GB2396268A

GB2396268A - Modifying image signals and optically modifying projected image dependent on orientation of screen

Info

Publication number: GB2396268A
Application number: GB0229131A
Authority: GB
Inventors: Dermot John Quinn; Graham Harry Moss; Steven James Chapman; Paul Antony Shires
Original assignee: Digital Projection Ltd
Current assignee: Digital Projection Ltd
Priority date: 2002-12-13
Filing date: 2002-12-13
Publication date: 2004-06-16
Anticipated expiration: 2022-12-13
Also published as: GB0229131D0; GB2396268B

Abstract

A projection system suitable for use in an underground station includes a projection lens system arranged to offset the projected image to reduce keystoning effects and an arrangement for modifying the electrical image signals applied to each light modulating element to further reduce keystoning effects and to take account of the curvature and orientation of the projection screen.

Description

- 1 PROJECTION SYSTEM

This invention relates to projection systems. In particular, the invention relates to projection systems 5 including one or more spatial light modulators which are used to produce an image which is projected onto a surface which is not a plane normal to the optical axis of the projection system, for example a screen formed on the tunnel wall at an underground station. Suitable 10 spatial light modulators may take the form of, for example, digital mirror devices (DMDs) or liquid crystal devices (LCDs).

The use of a projection system in an underground station 15 produces extreme demands on the projection system due to the necessity of mounting the projection system at a high position such that the projection system is normally out of reach. This then requires the downward tilting of the projection system, this causing keystoning of the 20 projected image. Further distortion of the projected image is caused by the curvature of the screen which may be complex. Relatively high ambient lighting conditions at the underground station will produce further problems for the projection system.

It is an object of the present invention to provide a projection system suitable for use in situations such as in an underground station which at least alleviates at least some of the above problems.

According to the present invention, there is provided a projection system comprising: at least one spatial light modulator comprising an array of electrically addressable light modulating elements; means for addressing each 10 light modulating element with an electrical signal representative of a portion of an image to be projected; means for optically modifying the image produced by the spatial light modulator dependent on the orientation of the surface on which the image is to be projected 15 relative to the optical axis of the projection system; and means for modifying the electrical signal applied to each light modulating element to create a further modification of the image produced by the spatial light modulator dependent on the orientation of the surface on 20 which the image is to be projected relative to the optical axis of the projection system.

An embodiment of the invention will now be described by way of example only with reference to the accompanying 25 figures in which:

Figure 1 is an overall view of a projection system in use in an underground station; Figure 2 is a sectional view of the underground station 5 including the projection system shown in Figure 1 in which there is no correction of the image projected on the projection screen; Figure 3 is a block schematic diagram of the components 10 of a projection system in accordance with an embodiment of the invention; Figure 4 is a schematic diagram of a DMD incorporated in the projection system of Figure 3; Figure 5 is a schematic diagram of the optical paths of light directed onto a mirror element of the DMD of Figure 4; 20 Figure 6 illustrates schematically the functional hardware and software components of the electronic system incorporated in the projection system of Figure 3; Figure 7 is a schematic ray diagram of the image 25 projected by the projection lens system in the projection

- 4 - system of Figure 3; Figure 8 corresponds to Figure 7 but shows the effect of the movement of the projection lens system relative to 5 the DMDs in order to offset the image; Figure 9 is a schematic view of the projection lens system included in the projection system of Figure 3; 10 Figure 10 is a sectional view of the underground station showing the image corresponding to Figure 2 but showing optical correction of the image projected by the projection lens system; 15 Figure 11 illustrates the residual distortion of portions of the image after the optical correction produced by the projection lens system; Figure 12 illustrates the principle of the electronic 20 correction of the keystone distorted image shown in Figure 11 produced by the warp geometry unit of Figure 6; Figure 13 shows further the details of the electronic 25 keystone correction illustrated in Figure 12;

- 5 Figure 14 illustrates the electronic correction of the distortion produced by the curved screen produced by the warp geometry unit of Figure 6; 5 Figure 15 shows further details of the electronic correction illustrated in Figure 14; Figure 16 illustrates the electronic correction of the image for a dual radius of curvature screen in which the 10 tunnel wall has different radii in the upper and lower cross-sections, using the warp geometry unit of Figure 6; Figure 17 illustrates the correspondence between the 15 light intensities represented by the input video signal and the projected light levels; and Figure 18 is a detail of the graph of Figure 17 at lowlight intensities.

