WO2002014904A1

WO2002014904A1 - Position-sensitive gamma radiation detector

Info

Publication number: WO2002014904A1
Application number: PCT/SE2001/001738
Authority: WO
Inventors: Thomas Elevant
Original assignee: Smt Hb
Priority date: 2000-08-11
Filing date: 2001-08-10
Publication date: 2002-02-21
Also published as: SE0002888D0; AU2001280387A1; SE0002888L; WO2002014904A8

Abstract

Device for position determined detection of radioactive radiation, comprising at least one pair of layers (1, 2), one of the layers in the pair here being called the primary layer (1) and the other the secondary layer (2), the layers comprising scintillating elements (3, 4) of some length called primary elements (3) in the primary layer (1) and secondary elements (4) in the secondary layer (2), the two layers (1, 2) being positioned on each other that the direction of the primary elements (3) in the primary layer (1) cross the direction of the secondary elements (4) in the secondary layer (2), the elements (3, 4) being connected to light sensitive means (21, 23, 25, 27, 39), which are arranged to react on light pulses that are generated in the scintillating elements (3, 4) by the radioactive radiation, and the light sensitive means are connected to a logic circuit system (38) for determining of where light pulses have been generated I the detection device. According to the invention a heavy material is arranged in or in the vicinity of the scintillating elements (3, 4), and/or the scintillating elements (3, 4) in the layers (1, 2) are connected to the light sensitive means (21, 23, 25, 27, 39) according to a multiplexing schedule, which will decrease the number of necessary light sensitive means (21, 23, 25, 27, 39).

Description

Position-sensitive gamma radiation detector

BACKGROUND TO INVENTION

The present invention relates to a device for the position-sensitive detection of radioactive radiation, consisting of at least one pair of layers, one of the said layers in the pair being referred to herein as the primary layer and the other as the secondary layer, the layers incorporating longitudinally arranged elements referred to as primary elements in the primary layer and secondary elements in the secondary layer, the primary elements at least being scintillating, the two layers being located relative to each other in such manner that the direction of the primary elements in the primary layer intersects that of the secondary elements in the secondary layer, the elements being connected to photosensitive devices which are designed to react to light pulses generated in the longitudinally arranged elements by the radioactive radiation, and the photosensitive devices being connected to a logic circuit system for determining the position of light pulses generated in the detecting device.

The invention also refers to a procedure for the position-sensitive detection of radioactive radiation, comprising the following stages:

^■ location of at least one pair of layers, referred to as the primary layer and secondary layer, of longitudinally arranged elements, referred to as primary elements in the primary layer and secondary elements in the secondary layer, in such manner that the direction of the primary elements in the primary layer intersects that of the secondary elements in the secondary layer, the primary elements at least being scintillating; ^■ connection of at least one section of the elements in each layer to photosensitive devices;

^■ analysis of the position of light pulses generated in the longitudinally arranged elements, based on output signals from the photosensitive devices.

STATE OF THE ART

A device of the above type is used, for example, in medical scans to determine the location and extent of a brain injury. SPECT (Single Photon Emission Computerized Tomography) is one type of scan in which the device described can be used. PET (Positron Emission Tomography) is another. In a PET scan, the patient whose brain is to be examined is dosed with positron-radiating isotopes by inhalation or injection. Those parts of the brain which may be damaged can be distinguished from the other parts either by an accumulation therein of positron-radiating substances or by an abnormally low concentration of such substances. The radioactive isotopes emit positrons which, in turn, produce photon pairs, each of which consists of two oppositely aligned photons (γ radiation), and each of which has an energy of 511 keV.

A known device, which is of the type described initially and which is used for detecting photon pairs of this type, for example in a PET scan, is described in US patent US 5103098.

A problem with that device is that it must be as thick as 10 cm to achieve a detection efficiency of 30%; in other words, to ensure that 30% of the incoming photons are detected. The thicker the detector, the more expensive it is since the fibres used are expensive. A thick detector also requires more advanced data processing and several photomultiplier tubes (PM tubes) to deal with parallax errors, compared with what is required by a thinner detector.

A further problem with this device and many others available on the market today is that a very high number of PM tubes is required to achieve a sufficiently high resolution in the detector. PM tubes are extremely expensive and currently account for the greater proportion of the cost, for example, of a PET scanner.

