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Photochromic lens mirror-coated with Cr
Optical Materials 30 (2007) 438–441 www.elsevier.com/locate/optmat
Photochromic lens mirror-coated with Cr Sungho Shin, Myeongkyu Lee
*
Department of Materials Science and Engineering, Yonsei University, 134 Shinchon-dong, Seodaemun-gu, Seoul 120-749, Republic of Korea Received 2 August 2006; received in revised form 14 November 2006; accepted 6 December 2006 Available online 20 February 2007
Abstract We have designed and fabricated mirror-coated photochromic lenses for use in sunglasses. These lenses consisted of a Cr thin film sandwiched between two SiO2 layers on the front surface and an anti-reflection (AR) coating on the backside. The SiO2 films above and below the Cr layer were introduced as the protection and buffer layers, respectively. The AR coating was to suppress back-reflection from the lens surface. Deposition of all coating layers were carried out by an e-beam evaporator under Ar atmosphere at P = 105 Torr and T = 70 °C. As expected, the overall transmittance decreased with increasing Cr thickness. For a Cr layer of 5 nm thickness, it changed from about 45% in the bleached state down to 25% after exposure to sunlight. This is consistent with the transmission range typically required for sunglasses. Ó 2006 Elsevier B.V. All rights reserved. PACS: 78.40.q; 7820.Ci Keywords: Chromium; Mirror coating
1. Introduction Materials that change color reversibly under illumination of different wavelengths have many potential applications in optical systems such as information storage, holograms, optical filters and eyeglasses [1–5]. In general, photochromic materials become colored with irradiation of ultraviolet (UV) light and get bleached by visible irradiation. This photochromic effect is mostly based on the photo-induced charge redistribution among molecules, ions or defects. Photochromic plastic lenses have already been developed for eyeglasses and are commercially available. These lenses completely block mid-UV light (290–320 nm) which may cause a damage to our eyes and partially transmit near-UV (330–400 nm). This near-UV light plays a role of pump light for coloration. Thus, they are transparent indoors and get colored outdoors where the UV density is relatively high. These photochromic lenses, with a variable
*
Corresponding author. Tel.: +82 2 2123 2832; fax: +82 2 312 5375. E-mail address: [email protected] (M. Lee).
0925-3467/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2006.12.005
transmittance depending on the UV density, have the biggest potential market on sunglasses. However, photochromic sunglasses have not been commercialized yet. Since photochromic lenses were originally developed for eyeglasses, they are almost clear in the bleached state and the overall transmittance is still rather high to be used for sunglasses. The main function of sunglasses is to protect our eyes from strong sunlight. Although it depends on many factors, the transmittance typically required for sunglasses is 20–40%. The transmittance of the photochromic lens changes from about 90% in the clear state down to 40– 60% after coloration, the latter actually depending on the UV density of sunlight. Therefore, its overall transmittance should be lowered in order to be used for sunglasses. One solution is to form a highly reflecting mirror coating on the lens and reduce the transmittance as a result. ‘‘Mirror coating’’ does not always mean a 100% reflection. It here refers to the coating by which the reflectance is increased over the uncoated case. Mirror coatings are often made of alternating high-index and low-index dielectric films [6–8]. In dielectric mirrors, the reflectance can be increased
S. Shin, M. Lee / Optical Materials 30 (2007) 438–441
by increasing the number of the layers, but the wavelength range for high reflectance becomes narrower as more and more layers are stacked. Thus the required ‘‘white mirror coating’’ is not achieved. Metallic mirror is an alternative which can maintain the high reflectance over a wide wavelength range. We here discuss photochromic lenses mirrorcoated with Cr, along with the feasibility for sunglasses.