Overview of the projection system in use at an underground station Referring firstly to Figure 1, this figure illustrates 25 a projection system 1 mounted in an underground station

- 6 - so as to project a series of images representing, for example, an advertisement on a screen 3 on the opposing tunnel wall. The projection system is mounted using a mounting mechanism 4 mounted on the tunnel wall.

5 Electric supply cables, a video data input cable and a cooling air input (not shown) run through the mounting mechanism. In order to be out of reach, the projection system 1 must be mounted via the mounting mechanism 4 high on the tunnel wall with the projected image being 10 directed downwards from the projection system.

Furthermore, the surface of the tunnel providing the screen surface is curved in a relatively complex manner which may change from station to station.

15 The effects of the tilted projection system and the curved screen surface on the projected image are shown in Figure 2 which represents a cross-section through the station shown in Figure 1. An outline of the projected image in the direction perpendicular to the section in 20 the absence of any correction is indicated on the left hand side of the figure. As can be seen, the tilting of the projection system produces a keystone distortion of the projected image, whilst the curvature of the tunnel produces a curvature of the edges of the image.

- 7 - In accordance with the embodiment of the invention, there is provided a projection system in which the distortion of the projected image as shown in Figure 2 is reduced both optically and electronically as will be described 5 hereafter. Overview of the functional units of the projection system Turning now to Figure 3, this figure represents a 10 schematic drawing of the basic functional units of a projection system 1 in accordance with an embodiment of the invention. The optical path of light through the system is indicated by bold arrows, the image produced by the projection system being projected onto the screen 15 3. The electrical supply system path is indicated by single open-head arrows. The electrical data signals path is indicated by closed-head arrows. The air cooling system is indicated by double-head arrows.

20 The optical system includes a lamp, for example a Xenon arc lamp 5 of typically 700 watts, powered by a lamp power supply 7. The lamp 5 is arranged to provide a substantially parallel beam of white light, which is filtered by optical filters 9 to remove unwanted infra 25 red light and passed through an optical relay lens system

- 8 - 11 to enter a prism arrangement 13.

The prism arrangement 13 includes a series of crossed dichroic splitters and optical combiners (not shown in 5 the drawing) which are effective to split the white light from the lamp 5 between three digital mirror devices (DMDS) which are indicated collectively as 15 and to recombine the spatially modulated light produced by each DMD 15 to produce a single lightbeam spatially modulated 10 with a coloured image. This image is projected by a projection lens system 17 onto the screen 3.

Whilst in order to simplify the drawing in Figure 3 the DMDS 15 have been shown as a single separate block, in 15 reality each of the three DMDS is responsive to light of a different colour, that is red, green or blue and is mounted on a different surface of the prism arrangement 13. Details of suitable prism arrangements are shown, for example, in European Patent Application Nos. EP 20 0746948 or US 6250763. It will be appreciated, however, that other prism arrangements may be used, and indeed separate light sources may be used for each colour light.

Furthermore, one or more DMDs may be used to spatially modulate light of different colours sequentially using 25 for example a colour wheel arrangement. Equally, colour

channels other than red, green and blue may be used, for example cyan, magenta and yellow.

