Digital Light Processing

Back of the housing with connection contacts

Digital Light Processing (DLP, English) is one of the U.S. company Texas Instruments (TI) developed and registered as a trademark projection technology, such as video projectors and rear projection screens for home cinema and presentation area; and, under the name " DLP Cinema ," in the digital cinema field. The technology has been licensed to several manufacturers.

Digital Micromirror Device

A central component of DLP projectors is a Digital Micromirror Device (DMD ) called microsystem. It is a spatial light modulator (SLM dt spatial light modulator ). This consists of arrayed micro mirror actuators, that is tiltable reflective surfaces having an edge length of about 16 microns. The movement is caused by the action of force of electrostatic fields. Each micromirror can be individually adjusted in its angle and usually has two stable final states between which it can switch up to 5000 times within one second. The number of the mirrors corresponds to the resolution of the projected image, a mirror may represent one or more pixels. Meanwhile DMD chip available with resolutions up to 4160 × 2080 pixels.

Since the introduction of the DMD technology was an important aspect of the development of technology, in addition to increasing the resolution that in 2010 reached 4K, an improvement of the contrast. For this to around 2002 in particular, two improvements have been developed, which are referred to as "small Rotated via " (SRV ) and "small mirror gap" (SMG ). Via the tube structure connecting the mirrors to the base. Here a reduction of this hollow member has resulted in fewer reflections / scattering of light, and thus a 50 percent improvement of the contrast. The mirror between the falling and reflected light from the substructure could be reduced by reducing the spacing between the mirrors by SMG (30% increase in contrast ), which also improved the filling ratio. Finally, a new, inorganic coating of the metallic substructure was called "Dark Metal 3" introduced. Another change for contrast improvement was the increase of the tilt angle of the mirror from 10 ° to 12 °.

These improvements appear to have been in 2004 with the "Dark Chip 2 " integrated.

The following, introduced in 2005, "Dark Chip 3" continued the above measures.

The presented end of 2007, "Dark Chip 4" finally brought, according to Texas Instruments a further increase in the contrast of 30 % through advances in lithography and other process changes.

In 2011, can be found on the market the majority of models with the Dark Chip 2 or the Dark Chip 3; the Dark Chip 4 was less spread. Factors that together determine the contrast of a certain projector are, besides the fact DMD, inter alia, the iris is used, the light generation and bundling portion, the light trap for the deflected light and the remuneration and reflection poverty of glass and other surfaces in the lens.

Imaging

Different levels of brightness of individual pixels are generated by a binary pulse-width modulated control of the mirror. To illustrate, for example, 32 (= 25 ) levels of brightness 5 states are needed. These differ by how long the DMD is connected (see also binary code and binary system ). In the first state ( bit 0) is a mirror, the shortest possible time. On or off ( 1 or 0) Next state ( bit 1) is doubled, the time and so on. The total time for one cycle is thus at 5 bits total of 496 microseconds. The principle is illustrated in the diagram opposite. In practice, a slightly modified control is used for a better visual representation. All bits except bit 0 and 1 are subdivided into sections, which are distributed over the entire cycle ( so-called bit allocation ).

Color representation

Since the DMD chip reflect the white light from the projection lamp, extra steps are required for a color image. There are the following two methods: the one-chip and three-chip technology.

When this projector is in a single-chip technology (also called mono chip technology) is connected a color wheel in the light path before the DMD chip, on the color filter of the primary colors (red, green and blue in the rule, but in some cases even more ) rotate. To achieve better brightness values ​​in the White, the color wheel, a white sector can also be added. With the position of the color filter, the electronics to switch the sub-image that is reflected from the DMD. Due to the rotational speed of the color wheel and the inertia of the human eye, the partial images are added to a colored image impression. Since the detection frequency is different from person to person, it was mainly the first models (x1) reports of a so-called rainbow effect, which then occurred when the viewer perceives the individual colors. Therefore, the rotational speed of the wheel was in a first step doubled (x2 ), and in more recent models, the color segments of 4 ( RGB and white ) to 7 (2 × 1 × RGB and white) increased ( x4).

A dark green sector seems to counteract color noise in the shadow areas.

To (up to 50 % luminance gain) to increase the gamut and image brightness, Texas Instruments offers since about 2005, marketed under the name " Brilliant Color" technology to where permitted the coordination between the mirror circuits of the DMD and the color wheel more options. Originally color wheels were provided with three equally sized segments colored in red, green and blue. However, this limitation brought with it that particularly strong hues in the intermediate colors, such as yellow, could not be displayed correctly. Moreover brought the filtering out those parts of the light spectrum of the lamp that are not red, green or blue, a brightness loss. BRILLIANTCOLOR enabled however, the use of color wheels with more segments, and additional colors. In the original version, these were, in addition to the above basic colors, cyan, magenta and yellow (see CMYK ), but some manufacturers vary the colors and let eg magenta away in favor of a white sector. BrilliantColor enables a flexible selection of color segments, for example, RGBRGB. Even with the use of such color wheels is still an enlarged color space depicted, as BrilliantColor allows to exploit the transition region between two color segments, ie one in which two adjacent segments at the same time " sweep " the DMD for color mixing. Originally an expensive technology, Texas Instruments incorporated BrilliantColor control from 2007 cheaper chipsets, so the technique is now widely use.

