Mobile phones have become indispensable in daily life through their diverse features and services. In particular, camera-phones have spread at a remarkable rate. Just after they appeared, in 2001, worldwide shipments were about 3 million, but in 2006, shipments had grown rapidly to 500 million^*1. During the last few years, camera-phone resolution has increased, and some models now have resolution of over five megapixels, in the same class as high-performance digital cameras. Such high resolution is supported by PanliteSP-1516, a special optical polycarbonate plastic from Teijin Chemicals Ltd.

Until now, most camera-phone lenses have been made of glass. Now, however, there is increasing demand for plastic lenses, due to their lower cost (one-tenth that of glass lenses), ease of molding, compactness, and lightweight. Mobile camera-phone lenses consist of 3-4 lens elements with different refractive indices. If one of the elements has a high refractive index and low optical distortion, high-quality photos with excellent sharpness and depth can be taken. However, conventional polycarbonate plastics, although used in CD players, DVD players, and eyewear, have not been suitable for camera lenses due to their high optical distortion. Therefore, Teijin Chemical has been researching polycarbonate plastics for camera lenses since 2003 and in 2007, the company succeeded in developing a new polycarbonate plastic, PanliteSP-1516, with minimal optical distortion.

In the development of new polycarbonate plastics, the process was reviewed from the molecular design level, and several new bisphenols were used in combination. This enabled optical distortion to be substantially have reduced and a high refractive index of over 1.61 to be achieved, while maintaining the same level of transparency as general-purpose polycarbonate plastics.

A feature of PanliteSP-1516 is that the photoelastic constant^*2, which is closely related to optical distortion, is about half that of general-purpose polycarbonate plastics, so optical distortion is low even in molded products. Moreover, the glass transition temperature^*3 of PanliteSP-1516 is 156ºC, so heat resistance is similar to that of general-purpose polycarbonate plastics. Consequently, even for injection molding, processing can be done under the same conditions as for general-purpose polycarbonate plastics. PanliteSP-1516 also has superior resistance to hydrolysis, which means that its performance does not tend to deteriorate even with long-term use and changes in usage environment. Therefore, this product is suitable for not only camera lenses, but also other applications such as optical sensors, industrial lenses, and optical films.

In addition to PanliteSP-1516, Teijin Chemical offers PanliteSD-1414, which has a similar refractive index and a glass transition temperature of over 180 degrees ºC. This product is being marketed for applications that require high heat resistance, such as automotive camera lenses.

^*1 Survey by Strategy Analytics, Inc. (U.S.).

^*2 Photoelastic constant: Expresses the relationship between stress and optical path difference due to birefringence

^*3 Glass transition temperature: Temperature at which the glass transition occurs. When plastics and other polymer materials are heated, they change from a hard, glassy state to a rubbery state; this is called the glass transition.

Click to enlarge

Comparison of birefringence (optical distortion)

Left: SP-1516, Right: General-purpose polycarbonate plastic
Thickness of molded product: From top 3 mm, 2 mm, 1 mm

^*The crossed Nicol method was used for the comparison. With this method, distorted parts appear white and undistorted parts appear black.