Referring now also to Figures 4 and 5, each DMD 15 5 comprises an array of pivoted mirror elements Ml1 to M;n as indicated in Figure 4. Whilst the figure shows a relatively small number of mirror elements, it will be appreciated that each DMD Will typically include 1024 x 768 mirror elements for a low resolution DMD, the number 10 of mirror elements increasing dependent on the resolution required. Each mirror element M is individually addressable by electrical data address signals representative of a pixel of one of the red, green or blue channels of a colour video signal representing 15 successive image frames from the electronic system 21 as will be described in more detail hereafter, and addresses each of the mirror devices M1, to Mmn.

As best seen in Figure 5, dependent on the applied 20 electrical data address signal, each mirror device may take one of two different orientations relative to the incident light beam corresponding to: (1) an. ' OFF '' orientation in which the reflected light 25 is directed along an. .OFF" path to a beam dump (not

À 10 shown), or (2) an "ON" orientation (shown dotted) in which the reflected light is directed along an ICON '' path 5 towards the projection lens system 17 and thus passes into the projection lens system 17.

Thus, each DMD 15 is capable of spatially modulating the incident light beam with a two-dimensional image in 10 respect of one of the colours, red, green and blue, with those mirror devices M which are tilted to the "ON" orientation providing bright portions of the image and those which are tilted to the '' OFF 't orientation providing dark portions of the image. By varying the ratio of the 15 duration of the ''ON" orientation periods to the "OFF" orientation periods for each mirror device M, display of a grey scale image can be achieved as will be described in more detail hereafter.

20 Referring now to Figure 6, this figure illustrates the functional electrical, electronic and software components included in the electronic system 21. The primary components of the electronic system are: 25 (1) a warp geometry correction system 61 responsive to

the input video signal 10 electronically correct the distortion of the projected image; (2) a digital electronics system 63 for correcting the 5 grey scale levels of the red, green and blue images to be projected; (3) a formatter unit 69 for supplying the electrical data address signals to the mirror elements M of 10 the DMDs 15; (4) a projection lens system positioning system 67 effective to provide electrical control signals to the lens mount unit 19 shown in Figure 3 in order 15 to adjust the position of the projection lens 17 relative to the prism arrangement 13 on the DMDs 15; and (5) a power supply unit 69 for powering the electronic 20 system 21 overall.

Further details of the components of the electronic system 21 will be described hereafter.

25 Turning now to the air cooling system as indicated by the

- 12 double-head arrows in Figure 3, this is effective to provide lightlyfiltered but powerful air flow in order to cool the lamp 5 and lamp power supply 7, together with finely filtered but less powerful air flow effective to 5 cool the optical filters 9 and the prism arrangement 13 carrying the DMDs 15.

Optical adjustment of the image displayed on the display screen Referring now to Figures 7, 8 and 9, these figures illustrate how the distortion of the image as illustrated in Figure 2 may be reduced optically. The projection lens 17 is designed to have an oversized lens field 71

15 as illustrated schematically in Figures 7 and 8 and to be movable relative to the position of the prism arrangement 13 carrying the DMDs 15. This enables the image projected by the projection lens system to be located below the optical axis of the projection system 20 1 without having to tilt the projection system 1 itself, reducing the amount of keystone distortion. In Figure 8, the projection lens system 17 is shown displaced vertically from the axially aligned position shown in Figure 7, the axially aligned position being shown as a 25 dotted line in Figure 8. The relative displacement of

- 13 the object plane 73 defined by the output of the prism arrangement 13 then shifts the position of the projected image 75 downwards relative to the optical axis of the projection lens system 17.

Referring now also to Figure 9, this figure shows schematically the lenses incorporated in the projection lens system 17. The projection lens system is formed as a series of spherical lenses 91, 93, 95, 97, 101, with 10 the front lens 99 being a non-spherical symmetric lens designed to a complex power series. The projection lens system is an inverted telephoto lens system in which the physical length of the lens system is longer than the focal length of the lens system. The projection lens 15 system is also telecentric in that in the image plane the cones of light from each lens are parallel. The ray path for the projection lens system 17 when axially aligned with the object plane 73 is shown in dotted lines, with the ray path for the displaced projection lens system in 20 the position indicated in Figure 8 being shown in full lines. Referring now again to Figure 6, during set up of the projection system 1 at a new underground station 25 location, the projection lens system 17 is moved by the

lens mount 19 relative to the DMDs 15, with the optimum position being set and stored in a lens position store 103 in the projection lens positioning system 67 in the electronic system 21. The lens mount control unit 105, 5 then provides suitable electrical signals to the lens mount unit 19 to cause the unit 19 to move the projection lens system 17 to the appropriate position relative to the prism arrangement 13 to produce the offset image illustrated in Figures 8 and 9.