SUMMARY

One purpose of the invention is to achieve a g radiation detecting device with a higher detection efficiency and better spatial resolution in the object than existing devices.

This is achieved in a device of the type described initially, in which a heavy material is disposed in or adjacent to the longitudinally arranged elements, the heavy material being chosen so as to generate an electron through the photoelectric effect of an incoming photon arising from the radioactive radiation, which electron leaves the heavy material and enters at least one primary and one secondary layer, and generates a light pulse in each layer.

The following is also achieved by a procedure of the type described initially:

^■ disposal of a heavy material in or adjacent to the longitudinally arranged elements so that an electron generated through the photoelectric effect of an incoming photon originating from the radioactive radiation is emitted by the heavy material and enters at least one primary and one secondary layer, generating a light pulse in each layer;

^■ generation of at least one light pulse in a primary layer and one light pulse in a secondary layer adjoining the said primary layer by the said electron emitted by the heavy material;

^■ determination of the position of the light pulses generated as the intersection of the two elements which have delivered signals.

The heavy material adjacent to the longitudinally arranged elements increases the efficiency of detection of the detector, the reason being that the incoming photons to be detected in the said longitudinally arranged elements by photoelectric effect or by Compton scatter must first displace an electron, which can then be detected. Since the probability of the photons displacing electrons is higher in a heavy material, the detection efficiency is higher when a heavy material is used together with the scintillating material. the heavy material also helps to screen the secondary radiation which occurs when the free electron is generated. Screening helps to reduce disturbances in position determination.

The spatial resolution achieved with the detecting device described is good since a number of crosslaid scintillating elements is used. When a light pulse has been generated in two adjoining, crosslaid scintillating elements, the position of the event can be calculated as the intersection of these two elements.

The longitudinally arranged elements should preferably be made of scintillating fibres.

Alternatively, the primary elements may be made of scintillating fibres and the secondary elements of wavelength shifting fibres. This enables the fibres to be made thicker and, consequently, easier to manage. The efficiency can also be improved.

This layers of the heavy material should preferably be laid between at least some layers of the longitudinally arranged elements.

Alternatively, the fibres may be of a type which incorporates the heavy material.

The heavy material should preferably be lead.

A further purpose of the invention is to achieve a detecting device which requires as few photosensitive devices as possible and, consequently, is cheaper.

This is achieved in a device and by means of a procedure of the type described initially, in which the longitudinally arranged elements in the layers are connected to the photosensitive devices in accordance with a multiplexing arrangement.

The photosensitive devices should preferably be photomultiplier tubes.

It is also advantageous if a predetermined number of the longitudinally arranged elements are connected in common to a light mixer, which then distributes the light among a number of photosensitive devices. This enables very thin scintillating elements to be used in the detector. Thin scintillating elements are necessary in this type of detector since it is desired that the incoming photons should generate an electron in two adjoining, crossing, scintillating elements in order to permit position detection.

The longitudinally arranged elements should preferably be arranged parallel to and adjacent to each other in the respective layers.

Both layers of longitudinally arranged elements should preferably be placed on each other in such manner that the direction of the primary elements crosses that of the secondary elements at right angles, the directions of the elements in the primary elements and in the secondary elements defining the X-axis and Y-axis respectively in an X-Y system of rectangular coordinates.

In addition, the device may suitably be provided with several similar pairs, each consisting of a primary layer and a secondary layer of longitudinally arranged elements, the said pairs of element layers being placed on each other in such manner that the primary elements in the primary layer in all of the pairs are arranged parallel to each other and that the secondary elements in the secondary layer in all of the pairs are arranged parallel to each other.

The elements in both layers of each element pair should preferably cross each other at right-angle, the one element direction and the other element direction defining the X- axis and Y-axis respectively in an X-Y-Z system of rectangular coordinates, the Z-axis of which is defined by the direction perpendicular to the layers of longitudinally arranged elements, that is, by the direction of thickness of the bundle formed by the layer pairs. BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 is a schematic view showing one side of a block extracted from a detecting device in one embodiment of the invention.

Fig. 2 is a schematic view showing the extracted block in Fig. 1, viewed in the direction of arrow II.

Fig. 3 is a schematic view showing the extracted block in Fig. 1, viewed in the direction of arrow III.

Fig. 4 is a schematic view showing how the fibres in an embodiment in accordance with the invention are connected to PM tubes.

