Pressure-sensitive light pen

A light pen which includes a transparent rod at its pointing end through which light is directed to a photodetector that is fixedly positioned within the light pen. The rod is spring loaded and axially slidable in response to applied pressure, resulting in different light intensities being applied to the photodetector and further resulting in an output voltage generated by the photodetector that is directly related to the applied pressure.

BACKGROUND OF THE INVENTION 
This invention relates to a new and improved light pen system for use with 
graphic displays. More particularly, it relates to a light pen having a 
new and improved design which permits continuous control of light pen 
output based on user applied pressure. 
In the prior art, light pens have been used as a pointer device in 
association with computer controlled cathode-ray tube video displays. The 
pen is most often an object about the size and shape of a fountain pen and 
inlcudes a means for sensing light and a means for converting this light 
into an electrical pulse. The pen is pointed by hand to some portion of 
the picture on the screen, and when the electron beam which is tracing the 
image causes that portion of the sceen to light up, the pen senses the 
light and generates an electrical pulse which serves as a signal to the 
computer. Typically, the computer circuit responds to this signal by 
reading an address counter having data which tracks the position of the 
cathode ray at that particular instant. The address in the counter, thus, 
corresponds to the location on the screen to which the light pen points. 
A light pen typically includes an elongated channel for directing light 
rays emanating from the video to a photodetector within the light pen. One 
example of a light pen can be found in U.S. Pat. No. 3,569,617 issued to 
Allen on Mar. 9, 1971. The light pen disclosed by Allen includes a 
collimation tube having an internal channel with non-reflective walls. 
Only light rays that are axially aligned with the channel are transmitted 
from the video screen to the photodetector. Non-axial light rays emitted 
from the screen are either not received through the channel opening or 
absorbed by the non-reflective walls of the channel. Various other light 
pens have been developed which include a lens for focusing the axially 
received light. Examples of such pens can be found in U.S. Pat. No. 
3,271,515 issued Sept. 6, 1966 to Harper, U.S. Pat. No. 3,917,955 issued 
Nov. 4, 1975 to Inuiya and U.S. Pat. No. 4,454,417 issued June 12, 1984 to 
May. 
These pens rely heavily on the ability to detect only the light coming from 
a small area. In addition to the issue of accepting narrow beams of light 
from the video screen, prior art light pens also address the issue of 
light pen activation. A light pen is normally kept electrically 
disconnected, or off, so that stray light impinging on the pen does not 
send an unwanted signal to the computer. Only when the light pen needs to 
be active does the user normally turn the pen "on". Although some light 
pens employ separate mechanical means to actuate the pen, improved means 
are disclosed in the Allen, Inuiya and May patents, in which the 
collimation tube is slidably mounted within an outer housing. When the 
user presses the collimation tube against the video screen, a miniature 
switch in the rear of the housing is actuated, thereby signaling the 
computer that the light pen is active. 
None of the prior art light pens permit light from large areas to be 
accepted and none have means for controlling the electrical output of the 
pen in response to the pressure applied thereon, save for the on-off 
switching done by the aforementioned light pens. It is an object of this 
invention, therefore, to provide simple and effective means for 
translating the pressure applied on a light pen into a continuously 
variable control of the light pen output. 
SUMMARY OF THE INVENTION 
The object of this invention is achieved with a light pen arrangement, 
generally shaped like a conventional ball point pen, that includes a 
transparent rod protruding from the light pen in a manner similar to that 
of the protruding refill in a ball point pen. Also included within the pen 
is a photodetector that is mounted at a fixed distance from the end of the 
pen. A cable, connected to the photodetector, is extended through the end 
of the pen opposite the end containing the protruding rod, adapted to be 
connected to the electronic circuitry associated with our computer screen. 
The transparent rod has a rounded end at the "pointing" end, outside the 
pen, and a polished face at the opposite end, facing the photodetector. 
Light entering the transparent rod at the pointing end is contained within 
the rod and travels through the rod until it exits at the polished end. At 
that point the light begins to disperse. Because of the dispersion, the 
amount of light reaching the photodetector is inversely related to the 
distance between the polished face of the transparent rod and the 
photodetector. Normally, the transparent rod is kept at its protruding 
position by the action of a spring which is situated within the pen and 
arranged to push the transparent rod outward. When pressure is applied on 
the pen, the transparent rod is pushed inward against the spring pressure. 
At that time, the polished end of the rod is closer to the photodetector 
and, consequently, more light is detected by the photodetector. The pen 
thus produces an output voltage that is directly proportional to the 
pressure applied on the pen.

DETAILED DESCRIPTION 
FIG. 1 illustrates the external structure of our light pen. It includes a 
tubular outer casing, or housing, that is made up of a front housing 10 
and a back housing 20. Housing portions 10 and 20 are screwed together. 
