LIGHT PEN APPARATUS FOR INTERACTIVE DISPLAY

Abstract: The present invention relates to a light pen apparatus, for use with an interactive display having a scanning electron beam scanning in lines (108) across a screen on which there are light emitting regions, and of the type comprising a light pen (36) for contacting the screen (6) of the interactive display and for collecting light from the screen, the light pen having an optical field of view (112) when in contact with the screen which is larger than a single scan line of the display, means for sensing the intensity of the light collected by the light pen, and means for generating an output signal dependent upon the intensity. Apparatus in accordance with the invention is characterised in that the output signal generating means comprises first threshold detection means responsive to the output of the sensing means for detecting when the intensity of the light collected by the light pen exceeds a first threshold level, second threshold detection means responsive to the output of the sensing means for detecting when the intensity of the light collected by the light pen exceeds a second threshold level, and light pen position determining apparatus including means responsive to the second threshold detection means for determining the vertical position of the light pen with respect to the display screen, and means responsive to the first threshold detection means for determining the horizontal position of the light pen with respect to the display screen.

Description: The present invention relates to light pen apparatus for use with an interactive display having a scanning electron beam scanning a line across a screen on which there are light emitting regions, and including means for determining the light pen position on the screen with high accuracy. Light pen input systems provide increased ease and convenience of use for computer operators. Input systems of this type require little or no training and enable the operator to quickly and easily interact with the computer. Instead of laboriously typing commands and data into the machine with a conventional keyboard, the operator can simply point to prompts supplied by the display, or write the data to be entered directly on the screen. Unfortunately, typical light pen systems are expensive, provide limited resolution and feel awkward to use. Conventional light pens have a lens system in the tip of the pen to image a small region of the display screen upon a light sensor in the pen. When the raster scanning electron beam of the display reaches the small region of the display imaged by the lens system, light emitted by the screen phosphor in that small region is detected by the light sensor in the pen. The pen immediately signals the computer system that light has been sensed. Since the position of the light pen at that time coincides with the position of the raster scanning electron beam, the computer can easily determined where the pen is located (i.e. at the location of the scanning electron beam when the light detection signal is received by the computer). Unfortunately, the lens system not only adds to the pen's cost, but also makes the pen bulky so that it has an unnatural and awkward feel. Light pen designs have been proposed which avoid some of these difficulties, but have other drawbacks instead. In particular, light pens have been proposed which include fibre optic elements so that the need for a lens system can be avoided. US-A-3,498,692, for example, describes a light pen design, which features the use of a fibre optic element for collecting light from a cathode ray tube display. The light collected when the pen is placed on the screen is correlated with information relating to the electron beam sweep that produced the display so that the pen position can be identified. However, while fibre optic pens are lower in cost and more reliable, and provide a better feel, they suffer from poor resolution. Due to the wide angle of acceptance of the fibre optic element, light is collected from a large number of screen picture elements (pels), which reduces the system's ability to correlate the collected light with the display sweep accurately. This in turn reduces the system's ability to identify the pen's position coordinates, that is its location on the screen, and, accordingly, the pen's resolution. Because determination of light pen position depends upon collected light, it is important that only relevant light be considered; that is, light collected when the pen is placed on the display screen. Thus, it is desirable for a light pen to include means for determining when the pen has engaged the screen. Conventional light pen systems rely upon the collected light used for screen coordinate position determination to provide also an indication of screen engagement. This is not very suitable for some applications, such as for hand writing input. For such applications, it is desirable to have a light pen system which can sense screen engagement even in a dark portion of the screen, independently of any particular image on the display. US-A-3 599 003 describes a light pen apparatus for use with an interactive display having a scanning electron beam scanning in lines across a screen in which there are light emitting regions. The light pen includes a lens system to ensure that the light received is collimated and therefore only light rays parallel to the axis of the pen are received. WO-A-83102849 describes a light pen apparatus with a narrow field of view that comprises only 1 pixel. There, each incoming pulse is tested against an upper threshold level to determine if that pulse is valid. Further, each pulse is also tested against a substantially lower second threshold to provide more accurate information regarding the timing of the pulses. The object of the present invention is to provide an improved light pen apparatus for use with an interactive display. The present invention relates to a light pen apparatus, for use with an interactive display having a scanning electron beam scanning in lines horizontally across a screen on which there are light emitting regions, and of the type comprising a light pen for contacting the screen of the interactive display and for collecting light from the screen, the light pen having an optical field of view when in contact with the screen which is larger than a single scan line of the display, means for sensing the intensity of the light collected by the light pen, and means for generating an output signal dependent upon the intensity. Apparatus in accordance with the invention is characterised in that the output signal generating means comprises first threshold detection means responsive