Calculator Technology

Home > Calculator Technology

Calculator Displays

The type of display used in a calculator depended on the technology available at the time, the cost of the display, the power consumption of the display if being used in a portable machine, and the legibility of the display.

Early 1960s - (1) Cold-cathode numerical display tubes, such as the "Nixie" tube, (2) cathode ray tubes (CRTs), and (3) incandescent filament lamps were the only display technologies available.

About 1967 - (4) Vacuum fluorescent display tubes were pioneered by Sharp.

1971 - (5) Light Emitting Diodes (LEDs) first used in a commercial calculator - the Busicom LE-120 "Handy".

1971 - (6) First-generation Liquid Crystal Displays (LCDs) used in a commercial calculator - Rockwell.

1973 - (7) Second-generation Liquid Crystal Displays (LCDs) introduced.

 

On this page, click on an item in the list below to go to that section

 

See also the excellent article by Rick Furr "Electronic Displays - An Overview of Technology" in the Collecting Calculators section of this site.

1) Cold Cathode Numerical Display Tubes
Often referred to as "Nixie tubes"

Cold Cathode Tube Display

The cold-cathode display tubes of an Anita 1011LSI calculator in use. Note also the small neon lamps used to indicate the decimal point (the third from the right is energised).

Cold-cathode display tubes were developed in the early 1950s and were used in the first electronic desktop calculator, the Anita Mk VII of 1961. Requiring high voltages and having a high power consumption they continued to be used into the early 1970s in AC powered calculators. Their use in battery powered calculators is rare, see Anita 1011B LSI.

The cold-cathode display tube and the neon lamp operate on the same principal.
The simple neon indicator lamp consists of a sealed glass tube with two closely spaced non-heated electrodes, an anode and a cathode. The tube contains neon gas at very low pressure together with a small percentage of mercury vapour. They are often used in AC power sockets to indicate that they are switched on.
The gas in the tube initially acts as an insulator and as the voltage between the anode and cathode is increased practically no current flows. However, a little of the gas in the tube will be ionised by naturally occurring cosmic rays, radioactivity, or ambient light into positive ions and negative electrons. The electric field being applied to the electrodes will cause the positive ions to move towards the cathode and the electrons towards the anode. As the applied voltage is increased so these will start to move faster and faster, colliding with other gas atoms and ionising them by knocking off electrons.
At the "striking" or "ignition" voltage the ions and electrons move which such high speeds that an avalanche of further ions and electrons is produced by all the collisions. This leads to a high current flow, which is kept in check by an external resistor in series, and an amber glow at the cathode.

The cold-cathode display tube is a neon lamp with multiple cathodes. Each cathode is shaped like one of the digits 1 to 9, and they are mounted in a closely spaced stack.
In front of the stack is the anode, formed from an open mesh grid visible in the photograph above. When the striking voltage is applied between the anode grid and any of the cathodes a discharge is formed and the gas around the cathode glows. Since the cathode is shaped like a digit the glow is also in the shape of that digit - see the photographs.
In use it can be seen that the numbers are in a stack since some numbers appear further forward in the tube than other numbers.

The life performance of a numerical display tube depends to a great extent on the length of time the discharge is maintained on a single cathode (ie. number). This is because in any gas-discharge device the cathode is subjected to constant ion bombardment which removes material from the cathode and deposits it elsewhere in the tube. This "sputtering" is unavoidable, but is limited by keeping the peak current as low as possible, consistent with visibility of the display.
If a display tube is kept with one cathode constantly glowing (ie. one number displayed all the time) then material is sputtered from that cathode. This only affects the glow of that cathode a little, but the sputtered material lands on the other cathodes and affects the current required to make them glow, and can lead to uneven illumination.
If the discharge is cycled between characters regularly, this gives a much improved life since each cathode, although receiving some sputtered material, is subjected to the cleansing action of bombardment.
The normal figures quoted in published data for the life of a numerical indicator tube is 5,000 hours with a continuous display of one character, and 30,000 hours when sequentially changing from one digit to the next every 100 hours or less.

