GB2449109A - Image display device using persistence of vision effect - Google Patents

Abstract

An image display device 30a, 30b comprises at least one light source movable along a predetermined path. A controller is adapted to modulate the intensity of light emitted by the at least one light source as it moves along the predetermined path so as to cause a desired image to be visible by virtue of persistence of vision. The image display device 30a, 30b is disposed between two sheets 31a, 31b of polarising material oriented with respect to each other such that their respective transmission axes are crossed, ideally at right angles. Preferably the image display device 30a, 30b comprises one or more arrays of light sources, being rotatable around a common axis with each light source traversing a unique path that may be interlaced with the paths traversed by the other light sources. The arrays of light sources may alternatively be arranged in groups, with each group being rotatable about a separate axis.

Description

DISPLAY DEVICE

The invention relates to an image display device comprising at least one light source movable along a predetermined path and a controller adapted to modulate the intensity of light emitted by the at least one light source as it moves along the predetermined path so as to cause a desired image to be visible by virtue of persistence of vision.

A typical device of this nature comprises a single arm along which is disposed an array of light emitting diodes (LEDs). The arm rotates in a continuous or oscillatory fashion such that each LED traces out a path. The LEDs are individually turned on and off as the arm rotates in such a manner as to cause an observer to see a dot matrix image in the area swept out by the arm.

Such devices may be adapted by incorporating red, green and blue LEDs to display.

a full colour static or moving picture. This technique is particularly useful since by using a relatively small number of LEDs, it is possible to recreate an image that would normally require several orders of magnitude more LEDs. All that is required is that the arm of LEDs spins sufficiently fast that persistence of vision causes an image to be visible to the observer. This kind of device typically requires that the LEDs on the rotating arm are closely spaced in order to obtain high resolution, and hence the LEDs must typically be small in size. As a result, the intensity of light omitted by these LEDs can be low.

An improvement on this type of device may be made by providing two or more arrays of light sources, each of which is rotatable around a common axis. The light sources in each array are arranged so that each light source traverses along a unique path around the common axis, whereby the light sources of each array are.

interlaced. A controller is provided which is adapted to modulate the intensity of light emitted by each light source as it traverses its respective unique path, such that the light sources in combination cause a desired image to be visible to an observer by virtue of persistence of vision. This type of display device is fully described in our co-pending international publication WO 2006/021788, which is herein incorporated by reference in its entirety.

By providing two or more arrays of light sources, each of which traverses a respective unique path, larger, brighter light sources may be used than in devices having only a single array, without reducing the resolution of the image that is produced. All that is required is that the arrays rotate around the common axis sufficiently quickly. The minimum acceptable rotational speed depends on the ambient light levels and the brightness of the light sources.

A further type of known image display device comprises a plurality of devices of the type having two or more arrays of light sources. The arrays of the plurality of display devices are configured to rotate around their respective common axes in synchrony such that the desired images caused to be visible by each device together form a composite image. Such image display systems are also fully described in co-pending international publication WO 2006/027188, which is herein incorporated by reference in its entirety.

One feature of the abovementioned types of display devices is that the image created may be viewed equally well from any angle. The display is therefore particularly suited to advertising applications in, for example, bus shelters where it can be viewed from either side. However, there is an attendant disadvantage, which is that the image created appears transparent. Therefore, by enabling the image created to be seen from both sides, for example by providing a housing having opposed, parallel viewing windows, the display is also rendered transparent. Thus, two viewers on either side of the display would be able to see the image and each other. This obviously detracts from the image.

In accordance with a first aspect of the present invention, there is provided an image display device comprising at least one light source movable along a predetermined path and a controller adapted to modulate the intensity of light emitted by the at least one light source as it moves along the predetermined path so as to cause a desired image to be visible by virtue of persistence of vision, characterised in that the image display device is disposed between two sheets of polarising material oriented with respect to each other such that their respective transmission axes are crossed.

By orienting the two sheets of polarising material such that their respective transmission axes are crossed, it is possible to ensure that light transmitted by one sheet is not transmitted by the other. Thus, the only thing visible to a person viewing the display will be the image created by the display device itself, and two people viewing the display from either side, for example, can be rendered invisible to each other.

However, it should be understood that it is not absolutely necessary for the transmission axes to be crossed such that they are perpendicular. The described effect may be achieved with much smaller angles between the respective transmission axes because the vast majority of light transmitted by one sheet will still not be transmitted by the other even at these much smaller angles.

Furthermore, it may be desirable in some circumstances to allow a vestige of the light transmitted by one sheet to be transmitted by the other, for example by causing the transmission axes to cross at a fairly low angle (say 200 to 300). This might be desirable where the display is to be used at a road junction to ensure that car drivers can still see cars approaching the junction along another road.

Nevertheless, by using the invention the detraction caused to the image by being able to see through it may be mitigated even if the circumstance does not allow it to be eliminated. In effect, by orientating the two sheets of polarising material to a desired angle, it is possible to obtain a desired contrast ratio between the image created by the display device and objects visible through the display device.

