When we think of displays these days, the first technologies that come to mind are LCDs, LEDs and organic LEDs (OLEDs), and touchscreens. These components, though not new, proliferate and evolve because of a number of factors.
Constant research and development in the electrical, mechanical, and chemical/materials disciplines uncovers ways to make these displays cheaper, more functional and energy efficient, and easier to integrate into new and existing designs. Incessant R&D also discovers unique design possibilities, by chance or intent, as well as commercially viable aesthetics.
Yet vacuum fluorescent display (VFD) technology, which dates back to the late 1960s, retains a solid foothold in many markets. Relatively lower in cost, VFDs exhibit high brightness, wide viewing angles, and ease of configuration. They also operate reliably across a wide temperature range. Typical applications include VCRs, audio equipment, and, based on their low cost, medical devices (Fig. 1).
INSIDE VFDs Tadashi Nakamura of Noritake Itron Corp. takes credit for the invention of VFD technology. The company’s research and development of VFDs began circa 1966 with single-digit components under production the following year, licensed to NEC.Noritake vice president Albert Smith describes VFDs as variations of the triode vacuum tube: a cathode, grid, and anode vacuum-sealed in a glass tube. When heated, the cathode emits electrons, which then race toward the positively charged grid and anode.
The electrons collide with the phosphor on the anode, causing the emission of bright light. The creation of specific characters and segments rests with the control of positive or negative potentials on the grid and anode. And since VFDs emit light, there’s no need for backlighting.
For the anode, color phosphors such as blue, green, neo green, lemon, amber, mandarin, and red are viable candidates. However, due to compatibility with a wider array of optical filters, blue-green is the more common choice.
Typical VFD modules operate from a 5-V ±5% dc supply with current consumption dependant on the particular component. They are compatible with transistor-transistor level (TTL) or CMOS I/O signals and deliver a typical luminance of 700 cd/m2 with a total viewing angle of 140°, mean time before failure (MTBF) of 80,000 to 100,000 hours (300,000 hours with design modifications), and a typical brightness degradation of 50,000 hours to half brightness.
ON THE VFD BANDWAGON Offering both standard and custom VFD modules, Densitron Corp. also sees opportunities for the technology (Fig. 2). “Character and graphic VFDs are still finding excellent adoption in select markets such as pro audio, rack instruments, point of sale, and automotive. They are still popular in white goods,” says Greg Hayes, general manager at Densitron.“Much of the appeal is the outstanding brightness of VFDs and their extraordinary viewing angle. Both white goods and automotive designers are also keenly interested in the wide operating temperature characteristics of VFD technology. It lends itself to very wide aspect ratio displays. Their inherent brightness and viewing angle make competing with less distinctive LED and LCD character modules quite attractive,” he says.
“The current mature price point of VFDs is now comparable to these less compelling display technologies, making them a strong candidate when ultrawide formats are highly desirable such as 1U and 2U rack applications. Also, the cost to custom tool VFDs is considerably less than OLEDs or TFTs and therefore more attractive to mid-market volume applications,” Hayes says.