Abstract: This note compares advantages and disadvantages of Cathode Ray Tubes, Electro-Luminescent, Flip-Dot, Incandescent Light Bulbs, Liquid Crystal, Light Emitting Diode, Organic LEDs, Polymer LEDs, Glow Discharge, Plasma Display Panels, and Vacuum Fluorescent Display technologies.
This note compares the main electronic displays technologies
in the table below. Each display type is described briefly, and the relative
advantages and disadvantages are reviewed.
Display
Type
Acronym
Emissive
or Reflective
Technology
Advantages
Disadvantages
Cathode Ray Tube
CRT
Emissive
The CRT is a vacuum tube using a hot filament to
generate thermo-electrons, electrostatic and/or magnetic fields
to focus the electrons into a beam attracted to the high voltage
anode which is the phosphor coated screen. Electrons colliding with
the phosphor emit luminous radiation. Color CRTs typically use 3
electron sources (guns) to target red, green, and blue phosphor
patterns on the screen.
Very bright
Wide viewing angle
No mask, so no pixel size limitation for mono
Minimum pixel size 0.2mm (color)
Low cost standard sizes
Low cost high-res color
Wide operating temperature range
Moderate (20khrs+) life
High (5kV to 20kV+) drive voltages
Not a flat panel (rare exceptions)
Can be fragile, particularly neck-end
Heavy
Source of X-rays unless screened
Affected by magnetic fields
Difficult to recycle or dispose of
Electro-Luminescent Display
ELD
Emissive
EL displays are solid state displays which use a
phosphor to emit light in the presence of an electric field. Phosphors
used are usually yellow-orange or green.
Flat panel
Bright, light emitting mono displays
150V+ operating voltages
Color panels difficult to make
Development largely overtaken by PDPs
Flip-Dot Display
Reflective
Each pixel comprises a hinged disk. The disk is
matt black on one side (unlit), and fluorescent or reflective on
the other (lit). Various techniques are employed to flip the disk.
One method balances the disk around an electromagnetic soft iron
armature. The winding is energized one way or the other to attract
the disk to one side or the other. Remanent magnetism holds the
position, so only a short power pulse is required to flip the pixel.
Only draws power during a transition
Retains display pattern when off
Very wide viewing angle
Low cost for large segments
Can have long life (100khrs+)
High power required to change pixel state
Difficult to make segments smaller than 8mm diameter
Limited to mono color
Requires some ambient light to be visible
Incandescent Display
Light Bulb
Emissive
A coated tungsten filament is run white hot in vacuum,
and it radiates both visible light and infrared (heat).
Mature technology, very available
Lowest cost for moderate to high light output
Standard sizes easy to obtain
Short life (1khrs to 10khrs); socket adds cost
Usually inefficient compared with LED
Narrow operating voltage range
High peak current when starting
Usually run hot
Usually fragile
Liquid Crystal Display
LCD
Reflective
An LCD uses the properties of liquid crystals in
an electric field to guide light from oppositely polarized front
and back display plates. The liquid crystal works as a helical director
(when the driver presents the correct electric field) to guide the
light through 90° from one plate through the other plate.
Small, static, mono panels can be very low cost
Both mono and color panels widely available
Static panels offer lowest power/voltage display
Reflective panels in general are low power
Very easy custom segment shapes, sizes
Reverse backlit mono panels are attractive
Backlight adds cost, and often limits the useful life
Requires AC drive waveform
Fragile unless protection added
Can have narrow temp range (0°C - 50°C)
Temperature compensation usually required
Can have narrow viewing angle
Low yields raise cost for larger (17"+) displays
Light Emitting Diode
LED
Emissive
LEDs are photon emitting semiconductors which emit
light due to the injection electroluminescence effect. The wavelength
of the emitted light varies primarily due to the choice of semiconductor
materials used, and is commonly in visible spectrum or infrared.
Lowest cost red or green emissive indicator
Available in very small sizes
Very bright versions available (higher cost)
Red and green types work from 3V supply
LED is point source, so light shaping required to make segment
shapes
White and blue LEDs expensive, need >3.6V supply
Can have narrow viewing angle
Color and efficiency vary with temperature and current
Care required to achieve 50 khrs+ life
Organic LEDs Polymer LEDs
OLED
PLED
Emissive
These displays use organic electroluminescent materials
deposited on a glass or flexible substrate. Devices based on small
molecules are usually referred to as OLEDs. Those based on large
organic 'polymer' molecules are usually called PLEDs. Light is generated
by injection electroluminescence, like LEDs. The choice of organic
material sets the emission color OLED pixels are capacitive (tens
to hundreds of picofarads) leading to significant switching losses
for large displays with high multiplex ratios.
Moderate cost for small (<4") color panels
Wider viewing angle than LCD
Faster element response than LCD
Emissive, unlike LCD color panels
RGB and mono displays
Can be built on a flexible substrate
6V to 16V operating voltages
Differential aging effects limit life
Power consumption high for matrix panels >128 x 64
Glow Discharge (Plasma) Indicators
Nixie
Emissive
A high voltage between two electrodes encapsulated
in a tube with an inert gas (typically neon) ionizes the gas allowing
current to flow and a glow discharge to appear around the cathode.
If the tube is driven from an AC source (e.g., a neon indicator),
the glow discharge appears around both electrodes. Complex displays
use multiple cathodes shaped either as segments (one or more lit
to make a character) or complete numerals (one lit at a time).
Wide operating temperature range
No phosphor (gas choice sets discharge color)
Long (40khrs+) life
AC or DC operation (indicators)
Unregulated DC operation (digits)
Obsolete but for specialized applications
Not suitable for small, detailed display
40V to 150V operating voltages
Plasma
Display
Panel
PDP
Emissive
Plasma displays use multiple controlled gas discharge
paths (one path per color per pixel) but the inert gas is chosen
to glow outside the visible spectrum. Each local current discharge
path terminates with its own phosphor coated cathode, which in turn
emits luminous radiation. The cathodes can be arranged as RGB triads
to make a color display.
Very bright, thin, color display
Available in large (30"+) sizes
Wider viewing angle than TFT color LCD
Phosphors limit life (5khrs to 20khrs)
150V+ operating voltages
Moderate power consumption
Entire display must be exercised to avoid phosphor differential
aging effects
Vacuum Fluorescent Display
VFD
Emissive
The VFD is a vacuum tube using hot filaments to
generate thermo-electrons, A grid (static display type) or multiple
grids (multiplexed display type) control and diffuse the thermo-electrons,
which are attracted to one or more high voltage phosphor coated
anodes, which then emit light. The anodes are at the back of the
display, so the emitted light passes through the grid(s) and filaments
and the display front to be seen by the user. The filaments are
not run hot enough to be usually visible.
Wide operating temperature range
Long (40khrs+) life
Wide viewing angle
Very bright, attractive, typically green display
Very easy custom segment shapes, sizes
Different colored segments easy
12V grid/anode voltage versions available
Filament supply (±8% typ. tolerance) required
10V to 60V grid/anode operating voltages
RGB displays available, but expensive
Phosphors other than green limit display life
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