A Thesis Presented to

Faculty of Alfred University

PHOTOCHROMISM

IN RARE-EARTH OXIDE GLASSES

by

Charles H. Bellows

In Partial Fulfillment of

the Requirements for

The Alfred University Honors Program

May 2016

Under the Supervision of:

Chair: Alexis G. Clare, Ph.D.

Committee Members:

Danielle D. Gagne, Ph.D.

Matthew M. Hall, Ph.D. SUMMARY

The following thesis was performed, in part, to provide glass artists with a succinct listing of colors that may be achieved by lighting rare-earth oxide glasses in a variety of sources. While examined through scientific experimentation, the hope is that the information enclosed will allow artists new opportunities for creative experimentation.

Introduction

Oxides of transition and rare-earth metals can produce a multitude of colors in glass through a process called doping. When doping, the powdered oxides are mixed with premade pieces of glass called frit, or with glass-forming raw materials. When melted together, ions from the oxides insert themselves into the glass, imparting a variety of properties including color. The color is produced when the electrons within the ions move between energy levels, releasing energy. The amount of energy released equates to a specific wavelength, which in turn determines the color emitted. Because the arrangement of electron energy levels is different for rare-earth ions compared to transition metal ions, some interesting color effects can arise. Some glasses doped with rare-earth oxides fluoresce under a UV “black light”, while others can express photochromic properties. Photochromism, simply put, is the apparent color change of an object as a function of light; similar to transition sunglasses.

This thesis explores such photochromism in rare-earth oxide glass samples. I hypothesized that the reason for this is small ranges of color the glass absorbs, combined with gaps in the ranges of color certain light sources emit.

[ii]

Procedure

The samples were made by mixing various rare-earth oxides with the glass frit used in the art department, allowing for the most compatible data for the artists, and then melting the mixture. Eight samples, each with different dopants, were produced.

The samples were then observed under isolated lighting conditions to see what color they were. The light sources included: sunlight, incandescent, fluorescent, and LED. A photograph of each sample, under each light source was taken.

As anyone skilled in photography knows, the colors you observe don’t always match the colors in the photo. Cameras register color and light differently than the human eye, adding discrepancies in color between the two devices. Additionally, a picture viewed on a computer screen can look completely different than the same picture after it has been printed out. To correct for these discrepancies, a color standard was used to normalize the observed pigments. The color standard assigns numbers and coordinates to colors to ensure exact replication. Determining the standard color of each sample under each light source was done by a RAL color standard wheel. RAL color scaling is a German color standard that assigns a 4-digit number to each color. The number can then be converted to other color standard systems, allowing one to choose the best system that fits their needs. The RAL color standard wheel itself resembles a paint swatch book; but the colors are guaranteed standards. It should be mentioned that, while standardized, the color assignments are still only approximations of the true color of the samples. At first glance, the RAL color standard containing colors seems like a lot; but it does not account for every possible color. For example, the praseodymium oxide sample appeared a yellow- green color. The RAL color scaling did not even have an adequate approximation, and therefore another color standard, Hex, had to be used for that particular sample.

[iii] Through color leveling in Photoshop, colors were balanced similar to setting a white balance on a camera. This allowed the photographs to appear closer to what was observed with the naked eye. Correction was based on using the terbium oxide sample, which was clear under all lighting conditions, as the standard. Additionally, a white sheet of paper was used as a standard, neutral background. All photographs, raw and color corrected, can be seen in the Appendix. While the corrected images are more accurate to the observed colors, the raw images were included to showcase the colors and variations in “white light” emitted by the different light sources.

Using a special type of film over the camera lens, spectra of the different light sources were photographed. The film, known as a diffraction grating, acts similarly to a crystal prism; separating the white light into the component rainbow colors. Thus, one could see what colors combined to make the white light of each source.

Results

Most of the samples did not exhibit photochromism, staying relatively the same color under every lighting condition. However, the samples containing samarium ions (Sm3+) and dysprosium ions (Dy3+) fluoresced under black light. The Sm3+ sample glowed a bright red orange and the Dy3+ sample glowed a pale ivory color. These, however, were not investigated further, because the focus was placed upon reasons why certain glasses changed color with variations in visible light.

The only two samples to produce any photochromic qualities were the two samples containing neodymium ions (Nd3+) and holmium ions (Ho3+). In every light source except fluorescent, the Nd3+ sample appeared violet; while appearing blue under fluorescent lighting. The Ho3+ sample exhibited a similar trait; salmon pink under fluorescent, yellow under all others.

[iv] Because one of the samples was doped with both Nd3+ and Ho3+, it was expected that that sample would also show photochromic qualities; however, the sample, for the most part, remained a neutral blue-grey without any drastic color changes.

