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LCD Viewing Modes & Polarizers


Each part number for Pacific Display Devices require that the Liquid Crystal Display Viewing mode and Polarizers be defined.  The following section on Viewing Modes and Polarizers will explain how the basic Liquid Crystal Display will appear with the Operating Mode selected.

  LCD Viewing Modes
The type of image the display will create is a cosmetic issue the engineering and marketing department work out. This is the easiest choice since there are only two basic selections, and we will go over each of them and what they mean:

POSITIVE IMAGE

LCD Module Positive ImageA positive image on an LCD display is when the pixel is "OFF" it is transparent, when a pixel in "ON" it is opaque. On almost all displays the image is smaller than the background, so this mode of operation is favored in an application where ambient light is high and it will help with the contrast of the display, especially for a display utilizing a Reflective rear polarizer.  An example would be a alpha-numeric character on a larger background.  The segments or dots on the character would absorb light (appearing dark) and the background (the larger area) would reflect light enhancing the characters.  Here are several typical Operational Mode & Viewing Mode combinations and the resulting images (assuming no backlighting which can color the background):

     TN: Black characters on a Gray background

     STN-Green: Dark Violet / Black characters on a Green background.
     STN-Silver: Dark Blue / Black characters on a Silver background
     FSTN:  Black characters on a White / Gray background
 

NEGATIVE IMAGE

LCD Module Negative ImageA negative image on an LCD display is when the pixel is "OFF" it is opaque, when a pixel in "ON" it is transparent. Since the image area is typically smaller than the background, the portion of the display that could reflect light and give the characters definition in this mode is minimized.  Therefore, this mode is typically only used when there is a backlight and the ambient lighting conditions are medium to dim. Using a backlight, the transparent segments of the display will "glow" because the backlight will be viewable only when the pixels are turned on. A high ambient light condition could wash out the backlight.  Here are several typical Operational Mode & Viewing Mode combinations and the resulting images (assuming a backlight with the specified coloration listed):

     TN: Glowing Green-Yellow characters on a light Gray background  (Green-Yellow Backlight)
     STN ("Blue-Negative"): Glowing Green-Yellow characters on an light Blue background (Green-Yellow Backlight)
     FSTN:  Glowing White characters on a Black background (White Backlight)
 

  LCD Polarizers
Each LCD has 2 polarizers, the front and rear polarizers, applied accordingly across the front of the display viewing surface and across the rear of the display to determine how the infuse light into the display.  The front polarizer is always Transmissive and not selectable by the user, however the rear polarizer has 3 choices and two grades for each choice.  The rear polarizer selection is as follows:

REFLECTIVE POLARIZER
LCD Reflective Polarizer
Reflective
displays have an opaque rear polarizer that includes a diffuse reflector, such as brushed aluminum. This layer reflects polarized ambient light that has entered the front of the display back trough the LCD cell. Reflective displays require ambient light to be seen. They exhibit high brightness, excellent contrast, and wide viewing angles. They are particularly suitable for use in battery operated equipment where an adequate level of light is always available. Reflective LCD's cannot be backlit, however they can be front lighted in some applications.

 

 



TRANSMISSIVE POLARIZER
LCD Transmissive Polarizer
Transmissive displays have a clear polarizer on the front and the back. The display therefore depends on light coming through from the back of the display toward the observer in order to be seen. Most, but not all transmissive displays are negative image, and we sometimes add colored filters to different areas of the display to highlight different annunciators. Another example of a transmissive polarizer display would be a transparent window where you could see the segments superimposed over your line of vision through the display window (this assumes a sufficient ambient light source exists on either side of the window).

 



TRANSFLECTIVE POLARIZER
LCD Transflective Polarizer
Transflective displays have a rear polarizer which includes a translucent material which reflects part of the ambient light, and also transmits backlighting. As the name implies, it is a compromise between the transmissive and reflective viewing mode. Used in reflection, it is not as bright and has lower contrast than the reflective type LCD, but it can be backlit for use in low light conditions. This polarizer is the best selection for a display that can be used in all lighting conditions with a backlight.

 



COMMERCIAL GRADE POLARIZER
The grade of a polarizer determines its operational and storage temperature range.  The commercial grade polarizer operates typically between -10C to +60C.

INDUSTRIAL GRADE POLARIZER
The grade of a polarizer determines its operational and storage temperature range.  The industrial grade polarizer operates typically between -30C to +80C. Other temperature ranges are also available.
 

LCD Viewing Mode Summary

Viewing Mode Display Description Application Comments Direct Sunlight Office Light Subdued Light Very Low
Light
Reflective
Positive Image
Dark segments
on light background
Not backlit. Provides best head-on contrast and environmental stability Excellent Very Good Average Poor
Transflective
Positive Image
Dark segments
on grey background
Can be viewed by reflected ambient light or with backlighting Excellent
No backlight
Good
No backlight
Good
Backlit
Very Good
Backlit
Transflective
Negative Image
Light grey segments
on dark background
Needs high ambient light or backlighting. Frequently used with color and multicolor transflector Good
No backlight
Fair
No backlight
Good
Backlit
Very Good
Backlit
Transmissive
Negative Image
Backlit segments
on dark background
Cannot be read by reflection Poor
Backlit
Good
Backlit
Very Good
No backlight
Excellent
Backlit
Transmissive
Positive Image
Dark segments
on backlit background
Designed for very low light conditions, yet able to be read in bright ambient lights Good
No backlight
Good
Backlit
Very Good
Backlit
Excellent
Backlit

 


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