Like conventional ball-point pens, our pen's outer housing can be 
constructed out of metal, plastic, or any other like substance. The far 
end of back housing 20 contains a porthole 22 to which a spring 41 is 
connected. An electrical cable 40 is extended through spring 41 and 
porthole 22 and into back housing 20. Spring 41 prevents cable 40 from 
bending (at its point of entry into the pen) at a radius that is too small 
from the reliable operation of the pen. 
The near end of front housing 10 also contains a porthole, or an opening, 
through which a transparent rod 30 is extended. As described hereinafter 
in greater detail, rod 30 is mounted within the light pen of FIG. 1 in a 
manner that permits it to slide into the pen's body in response to 
pressure applied on the pen and communicated to rod 30 in a direction 
parallel to its axis and the axis of the pen. 
The output of our light pen is applied, as shown in FIG. 1, to electronic 
circuits, 50, that amplify the light output, interpret it, and communicate 
to subsequent circuitry that controls, for example, the width of the 
meandering line drawn to follow the meandering movement of the light pen 
across the video screen. Alternatively, the light output can be used to 
control the color drawn on the video screen. 
FIG. 2 presents a cross-sectional view of our pen, while FIG. 3 presents an 
exploded view. The parts internal to the pen are shown both in FIG. 2 and 
in FIG. 3, but the following discussion concentrates mostly on the FIG. 3 
representation because of the ease with which the various elements can be 
identified. Thus, the internal parts of our pen are found in FIG. 3 
between elements 10 and 20. 
Element 21 is the largest element within the pen. Electrical cable 40 
connects to the back of element 21, while appended to the front of element 
21 is a photodetector 25 and rod 30 together with sleeve 33 and spring 34. 
More specifically, body element 21 is cylindrical in shape, with a radius 
smaller than the radius of the cylindrical cavity within the tubular front 
housing 10. The length of body element 21 is such that, when the pen is 
assembled, as shown in FIG. 2, the elements within the pen are securely 
positioned. At the back portion of body element 21 there is a hollow 
cylindrical nipple, or protrusion 27 of a radius slightly smaller than the 
radius of element 21. When our pen is constructed of a metallic material, 
protrusion 27 may advantageously be metallized. This is accomplished by 
constructing protrusion 27 out of copper or the like, or by sleeving a 
copper washer into protrusion 27 (not shown). 
The front end of back housing 20 contains a hollow cylindrical protrusion 
24, whose inside diameter is slightly greater than the outside diameter of 
protrusion 27. This allows protrusion 27 to slide within cylindrical 
protrusion 24 when our pen is assembled, as shown in FIG. 2. The outside 
diameter of protrusion 24 is larger than the diameter of body element 21, 
and the outside surface is threaded to permit the mating of back housing 
20 with front housing 10. Because the outisde diameter of protrusion 24 is 
larger than its inside diameter, a washer-like surface 28 is created at 
the end of protrusion 24. A spring 23, interposed between body element 21 
and housing element 20 is adapted to slide over protrusion 27 and rest on 
one end on body element 21 and, rest on the opposite end on surface 28. 
Thus, cable 40 passes through spring 41, porthole 22, housing element 20 
(which has a through passage for cable 40), protrusion 24, spring 23, and 
hollow protrusion 27 before it reaches element 21. 
Element 21 is made of a generally insulating material (electrically) and 
includes a cavity 29 at its juncture with protrusion 27 to allow the 
terminal end of cable 40 to reside within cavity 29. Along the length of 
body element 21 there exist two metallized strips 26 (only one of which is 
shown) that are diametrically positioned from each other. Each strip 26 
extends from cavity 29 to the front portion of body element 21. Cable 40, 
which contains two wires enclosed in an electrically conductive shield, 
terminates with short exposed portions of the wire ("pigtails") within 
cavity 29. The shiled is connected to the metallized portion of protrusion 
27, and the two pigtails are each connected to one of the metallized 
strips 26. When the pen is assembled as shown in FIG. 2, protrusion 27 
extends into the cylindrical cavity of protrusion 24, spring 23 is 
compressed, and the shield of cable 40 makes electrical contact with 
protrusion 24. Consequently, when our pen is constructed out of a metallic 
substance, the above-described construction causes the pen to be at the 
electrical potential of the shield (e.g., electrical ground). Of course, 
when the pen is constructed from an electrically insulating material, the 
shield in cable 40 and the metallization of protrusion 27 may be dispensed 
with. 
The front portion of body element 21 contains a hollow cylindrical 
protrusion 22 of a diameter smaller than that of element 21, whose outside 
surface is threaded. Metal strips 26 extend to within the cavity formed by 
the hollow portion of protrusion 22, and a conventional photodetector 25 
is permanently affixed within that cavity and connected to metallized 
strips 26. Advantageously, photodetector 25 is centered on the axis of 
element 21 in a plane that is perpendicular thereto. 