to the output of the sensing means for detecting when the intensity of the light collected by the light pen exceeds a first threshold level adapted to a first set of specific variations resulting from the horizontal scan, second threshold detection means responsive to the output of the sensing means for detecting when the intensity of the light collected by the light pen exceeds a second threshold level adapted to a second set of specific variations resulting from the horizontal scan, and light pen position determining apparatus including means responsive to the first threshold detection means for determining the horizontal position of the light pen with respect to the display screen, and means responsive to the second threshold detection means for determining the vertical position of the light pen with respect to the display screen. Light pen apparatus in accordance with the invention utilises a light pen having a comparatively wide optical field without a lens system adding to the bulk of the pen. By including the first and second threshold detection means the vertical and horizontal positions of the light pen can be determined accurately. In order that the invention may be more readily understood an embodiment will now be described with reference to the accompanying drawings, in which: FIG. 1 is an isometric view of apparatus including a light pen and a companion display embodying the principles of this invention, FIG. 2 is an end view of a light pen which can be used in the apparatus illustrated in FIG. 1, sectioned in a plane perpendicular to the pen's longitudinal axis at a point approximately midway along the pen's length, FIG. 3 is a side view of a first form of the light pen illustrated in FIG. 2, sectioned along the line 3-3 of FIG. 2, FIG. 4 is an end view of another light pen which can be used in the apparatus illustrated in FIG. 1 sectioned in a plane perpendicular to the pen's longitudinal axis, at a point approximately midway along the pen's length, FIG. 5 is a side view of a second form of the light pen illustrated in FIG. 2 sectioned along the line 3-3 of FIG. 2, FIG. 6 is a side view of the light pen illustrated in FIG. 4 sectioned along the line 6-6 of FIG. 4, FIG. 7 is a fragmented isometric view of a principal fibre optic element in a light pen of the type illustrated in FIGS. 1 to 6 in relation to a display screen of the type illustrated in FIG. 1, FIG. 7A is a graphic representation of the light intensity that would be produced by an electron beam scanning several times across the field of view of the principal fibre optic element illustrated in FIG. 7, FIG. 7B is a graphic representation of the instantaneous light intensity that would be produced by an electron beam scanning once across the field of view of the principal fibre optic element illustrated in FIG. 7, FIG. 8 is a block diagram of a light pen coordinate detector which can be used in the apparatus illustrated in FIG. 1, and FIG. 9 is a representation of the signals generated in time at various points within the detector illustrated FIG. 8. As noted above, light pens provide ease and convenience of computer use. With an interactive display, an operator can conveniently enter commands and data without need for resorting to a burdensome keyboard. A basic interactive computer display featuring a light pen is depicted in FIG. 1. As shown, the apparatus includes a light pen 1, a computer system 2, a display 4 and a screen 6. Pen 1 is connected to computer system 2 by means of cabling 8. Various different forms of the pen 1 are illustrated in FIGS. 2 and 6. The different pens illustrated are generally similar and vary primarily in the form of the screen sensor that each includes. Among the illustrations of the different pens like elements are referred to with like numerals. A first form of pen 1 is shown in FIGS. 2 and 3. With reference to these figures, pen 1 is seen to include a generally cylindrical body 10 suitable to be hand held, and having the general shape of a slender pen or pencil. Pen body 10 has a first end 12 in which a pen tip 14 is provided. Tip 14 is coaxially and movably mounted in body end 12. Tip 14 has a generally conical forward section 16 that extends beyond body end 12, and is suitable for engaging the screen 6 of the display 4 shown in FIG. 1. Tip 14 also has a rearward cylindrical section 18 that is received in a generally cylindrical bore 20 formed in body end 12. The rear section 18 of tip 14 has an annular shoulder 22 which cooperates with an annular stop 24 in cylindrical bore 20 to prevent the tip from passing out of bore 20. A restoring spring 26 is provided between stop 24 and the rear face 28 of the front section 16 of the tip 14. Spring 26 urges tip shoulder 22 against stop 24 to form a cavity 30 between tip rear face 32 and a front internal bulkhead 34 of body 10. Accordingly, when tip 40 is moved into engagement with screen 6, tip 14 moves inwardly of bore 20, until tip face 32 abuts bulkhead 34. On removal of pen 1 from screen 6, spring 26 urges tip shoulder 22 against stop 24 and restores cavity 30. Continuing with reference to FIG. 3, pen 1 is seen to include a principal fibre optic element or conduit 36 that is axially mounted in body 10. Conduit 36 is received in fixed relation to body bulkhead 34 and a rear internal bulkhead 38. Conduit 36 is also movably received in bore 40 of tip 14 so that when tip 14 engages display 6 and moves rearwardly within body 10, conduit end 42 moves along bore 40 until its outer end becomes coextensive with end 44 of tip 14 and actually engages screen 6. Conduit 36 as shown is optically coupled at conduit end 43 to a detector 46 located in a compartment 48 at end 50 of body 10. The features of detector 46 will be more fully described hereafter. As will be apparent to those skilled in the art, detector 46 could alternatively be located in computer 2, with appropriate optical coupling made within cabling 8. In operation, light received at conduit end 42 passes down conduit 36 where it is evaluated to determine the pen position coordinates as described below. The light pen 1 includes a screen sensor that provides an indication to the computer system 2 when pen 1 has been moved fully into engagement with display screen 6. The screen sensor features an optical circuit that is independent of the principal conduit 36 so that determination of screen engagement can be made independently of pen position determination. The screen sensor shown in FIG. 3 includes a light source 52, a screen sensor detector 54 and an optical conduit that provides an optical path between light source 52 and detector 54. Light source 52 and detector 54 are of any suitable type as is known in the art. As