High resolution light pen for use with graphic displays

S ~ HIGH RESOLUTIO~I LIGIIT PEN FOR USE 6 ¦ WIT~I GR~PHIC DISPLAYS 7 I BACKGROUND OF THE INVENTION 8 I The subject invention relates to a new and improved 9¦ light pen for use with graphic displays. More particularly, 10¦ a light ~en is disclosed having a new and improved optical 11¦ deslgn for use in conjunction ~ith unlque electrical circuitry, 12 which provides high resolution and substantially reduces the 13¦ ef~ects of jitter.

In the prior art, light pens have been used as a pointer device in association with computer controlled, cathode16 ray tube video displays. It is an object about the size and 17¦ shape of a fountain pen and includes a means for sensing light 18 and a means for converting this light into an electrlcal pulse.

19~ The pen is held in the hand and pointed at some portion of a picture, symbol or the like being displayed on the screen of 21 the cathode-ray tube. When the electron beam which is tracing 22 the image causes a portion of the screen next to the point of 23 the pen to light up, the pen senses this light and generates an 24 electrical pulse ~Ihich serves as a computer interrupt signal.

25 ¦ Typically, the computer circuitry responds to the interrupt 26 signal by reading an address coun'.er having data which tracks 27 the position of the cathode ray at that particular moment.

28 The address in the counte~ corresponds to the location of the 291 light pen.

301 The accuracy or resolution of the light pen is 311 affected by a number of factors. One of the primary factors 121 relates to the optics of the 1~ght pen ltselL. A light ~en 1~34935 1 typically inclu~es an elongated channel for directing light rays 2 I emanating from the video scteen to a photodetector within the 3 ~ light pen. One example of a light pen can be found in U.S.

Patent No. 3,569,617 issued March 9, 1971 to Allen. The light 5 I pen disclosed in the ~llen patent includes a collimation tube 61 having an internal channel with non-reflective walls. The light 71 pen will only accept light rays emanating from the video screen 81 which are axially aligned with the channel. Non-axial light 91 rays ernitted from the screen are either not received through the channel ~pening or ahsorbed by the non-reflective walls of the 11¦ channel.

12¦ Various other light pens have been developed which 13¦ include a lens for focusing the axially received light.

14¦ Examples of such pens can be found in U.S.

Patent No. 3,271,515 15¦ issued September 6, 1966 to Harper, U.S.

Patent No. 3,599,003 161 issued August 10, 1971 to Price and U.S.

Patent No. 3~917,955 17¦ issued Novernber 4, 1975 to-Inuiya. Al] of the light pens 1~¦ ~isclosed in the latter patents include a single lens which 19 is used primarily to focus axially received light rays onto 2~1 a photodetector. ~ach of these devices are limited to detecting 21¦ light rays which travel along the longitudinal axis of channel.

22~ Accordingly, it is necessary to provide the channel with a large 231 diameter so that light rays from a number of pixels on the video 24 ¦ screen will be accepted. Only in this manner will the number of 25 1 light rays received by the photodetector be sufficient to 26 ¦ generate a pulse.

27~ The above described configuration is acceptable when the 28 I light pen is used merely to define areas on the video screen.

2~ I However, for more sophisticated applications, higher resolution 30~ would be desirable. More specifically, in applications suc~ 31~ as on screen drawing, it is necessary to be able to detect light 32, rorn indiqidual pixels. In the l~tter situation, the channel of .1 . . I ~2349;~5 1 the light pen must be restricted to a diameter less than the 21 width of two pixels such that only li~ht rays emanating a sin~le 31 pixel are received by the photodetector. Thus it would be 41 desirable to provide an improved optical system or maximizing sl the reception of light rays emanating rom a sinyle pixel such 61 that they may be adequately sensed by the photodetector.

71 Another shortcominq of the prior art light pens 81 relates to the mechanical means utilized to actuate the device.

9 Frequently, separate independent triggering means are provided on the light pen for providing an actuation slgnal to the 1l¦ computer. Switch means of the latter type are disclosed in the 12 patents to Harper and Price cited above. However, to facilitate 13¦ the use of the light pen, it is preferable to provide some form 14 of automatic switching. For example, in the above cited patent to Allen, the colli~ation tube is slidably mounted within an 16 outer housing. When the user presses the collimation tube 17 against the video screen, a miniature switch in the rear of the 18 housing is actuated, signaling the computer. Similarly, in the 19 patent to Inuiya, a collimation tube is slidably mounted within the housing and connected to an internal switch. In the 21¦ Inuiya device, the photodetector Is mounted on the movable 22 collimation tube. Accordingly, in order to electrically connect 23 the photodetector~ wires must be affixed to the movable members.

