High resolution light pen for use with graphic displays

• US 4,454,417 A
• Filed: 02/05/1982
• Issued: 06/12/1984
• Est. Priority Date: 02/05/1982
• Status: Expired due to Term

• Current Assignee: George A. May
• Original Assignee: George A. May
• Inventors: May, George A.
• Primary Examiner(s): Nelms, David C.
• Assistant Examiner(s): Westin, Edward P.
• Application Number:
  US06/346,368
• Time in Patent Office: 858 Days
• Field of Search: 250/216, 250/227, 250/555, 250/566-568, 235/471, 235/472, 235/473, 340/706-708, 350/415, 350/562
• US Class Current: 250/216
• CPC Class Codes: G06F 3/03542 Light pens for emitting or receiving light

Abstract

A high resolution light pen comprises a collimation tube slidably mounted within a housing. The collimation tube includes an axially extending channel having a non-reflective surface. A photodetector is fixably mounted within the housing aligned with the channel. A lens is mounted adjacent to the front end of the channel and has a focal length equal to the spacing between the lens and an aligned pixel of the video screen. By this arrangement, all light rays emanating from an aligned pixel and passing through the lens are refracted axially along the channel and directed to the photodetector thereby maximizing the input thereto. The light pen further includes a discrimination circuit which generates computer interrupt signals for reading an address counter. The discrimination circuit generates an interrupt signal when the signal from the photodetector is valid. The interrupt signal is generated at a time which is substantially independent of the slope of the signals thereby reducing the effects of jitter.

Claims

1. A high resolution light pen for sensing light rays emitted from a pixel on a video screen comprising:

an elongated cylindrical housing having opposed front and rear ends;

a collimation tube mounted within the housing and projecting outwardly from the front end thereof, said collimation tube having a cylindrical channel extending along the longitudinal axis thereof, with the inner surface of said channel being nonreflective;

photodetector means fixedly mounted within said housing to the rear of said collimation tube and axially aligned with the channel thereof; and

lens means disposed within the channel of the collimation tube adjacent the front end thereof, said lens means having 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, whereby all light rays emanating from a pixel axially aligned with the channel and passing through the lens means are refracted axially along the channel and directed to said 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 said photodetector.

2. A light pen as recited in claim 1 further including a second lens means disposed adjacent the rear end of said channel for focusing said axially directed light rays within said channel directly onto said photodetector means.

3. A light pen as recited in claim 2 wherein said second lens means is fixedly mounted to said photodetector means.

4. A light pen as recited in claims 1, 2 or 3 wherein said collimation tube is slidably mounted within said housing and wherein said light pen further includes a switch means operatively connected to said photodetector, said switch means being disposed behind said collimation tube in a manner such that when the front end of said tube is pressed against the video screen it is moveable rearwardly, relative to said housing, to actuate said switch means.

5. A light pen as recited in claim 4 wherein said collimation tube includes a recess for receiving a stop pin fixedly mounted on said housing, said recess and stop pin combination for restricting the movement of said collimation tube in an axial direction.

6. A high resolution light pen system including a light pen for sensing light rays emitted from a pixel on a video screen and generating input pulses which are tested for validity by a discrimination circuit to reduce jitter, said system comprising:

a collimation tube mounted within the housing and projecting outwardly from the front end thereof, said collimation tube having a cylindrical channel extending along the longitudinal axis thereof, with the inner surface of said channel being nonreflective;

photodetector means fixedly mounted within said housing to the rear of said collimation tube and axially aligned with the channel thereof, said photodetector means generating input pulses in response to the reception of light rays;

lens means disposed within the channel of the collimation tube adjacent the front end thereof, said lens means having 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 such that all rays emanating from a pixel axially aligned with the channel and passing through the lens means are refracted axially along the channel and received by the photodetector;

first comparator means operatively connected to and receiving the input pulses generated by the photodetector means, said first comparator means for generating a first signal whenever the voltage of an input pulse exceeds a first threshold level;

second comparator means operatively connected to and receiving the input pulses generated by the photodetector means, said second comparator means for generating a second signal when the voltage of an input pulse exceeds a second threshold level, said second threshold level being lower than said first threshold level;

delay means connected to said second comparator means for delaying said second signal a fixed amount; and

gate means operatively connected to said first comparator means and said delay means, said gate means generating an output pulse whenever said gate means receives the first and second signals from said delay means and said first comparator means simultaneously, to reduce the difference in time at which output pulses are generated by input pulses of different amplitudes all exceeding the first threshold, thereby also reducing jitter.

7. A light pen system as recited in claim 6 further including a second lens means disposed adjacent the rear end of said channel for focusing said axially directed light rays within said channel directly onto said photodetector means.

8. A light pen system as recited in claim 7 wherein said second lens means is fixedly mounted to said photodetector means.

9. A light pen system as recited in claims 6, 7, or 8 wherein said collimation tube is slidably mounted within said housing and wherein said light pen further includes a switch means operatively connected to said photodetector, said switch means being disposed behind said collimation tube in a manner such that when the front end of said tube is pressed against the video screen it is movable rearwardly, relative to said housing, to actuate said switch means.

10. A light pen system as recited in claim 9 wherein said collimation tube includes a recess for receiving a stop pin fixedly mounted on said housing, said recess and stop pin combination for restricting the movement of said collimation tube in an axial direction.

11. A light pen system as recited in claim 6 wherein said gate means comprises a logical "and" circuit such that a positive output pulse is produced only when positive signals are simultaneously received from said first comparator means and said delay means.

12. A light pen system as recited in claims 6 or 11 further including an amplification means connected to both of said first and second comparator means, said amplification means for amplifying said input pulses prior to being supplied to both of said first and second comparator means.

13. 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, comprising:

an elongated housing having opposed front and rear ends and formed with a hollow channel extending therethrough;

a radiation detection device mounted in said housing adjacent the rear end of said channel;

lens means mounted within said channel adjacent the front end thereof, said lens means having a focal length such that when the front end of said 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 said detection device, while the radiation received from pixels displaced from the axis is directed by said lens towards the sides of the elongated channel and dissipated, whereby said detection device functions to detect the on-axis pixels to the exclusion of the off-axis pixels; and

a second lens means disposed adjacent to the rear end of the channel for focusing the axially directed radiation directly onto said radiation detection device.