When a dielectric thin film of refractive index nf is deposited on a substrate of nsub, the admittance locus of the film can be represented on the complex coordinate. For normal incidence, the normalized electric field B and magnetic field C is given by [6] B 1 cos d i sinnf d ð1Þ ¼ C nsub inf sin d cos d n d and d is the thickness of the film. If the where d ¼ 2p k f admittance Y, defined as the ratio of normalized magnetic to electric field, is written as C ¼ x þ iy B
the reflectance R is then given by Y 0 Y 2 1 Y 2 ð1 xÞ2 þ y 2 ¼ R¼ ¼ 2 Y0 þ Y 1 þ Y ð1 þ xÞ þ y 2
Quarter-wave thickness
Im
(
n 2f nt nt2
+ kt2
,
n 2f kt nt2
+ kt2
)
(1,0) Admittance of air
2. Theoretical background
Y ¼
439
ð2Þ
ð3Þ
where Y0 = 1 is the admittance of the incident medium, i.e., air. As the film thickness varies, the 2admittance of the film n Y also changes and becomes Y ¼ nsubf when the film thickness is equal to a quarter-wave thickness, i.e., d ¼ 4nk f . When pffiffiffiffiffiffiffiffi an additional condition of nf ¼ nsub is satisfied, the reflectance becomes zero. In cases of multilayered structures, Eqs. (2) and (3) are still effective and only the characteristic matrix of Eq. (1) is replaced by the product of characteristic matrices for each layer. The admittance Y of the film starts from the substrate admittance (nsub,0) and rotates clockwise on the complex as the thickness in 2 coordinate n creases. It arrives on nsubf ; 0 when the film becomes a quarter-wave thickness. As given in Eq. (3), the reflectance decreases as Y gets closer to the admittance of air, Y0 = (1, 0), and increases in the opposite case. Metal has a complex refractive index (N = n ik) and thus reveals an admittance locus different from that of a dielectric material. Since the imaginary term k is generally much greater than n, the admittance of bulk metal (n, k) is located near the negative imaginary axis of the complex coordinate and thus R 1. When a metal film is deposited on a substrate of nsub, the admittance Y starts from (nsub, 0) and moves towards the admittance of bulk metal as the thickness increases. For a metallic film thicker than the skin depth, the admittance is located near the bulk admittance. Thus a thick metal film does not function optically. The admittance of a thin metallic layer, (nt, kt) will exist in between (nsub, 0) and (n, k). The location of
Reflectance decreasing
Re
(nt ,− kt ) Admittance of uncoated metal film
Fig. 1. Admittance diagram of a dielectric layer deposited on a metal film (Ref. [6]). The locus starts at the admittance of the uncoated metal film and moves clockwise as the dielectric layer becomes thicker.
admittance and the resulting reflectance are determined by the thickness of the metal film. As illustrated in Fig. 1, the optical admittance of a metal film is always in the fourth quadrant. When a dielectric layer of refractive index nf is added on the metal film, the admittance rotates clockwise, starting from (nt, kt). As the dielectric layer gets thicker, the reflectance is decreased because the admittance moves towards the admittance of air. The minimum in reflectance will occur at a dielectric layer thickness less than a quarter-wave, i.e., when the locus of the admittance crosses the real axis. For a very thin dielectric film, however, the reflectance is nearly equal to that of the uncoated metal film. 3. Experimental procedures Two different kinds of photochromic lenses, smoke and brown lenses, were used for this work and they were supplied from Seeworld Optics Inc. Commercial plastic lenses are usually covered with hard coating and anti-reflection (AR) coating layers on both sides. Hard coating layer is to improve a resistance to scratch and is generally made by dip-coating the lens into a polysiloxane solution. Aspurchased photochromic lenses are only hard-coated on both sides. A metal layer sandwiched between two SiO2 layers was deposited on the front side of the lens. The backside, facing our eyes, was covered with an AR coating. Chromium (Cr) and silver (Ag) were chosen for metal layers, which are known to have a high reflectivity over the visible range [6,7]. The AR coating was made of 5 layers of SiO2 and ZrO2 films. Deposition of all coating layers was carried out inside the chamber of an e-beam evaporator under Ar atmosphere at P = 105 Torr and T = 70 °C. The thickness of the layers was controlled by an in situ monitoring system equipped in the evaporator. Transmission spectra of the lenses were measured with a UV–visible spectrophotometer.
440
S. Shin, M. Lee / Optical Materials 30 (2007) 438–441
4. Results and discussion
100
SiO2 10 nm Cr SiO2 50 nm Hard coating Photochromic lens Hard coating AR coating Fig. 2. Cr-based mirror coating structure.
