of Red LD
8.6 Measurement phosphor converted LED – Quality
8.7 Measurement phosphor converted LD – Quality
8.8 New calculation methods
9 Summary and Outlook (Innovations in Worklamp Light Quality)
10 Bibliography
11 List of figures
12 List of tables
Appendix 1
Appendix 2
Appendix 3
Danksagung
Zuerst möchte ich mich sehr herzlich bedanken bei Herrn Manfred Gerger, MBA sowie Herrn Dr. Hiebaum, Geschäftsführer der Hella Fahrzeugteile Austria GmbH, für die Möglichkeit, am Studium teilzunehmen, sowie für die Bereitstellung von Ressourcen seitens Hella Fahrzeugteile Austria GmbH. Besonderer Dank gilt auch Herrn Pfingstl, MSc, welcher, seinerseits Experte in sämtlichen Belangen der Lichttechnik und Optoelektronik, stets für Fragen ein offenes Ohr fand und immer Zeit für anregende Diskussionen fand.
An dieser Stelle auch vielen Dank an Herrn Univ. Prof. i.R. Dr. Jantsch, Institut für Halbleiter- und Festkörperphysik an der Johannes Kepler Universität Linz, welcher mit guten Hinweisen und fachlichen Anregungen zum guten Gelingen dieser Master Thesis wesentlich beigetragen hat. Durch die fordernde, aber dennoch familiäre Herangehensweise des gesamten Lehrkörpers wurde das fundamentale Wissen und Motivation für das Verfassen dieser Thesis vermittelt, vielen Dank dafür. Vielen Dank auch an Frau Dr. Becker- Unger, Leitung Zentrum für Interkulturelle Studien Fürstenfeld, welche den Lehrgang durch ihren Einsatz möglich gemacht hat sowie die Studenten stets wohlwollend betreut hat.
Auch allen Kollegen und Kommilitonen, sowie meiner Familie und Freunden möchte ich meine Dankbarkeit aussprechen, ohne die unzähligen anregenden Gespräche und Diskussionen sowie die andauernde Unterstützung wäre die Teilnahme am Studium kaum möglich gewesen.
Albert Krammer
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Großpetersdorf, am 31.03.2016 …..
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Bibliographic description
Albert Krammer:
Advanced light sources for premium worklamp systems
March, 2016 – Volume 90 pages
Hinweis zum Urheberschutz – Sperrvermerk
Auf Antrag des Verfassers wird diese Arbeit unter Berücksichtigung der Interessen des Unternehmens, an dem die Arbeit verfasst wurde, für 5 Jahre gesperrt, das heißt, sie wird bis zum Ablauf der Sperrfrist nicht öffentlich zugänglich gemacht und die Beurteiler und Betreuer dieser Arbeit sind zur Geheimhaltung verpflichtet.
Note about copyright laws – Blocking notice
This present study, including all its parts, is copyrighted and locked for a period of 5 years. Any beyond the limits of copyright law is not allowed without the permission of the author.
Kurzdarstellung
Die vorliegende Arbeit befasst sich mit der Untersuchung und Potenzialanalyse von verschiedenen Möglichkeiten zur Weißlichterzeugung in Arbeitsscheinwerfersystemen mittels Halbleitern. Untersucht wurden phosphorkonvertierende (PC) Systeme (Stand der Technik) sowie ein System zur RGB Farbmischung. Konkrete Parameter für die Bewertung sind die Farbtemperatur und der Farbwiedergabeindex. Ausgearbeitet wurden ein Light Emitting Diode (LED) sowie ein Laserdioden- Konzept für die RGB Weißlichterzeugung sowie als Referenz ein vorhandenes System basierend auf Farbkonversion durch Phosphore. In der Literatur werden die Möglichkeiten zur Optimierung von Weißlichterzeugung über Laserdioden mittels 4, 5 oder 6 Laserdioden für Farbwiedergabeindices von >80 beschrieben. 1 Die vorliegende Untersuchung arbeitet mit lediglich 3 Laserdioden, welche als Ergebnis einen Farbwiedergabeindex (Color Rendering Index, CRI) von bis zu 45 hervorbringt. Verglichen mit phosphorkonvertierenden Modulen mit blauen Laserdioden mit einem CRI von 71, ist dies ein verbesserungswürdiger Wert. Weiters werden die Probleme und Hürden genauer analysiert, welche eine eventuelle Industrialisierung verhindern können. Für ein Laser basierendes System sind dies die starke Temperaturabhängigkeit sowie die Schwierigkeit, homogen weißes Licht auf die Straße zu bringen. Des Weiteren wird eine neue Methode (TM30) zur Berechnung neuer Farbwiedergabekriterien beschrieben und mit der CRI Methode verglichen. Diese neue Methode, bekannt aus der Allgemeinbeleuchtung, zeigt eine Aufwertung von RGB Weißlichtquellen (Weißlichterzeugung mittels Farbmischung: Rot, grün und blau). Der vorgestellte RGB LED Prototyp erzielt mit TM30 um durchschnittlich 2% bessere Parameter als ein vergleichbares PC System.