Thus, referring now also to Figure 10, an optically corrected image as indicated on the left-hand side of Figure 10 can be achieved. Whilst the effect of keystoning on the image has been reduced in comparison 15 to the image shown in Figure 2, it will be appreciated, however, that this image still requires adjustment as can be seen by the superimposed "ideal" rectangular image which is also shown in Figure 10. Distortion is generally produced due to the curved surface of the 20 screen 3, together with some remaining keystoning in some circumstances where it has not been possible to offset the projected image relative to the optical axis of the projection system sufficiently to avoid the necessity of slightly tilting the projection system. This remaining 25 distortion is corrected electronically as will now be

described. Electronic adjustment of the image 5 The principle of electronic reduction of the distortion of the image after optical reduction of the keystoning effect has taken place will now be described with reference to Figures ll, 12 and 13 which illustrate, on the lefthand side of the figures, the spatially 10 modulated light beam produced by each DMD for an image consisting of five circular spots, this corresponding to the pattern of "ON" mirror elements within each DMD. On the right-hand side of each figure, the corresponding image as projected on the screen 3 is shown.

Referring firstly to Figure 11, this figure illustrates the distortion of the five circular spots produced by the orientation of the surface of the screen 3 relative to the projection system 1 due to the keystoning effects 20 when no electronic correction of the image has taken place. As can be seen in the right-hand figure, the projected image corresponding to each spot is distorted, this happening throughout the image overall to cause the spreading of the lower portion of the projected image 25 relative to the top of the image. In order to overcome

- 16 this, as illustrated in Figure 12, the electrical address signals to each mirror element M in each DMD is modified such that at least some of the light which would be reflected along the ''ON.' path indicated in Figures 4 and 5 5 by some of the mirror elements M, is actually reflected by different mirror elements towards the centre of each DMD. In particular, round the edges of the DMD, as indicated by hatching in Figure 12, the mirror elements M are arranged to remain in an "OFF" orientation 10 regardless of the corresponding electrical data address signal, the light which would have been reflected by these mirror elements as determined by the electrical data address signal for each element, being added to the light reflected by the inner mirror elements.

Turning now to Figure 14, this figure illustrates the effective mapping function which maps the original rectangular array of pixels in the image as determined by the input video signal which would normally be 20 displayed by a corresponding rectangular array of mirror elements M, to the actual mirror elements of the DMD which will be used to reflect the light corresponding to each pixel. These points are calculated by means a warp calculation which defines the geometric relationship 25 between each pixel in the input image and the mirror

- 17 elements which will be used to display the input image.

This is performed by defining a series of vertically and horizontally aligned points within the image, known as "Anchor Points'' and calculating, using 3rd-order 5 polynomial equations, the corresponding mirror element correction to effectively shift the position of the input image. The corrections for the pixel positions between the "Anchor Points" are then extrapolated in order to reduce processing time.

Figure 13 illustrates the effects of a further horizontal barrel correction imposed on the keystone corrected image shown in Figure 12. In the left hand part of the figure, those portions of the DMD which are arranged to remain 15 in an ''OFF'' orientation are again shown hatched. As can be seen from the right hand part of the figure, the resultant projected image has now the undistorted form which would have been produced on a planar screen perpendicular to the optical axis of a conventional 20 projection system. Figure 15 shows an equivalent mapping function for the correction of a single radius warp where there is a single radius of curvature. Screen, i.e. a cylindrical tunnel wall. Figure 16 shows a dual radius warp map for use with a cylindrical tunnel wall with 25 different upper and lower radius.

