Anti-refection coatings have proven to be one of the most important inventions in modern optics. By reducing the natural tendency of glass-to-air surfaces to reflect a portion of the incoming light, they ensure that the maximum amount of light reaches the film instead of being reflected away from the lens surfaces or, worse, bouncing around inside the lens. Especially modern lenses with their large number of single lens elements would otherwise not be able to transmit enough light to the film. At the same time, anti-refection coatings suppress "false light" (internal reflections including veiling glare, flare, narcissism), resulting in higher contrast and deeper blacks in the image. Last but not least, they are an important contributor to a pleasing, gentle color balance. Thanks to modern anti-refection coatings we are used to brilliant images in almost all lighting situations on the film set.

History

First developed in the Carl Zeiss laboratories in 1935, anti-refection coatings found widespread adoption after 1945. These first coatings were single layer coatings which optimized transmission for one color only, leading to an uneven transmission behavior across the color spectrum. A significant improvement was introduced in the 70s when multilayer coatings were introduced, offering a further reduction in reflectance from glass-to-air surfaces in a broader spectral range. A highly sophisticated version of this technology is the Zeiss T* coating used in the ARRI/Zeiss Ultra Primes and Variable Primes.

This graph shows the reflectance of a lens surface with a conventional Anti-refection coating at different frequencies of the light spectrum. Notice how the red line, which represents the reflectance at the edge of a strongly curved lens, indicates a severe increase in reflectance starting at 580 Nanometers.

click images to enlarge This graph shows the reflectance of a lens surface with the T* XP Anti-refection coating at different frequencies of the light spectrum. Notice how the red line, which represents the reflectance at the edge of a strongly curved lens, essentially follows the yellow line, which represents the reflectance at the center of the lens.

T* XP

Originally developed for the ARRI/Zeiss Master Prime lenses, the multi layer T* XP (Extended Performance) anti-reflection coating ensures maximum transmission in a wide spectrum of wavelengths. The T* XP coating has been optimized with respect to the spectral sensitivity of motion picture film and the sensitivity of the human eye.

Designing the proper coating formula is one part of the high art of anti-refection coatings. The second part is the equally tricky art of applying the coating in the proper and even thickness to the lens elements. Zeiss uses a carefully monitored, elaborate process in high vacuum where special optical substances are evaporated one after the other and deposited on the lens surface with precisely controlled thickness. For the T* XP coating this process was further refined to assure a perfectly even and symmetrical application. It assures uniform performance across the whole lens surface from the center all the way to the edges. This is especially important on lenses with large, strongly curved surfaces. When compared to conventional multilayer coatings, the T* XP coating has up to five times better transmission at the edges.

The result is higher contrast, deeper blacks and a great reduction of false light effects such as internal reflections, veiling glare, flare and narcissism. In combination with proper internal lens construction, the T* XP coating ensures that the lens can easily handle tricky lighting situations like strong backlight, sunsets or car headlamps. Lenses using the T* XP coating can catch subtle tones in the deepest shadows and fully utilize the high dynamic range of modern film stocks.