DETAILED DESCRIPTION OF INVENTION

The embodiment of the invention shown schematically in figs. 1, 2 and 3, and described in further detail below, is dimensioned and adapted specifically for use in the medical applications described initially, that is, in PRT scans in which the photons have an energy level of 511 keV and in SPECT scans in which the photons have an energy level of between 150 keV and 300 keV approximately.

The embodiment of the detecting device in accordance with the invention as shown in figs. 1, 2 and 3 is of the sandwich type, and consists of several pairs of parallel layers

1 and 2. It may be advantageous to provide the detector with curved layers so that it (the detector) can be shaped more closely to the patient. The first layer 1 in each pair is referred to below as the primary layer 1 and the second layer 2 as the secondary layer 2. Both layers 1,2 are comprised of longitudinally arranged, scintillating elements referred to as primary elements 3 in the primary layer 1 and as secondary elements 4 in the secondary layer 2. In this embodiment, the elements 3,4 consist of fibres 3,4. The fibres in the primary layer 1 are referred to a primary fibres 3 and the fibres in the secondary layer 2 as secondary fibres 4. The fibres 3,4 are arranged parallel to and adjacent to each other within the respective layers. The number of fibre layer pairs in the application described is 15-50. In this embodiment, both fibre layers 1,2 in each fibre layer pair are placed on each other in such manner that the primary fibres 3 in the primary layer 1 cross the secondary fibres 4 in the secondary layer 2 at right angles. The angle can naturally be varied in different embodiments. The fibre layer pairs are placed on each other in such manner that the primary fibres 3 in the primary layer 1 in each pair are arranged parallel to each other (X-axis in Fig. 3), that the secondary fibres 4 in the secondary layer 2 in each pair are arranged parallel to each other (Y- axis in Fig. 3), and that the points of intersection between the primary fibres 3 and the secondary fibres 4 in all of the fibre pairs are located directly in front of each other as viewed at right angles to the layers 1 and 2 (cf. Fig. 3).

The scintillating fibres 3 and 4 are of a known type and may, for example, consist of fibres marketed by the Bicron Corporation of Ohio, USA under the designation BCF- 12. In this case, the fibres 3 and 4, which are identical, are provided with a scintillating core of doped polystyrene, an inner sleeve which provides an internal, total reflection, and an outer sleeve which prevents optical crosstalk between the fibres. In this embodiment, the fibres have a diameter of approximately 0.25 mm, although the diameter may be varied. Thicker fibres may be used to detect photons with higher energies. The scintillating fibres 3,4 deliver very short light pulses (approx. 3 ns), which contributes to reducing the dead time of the system.

These fibres 3,4 with a circular cross-section may be replaced by equivalent fibres with a square cross-section. The latter are superior in the sense that they can be laid adjacent to each other without any space between the fibres. On the other hand, they are more expensive. Some other type of longitudinally arranged elements of scintillating material may be used instead of fibres.

A thin layer of a heavy material, lead in this embodiment, is, in a first embodiment, disposed between the two fibre layers 1 and 2. A thin layer 6 of the same type is also disposed between the fibre layer pairs placed on top of each other. In another embodiment, only one layer of a heavy material, such as lead, is placed between the fibre pair layers placed on top of each other. In this case, this layer should be somewhat thicker than would be the case if a layer of lead were disposed between all of the fibre layers. It is also possible to place the layer of heavy material between every second pair of fibre layers or in an another manner between the fibre layers, The probability that a photon will displace an electron on striking the light material from which the fibres 3,4 are made, that is, the probability of reaction, is low. The purpose of the lead layers 5,6 is to greatly increase the probability that the photon will displace an electron in the detecting device. Although the thickness of the lead layers 5,6 may also be varied, it is normally about 0.05-0,3 mm. The layers must be sufficiently thick to enable many electrons to be displaced, but also thin enough to ensure that the electrons are not trapped by the layer of lead.

In a third embodiment of the invention, a heavy material, such as lead, may be mixed with the fibres rather than disposing a number of layers of a heavy material between the layers of fibre. This affords an increase in detection efficiency in the same manner as the layers of lead described above.

The fibres 3,4 are connected to photosensitive devices which, in this embodiment, consist of PM tubes. These, in turn, are connected to a logic circuit system for determining the position of the light pulses generated.