A fundamental problem in display devices (not just projectors, but also television sets) with an extended color space as with BrilliantColor however, is that on Blu -Ray discs, the video is recorded with the much narrower color space BT.709. Therefore benefit BrilliantColor projectors, Blu- ray movies to play, first of all by the extended color space. To use the extended color space, so use the projector manufacturer algorithms that represent the colors of the BT.709 on a wider color gamut. This usually leads to stronger colors, especially with the secondary colors cyan, yellow and magenta, but carries the risk of an unnatural color display ( "Wild gamut " ), which also can be different from the intentions of the director of a film ( in the conversion of movie on Blu - Ray already had to make compromises in terms of color representation). In digital cinema, the films therefore with a much wider, as defined by the Digital Cinema Initiative color space ( DCI/P3 ) are coded.

Another technique for expanding color gamut and contrast, the dynamic control of the projection lamp dar. This regulation is per color wheel segment and thus allows an individual up-and down regulating the brightness of each of the colors used in the color wheel, in addition to the time specified by the Spiegeloszillationen brightness. This technique is marketed by Osram under the name " UNISHAPE " and Philips as " VIDI ".

For each projector can be the color wheel by the end user to one with a different color assignment exchange, which allows a more flexible use.

In a projector in the three-chip technology, the light after the light of red, green and blue divided by dichroic mirrors in three primary colors and separately over three DMD chips. The respective partial reflection of the individual DMD is added back in a so-called dichroic prism, which includes two cross dichroic mirror to the complete color image. The rainbow effect can not occur in these models. Due to the over one-chip models substantially higher price, these devices are used because of their high color fidelity, especially in film studios, cinemas and other color-critical applications. The disadvantage is that the image is composed of only three single colors. For color wheels, each manufacturer can decide which segments get what color. Thus, one can also offset inks project. In early 2012, a three-chip projector for cinema was first demonstrated, in which the lighting is not performed by means of lamps, but by three laser. With this approach, a brighter image can be generated. As further advantages improvements in the sharpness be specified (probably due to ending oblique rays), contrast (elimination of stray light ) and color gamut ( purer colors with lasers ). A launch could take place by 2015, which is currently the cost of cinema operators are still too high and regulatory issues as well as problems to be solved with " speckles " in the image.

Pros and Cons / artifacts

Thanks to the more direct light path compared to LCD technology and the lack of polarization of the light higher light output powers are achieved than with an LCD projector. Comparing the image of a DLP projector with an LCD projector, a fall on the softer rasterization of the image, which has a positive effect on the impression. Thanks to the large inclination angle of the micromirror high contrast values ​​are achieved. In addition, the DLP technology on the order of micro seconds, so that streaking can be prevented. This is noticeable especially in stereo 3D mode, in which the rapid pace of change is no " ghosting " (crosstalk between left and right 3D image) are generated.

In dark areas DLP projectors can show a noise " like a fine swarm of flies " acts, and which is caused by different temporal control of the individual mirror.

Older A DMD chip projectors show high-contrast transitions ( mostly black and white) but a rainbow effect, especially when the images change rapidly or eyes wander rapidly over the image. The basic colors of the color wheel are visible on the contours of the object, which can be very disturbing to many viewers. This effect can be more clearly seen when one puts his hand into the beam path, the fingers spread open and back and forth.

The manufacturers try to reduce this effect by that color wheels are used with more than three segments or at a higher speed. Devices that use a color wheel instead of three sets of differently colored LEDs, such as the Samsung HL- S5679W should, because of the potential for LEDs higher switching speed for color changes no longer show a rainbow effect. A test in 2011 but still showed a series of LED DLP projectors " clearly be perceived " rainbow effects.

For 3D projectors ( alternately projecting the images for the left and the right eye after the other ) is the ability for faster switching between images as compared to LCD and LCoS projectors advantageous since this crosstalk between the images ( with one eye at least a part of the image seen, which is intended for the other eye ) can be avoided. Crosstalk can however still occur through the shutter glasses used.

Life

In contrast to LCD technology DLP projectors are less or not at all affected by fading and branding colors. However, this is not because the colors fade, but due to the engines, their lifetime is limited by the bearings - The color wheel, constantly spinning during the projection, has a service life of approximately 20,000 hours. However, the LCD panel or the polarization filter of LCD projectors have faded after 4,000 to 6,000 hours and no longer suitable for use. The fact that a lower light intensity is needed, the light source of the DLP projectors have a life of up to 6000 hours, after which they must be replaced.

Tests from 2012 have shown that DLP Projectors - regardless of manufacturer - are superior to LCD projectors, as they have still not projected without any impairments and loss of quality, while the colors could not be more different for LCD projectors or image had lost all shape.