In one embodiment, the at least one light source comprises an array of light sources rotatable around a common axis, the light sources in the array being arranged such that each traverses along a unique path around the common axis, and the controller is adapted to modulate the intensity of light emitted by each light source in the array as it traverses its respective unique path such that the light sources in combination cause a desired image to be visible by virtue of persistence of vision.

In another embodiment, the at least one light source comprises two or more arrays of light sources, each array being rotatable around a common axis, the light sources in each array being arranged such that each traverses along a unique path around the common axis, and the controller is adapted to modulate the intensity of light emitted by each light source as it traverses its respective unique path such that the light sources in combination cause a desired image to be visible by virtue of persistence of vision.

The paths traversed by the light source of each array are preferably interlaced.

In yet another embodiment,the at least one light source comprises a plurality of arrays of light sources arranged in groups, each group of arrays being rotatable around a respective common axis, the light sources in each array being arranged such that each traverses along a unique path around the respective common axes in synchrony, and the controller is adapted to modulate the intensity of light emitted by each light source as it traverses its respective unique path such that the light sources in each group of arrays in combination cause a desired image to be visible by virtue of persistence of vision, the desired images together forming a composite image.

Typically, each group of arrays comprises a pair of arrays.

The paths traversed by the light source of each array within a group may be interlaced.

The sheets of polarising material may comprise a dichroic material.

In a typical application, the sheets of polarising material lie substantially parallel to each other.

Normally, the sheets of polarising material are oriented with respect to each other such that light transmitted by one of the sheets of polarising material is rendered substantially invisible through the other sheet of polarising material.

To achieve this, the sheets of polarising material are preferably oriented with respect to each other such that their respective transmission axes tie at right angles.

It has been found advantageous to orientate the transmission axis of each sheet of polarising material such that it lies at 450 to the vertical, such that the transmission axes lie at right angles to each other. This ensures that the image formed by the image display device is visible to an observer wearing polarising sunglasses, which are typically vertically polarised. In fact, it is not strictly necessary for the transmission axes to lie at exactly 450 to the vertical to enable such observers to view the image. The axis of transmission of each sheet of polarising material may lie between 30 and 60 to the vertical whilst still allowing them to view the image.

The sheets of polarising material may be transparent, or they can be partially transparent whilst incorporating a coloured tint. A suitable material is sold by Optical Filters Limited of 13-14 Thame Park Business Centre, Thame, Oxfordshire, 0X9 3XA, United Kingdom with reference POLA-XP38-600X900X0.18.

The invention will now be described with reference to the accompanying drawings, in which: Figures Ia, lb and 2 show different views of a prior art image display system; and Figure 3 shows an image display device according to the invention which incorporates the image display system of Figures Ia, lb and 2.

Figures lb and 2 show a prior art image display system I comprising a transparent enclosure 2 made, for example, from perspex. This system comprises six displays 3a-3f, although this quantity is arbitrary and more or fewer displays may be provided.

Three of these displays 3a-3c are also shown in a perspective view in Figure Ia.

The remaining three displays 3d-3f are not shown in this view for clarity. However, the structure of each of the displays 3a-3f is identical. As can be seen from Figure la each display 3a-3c has a corresponding upper arm 4a-4c and lower arm 5a-5c, each mounted on a corresponding shaft 6a-6c. A pair of arrays are attached at opposite ends of each of the upper and lower arms. For example, arrays 7a and Ba are attached to opposite ends of upper arm 4a and lower arm 5a. Each array7a-7c is offset by half a pixel with respect to its corresponding array 8a-8c so that the paths described by the LEDs on each array interlace, thereby increasing the resolution without requiring the LEDs to be densely packed.

All of the displays 3a-3f are coupled together by a suitable non-slipping coupling, such as the gear arrangement shown in Figure 2. Here a respective gear 9a-9f is attached to one end of each shaft 6a-6f, and each gear meshes with the gear attached to the neighbouring display 3a-3f. Thus, all of the displays may be driven from a single motor in synchrony.

Each upper arm 4a-4f and lower arm 5a-5f is mounted on its respective shaft 6a-6f such that they are always at an orientation that is offset from the adjacent upper and lower arms (i.e. each pair of arrays is disposed about its respective shaft such that they do not collide with the arrays of adjacent displays as they rotate). The purpose of this is to allow the shafts 6a-6f (and hence arrays 7a-7f and 8a-Bf) of each display 3a-3f to be closely spaced (i.e. the volume of rotation described by each pair of arrays as they rotate overlaps or intersects the volume of rotation described by the pairs of arrays on adjacent displays) such that a continuous composite image may be created from the images generated by each individual display but without collision between neighbouring arrays as they rotate.

