EP0358252A1

EP0358252A1 - Display device

Info

Publication number: EP0358252A1
Application number: EP89202028A
Authority: EP
Inventors: Arthur Marie Eugene Hoeberechts
Original assignee: Philips Gloeilampenfabrieken NV; Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 1988-08-09
Filing date: 1989-08-03
Publication date: 1990-03-14
Also published as: KR900003947A; JPH0282435A; NL8801983A

Abstract

A flat display tube provided with a display screen (4) and a generation system for generating a row of electron beams (7) which move substantially parallel to the display screen (4), and a deflection system for deflecting the electron beam (7) towards the display screen (4), and generation means (11a, 11b) for generating calibration beams (12a,12b). The calibration beams are deflected towards target bands (13a, 13b). The target spot of the calibration beams on the target band is detected and deviations from an ideal position and/or shape are used as a standard for correcting the paths of the electron beams (7).

Description

The invention relates to a display device comprising an evacuated envelope having a substantially flat, front wall, a layer of a luminescing material on the inner surface of the front wall, a generation system for generating several electron beams which are arranged in a row and which move predominantly parallel to the front wall, and a deflection system for deflecting the electron beams towards the layer of luminescing material, each electron beam scanning a column of picture elements.
Such a display device is known from United States Patent Specification 2,858,464, in which a display device is described which comprises to one side of the layer of luminescing material, hereinafter also called "the display screen", a generation system having a line cathode surrounded by an inner and an outer cylindrical electrode which are provided with rows of apertures. The number of apertures in the outer electrode corresponds to the number of vertical columns of picture elements. The generation system further comprises two deflection coils which are located outside and along the outer electrode. In an embodiment shown in Fig. 3 of the said Patent Specification, the apertures in the electrodes are located in a manner and the deflection coils, the line cathode and the electrodes are formed in a manner such, that during operation an electron beam is always selectively generated in a plane substantially parallel to the display screen from one aperture in the outer electrode. The deflection system comprises deflection electrodes on the rear wall substantially parallel to the front wall. By selectively energizing the deflection electrodes an electron beam is deflected towards the display screen. In this way an image is formed on the display screen.
Picture errors occur in display tubes, which are caused by, amongst others, magnetic fields. The visible effect of such errors is a distortion of the position of the image and/or the colour. The influence of magnetic fields is greater according as the energy of the electrons in the electron beams is smaller. In display tubes of the type described in the opening paragraph relatively low-energy electron beams (having less energy than a few KeV) are generally generated.
It is an object of the invention to provide a display device in which the number of picture errors is reduced.
To this end, a display device of the type mentioned in the opening paragraph is characterized in that the display device comprises generation means for generating at least one separate calibration beam, deflection means for deflecting at least the one calibration beam towards an associated target band, and detection means for detecting a target spot of at least the one calibration beam on the associated target band, and correction means which can be controlled by means of the detection means, and which are used to influence the electron beams.
This enables the influence of magnetic fields on the deflection to be compensated. During operation the target spot of the calibration beam is detected by means of the detection means. The target spot is correlated with the target spots of the electron beams on the display screen. Deviations of the target spot of the calibration beam from an ideal target spot form a criterion for the picture errors occurring. In the display device in accordance with the invention, during operation the deflection of the electrons towards the display screen is corrected by means of these deviations. A separate calibration beam is to be understood to mean herein, an electron beam which does not form part of the row of electron beams by means of which an image is formed on the display screen. Correction of the paths of the electron beams is to be understood to mean herein, a correction of the direction of the path as well as a correction of the shape of the electron beam, for example a correction of the focussing operation.
An embodiment of the display device in accordance with the invention is characterized in that the generation means for generating at least one calibration beam are arranged to generate at least the one calibration beam parallel to and next to the row of electron beams and that the associated target band extends next to the display screen.
The calibration beam may extend between the electron beams, in which case the target band associated with the calibration beam extends between the phosphor lines. However, preferably, the calibration beam extends next to and parallel to the row of electron beams, and the target band extends next to the luminescing layer. Since the calibration beam extends next to and parallel to the row of electron beams, the deviations in the calibration beam are readily correlated to the deviations in the row of electron beams, without the calibration beam disturbing the image because the beam is located outside the image.
A further embodiment of the display device in accordance with the invention is characterized in that the means for generating are arranged to generate at least one calibration beam next to both ends of the row.
This enables picture errors to be determined on two sides of the display screen. An average picture error across the display screen and a variation of a picture error across the display screen can be determined. In this way an increase of the possibilities of correcting the paths of the electron beams is obtained.
A still further embodiment of the display device in accordance with the invention is characterized in that the generation means, the target bands and the deflection means are symmetrically formed relative to a line through the centre of the luminescing layer.
