Displaying readings on an analog scale, where the digital characters are printed on the scale in the different locations that the pointer swings is not an issue. But, putting digits on a digital readout where different digits must share the same location, is more difficult. Three ways have been used: the solid character, the dot matrix, and the bar segments. The solid character can be the most pleasing way of displaying digits, but it is problematic since they must all share the same space. This was the way the servo wheels worked, and electrically, it was done with Nixie tubes. These used ten transparent in-line wafers enclosed in neon gas. Each wafer had electrodes in the shape of numerals, which glowed when the wafer was electrified. Although the unused wafers were still discernible while one wafer glowed, it was tolerable. The Nixie tube was used for years and can still be found on special equipment. However, this method is costly and the tubes are fragile.
The dot-matrix display is used on computer screens and printers and was used on test equipment. Small lamps arranged in a block form allow the lighting of specific ones to display a digit. The lamps can be miniature incandescent, neon gas plasma, LED, LCD, or phosphorescent. The dot matrix is more helpful when alphabetical characters are displayed. However, many costly switching circuits are required: The 5 x 7 matrix, for example, needs 35 switching circuits.
Solid Character Display (Gas Plasma-Nixie)
The bar segment display method is an inexpensive compromise. The character appearance is passable and only 7 switching circuits are needed.
There are numerous types of phenomena, which can be used to illuminate a display:
Plasma (ionized gas)
Incandescence is the way an ordinary light bulb works (a heated filament glows). Tiny incandescent lamps can be used in the dot matrix, but too much heat is generated, and the lamps usually fail. The neon lamp is the most popular of the gas plasma or ionization lamps. It was used in the Nixies. Tiny neon lamps can be used in the dot matrix or even the bar segments. But higher voltages are necessary to trigger gas ionization.
Fluorescence uses the combination of plasma and phosphorescence. An ionized gas, such as mercury vapor, emits ultraviolet radiation, which strikes a phosphorescent coating to produce light. This is the way the standard fluorescent bulb works. For fluorescent character displays, the 7-bar segment device can be used. Again, this is a costly and fragile display method. Straight phosphorescence uses an electron beam to strike a coated screen to produce light in a dot matrix, raster (line), or bar segment configuration. This is how TV and computer displays work.
Electroluminescence is light emitted by a solid carrying an electric current. The light-emitting diode (LED) is a perfect example. It is a semiconductor PN junction diode biased in the forward (high current) direction, which dissipates energy in the form of light. It is used expansively in the bar segment display.
The liquid-crystal display (LCD) does not generate light, but it uses ambient light with its varying optical qualities to create displays. The transmissive LCD has its opacity altered when electrified. The reflective LCD becomes reflective when it is energized. LCDs use the bar segment display method and have become the most popular displays for digital multimeters. The 7-bar-segment display consists of eight terminals, including a decimal point, used to create the digits. The backplate is used to bias every segment. After the meter counts the pulses, and the BCD codes are interpreted, they are fed to the display drivers, which select the correct combination of segments to create the digit.
Both LCDs and LEDs use the same configuration. The LED is a light source; thus, it provides a much brighter display than the LCD, and can also be more colorful: red, green. orange, or yellow. The transmissive LCD has its own backlighting, but since it is based on variable opacity, the display lighting is not as bright. The reflective LCD relies on high ambient light conditions to provide a good reflection. With low ambient light, the unit often must be tilted to a better reflection angle. Overall, the LCD display is a great display. It is inexpensive and requires low voltages, so it is the most common display on electrical multimeters. LEDs, on the other hand, are often used in lab equipment.