JP2000267127A - Wavelength variable color filter using liquid crystal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To use one voltage applying device as a common device for all the cells of a wavelength variable color filter using a liquid crystal. SOLUTION: A plurality of ECB cells 1 are disposed between a first and second polarizers 10, 10A with the transmission axes perpendicular to each other and disposed along the transmission direction. The cells 1 are disposed in such a manner that the angle between the optical axes of the cells 1 and the transmission axis of the first polarizer 10 is controlled to 45 deg. and that third polarizers 10B are interposed between every pair of cells 1 along the transmission direction with the transmission axis of the polarizer 10B parallel to that of the first polarizer 10. The thickness of the cells in this order is controlled to show geometric progression with a common ratio of 2 except for the last cell, and the thickness of the last cell is 1.5 times that of the first cell.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a tunable color filter using liquid crystal.

[0002]

2. Description of the Related Art Conventionally, generally used color filters have a fixed transmission wavelength. That is, the red filter only has red light, the green filter has only green light,
The blue filter transmits only a specific wavelength such as blue light only, and changing the transmission wavelength requires replacing the filter. As a technique for solving this inconvenience, there is known a wavelength variable color filter in which the birefringence effect of a nematic liquid crystal is applied to the principle of a Lyot-filter.

As shown in FIG. 4A, for example, a Rio filter is provided between parallel Nicol polarizers 10 (between parallel polarizers).
The angle between the optical axis and the transmission axis is 45
°, and the thickness of each birefringent plate 11 is doubled so as to increase the birefringence by a power of 2 so that the birefringence increases by a power of 2. Be composed. Note that adjacent birefringent plates can share a polarizer therebetween (the same applies hereinafter).

The transmission spectra of F1, F2 and F3 are shown in FIG.
As shown in (b), (c) and (d), the specific wavelength λ
_{At 0} (relative wavelength λ ₀ / λ = 1), a behavior is exhibited in which the peak interval is sequentially reduced by half while maintaining the peak of the transmittance 1. When these are superimposed, as shown in FIG. 4 (e), a sharp peak of transmittance 1 is obtained at a specific wavelength λ ₀ (B. Lyot: Comptes Rendus 197, 1953 (1933)).
). That is, this configuration provides good characteristics as a bandpass filter. The number of films is 4
It may be the above.

As a wavelength-variable color filter that applies the birefringence effect of a nematic liquid crystal to the principle of the lio filter, a modified liquid crystal lio filter has been proposed (K. Sat).
o, N.Kato, Y.Hanazawa and T.Uchida: Proc.Japan Di
splay, 392 (1989); Proc. SID 32,183 (1991)). This is, for example, as shown in FIG. 5, an electric field controlled birefringence cell using a nematic liquid crystal instead of a uniaxial crystal as a birefringent plate.
l; abbreviated as ECB cell) 1
The angle between the optical axis and the transmission axis is 45 ° between the B cell 1 and the parallel polarizer.
Parallel Nicol liquid crystal cells C1, C
2 in order of transmission, and after C2, in order to exclude wavelengths other than the desired wavelength from the visible region, the ECB cell 1 is placed between the vertical polarizers with the optical axis and the transmission axis (the transmission axis of the polarizer 10). Is added to a vertical Nicol liquid crystal cell C3 arranged so that the angle is 45 °.

The thickness (cell thickness) d of each ECB cell 1 is the same, and a voltage applying device 30 is individually connected to each ECB cell 1. According to this configuration, the parallel Nicol liquid crystal cell C
1, C2, and the voltage applied to the vertical Nicol liquid crystal cell C3 are adjusted to meet the birefringence condition of the lyo filter, whereby a transmission spectrum with a sharp peak is obtained.
By changing the combination of these voltages, the transmission peak wavelength (the main wavelength of the transmission spectrum) can be changed, and a sufficient amount of R (red), G (green), and B (blue) transmitted light can be obtained.
The number of parallel Nicol liquid crystal cells may be three or more.

[0007]

However, the following problem remains in the modified liquid crystal lyo filter. First, voltage application devices are required for the number of cells. In addition, it depends on the birefringence effect of the liquid crystal and thus is easily affected by temperature. That is, the transmittance and the transmission peak wavelength change due to the temperature change. Further, even if an attempt is made to compensate for the influence of temperature by voltage, the circuit becomes complicated because compensation is required for each cell.