In this way a variation of a picture fault across the display screen can be determined more readily.
An embodiment of the display device in accordance with the invention is characterized in that the generation means can suitably be used to generate at least two calibration beams next to an end of the row.
Various picture errors may occur: displacements of the electron beams parallel to the target strips, displacements transversely to the target bands and focusing errors. Owing to the use of two calibration beams various errors can more readily be determined.
In a further embodiment two target bandss extending next to one another overlap each other partly. Consequently, the target bands occupy less space.
Preferably, the generation system is composed of substantially identical units, each unit being suitable for generating one electron beam. This simplifies the composition of the generation system as well as the correction of the paths of the electron beams. In an embodiment each unit comprises a semiconductor cathode having an emissive surface and a stack of electrodes. Semiconductor cathodes operate at relatively low temperatures. This reduces thermal stress and, hence, picture errors caused by thermal stress.
Preferably, the electrodes are integrated into a stack of plate electrodes. This simplifies the composition of the generation system and reduces thermal stress.
A further embodiment of the invention is characterized in that the generation means comprise modulation means for modulating the intensity of at least the one calibration beam at a modulation frequency, and in that the detection means comprise filtering means for measuring a frequency component of a signal in a frequency range about the modulation frequency.
During operation the intensity of the calibration beam is modulated at a modulation frequency. The detection means can suitably be used to detect the frequency component of a signal about the modulation frequency. This has the advantage that a signal induced by the calibration beam impinging on the target band is detected without much attenuation while disturbing signals produced by the electron beams which form the image on the display screen are filtered out because, in general, such disturbing signals comprise no or only a very small frequency component in the frequency range about the modulation frequency.
If the generation means can suitably be used to generate calibration beams which extend next to one another, these generation means are preferably provided with means for modulating calibration beams extending next to one another at different modulation frequencies, and the detection means for the two different calibration beams are each provided with filtering means to measure a frequency component of a signal in a frequency range about the modulation frequency of the relevant calibration beam.
The signals of different calibration beams are modulated at different modulation frequencies. In this way, signals of different target bands can readily be distinguished.
In an embodiment, the target band comprises cathodoluminescing material and the detection means comprise means for detecting light emitted by the cathodoluminescing material and impinging on the target band.
In another embodiment the target band comprises a material having a large coefficient of secondary electron emission and the detection means comprise means for detecting secondary electrons emitted by the material having a large coefficient of secondary electron emission.
In yet another embodiment the target band comprises a conductive material and the detection means comprises current meters which are connected to the conductive material. The target spot can be readily detected by measuring the current, thereby rendering all additional detection means superfluous.
Relative to embodiments in which light or secondary electrons are detected, such an embodiment has the advantage that no or few disturbing signals are generated by the display screen: detection means for light or secondary electrons are also sensitive to light or secondary electrons emitted by the display screen.
Relative to emodiments in which light or secondary electrons are detected, an embodiment in which the target band contains conductive material has the additional advantage that the target band also serves a a detection means. By virtue hereof, the detection efficiency and the accuracy with which the location of the electron beam can be determined are increased.
The target band may be composed of a number of separate target regions. The simplest embodiment is the one in which the target band comprises a target strip which is coherent over substantially the entire length of the target band. This is particularly advantageous if the target band contains conductive material, because in this case few electric connections are required.
Preferably, the target strip is formed such that the target spot of the calibration beam can be detected in a direction along the target strip.
To this end, in an embodiment of the display device in accordance with the invention, the section of the target strip varies transversely to the target strip viewed along the strip, and in another embodiment the target strip is formed such that it has a staggered structure in the direction along the target strip, and in yet another embodiment the target strip is provided with regions having different coefficients of electron acceptation. Embodiments combining the different characteristics of the above-described target strips are also possible.
In an embodiment, the detection means comprise a pair of parallel, target strips which are coherent substantially throughout the target band and which are separated by a non-conductive channel. The target spot of the calibration beam on the target strips can be detected by comparing the signals of both target strips to one another. Preferably, the pair of target strips is formed such that they are mirror symmetrical relative to a line between them. In this way the detection of the target spot is simplified. The pair of target strips is preferably formed such that the target spot can be detected in a direction transverse to the target strips. For example, the section of the channel varies transversely to the channel viewed along the channel.
The invention will now be explained in greater detail by means of a few embodiments and with reference to a drawing, in which

