GB2034917A

GB2034917A - Wave plate retarders and methods of making such retarders

Info

Publication number: GB2034917A
Application number: GB7931345A
Authority: GB
Original assignee: Sony Corp
Current assignee: Sony Corp
Priority date: 1978-09-09
Filing date: 1979-09-10
Publication date: 1980-06-11
Also published as: JPS5538514A; GB2034917B; FR2435731A1; CA1131058A; AU5068179A; FR2435731B1; NL7906736A; AU527174B2; DE2936483A1

Abstract

A wave plate retarder 10 comprises two or more birefringent high molecular weight films 11 and 12 arranged with a predetermined angle between their respective optical axes to establish a desired phase difference during transmission through the films 11 and 12 of components of light polarized along orthogonal axes. The predetermined angle may be selected to provide a phase difference of 1 DIVIDED 4 or 1 DIVIDED 2 wavelength of light thus to produce a composite wave plate retarder which functions as a 1 DIVIDED 4 or 1 DIVIDED 2 wave plate regardless of the phase difference of components of light produced by transmission through a single film 11 or 12. As shown, polypropylene films 11, 12 are protected by glass or resin plates 13, 14. Figs. 3, 4 (not shown) show apparatus for measuring the phase difference introduced by single and paired plates. Fig. 1 (not shown) shows the retarder used in a video disc player. <IMAGE>

Description

SPECIFICATION Wave plate retarders and methods of making such retarders This invention relates to wave plate retarders and methods of making wave plate retarders.

Embodiments of the invention may form 2 wave plates or 4 wave plates suitable for use, for example, in an optical video disc reproducing apparatus.

In a reproducing apparatus for an optical video disc, an apparatus is known in which light from a laser is caused to impinge on a signal track recorded on the video disc. The reflected laser light therefrom, which is modulated by recorded pits in the signal track, is detected, and the recorded signal is read out. In order that the laser light reflected from the video disc is effectively supplied to the detector, and to avoid the reflected laser light returning to the source and causing noise, a combination of a 41 wave plate and a polarizing beam splitter has been proposed for effectively separating the laser light reflected from the video disc from the incoming light path.

Any suitable birefringent material such as, for example, mica, rock crystal or high molecular weight film may be used to make a W or 3 wave plate. Unfortunately, mica and rock crystal are rather expensive and, while high molecular weight film is relatively inexpensive, it normally has unstable characteristics. High molecular weight films, such as for example, polypropylene film which has been elongated by being stretched during manufacture on an elongation roll, has an optical axis which is aligned with the longitudinal direction of stretch of the film. Such a high molecular weight film has birefringent or double-refractive properties. A material having birefringent properties has different refractive indexes and light transmission speeds for light polarized in different, usually orthogonal, directions.When a beam of light is passed through a birefringent material normal to the optical axis, the birefringent material breaks up the light, into an ordinary ray and an extraordinary ray which travel through the material at different speeds. It is also a property of birefringent material that the ordinary ray and the extraordinary ray are linearly polarized in mutually perpendicular directions. If the thicknes of a high molecular weight film can be selected such that one of the rays passing through the film is retarded by a 41 or wavelength relative to the other ray, the resulting phase difference (retardation r) between the two rays is 90 or 1 80' respectively.

In practice, however, it is difficult to control the thickness of stretched high molecular weight films accurately enough to use them for $ wave or 3 wave plates. Even if the thickness could be controlled with sufficient accuracy, the birefringent properties of the material are not uniform from lot to lot due to unavoidable differences in composition and manufacturing conditions. It is therefore very difficult to select or to manufacture high molecular weight films having a desired phase difference r for a particular wavelength of light.

According to the present invention there is provided a method of making a wave plate retarder comprising the steps of: placing a film of birefringent material having an optical axis with its surface normal to a linearly polarized beam of light; rotating said film about an axis parallel to said beam of light; measuring a linearly polarized component of said beam of light after passing through said film; calculating a retardation phase angle based on said measuring step; forming at least two sheets each having optical axes from said film; and overlapping said at least two sheets with an angle between their optical axes such that a wave plate retarder is formed having a retardation of an integral multiple of 41 wavelength.