The present invention has been made in view of the above-mentioned problems of the prior art,
It is an object of the present invention to enable one voltage application device to be shared by all cells in a wavelength variable color filter using liquid crystal.

[0009]

The present inventors first studied the optical characteristics of the liquid crystal layer in order to achieve the above object. The contents of this study will be described. The retardation δ (V) indicating the magnitude of birefringence is expressed by the following equation (1). That, [delta] (V) is multiplied by the cell thickness d to the difference in the vertical direction of the refractive index n _o of the refractive index n _e and the optical axis of the direction parallel to the optical axis. Where n _o is constant, n _e is different for each liquid crystal molecule orientation state is represented by the equation (1) (2). Also, n
(Θ) is the angle θ between each liquid crystal molecule and a plane perpendicular to the transmission direction.
Is given by Equation (3).

[0010]

(Equation 1)

On the other hand, the orientation of the liquid crystal is determined by the elastic continuum theory. According to the elastic continuum theory, the relationship between the angle θ between each liquid crystal molecule and a plane perpendicular to the transmission direction and the distance x in the cell thickness direction is expressed by a differential equation (Equation 2) (P. Sheng).
: RCA Review 35, (1974) p.408).

[0012]

(Equation 2)

By solving the above equations (1) to (4), the retardation δ (V) at the time of applying a voltage is expressed by the following equation (5). Here, θ _M is a function of voltage, and the other variables are eigenvalues of the liquid crystal or liquid crystal cell. Therefore, although the retardation can be changed by changing the voltage, the point to be noted here is that δ (V) is proportional to the cell thickness d. This indicates that when the same voltage is applied to a plurality of cells having different cell thicknesses, the ratio of the individual retardations becomes equal to the ratio of the cell thicknesses.

[0014]

(Equation 3)

From the above, in the modified liquid crystal lyo filter, if a combination of cell thicknesses that satisfies the birefringence condition of the lyo filter under the condition that the same voltage is applied to each cell is adopted, one voltage can be obtained. It is considered that a wavelength tunable color filter capable of selectively transmitting each of RGB light beams by the application device can be realized. Therefore, when such a combination of cell thicknesses is obtained by calculation, the cell thickness is determined between the parallel polarizers by d and d.
2d and 1.5d between the vertical polarizers.

The present invention has been made based on the above findings, and the gist of the present invention is that a plurality of ECs are provided between first and second polarizers arranged in transmission order with transmission axes orthogonal to each other.
B cell, the angle between the optical axis of the cell and the transmission axis of the first polarizer is 45 °, and a third polarizer having a transmission axis parallel to the first polarizer is interposed between the cells, A wavelength tunable color filter using liquid crystal, wherein cells are arranged in transmission order, and the cell thickness in the arrangement order forms a geometric progression having a common ratio of 2 except for the last, and the last cell thickness is 1.5 times the first. It is in.

In the present invention, 22.5 to 67.5
° and a common ratio of 1.6 to 2.4 instead of the common ratio of 2,
1.1 to 1.9 times can be used instead of 1.5 times. Further, the ECB cell is preferably an OCB cell formed by superposing a retardation film on a bend cell.

[0018]

FIG. 1 is a schematic diagram showing a cell structure of a prototype of the present invention. Three ECB cells 1 having different cell thicknesses are arranged in a transmission order between a vertical polarizer (a first polarizer 10 and a second polarizer 10A arranged in a transmission order with their transmission axes orthogonal to each other). In this example, a third polarizer 10B having a transmission axis parallel to the first polarizer 10 is interposed, and the first (between parallel polarizers) cell thickness is d, and the next (between parallel polarizers) cell thickness. Is 2d, and the final cell thickness (between the vertical polarizers) is 1.5d. Each cell is arranged such that the optical axis forms an angle of 45 ° with the transmission axis of the first polarizer 10. Each cell is connected in parallel to a common voltage application device 30, and the same control voltage is applied to each cell.

In this example, LIXON GR41 manufactured by Chisso Corporation is used as the liquid crystal of the ECB cell 1, and the cell thickness is 4 μm (= d), 8 μm (= 2d), 6 μm (= 1.5) in the order of transmission.
d). The transmission characteristics were investigated using this prototype.
The result is shown in FIG. When the control voltage is 1.4 V, 1.7 V, 1.
When the voltage is changed to 9 V, transmittance peaks appear at wavelengths of R, G, and B sequentially. Note that the dotted line in the figure is a theoretical value, and the solid line is an actually measured value.