Both AR coated
60
Mirror (5 nm Cr) + AR Transmission (%)
Transmission (%)
80
40 20
20
0 325
0 300
400
350 375 Wavelength (nm)
500 600 Wavelength (nm)
400
700
100 Both AR coated
80 Transmission (%)
Both of Ag and Cr films were investigated for potential metal layers. However, Ag revealed a poor adhesion to the substrate and a very low deposition rate. This made it difficult to achieve uniform Ag films. The designed coating structure is shown in Fig. 2. A thin Cr layer sandwiched between two SiO2 layers of different thickness was deposited on the front side of the lens. Since most metal films are mechanically soft, they can be easily scratched if wiped with a cloth. In fact, Cr films without any protection layer were found to be easily scratched. It is thus necessary to protect the metal film with a harder dielectric layer. A SiO2 film of 10 nm thickness was put on top of the Cr film as a protection layer. It can also protect the metal film from atmospheric corrosion. As discussed earlier, the admittance of a metal film is in the fourth quadrant and so the reflectance falls when a dielectric layer is added. A thick dielectric layer will result in a substantial decrease in the reflectance, canceling out the effect of increasing the reflectance by a metal film. This is the reason why the protection layer was made relatively thin (10 nm). The SiO2 layer below the Cr layer was introduced as a buffer layer to enhance adhesion. The light incident from the backside of the lens will be partially reflected back by the Cr layer and this back-reflection can give rise to an eye-strain. Thus, the buffer layer was made rather thick (50 nm) in order to reduce the back-reflection. The AR coating on the backside is to suppress the surface reflection. It consisted of 5 layers of SiO2 (low index) and ZrO2 (high index) films. Since the AR coating design is already well established, it is not here discussed in detail. The dependence of transmittance on the thickness of Cr film was investigated. As expected, the transmittance decreased with increasing thickness. For a Cr thickness over 10 nm, the transmittance was too low to be used for sunglasses. The appropriate thickness was found to be 4– 6 nm. Fig. 3 compares the transmission spectra of mirrorcoated lenses (5 nm Cr) with those of commercial lenses. The compared commercial lenses have AR coatings on both sides. As shown, the transmittance was decreased uni-
60 40
Mirror (5 nm Cr) + AR
20 0 300
400
500 600 Wavelength (nm)
700
Fig. 3. Transmission spectra of mirror-coated lenses (5 nm Cr) and commercial lenses: (a) brown lenses; (b) smoke lenses.
formly over the visible range with a Cr film in both brown and smoke lenses. The UV transmission (320–400 nm) in the mirror-coated lens was also reduced to almost half that of a commercial lens (inset of Fig. 3a). A complete blocking of the UV light in this range may be achieved by increasing the thickness of the Cr film. But this will make the visible transmittance of the lens too low to be used for sunglasses. In addition, the photochromic effect will be weakened because this near-UV light plays a role of pump light for coloration. Fig. 4 compares the photochromic behavior of a mirror-coated brown lens with the result of a commercial lens, in which the colored spectra were obtained immediately after exposure to sunlight for 1 min. Of course, the colored spectrum will depend on the strength of the sunlight, but it is possible to compare the results obtained under the same condition. While the transmittance of the clear lens varies from 90% to 50% at k = 500 nm, the mirror-coated lens has a range of 45–25%. The obtained transmission range is consistent with the value required for sunglasses. The smaller transmission variation in the mirror-coated lens is because the amount of incident UV photons is decreased by the Cr film. Fig. 5 illustrates the reflections from both surfaces of a mirror-coated lens. While the front surface is highly reflecting, reflection from
S. Shin, M. Lee / Optical Materials 30 (2007) 438–441
a
441
100 Before exposure to sunlight
Transmission (%)
80 60 40
After exposure to sunlight for 1 min
20 0 300
400
500 600 Wavelength (nm)
700
60
Transmission (%)
50
Before exposure to sunlight
40 30 After exposure to sunlight for 1 min
20 10 0 300
400
500 600 Wavelength (nm)
700
Fig. 4. Photochromic behavior of (a) a commercial brown lens and (b) a mirror-coated brown lens.