Cold-cathode numerical display tubes are often called "Nixie" tubes though this was a trade name of Burroughs Corporation which was an early developer of this technology. Other names used are Pixie tube and Numicator tube.
For more information see Tom Jenning's "Nixie Indicators and Decimal Counting Tubes" pages at http://www.wps.com/projects/decimal-tubes/.

Several Sanyo models of the early 1970s use a type of discharge tube, shown above and below, which at first sight appear to be filament lamps. However the shiny wires are actually electrodes, with the surrounding black metal being the other electrode. These operate in the same way as small neon lamps, so with an applied voltage of about 60v an amber discharge is generated around the energised wire electrode, as shown below.

The cost of a Burroughs Nixie tube in 1971 was about $2 for lots of 10,000, which made them very competitive. However their size and high power and voltage requirements were disadvantages.
Around this time Burroughs introduced the "Panaplex" display providing multiple digits in a single planar tube, see below. Also based on cold-cathode technology, it used the familiar 7-segments to produce each digit, and required less manual assembly during manufacture and so was cheaper per digit. It also made more efficient use of space so that more digits could be packed into a smaller size. Although more common in desktop calculators the Panaplex was also used in some hand-held calculators. See the Compucorp 324G and Keystone 88.

Above is a Burroughs Panaplex display in use in a Keystone 88 hand-held calculator of about 1974. The digits are larger than those of LEDs of the time.

The sandwiched glass plates of a Panaplex display. The rear has a sealed glass nipple where the vacuum was achieved during manufacture.
This photograph illustrates that depending on how the ambient light falls on an unlit Pananplex display it characteristically shows the individual 7 segments of each digit or the cell of each digit.

Above is another, less common, amber gas-discharge display. This example is made by NEC (Nippon Electric Company) and is in a Sanyo ICC-809 hand-held calculator.

2) Cathode Ray Tube (CRT) Displays

Friden EC-132 CRT calculator

A Friden EC-132 calculator showing the Cathode Ray Tube (CRT) display and 4 lines of a calculation.

 

The cathode ray tube has been in use since the 1920s and was used in televisions, radar displays, and oscilloscopes. Its first use in a desktop calculator was in the Friden EC-130 (1963) and EC-132 (with square root).
Although they can display several lines of a calculation they are bulky and have high power requirements, which restricted their use to a few AC powered desktop calculators of the mid to late 1960s.

The four lines on the crt of an EC-132.

CRT in Friden EC-132

The photograph above shows the typical bottle shape of the early instrument type CRT and its metal shielding casing. Anyone who has seen inside a modern television or computer monitor (danger - very high voltages present when in use) will have seen the much fatter, squatter versions which are still used.

On the right of the tube is the electron gun, where a heated filament produces a cloud of electrons. These are focused into a beam and accelerated towards the fluorescent display screen on the left by an anode with a high positive voltage. The beam is deflected to write the electron beam on the fluorescent display and so produce the visible numbers.

These CRT display tubes were used in a few calculators of the mid to late 1960s, including Friden EC-130, Friden EC-132, Hewlett Packard HP9100A.

3) Incandescent Filament Lamp Displays

Incandescent filament lamps were employed in two different methods

Method 1 - Seven-segment displays.
Each digit makes use of 7 separate filaments arranged in the familiar pattern so that all numbers 0 to 9 can be displayed. Very few calculators used this type of display which can easily be confused with the amber-coloured cold cathode numerical display tubes shown above in Section 1.

Method 2 - "Light-Pipe" Display Modules.
Each digit consists of a stack of clear, flat plastic sheets each with one digit (0 to 9) inscribed. When a sheet is illuminated at its end by a small filament lamp the light is piped along the sheet and is scattered by the engraved number which then can be seen quite brightly.

Canon Canola 130S display

The "light-pipe" display modules of a Canon Canola 130S, from about 1968.

