FIELD OF THE INVENTION

This invention relates to an autostereoscopic display device with a display panel having an array of display pixels, and an arrangement for directing different views to different physical locations.

BACKGROUND OF THE INVENTION

A known autostereoscopic display device is described in GB 2196166 A. This known device comprises a two dimensional emissive liquid crystal display panel having a row and column array of display pixels acting as an image forming means to produce a display. An array of elongate lenticular lenses extending parallel to one another overlies the display pixel array and acts as a view forming means. Outputs from the display pixels are projected through these lenticular lenses, which function to modify the directions of the outputs.

The lenticular lenses are provided as a sheet of elements, each of which comprises an elongate semi-cylindrical lens element. The lenticular lenses extend in the column direction of the display panel, with each lenticular lens overlying a respective group of two or more adjacent columns of display pixels.

In an arrangement in which, for example, each lenticular lens is associated with two columns of display pixels, the display pixels in each column provide a vertical slice of a respective two dimensional sub-image. The lenticular sheet projects these two slices, and corresponding slices from the display pixel columns associated with the other lenticular lenses, to the left and right eyes of a user positioned in front of the sheet, so that the user observes a single stereoscopic image.

In other arrangements, each lenticular lens is associated with a group of three or more adjacent display pixels in the row direction. Corresponding columns of display pixels in each group are arranged appropriately to provide a vertical slice from a respective two dimensional sub-image. As a user's head is moved from left to right a series of successive, different, stereoscopic views are observed creating, for example, a look-around impression. The above described autostereoscopic display device produces a display having good levels of brightness. However, one problem associated with the device is that the views projected by the lenticular sheet are separated by dark zones caused by "imaging" of the non-emitting black matrix which typically defines the display pixel array. These dark zones are readily observed by a user as brightness non-uniformities in the form of dark vertical bands spaced across the display. The bands move across the display as the user moves from left to right and the pitch of the bands changes as the user moves towards or away from the display. Another problem is that the vertical lenses result in a much greater reduction in resolution in the horizontal direction than in the vertical direction.

Both of these issues can be at least partly addressed by the well known technique of slanting the lenticular lenses at an acute angle relative to the column direction of the display pixel array. The use of slanted angles lenses is thus recognised as an essential feature to produce different views with near constant brightness, and a good RGB distribution behind the lenses.

Traditionally, display panels are based on a matrix of pixels that are square in shape. In order to generate images in colour, the pixels are divided into sub-pixels.

Traditionally, each pixel is divided into 3 sub-pixels, transmitting or emitting red (R), green (G) and blue (B) light, respectively. Sub-pixels of equal colour are typically arranged in columns.

Recently, display manufacturers have started looking into alternative pixel layouts with the aim, given the same number of sub-pixels, to achieve:

a higher perceived resolution and/or

a larger colour gamut, and/or

a higher brightness (or reduced power consumption).

Several of the alternative pixel layouts have made it to the market. However, these changes in the pixel design require adaptation to the lenticular design, for optimal performance with these alternative pixel layouts. This is a costly process.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided an autostereoscopic display device comprising:

a display having an array of display pixel elements for producing a display, wherein one or more adjacent display elements together from a display pixel such that an array of pixels is defined, wherein the display pixels are arranged generally in rows and columns;

a lenticular array arranged in registration with the display for projecting a plurality of views towards a user in different directions, and comprising lenticular lenses to focus outputs of groups of the display pixels into the plurality of views projected towards a user in different directions, thereby enabling autostereoscopic imaging,

wherein the centroids of the display pixels define a grid of points in rows and columns, wherein the rows and columns are non-orthogonal.

The grid of points having non-orthogonal rows and columns is such that there is no orthogonal pair of lines with pass through all pixels centres in a row (i.e. through a number of pixels corresponding to the number or columns) and through all pixel centres in a column (i.e. through a number of pixels corresponding to the number of rows). It is of course possible to pick non-orthogonal lines even in a square grid of points, and arbitrarily name these "rows" and "columns". However, the grid of points in the pixel layout of the invention is not a square or rectangular grid of points - if a pair of orthogonal row and column lines are selected, one of these lines will skip pixel elements. By "display pixel" is meant the emission area of the pixel.