According to the present invention there is also provided a wave plate retarder having transmission along a slow axis which is slow relative to transmission along a fast axis orthogonal to said slow axis and comprising: a first sheet of birefringent high molecular weight film having a first optical axis; at least a second sheet of birefringent high molecular weight film having a second optical axis; said first sheet and said at least second sheet being stacked to form said wave plate retarder; and said first and second optical axes being angularly displaced from each other whereby said retardation of said wave plate retarder along said slow axis relative to said fast axis is an integral multiple of X wavelength of light.

The invention will now be described by way of example with reference to the accompanying drawings, throughout which like reference numerals designate like elements, and in which: Figure 1 is a schematic diagram showing a reproducing apparatus for an optical video disc in which an embodiment of wave plate retarder according to the invention may be used; Figure 2 is an enlarged cross-sectional view showing an embodiment of wave plate retarder according to the invention; Figure 3 is a schematic diagram showing an apparatus for measuring the phase difference of the ordinary and extraordinary ray propagated through a single sheet of high molecular weight film; Figure 4 is a schematic diagram showing an apparatus for measuring the characteristics of a pair of sheets of high molecular weight film having their optical axes at various angles to each other;; Figure 5 is a graph showing outputs measured with the apparatus of Fig. 3; Figure 6 is a graph showing the outputs of the apparatus of Fig. 4 with the optical axes of two sheets having an angle of 0 between them; Figure 7 is a graph similar to Fig. 6 except with the optical axes of the two sheets displaced 30 apart; Figure 8 is a graph similar to Fig. 6 with the optical axes of the two sheets displaced 40 apart; Figure 9 is a graph similar to Fig. 6 with the optical axes of the two sheets displaced 50 apart; Figure 10 is a graph similar to Fig. 6 with the optical axes of the two sheets displaced 60 apart;; Figure 11 is a graph similar to Fig. 6 with the optical axes of the two sheets displaced 70 apart; Figure 12 is a graph similar to Fig. 6 with the optical axes of the two sheets displaced 90 apart; Figure 13 is a graph similar to Fig. 6 with the optical axes of the two sheets displaced 1 20' apart; Figure 14 is a graph showing outputs measured with the apparatus of Fig. 3 using a different high molecular weight film; Figure 15 is a graph showing measured output curves of the apparatus of Fig. 4 wherein two sheets of said different high molecular weight film have their optical axes with an angle of 0 between them; Figure 16 is a graph similar to Fig. 15 except with the optical axes of the two sheets displaced 30 apart;; Figure 17 is a graph similar to Fig. 15 with the optical axes of the two sheets displaced 45 apart; Figure 18 is a graph similar to Fig. 15 with the optical axes of the two sheets displaced 60 apart; and Figure 19 is a graph similar to Fig. 15 with the optical axes of the two sheets displaced 90 apart.

Before describing the invention, a description of an apparatus in which an embodiment of wave plate retarder according to the invention may be used will be given with reference to Fig.

1.

A beam of laser light 2 emitted from a Ne-He laser 1 is introduced through a lens system 3, a beam splitter 4, a wave plate retarder having characteristics of a wave plate 5, a mirror 6 and an objective lens system 7 onto a signal track formed on a video disc 8. The beam splitter 4 is a polarizing beam splitter 4 which transmits light which is linearly polarized in a first direction, normal to the plane of Fig. 1, and reflects light which is linearly polarized perpendicular to the first direction. The light reaching the i wave plate 5 is linearly polarized along an axis normal to the plane of Fig. 1. The optical axis of the T wave plate 5 is disposed at 45 to the axis of polarization of the linearly polarized light from the beam splitter 4.As is well known, the output of the 41 wave plate 5 is circularly polarized light having right circular or left circular polarization.

The light beam 2 reflected from and modulated by recorded pits (not shown) in a signal track on the video disc 8 returns through the objective lens system 7, the mirror 6 and the X wave plate 5 5 to the beam splitter 4. Upon reflection from the video disc 8, the "handedness" of the light beam 2 is reversed. That is, if the impinging light beam is right-handed, the reflected light beam is left-handed. When the reflected light beam 2 passes through the X wave plate 5, the circularly polarized reflected light is reconverted to linearly polarized light, but with the axis of polarization at 90 to the axis of polarization of the light beam 2 originally supplied to the k wave plate 5 from the beam splitter 4.Since the reflected light enters the beam splitter 4 with the polarization axis rotated by 90 , it is unable to pass in a straight line through the beam splitter 4, but instead is reflected by 90 onto a detector 9 which may be, for example, a photodiode. The signal recorded on the video disc 8 is thus read out as an electric signal which is supplied to an output terminal t.