Fig. 1 is a partly cutaway perspective view of a display device in accordance with the invention;
Fig. 2 is a cross-sectional view of a display device in accordance with the invention;
Fig. 3 is a top view of a display screen of a display device in accordance with the invention;
Figs. 4 and 5 are top views of display screens of further examples of display devices in accordance with the invention;
Fig. 6 shows a detail of a display device in accordance with the invention;
Figs. 7a and 7b are front views of an example of a correction means suitable for a display device in accordance with the invention;
Fig. 8a diagrammatically shows a target strip suitable for a display device in accordance with the invention, and Fig. 8b is a represents alion of the strength of a signal induced by a calibration beam impinging on the target strip as a function of the location of the target spot;
Figs. 9a, 10a and 11a show a further target strip suitable for a display device in accordance with the invention, and Figs. 9b, 10b and 11b are represent alions of the strength of a signal induced by a calibration beam impinging on the target strip as a function of the location of the target spot;
Figs. 12a, 13a, 14a, 15a, 16, 17, 18 and 19 show further examples of target bands and target strips suitable for a display device in accordance with the invention; and
Figs. 12b, 13b, 14b and 15b show signals generated by target spots.

The Figures are diagrammatic and not drawn to scale, corresponding parts in the different embodiments generally bearing the same reference numerals.
Fig. 1 is a partly perspective view of a display device 1 in accordance with the invention. This display device comprises in an envelope 2a display window 3 which is provided on the inside with a display screen 4. The display device 1 further comprises a generation system 5 which comprises a number of elements 6 for generating a row of electron beams 7. The generation system further comprises a number of plate electrodes 8. These plate electrodes make sure that, amongst others, the electron beams 7 emitted by the elements 6 are accelerated and eventually emitted in a direction parallel to the display screen 4. The electron beams 7 are deflected towards the display screen 4 by deflection electrodes 9. In the present example, a shadow mask 10 is positioned in front of the display screen 4. The generation system comprises a number of substantially identical units, each unit comprising an emitting element 6 and associated parts of the plate electrodes 8.
Fig. 2 is a sectional view of the display device shown in Fig. 1.
Fig. 3 is a diagrammatic front view of a display device in accordance with the invention. Next to the elements 6 there is a generation means 11 for generating a calibration beam 12 next to and parallel to the electron beams 7. A target band 13 extends next to the display screen 4. The deflection means for deflecting the calibration beam towards the target band are not shown. Each electron beam scans one column of display elements 7a. A column of display elements may consist of a phosphor strip of cathodoluminescing material of one colour. A colour picture may be obtained, for example, by forming each third phosphor strip of a red phosphor, the neighbouring phosphor strip of a blue phosphor and the phosphor strip next to the latter of a green phosphor. In such an embodiment each electron beam scans one strip of one colour. The advantages of such an embodiment are that no shadow mask is required and the elements can be readily controlled. The disadvantage is that a slight deviation of an electron beam transverse by to the electron beam leads to colour differences. In the preferred embodiment shown in Fig. 2, the display device comprises a shadow mask 10. Each column of display elements 7a is composed of a pattern of red (R), green (G) and blue (B) phosphors. The phosphor strip contains a triad of phosphors (R, G, B) for each aperture in the shadow mask. In addition, the shadow mask at least partly screens the electron beams from magnetic fields.
Fig. 4 is a top view of a further example of a display device in accordance with the invention. This display device comprises means 11a and 11b for generating two calibration beams 12a and 12b, respectively, on either side of the row of electron beams 7. Two target bands 13a and 13b extend next to the display screen 4. The target bands may be located, for example, on the display window or on the shadow mask. Deviations can be determined on both sides of the display screen. It is possible to obtain an improved determination of the average picture error as well as to determine a variation of a picture error across the display screen 4.
Fig. 5 is a top view of another example of a display device in accordance with the invention. This display device comprises generation means 11c and 11d for generating calibration beams 12c and 12d. Target bands 12c and 12d extend next to the display screen 4. Owing to the use of two calibration beams, picture errors can be determined more accurately. Moreover, it enables a picture error to be determined even if one of the generation means fails.