The arrangement of the invention means that a standard lenticular design can be used, and the pixel layout can be selected to optimise the optical performance depending on the sub-pixel layout.

A set of three or more display pixel elements can together define a colour pixel.

For example, each pixel can comprise four columns of pixels elements and two rows of pixels elements, each row having red, green, blue and white pixel elements. This can define an RGBW pixel layout for example.

Each pixel can comprises three columns of pixel elements of red green and blue and one row of pixel elements. This defines an RGB pixel layout.

The display pixels define a rectangular display area, and preferably the rows of points are parallel to the top and bottom edges of the display area, and the columns of points are at an acute angle to the side edges of the display area.

In this way, the staggered pixel layout provides (or contributes to) the desired slant between the lens elements and the columns of pixels.

An array of row and column conductors is preferably provided, and the column conductors extend generally parallel to the side edges of the display area but with a stepped sawtooth shape. This sawtooth shape jumps between adjacent columns at each tooth step, and the ramp part of the sawtooth follows the direction of the pixel column.

Instead of a sawtooth shape, there can be straight row and column lines, and additional spurs to extend to the staggered pixels (in the row or column direction).

In one implementation, the separation between the centroid of the pixel and the centroid of the drive electronics for the pixel is not equal for all pixels. In this case, the drive electronics can be formed in a regular grid connected by straight orthogonal row and column lines, and the slanted columns (or rows) of pixel emission areas connect to the associated drive electronics by spur conductors.

In another implementation, the separation between the centroid of the drive electronics for a pixel and the associated crossing point of the rows and columns is not equal for all pixels. In this case, there can be an orthogonal set of row and column lines, and the spurs are used to connect to pixel drive electronics at the location of the pixel emission areas.

When using spurs, the column conductors can comprise a main line which extends parallel to the side edges of the display and the spurs can extend in the row direction to the pixel elements, the spurs having different lengths to different pixel elements depending on the relative position of the pixel element and main line.

This enables the pixel designs to be more uniform.

The lenticular lenses can have a long axis which is parallel to the sides of the display area, so that all of the desired slant is provided by the pixel array. This enables standardised non-slanted lenses to be used.

The lenticular lenses can instead have a long axis which is slanted with respect to the sides of the display area. The slant is then provided by the combination of the lens angle and the pixel configuration.

The display preferably comprises an electroluminescent display device, such as an OLED device. If a top emitting OLED device is used, it becomes particularly easy to implement the desired row and column conductor patterns without reducing pixel aperture or introducing visible non-uniformities into the pixel structure.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, purely by way of example, with reference to the accompanying drawings, in which:

Fig. 1 is a schematic perspective view of a known autostereoscopic display device; Fig. 2 is a schematic cross sectional view of the display device shown in Fig. i;

Fig. 3 shows how the known RGB pixel is projected by the lenticular arrangement in a known display;

Fig. 4 shows the known RGB pixel layout and a known RGBW pixel for a display to which the invention can be applied;

Fig. 5 shows other known RGB pixel layouts;

Fig. 6 shows three possible RGBW pixel layouts (including that of Fig. 4) for a display to which the invention can be applied;

Fig. 7 shows an RGBY pixel layout for a display to which the invention can be applied;

Fig. 8 shows a set of display pixel layouts for use in a display device of the invention;

Fig. 9 shows a first way to arrange the display driver electrodes; Fig. 10 shows a second way to arrange the display driver electrodes; and

Fig. 11 shows a further alternative pixel layout to which the invention can be applied.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention provides an autostereoscopic display device in which the centroids of the display pixels define a grid of points in rows and columns, wherein the rows and columns are non-orthogonal. This means that a standard lenticular design can be used, and the pixel layout can be selected to optimise the optical performance depending on the sub-pixel layout.

Before describing the invention in detail, the configuration of a known autostereoscopic display will first be described.

Fig. 1 is a schematic perspective view of a known multi-view autostereoscopic display device 1. The known device 1 comprises a liquid crystal display panel 3 of the active matrix type that acts as an image forming means to produce the display.