Referring now to Fig. 2, there is shown an embodiment of t wave plate 10 according to the invention which can be used as the i wave plate 5 in the apparatus of Fig. 1. Two films 11 and 12 are prepared from high molecular weight film taken from the same lot. The films 11 and 12 have thickness dand a maximum refractive index nO for the ordinary ray and a minimum refractive index ne for the extraordinary ray along orthogonal axes as previously described. The films 11 and 12 are stacked or overlapped with a predetermined angle between their optical axes, and are optionally protected by being encased between two protective plates 13 and 14.

The protective plates 13 and 14 may be bonded by adhesive or supported as a unitary body by a frame (not shown) to form the i wave plate 10. The protective plates 13 and 14 are made of glass, resin or the like through which light having the wavelength used in optical video disc reproduction can pass without double refraction. The use of the high molecular weight films 11 and 12 from the same lot ensures that, even although it is difficult to produce high molecular weight film with precise thickness and optical properties, the two films 11 and 12 will have the same thickness and optical properties, which permits them to be combined into a composite wave plate retarder as described.

The following paragraphs describe how the optical properties of a single sheet of high molecular weight film are measured. The manner in which the measured optical properties are used to construct a i wave plate 10 employing two high molecular weight films 11 and 12 is described later herein.

When a sheet of high molecular weight film transmits light of a particular wavelength A, components of the light polarized parallel to the direction of the maximum refractive index nO are transmitted at a different speed from components of the light polarized parallel to the direction of the minimum refractive index ne. Thus, when the two components of light emerge from the film, a phase difference r exists between them which is proportional to the difference between the refractive indices and the distance the light travelled through the film. The relationship is given by the following well known equation:

Where d= distance.

The method of measuring the phase difference rof a film sheet will be described with reference to Fig. 3. A light source 21, such as a laser tube, a semiconductor laser or the like is provided from which linearly polarized laser light 20 is emitted using, for example, a polarizing plate (not shown). A polarizing beam splitter 22 is located on the light path axis Z of the linearly polarized laser light 20 from the light source 21. A first detector 23 including, for example, a photo-diode, is provided coaxial with the laser light 20 to receive light transmitted through the polarizing beam splitter 22. A second detector 24 may optionally be provided to receive light reflected by the polarizing beam splitter 22.The polarizing beam splitter 22 has the axis of polarization aligned with the axis of polarization of the linearly polarized laser light 20 and functions to transmit therethrough the components of the linearly polarized laser light 20 from the light source 21 and to reflect the component of the laser light 20 polarized at right angles to the axis of polarization.

A film 25 cut from a high molecular weight film sheet, the phase difference r of which is to be measured, is prepared with a mark thereon to indicate the longitudinal direction of the film sheet. The film 25 is located on the axis Z between the light source 21 and the polarizing beam splitter 22 such that the surface of the film 25 intersects the axis Z at right angles. The direction perpendicular to the plane of Fig. 3 is taken as the X direction and the downward direction as the Y direction.The film 25 is supported so that it can be rotated about the Z-axis such that an angle 0 between the direction therein having a refractive index nO and the Y-axis may be varied from 0 to 360 . The linearly polarized laser light 20 from the light source 21 is assumed to be a wave vibrating in the Y-axis direction, and having an amplitude value E of the Y-axis.If it is assumed that the amplitude components of the linearly polarized laser light 20 along the axes which show the refractive indices nO and ne after the linearly polarized laser light 20 passes through the film 25 are Eo and Ee, these amplitude components Eo and Ee are expressed as follows: Eo= E cos O.eieo (2) Ee=-E sin @.ei e (3) Where:: ejso = (cos ~0 + i sin), and eife = (cos ~e + i sin ~e) represent the phases of the light wave immediately after the light passes through the film 25 (the path lengths of the light through the film 25 measured in wavelengths), and Ecos 8 and Esin 0 represent the amplitudes of the light wave, respectively. Accordingly, Eo and Ee represent the effective amplitudes of the light wave immediately after passing through the film 25. In equations (2) and (3), 2, and ~e are expressed as follows:

The phase difference r is qElo~+e as used in equation (1).

