Interactive projection display system

An interactive projection display system having a light pen device for use thereon is described. The projection display system has an image of a light valve on a projection screen which is formed by visible light of specified wavelengths. The projection screen also has a translatable crosswire image thereon which is formed by radiation which includes the visible light used to form the light valve image and an additional spectral component. The light pen device is sensitive to this additional spectral component. In a preferred embodiment, the image of the light valve and the crosswires are formed with white light (0.3 to 0.7 microns). The additional spectral component used for the crosswire image and the light pen sensing is a band of near-IR radiation (0.7 to 1 micron).

FIELD OF THE INVENTION 
This invention relates to interactive projection display systems and more 
particularly to a system suitable for having a light pen device used 
therewith. 
DESCRIPTION OF THE PRIOR ART 
Some projection display systems are interactive, that is, they permit 
operator selection of control options from a large library of available 
options. Interactive projection display systems generally employ a 
selection means and/or a position indicator. A keyboard, control lever or 
a stylus-type device, for example a light pen, are commonly used selector 
means. A cursor is an example of a position indicator. 
An interactive projection display system using a light pen device is 
described in the patent to Worthington et al, U.S. Pat. No. 3,825,730. 
This two-way projection system selects desired input messages from a 
library of selectable messages by locating the position of the light 
emitting light pen with an array of photodetectors. This system is limited 
to operation in a read only mode as well as to a fixed number of options 
which is determined by the number of detectors in the array. 
The stylus function for use as a graphic input is described in the patent 
to Ellis et al, U.S. Pat. No. 3,399,401 where the user interacts directly 
with a tablet separate and distinct from the display screen. Since the 
system does not deal directly with the display screen, it needs to 
correlate the tablet with the display screen. 
The cursor function in a data input generator is described in the patent to 
Heartz, U.S. Pat. No. 3,883,861 and is used for encoding information in a 
digital form to produce a data base descriptive of a land mass or the 
like. This system does not utilize a light pen function and requires a 
beamsplitter which would prohibit the use of a light pen function which 
would touch the screen. 
Stylus devices which are used to indicate coordinates directly on the 
display screen fall into two basic categories. The first type acts 
independently of the image on the screen such as the acoustic stylus 
device described in U.S. Pat. No. 3,692,936 (Moffitt Sept. 19, 1972). The 
second type, of which the CRT light pen is the most common example, uses a 
photoresponsive stylus which senses the photo emission from the display 
screen. The coordinates of the light pen are determined from timing 
information using the fact that the screen image is being refreshed 
(periodically rewritten) at a rapid rate. This basic light pen system has 
two limitations: it cannot operate with a storage type or long persistence 
display, and it can only detect in luminous areas of the screen. The first 
limitation is circumvented by the patent to Davis, U.S. Pat. No. 4,020,281 
in which a storage display is partially erased and rewritten during the 
light pen operation. The second limitation is circumvented by several 
means which employ the writing of a special luminous pattern on the screen 
during the light pen operation, as referenced in the aforementioned Davis 
patent. 
SUMMARY OF THE INVENTION 
It is a primary object of this invention to provide an improved interactive 
projection display system. 
It is another object of this invention to provide means for interacting 
with the projected image upon a screen. 
It is still another object of this invention to provide an interactive 
projection display system having a light pen function which is not 
restricted to predetermined portions of the display field. 
It is still another object of this invention to provide an interactive 
projection display system having a light pen function which can detect on 
both written and unwritten portions of the display field. 
It is yet still another object of this invention to provide an interactive 
projection display system having a light pen function which interacts 
directly with the projection screen. 
It is a further object of this invention to provide an interactive 
projection display system having both a light pen function and a cursor 
function. 
These and other objects are accomplished by an interactive projection 
display system which has an image of a light valve on a projection screen 
formed by visible light of specified wavelengths, and which has a 
translatable crosswire image on the projection screen formed by radiation 
which includes the visible light used to form the light valve image and an 
additional spectral component. This additional spectral component is an 
arbitrary wavelength band, and is preferably generated by the projection 
lamp. The system also has a light pen device sensitive to this additional 
spectral component which when used in conjunction with a translatable 
crosswire image senses the light pen position on the screen. In a 
preferred embodiment, the image of the light valve and the crosswires are 
formed with white light (0.3 to 0.7 microns). The additional spectral 
component used for the crosswire image and the light pen sensing is a band 
of near-IR radiation (0.7 to 1 micron). The cursor function is provided by 
the visible image of the crosswires on the projection screen. The 
intersection of the images of the two wires indicates the reference 
coordinates on the screen. In the light pen mode, the horizontal and 
vertical images of the cursor wires are translated rapidly across the 
screen in succession. The light pen device when activated in a 
conventional manner detects the passage of the image of the wires formed 
in the near IR band. The position of the light pen is determined by 
knowledge of the position of the cursor wires at the time the images are 
detected. 