The display panel 3 has an orthogonal array of display pixels 5 arranged in rows and columns. For the sake of clarity, only a small number of display pixels 5 are shown in Fig. 1. In practice, the display panel 3 might comprise about one thousand rows and several thousand columns of display pixels 5. The structure of the liquid crystal display panel 3 is entirely conventional. In particular, the panel 3 comprises a pair of spaced transparent glass substrates, between which an aligned twisted nematic or other liquid crystal material is provided. The substrates carry patterns of transparent indium tin oxide (ITO) electrodes on their facing surfaces. Polarising layers are also provided on the outer surfaces of the substrates.

Each display pixel 5 comprises opposing electrodes on the substrates, with the intervening liquid crystal material therebetween. The shape and layout of the display pixels 5 are determined by the shape and layout of the electrodes and a black matrix arrangement provided on the front of the panel 3. The display pixels 5 are regularly spaced from one another by gaps.

Each display pixel 5 is associated with a switching element, such as a thin film transistor (TFT) or thin film diode (TFD). The display pixels are operated to produce the display by providing addressing signals to the switching elements, and suitable addressing schemes will be known to those skilled in the art.

The display panel 3 is illuminated by a light source 7 comprising, in this case, a planar backlight extending over the area of the display pixel array. Light from the light source 7 is directed through the display panel 3, with the individual display pixels 5 being driven to modulate the light and produce the display.

The display device 1 also comprises a lenticular sheet 9, arranged over the display side of the display panel 3, which performs a view forming function. The lenticular sheet 9 comprises a row of lenticular lenses 11 extending parallel to one another, of which only one is shown with exaggerated dimensions for the sake of clarity. The lenticular lenses 11 act as view forming elements to perform a view forming function.

The lenticular lenses 11 are in the form of convex cylindrical elements, and they act as a light output directing means to provide different images, or views, from the display panel 3 to the eyes of a user positioned in front of the display device 1.

The autostereoscopic display device 1 shown in Fig. 1 is capable of providing several different perspective views in different directions. In particular, each lenticular lens 11 overlies a small group of display pixels 5 in each row. The lenticular element 11 projects each display pixel 5 of a group in a different direction, so as to form the several different views. As the user's head moves from left to right, his/her eyes will receive different ones of the several views, in turn.

Fig. 2 shows the principle of operation of a lenticular type imaging

arrangement as described above and shows the light source 7, display panel 3 and the lenticular sheet 9. The arrangement provides three views each projected in different directions. Each pixel of the display panel 3 is driven with information for one specific view.

The above described autostereoscopic display device produces a display having good levels of brightness. It is well known to slant the lenticular lenses at an acute angle relative to the column direction of the display pixel array. This enables an improved brightness uniformity and also brings the horizontal and vertical resolutions closer together.

Whatever the mechanism used to obtain an auto-stereoscopic display system, resolution is traded for depth: the more views, the higher the loss in resolution per view. This is illustrated in Fig. 3, which shows the native pixel layout of the 2D display panel as well as, on the same scale, the pixel layout in a 3D view obtained by putting a lenticular in front of the panel. The lenticular has a slant tan(0)=l/6 and a lens pitch PL=1.5 px (where px is the pixel pitch in the row direction) resulting in 9 views. In this case, px=py. The 3D image has a repeating pattern of sub-pixels, and the colours of a few sub-pixels (R, G and B) are shown so that all colours in the pattern can be understood. Each colour is output as a diamond shaped grid of sub-pixels which are interleaved with each other. In this case, the resolution of the single 3D image of the pair is 1/9 of the resolution of the native display.

The slant angle of the lenticular as well as its pitch should be chosen such that a number of requirements are fulfilled as much as possible:

(i) A favourable distribution of pixels should be obtained for each 3D view.

In each of the 3D views the sub-pixels of each colour should be distributed in a pattern that is regular and having a resolution that is similar for the horizontal and vertical direction. As shown in Fig. 3, the horizontal distance between neighbouring green pixels (labelled A in Fig. 3) should be comparable to the vertical distance between neighbouring green pixels (labelled B). This should hold for the other colours as well.

(ii) The surface area occupied by pixels of the same colours should be equal for each 3D view.

(iii) Absence of moire.