Fig. 6 shows a detail of the display device shown in Figs. 1 and 2. In this example, the element 6 is an element of a row of semiconductor cathodes having a generative surface for generating electrons. The elements 6 may also comprise one or a number of field emitter cathodes. The group of electrodes 8 comprises a number of electrodes 14 up to and including 19 for accelerating, focusing and directing the electron beams 7 emitted by the generation means 6. The electron beams 7 are deflected over a very short distance through an angle of 90^o by the electrodes 8. Consequently, the electron beams are emitted in paths parallel to the display screen. In this example, a correction means 21 in the form of two comb-shaped electrodes is located on the electrode 19. The correction means 21 is separated from the electrode 19 by means of an insulating intermediate layer 20.
Fig. 7a is a front view of the correction means 21. In the present example, this correction means 21 is composed of two cam-comb-shaped electrodes, one electrode comprising teeth 21a and a connection piece 21c, the other electrode comprising teeth 21b and a connection piece 21d. Electron beams 7 pass through apertures 22. By energizing the comb-shaped electrode at different potentials the paths of the electron beams can be corrected, in this example, in a direction parallel to a plane through the electron beams. Fig. 7b is a front view of a correction means for correcting the paths of the electron beams in a direction transverse to a plane through the electron beams. In this example, the correction means 21 is located on the electrode 19. This is not to be regarded as limitative. The correction means may form a unit which is separate from an electrode, possibly outside the electrodes 8, or it may be fitted to one of the other electrodes or it may be integrated in the element 6. It is alternatively possible that both a correction means as shown in Fig. 7 and a correction means as shown in Fig. 8 are contained in the display device. Neither should the shape of the correction means be regarded as limitative. The electrodes shown in the Figs. 7a and 7b are only examples. For example, it is possible that correction of the paths of the electron beams takes place by means of electromagnetic fields. In these drawings correction means are shown which influence all electron beams equally. Such an arrangement can only correct average deviations. It is alternatively possible to divide correction means 21 into sub-correction means, for example by interrupting the connection pieces 21c and 21d as is indicated in Figs. 7a and 7b by broken lines. In this case the number of separate connections needed increases accordingly. This also allows a variation of a picture error to be corrected.
Preferably, the calibration beams differ only little and in a controlled manner from the electron beams used to form the image. This can be readily achieved if the generation means for generating the calibration beam and the deflection means for deflecting the calibration beam towards the display screen are substantially identically shaped as the generation system for generating the electron beams and the deflection system for deflecting the electron beams towards the display screen. The best correction can then be determined on the basis of the deviation or deviations of the positions of one or more target spots of the calibration beams.
Fig. 8a shows a target strip 23. A calibration beam 24 impinges on this target strip. This leads to the generation of a signal. If the target strip comprises cathodoluminescing material a light signal is generated, the intensity of the light can be measured by means of light sensors. If the target strip comprises material having a high secondary electron emission, secondary electrons are emitted which can be measured by means of an electron sensor. However, the electrons or photons emitted by the target strip may adversely affect the image displayed. Besides, the detection efficiency may be low. Not every electron which is incident on the target band creates a photon (or electron), not every photon (or electron) is incident on a detection means, and not every photon (or electron) which is incident on a detection means is detected by said detection means. Consequently, the detection efficiency may be low. Moreover, inhomogneities in, for example, the cathodoluminescing material and/or the sensors and/or variations in the positions of cathodoluminescing material and sensors relative to each other may adversely affect the accuracy with which the locations of the electron beams is determined. Preferably, the target strip therefore consists of conductive material. The number of electrons impinging on the target strip can be measured by means of a current meter. In this case, the target strip also serves as a detection means. Fig. 8b shows the signal (I) generated by the electrons impinging on the target strip as a function of place (y=along the target strip, x=transverse to the target strip). By means of the target strip 23 the position of the target spot of electron beam 24 transversely to the target strip can be determined, but the position in a direction along the target strip cannot be determined.
Fig. 9a shows a target strip whose section changes transversely to the target strip viewed along the target strip. The signal I changes both as a function of x and as a function of y, such that the position of the target spot transversely to as well as along the target strip can be determined. This is diagrammatically shown in Fig. 9b.
Fig. 10a shows a target strip, the hatched areas of which have a larger coefficient of electron reflection than the unhatched areas. The signal I depends on both x and y, as is shown in Fig. 10b.