Fig. 4 shows how the primary fibres 3 in a first embodiment of the invention are connected across the light mixers 11 ,29 described below and, if appropriate, through optical fibres in accordance with a multiplexing technique, to photosensitive devices, such as PM tubes 21,23,25,27,39 for detecting the light signals generated in the fibres by scintillation. (This is illustrated in the lower part of Fig. 4.) The secondary fibres 4 in the secondary layers 2 are connected in similar manner to another combination of PM tubes. This may be another set of PM tubes, otherwise the same set of PM tubes used by the primary fibres 3 may be used. (The first alternative is illustrated without a detailed description and without numbering in the upper part of Fig. 4.)

In addition, the other end of all fibres in both the primary layer 1 and the secondary layer 2 may be connected to PM tubes in similar manner (not illustrated). An alternative to this is to reflect the light at the other end of the fibre so as to obtain a stronger light signal at the first end. This may be achieved by providing the other end of the fibres with some type of reflective material, such as aluminium, as illustrated by a plate A at one end of the secondary fibres 4. In the first embodiment shown in Fig. 4, the fibres are connected in groups of 16 to a first light mixer 11. In Fig. 4, this connection is shown by lines, even if some type of fibre, possibly an optical fibre, is probably used to make the connection. The 16 fibres assembled in the first group (on the right in the figure) consist, as shown in Fig. 4, of four adjoining primary fibres 3 in four successive primary layers 1. This first light mixer is, furthermore, in this embodiment, provided with four outgoing fibres 13,15,17, 19, also illustrated by lines, each of which is connected to its own photosensitive device, for example PM tubes 21,23, 25, 27. The second group of 16 primary fibres 3 shown is connected to a second light mixer 29, which is also provided with four outgoing fibres 31,33,35,37. These outgoing fibres 31,33,35,37 are connected to three of the PM tubes 21,23,25, to which the first light mixer is connected, and to a further PM tube 39. The next group of 16 fibres, which is not shown in this figure, is connected to a third light mixer, which divides the light into four components, which components are supplied to a new combination of the five PM tubes 21,23,25,27,39. Thus, different combinations of the PM tubes 21,23,25,27,39 will detect the signal depending on the group of 16 fibres in which the light has originated. This is in accordance with known multiplexing technology and it is easily seen that the variations whereby the light mixers can be connected to the PM tubes 21,23,25,27,39 are many and easy to compute.

The multiplexing technique described above can be used in detectors of the type described initially, with or without the aforementioned layers of a heavy material, such as lead.

The number of PM tubes to which each light mixer can be connected need not be four. It may be advantageous to reduce the number to two since the signals in each PM tube will then be increased. The number of fibres connected in common to each light mixer may also be varied. It is also possible to use this type of multiplexing technique without using light mixers. In this case, the fibres can be connected directly or by means of clear optical fibres to the PM tubes in accordance with the multiplexing arrangement described. This multiplexing technique reduces the number of PM tubes considerably. Arrays of up to 1000 tubes are usual in detecting devices commonly used in PET scans today. In this first described embodiment, in which 16 fibres are combined in a light mixer which supplies the light to four PM tubes and mutiplexing is employed, the number of PM tubes used would not exceed approximately 50- 100. Since PM tubes are very expensive and normally account for the greater proportion of the cost of a PET scanner, this reduction in the number of PM tubes is highly significant.

The method of using light mixers 11,29 to mix the light from a number of fibres before supplying it to the PM tubes permits the use of very thin fibres in the detector, which is otherwise problematic since the signals are very small when thin fibres are used. Thin fibres are necessary in this type of detector since the incoming photons are required to generate an electron in two adjacent, crossing fibres for the purpose of position determination

As described above, this type of detecting device in accordance with the invention is dimensioned and adapted specifically for use in PET scans, in which the photon energy is approx. 500 keV or, in the case of a SPECT scan, approx. 150-300 keV. In this instance, the diameter of the fibres 3 and 4, the thickness of the lead layers 5 and 6, and the number of double layers is chosen so that the probability that a free electron generated in the device by the photon will achieve a light pulse in two successive fibre layers 1,2 is relatively high.