shown, the optical conduit includes a first separate segment 56, which is preferably a fibre optic element, and a second separate segment 58, which also is preferably a fibre optic element. The two segments are optically coupled by a suitable coupler, for example, as shown in FIG. 3, another fibre optic element 60, arranged within tip 14 and moveable therewith. As shown in FIG. 2, fibre optic elements 36, 56 and 58 are suitably spaced apart within the body 16 of the pen in order to provide for convenient mounting. In accordance with the screen sensor design shown in FIG. 3, fibre optic element 56 has a first end 62 in optical communication with light source 52. Further, element 58 has a first end 64 in optical communication with detector 54. The opposite ends 66 and 68, respectively, of elements 56 and 58, are arranged in proximity to each other at body bulkhead 34. To provide for optical coupling of ends 56 and 58, a first end 70 and a second end 72 of optical coupler element 60 are arranged in axial alignment with the ends 56 and 58 respectively. With this arrangement, when tip 14 is brought into engagement with screen 6 and the body 10 of the pen 1 is further moved towards the screen 6, ends 70 and 72 are moved into positions adjacent to element ends 66 and 68 to complete the optical circuit which joins light source 52 and detector 54. This optical circuit is, of course, interrupted when the pen is removed from screen 6. Signals corresponding to full screen engagement are forwarded to the computer system for evaluation along with the pen position information. A pen similar to that shown in FIG. 3 is shown in FIG. 5. The difference between the pens of FIG. 5 and FIG. 3 is that in the pen shown in FIG. 5, in addition to the coupler element 60, tip cylindrical section 18 is provided with a notch 74 that defines a radially flexible member 76 in which end 70 of the coupler element means is located. In this arrangement, coupler element end 70 is normally misaligned with companion end 66 of element 56. Member 76 further includes a cam surface 78 which cooperates with a companion surface 80 provided in body 10. As a result, when tip 14 is moved into bore 20, member 76 is radially deflected inwardly relative to the centre axis of tip 14 until end 70 becomes aligned with end 66 of element 56 to complete the optical circuit coupling light source 52 and detector 54. A still further arrangement for coupling fibre optic elements 56 and 55 is shown in FIGS. 4 and 6. Here, a mirror or other reflective surface 82 is provided on surface 32 of the rear section 18 of the tip 14. The elements 56 and 58 are positioned close together, as best seen in FIG. 4. In this arrangement, the optical circuit is fully completed when the mirror is spaced approximately a fibre diameter from element ends 66 and 68. The optical circuit is interrupted when tip 14 is advanced toward bulkhead 32 as when tip 14 engages screen 6 and the pen 1 is moved further towards the screen. A simple inversion of the detector output is all that is required to make operation compatible with the sensor forms shown in FIGS. 3 and 5 as would be appreciated by those skilled in the art. It should be noted that the very slight spacing involved between the mirror 82 and the element ends 66 and 68 gives this arrangement the feel of a conventional ball point pen. As noted previously, in order for the pen 1 to interact with the display 4, it is essential the computer system 2 knows where on the screen 6 the pen 1 is located; that is, the pen position coordinates. As is well known in the art, pen position is conveniently determined with reference to the coordinates of the activating sweep that produces the display. For example, in the case of a cathode ray tube display, (CRT), which for simplicity will be assumed monochromatic, the CRT is provided with a screen including a matrix of picture elements (pels) in the form of vertical columns and horizontal rows of phosphor dots. These pels are caused to radiate light when excited by the electron beam of the CRT. The electron beam is horizontally swept over each row of pels successively, beginning at the top of the screen. As the beam is swept horizontally and then vertically between successive horizontal screens, the computer system modulates the beam with picture information which is subsequently presented on the screen in patterns of light and dark. Because the computer system knows where the electron beam is at any time, the sweep start time and sweep rate being known, the system can identify the coordinates of points on the screen by reference to the beam. Reference to the beam can be simply made by noting when the beam excites a pel. Therefore, by providing the light pen with a photo sensitive device and correlating the output of the photosensitive device with the electron beam sweep, the computer can identify the pen location with reference to the beam position coordinates. As described, in operation, the display provides the light required to locate the pen. However, if the screen picture information is likely to interfere with or be unable to satisfy the pen position determination process, another approach must be used such as, for example, controlling screen illumination for at least one picture period to permit pen position determination. This approach, however, has been found to have some drawbacks. As known in the art, an alternative to screen control is use of a display having a plurality of different phosphor arrays that are separately addressable. As described in DE-A-3037719, the screen may include addressable phosphors, in addition to the picture phosphors, which emit light outside the visible spectrum; for example, IR or UV. For example, this could be a shadow mask or beam index system. The light pen in such an arrangement would be made sensitive to the IR or UV wave length, thus avoiding interaction with the picture information. A pictorial representation of the pen position determination process in accordance with this invention is shown in FIGS. 7, 7A, and 7B. FIG. 7 is a fragmented view of CRT display screen 6. As shown, screen 6 includes an array of pels 100 comprising phosphor dots 102 arranged in columns 104 and rows 106. As described above, picture information is displayed by selectively exciting phosphors 102 with an electron beam 108 shown schematically to the left of screen fragment 110. As noted above, the position of the light pen can be determined by correlating the light collected by the pen with the known coordinates of the beam as it is swept over the screen. As described above with reference to FIGS. 1 to 6, in order to reduce cost, increase reliability and improve feel, the light pen uses a principal