“light-pipe” display module “light-pipe” display module

Different numbers being displayed by the "light-pipe" display modules of a Canon Canola 130S.

The photographs below are of a similar, though larger, "light-pipe" display module to those in the Canon Canola 130S, which are of a more compact design but work in an identical way.

Light pipes

Removing the cover reveals the stack of plastic light-pipe sheets, one for each number 0 to 9 in this module. Decimal points sheets could also be fitted.
Each sheet carries its number marked out in an array of conical pits in its surface. When a light is shone into the edge of the short side of a sheet the light is piped round the corner, as with fibre optics, and illuminates the pits and so the number is seen.

Lamps

The bottom of the module can be removed to allow replacement of the tiny filament lamps. There is one lamp to illuminate each "light-pipe" sheet.

These "light-pipe" numerical display modules only require the low voltage drive of the filament lamps. But the lamps have the disadvantages of high power consumption (though not much of a problem in an AC-powered calculator), short operating life, and a slow response. They were only used in a handful of AC powered desk calculators in the late 1960s early 1970s.

4) Vacuum Fluorescent Displays (VFD)

Vacuum fluorescent displays in Japanese calculators made in the late 1960s & early 1970s often have stylised digits and half-height zeros. This one shows 1234567.0. The half-height zeros make the display more readable when the calculator has no leading-zero suppression.

Vacuum Fluorescent Displays were developed in Japan jointly by Hayakawa (Sharp) and the Ise Electronics Co. These "Digitron" tubes were used first in the Sharp Compet CS-16A, launched at the end of 1967. These tubes had stylised digits, as shown above, which are often found in Japanese calculators of the late 1960s and early 1970s.
"Electronics" journal explained: "For the desk calculator, the digit patterns of the tubes were redesigned for better readability. The "0", for example, has only half the height of other digits. That way, the string of "0's" before the first significant number in the display is no longer a nuisance and there's no need to blank them out."


In June 1967 "Electronics" reported that Japanese calculator manufacturers were battling the high royalties that Burroughs Corporation was asking for them to produce copies of its Nixie tubes. This was one of the reasons for them developing the vacuum fluorescent display.
Their use continues to this day in calculators, video recorders, Hi-Fi systems, and other equipment where the display glows. These displays are quite bright and their power/voltage requirements are moderate so they were used widely in AC and battery powered calculators, but are now confined almost exclusively to AC calculators.

1st generation VFD display

First generation vacuum fluorescent display (VFD) using a separate tube for each digit. Each tube shows the fluorescent anodes arranged in the standard 7 segments, together with a decimal point, and an extra short segment so that "4"s look better.

Vacuum Fluorescent displays (VFDs) can be considered to be flattened cathode ray tubes (see section above).
In the original design each digit of a display required a separate tube. At the front two fine parallel wires stretching the height of the tubes form the cathode and are heated just to the point where they emit electrons but do not glow. Typically there are seven anodes arranged in the typical 7-segment pattern so that all numbers 0 to 9 can be generated.
By applying a voltage between any anodes and the cathode electrons are attracted to the anode and to accelerate high speed. The anodes are coated with a fluorescent coating which glows brightly when struck by the electrons. The colour produced is typically green or blue, though modern displays for Hi-Fi systems produce other colours such as white, red, yellow.
Between the anode and cathode is an open grid. Applying a negative voltage to the grid switches the digit off completely.

Originally each digit of the display required a separate display tube - these were used in both AC and battery powered models.
The next development was to reduce costs by squeezing all the digits into one long horizontal tube. The final developments were to do away with the tube and pack everything into a flattened glass package.

2nd generation VFD display

Second generation vacuum fluorescent display with all the digits in a single round tube.

3rd generation VFD display

Third generation vacuum fluorescent display with the digit assembly sandwiched between bonded glass plates.

4th generation VFD display

Fourth generation vacuum fluorescent display in a flattened package made by welding a domed piece of glass onto a flat piece.