An amplitude component ET of the laser light 20 in the Y-axis direction, which passes through the polarizing beam splitter 22 and arrives at the first detector 23, is given by the following expression: ET = Eo cos #-Ee sin H (4) If equations (2) and (3) are substituted into equation (4), the latter is rewritten as follows:

The energy IT of the component ET is expressed as follows:

Accordingly, an output lout from the first detector 23 is expressed by the following equation:: lout = ITα1- sin22# (1-cos r) (5) When 0 = 45 is substituted into equation (5), the output lout given by equation (5) is a minimum output (10ut)min, while when a = 0 is substituted into equation (5), the output loutgiven thereby is a maximum output (IOu,)max, respectively.Thus, a ratio R of both the outputs is given as follows:

Therefore, if the relationship between the rotation angle 8 and the output lout is measured by the first detector 23 and then the maximum and minimum outputs (IOut)max and (lout)min are obtained, the phase difference r is obtained from equation (6). When the film 25 is a X wave plate, r = 90 and R = 2. When the film 25 is a > wave plate, r = 180 and R = 1.

As set forth above, the phase difference r of the film 25 can be measured. Then, first and second films 11 and 12, each made of measured film 25, are overlapped with a predetermined angle between their optical axes to provide a wave plate 10.

The manner in which the angle (#-#) between the optical axes of first and second films 11 and 12 is established to produce a i or < wave plate is described in the following with reference to Fig. 4. The films 11 and 12 are stacked and rotatably located in the light path of the laser light 20 between the light source 21 and the polarizing beam splitter 22. The angles between the optical axes of the first and second films 11 and 12 in the Y-axis are assumed to be H and .The energy IT of the polarization laser light 20, which passes through the polarizing beam splitter 22 and arrives at the first detector 23, is expressed as follows:

When the first and second films 11 and 12 are rotated about the Z-axis while their angle difference (#-#) is held constant at a predetermined value, only the term cos(# + fl) of equation (7) is varied. In order to obtain the maximum and minimum values of the energy when both the films 11 and 12 are rotated about the Z-axis, equation (7) is differentiated with respect to zip so that an angle which will make the differentiated value zero can be calculated.If it is assumed that #-# = C (constant), the following equation is derived from equation (7):

Accordingly, sin (4g-2C) = sin 2( + H) = 0. Thus, when 2 (# + @) is 0 or':', the energy IT becomes a maximum or a minimum. At this time, cos 2 (# + #) becomes + 1.If this relationship is substituted into equation (7), the minimum or maximum value IT(min) or IT(max) of the energy IT can be obtained as follows:

In this case, cos2(#-0) > 0, Icos 2#| < 1 or cos 2r-1 < 0, and cos(#-#)(cos 2r-1) < 0.

From the above, it will be apparent that the minimum value lT(min) is obtained when cos 2 (# + #)= - 1 and hence is expressed as follows:

If #-# = C is substituted into equation (8') and rearranged, the following equation is obtained:

The maximum energy IT(max) becomes equal to E2 if optical loss is neglected.

Accordingly, the ratio R between the minimum and maximum values IT(min) and IT(max) is expressed as follows:

As described in connection with equation (6), the condition for a 4 wave plate is R = Therefore, for a composite wave plate retarder consisting of stacked films 11 and 12 to be a i wave plate, it is necessary that the value of C, that is the angle -S between the optical axes of the films 11 and 12 must satisfy R = + in equation (10). That is, if the above is satisfied, the composite wave plate retarder is a i wave plate.

Now, when equation (10) is set equal to 3 and solved: R = ((1/2 cos 2r-2 cos r + 3/2) cos4C + 2(cosr-1) cos2C + 1) = 1/2.

Since the value of r is determined from equation (6) and the value of R is known from measurement of the film 25, the value C necessary to produce a i wave plate may be determined by substituting the value of r into equation (11). Thus, a composite i wave plate consisting of two stacked films 11 and 12 can be formed by properly selecting the angle ( between the optical axes of the two films 11 and 12, irrespective of the value r of the films 11 and 12.

The above example assigns a value to R in equation (1) of + and hence produces a i wave plate. However, if a value of C is chosen which makes R in equation (10) equal to 1, a + wave plate is produced.

Two examples of the manner in which the invention can be employed will now be described.

In each example, a i wave plate for linearly polarized laser light with a wavelength A of 8300A is desired.