24 Similarly, in the patent to Allen, electr1cal connections are made through the movable member. In addition, in the Allen 26 I device, a fiber optics bundle is also connected through the 27 ¦ collimation tube. As can be appreciated, the actuation of 28 ¦ either of these light pens results in the repeated movement of 29 ¦ the connection means which frequently leads to the failure of 30 ¦ the device due to the breakdown of the connectors. Accordingly, 31 it would be desirable to provide a new and improved light pen 32 /// . . . I , ! '123~935 . 11 which includes an automatic actuation means that does not 21 require the movement of electrical connections thereby reducing 31 the liXelihood of breakdown.

41 As discussed above, when the photodetector of the 51 light pen is actuated, it generates an interrupt signal which 6 typically causes the computer to read a counter having an 71 address corresponding to the location of the cathode ray.

¦ Theoretically, if the interrupt signal were generated simul91 taneously upon the excitation of the phosphor of the pixel, the 10¦ location of the light pen could be accurately determined.

11¦ However, in practice, this result has been difficult to achieve.

12 More particularly, the light rays from an excited pixel that ~re 13¦ received by the light pen cause a pulse to be generated having 1~¦ an amplitude which varies according to the intensity of the 15¦ light received. For example, if the light pen is positioned 16 ¦ directly over the desired pixel, the intensity will be maximized 17 ¦ and the amplitude of the pulse which is generated by the photo1~ 1 detector will be fairly high. In contrast, if the light pen is 19 ¦ not directly aligned with a pixel, the intensity of the light rays received will be reduced such that a pulse having a rela21¦ tively smaller amplitude will be generated.

- 22¦ While the amplitude of a pulse varies with intensity, 23 the "rise time" of all pulses is constant. Rise time is defined 2~1 as the length of time it takes for the amplitude of a pulse to 25 ¦ rise from 10 per cent of maximum to 90 per cent. The rise time 26 ¦ of a system is governed by factors such as the tracking speed of 27 ¦ the video beam and the type of phosphor used. The problems of 28 ¦ detection occur because pulses having the same rise time but 29 ¦ different amplitudes will have different slopes. For example, 30 ! since a large amplitude pulse will reach a maximum value in the 31 ¦I same time period as a smaller p~lse, the slope of the larger 32 /// -4 .

~ ~,234935 1¦ pulse must be greater. ~leasurement uncertainties arise ~ecause 2 pulses having different slopes will exceed detection threshold 3 limits at different times.

41 Light pens are generally provided with discrimination 51 circuitry to deter~ine i a pulse generated by a photodetector 61 validly represents an excited pixel. Typically, each pulse 71 is compared to a threshold voltage level to determine if 81 the pulse is valid. In order to generate accurate position 91 data, the time between the start of a pulse and the point at 10¦ which it crosses the threshold level must be constant for all 11¦ pulses. However, as mentioned above, the time it takes an 12¦ incoming pulse to exceed the threshold level will vary with the 13j amplitude, which is in turn dependent upon the intensity of 14¦ the light received by the photodetector. Accordingly, for 15¦ an aligned pixell where the amplitude of the pulse is large and 16¦ the slope is great, the interrupt signal will be generated 17 fairly quickly. However, if the light pen is not directly & al1gned with the pixel, a pulse having a smaller amplitude and a 19j shallower slope is produced such that the interrupt signal will 20 ¦ be qenerated at a later time.

21 I This ti~ing uncertainty is the cause of ji~ter. More 22 particularly, the interrupt signal generated b~ the light 231 pen causes a counter to be read having an address corresponding 24 ¦ to the position of the light pen. However, the time the inter25 I rupt signal is generated is a function not only of the light 2~ ~ pen position, but of the intensity of the light rays received.

27 ¦ For example, since a non-aligned pixel will produce a slower 2~ I rising pulse that exceeds the threshold level at a later time, 29 the address counter will advance beyond the point which would 30 ! occur with an aligned pixel. Thus, a slight displacement 31 ¦ of the light pen can cause the computer to generate a substan32 tially different address.

-5. . I ~ 34~3~ ' 1 ¦ Jitter may occur even if the light pen is held station- i 2 ~ ary relative to the video screen. More particularly, the scanning 3 1 position of the cathode ray includes a predictable amount of 4 error such that in one sweep the ray might be directly aligned 5 1 with the light pen while on the next sweep it might be slightly 6 I off alignment. Accordingly, even if the light pen is held 71 stationary, the interrupt signals used to produce the location 8 data will vary depending upon the alignment of the excited pixel.

9 ¦ In many prior art applications, this jitter effect 10 ¦ is not a major disadvantage. For example, light pens are 11 ¦ frequently used merely to point to an area on the video screen.

12 1 Accordingly, precise data regarding the position of the light pen 13 ~ is not necessary since a location determination anywhere within 14 ¦ the field was sufficient. However, in other applications, there 15 ¦ is a need for a more accurate light pen. For example, if the 16 ¦ light pen were to be used for real time drawing r the precise 17 ¦ location of each light pen hit must be determined in order to 18 I define continuous line segments.

19 ¦ - One example of the prior art circuitry utilized to 20 ¦ provide more accurate location data can be found in U.S.

