Information is provided to a computer in a number of ways.  One way of providing information is by direct input from a user.  The most common user interface for inputting information is the keyboard.  The keyboard requires the user to type information in the same manner as a typist with a typewriter.  However, this is not the only way a user can provide information.

Another method of providing information to the computer relies upon the relative position of a cursor to a location on a video display such as a cathode ray tube (CRT).  This can be accomplished by use of a mouse, light pen or other conventional devices.  A mouse is a device that rests on a pad next to the computer and generally includes a ball which responds to the movement of the mouse by the user’s hand in an x-y plane.  This motion is translated onto the CRT by way of a cursor which points to a specific location.  When positioned correctly, the user may use a button, or buttons, on the mouse to direct the computer to perform an action dictated by the cursor’s location.  That action can take many forms.  Three such options are; 1) to issue a command to perform some function, or; 2) request information, or; 3) select from a menu of available options.

A light pen is similar in appearance to a pen or pencil-type writing instrument.  It differs from the mouse in that the tip of the light pen when placed in close proximity with the surface of the CRT receives light signals, via a photo detector, emanating from the illumination of the phosphorous coating as the electron gun within the CRT scans the pixels on the internal surface of the CRT.  Light pens interact directly with the video monitor’s screen like a touchscreen, in providing the absolute positioning and pen-type input of a digitizing tablet, but do not suffer from the confusion and ambiguity of a relative positioning mouse.  For most operating systems light pens are used as pointing devices with the same basic functional characteristics (to the operating system) as a mouse.  Special device driver software supplied by the light pen manufacturer provides the light pen with these characteristics.  The light pen is typically provided with a button, or buttons, that allow the user to execute a number of options in a manner similar to the buttons on a mouse.

Light pens are preferred over a mouse by many users.  One reason for this is that the light pen does not require a pad and thus occupies less space on a desk.  Another reason is that the use of a light pen for drawing is more comfortable as it is closely aligned with the method by which most people learn to draw.  That is, with a pen, pencil, or brush.


A computer light pen system is a man-machine interface control system.  In addition to a human operator and a conventional computer, with a Cathode-Ray-Tube (CRT) display screen, the system includes a CRT light pen, light pen interface hardware, and a light pen software driver.

The operator initiates functions of the control system by pointing the light pen to an area of the CRT screen, related to the functions, and depressing one of the light pen buttons.  By repeating this simple, intuitive pointing to the screen and depressing buttons, the operator may control the performance of complex hardware, software, and equipment control functions determined by application software.  The utility, efficiency, and cost effectiveness of the computer light pen systems in many applications is well established in the industry.

Most conventional computers today may be configured with CRT displays.  CRT displays, or monitors, are the display of choice for most computer systems requiring high data display density.  A light pen is ideally suited for pointing on high data density CRT screens because of the high accuracy and resolution attainable with light pen systems.

The CRT light pen detects light in its field-of-view on the CRT screen, and provides light and button data to the light pen interface hardware.  The interface in addition receives CRT synchronization data, and provides pen position and button data to the light pen driver.  The light pen driver software then provides input to the computer operating system to accomplish the desired application function.


The CRT display or monitor is the key element in the man-machine interface of the computer light pen system.  The data displayed on the CRT each frame is defined by the computer video controller.  The CRT receives frame video data and synchronization signals, and illuminates the CRT screen using the raster scanning method.  Video frame rates are typically 60 +/- 10 Hertz.

The video of each frame is raster scanned onto the CRT screen beginning in the upper left-hand corner. Video is scanned onto the screen one dot, or pixel, at a time, until the uppermost line is formed by the sequence of video dots.  When the first line is completed, the raster scan then forms the next line below, again beginning at the left of the screen, but one pixel lower than the first line. This process continues down the screen until all lines of the frame are scanned, completing one frame of video data.  Due to the high video frame rate of data scanned on the CRT screen, compared to the light sensing rate of the human eye, the operator sees an apparently static display which the brain interprets accordingly, and the operator uses to his end.  The light pen, however, processes light data at rates exceeding the video data rate, and thus easily differentiates pixels and lines necessary to the pointing function.

Rather than averaging visual CRT screen data, as the human eye does, the CRT and the lightpen function essentially upon one pixel of video at a time.  The CRT illuminates a screen pixel by directing electrons to the phosphor material behind the glass of the screen.  Electrons striking the phosphor produce photons of visible light.  The light pen pointed at the CRT screen, detects light within its field-of-view.


CRT light pen was invented at IBM in the late 1950s. In the 1960s the founders of Interactive designed the first CRT light pen using semiconductor detectors. Since then there have been significant improvements in components and technology resulting in mature, low cost products of great utility in many applications.