Other objects of this invention will be apparent from the following 
detailed description, reference being made to the accompanying drawings 
wherein various embodiments of the invention are shown.

DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS 
This invention deals with an interactive projection display system as shown 
in FIG. 1. The interactive system has a projection system 10 which 
projects an image from a light valve (not shown) on a rear projection 
screen 12. On the screen 12 are images 14 and 16 of the cursor wires which 
intersect each other at point 18 as the crosswire image to indicate the 
reference coordinates on the screen 12 and thereby provide the cursor 
function. A light pen 20 has an end 22 which interacts directly with the 
projection screen 20. During operation of the light pen 20, the images 14 
and 16 of the cursor wires are translated rapidly across the screen 12 in 
succession. Activation of the light pen 20 in a conventional manner 
detects the passage of the image 14 and 16 of the cursor wires by the 
light pen device being sensitive to the additional spectral component 
which is used to form the cursor wire images 14 and 16 as will hereinafter 
be more fully discussed. The position of the light pen end 22 on the 
screen 12 is determined by knowledge of the position of the cursor wire 
images 14 and 16 at the time the images 14 and 16 are intercepted as will 
hereinafter be more fully discussed. 
The cursor wires will be opaque to the radiation and hence their images 14 
and 16 on the screen will always be black. The light valve (LV) may be 
either subtractive or additive. With a subtractive LV, the screen will 
normally be luminous and written areas of the image will be darker. With 
an additive LV, the screen will normally be dark and written areas of the 
image will be luminous. In either case, the dark areas of the screen image 
will not be black because of the limited contrast ratio of the image. 
Thus, the crosswire image 18 will be visible at all points of the screen 
and in principle it could be detected by a light pen device 20 sensitive 
to the visible spectrum. However, the signal level would vary widely from 
dark to luminous areas of the screen, and hence it is preferable to flood 
the cursor wires uniformly with an additional spectral component which is 
distinct from the visible light used to form the image of the LV. 
The light or radiation output from the projection system 10 is critical for 
the successful operation of the cursor wire images and the light pen 
device and can be more readily understood by describing the radiation in 
the framework set forth in FIG. 2. Typically, a lamp of the type used in 
projection display systems has an output which covers the spectrum shown 
in FIG. 2, that is, it has UV light having wavelengths less than 0.4 
microns, visible light in the range of 0.4 to 0.7 microns, near-IR light 
having a wavelength between 0.7 and about 2.0 microns and IR radiation or 
heat having a wavelength greater than about 2 microns. In general, the 
image in the projection display system on the screen is formed with the 
visible light. This visible light is also used to form the image of the 
cursor wires. In addition, the crosswire image contains an additional 
spectral component. In the preferred embodiment, this additional spectral 
component is in the near-IR wavelength, that is, between 0.7 and 1.0 
microns. The light pen device is only sensitive to this additional 
spectral component. 
It should be recognized that the additional spectral component is not 
limited to the near IR band, but could be among other things a band of 
visible light, for example, red light. The LV image could be formed with 
the remainder of the visible spectrum, namely, cyan light. Dark areas of 
the LV image would appear red and light areas would be white. The light 
pen would be made sensitive to red light only. 
As shown in FIG. 3, the X-cursor is a wire 16A which is connected to motor 
and encoder means 24. The Y-cursor is a wire 14A which is connected to 
motor and encoder means 26. The mechanical and electromechanical operation 
of the X and Y-cursor wires is performed in a conventional manner and 
other alternative means may be used if desired. The image of the X-cursor 
wire moves across the screen from left to right or right to left. The 
image of the Y-cursor wire moves across the screen from top to bottom or 
bottom to top. 
An enlarged view of the light pen 20A is shown in FIG. 4. The light pen 20A 
has an end 22A which is positioned directly on the projection screen. When 
the light pen end 22A is placed on the screen, the lens 24 images a small 
area of the screen onto the aperture 26. As the image of the X-cursor 
crosswire crosses this area, and passes through filter 28, the output of 
the photodiode 30 will momentarily dip as shown in FIG. 5. Using the known 
position of the X-cursor at this instant (T.sub.1) (from a shaft encoder 
or by using a stepping motor as is well known in the art), the 
X-coordinate of the light pen can be found. Immediately following T.sub.1, 
the Y-cursor can be made to move in order to determine the Y-coordinate of 
the light pen. 
The essential characteristics of the light pen 20A is that the photodiode 
30 is responsive to the additional spectral component which is used to 
form the translatable crosswire image on the projection screen. The 
preferred additional spectral component is in the near IR spectrum 
portion, that is, between 0.7 microns and 1.0 microns. 