Patent 21¦ 3,512,037 issued ~ay 12, 1970 to Eckert et al~ The Eckert 22 patent discloses a fairly complex two step scanning system 231 whereln a first scan is used to obtain the gross position data.

2~1 Thereafter, a second scan is used to disclose horizontal and 25 ¦ vertical tangent points. As can be appreciated this two step 26 ¦ scan system is complex and difficult to use. More importantly, 27 ! it requires that the cathode-ray tube perform special searching 281 techniques. Therefore, this method could not be used where 29 ¦ the cathode-ray tube was operating in a standard fashion.

301 Accordingly, it would be desirable to provide new and improved 311 discrimination circuitry that is capable of providing accurate 32l position data of the light pen.

-6I . ". I ~LZ349~5 SUMMARY OF THE INVENTION The invention provides a high resolution light pen for sensing light rays emitted from a pixel on a video screen. The light pen comprises an elongated cylindrical housing having opposed fron~ and rear ends. A collimation tube is mounted within the housing and projects outwardly from the front end of the housing. The collimation tube has a cylindrical channel extending along its longitudinal axis, with the inner surface of the channel being non-reflective. A photodetector means is fixedly mounted within the housing to the rear of the collimation tube and is aligned with the channel. A lens means is disposed within the channel adjacent the front end thereof. The lens means has a focal length equal to the distance defined between the lens means and the virtual image of the pixel on the video screen when the front end of the collimation tube is placed against the outer surface of the screen.

Accordingly, all light rays emanating from an aligned pixel and passing through the lens means are refracted axially along the channel and directed to the photodetector, while light rays passing through the lens means from a displaced source are refracted at an angle and absorbed by the walls of the channel such that only light rays from an aligned pixel are sensed by the photodetector.

A second lens means may be disposed adjacent the rear end of the channel for focusing the axially directed light rays within the channel directly onto the photodetector means. The second lens means is preferably fixedly mounted on the photodetector means.

1.;~34~ The collimation tube may be slidably mounted within the housing and the light pen may further include a switch means operatively connected to the photodetector, the switch means being disposed behind the collimation tube in a manner such that when the front end of the tube is pressed against the video screen it is moveable rearwardly, relative to the housing, to actuate the switch means. The collimation tube may also include a recess for receiving a stop pen fixedly mounted on the housing, the recess and stop pen combination for restricting the movement of the collimation tube in an axial direction.

The invention further provides a light pen having an end adapted to be abutted against a video screen for detecting the presence of an illuminated pixel located adjacent the abutting end of the pen. The light pen comprises an elongated housing having opposed front and rear ends and formed with a hollow channel extending therethrough. A radiation detection device is mounted in the housing adjacent the rear end of the channel. A lens means is mounted within the channel adjacent the front end thereof. The lens means has a focal length such that when the front end of the pen abuts against the screen radiation from an illuminated pixel located near the axis of the channel is transmitted along the axis of the channel in a collimated beam to the detection device, while the radiation received from pixels displaced from the axis is directed by the lens towards the sides of the elongated channel and dissipated, whereby the detection device functions to detect the on-axis pixels to the exclusion of the off-axis pixels.

93~ A second lens means may be disposed adjacent the rear end of the channel for focusing the axially directed radiation directly onto the radiation detection device.

BRIEF DESCRIPTION OF T E DRAWINGS Figure 1 is a cross-sectional view of the new and improved light pen of the subject invention.

Figure 2 is an enlarged cross-sectional view illustrating the optics of the new and improved light pen of the subject invention.

Figure 3 is a graphical representation of the amplitude of input pulses generated by a light pen.

Figure 4 is a graphical representation of the output pulses produced using the new and improved discrimination circuit of the subject invention.

Figure 5 is a block diagram of the new and improved discrimination circuit of the subject invention.

~3q93~ 1 I DETAIr,EL) _F.SCRIPTION C)F Tl`lE P~FE!~RED EMBODI'~ENTS 2 ~ Referring to Fi,gures 1 and 2, the li~ht pen 10 oE the 3 ¦subject invention is il]ustrated. The light pen 10 include~s 4 ¦an elongated cylindrical housing 20 adapted to contain the 5 ¦optical and electrical elements. A cylindrical collimation tube 6 ¦22 is slidably mounted within the front end of housing 20.

7 ¦Preferably, a recess 26 is provided for receiving a stop pin 28 8 Ifixably mounted to the inner surface of the housing 20. The 9 ~ combination of the recess 26 and the stop pin 28 functions to lO ¦restrict the movement of the collimation tube reiative to the ll 1 housing, wi'ch regard to both the length and direction of travel~ 12¦ In the preferred embodiment, the movement of the tube is limited 13¦ to a direction parallel to the longitudinal axis of the housing.

14¦ The light pen 10 includes a photodetector means 30 l~ mounted to the rear of the collimation tube 22. A miniature 16 ~ switch 32 is also disposed adjacent the rear of the collimation 17 tube. The remainder of the light pen housing is adapted to ' 18¦ contain circuitry 34 for acting upon the signals received by the 19¦ photodetector 30. Electrical cables 36 and 38 are provided to 20¦ connect the circuitry 34 to the photodetector and supporting 21~ computer respectiveiy.

