Vacuum fluorescent displays (VFD) are voltage-controlled devices that provide high brightness and wide viewing angles. These relatively low-cost displays consist of a cathode, grid and anode sealed in a high-vacuum glass envelope. Heating the cathode provides the electrons with energy for emission. If both the grid and the anode are at a positive potential with respect to the cathode, the emitted electrons pass through the grid and onto the anode. Typically, the anode terminal is coated with a fluorescent material that emits light when bombarded by electrons. The anode areas correspond to individual segments of the display, and the grid is used to switch on and off whole digits. Brightness is a function of the voltage difference between the cathode and the anode. Vacuum fluorescent displays require high-voltage drivers that are capable of large output voltage swings between the on and off conditions. Most devices can produce output swings of at least 50 V and use metal oxide semiconductor (MOS) and bipolar technologies.
Types of Vacuum Fluorescent Displays
There are two basic types of vacuum fluorescent displays: alpha-numeric and graphic. Alpha-numeric devices display letters and numbers in a sixteen-segment or 5 in. x 7 in. dot matrix display. Graphic devices display numbers and visual elements. Display colors for vacuum fluorescent displays vary by wavelength and are measured in nanometers (nm). Color choices include blue (475 nm), green (510 nm), yellow (570 nm), orange (620 nm) and red (650 nm). Blue-green is the most popular display color because of its compatibility with a wide range of optical filters. Operating temperature, optical characteristics, and external dimensions are additional specifications to consider. Optical characteristics include the number of rows, number of characters per row, viewing area width, viewing area height, and luminance or brightness. Width, height and depth are important external dimensions.
Vacuum fluorescent displays vary in terms of features. Some devices include a dedicated, on-board microcontroller that acts as an interface between a central processing unit (CPU) and row and column drivers. The microcontroller generates characters, refreshes the display, and performs other related functions. Vacuum fluorescent displays with a keypad or serial interface such as RS232 or universal serial bus (USB) are also available. Vacuum fluorescent displays do not require backlighting, but often offer it as an option. Electro-luminescent backlighting is powered by high-voltage, high-frequency alternating current (AC) and provides a cool glow. Fluorescent backlighting is not suitable for environments with relatively high amounts of ambient light.