the other surface is relatively weak. The weak reflection from the backside can be explained by the existence of an AR coating and a thick SiO2 buffer layer. 5. Conclusion We have designed and fabricated mirror-coated photochromic lenses for use in sunglasses. These lenses consisted of a thin Cr film sandwiched between two SiO2 layers on the front surface and an AR coating on the backside. Deposition of all coating layers were carried out by an e-beam evaporator under Ar atmosphere at P = 105 Torr and T = 70 °C. The overall transmittance of the lens was controlled by the thickness of the Cr film. A SiO2 film of 50 nm was inserted as a buffer layer to enhance Cr adhesion and the Cr film was covered with a SiO2 protection layer of 10 nm. The AR coating on the backside was introduced to reduce the surface reflection. As expected, the overall transmittance decreased with increasing Cr thickness. For a Cr layer of 5 nm thickness, it changed from about 45% before coloration to 25% after coloration. This is consistent with
Fig. 5. Reflection from the front surface (upper) and the backside (lower).
the transmission range typically required for sunglasses. While the front surface was highly reflecting, reflection from the other surface was relatively weak. The weak reflection from the backside can be explained by the existence of a thick SiO2 buffer layer and an AR coating. Throughout the work, the thicknesses of SiO2 layers were fixed and the optimum values may be different from those given here. Nevertheless, the obtained results indicate that the suggested coating structure is an effective design for photochromic sunglasses. References [1] [2] [3] [4] [5]
T. Gong, J. Feng, W. Wei, W. Huang, Prog. Chem. 18 (2006) 298. A. Toriumi, S. Kawata, M. Gu, Opt. Lett. 23 (1998) 1924. F. Raymo, M. Tomasulo, J. Phys. Chem. A 109 (2005) 7343. K. Buse, A. Adibi, D. Psaltis, Nature 393 (1998) 665. M. Lee, S. Takekawa, Y. Furukawa, K. Kitamura, Phys. Rev. Lett. 84 (2000) 875. [6] H. Macleod, Thin Film Optical Filters, third ed., IOP, 2001. [7] C. Huangbo, Thin Film Optics, Techmedia, 2005. [8] E. Hecht, Optics, second ed., Addison Wesley, 1990.
Photochromic lens mirror-coated with Cr Sungho Shin, Myeongkyu Lee
*
Department of Materials Science and Engineering, Yonsei University, 134 Shinchon-dong, Seodaemun-gu, Seoul 120-749, Republic of Korea Received 2 August 2006; received in revised form 14 November 2006; accepted 6 December 2006 Available online 20 February 2007
Abstract We have designed and fabricated mirror-coated photochromic lenses for use in sunglasses. These lenses consisted of a Cr thin film sandwiched between two SiO2 layers on the front surface and an anti-reflection (AR) coating on the backside. The SiO2 films above and below the Cr layer were introduced as the protection and buffer layers, respectively. The AR coating was to suppress back-reflection from the lens surface. Deposition of all coating layers were carried out by an e-beam evaporator under Ar atmosphere at P = 105 Torr and T = 70 °C. As expected, the overall transmittance decreased with increasing Cr thickness. For a Cr layer of 5 nm thickness, it changed from about 45% in the bleached state down to 25% after exposure to sunlight. This is consistent with the transmission range typically required for sunglasses. Ó 2006 Elsevier B.V. All rights reserved. PACS: 78.40.q; 7820.Ci Keywords: Chromium; Mirror coating
1. Introduction Materials that change color reversibly under illumination of different wavelengths have many potential applications in optical systems such as information storage, holograms, optical filters and eyeglasses [1–5]. In general, photochromic materials become colored with irradiation of ultraviolet (UV) light and get bleached by visible irradiation. This photochromic effect is mostly based on the photo-induced charge redistribution among molecules, ions or defects. Photochromic plastic lenses have already been developed for eyeglasses and are commercially available. These lenses completely block mid-UV light (290–320 nm) which may cause a damage to our eyes and partially transmit near-UV (330–400 nm). This near-UV light plays a role of pump light for coloration. Thus, they are transparent indoors and get colored outdoors where the UV density is relatively high. These photochromic lenses, with a variable
*
Corresponding author. Tel.: +82 2 2123 2832; fax: +82 2 312 5375. E-mail address: [email protected] (M. Lee).
