With the advent of desktop and portable computer systems, the problem of maintaining the confidentiality of secure data is increased. This is a particular problem for laptop computers and hand-held personal digital assistants (PDAs) that are frequently used in public locations. Data security is also a problem for other display systems, such as automated teller machines, and Internet terminals in public locations, such as Internet shops and airports.
In recent years, a great deal of effort has been expended on making flat panel display screens as readable as CRT screens by using active matrix technology. However, enhanced readability of displayed data increases the risk of confidential information being viewable by unauthorized persons when portable displays are used in public locations.
One solution is to provide the display with physical “blinders” mounted on the side of the display to limit the angle at which the display can be seen. Another type of mechanical solution uses microscopic louvers to obscure the screen to any viewer not along the axis of the louvers. However, this does not prevent viewing by a person sitting directly behind the user of the display. In addition, this type of arrangement does not allow the user to leave the equipment unattended.
One manufacturer, InvisiView Technologies, Inc., Boca Raton, Fla., removes the front polarizer from a LCD type of device so the displayed image is no longer visible. If the display is viewed through polarized lenses, it becomes visible. This is a partial solution because anyone wearing consumer-grade polarized sunglasses can defeat the system.
U.S. Pat. No. 5,528,319 “Privacy filter for a display device” issued to Austin on Jun. 18, 1996 describes a privacy filter constructed of spaced-apart opaque grids that can be fitted to a display device. The problems with this arrangement is that it requires physical modification of the device, and like the blinders above, only limits the angle at which the display can be viewed.
U.S. Pat. No. 5,629,984 “System and method for data security” issued to McManis on May 13, 1997 describes a display system that alternates data frames with flash frames where an overwhelming majority of pixels are illuminated so that the flash frames have an average intensity substantially greater than the data frames. The user views the display with a shutter device that is synchronized to the displayed frames. The shutter is open for the data frames, and closed for the flash frames. The interspersed flash frames are intended to make it difficult for a viewer without the optical shutter device to intelligibly read the data frames.
The problem with this system is that most people can perceive images even is the relative intensity of darkest elements is only about 1/100 that of the brightest elements. In other words, the intensity of the flash frames would have to be increased by at least 20 db in order for the device to be effective. In a practical LCD applications, the display elements are usually driven at full power to maximize brightness. Therefore, it is problematic whether the driving voltage can be increased by a factor of a hundred. Even if the flash frames can be displayed, it is well known that over illuminating the display screen greatly shortens its useable life-span. In addition, the flash frames would attract attention to bystanders, and the device is more susceptible to counter attacks.