- 18 Correction for high ambient lighting The inventors have appreciated that in situations of high ambient lighting, as in a tube station, low projected 5 light levels are washed out, and accordingly have devised a means for enhancing the contrast of the projected images at the higher projected light levels. This improves the appearance of low saturation colours in the projected image, and in particular enables the better use 10 of existing advertising video material which has been developed for use in different situations.

Referring to Figure 17 and 18, these figures represent the relationship between the video input level light 15 level for each colour, ( i.e. red, green and blue) for each pixel of each frame of the image. The input video signal for each colour of each pixel will be representative of a binary number, typically of 9 bits, representing the time for which the mirror element must 20 be in the "ON.' orientation, each time interval being a multiple of a length of the time period corresponding to the least significant bit in the input video signal. All light intensities can be expressed as a sum of the various bits provided that the time periods are included 25 within a display frame period of less than about 20 msecs

- 19 duration, in which case the human eye can integrate the periods and respond as if to a single period having a level of brightness according to the sum of binary signal brightnesses. Figures 17 and 18 show a conventional transfer function in which the relationship between the video input level Vin and the light intensity output VOut of each colour produced by each mirror element of the DMD can be 10 expressed by: VoUt = Vine where y is a function known as the gamma function and is conventionally 2.2. Thus, on a log-linear scale, the 15 relationship between VOut and Vin is linear.

Figure 17 shows the overall shape of the modified transfer function which at low and high light level portions comprises, respectively, inverse exponential and 20 exponential curves, the low and high level portions being connected by a straight line. The gradual changes in gradient at the upper and lower portions of the transfer function are designed to avoid visible light level intensity steps in the projected image.

- 20 The inventors for the present application have appreciated that in mid to high ambient light conditions, superior performance can be achieved by increasing the number of light level steps produced by the DMDs at mid 5 light levels for a particular video input intensity compared to the low light levels. From the enlarged scale transfer function shown in Figure 18 it can be seen that at low light levels the gradient of the transfer function for the embodiment of the invention is less than 10 that of the conventional transfer function, whilst at the low to mid range light intensities, the gradient of the transfer function is greater than that of the conventional transfer function. This means that less low intensity or high intensity output bits are provided for 15 each increment in the input video signal level, whilst more light levels are provided in the 80% mid range levels, thus boosting the output light levels at the mid range light levels and increasing resolution at the mid range to high intensity light levels.

For an input video signal of 9 bits, the transfer function of the embodiment of the invention will typically produce 13 corresponding output bit levels.

- 21 Implementation of electronic processing of image Referring now again to Figure 6, in order to implement the electronic corrections to the image as described 5 above, the coefficients defining the relevant warp maps as shown in Figures 14 15 and 16, which are likely to be used in any particular tube station, are stored in a warp coefficients look up table 161 in the warp geometry unit 161. The input video signal is buffered in a video 10 signal input unit 163, the red, green and blue intensity levels for each frame being passed to respective red, green and blue frame stores 165, 167, 169. The relevant coefficients for each of the mirrors M in each image frame as determined by the warp maps stored in warp 15 coefficients store 161 are multiplied by the video signal values in each of the red, green and blue frame stores 165, 167, 169 and the modified data signals passed to further red, green and blue frame stones 171, 173, 175 in the digital electronics unit 63.

In the digital electronic unit 63, appropriate transfer functions as illustrated in Figures 17 and 18 for each of the red, green and blue channels are stored in transfer functions store 177. The binary bit levels for 25 each of the red, green and blue channels for the modified

- 22 data values stored in each frame store 171, 173, 175 are calculated on the basis of each transfer function, and data representative of the bit maps for each of the red, blue and green data for each frame is loaded into one of 5 a pair of sets of thirteen bit frame stores 179, 181 corresponding to the 13 bit output produced by the transfer function in the formatter unit 65. It will be appreciated that a pair of bit frame stores is provided in respect of each colour, that is, each DMD: only one 10 pair of bit frame stores is shown in order to simplify the drawing.

