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Low-Reflective Touch Panels

Gunze patented Indium Tin Oxide technology reduces troublesome reflectivity.

High visibility

Light reflection is one of the major factors that can considerably lesson the visibility of LCDs. As a pioneer in the touch panel industry, Gunze has developed a proprietary Indium Tin Oxide (ITO) low-reflective layer that improves touch panel transparency. The ITO layer allows light to pass through the horizontal surfaces with little or no reflection.

The aggregate total of light reflection rate is 20 - 22%, cumulative from each reflective surface R 1-R5 of a section of standard film-glass (Figure 6). R5 is the light reflection rate at the LCD surface. The contrast ratio (CR) is shown below the reflective-type LCD when a touch panel is mounted:

Rwhite and Rblack are the monochromatic reflection rates. The contrast ratio of the reflectivetype LCD alone is about 10-to-1. When a touch panel is mounted on the LCD, the contrast ratio decreases to about 5-to-1.

Figure 1. Transition of polarized light in circular polarizer structure
Click on drawing for larger view.

Reflection reduction methods

Gunze low-reflective touch panels employ anti-reflection film and polorization to maximize reflection.

Anti-reflection film
Surface light reflection is lowered by a layer of anti-reflective (AR) film composed of multiple transparent materials, each of which has a different light reflection rate. The film coherently adds an amplitude of light reflection at the film boundaries, reducing touch panel light reflection by as much as 10%.

AR film reduces reflection by controlling the amplitude and phase of light waves.

Note: Coherency is a condition in which the wave phases of light waves synchronize with each other.

Polarization
In addition to AR film, Gunze touch panels utilize polarization, which reduces reflection even more. A polarizer is a type of light isolator that reduces reflective light from inside the touch panel. The integrated touch panel and LC cells are covered by the polarizer on both sides (referred to as the inner type of polarizer) (Figure 3).

Note: An optical isolator is a component of a light element circuit that has a pair of input/output terminals. It is a passive optical component that passes light only in a designated direction, letting light in with minimal loss while greatly reducing output light.

Materials such as glass that are optically equiangular and therefore have no complicative light refraction are covered with the polarizer, which replaces the standard polytelephthalate (PET) layer. Gunze's innovative resin-based light refracting material offers superior reflection reduction.

Figure 2. Cross-section of fi lm-glass (standard) touch panel and its refl ective rate

Figure 3. Cross-section of Inner type touch panel and its refl ective rate

Figure 4. Cross-section of circular polarizer touch panel and its refl ective rate

Circular type polarizer

The circular polarizer converts certain linear light waves (such as 550 nm) to circular light.

A linear ploarizer and 1/4 retardation sheet form an adhesive layer (Figure 1). Linear light and circular light can be combined in a clockwise or counterclockwise direction, crossing each other squarely, with one type of crossing being transparent while the other is not.

Light from inside the touch panel is absorbed by the circular polarizer. Because the transparency of light absorbed by the polarizer is reduced, the circular polarizing sheet attached to the surface of the LCD has a 1/4 retardation rate, enabling the required transparency.

Any type of LCD can be used with a touch panel/circular polarizer assembly.

Film-Glass (F-G) and Film-Film (F-F) inner type polarizer

Integrating a touch panel and an LCD between two polarizers retains the LCD's brightness while using fewer polarizing films than are necessary with circular type polarizers, resulting in a high-visibility, cost-effective solution. The polarizer is integrated with the LCD during the manufacture of the LCD.

With F-G inner type polarizers, film and glass are both adhered to the LCD by a double-sided adhesive tape. Light reflection occurs in the layer between the lower side of the touch panel and the surface of the LCD, lowering the reflection rate.

With F-F inner type polarizers, the polarizing film is adhered to the complete surface of the LCD so that no light reflection occurs. This solution creates a touch panel with high visibility that is also lightweight, thin, and durable.

Layered Anti-Reflective film

Anti-reflection (AR) treatment to the surface of the touch panel is a highly effective means of reducing light reflection.

Bottom light waves' phases are shifted by 1/2 λ (λ = wavelength) causing top light waves to undergo interference. The AR film thickness is designed so that light reflection on the top surface differs from that on the reverse side by only 1/2.

A single layer of AR film affects only a narrow range of visible light waves; multiple layers of AR film prevent light reflection in a broad range of light waves.

Gunze's AR technology now makes it possible to manufacture touch panels with low reflection that are also highly durable and that can utilize both pen and finger input.

Gunze's AR technology prevents light reflection on the surface, with a high transparency rate. With standard touch panels, the AR coating is placed on the upper electrode film. With low reflective touch panels, the AR coating is on the polarizer.

Figure 5. Circular polarizer cross-section
Click on drawing for larger view.

Figure 6. F-G Inner cross-section
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