0925-3467/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2006.12.005
transmittance depending on the UV density, have the biggest potential market on sunglasses. However, photochromic sunglasses have not been commercialized yet. Since photochromic lenses were originally developed for eyeglasses, they are almost clear in the bleached state and the overall transmittance is still rather high to be used for sunglasses. The main function of sunglasses is to protect our eyes from strong sunlight. Although it depends on many factors, the transmittance typically required for sunglasses is 20–40%. The transmittance of the photochromic lens changes from about 90% in the clear state down to 40– 60% after coloration, the latter actually depending on the UV density of sunlight. Therefore, its overall transmittance should be lowered in order to be used for sunglasses. One solution is to form a highly reflecting mirror coating on the lens and reduce the transmittance as a result. ‘‘Mirror coating’’ does not always mean a 100% reflection. It here refers to the coating by which the reflectance is increased over the uncoated case. Mirror coatings are often made of alternating high-index and low-index dielectric films [6–8]. In dielectric mirrors, the reflectance can be increased
S. Shin, M. Lee / Optical Materials 30 (2007) 438–441
by increasing the number of the layers, but the wavelength range for high reflectance becomes narrower as more and more layers are stacked. Thus the required ‘‘white mirror coating’’ is not achieved. Metallic mirror is an alternative which can maintain the high reflectance over a wide wavelength range. We here discuss photochromic lenses mirrorcoated with Cr, along with the feasibility for sunglasses.
When a dielectric thin film of refractive index nf is deposited on a substrate of nsub, the admittance locus of the film can be represented on the complex coordinate. For normal incidence, the normalized electric field B and magnetic field C is given by [6] B 1 cos d i sinnf d ð1Þ ¼ C nsub inf sin d cos d n d and d is the thickness of the film. If the where d ¼ 2p k f admittance Y, defined as the ratio of normalized magnetic to electric field, is written as C ¼ x þ iy B
the reflectance R is then given by Y 0 Y 2 1 Y 2 ð1 xÞ2 þ y 2 ¼ R¼ ¼ 2 Y0 þ Y 1 þ Y ð1 þ xÞ þ y 2
Quarter-wave thickness
Im
(
n 2f nt nt2
+ kt2
,
n 2f kt nt2
+ kt2
)
(1,0) Admittance of air
2. Theoretical background
Y ¼
439
ð2Þ
ð3Þ
where Y0 = 1 is the admittance of the incident medium, i.e., air. As the film thickness varies, the 2admittance of the film n Y also changes and becomes Y ¼ nsubf when the film thickness is equal to a quarter-wave thickness, i.e., d ¼ 4nk f . When pffiffiffiffiffiffiffiffi an additional condition of nf ¼ nsub is satisfied, the reflectance becomes zero. In cases of multilayered structures, Eqs. (2) and (3) are still effective and only the characteristic matrix of Eq. (1) is replaced by the product of characteristic matrices for each layer. The admittance Y of the film starts from the substrate admittance (nsub,0) and rotates clockwise on the complex as the thickness in 2 coordinate n creases. It arrives on nsubf ; 0 when the film becomes a quarter-wave thickness. As given in Eq. (3), the reflectance decreases as Y gets closer to the admittance of air, Y0 = (1, 0), and increases in the opposite case. Metal has a complex refractive index (N = n ik) and thus reveals an admittance locus different from that of a dielectric material. Since the imaginary term k is generally much greater than n, the admittance of bulk metal (n, k) is located near the negative imaginary axis of the complex coordinate and thus R 1. When a metal film is deposited on a substrate of nsub, the admittance Y starts from (nsub, 0) and moves towards the admittance of bulk metal as the thickness increases. For a metallic film thicker than the skin depth, the admittance is located near the bulk admittance. Thus a thick metal film does not function optically. The admittance of a thin metallic layer, (nt, kt) will exist in between (nsub, 0) and (n, k). The location of
Reflectance decreasing
Re
(nt ,− kt ) Admittance of uncoated metal film
Fig. 1. Admittance diagram of a dielectric layer deposited on a metal film (Ref. [6]). The locus starts at the admittance of the uncoated metal film and moves clockwise as the dielectric layer becomes thicker.