fibre optic conduit instead of a lens system. FIG. 7 shows a principal fibre optic conduit 36 of a light pen in position to collect light from screen fragment 110. In this position, conduit end 42 would be placed upon the screen face plate (not shown), and would have a field of view represented by circle 112. As seen, field of view 112 is relatively large, i.e. greater than 20 degrees full width at half maximum, in that it encompasses a number of pels 102, typically, a linear dimension of ten. Because pels have a persistence, that is, their light continues for a time after excitation, the light collected by conduit 36 is a summation of the light generated as beam 108 sweeps horizontally over each row of pels within field 112. FIG. 7B is a graphic representation of this and shows that if the persistence is greater than the sweep time across field 112, the intensity of the light collected continues to increase until the sweep leaves field 112. The light intensity would thereafter decay at a rate determined by the pel persistence. The sweep shown corresponds to row 106a which is centrally located relative to field 112. FIG. 7A is a plot of the light intensity collected in sweeps along successive rows within field 112 as beam 108 progresses down the screen fragment. As would be expected, the peaks of the curve correspond to the sweeping across the individual rows within field 112. The heights of the peaks are proportional to the number of pels in each row within field 112. Additionally, peak height is dependent on the proximity of the pels to the centre of field 112. Because the light collection efficiency of conduit 36 is geometrically dependent, light generated away from the centre of conduit 36 is less readily collected. In FIG. 7A, the central peak A' corresponds to the maximum light intensity generated by the sweep along pel row 106a, and therefore corresponds to the maximum of the instantaneous light intensity B' shown in FIG. 7B. Because field of view 112 encompasses so many pels, the resolution of a conventional fibre optic light pen is low. As noted, a light pen's resolution is determined by the ability to indicate its position, which in turn, depends upon the correlation of light collected with the display sweep. Where light is collected from many pels, as in the case of a fibre optic pen, it is ordinarily very difficult to tie the sweep to any particular pel or group of pels. While at first glance, it may seem possible to identify central peak A' shown in FIG. 7A by means of a single level light detector, and thereby to correlate the collected light with the sweep, in practice it is not possible to trigger a detection circuit using peak A' reliably. To be effective, the detector level would have to be set close to the anticipated height of the peak. But because the top of the peak is relatively flat as compared with the slope on either side, trigger jitter occurs in the detection circuit, which renders this approach undesirable. The fibre optic light pen described with reference to FIGS. 1 to 6, however, overcomes the difficulties associated with the wide angle of view inherent in a fibre optic light collection element, and provides for repeatable pen position determination to within one pel of the actual. The fibre optic light pen described above is used in combination with a two-level light detector for correlating the collected light with the display sweep. As shown in FIG. 8, the detector 46 features a transducer 200 for receiving light from conduit 36 at input 202. Transducer 200 converts the light received into an electrical signal, having an amplitude proportioned to the light intensity, at transducer output 204. A first level sensor 206 and a second level sensor 208 are connected to transducer 200. The input 210 to sensor 206 and the input 212 to sensor 208 are connected in parallel circuit relation to output 204 from transducer 200. Sensor 206 produces a signal at its output 214 on receipt of a signal from transducer 200 in excess of a first predetermined amplitude. Sensor 208, on the other hand, produces a signal at its output 216 on receipt of a signal from transducer 200 in excess of a second predetermined amplitude. Continuing with reference to FIG. 8, detector 46 further includes a driver 218. Driver 218 has a first input 220 electrically connected to output 214 from sensor 206 and a second input 222 electrically connected to output 216 from sensor 208. In operation, driver 218 produces a signal at its output 224 in response to a predetermined combination of the signals from sensors 206 and 208. In a preferred form, transducer 200 includes a conventional photodiode 226 optically coupled to pen fibre optic conduit 36 through input 202 for converting the light received into an electrical signal in a manner well known in the art. Additionally, transducer 200 includes a transimpedance amplifier 228 having an input 230 electrically connected to the output 232 from photodiode 226 for electrically coupling the low impedance output from diode 226 to the inputs of sensors 206 and 208 connected to transducer output 204, as also well known in the art. Continuing with reference to FIG. 8, in a preferred form, first level sensor 206 includes a comparator 234 having a first input connected through sensor input 210 to transducer output 204 of amplifier 228 and a second input 236 connected to a first reference signal 238. Reference signal 238 constitutes the (first) predetermined level of sensor 206. Comparator 234 produces a signal at its output 240 when the amplitude of amplifier 228 exceeds the amplitude of reference signal 238. Sensor 206 further includes a one-shot multivibrator 242 having an input 244 connected to the output 240 from comparator 234. Multivibrator 242 is of the edge-triggered type and produces an output pulse at output 214 from sensor 206, which pulse terminates a predetermined time interval after receipt of a signal from comparator 234. This multivibrator output pulse therefore serves as a timing reference, the termination of which serves to signal the occurrence of an event during such predetermined time interval which satisfies predetermined conditions specified hereinafter. Like sensor 206, sensor 208 also includes a comparator 246 having a first input connected through sensor input 212 to transducer output 204 of amplifier 228 and a second input 248 connected to a second reference signal 250. Reference signal 250 constitutes the (second) predetermined level of sensor 208. Comparator 246 produces a signal at output 252 when the signal from amplifier 228 exceeds the amplitude of reference signal 250. Sensor 208 further includes a one-shot multivibrator of the edge-triggered type having an