In the formatter unit 65, a sequencer 189 provides appropriate address data signals to each of the red, 15 green and blue DMDs 15 so as to sequentially address all pixels of each DMD with the appropriate data under the control of a clock 191, a reset circuit 187 applying reset pulses to each mirror element M at the end of each set of bit data as described, for example in EP-A 20 0557360. It will be appreciated however that other address sequences may be used to address the mirrors M of the DMDs 15.

It will be appreciated that the above description of a

25 separate warp geometry unit, digital electronic unit and

formatter unit is given to simplify the explanation and these units may be integrated. In particular, it will be appreciated that some or all of the electronic system may be implemented by computer software running on a 5 suitable processor. Such computer software may be provided on a storage means such as a disc or downloaded as a signal from a network.

It will be appreciated that whilst the above embodiment 10 has been described in relation to a DMD, other spatial light modulators may be used to implement the invention, for example liquid crystal displays. Furthermore, a projection system in accordance with the invention finds application in other situations to a tube station where 15 the projection screen is not planar and normal to the optical axis of the projection system. In some cases the image may be required to be projected above the optical axis of the projection system, rather than below.

20 It will be appreciated that it is preferable to perform optical correction of the image prior to electronic correction of the image as electronic correction will lead to a reduction in resolution of the image. However, in principle and in accordance with the invention, 25 electronic correction or optical correction may be

performed alone in some circumstances where the orientation of the screen relative to the projection system permits this.

Claims

1. A projection system comprising: at least one spatial light modulator comprising an array of electrically 5 addressable light modulating elements; means for addressing each light modulating element with an electrical signal representative of a portion of an image to be projected; means for optically modifying the image produced by the spatial light modulator dependent on the 10 orientation of the surface on which the image is to be projected relative to the projection system; and means for modifying the electrical signal applied to each light modulating element to create a further modification of the image produced by the spatial light modulator 15 dependent on the orientation of the surface on which the image is to be projected relative to the optical axis of the projection system.

2. A projection system according to claim 1 wherein 20 said means for optically modifying comprises means for moving said light modulator elements and a projection lens system effective to project the spatially modulated light produced by the spatial light modulator relative to each other so as to offset the image projected by the 25 projection lens system relative to the optical axis of

- 26 the projection system.

3. A projection system according to either of the preceding claims wherein said means for modifying the 5 electrical intensity of signals including means for multiplying the intensity to be displayed for each pixel of the image modulator element with a factor effective to reduce keystone distortion of the projected image.

10 4. A projection system according to any of the preceding claims wherein said means for the electrical intensity of signals includes means for multiplying the intensity to be displayed for each pixel of the image modulator element with a factor effective to reduce 15 barrel distortion of the projected image.

5. A projection system according to any one of the preceding claims comprising means for increasing the number of available bit levels for displaying intensity 20 levels of an image at mid intensity levels relative to the low and high intensity levels.

6. A projection system according to any one of the preceding claims wherein said array is a DMD.

- 27 7. A projection system according to any one of the preceding claims wherein said array is an LCD.

8. A projection system according to any one of the 5 preceding claims for use in an underground station wherein said surface is formed on the wall of the underground station.

9. A projection system according to any one of the 10 preceding claims including a projection lamp and an air cooling system arranged to provide relatively high intensity but substantially unfiltered cooling air to the air cooling system, and relatively low intensity but highly filtered cooling air to the spatial light 15 modulator. 10. A method of use of a projection system according to any one of the preceding claims.

20 11. A computer program including processor implementable instructions for causing a processor to perform a method according to claim 10.

12. A projection system substantially as hereinbefore 25 described with reference to the accompanying drawings.