Spectroscopy was then conducted to determine what colors Nd3+ and Ho3+ absorbed, and what colors were transmitted. It turned out that the two ranges of wavelengths Nd3+ absorbs correspond to a blue and reddish-purple color. Similarly, Ho3+ possesses wavelength ranges that have complementary colors of yellows, reds, and oranges.

When looking at the photographs of the light spectra, most showed the standard, continuous rainbow gradient from red to violet. Interestingly, fluorescent lighting did not. Instead, its spectra showed black bars separating the colors. This indicated that there were gaps of wavelength ranges where the fluorescent lighting did not emit that color of light.

Conclusions

As expected, the different arrangements of electron energy levels cause the rare-earth ions to absorb only very small ranges of wavelengths. When photochromism is exhibited, it is due to the fact that these narrow ranges coincide with small gaps in the emission spectra of various lights. If the light does not emit that wavelength range, it cannot be absorbed by the glass sample. If it is not absorbed, its complementary color is not observed. When under fluorescent lights, the light does not emit the wavelengths necessary for the reddish-purple of the Nd3+. Therefore only the blue can be seen. Likewise, the light does not emit the wavelengths corresponding to Ho3+’s yellow, leaving some red and orange that appears as salmon pink. Under other light sources, the reds and oranges are usually canceled out, leaving only the yellow.

[v] Further work

As the sample containing both Nd3+ and Ho3+ contained the same amount of 2g of dopant as the other samples, it only contained 1g of each of the individual oxides. Due to the lower dopant concentration, there may have not been enough rare-earth oxide to produce a noticeable photochromic effect. The two ion types may have also negated each other’s coloration, resulting in the greyer tones. It would be an interesting study to further explore the potential photochromism in samples containing both Nd3+ and Ho3+.

[vi] ACKNOWLEDGEMENTS

I would like to extend my thanks to:

Dr. Alexis Clare, my thesis advisor, for her guidance and expertise in the subject matter,

Drs. Danielle Gagne and Matthew Hall, my Honors committee members, for their support and suggestions,

Professor Angus Powers and the Alfred University Hot Glass Studio for the System 96® glass frit used in the fabrication of the samples,

The Kazuo Inamori School of Engineering, NYS College of Ceramics at Alfred University, for access to laboratory equipment and the rare-earth oxides,

As well as everyone else who helped in this process.

[vii] TABLE OF CONTENTS

Page

I. Introduction 1 II. Experimental Procedure 3 A. Fabrication 3 B. Photography 3 C. Color Analysis 5 D. Spectroscopy 6 III. Results 6 A. Tb3+, Sm3+ & Dy3+ 7 B. Pr3+ & Er3+ 8 C. Nd3+, Ho3+ & Nd3+-Ho3+ 9 D. Emission Spectra 11 IV. Discussion 12 A. Nd3+ 14 B. Ho3+ 16 V. Conclusion 17 VI. Future Work 18 VII. Appendix 19 VIII. References 37

[viii] LIST OF TABLES

Page

Table I: Spectral colors and their complements 2

Table II: Coloration of each sample under all lighting conditions 7

Table III: All colors observed with CIELab and RGB conversions 19

Table IV: Specifications of all light sources utilized 20

[ix] LIST OF FIGURES

Page

Figure 1: Visual representation of CIELab coordinates 2

Figure 2: Setup for photographing samples under various lighting 4 conditions

Figure 3: Tb3+ sample under soft white incandescent a) raw and b) 8 corrected

Figure 4: Nd3+ sample under a) soft white CFL and b) soft white 9 incandescent

Figure 5: Absorption/Transmittance spectrum for the Nd3+ sample 10

Figure 6: Ho3+ sample under a) soft white CFL and b) soft white 10 incandescent

Figure 7: Absorption/Transmittance spectrum for the Ho3+ sample 11

Figure 8: Emission spectra of a) fluorescent b) daylight CFL c) soft white 12 CFL d) daylight incandescent e) soft white incandescent f) daylight LED g) soft white LED

Figure 9: A graphical representation of emission spectra from a variety of 13 lightings

Figure 10: Comparison of the absorption/transmittance spectrum of Nd3+ 15 and the emission spectrum of a CFL

Figure 11: Comparison of the absorption/transmittance spectrum of Ho3+ 17 and the emission spectrum of a CFL

Figure 12: Corrected Pr3+ sample under a) sunlight b) daylight 21 incandescent c) soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

[x] Figure 13: Corrected Nd3+ sample under a) sunlight b) daylight 22 incandescent c) soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

Figure 14: Corrected Sm3+ sample under a) sunlight b) daylight 23 incandescent c) soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

Figure 15: Corrected Tb3+ sample under a) sunlight b) daylight 24 incandescent c) soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

Figure 16: Corrected Dy3+ sample under a) sunlight b) daylight 25 incandescent c) soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