input 256 connected to the output 252 of comparator 246. Multivibrator 254 produces an output pulse at output 216 from sensor 208, which pulse terminates a predetermined time interval after receipt of a signal from comparator 246. Also in a preferred form, driver 218 includes a gate 258 having a first input at driver input 220 electrically connected to multivibrator 242 through sensor output 214. Additionally, gate 258 has a second input at driver input 222 electrically connected to the output of multivibrator 254 through sensor output 216. Gate 258 is of the conventional "AND" type and produces a signal at output 260 only when signals from both multivibrators 242 and 254 are present. Driver 218 further includes a one-shot multivibrator 262 of the edge-triggered type having an input 264 electrically connected to the output 260 of the gate 258. However, unlike multivibrators 242 and 254, multivibrator 262 produces an output at driver output 224 when the signal at output 208 of gate 258 turns OFF. Before considering the operation of detector 46 it will be instructive to consider how the levels of reference signals 238 and 250 and the duration of the outputs of multivibrators 242 and 254 are selected. Satisfactory results for the determination of the light pen position can be obtained if a two level detector is used wherein the first level is set to the light intensity expected when the sweeping beam first enters field 112 and the second level is set to the maximum intensity expected if the beam were to sweep along a pel row (for example row 106a) while traversing a diameter of field 112. If the test for satisfaction of the second level is made during a time period following satisfaction of the first level, and this time period is greater than the time necessary for the sweep to traverse a diameter of field 112, then it can be concluded that, if the second condition is satisfied, the sweep will have just moved out of field 112 after having passed directly under the centre of conduit 36. The position of the centre of conduit 36 can then be determined by utilising the timing reference provided by the output pulse from multivibrator 242. The first reference level signal 238 is set to correspond to an input light level at 202 just above the system noise. In this way, comparator 234 will react to the excitation of a pel or part thereof first excited within field 112. The second reference level signal 250 is set to correspond to the input light level expected when the beam sweeps along a row of pels when it traverses a full diameter of field 112. Multivibrator 242 of sensor 206 is set to have a pulse width corresponding to the time required for the sweep to traverse a diameter of field 112 and multivibrator 254 of sensor 208 is set to have any convenient pulse width. Operation of detector 46 may best be understood with reference to FIG. 9. FIG. 9 shows the relationship of signals "a" to "g" generated in detector 46 during operation and illustrated in FIG. 8. Referring to FIG. 9, and assuming electron beam 108 is sweeping along pel row 106a, the output of transducer 200 shown at "a" increases up to level 1 as the beam enters field 112. With a transducer output in excess of reference signal 238 (LEVEL 1), the output of comparator 234, shown at "b", goes up. As the output of comparator 234 goes up, this pulse edge triggers multivibrator 242 whose output, shown at "d" also goes up. Since, however, only the output of multivibrator 242 is presented to gate 258 of driver 218, the output of gate 258 at "f", and accordingly the output of driver 218 at "g", remains low. Thereafter, as the beam continues to sweep along row 106a, the output of transducer 200 continues to increase until reference signal 250 (LEVEL 2) is exceeded as the beam is about to pass out of field 112. The output of comparator 246 at "c" goes up causing multivibrator 254 to be triggered, and its output at "e" goes up. With the presence of the "up" outputs of multivibrator 242 and 254 at the input to gate 258, the output of gate 258 at "f" goes up. Since the duration of multivibrator 242 is set to correspond to the transit time of the beam across a full diameter of field 112, as the beam leaves field 112 the output of multivibrator 242 at "d" goes down. On termination of the output from multivibrator 242, the output of gate 258 "f" goes down causing multivibrator 262 to be triggered. Since both reference conditions were satisfied during the prescribed time period, the beam is judged to have travelled along a row of pels while traversing a full diameter of field 112. With the triggering of multivibrator 262, a signal at "g" is provided to the computer system indicating the beam has just passed beneath conduit 46 and is leaving field of view 112. The system can thereafter correct for the time elapsed since the beam actually passed beneath conduit 46 to provide the exact coordinates of the position of the pen. As is immediately apparent, by setting the second reference level to the maximum intensity expected for a sweep along a row of pels occupying a full diameter based on the geometry of field 112 no other row positioned off a diameter of the field can satisfy the second reference level condition. As a result, no output is realised at gate 258 and no activation of multivibrator 262 can be effected for any sweep other than the one that traverses a diameter of field 112. In the apparatus described above, no bulky imaging optics are needed in the light pen tip because a novel two-threshold light sensing scheme is used to accurately determine the position of the light pen from the light collected by the principal fibre to an accuracy which is not limited by the size of the field of view of the light collecting fibre. The two-threshold sensing scheme works to resolve the position of a fibre optic light pen (or any other kind of light pen) to virtually one pel (picture element) resolution by determining the horizontal position of the pen (along the electron beam scan lines) using at least one horizontal position measuring event and by separately determining the vertical position of the pen (perpendicular to the electron beam scan lines) using at least one different vertical position measuring event. A "first" threshold (for the amplitude of the light sensed by the pen) is used to determine the horizontal position of the pen while a "second" threshold (for the amplitude of the light sensed by the pen) is used to determine the vertical position of the pen. The electron beam scan line which corresponds in position to the position of the light pen (with possibly a substantially constant offset) is chosen through use of the second threshold, while the position of the light pen along