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Thin film polarization colour-selective beamsplitter
ELSEVIER
Thin Solid Films 268 ( 1995) 137-139
Thin film polarization colour-selective beamsplitter P.F. Gu, X. Liu, H.F. Li, Z.D. Zheng, J.F. Tang Department
of Optical Engineering. Zhejiang University, Hangzhnu 310027, People’s Republic ofChinu Received 16 December
1994; accepted
16 June 1995
Abstract A thin film polarization-selective beamsplitter system used for a liquid-crystal light-valve image projector has been presented. The system involves three beamsplitters which serve as either a polarizer/analyzer or a separator/synthesizer for the three primary colours. These filters have been realized with multilayer optical coatings. The design, manufacture and performance measurement of the filters have been described. Keyword.r:
Optical coatings;
Coatings
1. Introduction
The application of the liquid-crystal light valve (LCLV) to the large screen display has made substantial progress in the past decade I 1,2]. The light-addressed, reflective-mode LCLV provides the possibility to construct a compact fullcolour large-screen projector, for displaying high-brightness and high-resolution images. In such a projector, the role of a colour-selective polarization beamsplitter is the essential part to separate an input light beam into different optical paths for polarization and colour processing, because the LCLV is worked in the polarized light state. The function of the polarization-selective beamsplitter in a large-screen display system involves two aspects: polarizing light radiation and separating the light beam into three primary colours. The properties of this system have a great influence on the brightness, contrast and colour distortion of the display images. A polarization-selective beamsplitter system with good performance has been designed, which consists of three polarizer/colour separators and trimming filters. The whole system is immersed in an oil tank, where the oil index matches the index of the glass substrates of the beamsplitters. This beamsplitter system has the advantage of uniform, low birefringence compared with that of the glass-prism beamsplitters [ 3,4]. A system with an extinction ratio greater than 200 for red and green colours and of about 80 for blue colour has been achieved. The ratio of projection energy between red, green and blue radiation is about 1:4.5:0.06. 0040-6090/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDIOO40-60900040-6090(9.5)06819-8
2. Large-screen
display system
A conceptual sketch of a LCLV large-screen display system is shown in Fig. 1. The light from a xenon arc lamp is collimated by a lens LO, and is polarized, and separated into three primary colours by the polarization selective beamsplitter. The three polarized colour light beams illuminate each light valve in the red, green and blue channels respectively, then are modulated by the light valves which are driven by the light images on cathode rays tubes (CRT), and are reflected back by the valves, synthesized to a full colour image by the main polarizer (f2), and projected to the screen by the object lens.
3. Beamsplitter
prism
The key to realizing a LCLV-based large-screen display system is the configuration of the polarizing selective beamsplitter prism. As shown in the Fig. 1, the system involves three polarization selective filters and three trimming filters. All filters were immersed in a liquid oil of which the refractive index matches that of glass substrates of the filters. Such an arrangement has the advantage of low birefringence and more compact structure in comparison with glass prisms. The first colour-selective polarization beamsplitter fl is a green colour prepolarizer which reflects the s-polarized green light, and transmits s-polarized red, blue light and the whole broadband white p-polarized light, as shown in Fig. 2(a). The second broadband polarization beamsplitter f2 serves as a main polarizer/analyzer, its desired transmittance curve is presented in Fig. 2(b) . This means that f2 reflects the white
138
P.F. Gu et al. /Thin Solid Films 268 (1995) 137-139
Sr n
Once the refractive indexes are selected, the angle of incidence will be determined. The design of the prepolarizer fl , for the designated angle of incidence, is given by: G/ (0.73H0.657L)
“/G
ng
= 1.47 nH= 1.62 nL= 1.46 h,=790nm
Fig. 1. Liquid-crystal light-valve large-screen display system: S, xenon arc lamp; D, diaphragm; FO, heat filter; Ml, mirror; fl, prepolarizer; f2, polarizer/analyzer; t3, colour separator; LO, lens; fo, fa. fn. trimming filters; L, projective lens; Sr, Screen; Lo, LK, La, light valves; Tr, Tg, Tb, R, G. B, CRTs.