In Summe handelt es sich bei der vorliegenden Arbeit um eine Gegenüberstellung der beiden Technologien (PC und RGB). Weißlichterzeugung mit RGB LEDs zeigt ein großes Potenzial, insbesondere bei speziellen Anwendungen, die veränderbare Spektren erfordern. RGB LD Systeme sind zum derzeitigen Stand der Technik durchaus mit zufriedenstellenden Werten realisierbar, jedoch muss für weitere Untersuchungen eine optimierte LD Auswahl und Anzahl getroffen, ein optimiertes Thermomanagement vorgesehen und ein optimiertes optischem System angedacht werden. Derzeit liegen die Qualitätsparameter eines RGB LD Systems durchschnittlich um 20% schlechter als bei vergleichbaren PC Systemen.
Abstract
The present work is concerned with the study and feasibility analysis of various options for generating white light for semiconductor based work light systems. Analyzed were phosphor converted (PC) systems (state of the art) and a system for RGB color mixing (generation of white light via red, green and blue light sources). Specific parameters for the evaluation are the color temperature and color rendering index (CRI). An LED and a laser diode concept for the RGB white light generation were developed. As a reference, existing systems based on phosphor conversion were evaluated. In the literature, options for optimizing white light generation via laser diodes by 4, 5 or 6 laser diodes for color rendering indices of > 80 are described. The present investigation uses only three laser diodes, which as a result produces a CRI of up to 45. Compared to phosphor- converted modules with blue laser diodes with a CRI of 71, this is an improvement worthy value. Furthermore, the problems and obstacles which can prevent a possible industrialization are analyzed in detail. For a laser-based system, these are the strong temperature dependence and the difficulty of bringing homogeneous white light onto the road. Furthermore I introduce a new method (TM30) for calculating new color reproduction criteria and I compare it with the CRI method. This new method, known from the solid state lighting, shows an appreciation of RGB white light sources. The presented RGB LED prototype achieved with the TM30 measurement method better parameters by an average of 2% than a comparable PC system.
In summary, the present work is a comparison of both technologies (PC and RGB). Generation of white light with RGB LEDs shows great potential, particularly in special applications (applications requiring variable spectra). RGB LD systems can be realized at the current state of the art well with satisfactory values. For further development there is a need of optimized LD type and number, optimized thermal management and an optimized optical system. At present, the quality parameters of RGB LD Systems are on average 20% lower than for comparable PC systems.
List of abbreviations
Abbildung in dieser Leseprobe nicht enthalten
1 Motivation, Objectives
At the moment, the tree most important aspects for developing innovative lighting products are to cover and integrate the issues “safety”, “comfort” or “integration”, illustrated in figure 1.
To cover the “Comfort” demand, more investigations are needed in the field of quality of the light – especially the Color Rendering Index (CRI) and the Correlated Color Temperature (CCT) are important topics.
Developing innovative products means monitoring the market, the competitors, potential technologies and investigating them. Pre- development is an important part of this process. In this thesis, the development of a concept for a LASER work lamp is a key component.