the chosen scan line (with possibly a substantially constant offset) is separately determined through use of the first threshold. It has been found that, with the two-threshold light sensing scheme described above, virtually one pel resolution can be achieved even though the field of view of the pen may be hundreds or even thousands of pels, because the vertical and horizontal positions of the pen are separately and independently determined, thereby allowing each of the two separate position detection functions to be individually designed for optimum accuracy and sensitivity. Heretofore, a single (though sometimes elaborately chosen) detection event has been used to determine the position of a light pen in both the vertical and horizontal directions simultaneously (i.e. a specific instant in time was somehow chosen to correspond with the position of the pen and the coordinates of the raster scan at that specific instant then determined the sensed position of the pen). With the two-threshold sensing system described above, at least two separate instants in time are detected. One of the detected time instants is used to determine only the electron beam scan line corresponding with the pen position. Inherently, a horizontal position along that chosen scan line also corresponds with that particular time instant, but this horizontal position information is deliberately not used to determine the horizontal position of the pen, because the raster scan position of that time instant has been selected for optimum accuracy in the vertical direction at the expense of accuracy in the horizontal direction. A separately detected time instant is used to determine the position of the light pen along this chosen electron beam scan line, a time instant which has been selected for optimum raster scan position accuracy in the horizontal direction. The corresponding scan line may be determined either before, during, or after the pen position along the scan line is determined. Furthermore, since the horizontal position determining threshold event(s) occur at a different time to the vertical position determining threshold event(s) and since they occur in a known sequence, it is possible to select the two threshold amplitudes totally independently of each other. In theory, either threshold could be greater than the other and with appropriate time gating the thresholds conceivably might even be equal to each other. However, the embodiments which result from some of these variations are inconvenient to implement in practice and have no apparent compensating advantages. Accordingly, it is preferred that the first threshold amplitude (the horizontal position determining threshold) be lower than the second threshold amplitude (the vertical position determining threshold). In order to avoid flicker, the visible light emitting phosphors used in CRT display screens all have a substantial persistence (i.e. the light emitted from the phosphors only gradually declines after the electron beam excitation is removed). As a result, when a light pen has a field of view covering many pels (such as is shown in FIG. 7), and the CRT electron beam sweeps horizontally across the field of view, the light sensed by the light pen gradually increases as more and more of the pels being viewed are turned on by the electron beam horizontally sweeping across the field (FIG. 7B illustrates a typical sensed light signal during a line sweep across a field of view covering many pels). As successive horizontal sweeps get closer to the centre of the field of view, the maximum light sensed during a horizontal sweep across the field of view becomes higher. This results not only from the larger number of pels contributing to the sensed light (i.e., the length of the scan line portion within the field of view generally increases as the scan line gets closer to the centre of the field of view) but also from the generally better light collection efficiency at the centre of the field of view. After the electron beam has scanned horizontally across the field of view, the light emitted by the phosphor in the field of view declines gradually until the next line sweep enters the field of view. Typical CRT phosphors are still emitting some light when the next line is scanned, so that the sensed light level typically does not completely drop to zero until after the whole field of view has been scanned (FIG. 7A illustrates a typical sensed light signal during a full scan of a field of view covering many pels). The second threshold is preferably selected such that the sensed light amplitude reaches that threshold just before the CRT beam leaves the field of view during the horizontal scan of a scan line which passes close to the centre of the field of view of the light pen. It is most preferred that this second threshold be selected such that it is reached only when the CRT beam traverses the scan line which passes most closely to the centre of the field of view of the pen (such a threshold L2 is illustrated in FIG. 7A). If the second threshold is so selected, the scan line sensed by the second threshold directly corresponds to the vertical position of the pen. A lower second threshold could alternatively be used and it too would be exceeded during the most central line scan in the field of view, but it would also be exceeded during one or more additional line scans. It should be apparent that the associated computer could still determine the central scan line from a group of threshold sensed scan lines (i.e. the central scan line in the field of view of the pen is the scan line in the centre of the group of scan lines sensed). Alternatively, the vertical position for the pen could be calculated as the average of the vertical positions of the group of threshold sensed scan lines. It is also possible to use (or sense) only the first scan line of two or more scan lines meeting the second threshold criterion. This obviously results in a vertical offset of the threshold sensed scan line with respect to the actual scan line which passes most closely to the centre of the field of view of the light pen. However, in many applications this offset would cause no serious problem because it remains substantially constant (i.e. does not change over a user's session). This offset also could be corrected for by the computer without resorting to an interpolation involving many sensed scan lines by applying a predetermined vertical offset correction (which could be calibrated or adjusted from time to time by the user or the computer). Selection of a second threshold high enough that it senses only the scan line closest to the centre of the field of view does have a disadvantage in that changes due to aging and