0,=48’
To increase the s-reflectance at wavelength of 500-600 nm, while maintaining the p-transmittance at or very near unity, the design approach for s-polarized radiation is based on the design technique of the minus filter, while for p-polarized light, the Brewster condition between high- and lowindex materials is satisfied. The extinction ratio of the main polarizer/analyzer f2 is most important for projection image contrast. The Brewster condition for p-polarized in the design is satisfied. To expend the reflective band of the s-polarized light, two multilayer stakes are necessary. The final design can be written: G/(0.611H0.4L)5(0.872H0.576L)5/G
ng= 1.47 nH
= 2.35 nL = 1.44 A0= 820 nm $ = 57” s-component light and transmits the white p-component light. Since the incident plane of the prepolarizer fl is oriented at 90” with respect to that of the main polarizer f2, the p-polarized light with respect to the first prepolarizer fl will be spolarized light with respect to the main polarizer f2, and vice versa. This change in orientation of the polarization plane through the combination of the prepolarizer and main polarizer, greatly improves the extinction ratio of the light. It can be seen that fl reflects the white s-polarized light into the green light valve and transmits blue and red p-polarized light to the blue and red channels. The green trimming broadbandpass filter (fo) reflects the blue and red s-polarization light back to the illumination system. Only the s-polarized green light illuminates the green LCLV (Lo). The illumination of s-polarized light is modulated (the polarization plane is changed) and reflected by Lo, then transmits through the analyzer f2 to the projection lens. A third colour-separating beamsplitter f3 transmits the blue p-polarized light to blue LCLV (Ln) and reflects the red p-polarized light to the red LCLV ( LR). Similar to the green channel, this p-polarized light is filtered by f, and f,, trimming filters and is modulated by L, and L, valves respectively, and then reflected by analyzer f2 to the projection lens. The required performance of f3 is shown in Fig. 2(c). All the prisms were constructed with multilayer thin films deposited on glass plates and immersed in an index matching oil tank. It is possible to find out the angle of incidence in the oil prism, by applying the Brewster condition to the interfaces between two materials of different refractive index. When the Brewster condition is satisfied, the reflectance of p-polarized light vanishes. In this case, the incident angle $ depends on the oil index no and the thin film indexes of nH and n,_ [ 51:
Since the green colour is split from the red and blue light by the prepolarizer fl, the design for the red and blue separating beamsplitter becomes easy. The only way is to select suitable high- and low-index materials, which are far from the Brewster condition, and have a low polarization effect, This design is as follows: G/0.54M(0.69H0.46L)i50.54M/G
ng = 1.46 nM
= 1.45 nH= 1.7 nL= 1.37 ho= 1125 nm $=57’
\Z]*
400
500
600
700
WAVELENGTH 0)
GREEN PREPOLARIZER
~‘I$y,-y
t-
400
b)
500 WAVELENGTH
BROADBAND POLARIZER/
700 ANALYZER
PrS pj-y~
*
400
c)
500 600 WAVELENGTH
RED-BLUE
700
COLOR SEPARATOR
Fig. 2. The idealized transmissioncurves splitter.
forthe polarization-selectivebeam-
*i; El@@ Q+J
P.F. Gu et al. /Thin Solid Films 268 (1995) 137-139
-- - -_-
IO
9
1I
-__ --_ __ -. _ ___ -. -- a - -- _ -. -
._
139
7
I
pJ1 5
.*
4
g60-
3
2 r-402 w
6
Fig. 3. Set-up for measurement of the polarized spectral performance: I, light source; 2. focus lens; 3, monochromator; 4, collimater; 5, polarizer; 6, spectrometer; 7, oil box; 8. specimen; 9, analyzer; 10, integrator; 11, PMT.
s
WAVELENGTH
(NM)
100‘ P
The design of the trimming filters is a common pass filter or short-wave-pass or broadband-pass
long-wavefilters.