Here, when the second cell (cell thickness 2d) is omitted (the polarizer 10B on one side is also omitted), the peaks are slightly broadened, while the second and third cells (last) are omitted. Insert a cell with a cell thickness of 16 μm (= 4d) between cells (a polarizer on one side)
When 10B is also inserted), the peak further sharpens, but in any case, the dominant wavelength of the transmission spectrum can be continuously changed by one voltage operation. That is, in the present invention, the number of ECB cells is not limited to three, but may be two or more.

As described above, the cell thickness in the transmission order except for the last cell is configured to form a geometric progression having a common ratio of 2, and the last cell thickness is set to 1.5 times the initial value (more generally, a half integer multiple). ) Realizes a tunable color filter capable of continuously changing the main wavelength of the transmission spectrum by one voltage operation. Even if the angle between the optical axis of each cell and the transmission axis of the first or third polarizer is changed within a range of ± 22.5 ° around 45 °, a transmission characteristic acceptable as a filter is obtained. However, when the value deviates from this range, the peak value significantly decreases. Also, the common ratio is centered on 2, and the magnification of the last cell thickness with respect to the beginning is centered on 1.5, and the transmission characteristics acceptable as a filter can be obtained even if they are changed within a range of ± 0.4, respectively. If the deviation deviates, a plurality of transmission peaks in the visible region appear, and the peak value is significantly reduced.

For this reason, in the present invention, instead of 45 °,
22.5 to 67.5 °, and the common ratio 1.6 to 2.4 instead of the common ratio 2
In place of the above 1.5 times, it can be made 1.1 to 1.9 times. In the present invention, the transmission order may be changed as long as the relative positional relationship between the liquid crystal cell and the polarizer sandwiching the liquid crystal cell is not changed. For example, assuming that the reference cell thickness is d,
It may be 2d between parallel polarizers, 1.5d between vertical polarizers, and d between parallel polarizers.

The cell thickness 1.5d is generally a half integral multiple of d, that is, (m + 1/2) d.
The case of m = 1 is 1.5d. If m is small, relative wavelength λ
_The effect of blocking wavelengths other than ₀ (blocking effect) is large, but the effect of sharpening the spectrum (sharpening effect) is small. Conversely, if m is large, the blocking effect is small, but the sharpening effect is large. In consideration of this point, m may be determined according to the application.

In other words, the generalized gist of the present invention is that a plurality of ECB cells are placed between first and second polarizers arranged in transmission order with their transmission axes orthogonal to each other, and the optical axis of the cell is set to the first axis.
45 ° angle with the transmission axis of the polarizer (allowable range is 22.5 to
67.5 °) and a third polarizer having a transmission axis parallel or perpendicular to the first polarizer is interposed between the cells, and the cells are arranged in the transmission order. The cell thickness between the parallel polarizers is 2 (allowable). The range is a power of 1.6 to 2.4), and the cell thicknesses between the vertical polarizers are half integer times ((m + 1/2) times; m is a natural number;
The tunable color filter using liquid crystal is characterized in that the allowable range of 1/2 (= 0.5) has a relationship of 0.1 to 0.9).

As described above, according to the present invention, the control voltage of each cell can be set to a common value, whereby a bandpass filter capable of arbitrarily adjusting the main wavelength of the transmission spectrum can be configured. . Therefore, one voltage operation is sufficient. In the case where a wavelength variable color filter is constituted by three ECB cells, the ratio of the cell thickness is at most 4 in the conventional modified liquid crystal lyo filter, but is at most approximately 2 in the present invention. According to this, excessive cell thickness can be avoided, and a realistic cell thickness design becomes possible. Then, even if the characteristics of the liquid crystal fluctuate due to a change in the temperature of the system, the voltage operation is reduced by one.
Therefore, the configuration of the temperature compensation circuit is easy.

The ECB cell is an OCB cell (optically compensated bend cell) formed by superposing a retardation film on a bend cell (a liquid crystal cell having a bend alignment).
l) is preferred. This is because OCB cells have the advantage of quick response and a wide viewing angle (eg, T. Miyashita, P. Vetter, M. Suzuki, Y. Yamaguchi a).
nd T. Uchida: Proc. Eurodisplay, p. 149 (1993), JP-A-7-84254, etc.).