22¦ ' The light pen 10 of the subject invention is a high 23¦ resolution device capable of focusing on an individual pixel ' , 241 of a video screen. This unique result is achieved at relatively 251 low cost due to the new and improved optical construction of the 26 ¦ light pen. ReEerring more particularly to Figure 2, it will be 27 ¦ seen that collimation tube 22 includes a cylindrical channel 38 28 ¦ extending along the longitudinal axis thereof. PreEerably, the 29 I diameter of the channel 38 is related to the spacing between 30 ¦ pixels on the screen and is on the order of 1.5 times the pixel 31 ¦ width. The inner surface of the collimation tube is coated with 32 I a non-reflective material such that any light rays which inter I - 1 0 ; ~23493~ l ~sect with the coating are absorbed rather than reflected.

2 ¦Channel 38 is aligned with sensor 40 of photodetector means 30.

¦ In accordance with the subject invention, a lens 42 is 4 Imounted within the channel 3~ of the collimation tube adjacent 5 ¦the front end thereof. Due to the unique construction of lens 142, all light rays entering the lens from an aligned pixel are 7 refracted and directed axially along the channel. In order to 8 construct the proper configuration of the lens the location of 9 ¦ the virtual image of the pixel must be calculated. As illuslOI trated in Figure 2, the pixels (P1 and P3) are actually ll¦ located in a phosphor layer 44 behind glass screen 46. ~owever, 12¦ due to the refraction of light caused by the glass screen, the l~¦ image of each pixel appears to be located within the glass 46.

14¦ This effect is similar to the apparent location of an object at 15¦ the bottom of a body'of water. The specific location of the 16¦ virtual image within the glass screen 46 is a function of a 17 number of factors including the thickness of the glass and its 18 refractive index. Thus, in order to focus on a particular 19 pixel, lens 42 must be configured to focus on the apparent 20¦ source (P2 and P4) of the light rays rather than the actual 21¦ source of light.

22¦ In accordance with the subject invention, the focal 231 length F1 of lens 42 is intended to be equal to the distance '24 ¦ between the lens'and the virtual image P2 of the pixel when 25 ¦ the light pen is pressed against the front surface of screen 26 ¦ 46. This relationship enables the subject light pen to capture 27 1 a far greater percentage of light rays emanating from the 28 ~ virtual image P2 of the pixel then the light pens known in the 29 ¦ prior art. More particularly, and as discussed above, light 30 ¦ pens in the prior art were only capable of capturing light rays' 31 ¦ which emanated parallel to the channel of the collimation tube.

32 As illustrated by light ray R1 emanating from pixel P2j the . 1~3~935 1~ subject light pell will similarly capture any light rays emanating 2 I parallel to the lon~itudinal a~is of channel 38. In addition, ¦ 3 ¦ the subject light pen will also capture light rays which emanate 4 ¦ at an angle relative to the longitudinal axis of the channel 38, S ¦ as illustrated by rays R2. As can be appreciated, since the 6 ¦ virtual image P2 f pixel P1 is located at the focal 7 I point of the lens 42, rays R2 which emanate at an angle and 8 I enter ]ens 42 are refracted into a path parallel to the iongi9 tudinal axis of channel 38. Thus, all rays which emanate from 10 I the virtual image P2 and pass through lens 42 can be captured 11¦ and transferred to the photodetector.

12¦ In contrast, light rays emanating from pixels which 13¦ are not aligned with the channel 38 and which enter lens 42 are 14¦ absorbed by the collimation tube. As illustrated in Figure 2, light from pixel P3 apparantly emanates from source P4. The 16 latter light rays ~shown as R3 and R4) are reracted by lens 17 1 42 such that they will intersect and be absorbed by the inner 18 surface of housing 38.

19 ~ In order to maximize the reception of light rays at the photodetector, a focusing lens 50 may also be provided~ ¦ 21 Focusing lens 50 is mounted to the photodetector using a casing 22 52. The focal length F2 f lens 50 is equal to the spacing 23l between the lens and the surface of sensor 40 of photodetector I 30. Accordingly, as illustrated in Fiyure 2, all parallel light 251 rays entering lens 50 will be focused directly on the center of 26l sensor 40. By this arrangement, any stray li~ht rays which are 271 not fully absorbed by the internal surface of channel 38 will be 28~1 refracted away from the photodetector by lens 50 thereby minimiz29l ing spurious signals. Preferably, the diameter of the rear end 80l of channel 38 is widened to accommodate casing 52.

31¦' One advantage of the subject construction is that 32l the lenyth OL the collimation tube 38 is not critical. More l -12. . I ~ lZ3~935 1 ¦particularly, since lens 50 will focus only parallel liqht rays, 21 it can be positioned at any distance frorn lens ~2. The length 31 of the collimation tube should be long enough to permit the 41 absorption o~ all non-aligned light rays. When used ~tith a 51 typical size video matrix, the collimation tube 38 can have a 61 length on the order of three centimeters.