Modern VFD

Vacuum fluorescent displays have been developed much further as can be seen in the photograph above of the display from a modern hi-fi sound system.

5) Light Emitting Diode (LED) Displays

LED display

An LED display with its intense red colour.

The LED (Light Emitting Diode) display appeared commercially in the late 1960s. American Calculator Corp., of Dallas, announced the first use of LED displays in a calculator in late 1970. "Electronics" journal stated that it "employs eight Monsanto gallium arsenide phosphide light emmitting diodes in its display". However, in April 1971 it was announced that the company had gone bankrupt, so it may never have sold any commercially. The first calculator with LED display to be successfully marketed was the small Busicom LE-120 "Handy" in early 1971.

Being based on semiconductor materials it is very compatible with calculator integrated circuits and has a moderately low power consumption.
The initial problems were high cost and small size. With large scale production the price rapidly reduced. The small size was alleviated by placing moulded plastic magnifying lenses in front, as can be seen below, however this gives a narrow viewing angle. Due to the small size of LED displays they were very rarely used in desktop calculators of the 1970s.

The LED eventually lost out to the Liquid Crystal Display (LCD, see below) which has a much lower power consumption (it is passive and does not emit light) and has a larger size at little extra cost.

Early 8-digit LED display in a Commodore Minuteman 2 using individual 7-segment array modules. The ninth module on the far left provides "-" sign and overflow indication.

Bare LED display

Early 8-digit LED display. This has nine bare 7-segment array LED chip dice mounted on two carriers, and does not use magnifying lenses. The die on the far left provides "-" sign and overflow indication.

LED display

LED module showing the typical plastic moulding with magnifying lenses.

LED display

LED module showing the 7-segment LED arrays viewed through the magnifying lenses.

LED display

LED module showing the number 12345678 being displayed. The array on the left is used for displaying the minus sign and other characters, such as to indicate overflow.

6) Liquid Crystal Displays (LCDs)

1st generation LCD display

First Generation LCD with silvery digits against a dark background.

Liquid Crystal Displays (LCDs) were developed in the late 1960s and early 1970s. Thomson-CSF of France was one company involved in their development and demonstrated a calculator with a 16-digit LCD in early 1971 (photograph in "Electronics", May 24 1971). However, they were selling the display at a price of $10 per digit at that time so it would have been expensive and was probably not sold commercially. Busicom announced a LCD version of the LE-120 (itself the first successful LED calculator) in 1971, but it too appears to have never been sold commercially.

The first successful use of LCD displays in calculators appears to have been in models made by Rockwell for Lloyds (Accumatic 100), Rapid Data (Rapidman 1208LC), and Sears in 1972. These use displays which operate in the transmissive mode and require a backlight, so removing much of the advantage of low power consumption.

COS

Sharp were also pioneers in calculators using LCD displays, and in 1973 introduced their COS (Crystal On Substrate or Calculator On Substrate) technology in the Sharp EL-805.

The true COS calculator has a circuit board which appears to be made of glass, as shown on the left, viewed from the rear of the calculator. The LCD display is formed directly on this circuit board, which also carries at least two layers of conductive tracks, separated by a white, insulating lacquer, and the electronic components. The user actually looks through the circuit board when viewing the display. Note that the glass circuit board has no holes in it, all of the components being surface-mount types.

The use of the glass circuit board was a dead-end in the development of calculators. Subsequent models from Sharp with this type of display have conventional circuit boards, though their LCD display modules have a similar construction to the display section on the glass circuit boards.

 

LCDs have the great advantage of very low power consumption since they are passive displays, altering the reflection of ambient light rather than actively generating light.

There was a lot of discussion about the stability of the early liquid crystal material, which was justified since many calculators of the period with this type of display no longer show the numbers properly.

2nd generation LCD display

Second Generation LCD with black digits and a yellow background - the yellow is actually a filter in front of the display to absorb damaging Ultra Violet light and prolong the life of the liquid crystal material.

