Physical Characteristics Questions
- Why can I see the picture elements in my LCD more clearly in my LCD than in my CRT?
- My LCD changes its color when I press on it. Is this a problem?
- My LCD runs slowly at low temperatures. Why?
- An image seems to be burned in to my LCD. What caused this and what can be done about this?
- Does the gap between a touch panel and the display cause a problem?
Q. Why can I see the picture elements in my LCD more clearly than on my CRT?
A. Each pixel area in an LCD has a well defined etched boundary where the electron beam of a CRT has a Gaussian shape that has much more gradual edges.
Q. My LCD changes its color when I press on it. Is this a problem?
A. By pressing on LCD, you are changing the cell gap, and in changing the gap, the polarizers do not extinguish the light the same way as the nominal cell gap for which it was designed. If you are using the display with a touch panel, you should keep the touch panel at a a reasonable distance from the display to insure you're not altering the cell gap and thereby the color when pressing on it.
Displays are relatively durable, you can press with a reasonable pressure without causing permanent damage. However a large amount of pressure on the display or repeated vibrational pressure on a display can destroy the alignment layer and cause permanent damage. This is generally not a problem for just the occasional touch.
Q. My LCD runs slowly at low temperatures. Why?
A. Liquid crystal material is a fluid and the like many fluids, the viscosity of the fluid changes as a function of temperature. At very low temperatures, the viscosity can be dramatically increased. Like any fluid under the influence of a force, it then moves more slowly. In LCDs, molecules are under the influence of an electric field. When the fluid becomes more viscous, the field has less impact and the result is a slower response. The specification for each display defines the range of temperatures of use and the response speeds.
If you need faster response at low temperatures, you may consider working with an integration partner to add a heater element to the display or add a heater to your enclosure to raise the minimum temperature.
Q. An image seems to be burned into my LCD. What causes this and what can be done about it?
A. Image burn-in or image retention is a problem with a variety of displays. Displays such as plasma panels or OLEDs where there is a differential level of brightness caused by simply running over a period of time are permanent effects. For LCDs, the effect is generally repairable. The cause is usually some kind of DC offset in the way the display is being driven. You should take great care that you're not violating any of the parameters in the specification for driving the panel that can cause image retention problems. Image retention is caused by migration of charged carriers in the display from one electrode to another which only happens under the influence of a DC bias. As these ionic contaminants migrate to the surface, the electric field is lowered where they are deposited, a causing the total field applied across liquid crystal display to be reduced in those regions. This exhibits itself as less drive in that region and a burned-in image.
In an LCD, if you write a completely white screen over a period of time, those contaminants are scrubbed from the surface and the image retention will go away. If you're using a display with adjustable DC offset, make sure you are adjusting for minimal flicker in the display when setting Vcom. In doing so you are minimizing the DC offset and will reduce the levels of image retention. At elevated temperatures when the viscosity of the liquid crystal material is reduced, the carrier mobility goes up and the image retention can be enhanced. If you know you are generally going to operate at elevated temperatures, you should adjust the DC bias settings at the expected operating temperatures rather than at room temperatures
Q. Does the gap between a touch panel and the display cause a problem?
A. If a touch panel does not have an anti-reflective coating, it is possible to see some loss of light and some internal reflection. We strongly encourage the use of an anti-reflective coating on the backside of the a touch panel. If you place a touch panel too close to the display, and there are internal reflections, then you may see Newton rings. Depending upon how you attach your touch panel to the display, you may get a differential expansion between a touch panel and the display as a function of temperature and variable levels of Newton rings.