71 In the preferred embodiment of the subject invention, 81 switch 32 is provided with a spring biased member 56 which is 91 disposed adjacent the rear-end of collimation tube 22. In the 10¦ inactive condition, the spring action of the member 56 biases 111 the collimation tube into its maximum forward position.

12¦ In use, the operator presses the collimation tube against the 13¦ front surface of the screen 46 causing the collimation tube to ~41 move rearwardly relative to the housing 20. The rearward ,51 movement of the collimation tube 22 causes member 56 to move 16¦ rearwardly, activating the circuits within the light pen~ ,71 Preferably, a switch 32 is selected which creates an audi~le ~¦ click and a tactile locking sensation to inform the operator 19¦ that the circuît has been activated. As discussed above, the 20 ¦ combination of recess 26 and stop pin 28 controls the movement 21¦ of collimation tube within the housing. In Figure 2, the light 22¦ pen is depicted in its rearward, activated position within the 23 ¦ housing.

24 ¦ The mechanical switching arrangement of the subject 25 ¦ light pen functions to reduce the likelihood of breakdown from 26 frequent use. More particularly, all of the electrical controls 27 ¦ are fixably mounted to the housing, and only the collimation 28 ¦ tube is mounted for slidable movement. Thus, the repeated 29 ~ movement of the collimation tuhe does not cause any movement of 30 ' electrical wiring or optical bundles as disclosed in the prior 31 art. Accordingly, the subject light pen is easy to use and is 32 1 /// -13. ~ ~IL234~3~ ¦long lasting. E~urther, the subject pen is capable of high 2 ¦resolution and maximizes the light received at the photodetector. I 3 ¦ Referring no,~ to Figures 3 thro~gh 5, the discrimina- ¦ 4 ¦tion circuitry of the subject light pen will described in more 5 ¦detail. Preferably, the circuitry is mounted within the light 6 ¦ pen rather than at the computer control to eliminate spurious 7 signals caused by such factors as high frequency noise.

8 I As mentioned above, one of the major shortcomings 9j of the prior art light pen circuitry relates to the ef~ects 10¦ of jitter. The jitter phenomenon may be more readily understood 111 with reference to Figure 3, where the horizontal axis represents 12¦ time, and the vertical axis represents the voltage amplitude of 13 incoming pulses. The curves which are illustrated represent 141 the voltage generated by the photodetector in re.sponse to the 15¦ reception of light rays.

16 A video screen may be thought of as an array of 17 horizontal lines illuminated by the sweeping mot.on o the 18 ¦ cathode ray. When the light pen is placed on the video screen 19 I it will detect light emitted from the phosphor excited by the 201 cathode ray. The cathode ray takes a finite period of time to 21¦ sweep through the field of the light pen. The tr~cking speed of 22¦ the cathode ray, along with the type of phosphor utilized, 23¦ contributes to the particular rise time of the video system. As 24 noted above, the rise time is defined as the length of time it 25¦ ta~es for the amplitude of a pulse to rise from 10 percent of 26¦ maximum to 90 percent. As illustrated in Figure 3, each of the 27 pulses labeled A, B, and C reach a maximum point at approximately 281 the same time indicated by line D.

29 The amplitude of the pulses is governed by the 30 I intensity of the light rays received by the light pen. When 31 I the light pen is placed on the video screen in direct alignment 32 with one of the lines traced by the cathode ray, a pulse having I -14 12~ 93~;

1 la maximu~n amplitude, as illustrated by curve A in Figure 3, will 2 ¦be produced. In contrast, if the light pen is placed on the 3 vldeo screen in a location displaced slightly from one of the 4 llnes traced by the cathode rayl a pulse having a lo~er amplitude 5 will be generated, as illustrated by curve B in Figure 3.

61 Pulses having varying amplitudes will be generated 71 even if the light pen is held stationary with respect to the 81 video screen. More particularly, the tracking of the cathode 91 ray is subject to error such that the degree of alignment with 10¦ the light pen will vary with each sweep. Accordingly, pulses 11¦ varying in amplitude will be generated which, in the prior art, 12¦ resulted in measurement uncertainties. Curve C, illustrated in 131 ~igure 3 is a low amplitude pulse representing a stray signal 14¦ from a pixel not aligned with the field of the light pen.

15¦ Discrimination circuitry must be capable of rejecting the latter 16¦ type of low amplitude pulse as an invalid si~nal.

17¦ Since each of the pulses illustrated in Figure 3 have lR ¦ different amplitudes, but identical rise t;mes to maximum, their 19 I slopes must be different. For example, pulse A, having a 20 1 maximum amplitude, has a relatively steep initial slopeO In 21 contrast, curve B, having a lower amplitude has a relatively 22 I shallower slope. As pointed out above, measurement uncertainties 23 arise because pulses having different slopes will exceed detection 24 ¦ threshold limits at different times.