$ Zt30k!? I’ zso,Q
4. Preparation
and performance
measurement
The layer thickness is the quarter-wave optical thickness for all the designs at normal incidence, so it is easy to monitor by means of the common turning-point method. Since the materials are oxides, electron beam evaporation is employed. To reduce the absorption of the films, oxygen ion-assisted deposition is used during the deposition. To measure the performance of the filters, the experimental set-up, as shown in Fig. 3, was constructed, which can be used for measuring the transmittances and the reflectances of s-and p-polarized light T,, Tp, and R,, R,. The light of an incandescent lamp is collimated by a lens, and then focused on the input slit of a monochromator. The output light from the monochromator is collimated again, and polarized by a polarizer, and then passes through the oil tank in which the sample lies. The output light from the oil tank is analyzed by an analyzer, and detected by a photomultiplier tube with an integrated sphere. The polarizer and analyzer can be rotated to produce both s-and p- polarization states, and the system extinction ratio is better than 1000: 1 from 420 nm to 750 nm. Fig. 4 shows the measured results of T, and Tp for the colour selective prepolarizer fl, main polarizer/analyzer f2 and colour-separating beamsplitter f3; the measured transmittance curves are close to the calculated ones. Using an illuminometer, we estimated the extinction ratio for the red, green and blue channels. A ratio of greater than 200 for the red and green channels and about 80 for the blue channel has been obtained in actual test measurements. Also, the ratio of projection visual energy between red, green and blue is about 1:4.5:0.06, which can give a satisfied full-colour image display.
5. Conclusion The above polarization colour-selective beamsplitter system has the characteristics of a high-polarization extinction
400
GO0 500 WAVELENGTH (NM)
700
P
WAVELENGTH
(NM)
Fig. 4. Measured polarized spectral transmission main polarizer; C, color separator.
curves: A, prepolarizer;
B,
ratio, and regularity in the thin film structure so that it can be easily deposited by the general optical-coating monitoring techniques. It has been produced and used for a full-colour large-screen display system. The projector system can project a light radiation of about 1500 lumens with a contrast ratio of better than 20: 1. The projected image size is 3 X 4 m2 with a projection distance of 15 m. The experimental results of the prototype LCLV large-screen projector indicate that the operation and performance of the colour-selective polarization beamsplitter system is very suitable in such kinds of projector systems.
References [l] R.M. Carbone and D. Maclver, Proc. SPIE, 76 ( 1987) 6. [ 21 H.F. Li, P.F. Gu, X. Liu, T. Mei and J.F. Tang, Proc. VIE, 2000 ( 1993) 13. [3] R.S. Gold and A.G. Ledebuht, Proc. SPIE, 526 (1985) 15. [4] J. Koda, A. Henderson, A. Ledebuhr, W. Blehaand K. Huelsman, Proc. SPIE, 76 (1987) 78. [ 51 H.A. Macleod, Thin Film Optical Filters, Chapter 12. ADAM Hilger LTD. London, 1969, p. 303.
Thin Solid Films 268 ( 1995) 137-139
Thin film polarization colour-selective beamsplitter P.F. Gu, X. Liu, H.F. Li, Z.D. Zheng, J.F. Tang Department
of Optical Engineering. Zhejiang University, Hangzhnu 310027, People’s Republic ofChinu Received 16 December
1994; accepted
16 June 1995
Abstract A thin film polarization-selective beamsplitter system used for a liquid-crystal light-valve image projector has been presented. The system involves three beamsplitters which serve as either a polarizer/analyzer or a separator/synthesizer for the three primary colours. These filters have been realized with multilayer optical coatings. The design, manufacture and performance measurement of the filters have been described. Keyword.r:
Optical coatings;
Coatings
1. Introduction
The application of the liquid-crystal light valve (LCLV) to the large screen display has made substantial progress in the past decade I 1,2]. The light-addressed, reflective-mode LCLV provides the possibility to construct a compact fullcolour large-screen projector, for displaying high-brightness and high-resolution images. In such a projector, the role of a colour-selective polarization beamsplitter is the essential part to separate an input light beam into different optical paths for polarization and colour processing, because the LCLV is worked in the polarized light state. The function of the polarization-selective beamsplitter in a large-screen display system involves two aspects: polarizing light radiation and separating the light beam into three primary colours. The properties of this system have a great influence on the brightness, contrast and colour distortion of the display images. A polarization-selective beamsplitter system with good performance has been designed, which consists of three polarizer/colour separators and trimming filters. The whole system is immersed in an oil tank, where the oil index matches the index of the glass substrates of the beamsplitters. This beamsplitter system has the advantage of uniform, low birefringence compared with that of the glass-prism beamsplitters [ 3,4]. A system with an extinction ratio greater than 200 for red and green colours and of about 80 for blue colour has been achieved. The ratio of projection energy between red, green and blue radiation is about 1:4.5:0.06. 0040-6090/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDIOO40-60900040-6090(9.5)06819-8
2. Large-screen
display system
A conceptual sketch of a LCLV large-screen display system is shown in Fig. 1. The light from a xenon arc lamp is collimated by a lens LO, and is polarized, and separated into three primary colours by the polarization selective beamsplitter. The three polarized colour light beams illuminate each light valve in the red, green and blue channels respectively, then are modulated by the light valves which are driven by the light images on cathode rays tubes (CRT), and are reflected back by the valves, synthesized to a full colour image by the main polarizer (f2), and projected to the screen by the object lens.
