Optical Performance Questions
Q. Why do I see a bleeding effect when my display is used outside?
A. This effect is sometimes referred to as purple bloom. It is seen only with active-matrix displays and is caused by very high levels of light - typically sunlight – photo-activating the transistors. In addition to applying an electrical signal to make a transistor conduct, if you apply enough light to a transistor, it will cause some flow of current and in doing so will apply a field to the liquid crystal material which turns it on. When turned on, a faint streaking purple color is typically displayed.
Some displays will exhibit this effect more than others. When a display is used above the ambient brightness specification level, it is possible to see this effect in almost any display - with slight variations depending how a protective black mask is applied. The accuracy of placement of this protective black mask is critical for preventing sunlight creating this unwanted activation of the transistors.
Sharp panels exhibit this effect far less than many competitive products. Each model should be tested to insure suitable operability in a bright ambient environment.
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Q. Why does my LCD turn dark at elevated temperatures?
A.
Each liquid crystal display is filled with a liquid crystal material. It is neither solid nor liquid but what is called a mesamorphic fluid. The name “liquid crystal” came from the property of having an ordered state like a crystal plus a liquid state. As one changes the temperature, it goes from something like a slushy solid to a state known as isotropic - much like water where there's no specific optical properties. In this state, it also doesn't have any dielectric properties that can be switched with an electric field.
When operated at very elevated temperatures, the material goes to this isotopic state and it does not behave with polarized light in the same way. The display will typically turn dark instead of normally white. However, this darkening is reversible. When you go back to normal room temperatures again it will operate correctly. If you're seeing a display turn dark at elevated temperatures, it means you are well above the specified temperature. [ Back to Top ]
Q. Why does the color shift when I look at the display off-axis?
A.
The most common type of display is a twisted nematic (TN) display. The liquid crystal molecules in the off state are oriented more or less parallel with the surface but as you look at micro-slices of that layer from the bottom electrode to the top, the liquid crystal direction or the director is rotated by 90 degrees. So imagine a small twist layer-by-layer from the bottom to the top of 90°. When one applies an electric field, these molecules reorient and essentially stand on end. Between cross polarizers, the light in the off state (the polarization direction) rotates by 90°. When looking at the displays somewhat off axis you do not get the same level of rotation of the light and the same level of extinction, so there is leakage of the light where there would otherwise be a black state.
That leakage will cause some color shift. By using a compensation film, display manufacturers can minimize and in some cases nearly eliminate the amount of shift off-axis. Displays using Sharp’s Advanced Super View technology exhibit a uniform coloration over at least a 170° viewing cone, and some to 176° – both horizontally and vertically. Other improved viewing cone technologies such as the PVA and Super IPS in competitive products also produce somewhat less color shift off-axis, but overall, the characteristics of Advanced Super View displays are superior because they do not compromise display response speed or contrast ratio.
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Q. My LCD washes out in sunlight. Why and what can be done to prevent this?
A.
This type of wash out is not due to temperature. This effect occurs when a bright light from the front overwhelms the backlight that is coming from the display. A transmissive type display depends upon seeing the light that comes from the back of the display, going through the liquid crystal, being switched correctly and going though the corresponding color filters. If the light from the front (that is reflecting from the front of the display) becomes the stronger component of what your eyes sees, then we experience washout.
To prevent washout, one should consider Sharp’s Advanced TFT, a type of transflective display. In such a display, when light comes from the back in a dark environment we see it as a normal transmissive display. But when light comes in the front, it is predominantly reflected off the highly reflective layer that we build in as part of the electrode in each picture element. That component of light gets stronger and stronger when a bright light is impinged on the display. The result is a fairly uniform non-washed out color at any ambient brightness level. It has a very uniform color palette from the brightness of sunlight to absolute darkness. LCDs in the Advanced TFT mode are far superior to other types emissive displays such as OLEDs which have no way to be used in a reflective mode, since they are entirely dependent upon seeing the light comes from the emissive source. They cannot compete with a very bright light coming from an external source.
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Q. My LCD screen seems to have lost brightness. What can be done about this?
A.
In a transmissive display, the light sources are typically a cold cathode fluorescent tube (CCFT), and in some cases an LED. In time, the light from these sources will slowly degrade. The lifetime of a CCFT is defined as the time at which it emits half the original brightness. In an industrial type display, this is typically 50,000 hours. In some new TV type displays it is 60,000 hours. In the not-too-distant future we will see as much as 80,000 hours before the light goes to half brightness.
If your display has been running this number of hours and the brightness has diminished, many displays have CCFTs that can be replaced. Replacement lamps are available from Sharp or through our distribution sources. Not all displays contain replaceable backlights, and these displays must be replaced in their entirety.
After you replace a backlight, an LCD display will perform like it was new - unlike other technologies such as OLEDs or plasma panels where there is no way to refurbish them. Another good point about display brightness decay in LCDs is that it happens uniformly over the entire display rather just in areas where you have written to it, so that one does not experience image burn-in. [ Back to Top ]
Q. My LCD looks dark when viewed through sunglasses. What can be done about this?
A. Displays are typically made with linear polarizers and sunglasses are also made with linear polarizers. If you cross the axis of the linear polarizer of your sunglasses to the one in the display, you will see a dark display. There are some displays that are made with the axis of polarization at 45° so when viewed through linear polarizers, one does not see any degradation in brightness. If your application is one where you are typically using it outside and if you're using a display that must have a linear polarizer, there are aftermarket integration companies that can add a quarter wave retarder to each side and make this display into one that responds to circular polarization. Circular polarized light is not extinguished with linear polarized sunglasses and may be a solution.[ Back to Top ]
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