Sleeve 33 is adapted for screwing onto protrusion 22. It has a shape of a 
truncated cone, or a tapered cylinder, with a through passage centered 
along its axis. The diameter of the through passage has one value at the 
narrow end of the truncated cone, extending for a distance into the cone, 
and another and slightly larger value at the wide end of the cone. Viewed 
in another way, sleeve 33 is generally conical in shape with a bored hole 
centered on its axis that runs through the cone, and is countersunk for a 
distance from the base of the cone. The larger diameter countersinking is 
such that the inside surface can be threaded and mated with the threading 
of protrusion 22. 
Rod 30, which is designed to be inserted into the through passage of sleeve 
33, is generally cylindrical in shape. It may be made out of Plexiglas 
plastic sheets (which is a registered trademark of Rohm&Has Co. of 
Philadelphia, Pa.), or any other transparent material and includes a front 
section 42 that extends to about 75% of the length of rod 30, a back 
section 31 that extends for about 15% of the length of rod 30, and a 
cylindrical center section 32 extending for the remainder of rod 30. The 
front section 42 has a radius that is slightly smaller than the small 
radius in the through passage of sleeve 33, the center section 32 has a 
radius that is slightly smaller than larger radius in the through passage 
of sleeve 33, and the back section 31 has a radius that advantageously is 
about equal to the radius of front section 42. The free end of front 
section 42 is spherically rounded and polished to readily accept light 
from a computer CRT or the like. The free end of back section 31 is flat 
and also polished, with a surface that is perpendicular to the axis of rod 
30. 
Spring 34 has an inside diameter that is slightly larger than the radius of 
back section 31 and an outside diameter that is slightly smaller than the 
radius of the cylindrical cavity in protrusion 22. Of course, the radius 
of back section 31 is smaller than the radius of the cylindrical cavity by 
at least the thickness of the material making up spring 34. 
When our pen is assembled, as shown in FIG. 2, one end of spring 34 is 
placed inside the cavity of protrusion 22, the back section of rod 30 is 
placed inside the other end of spring 34, and sleeve 33 is placed over the 
rod 30 - spring 34 assembly and screwed to protrusion 22. This causes 
spring 34 to be compressed, and the compression force exerted by spring 34 
is communicated to rod 30 through the center section of rod 30 (section 
32). This forces the front section of rod 30 to extend through sleeve 33. 
Front housing 10 sleeves over the entire assembly of spring 23, element 
21, spring 34, rod 30 and sleeve 33 and attaches to back housing 20 by 
screwing onto the threaded surface of protrusion 28. As a result, rod 30 
extends outside front housing 10. 
In operation, when no pressure is applied in our light pen, rod 30 extends 
to the point where center section 32 reaches the smaller diameter hole 
bored through sleeve 33. This position, as illustrated in FIG. 4 with pen 
100, results in a particular distance D between the polished flat end of 
rod 30 and the photodetector. When pressure is applied to our light pen, 
rod 30 is pushed inward, as illustrated in FIG. 4 with pen 200, and the 
distance between the polished flat end of rod 30 and the photodetector is 
reduced to d. Since light emanating from the end of rod 30 disperses with 
distance, less light impinges on the photodetector for light pen 100 than 
for light pen 200. The signal output of the ight pen is hence proportional 
to the applied pressure. 
One of the features of our light pen is that a large amount of light is 
accepted by the photodetector. A transparent rod (30) is used to effect 
the aforementioned dispersion, but it also has the additional property of 
reflecting light off its cylindrical wall, which channels more light to 
the photodetector. The rounded tip of rod 30 increases the amount of light 
reaching the photodetector still further. 
Such a condition, which is unacceptable to prior art light pens (and hence, 
they use only small openings and light channels of air rather than 
transparent materials of a higher index of refraction), is put to 
advantage in accordance with the principles of our invention by applying 
the output signal of our light pen to circuitry that utilizes the 
increased light. Specifically, circuit 50 (FIG. 1) takes the light output 
of the pen and calculates therefrom a highly accurate measure of our light 
pen's position. 
The manner in which the above is accomplished may be more clearly 
understood when it is observed that the light coming from our pen is 
really a sequence of signals from those pixels on the CRT screen whose 
light manages to reach the photodetector. That sequence of signals 
represents a two-dimensional array of light intensities from pixels in the 
neighborhood of the point at which our light pen is aimed. Having such an 
array, a centroid of the array can be computed which provides a good 
indication of the point towards which our pen is directed. Computation of 
the centroid can be had with a circuit 50 created for that purpose, which 
is described in "Moment Calculations by Digital Filters," by Z. L. 
Budrikis and M. Hatamian, in BSTJ Vol. 63, No. 2, pp. 217-229, (February 
1984). 
Although the operation of our pen is described in connection with a CRT 
display, it should be understood that our light pen can easily be employed 
in different circumstances without departing from the spirit and scope of 
our invention. For example, instead of relying on a CRT beam to provide a 
light to the photodetector which would identify the pen's position, a pad 
having fine cross marks in the conventional x and y directions can be 
used, and the photodetector can be caused to detect transitions across the 
cross marks. The light source can be ambient light or a light source built 
into the light pen.