Recently innovative front light systems have been introduced – e.g. for the BMW i8 passenger car. This system is based on a blue laser which is focused onto a phosphor.2 The latter acts as a “point source” for intense white light which then is projected onto some “work area” by suitable optical elements like mirrors or lenses.
Lasers have advantages as compared to light emitting diodes (LEDs) as they have high efficiency also at high optical power and they have a high beam quality. Phosphor based systems, which can be driven both by lasers and LEDs, have the advantage of a simple optical layout and the potential of a high color rendering index. Their disadvantage is the unavoidable reduction of efficiency which results from the intended red-shift in the phosphor: part of the photons produced by a primary blue or ultraviolet source (LED or laser) is absorbed by the phosphor which produces yellow, red or green light, i.e., with lower photon energy. The difference in photon energy is dissipated in the form of heat.
Therefore in this work I investigate the possibility to construct a work light based on a combination of different semiconductor light sources such as LEDs and lasers, i.e., an RGB system and I compare it to conventional phosphor conversion systems.
Other prototypes with LED work lamps and different color tuning options will be used to establish a benchmark. Goal of this thesis is to establish also a benchmark for LED and LASER concepts for worklamps. The results should demonstrate whether currently available laser technology has the potential for a high quality worklamp. The task here is to create the best spectrum for the optimum illumination.
Also motivated by medical lighting, where a CRI > 96 is required, this thesis discusses ways to generate both – optimized CRI and CCT – for the potential future- premium- workplace- illumination.
2 Introduction to Photometric Terms and Units
2.1 Colorimetry
Colorimetry intends to describe the perception of light by humans. Some historical milestones of colorimetry are depicted in Fig. 4. The first relevant investigations were done in the early 1900 years; the first definition was proposed in 1931. Some parameters are still not well defined as standard.3
Abbildung in dieser Leseprobe nicht enthalten
Figure 2 - Milestones CIE evolvement, colorimetry as pretty new field of investigation and still not completely defined
2.1.1 Normalized Colors X, Y, Z
To understand the color rendering attribute of light sources, it is necessary to introduce a number of terms. The color stimulus is some optical radiation, which causes the impression of color in the human eye. The color stimulation is expressed by the normalized color indices X, Y and Z. The measured spectral ray density is described by a weighted superposition of the tristimulus value functions , or . These functions, which describe the relative sensitivity of the human eye, are defined as reference [3] and depicted in Figure 3.
Abbildung in dieser Leseprobe nicht enthalten
Figure 3 - CIE 2° Tristimulus Value Function4
The spectral ray density is measured with a spectrometer and the normalized color indices, X, Y and Z are calculated with the help of equations ( 1 ), ( 2 ) and ( 3 ).
Abbildung in dieser Leseprobe nicht enthalten
In most cases a k value of 683 lm/W is adopted. L is the spectral radiance of the color stimulus. The spectral radiance defines the emitted radiant power of a body at a frequency in a defined direction (at some solid angle). As an example, the spectral radiance of an ideal thermal emitter (“black body”) is depicted in a double- logarithmic diagram in Figure 4.
Abbildung in dieser Leseprobe nicht enthaltenFigure 4 - Spectral radiance of a black body of different temperatures – vs. wavelength on double logarithmic scale. The visible colors are indicated5
The so called chromaticity coordinates x, y and z are defined in equations ( 4 ), ( 5 ) and ( 6 ).
Abbildung in dieser Leseprobe nicht enthalten
Summarized in a diagram, the colors of combinations of x and y are illustrated in the “CIE Chromaticity Diagram”, see Figure 5. The third coordinate, z, is obtained from x + y + z = 1.
2.1.2 CIE Chromaticity Diagram
Abbildung in dieser Leseprobe nicht enthalten
Figure 5 - CIE Chromaticity Diagram. For each point within the outer curve the color is indicated. The outer curve represents monochromatic light and the wavelength is indicated. The inner curved line indicates color points which are obtained for black body radiation and the corresponding temperatures are indicated.