many other factors causes a variation over time in what this second threshold amplitude should be. In order to prevent this second threshold over time from becoming too high to sense any scan line on the one hand, or so low that a plurality of scan lines are sensed on the other hand, typically will require some kind of user or computer adjustment to the second threshold from time to time. If it is critical that only one scan line be sensed by the second threshold, then, in a practical embodiment, some kind of automatic control of the second threshold probably will be needed. It should be emphasised that automatic adjustment of the second threshold by the computer can be implemented quite easily by those of ordinary skill in this field. The computer can readily determine when more than one successive scan line meets the second threshold criterion (so that the second threshold can then be incrementally raised by a small amount before the next full scan of the pen's field of view) or when no scan line has been sensed (so that the second threshold then can be incrementally lowered by a small amount before the next full scan of the pen's field of view). A computer can readily determine that the second threshold is too high when one or more valid first threshold detections are made during a screen scan without any second threshold detection. The first threshold is preferably selected such that there will be as little inaccuracy as possible in the horizontal position determination of the light pen. This will occur, for example, if the first threshold is selected such that when the light amplitude reaches this threshold during a horizontal scan across the field of view, the light amplitude is changing at a maximum rate (or nearly so). Variation in the time position at which the first threshold is exceeded due to noise and other unpredictable factors is minimised by making the first threshold correspond with a region of maximum slope in the light signal. It is necessary, in addition, to choose a first threshold which is not lower than the highest minimum light signal level between successive line scans (i.e., the detected light signal must fall below the first threshold between each successive line scan through the field of view) and it must obviously be low enough to be reached during the line scan or scans when horizontal position determination information is to be collected. A suitable first threshold L1 is illustrated, for example, in FIG. 7A. In general, there is no particularly convenient way of directly detecting the centre of a horizontal scan through the field of view of a light pen. The first threshold is detected at a horizontal position typically offset from the centre of the field of view, unless of course the value of the first threshold is deliberately adjusted by the user or the computer such that it falls midway along the particular rising slope in the light detector signal which corresponds to the scan line(s) being used for position determination. As described before in relation to the vertical position determination, an offset is not a serious problem in most applications because it does not significantly change during a user's session. As with the vertical position determination, a horizontal offset could be compensated for without resorting to an interpolation by applying a predetermined horizontal offset correction (which could be calibrated or adjusted from time to time by the user or the computer). Just as more than one event could be sensed for the vertical position determination, so also more than one event could be sensed to determine the horizontal position. As with the vertical position determination, interpolation also could be used to determine the horizontal position of the light pen from two or more first threshold detections. For example, an average of two or more first threshold detections could be used to make a horizontal position determination. The interpolation could involve first threshold detections in separate scan lines or in the same scan line (the fall of the light signal through the first threshold can be a detected threshold event also). Since the horizontal position determination and the vertical position determination are separate and independent functions, it should be clear furthermore, that the scan line chosen through use of the second threshold may or may not be involved in the horizontal position determination, depending upon the embodiment. It might be convenient, for example, to sense first a scan line using the second threshold and then determine the horizontal position of the light pen during the next scan line through the field of view. It is even possible to determine the vertical and/or horizontal position of the light pen through use of more than one screen scan. The two positions could be determined in separate screen scans or one or both of the position determinations could individually require more than one screen scan (for example as with a successive approximation method of position determination). In the preferred embodiment, a first threshold sensing event occurs during many if not all of the scan lines which traverse the field of view. However, only one of these first threshold sensing events will be used to determine the horizontal position of the light pen. The one which will be used is determined by the second threshold, which is set at a level so as to sense the scan line closest to the centre of the field of view of the pen. The first threshold sensing event associated with the scan line selected by the second threshold is used to determine the horizontal position. Since the first threshold sensing event in a scan line occurs before it is known whether that scan line will meet the second threshold criterion, the time position of the first threshold sensing event is temporarily stored through use of a time delay until a determination is made as to whether or not the second threshold criterion is satisfied. The time delay is sufficiently long that the second threshold will be exceeded during that time period if that scan line is the most central scan line. The pen is considered to have the same vertical coordinate as the scan line sensed by the second threshold and the same horizontal position as the time delayed first threshold sensing event (preferably offset corrected as described earlier). The light pen described above also features a screen sensor for determining when the light pen has fully engaged the display screen. The screen sensor is separate from the principal light conduit and the two-threshold light sensing apparatus associated with the principal light conduit and can, therefore, independently provide information regarding screen engagement.