EXAMPLE 1 A A high molecular weight film sheet, identified as P2262 (polypropylene sheet), having a thickness of 15 microns and made by Toyo Spinning Co., Ltd., was used. The film 25 to be measured was cut from a high molecular weight film sheet, and was located in the apparatus of Fig. 3. The light source 21 was a semiconductor laser source, operative to generate laser light at a wavelength A of 8300 . The film 25 was rotated about the Z-axis, and the angle between the Y Y axis and the optical axis of the film 25, and the outputs from the detectors 23 and 24 were measured. The measured results are shown in curves 31 and 32 of Fig. 5. Solid line curve 31 is the output from the first detector 23 and the dashed line curve 32 is the output from the second detector 24.

From the curve 31, it is found that the maximum and minimum outputs from the first detector 23 were 560 microwatts and 430 microwatts, respectively. However, it is also seen from the curve 32 that the minimum output from the second detector 24 was 60 microwatts. In general, it is known that the polarizing beam splitter 22 transmits the component of incident light which is polarized parallel to a surface determined by the incident and reflecting light paths, (called a P-wave), and reflects the component of incident light, which is polarized in a direction perpendicular to the above surface (called an S-wave). For linearly polarized light, a maximum transmitted value of light should coincide with zero reflected light. Since the reflected component detected by the second detector 24 never fell to zero, an unpolarized component must have existed in the laser light 20.

The laser light source 21 used in this example was a semiconductor laser source. In general, it is known that a semiconductor laser source operates in an unpolarized light-emitting diode (LED) mode at low drive currents and begins to operate in the laser mode only when the drive current exceeds a predetermined value. Accordingly, the detected value of the output power of a semiconductor laser source includes an unpolarized LED component as well as a polarized laser light component. Since the minimum detected value of reflected light was 60 microwatts, it was deduced that this amount, due to unpolarized LED emission, was included in the 560 microwatts transmitted light.When this LED component was subtracted from the measured transmitted values, the actual transmitted maximum and minimum outputs due to polarized laser light were expressed as follows: (lout)max = 560 microwatts-60 microwatts (lout) min = 430 microwatts-60 microwatts The above values were substituted into equation (6) and a phase difference r of 43 was calculated (Cos r + 0.48). When this value was substituted into equation (11), a value of C, that is (ç-8), required to produce a 4 wave plate, was calculated to be 41.2 .

The first and second films 11 and 12 were cut from the above measured high molecular weight film 25, and were overlapped or laminated in such a manner that the angle between their optical axes was successively established at angles of 0 , 30 , 40 , 50 , 60 , 70 , 90 , and 1 20'. Optical properties of the resulting stacks were measured by rotating each one about the Z axis in the apparatus of Fig. 4. The measured results are shown in the graphs of Figs. 6 to 13 respectively.In the graphs of Figs. 6 to 13, the solid line curves represent the outputs of the first detector 23, that is the amount of light which is transmitted through the polarizing beam splitter 22, and the dashed line curves represent the outputs from the second detector 24, that is the amount of light which is reflected by the polarizing beam splitter 22. As is clear from the graph of Fig. 8, when the angle between the optical axes of the films 11 and 12 is 40 (near the calculated angle 41.2 ), the minimum value of the light transmitted through the polarizing beam splitter 22, which occurs at an angle of ':'/4, is approximately coincident with the maximum value of light reflected by the polarizing beam splitter 22 as indicated by the solid and dashed line curves respectively.This indicates that the linearly polarized laser light from the light source 21 is subjected to circular polarization by the composite wave plate retarder consisting of the films 11 and 1 2. That is, the composite wave plate retarder acts as a T wave plate for the linearly polarized laser light, so that the light components transmitted and reflected by the polarizing beam splitter 22 become nearly equal. Thus, it is shown that a i wave plate if formed of the films 11 and 12 when their optical axes are disposed about 40 apart.

The graphs of Figs. 6 and 7 indicated that when the films 11 and 12 have their optical axes displaced 0 or 30 apart, respectively, certain rotational angles exist at which the transmitted light component and the reflected light component are equal. The rotary angle positions which accomplish this relationship appear at points where the solid line curves intersect the dashed line curves at steep angles instead of at the slowly changing angles at maxima and minima of the curves shown in the graph of Fig. 8.Therefore, in the cases shown in Figs. 6 and 7, although angles can be found at which characteristics of a g wave plate are observed, if the rotary angle of the wave plate retarder is deviated even a small amount about the Z-axis from the rotary angle positions at which the curves intersect, the transmitted and reflected light components are varied abruptly, and hence such a wave plate retarder ceases to function as a T wave plate. For practical purposes, wave plate retarders having characteristics shown in Figs. 6 and 7 require such high positioning accuracy that they are seldom used.