The light pen is more correctly called a CRT light pen. The light pen is designed specifically to detect light emanating from a CRT screen, and convert the light signals to equivalent electrical signals.  The CRT light pen is not suitable for use with flat panel or LCD displays, although they are used with pulsed LED display matrices.

The key element of a light pen in the conversion of light to electrical signals is the photodetector.  The photodetector is located near the tip of the pen with an aperture or other optics to receive CRT light emanations.  When the light pen tip is pointed at the CRT screen, light received by the photodetector produces video current in the detector, which is proportional to the light received.  The form and the timing of the photodetector current is identical with that of the CRT screen phosphor illuminance resulting from impinging CRT electrons associated with the spot of interest.

The photodetector video is amplified, differentiated, and formed into a light pulse output of the pen.  The light pulse so processed contains the basic light timing information used in the interface to provide precise position data to the light pen driver software, and thus to the computer OS.

The light pen also may include up to 3 switches operated by buttons. Operating these switches produce associated outputs provided to the interface unit.  These outputs are used to initiate computer operations associated with the spot of light under the light pen tip.  Further information may be indicated to the computer depending upon which button is pressed, while pointing at a particular screen spot.  For example pressing button 1, may indicate a left mouse function, button 2 a right mouse, etc.

Although the theory of operation as presented above is relatively simple, in practice light pen configurations involve considered trade-offs of optical, electrical, and mechanical parameters.  Details of design trade-offs are beyond the scope of this document; but include consideration of the high noise environment of the CRT screen, signal-to-noise, cross-talk between switching circuits and low level video circuits, and mechanical constraints on hand-held instruments.

Interactive has developed the 200 and 300 Series that overcome all of these constraints, and provide the industry the most cost effective and reliable product available.


The light pen interface hardware provides three basic functions: 1) It receives light pen video and button signals; video sync signals, and DC power inputs. 2) From these inputs, the light pen screen position and button press data is determined. 3) The data is then formatted and send to the host computer, via a suitable computer input port or bus.

The interface receives light pulses and button press inputs from the light pen via the pen cable/connector. The video horizontal and vertical sync signals are received by the interface via a “Y” sync cable connected between the computer video controller and the monitor. DC power is received from the computer port or bus, or from an external DC supply. DC power is conditioned, as required, and provide to both the interface and the light pen.

The absolute position of the light pen on the screen is determined by comparing the timing of the light pulse output of the pen with the state of vertical and horizontal counters synchronized by the video sync signals from the video controller. A vertical sync signal occurs at the beginning of each frame. A horizontal sync signal occurs at the beginning of each line in each frame. The coincidence of a vertical sync and a horizontal sync occurs at the beginning of each frame. Based of these facts, the combinations of synchronized counter states may be used to form a timing matrix, related directly the physical position of the pen on the screen, against which the light pulse timing can be compared, to determine the absolute position of the pen on the screen. The accuracy of the absolute position of the pen position is determined by size of the two counters and the clock rate used to increment the counters. The size of the counters and the clock rate is usually selected to provide accuracy of one pixel or less at the maximum screen resolution of the monitor.

The interface also processes light pen button press data, and forms interrupt, proximity, and interface address data. This data and the pen position data is then formatted to be sent to the light pen driver software via a defined computer port or bus. Light pen interface units are commonly available for serial port, USB port, and the ISA and PCI bus communication on IBM PCs and compatibles. Current interface designs may be implemented using micro controllers, PALs, and ASICs, as well as, more conventional devices.


The light pen software drivers for all windows operating systems (OS) function in the same general manner.  Common core code is used for Windows 95, 98, and NT.  Beyond the core code for the drivers, code additions and adjustments are made to accommodate the particular OS and the hardware interface used.  Specific light pen drivers are maintained by OS and by hardware interface type within each OS.

The light pen drivers all include the following interfaces:

Interfaces to the light pen hardware: This interface provides the driver with pen position, button press and other related data.

Interfaces to the OS: This interface provides OS cursor positioning for the user desktop and parsing light pen button presses to effect mouse emulation.  In addition, there are interfaces to the light pen, via OS APIs, which allow an application program to receive, manipulate, and alter light pen data.

Interface to the user: This interface provides the user screens for light pen calibration and the light pen control panel.  The control panel folders provide the user with the facility to map mouse buttons to pen buttons, adjust for offset, acceleration, pen response, and double click speed following installation and calibration.  Facility is also provided for flashing the screen for pen use in screen areas without light illumination, and for on-screen arrow-key type adjustment for systems without a keyboard.  The operation and use of the control panel folders is detailed in the user guide.

Each light pen system is delivered with a Users Guide which includes step-by-step instructions for the installation of both hardware and software.  In addition, a Readme file is included with each driver.  For ease of installation and successful light pen system operation the provided information should be read and understood before installation is performed.