Figure 17: Corrected Ho3+ sample under a) sunlight b) daylight 26 incandescent c) soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

Figure 18: Corrected Er3+ sample under a) sunlight b) daylight 27 incandescent c) soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

Figure 19: Corrected Pr3+-Ho3+ sample under a) sunlight b) daylight 28 incandescent c) soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

Figure 20: Raw Pr3+ sample under a) sunlight b) daylight incandescent c) 29 soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

Figure 21: Raw Nd3+ sample under a) sunlight b) daylight incandescent c) 30 soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

[xi] Figure 22: Raw Sm3+ sample under a) sunlight b) daylight incandescent c) 31 soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

Figure 23: Raw Tb3+ sample under a) sunlight b) daylight incandescent c) 32 soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

Figure 24: Raw Dy3+ sample under a) sunlight b) daylight incandescent c) 33 soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

Figure 25: Raw Ho3+ sample under a) sunlight b) daylight incandescent c) 34 soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

Figure 26: Raw Er3+ sample under a) sunlight b) daylight incandescent c) 35 soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

Figure 27: Raw Nd3+-Ho3+ sample under a) sunlight b) daylight 36 incandescent c) soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

[xii] ABSTRACT

Oxides of transition and rare-earth metals are commonly used as colorants, adding pigment to glasses. However, some rare-earth oxides can cause the glass to exhibit photochromism, changing color under different lighting conditions. This thesis explores the extent of this photochromism. To accomplish this, glasses were produced with a variety of select rare-earth oxides added. These samples were then observed and photographed under different lighting. Light sources utilized include: fluorescent, LED, incandescent, and sunlight. Further spectral analysis was conducted on samples of note. While slight variations in color were observed across several of the samples, only holmium and neodymium oxide doped samples exhibited strong photochromism. Additionally, the samples containing samarium and dysprosium oxide fluoresced when exposed to ultraviolet “black light”. Through research into technical literature, it was found that some light sources, such as fluorescent, possess gaps across certain wavelengths of the visible spectrum. This, in conjunction with very narrow discrete absorption spectra of some rare-earth elements, is the cause the photochromic qualities of the glasses.

[xiii] I. Introduction

Glass makers have both knowingly and unknowingly used oxides of transition metals and rare-earth metals give color to glass for millennia; from intentionally adding cobalt oxide to produce beautiful blues in vases and bottles to iron oxide contamination giving panes glass a green tint that one sees along the edge. While any artist knows the importance of lighting a piece, how much can the lighting actually change its appearance? In rare instances, oxides added to the glass can change its color depending on the light the glass is exposed to; this is what is known as photochromism. In this thesis, the potential for photochromic properties of rare-earth oxide glasses is explored. However, before this can happen, one must first consider color’s relation to light.

Observed color is the result of the colored object absorbing select portions of wavelengths within the visible spectrum. The color one sees is a combination of all the wavelengths transmitted or reflected by the material, i.e. not absorbed. This unabsorbed light appears as the color complementary to the color associated with the wavelengths absorbed. For example, an object which absorbs orange light appears to be blue. Table I details ranges of visible light wavelengths, as well as the color associated with them and their complements. Color, however, is rarely monochromatic, being composed of a singular color at full intensity. Therefore, there are three main criteria for which a color can be defined: hue, saturation, and value. Hue denotes what spectral range the color is contained within; this is also commonly known as the name of the color (red, blue, green…). Saturation indicates the amount of white within the color. The less white in a color, the more saturated it is. For example, there is no range of wavelengths that denote pink; it is instead an unsaturated red. Value describes how dim or bright a color is; characterized by the amount of grey that makes up the color1.

[1] Table I: Spectral colors and their complements1

Wavelength (nm) Spectral Color Complementary color 400-420 Violet Green-Yellow 420-460 Violet-Blue Yellow 460-480 Blue Orange 480-495 Blue-Green Red 495-530 Green Purple 530-570 Green-Yellow Violet 570-590 Yellow Violet-Blue 590-630 Orange Blue 630-700 Red Blue-Green

Because color is so dependent on lighting, several systems have been created in attempts to standardize the observed color. One such system is the CIELab color system. Fully developed in 1976, colors are defined based on a three coordinate system; L, a, and b. The L coordinate describes the lightness of a color from a scale of 0-100. 0 equates to black, while 100 represents white. Other systems, such as RGB, lightness is determined based on relative ratios of three defining colors. In the case of the aforementioned system, these colors are: red, green, and blue. The a and b coordinates are related to complementary colors. The a coordinate represents how red or green the color appears; red on the positive end, green on the negative. The b coordinate does the same with yellow and blue, respectively. This system is illustrated by the graph in Figure 1. Neutral grey is located at a: 0, b: 02.