When the OCB cell is used, it is desirable that the thickness condition of each of the bend cell and the retardation film be the same as the overall thickness condition in order to meet the birefringence condition of the Lyot filter. That is, the OCB cells having different thicknesses used in the present invention are preferably similar in the thickness direction.

[0028]

FIG. 3 is a schematic diagram showing a cell configuration of an embodiment of the present invention using an OCB cell. An OCB cell 2 in which a retardation film 2B is overlapped on a bend cell 2A was used as an ECB cell. The liquid crystal used for the bend cell 2A is TD6004XX manufactured by Chisso Corporation. The physical properties of this liquid crystal are: elastic constant ratio: κ = 0, dielectric constant ratio: γ = 1.88, Freedericksz threshold: V _c = 1.405 V. As the retardation film 2B, a uniaxial film manufactured by Nitto Denko Corporation was used.

The bend cell 2A and the retardation film 2B
The angle between each optical axis and the transmission axis of the first polarizer 10 is 45
°. The cell thickness of the bend cell and the film thickness of the retardation film were as shown in Table 1 according to the present invention. In this example, the theoretical values of the main wavelength of the transmission spectrum with respect to the applied voltage are as shown in Table 2.

[0030]

[Table 1]

[0031]

[Table 2]

When white light is applied from the back side of the wavelength tunable color filter of this embodiment and the applied voltage is changed in the range of 12.6 to 2.3 V by operating the voltage applying device 30, the front side becomes
According to the relationship in Table 2, it can be visually recognized that the color lights of various main wavelengths are transmitted alternately.

[0033]

As described above, according to the present invention, there is an excellent effect that a bandpass filter which can alternately transmit various colors of light by one voltage operation is realized.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a cell structure of a prototype example of the present invention.

FIG. 2 is a transmission spectrum diagram showing transmission characteristics of a prototype example of the present invention.

FIG. 3 is a schematic diagram showing a cell configuration according to an embodiment of the present invention.

FIG. 4 is a diagram illustrating the principle of a Rio filter.

FIG. 5 is a schematic diagram illustrating a configuration example of a modified liquid crystal Rio filter.

[Explanation of symbols]

 Reference Signs List 1 ECB cell (electric field control birefringent cell) 2 OCB cell 2A bend cell 2B retardation film 10 polarizer (first polarizer) 10A polarizer (second polarizer) 10B polarizer (third polarizer) 11 Birefringent plate (uniaxial crystal) 30 Voltage application device

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Tetsuya Miyashita 207 23 Matsuya Minami Yagiyama 1-9-9, Yagiyama-Minami, Taihaku-ku, Sendai City, Miyagi Prefecture (72) Inventor Moriyoshi Shinki 31 Matsugaoka, Taihaku-ku, Sendai City, Miyagi Prefecture −10 Matsugaoka Corporation 105 (72) Inventor Hidenori Chiba 4-25-7 Tsurugaya, Miyagino-ku, Sendai City, Miyagi Prefecture F-term (reference) 2H048 AA06 AA12 AA24 AA25 AA26 2H088 EA49 GA02 HA16 HA18 JA09 MA20 2H089 HA30 HA32 QA16 RA07 TA14 TA15

Claims (3)

[Claims] 1. A method according to claim 1, wherein a plurality of ECB cells are arranged between the first and second polarizers, the transmission axes of which are orthogonal to each other, and the optical axis of the cell forms an angle with the transmission axis of the first polarizer. A third polarizer having a transmission axis parallel to the first polarizer at 45 ° is interposed between cells and arranged in the transmission order. The cell thickness in the arrangement order is equal to the common ratio of 2 except for the last. A wavelength tunable color filter using liquid crystal, characterized by a series of rows, wherein the last cell thickness is 1.5 times that of the first cell. 2. The wavelength tunable according to claim 1, wherein the common angle is 22.5 to 67.5 ° instead of the 45 °, the common ratio is 1.6 to 2.4 instead of the common ratio 2, and the common ratio is 1.1 to 1.9 instead of the 1.5 times. Color filters. 3. The tunable color filter according to claim 1, wherein the ECB cell is an OCB cell obtained by superposing a retardation film on a bend cell.