The Planck’ curve in Figure 5 illustrates the chromaticity coordinates x, y and z obtained for black body radiation. x characterizes mostly the red part, y the green part and z the blue part. At the edges of the outer black curve, the spectral colors can be found. The so called white point is located at x = y = 0,333. The white point serves to define the color “white”.6 On the black body curve, all color temperatures of thermal light sources are located. Because it is useful to describe the color temperature of all light sources, the expression correlated color temperature is used.7
2.1.3 Correlated Color Temperature (CCT)
The correlated color temperature is that color temperature, whose locus on the Planck curve is nearest to the locus of the light source under consideration in a uniform chromaticity diagram. The latter term is explained below. The CIE chromaticity diagram is not uniform; therefore the uniform color scale UCS as given in Figure 6 is used.8
Abbildung in dieser Leseprobe nicht enthalten
Figure 6 – UCS u v Color diagram (uniform color scale)
The axes of this diagram (u and v) are calculated with the following equations ( 7 ) and ( 8 ).
Abbildung in dieser Leseprobe nicht enthalten
Uniform is an expression, which describes that a change of the color value must create the same change of the visual impression. It is related also to the term “color tolerance”. An example on how to describe the color tolerance in the CIE chromaticity diagram is given in Figure 7.
Abbildung in dieser Leseprobe nicht enthalten
Figure 7 - MacAdam diagram in the CIE 1931 color space9
Figure 7 depicts the tolerances in the CIE chromaticity diagram, including the so-called MacAdam ellipses. Those ellipses indicate the border of points, whose distance to the reference is less than the just-noticeable-difference threshold. The ellipses vary in size, therefor the CIE diagram is non-uniform.10
2.1.4 Color Rendering Index (CRI)
In the “International Lighting Vocabulary”, the color rendering index is defined as “the aftermath of light to the color impression of objects in a conscientious or an unconscientious comparison to the color impression of the same objects under some light source”11. The CRI attribute of a light source (of a test light source) always relates to a reference light source with the same correlated color temperature.12
The CRI has a very important role in the workplace illumination sector. The user wants to see the objects in “natural” colors from well-known light sources like filament bulbs or simple daylight. Measurements yield an value (the closer the value to 100, the better the color visibility).
Basis of the calculation method are the test colour samples (TCS) from the “Munsell Color Atlas”13, depicted in Figure 8.
Abbildung in dieser Leseprobe nicht enthalten
Figure 8 - Test Color Samples (TCS) for calculating CRI from the Munsell color atlas14
In fact, there exist some problems with the value. The first 8 TCS’ are unsaturated colors (compare Figure 9) and not representative for natural colors. Unsaturated and saturated are expressions for the vividness and pureness of a color. For example, the saturated TCS (compare Figure 10) would be very representative for RGB light sources; the reflection spectrum would increase with the discontinuous spectrum of an RGB light source. 15
Figure 9 and Figure 10 depicts the spectral reflectance curves of the TCS colors. Spectral reflectance describes the ability of reflecting radiant energy of defined wavelengths on an ideal diffusely reflecting surface.
Abbildung in dieser Leseprobe nicht enthalten
Figure 9 - Spectral curves of the TCS colors (unsaturated TCS01 - TCS 08)16
Abbildung in dieser Leseprobe nicht enthalten
Figure 10 - Spectral curves of the TCS colors (saturated TCS09 - TCS 14)17
2.1.5 Spectral Structure of Light Sources
Abbildung in dieser Leseprobe nicht enthalten
Table 1 – Examples of measured spectral structure of semiconductor based light sources – phosphor- converted LED and LD
2.2 Generation of White Light
State of the art semiconductor technology does not allow generating white light directly from the semiconductor LED, the so-called “die”. Therefore different possibilities exist to generate white light.
2.2.1 Phosphor conversion
State of the art LEDs and LDs for lighting applications are mostly realized with a blue- emitting chip and a phosphor, which convert part of the light.