Fig. 11a shows a target strip having a staggered structure. Figs. 11b show the signal I as a function of y for x=0 and for x≠0. It is clearly shown that for x≠0 the variation of the signal as a function of y becomes much more complex, i.e. the signal I is composed of more and higher frequency components, as is shown in the lower drawing of Figs. 11b, in which drawing the frequency is plotted on the horizontal axis and the value of the frequency components is plotted on the vertical axis. For x=0 the signal only contains components having a frequency f₀ and a higher harmonics of f₀, for x≠0 there is also a frequency component having for a frequency f₁.
Fig. 12a shows a target band comprising two parallel target strips 23a and 23b. Fig. 12b shows the difference between the signals I_a and I_b of target strip 23a and target strip 23b, respectively. This difference is 0 if the target spot 24 is located right between both target strips.
Fig. 13a shows a target band comprising two target strips between which there extends a channel of varying section. Also in this case the difference in intensity I_a-I_b is 0 if the target spot is located right between the target strips. The sum of I_a+I_b varies as a function of y so that the location of the target spot transversely to as well as along the target band can be determined. If the target spot is not symmetrical relative to a line along the target band but instead, for example, egg shaped it is to be noted that I_a-I_b will always variation, the value of which is close to nil. Consequently, it is also possible to obtain some information about the shape of the target spot.
Figs. 14a and 14b show how more properties of the calibration beams can be determined and, hence, more picture errors can be corrected by making use of two target bands 24 and 25, for two calibration beams, having target spots 26 and 27. In Fig. 14a the target spots differ in size due to a difference in focussing. The difference between the signals I₁ of target strip 24 and I₂ of targer strip 25 is nil if both target spots are equally large, and if not the difference will not be equal to 0. This is diagrammatically shown in Fig. 14b. I₁-I₂ is a function of Δ; the difference in diameter between the target spots 26 and 27. This can be used to control the focusing of the electron beams in the manner described below:
one of the beams is overfocused and the other is underfocused to the extent that both target spots are equally large. This enables "correct" focusing to be readily determined by means of interpolation. In this way the correct focusing can be determined much quicker and more accurate than when only one calibration beam is used.
Figs. 15a show another example of how several properties of the calibration beams can be determined by using two calibration beams having target spots 30 and 31, respectively. The target strips 28 and 29 vary in section so that of each of the target spots 30 and 31 the position along the target strip can be determined. One parameter (for example a potential at an electrode) of the emission means or deflection means for the calibration beam with target spot 30 slightly differs from a parameter of the emission means or deflection means for the calibration beam with target spot 31. As a consequence hereof the target spot 30 is located somewhat "above" the target spot 31. On account hereof, the signal I₁ (= signal of target strip 28) and the signal I₂ (= signal of target strip 29) differ in phase. The relation between this parameter and the location of the target spot can readily be determined. By means of these data an improved correction of the paths of the electron beams is possible.
Fig. 16 provides a further example of target bands suitable for a display device in accordance with the invention, in which a number of aspects of the other drawings are combined. The drawing shows two target bands, each having two target strips, 32 and 33, and 34 and 35, respectively.
As has been stated above, the intensity of the calibration beam is preferably frequency-modulated. By measuring the signal in a frequency range about this modulation frequency, disturbing signals of the electron beams or, for example, secondary electrons induced by the electron beams can be filtered out. If the generation means is suitable for generating two calibration beams extending next to one another, and the associated target bands extend next to one another, it is advantageous, in particular, if the two calibration beams are modulated at different modulation frequencies. In this case, the location of one calibration beam is not adversely influenced by the other calibration beam, and common target strips can be used. This is illustrated by two examples shown in Fig. 17 and 18. The signal of electrode 40 brought about by calibration beams 38 and 39, which are modulated at frequencies f₁ and f₂, respectively, has two frequency components. These frequency components can be readily measured in known manner. The advantages of common target strips are a simpler construction and a saving of space.
Fig. 19 shows another example of a couple of target bands. In the present example, these target bands comprise target strips 41, 43, 45 and 47, which are separated by channels 42, 44 and 46. The signal of a target strip X brought about by a target spot Y is hereinafter called I (X, Y).
Quantities which can be determined are, for example:

A) The location of the target spot 48 relative to the channel 42: this follows from I(41,48) - (I(43,48) + I(45,48))
B) The location of the target spot 48 along the channel 42: this follows from I(43,48) or I(45,48)
C) The size of the target spot 48: this follows from the ratios between I(41,48), I(43,48) and I(45,48)
D) The location of target spot 49 relative to channel 46: this follows from I(47,49) - (II(45,49) + I(43,49))
E) The location of target spot 49 along channel 46: this follows from I(43,49) or I(45,49)
F) The size of target spot 49: this follows from the ratios between I(47,49), I(45,49) and I(43,49)
G) the relative difference in size between the target spots 48 and 49: this follows from
I(41,48)+I(43,48)+I(45,48)-I(43,49)-I(45,49)-I(47,49)
H) The relative difference in position between the target spots 48 and 49 along the channels 42 and 43: this follows from comparing I(43,48) and I(45,48) with I(43,49) and I(45,49)

It will be clear to those skilled in the art that many variations are possible within the scope of the invention.

Claims

1. A display device comprising an evacuated envelope having a substantially flat, front wall, a layer of a luminescing material on the inner surface of the front wall, a generation system for generating several electron beams which are arranged in a row, and which move predominantly parallel to the front wall, and a deflection system for deflecting the electron beams towards the layer of luminescing material each electron beam scanning a column of picture elements, characterized in that the display device comprises generation means for generating at least one separate calibration beam, deflection means for deflecting at least the one calibration beam towards an associated target band, detecting means for detecting a target spot of at least the one calibration beam on the associated target band, and correction means which can be controlled by means of the detection means and which are used to influence the electron beams.