The combination of a lenticular in front of a display panel is very susceptible to the occurrence of moire ('banding'). This effect is caused by the combination of the periodicity of the pixel layout of the display panel and the periodicity of the lenticular. It is worsened by the fact that the sub-pixels of the display panel are surrounded by a black matrix. By means of slanting the lenticular and by choosing the lenticular to have a width that is not equal to an integer times the width of a sub-pixel, this moire effect can be minimised. As stated above, recently, display manufacturers started looking into alternative pixel layouts using more than 3 primary colours.

Figs. 4 to 7 show various pixel layouts. The sub-pixel colours for at least one pixel are identified with letter labels ("R", "G", "B" etc.). The pixels are in repeating patterns. Where whole columns of pixels have the same colour, these are identified from above the columns. The colours of only enough pixels have been shown for the repeating pattern to be identified.

The left part of Fig. 4 shows a conventional pixel layout of a 2x2 matrix of RGB pixels. Each pixel has three sub-pixels, hence the subscript "3" in RGB 3 (the same notation is used for all pixel layouts).

The right part of Fig. 4 shows an RGBW 8 pixel arrangement having 4 primary colours and 8 sub-pixels per pixel. In addition to RGB (Red, Green, Blue), also white (W) sub-pixels are provided. Comparing the layouts (occupying the same surface area), the perceived resolution of both layouts is similar whereas the RGBW layout results in a higher brightness (at least for LC panels): note that green and white are most important for generating a bright image (the human eye is more sensitive to green and white than it is to red and blue).

Pixel layouts using more than 3 primary colours will be termed "multi- primary" pixel layouts. Several such multi-primary layouts have reached the market and are expected to become mainstream.

For completeness, Fig. 5 shows several alternative RGB pixel layouts.

The left image is the conventional RGB 3 -sub-pixel layout. The RGB 6 layout in the middle essentially spatially inverts two pairs of RGB pixels. The RGB 8 layout on the right has made it to the market for 2D displays by Samsung. In the RGB 8 layout, the green sub-pixels are half as wide as the R and B sub-pixels. The green sub-pixels are in columns, whereas the R and B sub-pixels alternate along the column direction.

In Fig. 6 and Fig. 7, the most promising pixel layouts based on 4 primaries are shown.

Fig. 6 shows three RGBW pixel layouts, whereas Fig. 7 shows an RGBY (Y= Yellow) pixel layout, which has been used by Sharp.

In Fig. 6, RGBW pixels with 4, 6 and 8 sub-pixels are shown. The different proportions of different primary colours (and white) in the different pixel layouts give different brightness characteristics as well as different output colour gamut. In another favourable variant of Fig. 6, the white sub-pixels may be replaced by yellow sub-pixels, and this layout results in a larger colour gamut.

In Fig. 7, pxR=pxB=2 pxG=2pxY (the red and blue sub-pixels are twice as wide as the green and yellow sub-pixels in the row direction). Compared to the RGB layouts, this layout results in a larger colour gamut.

However, all of these new pixel designs require different lenticular layouts (e.g. slant angles). As a consequence, it is necessary to re-design and individually

manufacture the lenticular for each new pixel layout which is a costly exercise.

The invention provides an alternative approach to the problem of requiring a multiplicity of slanted lenticular designs, by providing a standardised non-slanted lenticular design and by adjusting the layout of the sub-pixels in a slanted manner to realise the benefits described above.

Alternatively, a new pixel structure can be matched to an (existing) slanted lenticular layout by providing both the lenticular and the sub-pixel structure with a slanted layout, whereby the effective slant angle is defined as the difference between the slant angle of the lenticular and the sub pixel structure.

The invention is based on the recognition that the sub-pixels do not necessarily have to be arranged on a rectangular grid. The unit cell of such a grid can also be trapezoid- shaped.

Some examples are shown in Fig. 8.

Fig. 8 shows examples of a sheared pixel layout, namely one in which the rows or columns (the lines passing through the area centres of the pixel elements) are not orthogonal. Fig. 8 shows RGB 3 and RGBW 8 pixel layouts (the sub-pixel colours are as identified in Figs. 4 and 6 respectively). For each pixel layout, a sheared row structure is shown on the left and a sheared column structure is shown on the right. The dotted line 80 represents the direction of the sub-pixel columns, i.e. the line passing through the area centres (centroids) of the column of sub-pixels. The sub-pixels themselves do not necessarily need to have the rectangular shape shown in Fig. 8, and they can have a trapezoidal shape.

