SCHOTT Group Home

High Definition TV

Ever larger television displays require modern HDTV cameras (see picture above) that are capable of zooming in on distant images without causing distortion. When used as an optical lens material, calcium fluoride helps avoid color aberration. Photo on top: Sony, Photo below: SCHOTT
Katrin Striegel

Close to the Action

High definition tv (HDTV) is the television of the future. Special optical materials in the form of calcium fluoride crystals provide the higher resolution that modern cameras require.

By the Soccer World Cup 2006 in Germany at the very latest, consumers have become accustomed to being extremely close to the action with five times more image data in 16:9 format. Even when the action on the field is actually taking place far away from the fans inside the stadium and the players appear as if they are only a few inches tall, television viewers at home experience every bead of sweat, thanks to HDTV. Nevertheless, larger and larger televisions require significantly higher resolution. Modern HDTV cameras that are capable of capturing distant images without any distortion or deviation are responsible for delivering these incredible zoom results. Here, however, the problem that results when color aberration occurs and many different lenses are placed behind each other had to be resolved. These aberrations occur because the transitory light beams are refracted to different degrees by a lens, depending on their wavelength, and therefore do not arrive at exactly the same point on the image plane. Fuzziness and color errors, or so-called chromatic aberrations, result. This undesirable effect can be corrected, however, by adding a second lens or optical material that behaves in exactly the opposite manner. This is where calcium fluoride crystals come into play.
Color correction in lens systems

Basically, this involves leveraging a long-known property of certain types of crystals. Even back in 1886, the optics pioneer from Jena, Professor Ernst Abbe, recognized that calcium fluoride crystals were uniquely suited for correcting colors in lenses and went on to develop the concept of apochromatic correction. Apochromatic corrected lens systems refract light in an entirely different manner and, thus, achieve very precise overlapping of the red, green and blue rays of light. The result is that no more chromatic aberrations occur. In the field of photography, the abbreviation APO is frequently used to refer to apochromatic lenses. Thus far, mainly higher quality, light-intensive telephoto lenses have been introduced as double lenses. But, now that HDTV technology has become popular, lenses for television cameras have become yet another market for calcium fluoride crystals.

”In color viewing systems, it really depends how cleverly highly refractive and low-refractive glasses are combined. Here, the objective is to retain the refraction that is needed to either increase or decrease the size of an object and, on the other hand, correct the color aberrations that occur. Here, calcium fluoride crystals, such as those produced in entirely new sizes and volumes by SCHOTT LITHOTEC in Jena, are well-suited, because they have a relatively low index of refraction and an extremely high Abbe Number, in comparison with optical glasses,” explains Peter Maushake, Product Manager of Optical Materials at SCHOTT LITHOTEC in Jena, Germany.

Calcium fluoride is known for being one of the hardest materials in the fluoride crystal family. The excellent processing possibilities that result from this are yet another reason why it is particularly well-suited for use in manufacturing a broad range of optical components.
Since the World Cup took place in Germany in 2006, viewers are now much closer to the action, thanks to HDTV. Photo: TV Skyline
Also ideally suited for laser lenses

Calcium fluoride is not only capable of standing up to temperatures of up to 800° C in dry atmosphere, due to its low absorption, it is also perfectly suited for use in lenses for high-performance lasers. The homogeneity of the index of refraction is yet another important parameter for this lens material in achieving images that are as free from distortion as possible. Here, calcium fluoride is capable of meeting even the highest demands for quality.

Even if microlithography for use in manufacturing computer chips remains the most important area of application for calcium fluoride, mainly due to its exceptional ultraviolet (UV) transmission, this material will also be used more and more often in visual, as well as infrared wavelength regions. Calcium fluoride offers a number of other important optical characteristics, including low axial and radial birefringence. Thanks to its extremely high purity and structure, calcium fluoride also offers high laser stability and is, therefore, popularly used in deep UV litho excimer laser lenses. Like fused silica, it has taken on great importance as a material for use as a material in semiconductor lithography in so-called wafer steppers. Today, these illumination and projection lenses are capable of projecting circuit structures with minimal structural widths of 45 nanometers onto silicon wafers.
Additional informations
Growing of monocrystals is unique
Download this article as a PDF file