Claims: Light pen apparatus, for use with an interactive display having a scanning electron beam scanning in lines horizontally across a screen on which there are light emitting regions, and comprising a light pen (1) for contacting said screen (6) of said interactive display (4) and for collecting light from said screen, said light pen having an optical field of view (112) when in contact with said screen which is larger than a single scan line of said display, means (200) for sensing the intensity of the light collected by said light pen, and means (206, 208, 218) for generating an output signal dependent upon said intensity, characterised in that said output signal generating means comprises first threshold detection means (234) responsive to the output of said sensing means (200) for detecting when the intensity of the light collected by said light pen exceeds a first threshold level adapted to a first set of specific variations resulting from said horizontal scan, second threshold detection means (246) responsive to the output of said sensing means for detecting when the intensity of the light collected by said light pen exceeds a second threshold level adapted to a second set of specific variations resulting from said horizontal scan, and light pen position determining apparatus including means (218) responsive to said first threshold detection means for determining the horizontal position of said light pen with respect to said display screen, and means (218) responsive to said second threshold detection means for determining the vertical position of said light pen with respect to said display screen. Light pen apparatus as claimed in Claim 1, characterised in that said first threshold level corresponds to a collected light intensity which changes rapidly so as to provide high resolution horizontal light beam position determination therefrom. Light pen apparatus as claimed in Claim 1 or Claim 2, characterised in that said second threshold level corresponds with a collected light intensity near the maximum collected by said light pen so that said second threshold detection means senses the scan line (106a) of said display which most closely corresponds with the centre of the field of view (112) of said pen. Light pen apparatus as claimed in any one of the preceding claims, characterised in that said horizontal position determination of said pen and said vertical position determination of said pen are independent of each other. Light pen apparatus as claimed in any one of the preceding claims 1 to 3, characterised in that said first low threshold detection means (234) senses when said scanning electron beam enters said field of view (112) of said light pen (1), said second high threshold detection means (246) senses when said electron beam has scanned across the centre region of said field of view (112) of said light pen (1), and said light pen position determining apparatus comprises means (242) for determining a fixed time interval following each low threshold detection, the position of said light pen being determined by the position of said scanning electron beam at the end of said fixed time interval when a high threshold detection has also been made during said time interval. Light pen apparatus as claimed in any one of the preceding claims, including sensor means for determining if said pen has fully engaged said screen and for supplying a signal indicative thereof. Light pen apparatus as claimed in Claim 6, characterised in that said sensor means comprises a light source (52), a fibre optic conduit and a detector (54), said conduit providing an optical path between said light source with said detector, and in that said conduit includes a first segment (56) and a second segment (58) and means for optically coupling said first and second segments. Light pen apparatus as claimed in Claim 7, characterised in that said light pen body (10) is of a generally cylindrical shape having a pen tip (14) extending from one end thereof and movably mounted therein, and in that said first and second segments (56, 58) of said conduit are fixedly mounted within said pen body, a first end (62) of said first segment (56) being coupled to said light source (52) and a first end (64) of said second segment (58) being coupled to said detector (54), a second end (66) of said first segment (56) and a second end (68) of said second segment (58) being arranged in proximity to one another. Light pen apparatus as claimed in Claim 8, characterised in that said means for optically coupling said first and second segments (56, 58) of said conduit includes a reflective element (82) located on a surface of said tip (14) proximate said second end (66) of said first segment (56) and said second end (68) of said second segment (58), said tip (14), said reflective element (82) and said conduit segments (56, 58) being arranged such that said light path from said light source (52) along said first segment (56) to said detector (54) through said second segment (58) is complete when said tip (14) has not fully engaged said screen (6), and such that said light path from said light source to said detector is not complete when said tip has fully engaged said screen. Light pen apparatus as claimed in Claim 8, characterised in that said means for optically coupling said first and second segments (56, 58) of said conduit includes an optical channel (60) disposed in said tip (14), a first end (70) of said channel being located proximate the second end (66) of said first segment (56) and a second end (72) of said channel being located proximate the second end (68) of said second segment (58), said tip (14), said optical channel (60) and said conduit segments (56, 58) being arranged such that the light transmitted from said light source (52) over said first segment (56) is coupled to said detector (45) through said second segment (58) when said tip (14) has fully engaged said screen (6), and such that light transmitted from said light source (52) is not coupled to said detector (54) when said tip (14) has not fully engaged said screen. Light pen apparatus as claimed in Claim 10, characterised in that said tip (14) includes a radially flexible member (76) containing at least one end of said optical channel (60), said channel being disposed in said member (76) such that at least one channel end (74) is misaligned with and spaced from a companion segment (56) of said conduit when said tip (14) is not in fully in engagement with said screen (6), said member (76) further including a surface (78) which cooperates with a surface (80) on said body (10) such that when said tip (14) fully engages said screen (6), said member (76) is caused to bring the at least one channel end (74) contained therein into alignment with and adjacent the respective companion conduit segment (56) so that light from said light source (52) may be coupled to said detector (54).

Inventors: CASWELL, NATHAN SAUNDERS, CHANG, IFAY FAY
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1986
Main classification: G06K11
IPC codes: G06F3