EXAMPLE 2 A high molecular weight film sheet of Mylar (Registered Trade Mark) (polypropylene) with a thicknes of 12 microns was used. The film 25 to be measured was cut therefrom in a manner similar to Example 1. Light transmitted and reflected by the polarizing beam splitter 22 was measured by the first and second detectors 23 and 24. The measured results are shown in curves 51 and 52 of Fig. 14. From the measured result, the maximum and minimum values of the transmitted light were obtained as follows: (IOu,)max = 500 microwatts-130 microwatts (lout)min = 130 microwatts-1 30microwatts = 0 From equation (6), r was 180 , and from equation (11), C was 67.5 and 22.5 for a wave plate.In this example, the films 11 and 12 were cut from the Mylar sheet, and laminated into a wave plate retarder 10. Angle C was successively selected at 0 , 30 , 45 , 60 and 90 , and the relationships between the rotary angle of the films 11 and 12 about the Z-axis and the measured outputs from the first and second detectors 23 and 24 for the light components transmitted and reflected by the polarizing beam splitter 22 are shown in the graphs of Figs. 15 to 19 by the solid line and dashed line curves, respectively. As shown in Figs. 16 and 18, angles of 30 and 60 (the closest values to 22.5 and 67.5 ) approached the requirements for a - wave plate.

Although the above illustrative examples of the invention are described using films 11 and 12 cut from a common film sheet and stacked with a predetermined angle between their optical axes to produce a desired retardation, it would also be possible to produce a composite wave plate retarder by stacking more than two film sheets or by combining films which are cut from different film sheets with similar effects.

Claims

1. A method of making a wave plate retarder comprising the steps of: placing a film of birefringent material having an optical axis with its surface normal to a linearly polarized beam of light; rotating said film about an axis parallel to said beam of light; measuring a linearly polarized component of said beam of light after passing through said film; calculating a retardation phase angle based on said measuring step; forming at least two sheets each having optical axes from said film; and overlapping said at least two sheets with an angle between their optical axes such that a wave plate retarder is formed having a retardation of an integral multiple of X wavelength.

2. A method according to claim 1 wherein said step of calculating includes determining the ratio of maximum and minimum values of said linearly polarized component in said measuring step and employing said ratio in said calculating step.

3. A method according to claim 2 wherein said retardation phase angle is determined from: said ratio = 3(1 + cosr) and r = the phase angle of retardation.

4. A method according to claim 1 wherein said film is polypropylene which has been uniaxially stretched during the manufacture thereof to define an optical axis parallel to the direction of stretch.

5. A method according to claim 1 wherein said angle is determined from:

where C = said angle between the optical axes r = the retardation phase angle.

6. A wave plate retarder having transmission along a slow axis which is slow relative to transmission along a fast axis orthogonal to said slow axis and comprising: a first sheet of birefringent high molecular weight film having a first optical axis; at least a second sheet of birefringent high molecular weight film having a second optical axis; said first sheet and said at least second sheet being stacked to form said wave plate retarder; and said first and second optical axes being angularly displaced from each other whereby said retardation of said wave plate retarder along said slow axis relative to said fast axis is an integral multiple of wavelength of light.

7. A wave plate retarder according to claim 6 wherein said first sheet and said at least second sheet are of polypropylene film which has been stretched in one of its dimensions during the manufacture thereof.

8. A wave plate retarder according to claim 6 wherein said first and said at least second sheets are from a single lot of high molecular weight film.

9. A wave plate retarder according to claim 6 further comprising means for forming said wave plate retarder into an integral unit.

10. A wave plate retarder according to claim 9 wherein said means for forming includes first and second transparent plates sandwiching said first and said at least second sheets.

11. A wave plate retarder according to claim 10 wherein said transparent plates are glass.

12. A method of making a wave plate retarder substantially as hereinbefore described in Example 1.

13. A method of making a wave plate retarder substantially as hereinbefore described in Example 2.

14. A wave plate retarder substantially as hereinbefore described with reference to Fig. 2 of 5 the accompanying drawings.