Since two light pulses are generated simultaneously in a fibre 3,4 in each of two successive fibre layers 1,2, a logic circuit system 38 connected to the PM tubes is used to verify that this has occurred and to identify the fibres in question. The position of the particular electron interaction which has given rise to the light pulses is thereby determined. The resolution will be a function of the chosen fibre diameter. If a diameter of 0.25 mm is specified and it is chosen to mix 16 (4 x 4) fibres in a light mixer, the resolution will be 1 (0.25 x 4) mm along the X and Y axes and twice that value, or 2 mm, along the Z-axis. This resolution should be sufficient in PET scans since the positron, when emitted by the positron-emitting element in the body, will travel approximately 1 mm before reacting with an electron, while two photons will be formed and transmitted in the opposite direction. Thus, the uncertainty is approximately 1 mm regardless of the quality of detection resolution. In a SPECT scan, collimators are used to ensure acceptance of the photons entering the detector in certain, specified directions. These collimators provide a sufficiently large opening relative to the patient to permit the passage of sufficient numbers of photons for detection. Since this method is also a source of uncertainty in position detection due to the collimator opening size, a resolution of 1 mm in the X and Y directions should also be sufficient in SPECT scans.

Two such simultaneous events, one for each photon in the photon pair, is used in actual PET imaging, to determine the position of the origin of the photon pair in the patient's brain. Because of the accurate position determination of electron interaction along the Z-axis, this determination can be carried out accurately, that is subject to a small parallax error, resulting in good resolution.

The logic circuit system used in this application for pulse discrimination, determination of the synchronicity of different pulses and identification of scintillator elements is well known to the professional and will not be described in detail here.

In another embodiment of the present invention, the heavy material is disposed in such manner that it covers three sides of the longitudinally arranged scintillating elements which, in this case, consist of square-section scintillating fibres.

In yet another embodiment of the invention, the primary layer only consists of scintillating fibres. In this case, the secondary fibres consist of wavelength shifting fibres. In other respects, the device is of the same appearance as in previously described embodiments. A heavy material is disposed, for example, between each pair of a primary and a secondary layer. As before, a gamma quantum generates an electron in the heavy material, which electron penetrates the primary layer and generates a light pulse in it. A high proportion (about 70%) of this light pulse strikes the next fibre layer, the secondary layer, which consists of wavelength shifting fibres, generating a light pulse of lower frequency which is transmitted along the fibre to a PM tube. The difference between this and the embodiments described previously is that the necessary light pulse in the secondary layer is generated by the first light pulse, which is generated in the primary layer, causing a wavelength shift in the secondary layer. Position determination is performed in the same manner as in previously described embodiments. The advantages are that the fibres can be thicker and, thereby, easier to manage, and that the efficiency can be increased. However, this variant is somewhat more expensive, while the position resolution is somewhat poorer.

The detecting device in accordance with the invention may naturally be used in applications other than the medical applications described above. For example, it may also be used to advantage for photon detection in material tests performed using the 2D-ACAR (2D-Angular Correlated Annihilation Radiation) technique. The detecting device in accordance with the invention may also be used to detect radioactivity other than g radiation, such as a radiation, b radiation, X-ray radiation, neutrons, positrons and protons. To detect charged particles, the device requires only one pair of fibre layers since these particles do not required a significant material thickness to interact.

Claims

PATENT CLAIMS

1. Device for position-sensitive detection of radioactive radiation, consisting of at least one pair of layers ( 1 ,2), one of the said layers in the pair being referred to herein as the primary layer (1) and the other as the secondary layer (2), the layers incorporating longitudinally arranged elements (3,4) referred to as primary elements (3) in the primary layer (1) and secondary elements (4) in the secondary layer (2), the primary elements at least (3) being scintillating, the two layers (1,2) being located relative to each other in such manner that the direction of the primary elements (3) in the primary layer (1) intersects that of the secondary elements (4) in the secondary layer (2), the elements (3,4) being connected to photosensitive devices (21,23,25,27,39), which are designed to react to light pulses generated in the longitudinally arranged elements (3,4) by the radioactive radiation and the photosensitive devices being connected to a logic circuit system for determining the position of light pulse generation in the detecting device, characterised in that a heavy material is arranged in or adjacent to the longitudinally arranged elements (3,4), the heavy material being chosen so as, by virtue of the photoelectric effect caused by an incident photon produced by the radioactive radiation, to generate an electron which, on emission from the heavy material, enters at least one primary layer (1) and generates at least one light pulse in each of the primary and secondary layer (1,2).