25 ¦ Typically, in the prior art discrimination circuitry~ 26 ¦ the amplitude of a pulse is compared to an arbitrarily set 27 ¦ threshold level depicted as Tl in Figure 3, to determine if 2~ ¦ the pulse is a valid signal. More specifically, unless the 29 ' pulse exceeds the threshold level T1, it is assumed that 30l the pulse is merely a spurious signal generated, for example 31!¦ by a pixel near to, but not aligned with the light pen. As 321i seen in ~igure 3, pulse ~, having a relatively steep slope, Il -15 3493~i I . ~ i 1 ~ exceeds threshold level T1 at point 60. Pulse B, having a - 2 ¦ relatively shallower slope, does not exceed threshold level T1 1 3 until point 62. Pulse C, representing an invalid sicJnal never 41 exceeds threshold level T1.

5¦ In the prior art, when a pulse exceeds threshold level 6 ¦ T1, an interrupt signal is generated causing an address in a 71 counter to be read which represents the location of the cathode 8 ray at that time. By this arrangement, the position of the 91 liaht pen is determined. As illustrated in Figure 3 however, 10¦ while pulses A and B beqan at the same point of origin 0, they 11¦ exceed threshold level T1 at different times. More particu12¦ larly, pulse s exceeds threshold T2 a time period W1 after 13¦ pulse A. Accordinaly, the address counter in the computer, 14 tracking the movement of the cathode ray will be permitted to advance such that the circuitry is supplied with an incorrect 16 light pen positio-n. This timing discrepancy is the cause of 17¦ jitter.

18¦ As can be appreciated, a finite time period will elapse 19¦ between the origin 0 of pulse A and the point 60 at which the 20¦ pulse crosses threshold level T1. Further, since electronic 21 communications are not truly instantaneous, a finite time 22 ¦ period will elapse between tirne the curve crosses threshold T~ 23j and the address counter is actually read. Accordingly, the 241 address in the counter does not actually correspond to the 2~1 position of the liqht pen. Rather, when the counter is read, 26 the address therein exceeds the actual position of the li~ht pen 271 by a fixed amount. This known time difference can be compensated !

28,1 for by logically subtractinq a fixed number from the address 291 counter to obtain the actual light pen position. However, the 30¦l timing uncertainty caused by variations in amplitude is not 31¦1 constant and cannot be corrected, and results in jitter.

32! /// ¦~ -16 ~L2~935 l ¦ In the prior art, the random timing error, illustrated 2 las W1 in Fi~ure 3, is generally a minimum of 100 nanoseconds 3 which corresponds to an approximately 1.5 pixel error.

In 4 ¦ contrast, the subject invention provides new and improved circuitry wherein the jitter ef~ects can be reduced to between 6 ~20 and 40 nanoseconds corresponding to a 0.5 pixel`spacing that 7 j effectively permits discrimination of exact pixels.

8 ¦ In accordance with the subject invention, each incoming 91 pulse is tested against an upper threshold level T1 to deterlO¦ mine if that pulse is valid. Further, each pulse is also tested ll¦ a~alnst a substantially lower threshold T2 to provide more 12¦ accurate information regarding the timing of the pulses. The 13¦ subject circuitry taXes advantage of the fact, as seen in Figure 14¦ 3, that the paths of pulses of varying intensity are fairly 15 ¦ similar in their early stages and diverge over time due to the 16 I di~ferences in slopes. Thus~ the spacing or timing difference l7 ¦ W2, between valid puises A and B when crossing a lower thres18 ¦ hold T2 (points 78 and 80 respectively), is substantially less 19 than the spacing W1 when the same pulses cross the higher 20¦ threshold T1.

21¦ It is apparent that the lower threshold level T2 22¦ could not be utilized alone to determine the validity of pulses 23 since this would result in the acceptance of spurious-signals~ 241 For example, pulse C, representing a spurious pulse, crosses 251 threshold level T2, at point 82. Thus, if threshold level 26~ T2 alone were used to test validly, pulse C would be improperl~ t 27 accepted as valid. Therefore, the determination of validity of 28 ¦ the pulse must be based on a comparison at a higher threshold 29 I level represented by T1. Accordingly, in the subject circuitry, 301 only pulses which exceed the upper threshold level T1 are 3ll accepted as valid pulses, while the lower threshold level T2 32 ~ .' . I 1~3493~ 1 is utilized as a timing mechanism for reducing the effects of 2 1I jitter.

j Referring to Fiaures 4 and 5, the circuits of the 4 suhject invention and its operation will be more fully described.

As illustrated in the block diagram of Figure ~, the output of 6 photodetector 30 is supplied to two amplification stages 70 and 7 72 respectively. Preferably, the preamplifier 70 has a differen- i 8 tial output while amplifier 72 has both a differential input and 9 output. The two amplification stages are intended to provide a 10¦ gain of between 200 and 40n.

