Structure of color filters

• What is a color filter?
• Basic structure of a color filter
• Color expression of a display
• Manufacturing process of a color filter

What is a color filter?

Color filters are what make pictures and videos “colorful.” By transmitting light through a color filter, color information is given to the light signal. Used mainly in slim TVs, computers and smartphones, these panels are the key component in the color reproduction of FPDs (flat panel displays). Color filters are used in various devices such as liquid crystal displays (LCDs) and organic light emitting diodes (OLEDs) displays for panel applications.

Basic structure of a color filter

A color filter consists of a slim glass substrate and color resist. Three types of color resist, red (R), green (G) and blue (B) are used to form a lattice pattern on the glass. In addition, the borders of each color resist are divided into grids by a black matrix (BM) to prevent color mixing between adjacent color resist.

There are two types of color filter patterns, the “Stripe Arrangement” and the “Mosaic Arrangement” (above diagram). In the stripe arrangement, subpixels of the same color are arranged in vertical lines. This pattern is suited for displaying linear models such as lines, rectilinear shapes, and letters. In the mosaic arrangement, the resist units of the same color are lined obliquely. This way, the display is able to express colors and gradations through a more sophisticated mixture of colors. Therefore, this pattern is suited for displaying complicated images such as photographs.

In the early stage of development, black lines would often show up in the displays using the stripe arrangement. This was due to the black matrix, and was a major drawback of this color filter pattern. However, the color filters we have reached a higher level of refinement where the black lines are hardly noticeable. Since this pattern is low-cost and has very few defects during operation, most of the color filters manufactured today are made with the stripe arrangement.

Color expression of a display

Most displays, including FPDs, express colors by the mixing the three primary colors of light. This is known as the additive color mixture method. The three primary colors of light are red, green and blue (RGB). These are the same colors you will find in a color filter (In addition to these three, color filters also use black.) For example, you can express yellow by mixing RGB in a 1:1:0 ratio and if you want to express magenta, then mix RGB in a 1:0:1 ratio. This way, we can express a vast variety of colors by adjusting the color combination and the RGB ratio.

In reality however, since the colors of the color filters (color resists) and the type of the light source differ in each product, the range of colors a certain display can express differs as well. This range of color expressed by mixing RGB is called the “color gamut.” The color gamut has several standards such as sRGB , Adobe RGB, and DCI. Normally, a color gamut is indicated by a triangle on the "xy chromaticity diagram" that represents the colors of light in plane coordinates (the diagram on right side). This certain region is called the “color triangle” and each apex indicates the limit of RGB each standard can express. When the color triangle is large, a wider range of colors can be expressed. In a display, the product responding to the standard with a larger color triangle can express colors more vividly.

Manufacturing process of a color filter

There are several manufacturing methods of color filters, but most are made by the “photolithography method” where the color resist is fixed onto the glass substrate by lithographic exposure or image development.

1. Apply the color resist onto a glass substrate (preliminarily processed with the black matrix).

2. Cover the glass with the photomask corresponding to the pattern of the color filter. Then, apply ultraviolet light to partially insolubilize the color resist.

3. After removing the mask, wash away the extra color resist with an alkali developing solution. Then, cure the color resist by post-bake treatment.

4. Repeat the above process (1)-(3) three times (the number of colors in a typical color filter). Finally, cover the color filter with an ITO layer.