3. Beamsplitter
prism
The key to realizing a LCLV-based large-screen display system is the configuration of the polarizing selective beamsplitter prism. As shown in the Fig. 1, the system involves three polarization selective filters and three trimming filters. All filters were immersed in a liquid oil of which the refractive index matches that of glass substrates of the filters. Such an arrangement has the advantage of low birefringence and more compact structure in comparison with glass prisms. The first colour-selective polarization beamsplitter fl is a green colour prepolarizer which reflects the s-polarized green light, and transmits s-polarized red, blue light and the whole broadband white p-polarized light, as shown in Fig. 2(a). The second broadband polarization beamsplitter f2 serves as a main polarizer/analyzer, its desired transmittance curve is presented in Fig. 2(b) . This means that f2 reflects the white
138
P.F. Gu et al. /Thin Solid Films 268 (1995) 137-139
Sr n
Once the refractive indexes are selected, the angle of incidence will be determined. The design of the prepolarizer fl , for the designated angle of incidence, is given by: G/ (0.73H0.657L)
“/G
ng
= 1.47 nH= 1.62 nL= 1.46 h,=790nm
Fig. 1. Liquid-crystal light-valve large-screen display system: S, xenon arc lamp; D, diaphragm; FO, heat filter; Ml, mirror; fl, prepolarizer; f2, polarizer/analyzer; t3, colour separator; LO, lens; fo, fa. fn. trimming filters; L, projective lens; Sr, Screen; Lo, LK, La, light valves; Tr, Tg, Tb, R, G. B, CRTs.
0,=48’
To increase the s-reflectance at wavelength of 500-600 nm, while maintaining the p-transmittance at or very near unity, the design approach for s-polarized radiation is based on the design technique of the minus filter, while for p-polarized light, the Brewster condition between high- and lowindex materials is satisfied. The extinction ratio of the main polarizer/analyzer f2 is most important for projection image contrast. The Brewster condition for p-polarized in the design is satisfied. To expend the reflective band of the s-polarized light, two multilayer stakes are necessary. The final design can be written: G/(0.611H0.4L)5(0.872H0.576L)5/G
ng= 1.47 nH
= 2.35 nL = 1.44 A0= 820 nm $ = 57” s-component light and transmits the white p-component light. Since the incident plane of the prepolarizer fl is oriented at 90” with respect to that of the main polarizer f2, the p-polarized light with respect to the first prepolarizer fl will be spolarized light with respect to the main polarizer f2, and vice versa. This change in orientation of the polarization plane through the combination of the prepolarizer and main polarizer, greatly improves the extinction ratio of the light. It can be seen that fl reflects the white s-polarized light into the green light valve and transmits blue and red p-polarized light to the blue and red channels. The green trimming broadbandpass filter (fo) reflects the blue and red s-polarization light back to the illumination system. Only the s-polarized green light illuminates the green LCLV (Lo). The illumination of s-polarized light is modulated (the polarization plane is changed) and reflected by Lo, then transmits through the analyzer f2 to the projection lens. A third colour-separating beamsplitter f3 transmits the blue p-polarized light to blue LCLV (Ln) and reflects the red p-polarized light to the red LCLV ( LR). Similar to the green channel, this p-polarized light is filtered by f, and f,, trimming filters and is modulated by L, and L, valves respectively, and then reflected by analyzer f2 to the projection lens. The required performance of f3 is shown in Fig. 2(c). All the prisms were constructed with multilayer thin films deposited on glass plates and immersed in an index matching oil tank. It is possible to find out the angle of incidence in the oil prism, by applying the Brewster condition to the interfaces between two materials of different refractive index. When the Brewster condition is satisfied, the reflectance of p-polarized light vanishes. In this case, the incident angle $ depends on the oil index no and the thin film indexes of nH and n,_ [ 51:
Since the green colour is split from the red and blue light by the prepolarizer fl, the design for the red and blue separating beamsplitter becomes easy. The only way is to select suitable high- and low-index materials, which are far from the Brewster condition, and have a low polarization effect, This design is as follows: G/0.54M(0.69H0.46L)i50.54M/G
ng = 1.46 nM
= 1.45 nH= 1.7 nL= 1.37 ho= 1125 nm $=57’
\Z]*
400
500
600
700
WAVELENGTH 0)
GREEN PREPOLARIZER
~‘I$y,-y
t-
400
b)
500 WAVELENGTH
BROADBAND POLARIZER/
700 ANALYZER
PrS pj-y~
*
400
c)
500 600 WAVELENGTH
RED-BLUE
700
COLOR SEPARATOR
Fig. 2. The idealized transmissioncurves splitter.
