Effect of Static Images on Two Display Types
In most Point Of Sale applications, it is common for a static image (perhaps the company logo or an advertisement) to be displayed on a video screen anytime the display is not in use. In many cases, this may be done for an extended period of time. Unfortunately, this will result in undesirable effects on the display.
Cathode Ray Tubes (CRTs)
CRTs are emissive, or light producing displays. The phosphor coating on the inside of the screen is excited by electrons and emits light. In doing so, the phosphor decays and emits less light over time, resulting in a dimmer display. This is inherent in the physics of phosphor emission and nothing can be done about it. Fortunately, the dimming process is very slow.
The problem arises when the same image is displayed on the screen for a long period of time. In that case, some areas of the phosphor are excited and some are not. The result is a non-uniform aging of the phosphor. Eventually, the static image will be "burned" into the screen and will even be visible when the display is turned off. This is a permanent effect and cannot be reversed.
This is a cumulative effect. The image does not have to be displayed for long continuous periods of time. If an image (such as a menu) is used frequently enough, it will eventually be burned into the screen. It is just a matter of time. Also, the effect is directly related to screen brightness - the brighter the image, the more rapidly the phosphor decays and the sooner phosphor burn will be evident.
The only way to prevent or postpone phosphor burn is to use a screen saver program that displays an image that randomly moves around the screen.
Liquid Crystal Displays (LCDs)
LCDs are non-emissive displays. Normally the liquid crystal material does not transmit light. When a voltage is applied to a pixel, the molecular alignment changes so that it will transmit light, either from a backlight (transmissive LCD) or from reflected ambient light (reflective LCD). Most POS LCDs are transmissive displays.
Although the backlight is an emissive device and will dim over time there is no inherent aging in the liquid crystal material. However, if a voltage is applied to a pixel for a long enough duration, a residual charge will be retained even after the voltage is removed. Thus some light will continue to be transmitted/reflected through that pixel, even when it is in its "off" state.
If a static image is displayed on an LCD screen for a long time, the result will be that the image will still be visible even when other images are displayed or when the display is turned off. The appearance is just like phosphor burn on a CRT, but there are two distinctions:
This is not a cumulative effective. It will occur only if the static image is left on the screen for a continuous period of time.
The effect is not permanent. If the voltage is removed for as long as it was applied, the effect will be reversed.
As with a CRT, the only way to completely avoid the phenomenon is to use a screen saver program that displays an image that randomly moves around the screen.
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