Figure 1: Visual representation of CIELab coordinates2

[2] II. Experimental Procedure

A. Fabrication Samples were created using roughly 100g of System 96® glass frit and about 2g of oxides of the following rare-earth elements: Praseodymium (Pr3+), Neodymium (Nd3+), Samarium (Sm3+), Terbium (Tm3+), Dysprosium (Dy3+), Holmium (Ho3+), Erbium (Er3+), and a 1:1 wt. % composition of Neodymium and Holmium (Nd3+-Ho3+). Lanthanum, Cerium, Gadolinium, Thulium, Ytterbium, and Lutetium were not tested, as they were not likely to produce any color. Europium was not tested due to an insufficient quantity of the oxide to produce a viable sample; however, it was previously known that Eu3+- doped glass fluoresces a bright scarlet under UV “black light” and is mostly colorless under all other sources. Promethium was excluded because of its radioactive nature.

Because of the size of the frit obtained from the Alfred University Hot Glass Studio, it was necessary to crush the glass to a finer size with a mortar and pestle; allowing for a better melt. The frit was then mixed with one of the rare-earth oxides and melted in a ceramic crucible. All samples were melted in Carbolite furnace at a temperature of 1250°C for an hour, poured into a cylindrical graphite mold, removed and annealed at 530°C for an hour in a Thermolyne furnace. At the end of the aforementioned duration, the annealer was shut off and the samples were allowed to cool overnight.

B. Photography Samples were photographed under nine lighting conditions using a Nikon D50 digital camera. The lighting conditions were as follows: direct sunlight, daylight incandescent, soft white incandescent, fluorescent tubing, daylight CFL, soft white CFL, LED UV “black light”, daylight LED, and soft white LED. With the obvious exception of sunlight, photographs were taken under all lighting conditions indoors at night. This was done to prevent light pollution from even indirect sunlight. Furthermore, all sources of artificial

[3] light were shut off save the source of the light being tested at the time. Specifications for the bulbs used can be seen in Table IV in the Appendix.

Samples were photographed using the setup shown in Figure 2 below. The aforementioned camera was positioned on a tripod over the sample; an architect’s lamp was positioned just outside of the field of view of the camera. This was done so the sample would be exposed to the light directly, while also preventing the camera from casting a shadow. To ensure a neutral background for side-to-side comparisons, a piece of Mohawk Color Copy 98, 28lb Bright White paper was placed beneath all samples. With the exception of sunlight, the fluorescent tubing, and the UV “black light”, samples under all lighting types were photographed in this manner. The fluorescent tubing was the ambient light source of the room, and therefore did not require the use of the architect’s lamp. Similarly, the UV “black light” condition was satisfied using a UV flashlight consisting of nine “black light” LEDs. Sunlight conditions were conducted in an open area around noon, when the Sun was directly overhead, and there was little to no cloud coverage. In each of the three cases, the tripod and sheet of paper background were still utilized.

Figure 2: Setup for photographing samples under various lighting conditions

[4] Photographs of the emission spectra from most of the light sources were also taken. UV “black light” and sunlight were not included due to an inability to be photographed. The UV “black light” did not provide bright enough spectral lines for them to show up in a photograph. Likewise, there was too much ambient sunlight for the spectral lines to show up in a photogenic manner, and looking directly at the Sun is not advised for any reason. In order to photograph the other sources, a black piece of card stock with a narrow slit in it was placed between the camera and the light source. The emission spectra were then photographed by placing an Edmund Scientific Co., 1000 lines/mm linear diffraction grating over the lens of the camera.

C. Color Analysis In addition to photographing each sample, samples were matched to a similar color using a RAL color standard wheel, for all light sources. Due to a lack of similar colors within the RAL scaling, Pr3+ samples were categorized using Hex scaling3. These colors and their corresponding RAL number were recorded and also converted to CIELab coordinates4, as well as RGB values5. Table III in the Appendix lists these colors and their respective values.

Due to discrepancies between observations made by the naked eye and what is imaged by the camera, some color correction was employed. Initially, this was intended to be achieved via white balancing on the camera itself, using the white sheet of paper as the reference. However, due to unknown malfunctions within the camera this was not possible and alternative measures were taken. Instead, images were level balanced in Adobe® Photoshop® Elements 9. The corrected images can be seen in Figures 12-19 in the Appendix; while the raw images can be seen in Figures 20-27 in the Appendix. The corrected images depict colors closer to those observed with the naked eye. Correcting the images also generates a relatively standard white background, and allows one to compare the colors side-by-side with little interference from the color of the cast light.

[5] Raw images are included to showcase the color given off by the light source, which can be seen clearly when looking at the color of the “white” background.

