Loading Content..

Of course, when visualizing 3D content, you want the colors to be displayed just as correctly as in two dimensions. However, because the INFITEC method is based on the fact that the intensity distribution over the wavelengths on the left and right sides is different, there is always the possibility that the colors in the left and right images do not match. This used to be even more pronounced with the first generation of INFITEC filters because only three wavelength bands were used per side. Therefore, in the early days of 3D projection using the INFITEC method, it was often necessary to insert a special color processor between the image source and the projectors. With the introduction of the Excellence® filters, it has become much easier, because the left and right images already match much better in terms of color. Special color processors no longer have to be used; the remaining differences between the two images can be corrected using the color settings available on each projector – if this is necessary at all. At the same time, the color balance already built into the filter makes the projected image brighter. The reason for this is explained below.

How do the Excellence® filters do this? The human eye has three different types of color vision sensory cells (cones), one each for red, green and blue. A green color sensation is triggered, for example, when the eye receives light wavelengths in a certain range that is different from the ranges for red and blue color sensations. Whereas in the past each of these areas was divided into two bands, the Excellence® technique now uses three bands per color. Let’s take green again as an example, where the following is just as applicable to red and blue. Of the three bands that divide the “green” wavelength range, the two outer ones are fed to the left eye and the inner one to the right eye. The two outer bands are now selected so that the color impression conveyed by the transmitted light is exactly the same as that produced by the middle passband. The fact that this is possible has been known for a long time and is called metamerism. The outer transmission bands of green are adjacent to one of the outer transmission bands of blue (on the short wavelength side) and red (on the long wavelength side) and are combined with these. Thus the (imaginary) nine bands merge into seven, four of them for the left and three for the right image. The spectral widths of these bands are adapted to the light source in a complex simulation process in such a way that the perceived colors on the left and right match as closely as possible.

Of course, this is not possible for every single projector (at least not at an affordable price), so that there are usually still slight color differences left, which can be more or less annoying depending on the application. However, these can be compensated with the adjustment possibilities that every modern digital projector offers. Professional projectors have additional and more comfortable settings than home projectors. In the professional sector, if the color rendering requirement is very high, the color adjustment can be quite complex. For this one needs a spectroradiometer, with which one measures the intensity over the wavelength of the radiation of the projector concerned both without INFITEC filter and also with. These spectra are then calculated with each other and with data of the desired color space; the results can be entered directly into the settings menu of professional projectors. This is usually not possible with consumer devices. But on the other hand, a visual adjustment based on special test images is usually sufficient. In the case of high demands, a spectroradiometer is used again, but in most cases several iterations have to be carried out until all colors are correct. It has to be considered that each color correction inevitably costs brightness, as the projectors are usually designed to deliver the maximum luminous flux without corrections. If, for example, the image is slightly greenish, one cannot make red and blue brighter, but must darken green. Frequently, a small color difference between the left and the right image can also be tolerated, so that a compromise between color fidelity and brightness can be made.

Conventional digital projectors use ultra-high pressure (UHP) Mercury lamps as light sources, except for cinema projectors which apply Xenon lamps. We will not talk about cinema projectors but only about consumer devices.

UHP lamp-based projectors suffer from containing hazardous substances (Mercury), comparatively low lifetime and thus the need to replace the lamp from time to time. They require cooling even after the lamp was switched off which determines the minimum shut-down time. In addition, the fans for cooling are a source of noise and subject to wear. All these reasons made the projector manufacturers look for other technologies to generate the light required.

One common technology feature of more efficient light sources are lasers. But behind this buzz word there are several different technologies.

Pure laser projectors use lasers for each of the primary colors red, green, and blue. As lasers are currently the most efficient light sources, laser diodes are preferred for small battery driven projectors to increase the on-time per battery charge. But single laser light sources show an unwanted phenomenon: speckle. Speckles result from interference effects of the laser light beam with itself when reflected at any diffusively reflecting surface – including projection screens. It can be avoided only by using several laser diodes per primary color, which makes the projectors more expensive but also brighter – and larger. In cinema this is applied to a large extent, but there are few speckle-reduced consumer laser projectors.

Instead there are many projectors called “laser” that use just one of these light sources to generate blue light. The red and green light required is produced otherwise. On one hand there are so-called laser phosphor projectors which use a part of the blue laser light to excite a substance called phosphor or luminophore to emit yellow light which can be split into green and red. In principle this substance is the same as that in fluorescent lamps. The phosphor – not to be mixed up with the chemical element Phosphorus! – has to be cooled to withstand the high power density of the blue laser light. In general, this is done by rotating a wheel which carries the phosphor – a mechanical part which is prone to wear. But other cooling methods and/or phosphors that withstand higher temperatures are under development. In any case active cooling will reduce the efficiency of the system.

Laser phosphor projectors are a kind of hybrid solution for generating light in red, green, and blue. On the other hand, there is another hybrid solution using a blue laser and a red LED. Again, the green light is produced by conversion of blue laser light using a phosphor wheel. Unless the manufacturer uncovers the light generation principle the user cannot discriminate the different ways because all of them are sold as “laser” projectors. The expert of course can tell by measuring the light spectra.

Still another way of producing red, green, and blue light is using exclusively LEDs. These devices are not quite as efficient as lasers (without cooling) but still much more than Mercury lamps. With LEDs there is no speckle problem at all because their wavelength range is much broader than that of lasers. In order to make it even broader the green LED often is actually a blue one combined with a green phosphor. Here the phosphor does not have to be cooled as the light of an LED is much less concentrated than that of a laser. The only limitation of LEDs currently is the achievable brightness.

Lasers and LEDs both have a much longer lifetime than UHP or Xenon lamps. Start-up is possible almost instantaneously, and shut-down is also much faster than with conventional light sources, especially when no active cooling is required.

Let us come to wavelength multiplex 3D (WMT). This is known to work well with conventional light sources though a lot of light is lost in the filters required. When the emission bands of the primary colors red, green, and blue are narrower but not too narrow to be split into two bands each, the efficiency of a WMT 3D system will increase. This is the case for LED light sources. With laser projectors we have got a problem: Laser lines are so narrow, they cannot be divided into two that can be separated by any kind of filters. The solution for pure laser projectors is simple but costly: double the number of wavelengths used in order to get two blues, two greens, and two reds, one of each for the left image and one for the right one. This technique is called 6P for six primaries and is extremely light efficient. In addition, INFITEC offers lightweight glasses with unbreakable film lenses for these projection systems.

But what about laser phosphor projectors? The original design cannot be used with wavelength multiplex 3D, because a single blue laser line cannot be split into two which could be separated by glasses. But in the meantime, there are first projectors with two different blue laser wavelengths reaching the market. They are specially designed to fit with the wavelength multiplex 3D technology and passive glasses specially designed for these projectors are available from INFITEC.