2. Device as per patent claim 1 , characterised in that the longitudinally arranged elements (3,4) consist of scintillating fibres (3,4).

3. Device as per patent claim 1, characterised in that the primary elements consist of scintillating fibres and the secondary elements of wavelength shifting fibres.

4. Device as per patent claims 1 or 2, characterised in that thin layers (5,6) of the heavy material are disposed between at least some of the layers (1,2) of longitudinally arranged elements.

5. Device as per patent claim 2, characterised in that the fibres (3,4) are fibres of a type incorporating the heavy material.

6. Device as per any of the foregoing patent claims, characterised in that the heavy material is lead.

7. Device as per any of the foregoing patent claims, characterised in that the longitudinally arranged elements (3,4) in the layers (1,2) are connected to photosensitive devices (21,23,25,27,39) in accordance with a multiplexing arrangement, which reduces the number of photosensitive devices (21,23,25,27,39) required.

8. Device as per any of the foregoing patent claims, characterised in that the photosensitive devices (21,23,25,27,39) are photomultiplier tubes (21,23,25,27,39).

9. Device as per any of the foregoing patent claims, characterised in that a predetermined number of the longitudinally arranged elements (3,4) is connected to a light mixer (11,29), which then distributes the light among a certain number of photosensitive devices (21,23,25,27,39).

10. Device as per any of the foregoing patent claims, characterised in that the longitudinally arranged elements (3,4) are arranged parallel to and adjacent to each other in the respective layers (1,2).

11. Device as per any of the foregoing patent claims, characterised in that the two layers (1,2) of longitudinally arranged elements (3,4) are so placed on each other in such manner that the direction of the primary elements (3) crosses that of the secondary elements (4) at right angles, the direction of the elements in the primary layer (1) and the secondary layer (2) being defined as the X-axis and Y- axis respectively in a system of X-Y rectangular coordinates.

12. Device as per any of the foregoing patent claims, characterised in that it is provided with several identical layer pairs, each consisting of a primary layer (1) and secondary layer (2) of longitudinally arranged elements (3,4), and in that these pairs of element layers are placed on each other in such manner that the primary elements (3) in the primary layer (1) in all of the pairs are arranged parallel to each other and that the secondary elements (4) in the secondary layer (2) in all pairs are arranged parallel to each other.

13. Device as per any of the foregoing patent claims, characterised in that the longitudinally arranged elements (3,4) in the layers (1 ,2) in each pair of element layers cross each other at right angles, the first element direction and the second element direction being defined as the X-axis and Y-axis respectively in a system of X-Y-Z rectangular coordinates, in which the Z-axis is defined as the direction perpendicular to the layers (1,2) of longitudinally arranged elements, that is the direction of thickness of the bundle formed by the layer pairs.

14. Procedure for position-sensitive detection of radioactive radiation comprising the following stages: ■ location of at least one pair of layers (1 ,2), referred to as the primary layer (3) and the secondary layer (2) of longitudinally arranged elements (3,4), referred to as primary elements (3) in the primary layer (1) and as secondary elements (4) in the secondary layer (2), in such manner that the direction of the primary elements (3) in the primary layer (1) in each pair crosses the direction of the secondary elements (4) in the secondary layer (2), the primary elements at least being scintillating;

■ connection of at least part of the elements (3,4) in each layer to photosensitive devices (21,23,25,27,39);

■ analysis of the position of the light pulses generated in the longitudinally arranged elements based on output signals from the photosensitive devices, characterised by

^■ location of a heavy material in or adjacent to the longitudinally arranged elements (3,4) so that an electron generated by photoelectric effect in the heavy material by a photon produced by radioactive radiation is emitted by the heavy material and enters at least one primary layer (1);

^■ generation of at least one light pulse in a primary layer (1) and at least one light pulse in a secondary layer (2) adjoining the primary layer (1) by the said electrode emitted by the heavy material;

^■ determination of the position of the generated light pulses as the intersection of the two elements which have delivered signals.

15. Procedure as per patent claim 14, characterised in that connection of the elements (3,4) to the photosensitive devices (21,23,25,27,39) is made in accordance with a multiplexing arrangement, which reduces the number of photosensitive devices (21,23,25,27,39).

16. Procedure as per either patent claim 14 or 15, characterised in that the light from a predetermined number of the longitudinally arranged elements is mixed in a light mixer, which then distributes the light among a specified number of photosensitive devices.