D. Spectroscopy Select samples that exhibited photochromism were them prepared for further spectral analysis. Samples were cut to have two flat, parallel sides using a Leco® Corp. Vari/CutTM VC-50 saw. They were then polished to a higher optical clarity using a Buehler MetaServ® 250 grinder-polisher. Silicon carbide pads of varying grades ranging from 120-1200 were utilized.

Spectral data was collected using a PerkinElmer Lambda 950 UV-Vis spectrometer and the PerkinElmer UV WinLab software. Analysis was conducted from 200-1000nm at a 2.00nm interval. The data was then used to generate spectral graphs using Microsoft® Excel 2010.

III. Results Table II provides a summary of the colors observed for each sample under the various light sources. Only two of the samples exhibited strong photochromic properties, while two others exhibited fluorescence with UV exposure. Considering the fluorescent tube source was the ambient light, there was a concern about the distance between the source and the sample. However, distance seemed to have no effect on the color of the sample, so long as it was the only source of light the sample was being exposed to at the time.

[6] Table II: Coloration of each sample under all lighting conditions

Dopant Light 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ Nd - Source Pr Nd Sm Tb Dy Ho Er 3+ Ho Green- Oyster Zinc Light Silver Sunlight Blue lilac Colorless Colorless yellow white yellow pink grey Daylight Bitter Oyster Zinc Light Platinum Blue lilac Colorless Colorless Incan. lemon white yellow pink grey Soft Bitter Oyster Rape Light Pastel White Blue lilac Colorless Colorless lemon white yellow pink violet Incan. Fluor. Green- Pastel Salmon Light Pigeon Colorless Colorless Colorless Tube yellow blue pink pink blue Daylight Green- Pastel Salmon Light Agate Colorless Colorless Colorless CFL yellow blue pink pink grey Soft Bitter Pastel Oyster Light Antique Pigeon White Colorless Colorless lime blue white pink pink blue CFL Daylight Bitter Zinc Light Oyster Blue lilac Colorless Colorless Colorless LED lime yellow pink white Soft Bitter Zinc Light Platinum White Blue lilac Colorless Colorless Colorless lemon yellow pink grey LED Bright UV Light N/A N/A red N/A N/A N/A N/A (LED) ivory orange

A. Tb3+, Sm3+ & Dy3+ For all nine light sources, the Tb3+ sample yielded no coloration. While this did not provide further information regarding photochromism, it did help to reinforce the color standard. In the raw Tb3+ images depicted in Figure 23, it is clear that the coloring of the sample appears to be different under each lighting condition. However, the sample was colorless to the naked eye when observed under every source. Therefore, in the raw images, any color observed is due to the camera’s detection of the color cast by the light itself. Comparatively, the corrected images in Figure 15 are more accurate to the observed coloration of the sample. While there is still a defined difference in colors, especially with the warmer-toned soft white bulbs, it is lessened with the correction. A side-by-side comparison of the correction can be seen in Figure 3 below.

[7] a b

Figure 3: Tb3+ sample under soft white incandescent a) raw and b) corrected

The Sm3+ sample, most notably, exhibited strong fluorescence under UV “black light”. Under this condition, the mostly colorless sample fluoresced a bright red-orange. Additionally, the sample appeared off-white to pale yellow under select lighting conditions. Coloration was only observed under sunlight, soft white CFL, and both incandescent sources. The coloration is so slight that, on its own, it is hard to distinguish whether the sample itself is a yellowish color or if it is colorless with a yellowish cast from the aforementioned sources. However, if compared with the Tb3+ sample, it becomes apparent there is indeed a difference in coloration of the two samples. While easy to see with the naked eye, this is difficult to determine based on the corrected images; but near impossible to gather from the raw images.

The Dy3+ sample, similar to Tb3+, was colorless under nearly all circumstances. However, under UV “black light” it fluoresced a pale yellow, ivory color. Dy3+ amd Sm3+ were the only two samples to exhibit any fluorescence under UV light.

B. Pr3+ & Er3+ Both the Pr3+ and Er3+ samples did not exhibit any interesting characteristics; remaining consistently green-yellow and pink, respectively. While there was some variation in color

[8] from source to source, there was not a significant enough change to warrant photochromism.

C. Nd3+, Ho3+ & Nd3+-Ho3+ For sunlight, both incandescent, and both LED, the Nd3+ sample was a light shade of violet. However, under all three fluorescent sources, the lilac shifted to a more blue color. This shift can be seen in Figure 4.

a b

Figure 4: Nd3+ sample under a) soft white CFL and b) soft white incandescent

Spectral analysis of the sample revealed three major absorption peaks within the visible spectrum (Figure 5). Peaks at 574nm and 586nm show absorption of yellow light, resulting in the observation of a violet-blue color. The third peak, at 524nm, absorbs green light, resulting in a purple color. These peaks provide a clear color profile for Nd3+ that agrees with the observed colorations.