admittance and the resulting reflectance are determined by the thickness of the metal film. As illustrated in Fig. 1, the optical admittance of a metal film is always in the fourth quadrant. When a dielectric layer of refractive index nf is added on the metal film, the admittance rotates clockwise, starting from (nt, kt). As the dielectric layer gets thicker, the reflectance is decreased because the admittance moves towards the admittance of air. The minimum in reflectance will occur at a dielectric layer thickness less than a quarter-wave, i.e., when the locus of the admittance crosses the real axis. For a very thin dielectric film, however, the reflectance is nearly equal to that of the uncoated metal film. 3. Experimental procedures Two different kinds of photochromic lenses, smoke and brown lenses, were used for this work and they were supplied from Seeworld Optics Inc. Commercial plastic lenses are usually covered with hard coating and anti-reflection (AR) coating layers on both sides. Hard coating layer is to improve a resistance to scratch and is generally made by dip-coating the lens into a polysiloxane solution. Aspurchased photochromic lenses are only hard-coated on both sides. A metal layer sandwiched between two SiO2 layers was deposited on the front side of the lens. The backside, facing our eyes, was covered with an AR coating. Chromium (Cr) and silver (Ag) were chosen for metal layers, which are known to have a high reflectivity over the visible range [6,7]. The AR coating was made of 5 layers of SiO2 and ZrO2 films. Deposition of all coating layers was carried out inside the chamber of an e-beam evaporator under Ar atmosphere at P = 105 Torr and T = 70 °C. The thickness of the layers was controlled by an in situ monitoring system equipped in the evaporator. Transmission spectra of the lenses were measured with a UV–visible spectrophotometer.
440
S. Shin, M. Lee / Optical Materials 30 (2007) 438–441
4. Results and discussion
100
SiO2 10 nm Cr SiO2 50 nm Hard coating Photochromic lens Hard coating AR coating Fig. 2. Cr-based mirror coating structure.
Both AR coated
60
Mirror (5 nm Cr) + AR Transmission (%)
Transmission (%)
80
40 20
20
0 325
0 300
400
350 375 Wavelength (nm)
500 600 Wavelength (nm)
400
700
100 Both AR coated
80 Transmission (%)
Both of Ag and Cr films were investigated for potential metal layers. However, Ag revealed a poor adhesion to the substrate and a very low deposition rate. This made it difficult to achieve uniform Ag films. The designed coating structure is shown in Fig. 2. A thin Cr layer sandwiched between two SiO2 layers of different thickness was deposited on the front side of the lens. Since most metal films are mechanically soft, they can be easily scratched if wiped with a cloth. In fact, Cr films without any protection layer were found to be easily scratched. It is thus necessary to protect the metal film with a harder dielectric layer. A SiO2 film of 10 nm thickness was put on top of the Cr film as a protection layer. It can also protect the metal film from atmospheric corrosion. As discussed earlier, the admittance of a metal film is in the fourth quadrant and so the reflectance falls when a dielectric layer is added. A thick dielectric layer will result in a substantial decrease in the reflectance, canceling out the effect of increasing the reflectance by a metal film. This is the reason why the protection layer was made relatively thin (10 nm). The SiO2 layer below the Cr layer was introduced as a buffer layer to enhance adhesion. The light incident from the backside of the lens will be partially reflected back by the Cr layer and this back-reflection can give rise to an eye-strain. Thus, the buffer layer was made rather thick (50 nm) in order to reduce the back-reflection. The AR coating on the backside is to suppress the surface reflection. It consisted of 5 layers of SiO2 (low index) and ZrO2 (high index) films. Since the AR coating design is already well established, it is not here discussed in detail. The dependence of transmittance on the thickness of Cr film was investigated. As expected, the transmittance decreased with increasing thickness. For a Cr thickness over 10 nm, the transmittance was too low to be used for sunglasses. The appropriate thickness was found to be 4– 6 nm. Fig. 3 compares the transmission spectra of mirrorcoated lenses (5 nm Cr) with those of commercial lenses. The compared commercial lenses have AR coatings on both sides. As shown, the transmittance was decreased uni-
60 40
Mirror (5 nm Cr) + AR
20 0 300
400
500 600 Wavelength (nm)
700
Fig. 3. Transmission spectra of mirror-coated lenses (5 nm Cr) and commercial lenses: (a) brown lenses; (b) smoke lenses.