The optimum perception as previously defined for slanted lenticulars in combination with a traditional square shaped matrix layout remains valid provided that the slant angle of the lenticular is now defined with respect to the average direction of the columns of pixels (lines 80 in Fig. 8).

This slanted layout of pixels is possible in any matrix-based display (such as an LCD, Plasma display, OLED). The invention is however particularly suitable for an active matrix OLED display, and more specifically for a top emitting OLED display, as in this type of display it is possible to laterally displace the emission area of the sub-pixel from its associated drive electronics. Why this is important is explained below.

There is increasing interest in the use of organic light emitting diode (OLED) displays generally, as these do not need polarizers, and potentially they should be able to offer increased efficiency since the pixels are turned off when not used to display an image, compared to LCD panels which use a continuously illuminated backlight.

Traditionally, the square matrix of rows and columns of sub-pixels are connected together by (horizontal) select lines and (vertical) data lines to associated Select (row) driver IC's and Data (column) driver IC's respectively. In this manner, every row and every column of sub-pixels extends across the full display and is connected to its associated driver IC. Such an arrangement is effective both in terms of costs (minimum number of driver ICs) and display image uniformity (as all data and select lines have equal electrical properties).

For the layouts shown in Fig. 8, for the staggered row designs, the most logical approach would be to connect all sub-pixels of the vertical columns to a data driver IC and all slanted rows to a select driver IC. However, this will result in a portion on the top left hand side of the display where the select driver IC's connect to progressively shorter rows, which may cause some image uniformity problems. Furthermore, at the bottom right hand corner, there will be a set of rows which cannot be connected by drivers on the left hand side of the display, as they terminate at some point across the display. In addition to image uniformity problems, it will be necessary to connect these rows from the other side (right hand side) of the display, which is particularly unsuitable.

For the staggered column designs, the most logical approach would be to connect all pixels of the slanted columns to a data driver IC and all horizontal rows to a select driver IC. However, this will result in a portion on the top right hand side of the display where the data driver IC's connect to progressively shorter columns, which may cause some image uniformity problems. Furthermore, at the bottom left hand corner, there will be a set of columns which cannot be connected by drivers on the top side of the display, as they terminate at some point across the display. In addition to image uniformity problems, it will be necessary to connect these columns from the other side (bottom) of the display, which is particularly unsuitable. For this reason, in a preferred embodiment of the invention, the stepped arrangement of sub-pixels (as shown in Fig. 8) is interfaced with the traditional row and column connections to the driver ICs. To enable this, it is necessary to either:

form row and/or column lines which meander along the stepped sub pixels and periodically step back as the sub-pixels shift position by either a row or a column spacing, or maintain the traditional straight row and/or column lines but to arrange the sub-pixels such that the sub-pixel emission areas are laterally displaced from the sub-pixel drive electronics.

The examples of the invention shown below are based on slanted columns of pixels and corresponding stepped column conductors, but the invention can be applied equally to designs with slanted rows of pixels and corresponding stepped row conductors. Thus, the references to rows and columns may be interchanged to arrive at alternative examples.

The first approach is shown in Fig. 9, which illustrates a meandering column conductor 90. Every third sub-pixel along the column direction, the column conductor 90 becomes associated with a sub-pixel from an adjacent column of pixels. Thus, the column line is associated with a group of columns of sub-pixels, with a number (3 in this example) of sub-pixels of each column in the group associated with the column conductor. The number (3 in this example) is of course a function of the slant of the sub-pixel columns.

The meandering column conductor has a stepped sawtooth shape. This sawtooth shape steps around the pixel elements but follows the direction of the column until the next sawtooth. Instead of a sawtooth shape, a chevron arrangement could be considered which first steps to the right from the vertical column direction and then symmetrically steps back to the column direction and then repeat this chevron structure.