[9] Nd

45

40 35 30 524 25 20 15 574 Transmittance 10 586

% % 5 0 380 480 580 680 780 Wavelength (nm)

Figure 5: Absorption/Transmittance spectrum for the Nd3+ sample

The Ho3+ sample exhibited photochromism similar to that of the Nd3+ sample. Likewise, the sample only changed under fluorescent lighting; appearing as a goldenrod color in all other lightings, then changing to a salmon pink. An example of this is provided in Figure 6.

b a Figure 6: Ho3+ sample under a) soft white CFL and b) soft white incandescent

The absorption/transmittance spectrum of Ho3+ (Figure 7) proved to be more complex and less clear than the Nd3+ spectral graph. There are two absorption peaks in the violet- blue range, 446nm and 460nm, adding yellow to the observed color. The 488nm peak

[10] absorbs in the blue-green, providing red. All these peaks are to be expected considering the two-tone Ho3+ sample. Surprisingly, there are also two green-yellow peaks, 520nm and 536, resulting in purple and violet contributions; as well as a red peak at 638nm, adding blue-green to the mix.

Ho

25

520 20 638 488 536 15

10 460

5

% % Transmittance 446 0 380 480 580 680 780 Wavelength (nm)

Figure 7: Absorption/Transmittance spectrum for the Ho3+ sample

Interestingly, the Nd3+-Ho3+ sample did not seem to produce any strong photochromic qualities. Instead it produced varying shades of blue-grey; with Neodymium’s cool tones slightly overpowering the warm tones of the Holmium. Additionally, there was no discernable pattern to the shifting tones as there was for the two constituent dopants.

D. Emission Spectra Photographs of the emission spectra for fluorescent, incandescent and LED sources are compiled below in Figure 8. The incandescent sources appear to be the most complete, having wholly continuous spectra from red to violet-blue. The LED sources, for the most part, also have continuous spectra. However, there appears to be a slight gap in the blue- green to blue range. Arguably the most significant of all were the spectra for the

[11] fluorescent sources. The spectra are more discrete than the other two types of light; possessing substantial gaps in the range of visible light.

a

b c

d e

f g

Figure 8: Emission spectra of a) fluorescent b) daylight CFL c) soft white CFL d) daylight incandescent e) soft white incandescent f) daylight LED g) soft white LED

IV. Discussion If the emission photographs are compared to a spectral graph of each source (Figure 9), the graph confirms the initial observations. The incandescent graph shows an even and continuous spectrum, inclining towards the redder tones; agreeing with the yellowish cast given off by the light source. The LED graph indeed has a decrease in the blue-green to

[12] blue range that matches the gap in the emission spectra collected. The fluorescent graph also agrees with the photographed spectra; exhibiting narrow spikes of light at wavelengths that match the discrete emission spectra.

Figure 9: A graphical representation of emission spectra from a variety of lightings6

Due to their location on the periodic table, rare-earth elements are dominated by valence electrons in the 4f shell. Because the filled 5s and 5p shells shield the 4f orbital, the environment a rare-earth ion is placed in has very little effect on the spectrum the ion generates. In fact, spectra closely resemble the spectra of elemental rare-earth atoms. This means that the type of glass used should not affect the color the ion imparts on the glass. Interactions between 4f orbitals also generate narrower bands in absorption spectra than d-group interactions. This makes rare-earth oxide glasses more susceptible to gaps in emission spectra than glasses doped with transition metal oxides. While rare-earth ions have d-orbitals as well as f-orbitals, 4f-4f transitions are the only ones necessary to study the cause of color. The 4f-5d transitions possess sufficiently high energy to produce wavelengths of 200nm or less; well within the ultraviolet range and outside the visible spectrum1.

[13] 4f-4f transitions possess weaker intensities compared to other color producing transitions. Therefore, a glass must be doped with more rare-earth oxide to achieve the same saturation as if it were doped with a transition metal oxide. This explains why all colored samples exhibited pale and pastel shades of color, and may be a potential explanation as to why the Nd3+-Ho3+ sample did not exhibit photochromism. Another possible explanation is a cancellation of wavelengths; seen in other rare-earth ions, such as Er3+. Within its absorption spectra, Er3+ has a green peak at 518nm which comprises the majority of its pink color. There is another peak in the blue-green region at 483nm; however it is neutralized by a red peak at 647nm, the first peak’s complementary color. Without either of these peaks, the sample remains a pink color due to the 518nm peak1. The combination of the Nd3+ spectrum and Ho3+ spectrum may have caused a similar cancellation in the resulting Nd3+-Ho3+ spectrum.