The interaction between the light pen and the cursor wire image is more 
easily understood by a discussion about FIG. 5. In FIG. 5, the upper curve 
34 shows the output of the photodiode and the lower curve 36 shows the 
position of the image of the X-cursor as a function of time. The position 
information is provided by a shaft encoder on the motor which drives the 
X-cursor, or by timing information, or by similar means. When the image of 
the X-cursor crosses the position of the light pen at 38, the output 40 of 
the photodiode will momentarily drop. The known position of the X-cursor 
at this time (T.sub.1) is the X-coordinate of the light pen. 
Similarly, a subsequent and similar operation of the Y-cursor will provide 
the Y-coordinate of the light pen. 
A preferred embodiment for a transmissive LV is shown in FIG. 6. A 
projection lamp 42 having visible light 44 represented by an arrow with a 
single line, near-IR radiation 46 represented by an arrow with a double 
line, and IR radiation 48 represented by an arrow with a wavy line. The 
radiation 44, 46, and 48 is collimated by the condenser lens 50. The 
collimated radiation is directed to hot mirror 52 which transmits the 
visible light 44 and reflects the IR radiation 46 and the near-IR 48. 
Radiation 46 and 48 is directed to a special cold mirror 54 which 
transmits the IR radiation 48 and reflects the near-IR radiation 46 to a 
condenser lens 56. The near-IR radiation 46 is reflected off of a folding 
mirror 58 to a hot mirror 60. The visible light 44 which is passed through 
the hot mirror 52 passes through a condenser lens 62 and a light valve 
which is located in plane 64. The visible light then passes through hot 
mirror 60 and continues along with the near IR radiation 46 to relay lens 
66 which relays an image of light valve in plane 64 into the plane of the 
cursor wires located at 68. The light then proceeds through a field lens 
70 which images the aperture of the relay lens 66 into the aperture of the 
projection lens 72. The projection lens 72 images plane 68 onto a screen 
(not shown.) Hence, the screen displays an image of the LV whose image was 
relayed into plane 68 and also an image of the cursor wires which are 
located in plane 68. 
The preferred embodiment described above in FIG. 6 utilizes the near-IR 
radiation as the spectral component which passes through the plane 68 to 
form an image of the cursor wires on a screen which is subsequently sensed 
by a light pen sensitive to the near IR radiation. 
It is understood that numerous variations of the embodiments shown in FIG. 
6 can be used. For example, the mirror 60 may also be used for directing 
on to the LV 64 a near-IR laser, such as Nd:YAG for the purpose of writing 
on the LV. It is also understood that instead of using a portion of the 
lamp spectrum for the near-IR, a separate narrow bandwidth source could be 
used. For example, the special cold mirror 54 could be removed and a GaAs 
IR diode could be put in its place. 
Another embodiment suitable for a reflective LV is shown in FIG. 7. A lamp 
80 transmits visible light 82, near IR radiation 84 and IR radiation 86 
through a condenser lens 88 which collimates the light from the lamp 80. 
The light passes through the condenser lens 89 and forms an image of the 
lamp filament on an elliptical mirror 90 on the aperture plate 94. The 
elliptical mirror 90 reflects visible light but transmits IR and near-IR 
radiation. The aperture plate 94 contains an aperture 91 and a front 
surface mirror 92 on the opposite side of the aperture plate to elliptical 
mirror 90. Aperture plate 94 is suitable for a subtractive LV. 
Alternatively, aperture plate 94A is for an additive LV and has an 
elliptical mirror 90A, a front surface mirror 92A on the opposite side of 
the aperture plate to 90A and an opaque stop 93A. The near IR radiation 84 
and the IR radiation 86 which passes through the elliptical mirror 90A 
continues and passes through condenser lens 96 to a special cold mirror 98 
which transmits the IR heat 86 but reflects the near IR radiation 84. The 
rear IR radiation 84 that is reflected by the mirror 98 passes through the 
condenser lens 96 and forms am image of the lamp filament on mirror 92A. 
The visible light 82 is reflected by mirror 90 (90A) and passes through a 
telecentric schlieren lens 100 to a reflective light valve 102. The 
reflective light valve 82 reflects visible light 82 through the lens 100 
to the aperture 91 on aperture plate 94 (or around the opaque stop 93A on 
aperture plate 94). The visible light 82 and the near IR radiation 84 
passes through relay lens 103 which along with lens 100 forms an image of 
the light valve 102 in plane 104, the location of the cursor wires. Field 
lens 106 images the aperture of relay lens 103 into the aperture of 
projection lens 108 which images plane 104 onto a screen not shown. 
It is understood that the invention could be utilized with an image from a 
CRT or similar photoemissive display element when used in a projection 
display mode. 
This invention is particularly useful for projection systems employing a 
reflective liquid crystal light valve. 
Although some preferred embodiments of this invention have been described, 
it is understood that numerous variations may be made in accordance with 
the principles of this invention.