formly over the visible range with a Cr film in both brown and smoke lenses. The UV transmission (320–400 nm) in the mirror-coated lens was also reduced to almost half that of a commercial lens (inset of Fig. 3a). A complete blocking of the UV light in this range may be achieved by increasing the thickness of the Cr film. But this will make the visible transmittance of the lens too low to be used for sunglasses. In addition, the photochromic effect will be weakened because this near-UV light plays a role of pump light for coloration. Fig. 4 compares the photochromic behavior of a mirror-coated brown lens with the result of a commercial lens, in which the colored spectra were obtained immediately after exposure to sunlight for 1 min. Of course, the colored spectrum will depend on the strength of the sunlight, but it is possible to compare the results obtained under the same condition. While the transmittance of the clear lens varies from 90% to 50% at k = 500 nm, the mirror-coated lens has a range of 45–25%. The obtained transmission range is consistent with the value required for sunglasses. The smaller transmission variation in the mirror-coated lens is because the amount of incident UV photons is decreased by the Cr film. Fig. 5 illustrates the reflections from both surfaces of a mirror-coated lens. While the front surface is highly reflecting, reflection from
S. Shin, M. Lee / Optical Materials 30 (2007) 438–441
a
441
100 Before exposure to sunlight
Transmission (%)
80 60 40
After exposure to sunlight for 1 min
20 0 300
400
500 600 Wavelength (nm)
700
60
Transmission (%)
50
Before exposure to sunlight
40 30 After exposure to sunlight for 1 min
20 10 0 300
400
500 600 Wavelength (nm)
700
Fig. 4. Photochromic behavior of (a) a commercial brown lens and (b) a mirror-coated brown lens.
the other surface is relatively weak. The weak reflection from the backside can be explained by the existence of an AR coating and a thick SiO2 buffer layer. 5. Conclusion We have designed and fabricated mirror-coated photochromic lenses for use in sunglasses. These lenses consisted of a thin Cr film sandwiched between two SiO2 layers on the front surface and an AR coating on the backside. Deposition of all coating layers were carried out by an e-beam evaporator under Ar atmosphere at P = 105 Torr and T = 70 °C. The overall transmittance of the lens was controlled by the thickness of the Cr film. A SiO2 film of 50 nm was inserted as a buffer layer to enhance Cr adhesion and the Cr film was covered with a SiO2 protection layer of 10 nm. The AR coating on the backside was introduced to reduce the surface reflection. As expected, the overall transmittance decreased with increasing Cr thickness. For a Cr layer of 5 nm thickness, it changed from about 45% before coloration to 25% after coloration. This is consistent with
Fig. 5. Reflection from the front surface (upper) and the backside (lower).
the transmission range typically required for sunglasses. While the front surface was highly reflecting, reflection from the other surface was relatively weak. The weak reflection from the backside can be explained by the existence of a thick SiO2 buffer layer and an AR coating. Throughout the work, the thicknesses of SiO2 layers were fixed and the optimum values may be different from those given here. Nevertheless, the obtained results indicate that the suggested coating structure is an effective design for photochromic sunglasses. References [1] [2] [3] [4] [5]
T. Gong, J. Feng, W. Wei, W. Huang, Prog. Chem. 18 (2006) 298. A. Toriumi, S. Kawata, M. Gu, Opt. Lett. 23 (1998) 1924. F. Raymo, M. Tomasulo, J. Phys. Chem. A 109 (2005) 7343. K. Buse, A. Adibi, D. Psaltis, Nature 393 (1998) 665. M. Lee, S. Takekawa, Y. Furukawa, K. Kitamura, Phys. Rev. Lett. 84 (2000) 875. [6] H. Macleod, Thin Film Optical Filters, third ed., IOP, 2001. [7] C. Huangbo, Thin Film Optics, Techmedia, 2005. [8] E. Hecht, Optics, second ed., Addison Wesley, 1990.