A. Nd3+ Research into technical literature confirmed that two main peaks are responsible for the coloration of the Nd3+ sample. The first is a peak at 521nm in the green region of the spectrum that matches well with the experimental peak at524nm; resulting in the reddish- purple portion of the sample’s coloration. The other peak is located within the region at 575nm, giving a violet-blue tint. However, when doped in a glass, the Nd3+ ion can interact with other ions through the nephelauxetic effect. This translates to a shift in that peak to 589nm at most. This explains the two experimental peaks at 574nm and 586nm. Interestingly, when the shift occurs the new peak position complements the yellow emission peak of Na+ ions. Due to their complementary nature, the two peaks cancel each other. Thus, Nd3+ doped glass is used in glassmaker’s goggles to negate the yellow light given off by molten glass, protecting the wearer’s eyes1.

In Figure 10, the aforementioned peaks are extrapolated down to compare with the emission peaks of fluorescent light. While the peak responsible for the violet-blue coloration overlaps a small emission peak, the peak resulting in reddish-purple coincides

[14] with a gap in the spectrum. Therefore, none of the purple color is present while the sample is under fluorescent lighting.

Nd

45 40

35 30 25 20

Transmittance 15

% % 10 5 0 380 480 580 680 780 Wavelength (nm)

Figure 10: Comparison of the absorption/transmittance spectrum of Nd3+ and the emission spectrum of a CFL7

[15] B. Ho3+ The Ho3+ spectrum consists of two main peaks at 535nm and 447nm, corresponding to the experimental peaks at 536nm and 446nm. The peak at 447nm generates the Ho3+ sample’s yellow color, while the violet of the 535nm peak is cancelled by a portion of the 447nm peak. The spectrum also contains minor peaks at 641nm, 481nm, 471nm, and 465nm, corresponding to peaks at 638nm, 488nm, and 460nm. The blue-green color of the 641nm peak is normally neutralized by the reds, oranges, and yellows of the complex band around 465-481nm1. Though, under fluorescent lighting some of the peaks coincide with emission spectrum gaps (Figure 11). The yellow contribution from the 447nm and 465nm peaks overlap a gap in the spectrum, as does the blue-green contribution at 641nm. The remaining peaks at 481nm (red) and 471nm (orange) and some of the 535nm peak (violet) combine to produce a salmon pink sample under fluorescent lighting.

[16] Ho

25

20

15

10

% % Transmittance 5

0 380 480 580 680 780 Wavelength (nm)

Figure 11: Comparison of the absorption/transmittance spectrum of Ho3+ and the emission spectrum of a CFL7

V. Conclusion It was determined that, as hypothesized, the photochromism in select rare-earth oxide glasses was due to a combination of narrow absorption bands and gaps in emission spectra of select light sources. The absorption peak that provides Nd3+ glass with a purple tint coincides with a gap in the emission spectrum of fluorescent light; causing the glass to appear blue due to Nd3+ only other significant peak. Likewise, the Ho3+ peaks that produce yellow, as well as the blue-green peak that cancels the reds and oranges overlap

[17] with fluorescent gaps. The resulting combination of reds and oranges yield a salmon pink color.

Possible sources of error arise from quality of the sample preparation for spectral analysis. Bubbles and suboptimal polishing due to human error may have caused some of the absorption peaks to appear broader and shallower. Additionally, the RAL color scaling wheel contains a finite variety of colors; the standards found for each sample the best approximations and not the exact color observed. Adding another level of difficulty was the fact that the colors were being matched to transparent colored glass samples, making it hard to determine the exact color if there were full opacity. Due to the RAL scaling’s lack of a decent approximation of the Pr3+ sample’s color, Hex scaling was needed. However, as the Hex color standard was observed online and not in a tangible form, such as the RAL color scaling wheel, it may vary from a physical Hex color.

VI. Future Work Further research can be conducted into the potentials of photochromism in the Nd3+-Ho3+ sample. Increasing the dopant level may provide a stronger color, allowing a more noticeable shifting in color. Additionally, changing the ratio of the two dopants may yield interesting results. Spectral analysis should also be conducted to test the viability of the wavelength cancellation theory. In that respect, how would combining non-photochromic colorants affect the photochromism of either Nd3+ or Ho3+ doped glass?

[18] VII. Appendix Table III: All colors observed with CIELab and RGB conversions

Color RAL CIE Lab Scaling RGB Scaling Sample Name Number L a b R G B N/A Bitter lemon 84.94 -28.58 82.60 202 224 13 Hex #cae00d N/A Bitter lime 92.84 -46.88 89.36 191 255 0 Hex #bfff000 Green- N/A 91.96 -52.48 81.87 173 255 47 yellow Hex #adff2f Oyster 1013 88.13 0.19 9.67 227 217 198 white

Light ivory 1015 86.40 2.06 15.48 230 210 181

Zinc yellow 1018 84.83 3.05 69.19 250 202 48

Rape yellow 1021 78.88 10.03 82.04 246 186 0

Bright red 2008 61.99 44.64 51.72 237 107 33 orange Antique 3014 60.17 32.49 12.58 203 115 117 pink

