Glossary of LCD Terms

LCD or LCM

Liquid Crystal Module (LCM) and Liquid Crystal Display (LCD) are often used interchangeably. To be pedantic, LCD is the actual glass display, while LCM describes the complete assembly (or module) including printed circuit board (PCB), electronics and back-light. The most used term is LCD or Liquid Crystal Display.

Character or Graphic

LCD's can be broadly broken into two groups describing how the screen is laid out and the normal format of sending data to the module.

Character

Typically displays discrete characters each 7x5 or 8x5 matrix, although other character matrices are less commonly available. A character cursor and/or icons may also be included on the display. Data is normally sent as text (with positioning and other information). Some controllers will allow custom characters to be created or have font-sets other than English available. While most controllers have near identical command sets, there may be differences in the required timing of received instructions and good design should make allowance to adjust timing.

Graphic

In a graphic display, the viewing area is laid out as a dot matrix, and data can be sent to control each individual dot, although some controllers have an in-built character generator to display alpha-numerics on the screen. Some modules have a controller on-board, while others are 'pure driver' and require an external controller. Except in high volume applications, it is rarely economical to use pure driver with an external controller. As modules may have different controller options and it is important to specify the controller you require as they are not generally compatible.

LCD type

TN

Twisted Nematic. Has a viewing range of approximately 90°. Has been largely superseded by STN and is normally only used for high volume, low cost products or by some hobbyist suppliers. Not recommended for general use.

HTN

High Twisted Nematic. Viewing range 100-110°. Not commonly available.

STN

Super Twisted Nematic. Viewing range 180°. Most common LCD type. The wide viewing range and high contrast makes it a good choice for many applications. Commonly available as very dark blue on yellow/green or grey background. As it is made in volume, cost is comparable to lesser types in all except highest volume.

FSTN

Formulated STN or Filtered STN. An optically compensated film is added to STN, and is used as a high contrast monochrome display. Normally makes the display 'negative', i.e. 'on' pixels are transparent. Used with CCFL backlight, can be paper-white with black pixels.

Color STN

STN display with R/G/B filter to display colours.

TFT

Thin Film Transistor. Active type display for Notebook and other colour applications.

OLED

Organic Electro Luminescent. Not strictly a LCD, but can be used in place of LCD in some applications (i.e. mobile phone colour displays). It is self illuminating and so has a wide viewing and high contrast. Currently limited availability.

LCD Structure

The following LCD types are available according to the construction of the display.

Reflective

No backlight, with display illumination from ambient or external light source. They are not suitable for use in a dark environment. Positive display ('on' pixels are dark)

Transflective

Allows light transmission from rear of the display, while retaining sufficient reflectivity to use ambient light. The backlight can be switched off in bright locations to conserve battery power or backlight service life. Normally positive display.

Transparent or Transmissive

Backlight is always required for viewing. Ambient light does not assist viewing. Negative display.

Backlight

A backlight allows the LCD to be viewed in dark conditions.

LED - Light Emitting Diode

Uses an array of LED's to illuminate the display. May be in a light box behind the display for maximum brightness, or around the edge for thinner display, but lower brightness. Low voltage DC supply. Usually yellow/green colour and other colours may be available, but often high volume is required. Long life span and relatively high power consumption.

EL - Electroluminescent

Light generated by an electric field on a high molecular weight film. Normally purple/blue or white. Requires a high voltage AC supply usually generated by an inverter. Low power consumption, but limited life. Can be added to most modules without increasing module thickness.

CCFL - Cold Cathode Fluorescent Lamp

Thin fluorescent tube along edge(s) of display. High brightness suited to graphic modules. Requires a CCFL inverter to operate.

Light Guide

Not quite a back-light, but a module housing with apertures to shine light into, often from surface mount (SMD) LEDs, which is then diffused across the rear of the LCD to illuminate it.

Viewing Angle

Specifies the viewing direction to get the best contrast. The viewing angle is a function of the orientation of the polarising film and is set during manufacture and cannot be altered later. Important with TN displays due to the lower viewing angle range, and while still specified for STN, it is a lot less important for most applications.

12 o'clock

Also called Top viewing. For displays that will be typically viewed from the direction of the top edge. Typically this might be a panel mounted display that is installed below the users eye-line.

6 o'clock

Also called Bottom viewing. A typical application would be in a hand-held unit where the display is viewed from the bottom edge as the unit is held away from the user.

9 o'clock & 3 o'clock

While theoretically possible, I have never seen a display in either configuration.

Normal or Extended Temperature

The LCD glass used determines the temperature range the modules can be stored and operated at. The values given below are most commonly used, but check with the supplier the actual allowable extremes.

Normal Temperature

0°C ~ 50°C

Extended Temperature

-20°C ~ 70°C. Usually requires a negative voltage on the contrast pin.

Touch Panel

A transparent overlay is affixed to the display. When the overlay is touched, an electrical signal identifies which area was pressed. Touch panels can be broadly grouped as analogue or digital type.

Analogue

It can identify where the screen was touched according to X-Y co-ordinates. It is suitable when there is a large number of 'hot-spots' or continuous co-ordinate input is required (i.e. character recognition)

Digital

Uses short circuiting key switches at 'hot spots' on the screen. It is most suitable to use when there is relatively few touch-pads and you wish to use key encoders or other existing circuitry. Also more suited to finger tip entry.

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All information is provided in good faith, but without any warranty or guarantee as to fitness to purpose. It is solely left to the discretion of the recipient or prospective user as to the suitability of information or product supplied by Computronics for their application.