A video projector (also projector , digital projector , data video projector ), colloquially mostly beamer [ ˈbiːmɐ ] ( pseudo-English word coining , derived from English beam ), is a special projector that produces still and moving images a visual output device ( television receiver , computer , DVD player , VCR , etc.) for an audience in enlarged form on a screen (also projected image) projected . The range of devices ranges from small presentation projectors for mobile use to stationary high-performance projectors.
Video projectors have evolved from previous types of projectors. The history of previous projectors has been described separately.
Video projectors can basically be differentiated with regard to the projection method used.
The Eidophor system was the first system that could deliver bright images in high resolution and was common in the professional sector until the late 1980s.
The first video projectors with wider distribution used special cathode ray tubes to display the image. These tubes were designed for very high brightness in order to provide enough light for projection. For color projectors, three tubes are used - one for each primary color - that have separate lenses . The brightness and the supported resolution usually increase with the tube size. There are three main sizes for projection tubes: 7, 8 and 9 inches. With regard to the highest possible sharpness of the electron beam, a distinction is made between ES (electrostatic) and EM (electromagnetic) focusing devices. With the correct setting, EM devices can project a very sharp image. 9-inch devices with EM focus are still used today in high-resolution flight simulators and high-end home theaters. 7 "and 8" sets were often used in rear projection televisions in the past. In addition to older, used models, there are only a few new devices on the market (Barco, VDC).
- Because of the picture tubes, the projectors are very variable in resolution. You can usually display from NTSC to 1080p and also 3D material (Bluray-3D, sequential 3D).
- The process does not know any pixels (picture elements). This means that the images are displayed a little more blurred, but more natural.
- There is almost no delay time. This are videos with interlaced scanning ( interlacing ) no problem.
- No additional light source is required, as the tubes themselves generate light.
- The tubes produce very high contrasts (10,000: 1 to 30,000: 1) and very good black levels.
- The typical service life of the tubes is over 10,000 hours.
- Tube projectors have a relatively low overall brightness. For most models, the room needs to be darkened as much as possible.
- The tubes are very sensitive to burn-in . If parts of the luminescent layer are stimulated too strongly or for too long, they gradually darken there.
- Since the three projection systems work separately in color projectors, they require a complex set-up procedure. This makes mobile use difficult.
- The projectors are very heavy due to the tubes.
- Because of the relatively long decay time of some types of phosphors, some tubes produce too much ghosting during 3D playback.
- Good used or new devices are still very expensive.
Liquid crystal projectors ( LCD ) work in principle like slide projectors. Instead of a slide, one or more transparent active matrix liquid crystal displays are used, driven by thin-film transistors . Devices common today use three separate LCD matrices ( 3LCD technology) - one for each basic color - whose projection is combined to form an image using a specially arranged projection system with dichroic mirrors . This enables the superimposed color composition (in contrast to the side-by-side display on LCD monitors). Particularly high-quality devices use an optical unit with four separate LC matrices, whereby in addition to red, blue and green a further matrix is used especially for the differentiated and bright representation of yellow. This allows you to combine a high light output with a good color balance, which is quite difficult for conventional devices with three LC matrices - their green display often has a slight yellow tinge, the correction of which is usually associated with a very high loss of brightness.
In devices with only one liquid crystal display , the three basic colors are represented by three integrated matrices arranged next to one another, as on LC flat screens , which, due to the reduced resolution, leads to a coarser image impression.
- LCD projectors are relatively inexpensive.
- They enable good readability of texts and graphics through the sharp delimitation of the pixels.
- The devices can be built small and light.
- Compared to single-chip DLP projectors of the same light output class, LCD projectors show a much better color intensity.
- The sharply defined pixel structure ("fly screen " effect, English screen door ) can be perceived as disturbing. This disadvantage existed especially in early devices with low resolution and comparatively large conductor tracks on the LC matrix. With current HD projectors, the fly screen hardly shows up or not at all.
- The LC matrices have a fixed resolution. Any deviating resolution must be adapted (interpolated), which reduces the image quality. This applies in particular to perspective corrections ( keystone ).
- In the case of sluggish LCDs, the image has a trailing effect. This disadvantage mainly affects old devices.
- Liquid crystals have a relatively limited operating temperature range. In order to keep the LCDs in this area, measures must be taken such as separate polarization and color filters (see Imagina 90 ) and an effective ventilation concept. It is thus possible that the clearing point (transition from the liquid-crystalline to the liquid phase) is not reached. The fans of LCD projectors can usually be heard clearly; this even applies to "whisper-quiet" home theater devices.