Light pink 3015 72.73 20.48 3.96 216 160 166

Salmon pink 3022 58.10 36.44 27.34 207 105 85

Blue lilac 4005 50.92 15.58 -23.06 118 104 154

Pastel violet 4009 60.59 10.38 -2.88 157 134 146

Pigeon blue 5014 53.79 -2.64 -15.59 99 125 150

Pastel blue 5024 60.50 -9.53 -17.38 96 147 172

Silver grey 7001 63.81 -2.22 -4.05 140 150 157

Platinum 7036 63.49 1.27 0.78 151 147 146 grey

Agate grey 7038 72.97 -1.50 2.97 176 176 169

[19]

Table IV: Specifications of all light sources utilized

Lighting Bulb Brightness Light Brand Energy Usage Type Type (lumens) Appearance Sunlight - - - - - Daylight GE 43 W A19 565 2900K Incan. Reveal® (60 W replacement) Soft White GE A15 560 60 W 2700K Incan. Fluor. GE F32T8 2800 32 W 4100K Tube Ecolux® Daylight 9 W EcosmartTM GP19 450 5000K CFL (40 W replacement) Soft White 9 W EcosmartTM GP19 550 2700K CFL (40 W replacement) Daylight GE 11 W A19 680 2700K LED Reveal® (60 W replacement) Soft White 7 W GE A-shape 470 2700K LED (40W replacement) UV (LED) Ultra-Light - - - -

[20] a b c

d e f

g h i

Figure 12: Corrected Pr3+ sample under a) sunlight b) daylight incandescent c) soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

[21] a b c

d e f

g h i

Figure 13: Corrected Nd3+ sample under a) sunlight b) daylight incandescent c) soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

[22] a b c

d e f

g h i

Figure 14: Corrected Sm3+ sample under a) sunlight b) daylight incandescent c) soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

[23] a b c

d e f

g h i

Figure 15: Corrected Tb3+ sample under a) sunlight b) daylight incandescent c) soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

[24] a b c

d e f

g h i

Figure 16: Corrected Dy3+ sample under a) sunlight b) daylight incandescent c) soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

[25] a b c

d e f

g h i

Figure 17: Corrected Ho3+ sample under a) sunlight b) daylight incandescent c) soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

[26] a b c

d e f

g h i

Figure 18: Corrected Er3+ sample under a) sunlight b) daylight incandescent c) soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

[27] a b c

d e f

g h i

Figure 19: Corrected Pr3+-Ho3+ sample under a) sunlight b) daylight incandescent c) soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

[28] a b c

d e f

g h i

Figure 20: Raw Pr3+ sample under a) sunlight b) daylight incandescent c) soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

[29] a b c

d e f

g h i

Figure 21: Raw Nd3+ sample under a) sunlight b) daylight incandescent c) soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

[30] a b c

d e f

g h i

Figure 22: Raw Sm3+ sample under a) sunlight b) daylight incandescent c) soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

[31] a b c

d e f

g h i

Figure 23: Raw Tb3+ sample under a) sunlight b) daylight incandescent c) soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

[32] a b c

d e f

g h i

Figure 24: Raw Dy3+ sample under a) sunlight b) daylight incandescent c) soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

[33] a b c

d e f

g h i

Figure 25: Raw Ho3+ sample under a) sunlight b) daylight incandescent c) soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

[34] a b c

d e f

g h i

Figure 26: Raw Er3+ sample under a) sunlight b) daylight incandescent c) soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

[35] a c b

d e f

g h i

Figure 27: Raw Nd3+-Ho3+ sample under a) sunlight b) daylight incandescent c) soft white incandescent d) fluorescent e) daylight CFL f) soft white CFL g) UV LED h) daylight LED i) soft white LED

[36] VIII. References

[1] K. Binnemans and C. Görller-Walrand, "On the color of the trivalent lanthanide ions," Chem. Phys. Lett., 235 [3-4] 163-74 (1995).

[2] "Technical Guides: Color Models, CIELAB" (2000) Adobe Systems Inc. Accessed on: March 2016. Available at

[3] M. Gallagher, Encycolorpedia. Accessed on: March 2016. Available at

[4] "List of RAL colors" (2016) Wikimedia Foundation. Accessed on: March 2016. Available at

[5] C. Cabo, "RAL Colours Classic" (2014). Accessed on: March 2016. Available at

[6] J. Herrman, "Ultimate Light Bulb Test: Incandescent vs. Compact Fluorescent vs. LED" (2011) Popular Mechanics. Accessed on: April 2016. Available at

[7] S. Watson, "Basics when Designing and Picking Light Sources" (2016) LEDinside. Accessed on: April 2016. Available at

[37]