Low Reflective Touch Panel is developed to improve visibility of outdoor applications. This touch panel prevents depression of contrast under sunlight and resolves viewing difficulty of display. It is necessary to consider preventing reflection of applications such as digital camera, video camera, and personal navigation device which will be used under outdoor environment.
Method and Structure
The detection principle of low reflective touch panel is the same as 4-wire / 8-wire touch panel, but only the material structure differs and is constructed using polarizer film which is a material used for LCD for the purpose of designing a low reflective model. (Fig.2 & Fig.3)
When designing a low reflective touch panel with polarizer film, an optical isotropic film is used for upper electrode since biaxial stretched PET film may not be used.
GUNZE is capable of forming ITO layer on this optical isotropic film using its own sputtering system that may form the ITO layer on various types of film. Therefore, GUNZE has been offering the low reflective touch panel ahead of other companies and have been recognized among customers.
The visibility of LCD is subject to composition of display-ability, sighting condition, lighting environment, and etc. It is desirable to minimize any loss of LCD display-ability. However, resistive touch panel has many surfaces exposed to the air, light reflection will increase and contrast ratio of LCD will be greatly depressed.
When looking at the cross section of film-glass type touch panel (Fig.4), the total reflection rate is about 20% which adds light reflection rate R1- R5 of each surface. Considering R5 as the light reflection rate at the surface of LCD, CR, contrast ratio of LCD with touch panel mounted on, is expressed as equation shown below. This is an example when reflective LCD is used.
Rwhite and Rblack are the monochromatic reflect ion rates on reflective LCD respectively. Contrast ratio of reflective LCD will be depressed when touch panel is mounted. Consequently, the technology of reducing the light reflection and improving the contrast ratio is required.
1. Using AR (Anti-Reflection) Layer
Light reflection is reduced by the layers of multiple transparent materials that have different reflective index. The layered film is called "AR film" which has a function of reducing light reflection by adding amplitude of light reflection at boundary of films to coherent. This means delaying optical phases to let maximum and minimum reflection light interfere in order to reduce reflection.
Recently, there is film-glass type touch panel that has reduced its reflection rate to about 10% by implementing AR layer on all surfaces, but this will result in a high price.
2. Using Polarizer
Controlling optical amplitude and phase by using AR film is one way to reduce optical reflection. Another way to reduce this reflection is to use the polarizer. There is an optical component called "optical isolator" which eliminates the ref lection by using the polarizer, and the circular polarizer film is a kind of optical isolator. By placing polarizer film on the surface, reflective light from inside the touch panel will be reduced. Development of "Inner Type", a type which covers both sides with polarizer films where touch panel and LC cell are integrated, has been progressed showing its effectiveness recently.
For the layer between two polarizer films, materials with no double refraction property such as glass or materials which anisotropic light refraction are controllable are necessary to be used since general stretched films such as PET film are unable to be used. Using optical isotropic materials that could control the double refraction for touch panel design is a distinction of GUNZE and this has lead to result GUNZE being the pioneer to offer the low reflective touch panel to the market.
Linear Polarizer Type
Having a directional character of passing the light, the polarizer absorbs and shut out the light oscillator with vertical direction to only let the light oscillator that matches its transmission axis pass through the polarizer. This is called "linear polarizer", and the light with a certain direction will be obtained. Low reflective touch panel applies this linear polarizer technology.
The optical reflection that has passed the polarizer will be reduced more than half of normal reflection (Fig.5).
By placing the polarizer on the top surface of film-glass type touch panel, the reflection is reduced to about half of its original.When combining this low reflective touch panel with LCD, there will be an extra polarizer on the top surface of LCD because of the structure. It is desirable to remove this extra polarizer considering the transparency, but this remodeling of LCD will be difficult. Thus, the low reflective touch panel is normally designed with the structure of an extra polarizer on the top surface of LCD. It is considered that the brightness of LCD will be depressed about 7% when combined with the low reflective touch panel due to an extra polarizer. Recent appearance of LCD backlight having high brightness has come to cover this depression of LCD brightness.
Brightness Transparency Rate
The transparency of a polarizer which passes through the light of only one direction is 50%. Therefore, the transparency will be 40% when combined with a touch panel which has the transparency of 80%. However, the light on the display is a linear polarizer which has passed through a polarizer meaning by adjusting and matching axis angles of the polarizer on the top surface of LCD, touch panel will let the light coming from LCD pass through a low reflective touch panel clearly. If both axis angles match, there will be no loss of light that come out of LCD, and transparency will only be affected by the transparency of a touch panel of 80%.
"Brightness transparency rate" is the transparency of only touch panel without the consideration of polarizer transparency which is 50%.
Circular Polarizer Type
The polarizer film is an element which transforms the natural light into linear polarizer. Circular polarizer is a film consists of a polarizer film and a 1/4retardation film through an adhesive layer.
When the phase waves of polarizer film and 1/4 retardation film are combined, the optical oscillator forms a shape of spiral. The spiral looks as concentric circle from the light traveling direction, and this state is called "circular polarization." (Fig.1)
Circular polarizer film has a characteristic of transforming linear polarizer into circular polarizer, and the touch panel that has applied this characteristic is called circular polarizer type touch panel. When reflection occurs to incident polarizer light (anti-clockwise or clockwise), it will be reflected as inverse polarizer light. This reflection light will be absorbed by the polarizer film and will not pass through. By placing circular polarizer film (polarizer film +1/4 retardation film) on the top surface of a touch panel will eliminate the reflection light from inside the touch panel. For circular polarizer type touch panel, an 1/4 retardation film is placed at the bottom side of touch panel to fulfill the condition of transparency of circular polarizer film in order to convert the outgoing light from LCD which is linear polarizer into circular polarizer.
The touch panel integrated with circular polarization film is capable of combining with various LCD regardless of transmissive or semi-transmissive type LCD. (Fig.7)
There is a method called "Anti-Reflection Treatment" for controlling the surface reflection of a touch panel.
AR (Anti-Reflection) treatment is a method of preventing reflection by using the characteristic of optical wave interference of which delaying phase for 1/2(=wave length) in order for maximum and minimum wave lengths of reflection light to interfere.
The thickness of Anti-Reflection layer is designed as the light reflection of top and bottom surface to have 1/2 differential from one another. It is expected to show its effectiveness by preventing reflection only in narrow wave length range of visible light (400 - 800nm) with one Anti-Reflection layer.
By implementing multiple Anti-Reflection layer enables to prevent the reflection of wave lengths of broad range. Surface with AR treatment will have reflection of reddish-violet. This treatment is implemented to lenses of glasses and camera in general.
It is possible to design a non-reflective touch panel by placing AR treated film which prevents the reflection at the top surface and improves the transparency. Although there is an issue of cost phase for AR treatment, it has a profound effect on surface reflection and is used as a means of low reflection besides using the low reflective type touch panel especially for applications for outdoor environment where product is used under direct sunlight.
Film-Glass and Film-Film Structure
At first, Film-Glass structure was common for low reflective touch panel. For Film-Glass structure, double-sided adhesive is used to bond LCD with touch panel. By fixing the outer frames using adhesive lead to the loss of transparency because of unavoidable air layer produced between the bottom side of touch panel and the top surface of LCD. Film-Film structured touch panel which directly bond a low reflective touch panel onto LCD in order to eliminate the air layer appeared following Film-Glass structure.
Although the countermeasures for contamination of foreign substances and removal of air-bubble are necessary, using Film-Film type structure makes it possible to reduce outside reflection up to about 4% and obtain display of high contrast. (Fig.6, 8)