By spinning the displays 3a-3f fast enough and modulating the intensity of light emitted by the light sources on them as they rotate, each display can be caused to generate an image. This brings about a composite image which may be viewed around the entire perimeter of the image display system 1. This display system 1 finds particular use in advertising and other large-scale display systems, where the cost of providing a large display using static LEDs or the like would be prohibitive.

Figure 3 shows how the prior art system of Figures Ia, lb and 2 may be modified to embody the invention. Specifically, the perspex housing 2 shown in Figure lb may be replaced by a different housing, part of which is shown in Figure 3.

In the example of Figure 3, two displays 30a and 30b (again, this number is arbitrary and more or fewer displays may be provided) of the same type as displays 3a-3f are disposed between two parallel sheets 31 a and 31 b of polarising material, typically a dichroic material such as POLA-XP38-600X900X0. 18 mentioned above.

These sheets 31a and 31b form part of a housing, the remainder of which is not shown and is not relevant to the invention.

The first sheet 31 a of polarising material is oriented such that the transmission axis lies at 45 from the vertical. Therefore, only light that is polarised to 45 from the vertical, that is in alignment with the transmission axis, is transmitted by the first sheet 31a.

The second sheet 31b of polarising material is also oriented such that the transmission axis is 45 from the vertical. However, the sheet 31b is also oriented such that its transmission axis lies at 90 to the axis of transmission of the first sheet 3la.

As a consequence no light can pass through both the first sheet 31a and the second sheet 31 b, and therefore the person 32 positioned behind the first sheet 31 a (or indeed any other object position on that side of the image display unit) cannot be seen by someone looking at the displays 30a and 30b through the second sheet 31 b.

Similarly, any object placed in front of the second sheet 31 b cannot be seen by the person 32 when he is looking at the displays 30a and 30b through the first sheet 31a.

In contrast, however, the image produced by the displays 31 a and 31 b can be seen through both the first and second sheets 30a and 30b because the light emitted from the displays 30a and 30b is not polarised. Thus, the image produced by the displays 30a and 30b is still "double sided".

The light emitted by the displays 30a and 30b will be polarised after it has been transmitted by either of sheet 31a or 31b. However, since their transmission axes have been oriented at an angle of 450 from the vertical a person wearing polarised sunglasses will still be able to see the image created by displays 30a and 30b because the transmission axes of sunglasses lenses is typically either vertical or horizontal.

Claims (13)

1. An image display device comprising at least one light source movable along a predetermined path and a controller adapted to modulate the intensity of light emitted by the at least one light source as it moves along the predetermined path so as to cause a desired image to be visible by virtue of persistence of vision, characterised in that the image display device is disposed between two sheets of polarising material oriented with respect to each other such that their respective transmission axes are crossed.

2. An image display device according to claim 1, wherein the at least one light source comprises an array of light sources rotatable around a common axis, the light sources in the array being arranged such that each traverses along a unique path around the common axis, and the controller is adapted to modulate the intensity of light emitted by each light source in the array as it traverses its respective unique path such that the light sources in combination cause a desired image to be visible by virtue of persistence of vision.

3. An image display device according to claim 1, wherein the at least one light source comprises two or more arrays of light sources, each array being rotatable around a common axis, the light sources in each array being arranged such that each traverses along a unique path around the common axis, and the controller is adapted to modulate the intensity of light emitted by each light source as it traverses its respective unique path such that the light sources in combination cause a desired image to be visible by virtue of persistence of vision.

4. An image display device to claim 3, wherein the paths traversed by the light source of each array are interlaced.

5. An image display device according to claim 1, wherein the at feast one light source comprises a plurality of arrays of fight sources arranged in groups, each group of arrays being rotatable around a respective common axis, the light sources in each array being arranged such that each traverses along a unique path around the respective common axes in synchrony, and the controller is adapted to modulate the intensity of light emitted by each light source as it traverses its respective unique path such that the light sources in each group of arrays in combination cause a desired image to be visible by virtue of persistence of vision, the desired images together forming a composite image.

6. An image display device according to claim 5, wherein each group of arrays comprises a pair of arrays.

7. An image display device according to claim 5 or claim 6, wherein the paths traversed by the light source of each array within a group are interlaced.

8. An image display device according to any of the preceding claims, wherein the sheets of polarising material comprise a dichroic material

9. An image display device according to any of the preceding claims, wherein the sheets of polarising material lie substantially parallel to each other.

10. An image display device according to any of the preceding claims, wherein the sheets of polarising material are oriented with respect to each other such that light transmitted by one of the sheets of polarising material is rendered substantially invisible through the other sheet of polarising material.

11. An image display device according to any of the preceding claims, wherein the sheets of polarising material are oriented with respect to each other such that their respective transmission axes lie at right angles.

12. An image display device according to claim 11, wherein the transmission axis of each sheet of polarising material lies at 450 to the vertical.

13. An image display apparatus substantially as hereinbefore described with reference to the accompanying drawings.