From moving mechanisms to glowing segments and electronic elegance.
July 10, 2026
At a Glance
- The segmented-display concept is far older than most people realize.
- Engineers spent decades debating how many segments were enough.
- VFDs became some of the most recognizable displays of the electronic age.
As a brief reminder before we hurl ourselves into the fray with gusto and abandon (and aplomb, of course), in Part 1, we perused and pondered the use of analog meters; in Part 2, we turned our attention to digital displays implemented using a variety of electromechanical technologies; and in Part 3, we contemplated large-scale electromechanical displays (scoreboards, split-flap, flip-card, flip-dot).
Rotating-prism displays (1970s onward)
Just to close out the topic of electromechanical display technologies, it behooves us to at least make mention of rotating-prism displays and their various cousins.
Where split-flap displays present information one character at a time, and flip-dot displays introduce the concept of mechanical pixels, rotating-prism displays occupy an interesting middle ground. Instead of flipping cards or dots, these systems employ triangular or polygonal prisms mounted side by side in long rows. Each face of a prism may carry a different color, symbol, letter, number, or pattern. Small electric motors, solenoids, or electromagnetic actuators rotate the prisms into position, allowing the display to change its appearance under electronic control.
One common implementation employs three-sided prisms, each presenting a different colored face. Arrays of these prisms can create large information displays visible from considerable distances. Other systems employ four-sided prisms, rotating cubes, cylindrical drums, and other geometric variants. Like their flip-dot cousins, many of these displays are bistable, consuming power only while changing state. They have proven particularly popular in transportation systems, industrial facilities, parking guidance systems, and roadside information signs, where their excellent visibility and low power consumption make them attractive alternatives to early electronic displays.
Although rotating-prism displays never achieved the iconic status of split-flap departure boards or flip-dot information panels, they represent one of the last major evolutionary branches of electromechanical display technology. By the 1980s and 1990s, advances in LEDs and other electronic display technologies rendered many electromechanical solutions increasingly unnecessary. Even so, there remains something oddly satisfying about watching physical objects rotate into place to convey information. In many ways, these devices embody the same engineering philosophy we have encountered throughout this series: when electronics cannot yet do the job economically, engineers simply persuade gears, springs, flaps, prisms, and other moving bits and bobs to do the heavy lifting.
Segment displays: From incandescent lamps to VFDs
Having now brought our tour of electromechanical display technologies to a close, it's time to turn our attention to a different family of devices: segment displays. Unlike flip-dots, rotating prisms, or split-flap mechanisms, which create information by physically moving parts into position, segment displays form characters by selectively illuminating predefined shapes called segments.
The idea is elegantly simple. A digit such as "8" can be broken down into a collection of straight lines. By turning different combinations of those lines on and off, the display can represent any numeral from 0 to 9. The same concept can be extended to letters, punctuation marks, and special symbols by increasing the number of available segments.
Although most people immediately think of the ubiquitous 7-segment light-emitting diode (LED) displays found in clocks, calculators, and test equipment, the segmented-display concept is far older than many realize. In fact, engineers were experimenting with segmented displays decades before the invention of the transistor, let alone the LED. Some of these early implementations were themselves electromechanical, employing cams, linkages, shutters, rotating vanes, and solenoids to assemble numerals from independently controlled display elements. As we shall see, the transition from electromechanical segmented displays to their electronic descendants was more evolutionary than revolutionary.
Early incandescent segmented displays
One of the earliest known segmented displays dates back to the late nineteenth century. In 1898, George Lafayette Mason filed a patent for a 21-segment display whose segments could be used to form both letters and numbers. Rather than using LEDs (which, of course, hadn’t been invented at that time), each segment was illuminated by its own miniature incandescent lamp. In some systems, rotating camshafts, switches, and electromechanical mechanisms selected which segments were energized, allowing the display to present changing alphanumeric information.
Viewed from a modern perspective, these displays appear surprisingly familiar. The underlying principle is essentially identical to that employed by later electronic technologies; only the methods of illumination and control differ. This is yet another reminder that many of the ideas we associate with modern electronics were actually conceived long before the electronics themselves existed.