By this arrangement the voltage 11¦ generated by the photodetector 30 can be boosted to approximately ¦ 12¦ one-half volt or more.

13¦ The output of amplifier 72 is supplied to a first 14¦ comparator means 74 which determines if the pulse is a valid 15¦ signal. Comparator 74 is confi~ured to output a first signal 16 ¦ when the input voltage exceeds the upper threshold level T1.

17¦ Referring to Figure 4A, the type of output of the first com13¦ parator means 74 is shown, when supplied with the input pulses 19 illustrated in Figure 3. Thus, for pulse A, a first signal A1, will be aenerated beginning when pulse A crosses the upper 211 threshold Tl at the point 60. Similarly, a signal B1 is ~2 generated when pulse B crosses upper threshold 1'2 at point 62.

231 The signals Al and Bl will remain high for as long as the 241 amplitude of the associated input pulses exceed threshold level 2~1 T1. As discussed abovel the time difference W1 between the 26 ¦ initiation of signals A1 and Bl corresponds to the jitter 27 ¦ effects of the prior art.

28 ¦ In order to reduce this jitter, the subject invention 29 I includes a second comparator means 76 which tests the incoming 30 ~ pulses against a substantially lower threshold level T2.

31 il Comparator 76 is configured to initiate a second signal when 321 the incominq pulse exceeds the lower threshold le~el T2.

~ 18~ ` ,i i 1~3~33~ , l , ~ 1l 1¦ Figure 4s illustrates the Olltput of second comparitor 7& when 2¦ supplied with the pulses illustrated in Figure 3. More particu31 larly, second signal P2, is generated when pulse A crosses the 41 lower thresholcl T2 at point 78.

Similarly, signal P,2, is 51 generated when pulse B crosses threshold T2 at point 80. As 61 seen in Fiqures 2 and 3, pulse B crosses threshold level T2 at 71 a fixed time W2 later than pulse A. As discussed above, fixed 8 time interval W2 associated with threshold level T2, is I suhstantially less than the interval W1 associated with upper 10¦ threshold level T1. This reduction enables the circuit to 11¦ substantially reduce jitter.

12¦ ~ecause comparator 76 is set at a low threshold level, 13¦ it ~7ill generate signals based on pulses which are not valid.

14¦ For example, since pulse C crosses lower threshold level T2 at point 82, a signal C2, will be generated as seen in Figure 4B.

16 ¦ Since comparator 76 produces sianals based on invalid puIses, 17 ¦ the information derived therefrom cannot be used solely to 18 I determine the validity of pulses. Rather, the signals generated 19 ~ by comparator 76 are combined with those produced by comparator 20 ¦ 74 to achieve the desired results.

21 I In accordance with the subject ~nventionr the signals 22 ¦ of the second comparator means 76 are supplied to a delay means 23 ¦ 84. Delay means P4 may be defined, for example, by a monostable 24 ¦ multivibrator. The length of the delay is calculated based on various factors such as the particular threshold levels which 26 I have been selected.

However, the time period must be suffi- !

27 I ciently long enough such that the output of delay means 84 28 ! occurs during the time when the pulses have exceeded the upper 24 1 threshold level T1. Figure 4C illustrates the output of ~ol¦ the delay means when supplied with the signals generated 31¦ by co~parator 76 shown in Figure 4B. The output (A3, B3 and 32 /// _19_ ' ~ 1 12349~5 1 ~C3) of delay means 84 is identical in character to the output 2 of com~arator 76 except for a uniform delay.

3 The output from delay means 84 and the output from 4 the first compara~or means 74 are then supplied to a gate means 90. Preferably, gate means 90 consists of a logical 6 circuit such as an "an~" gate. As well kno~in in the art, an 7 "and" gate will generate a positive going output pulse only when 8 both incoming signals are high. It is to be understood of 9 course, that if it is desired that a low signal be indicative of a valid pulse, a loaical "nand" gate could be used wherein a 11 neaative going pulse is produced only if both inputs are high 12 The output of gate 90 having "and" logic is illustrated 13 in Figure 4D. More particularly, an output pulse A4, based on 14 input pulse A, will be generated when the signals A1 and A3 from comparator 74 and delay means 84 are simultaneously high.

16 Similarly, an output pulse B4 will be generated when signals 17 B1 and B3 are both high.

The spacing between the output 18 pu1ses A4 and B4 corresponds to the timing interval W2. Gate 19 means 90 will not generate any output pulse for input pulse C since the gate is never supplied with a corresponding siynal 21 from comparator 74.

22 Each output pulse A~, B4 is utilized as an inter23 rupt signal causing the computer to read a counter having an 24 address corresponding to the position of the cathode ray.

~ Because the time difference W2, between pulses A4 and B4, 26 is relatively short, the measurement variations which cause 27 iitter are minimized. Of course, since there has been a delay 28 incorporated in the circuitry, the interrupt signal will occur 29 at a finite time after the origin of each pulse. However since the delay is a fixed value, the timing of the interrupt signal f 31 ¦ can be logically corrected by subtractin~ a fixed amount from 32 I the counter.