Abbildung in dieser Leseprobe nicht enthaltenAbbildung in dieser Leseprobe nicht enthalten
Figure 11 - Different Phosphors under daylight (upper row) and under UV excitation (lower row)18
Two possibilities exist to convert light with phosphors. First, it is possible to coat a mirror (reflector) with phosphor substances and secondly it is possible to coat a lens (transparent material) or the LED directly with a phosphor substance. In both cases, white light emission is the target.19 The underlying principle is indicated in Figure 12, where the so- called Stokes shift is explained. Figure 12 depicts the difference of the band maxima of the absorption spectra and the emission spectra.
Abbildung in dieser Leseprobe nicht enthalten
Figure 12 - Stokes Shift: Light absorbed by the phosphor material (blue curve) causes emission (red curve) at lower frequency (longer wavelength)20
In solid state lighting (SSL), the LED is currently the most efficient white light source with high color quality.21 Apart from all the advantages of the LED technology, they have some performance limitation at higher output power, the so called efficiency droop, illustrated in Figure 13: the efficiency decreases continuously with increasing power, in sharp contrast to a laser diode where the efficiency increases and finally it saturates at high power. This is a main reason, why researchers have great interest in developing laser driven lighting systems.
Abbildung in dieser Leseprobe nicht enthalten
Figure 13 - Power Conversion Efficiencies versus input power density of blue LED/LD22
2.2.2 Color Mixing
Advanced applications (medical, shop illumination, museum & art etc.) require high quality light. There are two possibilities to achieve more or less white light of appropriate quality. The combination (i) of a base color (blue) and its complementary color (yellow) enables the generation of white light. The combination of the 3 base colors (ii) enables the lighting engineer also to create an advanced spectrum, to be seen in Figure 14.23
Abbildung in dieser Leseprobe nicht enthaltenAbbildung in dieser Leseprobe nicht enthalten
Figure 14 - RGB Color mixing CIE Diagram. Left: When using a blue and a yellowish light source (e.g. PC systems), in theory every point on the blue line can be adjusted. Right: When using 3 different colors (here: red, green and blue) every point in the triangle can be adjusted.24
Depending of the three colors used, the realizable color field can be extended. In Figure 14 possible combinations are demonstrated. To get the biggest possible triangle, the ideal combination would be a combination of three corner points of the triangle. With an appropriate electronic control unit, all points within this triangle could theoretically be realized. In the case of only two light sources, all colors on the connecting line can be adjusted. For the impression of white light, particular intensity ratios of the 2 or 3 light sources are necessary.
2.3 Physiological Aspects
Kruithoff and Wald described a comfort zone for human visions. It is a zone limited by two curves, which describes the pleasant ratio of color temperature and illuminance (lux). In Figure 15 one can see, that for low color temperatures of light (warm light, <3000K) also relatively low illuminance values in a rather narrow range are agreeable and recognized as comfortable. Higher illuminance creates annoying conditions and lower ones impair cognition of details. This may be seen as compatible with evolution – for thousands of year, the light of fire was the only one generated and used by humans if sun light was not available. On the other hand, color temperatures above 5000K require higher illuminance, >500lx to be perceived as comfortable light.25
[...]
1 (Soltic 2013)
2 (Darmstadt, 10th Internatonal Symposium on Automotive Lighting 2013)
3 (Khanh, et al. 2013)
4 Compare website CIE
5 (Vienna 2014)
6 (Khanh, et al. 2013)
7 (Witting 2014)
8 (Witting 2014)
9 (Wikipedia 2016)
10 (Wikipedia 2016)
11 (CIE 1987)
12 (Khanh, et al. 2013)
13 (Khanh, et al. 2013)
14 (Khanh, et al. 2013)
15 (Khanh, et al. 2013)
16 Compare website CIE
17 Compare website CIE
18 (Khanh, et al. 2013)
19 (Gutdula, et al. 2013)
20 (Gutdula, et al. 2013)
21 (Herrmann 2013)
22 (Jonathan 2013)
23 (Ohta und Robertson 2005)
24 https://en.wikipedia.org/wiki/Dominant_wavelength
25 (Witting 2014)
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