Why 7 segments?
When displaying decimal digits, engineers eventually discovered that seven carefully arranged segments represent a sweet spot between simplicity and readability. Fewer segments make certain digits difficult to distinguish, while additional segments increase complexity without providing significant benefits for purely numeric applications.
Using 7 segments. CLIVE “MAX” MAXFIELD
The familiar arrangement consists of three horizontal segments, four vertical segments, and enough combinations to represent the digits 0 through 9. Although the resulting numerals are somewhat stylized, they are immediately recognizable. This balance of low component count, ease of manufacture, and good readability explains why 7-segment displays remain popular today, even in an age dominated by graphical displays.
Additional segment variants
Of course, displaying letters is more challenging than displaying numbers. While seven segments are adequate for decimal digits, they struggle to represent the full alphabet. As a result, engineers have experimented with a variety of more elaborate arrangements.
From left to right, 7, 9, 14, and 16 segments. ERRORAGE, CC0, via Wikimedia Commons
Nine-segment displays offered modest improvements, but 14-segment and 16-segment configurations eventually became particularly popular because they could represent both uppercase letters and numerals with reasonable legibility. Such displays were widely used in instruments, terminals, calculators, communication equipment, and consumer electronics.
As is so often the case in engineering, the "best" solution depended on the application. More segments improved appearance and flexibility, but they also increased cost, power consumption, and circuit complexity.
Panaplex displays
One particularly interesting branch of the segmented-display family emerged in the 1970s in the form of Panaplex displays. Rather than using incandescent lamps, these devices employed neon-based gas-discharge technology. Each segment formed a separate discharge region that glowed with the characteristic orange color associated with neon illumination.
Panaplex displays occupied an interesting middle ground between the digit-at-a-time Nixie tubes (which we will introduce in the next installment of this mega-mini-series) and later segmented electronic displays. They retained much of the visual charm of earlier gas-discharge technologies while offering the flexibility and efficiency associated with segmented architectures. As a result, they found use in calculators, instruments, industrial equipment, and a variety of consumer products before eventually being overtaken by LEDs and LCDs.
VFD displays
Among all the segmented-display technologies developed during the twentieth century, vacuum fluorescent displays (VFDs) arguably represent one of the most successful and visually distinctive. These devices employ heated filaments that emit electrons, control grids that select individual digits, and phosphor-coated segments that glow when struck by the electron stream. The resulting display is typically a brilliant blue-green color that remains instantly recognizable today.
VFD implementation of a 7-segment display. OSKAR (https://display-tubes.org/vfd/reflector-iv-12
VFDs became enormously popular from the 1970s onward, appearing in everything from video cassette recorders and microwave ovens to audio equipment, cash registers, automotive dashboards, and laboratory instruments. Compared to contemporary Nixie tubes and gas-discharge displays, VFDs required substantially lower operating voltages, offered excellent brightness, and could readily support complex segmented arrangements (we’ll discuss all these devices in greater detail in future columns).
For many engineers and hobbyists, VFDs represent the perfect blend of old and new. As I may have mentioned once or twice (or several dozen times), I've recently been playing with VFDs in some of my own hobby projects, which may explain why I find these glowing little scamps so appealing.
VFDs possess the unmistakable glow and visual character of vacuum-tube technology while delivering the flexibility and practicality of a modern electronic display. Indeed, their enduring popularity among retro-computing enthusiasts and hobbyists demonstrates that good display technology never truly goes out of style.
Next time
As fascinating as segmented displays may be, we still have a few more weird and wonderful display technologies lurking in the wings. Next time, we'll turn our attention to some of the most beloved retro displays, including Nixie tubes and their lesser-known cousins. Along the way, we'll encounter a menagerie of ingenious, quirky, and occasionally bonkers technologies before concluding our journey with a glimpse of the display technologies that may shape our future. Until next time, as always, please feel free to email me at [email protected] with any comments, questions, or suggestions.