-20 I 1 ~3~335 l I ~s illustrated in Eig-lres 3 and 4, the timing differ2 ence ~2~ between the outputs A4 arld B4, is substantially 3 less than the difference W1 between the upper threshold 4 crossings 60 and 62. In practice, the time period W1 is even shorter than illustrated. More particularly, amplifiers, such 6 as those used to boost the incoming pulses, have inherent 7 limitations (referred to as slope limits) which tend to inhihit 8 fast rising pulses Accordingly, in the early stages of the 9 pulses, the paths of the curves will be virtually coincident thereby substantially eliminating the effects oE jitter.

ll The amount of reduction of jitter can be affected 12 by adjusting the threshold levels T1 and T~. ~or example, 13 ¦ when the upper thresholds level T1 is set at 75~ of maximum 14¦ amplitude and the lower threshold T2 at 50~ of maximum, a 15¦ jitter of 40 nanoseconds can be achieved. When the lower 16 threshold level is reduced to 25~ of maximum amplitude, a jitter 17 of only 20 nanoseconds can be achieved. ~eduction of jitter to 18 ~ this range corresponds to an error of approximately 0.5 pixels.

19 By this arrangement, the accurate location of individual pixels can be achieved to permit the use of a light pen in far more 21 demanding applications such as drawing.

22 In summary, there is provided a new and improved 231 high resolution light pen for use with graphic displays In 24¦ addition, a unique circuit is disclosed for determining the 251 validity of incoming pulses and for reducing the effects of 26 ¦ jitter. The sub~ect light pen 10 includes a collimation tube 271 22 slidably mounted within a cylindrical housing 20. The 281 collimation tube includes a cylindrical channel 38 extending 291 along the longitudinal axis thereof. The inner surface of 301 /// 31 1 /// 3211 ", . I -21- j . . I ~23493S 1 1 the channel is non-reflective. A photodetector 30 is mounted 2 ¦ within the housing in alignment with the channel of the 3 ~ collimation tube. In accordance with the subject invention, 4 ¦ a lens means 42 is mounted in the channel adjacent the front 5 1 end thereof. The focal length of lens 42 is fixed such that it 6 ¦ is eaual to the spacing between the virtual image of a pixel and 7 ¦ the lens, when the liaht pen is placed adjacent a video screen.

8 I B~ this arrangement, all light rays emanating from the pixel ~ ~hich pass through the lens are refracted along the lon~itudina] axis of the housina and directed to the photodetector. Light 11¦ rays from non-aligned pixels which pass through the lens are 12¦ absorbed by the non-reflective surface of the-channel.

13¦ In the preferred embodiment of the light pen, a second 14¦ lens 50 is provided for focusing parallel light beams onto the 1~1 photodetector. By this arrangement, light received by the 161 ~hotodetector from an aligned pixel is maximized while light 17¦ from non-aliqned pixels, which can give rise to spurious signals, 18 I are absorbed. In the preferred embodiment of the light pen, a 19 ~ miniature switch 32 is provided behind the collimation tube 20 ¦ which is activated by the rearward movement of the tube as the 21 light pen is pushed aqainst the video screen. Accordingly, 22 1 light pen switching may be activated without movement of any 23 electrical wires.

24 ¦ The subject invention further includes new anfl 25 ¦ improved discrimination circuit for determining the validity 26 ¦ of incoming pulses while eliminating the effects o jitter.

27 ¦ The subject circuit includes a first comparator means 74 which 28 ¦ receives the input pulses and generates a first signal 29 I whenever the voltage exceeds an upper threshold level T1, 30 I corresponding to a valid pulse. A second comparator means 76 311¦ is provided for qenerating a second signal when the voltage 3~¦l of the input p~lse exceeds a lower threshold level T2. The -''2 G ~3 4 9 3 5 ~1 ' ` .i 1 second signal of thc second comparator is delayed and supplied 2 to a aate means 90 along with the first signal. The gate means 31 ~enerates an output pulse whenever it receives the first and 4 I second sianals simultaneously. The production of the output ¦ signal by the gate means is substantially independent of the 6 ¦ amplitude of the incomina pulses.

71 While the subject invention has been described with 8 reference to preferred embodiments, it is apparent that changes 9 and modifications could be made therein by one skilled in the 10¦ art without varying from the sccpe and spirit of th~ subject 111 invention as defir,ed by the appended claims. Further, it is 12¦ envisioned that the high resolution light pen and discrimination 13¦ circuitry may be used independently when desired. However, when 14¦ used in combination, maximum resolution and the elimination of the detrimental effects of jitter are achievedO 161 /// 17 a¦ /// ' 19 I /// //~ 211 ~// 221 /~/ ~31 /// 24 ~ /// 2~ ~ 27 /// 28 ! /// 29 /// 301~ /// 31 ~ 32l /// -23!~ !