Advances in the liquid crystal material greatly improved its stability, removing the need for the yellow, ultra-violet absorbing filter. These third generation LCDs are used for the displays of modern hand-held calculators and in conjunction with modern integrated circuit techniques result in calculators running for years on one button cell or just on solar power.

3rd generation LCD display

Third generation LCD with black digits on grey background.

7) The Display War

The display technology used in the 1960s was overwhelmingly the cold-cathode discharge numeric display tube, typified by the Burroughs "Nixie" tube, with its amber colour. By 1971 tubes of this type could be bought in quantity for US$1-to-$2-per-digit 1.

A few calculators, mostly from Friden, used cathode ray tubes (CRTs) which could economically give several lines of digits.

In the late 1960s vacuum fluorescent displays (VFDs), with a green/blue colour, were pioneered by Sharp. Having a reduced power/voltage requirement and a bright display these took over from the cold-cathode tube, especially among Japanese manufacturers, in the 1970s.They were also widely used in hand-held calculators till the late 1970s, and they remain the only light-emitting displays still used, in some AC powered desktop calculators.

Light emitting diode (LED) displays were introduced commercially in 1967, but were initially very expensive, costing about US$60 a digit. By 1971, in quantities of 1,000, 1/8 inch high LED dispplays could be bought for US$3.95 each 1.
Although more expensive than the numeric display tube the LED had the advantages of small size, low voltage, and lower power consumption, which made it very suitable for the newly appearing miniature pocket calculators. Although expensive at first, the price of LEDs soon dropped as production quantities increased and competitors entered the market. Within a year or two of their introduction in calculators in 1971 they were used extensively in hand-held calculatorafter till the late 1970s when they were largely replaced by liquid crystal displays (LCDs).

All of the light emitting displays have the disadvantage that they are difficult to read in bright ambient light. They must also use energy to generate light, but power consumption could often be reduced by pulsing them, which could give the same apparent brightness at a lower average current.

The liquid crystal display (LCD), also introduced in calculators in 1971, required a less technical production environment and cheaper materials than the LED, and so could be made much cheaper. It also had tiny power requirements, and being reflective was easily readable at all normal office lighting levels and in full sunlight. However, early manufacturing problems, the slow response speed of early liquid crystals, and concerns about the life and temperature stability of the liquid crystal material held up its wide acceptance till the mid 1970s. Then there was no stopping the LCD and by 1978/9 it dominated the calculator market and allowed credit card-sized calculators to be produced.

1) Electronics, May 1971.

8) Identifying the display type from its colour

A) If the digits glow -

  • Intense red -> Light Emitting Diodes (LED).
  • Delicate Amber -> Gas Discharge (Nixie, Panaplex, etc).
  • White -> Filament lamp/"Light-Pipe".
  • Bright green -> Vacuum Fluorescent (VFD).
  • Bright blue -> Vacuum Fluorescent (VFD).

There is no known calculator with a green LED display - green LEDs were developed after the red variety and were more expensive.

LED displays were very rarely used in desktop calculators.

 

B) If the digits do not glow then the calculator has a Liquid Crystal Display (LCD).

 

See also the excellent article by Rick Furr "Electronic Displays - An Overview of Technology" in the Collecting Calculators section of this site.

Home
1) Mechanical Calculators
2) Desk Electronic Calculators
3) Hand-held Calculators
4) Non-Decimal Calculators
5) The Pocket Calculator Race
6) Calculator Companies
7) Calculator Photo Library
8) Calculator Articles
9) British Calculator Companies
10) Puzzle Corner
11) Calculator Time-line
12) Calculator Technology
13) Index to the Calculators
14) Calculator Resources
15) Frequently Asked Questions
16) Update Information
17) Vintage Calculator News
18) Bletchley Park
19) About this site
20) Search this Site

Calculator Technology

Calculator Mechanisms
Calculator Electronics
Calculator Displays
Calculator Keyboard Layouts

Vintage Calculators

© Text & photographs copyright Nigel Tout   2000-2014 except where noted otherwise.