2. A display device as claimed in Claim 1, characterized in that the generation means for generating at least one calibration beam are arranged to generate at least the one calibration beam parallel to and next to the row of electron beams, and the associated target band extends next to the luminescing layer.

3. A display device as claimed in Claim 2, characterized in that the generation means for generating are arranged to generate at least one calibration beam next to both ends of the row.

4. A display device as claimed in Claim 2 or 3, characterized in that the generation means, the target bands and the deflection means are formed symmetrically relative to a line through the centre of the luminescing layer.

5. A display device as claimed in Claim 2, 3 or 4, characterized in that the generation means for generating are arranged to generate at least two calibration beams next to an end of the row.

6. A display device as claimed in Claim 5, characterized in that two target bands extending next to one another partly overlap each other.

7. A display device as claimed in any one of the preceding Claims, characterized in that the generation system is composed of substantially identical units, each unit being suitable for the generation of one electron beam.

8. A display device as claimed in Claim 7, characterized in that each unit comprises a semiconductor cathode having an emissive surface and a stack of electrodes.

9. A display device as claimed in Claim 7 or 8, characterized in that the electrodes are integrated into a stack of plate electrodes.

10. A display device as claimed in any one of the preceding Claims, characterized in that the generation means are provided with modulation means for modulating the intensity of at least the one calibration beam at a modulation frequency, and in that the detection means are provided with filtering means for measuring a frequency component of a signal in a frequency range about the modulation frequency.

11. A display device as claimed in any one of the preceding Claims, characterized in that the generation means are suitable for generating at least a pair of calibration beams extending next to one another and modulation means for modulating the intensity of each of the calibration beams at a modulation frequency which differs for each calibration beam, and in that detection means are provided with filtering means for measuring a frequency component of a signal in frequency ranges about the different modulation frequencies.

12. A display device as claimed in any one of the preceding Claims, characterized in that the target band comprises cathodoluminescing material and the detection means comprise means for detecting light emitted by the target band.

13. A display device as claimed in any one of the Claims 1 up to and including 11, characterized in that the target band comprises a material having a large coefficient of secondary electron emission and the detection means comprise means for detecting secondary electrons emitted by the target band.

14. A display device as claimed in any one of the Claims 1 up to and including 11, characterized in that the target band comprises a conductive material and the detection means comprise a current meter connected to the conductive material.

15. A display device as claimed in one of the Claims 12, 13 or 14, characterized in that the target band comprises a target strip which is coherent over substantially the entire length of the target band.

16. A display device as claimed in Claim 15, characterized in that the target strip is formed such that the location of the calibration beam can be determined in a direction along the target strip.

17. A display device as claimed in Claim 16, characterized in that the section of the target strip transversely to the target strip varies viewed along the strip.

18. A display device as claimed in Claim 17, characterized in that the section changes periodically.

19. A display device as claimed in Claim 16, characterized in that the target strip is formed so as to have a staggered structure.

20. A display device as claimed in Claim 16, characterized in that the target strip comprises regions having different electronacceptation coefficients.

21. A display device as claimed in Claim 15, characterized in that a target band comprises a pair of parallel target strips which are coherent substantially throughout the length of the target and which are separated by a channel.

22. A display device as claimed in Claim 21, characterized in that the pair of target strips is formed such that the position of the calibration beam can be determined in a direction transverse to the target strips.

23. A display device as claimed in Claim 22, characterized in that the section of the channel varies transversely to the channel viewed along the channel.

24. A display device as claimed in Claim 21, 22 or 23, characterized in that the pair of target strips is mirror symmetrically formed relative to a line between the target strips.

25. A display device as claimed in Claim 21 and 16, characterized in that neighbouring target strips of both target bands are separated by a meander-shaped channel.

26. A display device as claimed in Claims 21 and 6, characterized in that both target bands have one common target strip.