Electronic Calculator Displays
An Overview of Technology

INCANDESCENT FILAMENT DISPLAY

An Incandescent Filament display is usually housed in a vacuum tube like the either the Nixie tube or the early Vacuum Fluorescent tubes. This display is typically a seven segment style of display where each display segment is formed with a conductive anode tungsten filament. A small voltage placed across a filament will cause it to heat to incandescence. They emit a yellowish-white light that can be filtered to any desired color. The filament voltage (3-5vdc) can also be varied to change the brightness level of the display. The biggest problem with Incandescent displays is they have a slow response time and they consume a large amount of current. A popular version of this type of display was the RCA Numitron. Some early electronic kits used the Incandescent Filament display.

GAS DISCHARGE or PLASMA DISPLAY

A Planar Gas Discharge or Plasma Display Panels (PDP) display utilizes the same principle the Nixie tube does. It's construction consists of sandwiching a hollow center layer filled with neon and a small amount of mercury between a glass front and a ceramic back. A thick conductive paint forms the Cathodes on the inside of the ceramic back. The Cathodes form the segments of each digit. Each digit is covered by a separate Anode that is deposited on the inside of the glass front. The Anodes are formed from a thin transparent layer of tin oxide. When a sufficient voltage is applied between a cathode segment and it's anode, the gas around the cathode segment breaks down and begins to glow. Like the Nixie tube, the digits glow with a orange-red color. Voltage requirements for these displays are typically 180-200 volts DC).

Burroughs manufactured a common brand of PDPs called Panaplex II. PDPs were used in many early calculators including CompuCorp's Scientist series.

VACUUM FLUORESCENT DISPLAY MODULE

The Vacuum Fluorescent display (VFD) consists of a vacuum tube in which there are three basic types of electrodes, the filament (cathode), the anode (segment), and the grid. The VFD is essentially a small Cathode Ray Tube. The filament (or filaments) is a very fine wire that is heated to a temperature just below incandescence. At that temperature it remains virtually invisible but it emits electrons. A transparent metal mesh grid covers each digit and controls the electrons emitted from the filament toward that digit. Seven phosphor coated anodes, arranged in the seven-segment configuration (that form a square eight), glow when struck by the electrons. When a positive voltage of 12 to 25 volts is applied to the grid and the anodes, the electrons emitted by the cathode filament are accelerated and attracted to the positive anode segments which in-turn glow. If the grid has a negative potential then it will block the electrons from passing regardless of the potential of the anodes under the grid.

VACUUM FLUORESCENT DISPLAY TUBE

VFDs were developed in Japan in 1967. Early versions of VFDs were individual digits housed in vacuum tubes like the Nixie tube and Incandescent Filament displays. VFD Phosphors can be formulated to emit red, yellow, and green as well as the more common blue-green color. Later versions would house all of the digits (and other graphics and indicators) in one large glass assembly. Currently VCRs account for 30% of the VFD market and Audio/Video products account for another 30%. Many early series of calculators like the Commodore 412F, Brother 310, and the MITS 816 used the individual digit VFD tubes. Later manufacturers such as TI and Rockwell used the integrated multidigit VFDs in both handhelds and desktops.

ELECTROLUMINESCENT DISPLAY

Thin-film Electroluminescent Displays (ELDs) use a thin film of phosphor (zinc sulfide (ZnS); ZnSe; ZnSMn or other fluorescent materials) sandwiched between a dielectric layer that is sandwiched between two glass plates. Transparent electrodes (tin-oxide) are deposited on the insides of the glass plates. When a sufficient AC voltage (>100 volts) is applied to any of these electrodes the phosphors will be excited and will emit light. ELD phosphors can be mixed with pigments to emit many colors of light including green, blue-green, lemon-yellow, orange, red as well as white light.

This type of solid state display can endure extreme conditions with exceptional tolerance to shock, vibration, temperature, and humidity, while response times remain less than one millisecond. I have not seen ELDs used in calculators but they are used in some laptops, office machines and in the cockpit of the Spaceshuttle. They are also used to backlight LCD panels.

LED DISPLAYS

A Light Emitting Diode (LED) is an special type of diode that emits light when electricity applied to it's anode and cathode. A typical LED requires about 1 1/2 volts DC at 10 milliamps to begin emitting light. LEDs usually produce red light but yellow, green and blue versions are also now available. The LED was first marketed by Texas Instruments around 1962. LED displays (7 or more individual LEDs) were introduced around 1967 but were very expensive. Calculators used LEDs that were arranged to form either a seven-segment display or a dot-matrix display.

Early seven segment displays formed each segment with many LEDs, later seven-segment displays would use one LED per segment with a light pipe to spread it's light across the segment. Also early LED displays were made small in order to keep power consumption down. A clear plastic bubble lens was fabricated into the package to magnify the display for easier viewing.

NUMERIC REPRESENTATION

The dot-matrix style of display would form characters shaped similarly to that of a dot-matrix printer. A dot matrix of 4x7 or 5x7 is typically used. Notice how the 4x7 matrix makes up for the missing 5th column by slightly slanting the columns. LEDs require much more power than LCDs and are more expensive to manufacture. This is the simple reason for their demise from being used in calculators.

LIQUID CRYSTAL DISPLAY