forthe polarization-selectivebeam-
*i; El@@ Q+J
P.F. Gu et al. /Thin Solid Films 268 (1995) 137-139
-- - -_-
IO
9
1I
-__ --_ __ -. _ ___ -. -- a - -- _ -. -
._
139
7
I
pJ1 5
.*
4
g60-
3
2 r-402 w
6
Fig. 3. Set-up for measurement of the polarized spectral performance: I, light source; 2. focus lens; 3, monochromator; 4, collimater; 5, polarizer; 6, spectrometer; 7, oil box; 8. specimen; 9, analyzer; 10, integrator; 11, PMT.
s
WAVELENGTH
(NM)
100‘ P
The design of the trimming filters is a common pass filter or short-wave-pass or broadband-pass
long-wavefilters.
$ Zt30k!? I’ zso,Q
4. Preparation
and performance
measurement
The layer thickness is the quarter-wave optical thickness for all the designs at normal incidence, so it is easy to monitor by means of the common turning-point method. Since the materials are oxides, electron beam evaporation is employed. To reduce the absorption of the films, oxygen ion-assisted deposition is used during the deposition. To measure the performance of the filters, the experimental set-up, as shown in Fig. 3, was constructed, which can be used for measuring the transmittances and the reflectances of s-and p-polarized light T,, Tp, and R,, R,. The light of an incandescent lamp is collimated by a lens, and then focused on the input slit of a monochromator. The output light from the monochromator is collimated again, and polarized by a polarizer, and then passes through the oil tank in which the sample lies. The output light from the oil tank is analyzed by an analyzer, and detected by a photomultiplier tube with an integrated sphere. The polarizer and analyzer can be rotated to produce both s-and p- polarization states, and the system extinction ratio is better than 1000: 1 from 420 nm to 750 nm. Fig. 4 shows the measured results of T, and Tp for the colour selective prepolarizer fl, main polarizer/analyzer f2 and colour-separating beamsplitter f3; the measured transmittance curves are close to the calculated ones. Using an illuminometer, we estimated the extinction ratio for the red, green and blue channels. A ratio of greater than 200 for the red and green channels and about 80 for the blue channel has been obtained in actual test measurements. Also, the ratio of projection visual energy between red, green and blue is about 1:4.5:0.06, which can give a satisfied full-colour image display.
5. Conclusion The above polarization colour-selective beamsplitter system has the characteristics of a high-polarization extinction
400
GO0 500 WAVELENGTH (NM)
700
P
WAVELENGTH
(NM)
Fig. 4. Measured polarized spectral transmission main polarizer; C, color separator.
curves: A, prepolarizer;
B,
ratio, and regularity in the thin film structure so that it can be easily deposited by the general optical-coating monitoring techniques. It has been produced and used for a full-colour large-screen display system. The projector system can project a light radiation of about 1500 lumens with a contrast ratio of better than 20: 1. The projected image size is 3 X 4 m2 with a projection distance of 15 m. The experimental results of the prototype LCLV large-screen projector indicate that the operation and performance of the colour-selective polarization beamsplitter system is very suitable in such kinds of projector systems.
References [l] R.M. Carbone and D. Maclver, Proc. SPIE, 76 ( 1987) 6. [ 21 H.F. Li, P.F. Gu, X. Liu, T. Mei and J.F. Tang, Proc. VIE, 2000 ( 1993) 13. [3] R.S. Gold and A.G. Ledebuht, Proc. SPIE, 526 (1985) 15. [4] J. Koda, A. Henderson, A. Ledebuhr, W. Blehaand K. Huelsman, Proc. SPIE, 76 (1987) 78. [ 51 H.A. Macleod, Thin Film Optical Filters, Chapter 12. ADAM Hilger LTD. London, 1969, p. 303.