A new generation of lenses is becoming available from ARRI and Zeiss, all sharing a common trait: the new T* XP (Extended Performance) anti-reflex coating. This coating can be found on the Master Primes, Ultra Prime 8R, Ultra 16 lenses and on the Lightweight Zoom LWZ-1.

Here is a description of the function and history of anti-reflex coatings and how the T* XP coating provides great advances in coating technology.

What do Anti-reflex Coatings Do?

Anti-reflex coatings have proven to be one of the most important inventions in modern optics. By reducing the natural tendency of glass-air surfaces to reflect a portion of the incoming light, these coatings 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-reflex 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 proper color balance. Thanks to modern anti-reflex coatings we are used to brilliant images in almost all lighting situations on the film set.

Anti-reflex Coating History

First developed in the Carl Zeiss laboratories in 1935, anti-reflex 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-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. For the recently introduced new ARRI/Zeiss lenses, the T* coating has been refined even further, resulting in the T* XP coating.

The T* XP Coating

To achieve the excellent image quality of the new lenses (for the Master Primes even at T1.3!), the number of lens elements had to be increased. This could have lead to a decrease in light transmission and an increase in various false light effects such as veiling glare, flare and narcissism. Instead, the new T* XP coating as well as other measures allow a greater light transmission, and the false light effects have been attenuated even below the level of other modern cine lenses. Thus these new lenses can catch subtle tones in the deepest shadows and fully utilize the high dynamic range of modern film stocks.

The T* XP coating is optimized with respect to the spectral sensitivity of motion picture film and the sensitivity of the human eye. Additionally the color of any remaining minor ghosting effect will be rendered in magenta instead of green, which is considerably less noticeable on film.

A disadvantage of conventional multilayer coatings was their application on large, strongly curved surfaces, as are used in the Master Primes, for instance. On this type of surface the spectral reflectance of the coating usually shows a variation, so that it’s optical and mechanical performance close to the lens edge becomes worse than its performance at the optical center. Compared to conventional multilayer coatings the T* XP coating has a more uniform performance across the lens from optical center to the edges; in fact the T* XP coating has up to five times better transmission at the edges.

This graph shows the reflectance of a lens surface with a conventional anti-reflex 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.		This graph shows the reflectance of a lens surface with the T* XP anti-reflex coating at different frequencies of the light spectrum. Notice how the black line, which represents the reflectance at the edge of a strongly curved lens, essentially follows the blue line, which represents the reflectance at the center of the lens.

Designing an anti-reflex coating formula that ensures maximum light transmission in a wide spectrum of wavelengths is one part of the high art of anti-reflex coatings. The second part is the equally tricky art of applying the coating in the proper and even thickness onto 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 of the lens coating.

The design of the new T* XP anti-reflex coating and its sophisticated application techniques are yet another technology that contributes to the outstanding optical quality of the Master Primes, Ultra Prime 8R, Ultra 16 lenses and of the Lightweight Zoom LWZ-1. What counts in the end, of course, are the new creative option these technologies allow the cinematographer, are the images on the screen. So we are now looking forward to the new and exiting visuals cinematographers world wide will be able to produce with these new lenses.