Medical Device Link.

Medical Electronics Manufacturing Magazine
MEM Article Index

Medical Electronics Manufacturing Fall 1998

Display Technologies

Vacuum Fluorescent Display Technology

Albert Smith, vice president, Noritake Company, Inc.

The characteristics of vacuum fluorescent display (VFD) technology include high brightness, wide viewing angle, wide temperature range, and relatively low cost, making it particularly appropriate for medical uses. The high brightness and wide viewing angle produce excellent visibility in low light conditions, and in situations where the viewer cannot be directly in front of the display. The wide temperature range is suitable for emergency vehicle and portable equipment use. And at current pricing, made more competitive by recent manufacturing efficiencies, VFD technology has become an essential display option for engineers involved in designing electronic medical devices.

VFDs, introduced in 1967, are a variation of the triode vacuum tube consisting of a cathode, grid, and anode sealed in a high-vacuum glass envelope. The cathode is a directly heated, fine tungsten wire coated by an alkaline earth metal oxide. The grid is a thin metal mesh, and the anode is a segment or dot formed as a conductive electrode on which phosphor is printed. The shape of the phosphor segment or the arrangement of illuminated phosphor dots creates the characters or symbols. Electrons emitted from the cathode are accelerated with a positive potential applied to both the grid and anode, which upon collision excite the phosphor on the anode to emit a very bright light. Controlling the positive or negative potential on the grid and anode creates the desired characters or segments. Because VFD technology is based on self-emitting light, it does not depend on backlighting or other external light sources.

A typical graphics vacuum fluorescent display.

VFDs can use a number of different color phosphors, including blue, green, neo green, lemon, amber, mandarin, and red. The most popular color is blue-green because of its compatibility with a broad range of color optical filters, making possible a wide choice of display colors to enhance the cosmetic appearance of both the display and the electronic device. Also, the blue-green phosphor is much brighter given the same power consumption.

Chip-in-glass VFD (foreground) features fewer lead pins than standard modules (back) for easier mounting.

Tube or Module. Typically, VFDs can be purchased as a tube alone or premounted in a complete module, including all electronics needed to illuminate the tube. The medical device supplies 5 V power and sends data signals to the module. Both glass tubes and modules can be purchased as standard or custom-designed components.

Current VFDs are more flexible than previous models, offering features that reduce power, reduce the size of the envelope, and reduce the number of pin lead-outs as well as increase brightness and durability. In addition, improved technology has reduced costs for most applications.

Figure 1. Structural scheme of BD vacuum fluorescent display.

Figure 2. A 16 x 16–dot phosphor matrix built on top of a semiconductor chip that integrates memory functions and display driver circuits. These chips are arranged in a single or double stage format forming a highly precise graphic display.

Thinner Chip-in-Glass Design. A VFD was recently introduced that uses thin chips hidden inside the vacuum tube. The new technology, called chip-in-glass, requires very few lead-outs, making assembly easier and less costly. In addition, drivers come as part of the tube, so manufacturers no longer need to resource and stock them. This is particularly important when the space for a display is small and crowded with other components.

Active Matrix. Another VFD technology uses an array of chips, each having a 16 X 16 format of phosphor dots printed directly on the chip. The characters or graphic messages are created by the assignment of electrons to the appropriate phosphor dots on each chip. This technology provides higher resolution and higher brightness than is possible with conventional construction. In addition, active-matrix technology offers bright, clear illumination of smooth-flowing graphics and can reproduce characters from any language (see Figures 1 and 2).

VFD tubes can be purchased alone, which is the lowest-cost option. Tubes are available as standard models or may be custom designed to OEM specifications. The power supply, drivers, and voltage converters are designed into the device's PC board.

VFDs may also be purchased as full modules, either standard or custom designed. Modules include drivers, voltage converters and, in certain models, a built-in character generator. Only a 5-V current and the data signals are sent to the VFD module. Liquid crystal displays can be converted to VFD modules. Drop-in replacements with the same hole-to-hole mountings, voltage, and software requirements are available.