It is possible to apply this approach to all display types, particularly LCDs and bottom emitting OLED displays. One issue of such an approach is the additional space taken by the meandering row and/or column lines (which will reduce either the transmission aperture of an LCD or the emission size of a bottom emitting OLED) and in addition, creating the step back will introduce non-uniformity as the sub-pixel layout will be different where the step-back occurs, as space is required for the additional line length.

The steps are not needed if the column conductor does not interfere with the pixel, for example if it is beneath the pixel as in the example of Fig. 10 below. In that case, a ramped sawtooth shape is possible. The second approach is shown in Fig. 10, again for a staggered column structure.

The sub-pixel emission area is laterally displaced from the sub-pixel drive electronics. The columns conductors 100 and in particular the position at which they connect using spurs to the sub-pixels, are shown are vertically displaced for visualisation reasons. In practice, all columns may be positioned at identical vertical positions. The positions of the spurs may need to be offset due to the space required for vias to connect the underlying electronics to the top layer conductors, and in some cases to avoid longer spurs short circuiting.

The main column conductor 100 is vertical, and horizontal spurs extend from the main conductor 100 to the sub-pixels.

In the same way as in Fig. 9, every third sub-pixel along the column direction, the column conductor 100 becomes associated with a sub-pixel from an adjacent column of pixels. Thus, the column line is associated with a group of columns of sub-pixels, with a number (3 in this example) of sub-pixels of each column in the group associated with the column conductor. The number (3 in this example) is of course a function of the slant of the sub-pixel columns.

The spurs only need to extend to a maximum of one column width.

Fig. 10 also shows the arrangement of the electronics (for one pixel) in cross section. The driver electronics 102 is connected to the OLED sub-pixel 103 by a wire 104.

Short circuits are avoided as the horizontal connecting wire 104 is at a higher level than the columns and other electronics.

In Fig. 10, the electronics is shown spaced from the pixel emission area. This means the separation distance between the centroid of the pixel emission area and the location (e.g. the centroid) of the drive electronics for the pixel is not equal for all pixels.

This separation distance corresponds to the length of the spur. In this case, the drive electronics and the row and column conductors can be formed in a regular orthogonal grid, and the slanted columns (or rows) of pixel emission areas connect to the associated drive electronics by the spurs.

However, it is also possible for the electronics of the pixel to be located at the pixel emission area. In this case, the separation distance between the location (e.g. the centroid) of the drive electronics for a pixel and the associated crossing point of the rows and columns is not equal for all pixels. In this case, there can be an orthogonal set of row and column lines, and the spurs are used to connect to the pixel drive electronics at the location of the pixel emission areas.

Laterally displacing the column conductor (or row conductor in the alternative example) using spurs in this way has the advantage that the above issues do not occur, but this approach is most practical for displays where the light emission is not through the glass plate comprising the active matrix electronics - as is the case for top emitting OLED displays.

The approach of laterally displacing column conductors, when combined with OLED displays, enables various other pixel layouts to be used, as shown for example in Fig. 11. This shows an example of a 'Mondriaan'-type of pixel layout (an RGBW 4 pixel).

Fig. 11(a) shows a ingle pixel consisting of 4 sub-pixels, Fig. 11(b) shows a 3x3 array of such pixels, and Fig. 11(c) shows sheared 3x3 array of such pixels with the columns sheared.

In this case, within a single pixel, the centroids of the sub-pixels are not positioned on a rectangular grid or sheared rectangular grid. Each of the 4 different sub- pixels emit a different primary colour while the centroids of these sub-pixels do not lie on a rectangular grid. Furthermore, each of the sub-pixels constituting a pixel has a different size.

The centroids of the pixels themselves (shown with a star in Fig. 11) however are positioned on a rectangular grid for the version of Fig. 11(b) or a sheared rectangular grid for the version of the invention in Fig. 11(c). The grid of points defined by the centroids of all sub-pixels of the same type is on a skewed grid, as well as the grid of points defined by the centroid of each full pixel.

The angle between the row and columns is selected as a function of the lens design and number of views of the autostereoscopic display. Typically, the slant angle of a lenticular is arctan(l/3) or arctan(l/6) as will be known to those skilled in the art. The angle between the rows and columns of pixels will be of the same order of magnitude.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be capable of designing many alternative embodiments without departing from the scope of the invention as defined by the appended claims.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.