- LCD memory effect (LCD burn-in): If image areas are stimulated for too long with images that are too bright, these areas slowly become permanently dark. After a few 1000 hours of operation, the electro-optical properties of a layer with organic liquid crystals are permanently impaired due to the high light intensity (especially the short-wave light component). When using polarizing films with organic dyes, their optical properties can also be reduced. This affects the properties of the projector (contrast, color spectrum). Inorganic LCD panels, which have been increasingly used since 2008, no longer show this drawback according to the manufacturer and have a “far longer”, but so far unspecified lifespan.
- Sensitivity to dust and smoke as both the lamp and the LCDs need to be cooled with fresh air. Encapsulation or sealing of the optical unit is therefore not possible (in contrast to DLP projectors). The image quality therefore deteriorates quickly in very dusty or smoky environments.
- Since the images of several LC matrices are projected on top of one another, incorrect convergence can occur.
A digital micromirror device (DMD) is used as the image converter of a DLP ( Digital Light Processing ) projector , an integrated circuit on which a tiny mirror that can be tilted by an electrical pulse is located for each individual pixel. The image is generated by targeted control of the tilting mirror so that the light is directed or deflected in the direction of the projection optics. Since this type of image generation only knows the two states on and off , gradations of brightness must be achieved by rapid pulsing. The DLP mirrors switch up to 5000 times per second.
Most devices use a rapidly rotating color wheel to generate a color image, with a DMD projecting all three primary colors one after the other. In newer design DLP projectors, the number of color segments has been increased to six to seven in order to be able to achieve more exact results with color mixtures. The use of a transparent segment in the color wheel in all newer single-chip DLP projectors that are not specifically intended for home cinema use is noteworthy. This has the advantage of a higher light output when displaying white surfaces. However, the transparent segment is at the expense of color light output, which means that DLP projectors have a visibly poorer color display and intensity when projecting onto larger surfaces. Three separate image converters (DMD) are used in projectors for the professional sector (e.g. cinema). LED projectors switch colors electronically.
The sole technology owner of DMD production is Texas Instruments (TI).
- very high speed, therefore no afterglow or dragging of the image, therefore very well suited for 3D projection
- no burn-in of the image (e.g. in computer games)
- higher contrast (due to the deeper black) than with the LCD projector
- less pronounced pixel structure than with LCD projectors
- Encapsulated optics and durable DMD make it less sensitive to dust than LCD projectors.
- Due to the fixed resolution of the mirror matrix in the device, the best quality is only given with a certain resolution of the input signal. Otherwise a quality-reducing scaling is necessary.
- Rainbow effects on some devices with a color wheel if the color wheel does not rotate at a high speed (depending on the manufacturer).
- When displaying certain, individual gray or color values, there may be visible flickering.
- Color fidelity is sometimes not given. In particular, DLP projectors have a problem displaying rich green and also all red and orange hues. This mainly affects the devices for the private sector (single-chip DLP), since they use color wheel technology.
- Devices with a white segment offer a poorer color light output than LCD projectors of the same brightness class.
- Noise generated by fan and color wheel
Since the light output of light-emitting diodes is not particularly high, especially in the green between 540 and 610 nanometers wavelength , there are hybrid products in which the green light is generated by a laser diode instead of a light-emitting diode .
- Compared to conventional projector lamps, LEDs are more energy efficient: with the same light output, less energy is converted into heat, which reduces the need for cooling.
- The lower cooling requirement allows smaller housings and lower fan noise (in extreme cases passive cooling).
- The lower energy requirement enables operation with a battery.
- LEDs last over 20,000 hours, while conventional projector lamps last around 4,000 hours.
- Since the colors are formed by sequential lighting up of the RGB LEDs, the color wheel normally required for DLP projectors is also omitted.
- Because of the fixed resolution of the LCD in the device, the best quality is only given with a certain resolution of the input signal. Otherwise a quality-reducing scaling is necessary.
- The light output of LEDs is sometimes considerably lower than that of conventional projector lamps. In particular, inexpensive devices sometimes only have a hundredth of the light output of conventional projectors. In summer 2011 there was the first LED projector with 1000 ANSI lumens .
- A permanent decrease in the luminous flux of the LEDs makes the picture steadily darker, and a simple change of the LED light source is not always possible. An adaptive increase in the LED current that is sometimes used can compensate for this loss of brightness.
- Even without a color wheel, the DLP-based models produce a rainbow effect, as the basic colors are projected one after the other.
With LCoS technology ( Liquid Crystal on Silicon ), which also uses liquid crystal displays, the LCDs are used in reflective mode. In contrast to the DLP technology, this LCD technology is represented by different manufacturers by different brand names such as SXRD (Sony) or D-ILA or DLA (Sanyo). The projection optics are similar to those of a DLP projector because the liquid crystal displays are mounted on a reflector with control electronics underneath.
The main advantage of the LCoS chips is that the signal lines for controlling the individual pixels are hidden behind the mirror surface, so that the distances between the pixels are smaller compared to conventional LCDs, which significantly reduces the well-known " fly screen effect " and provides higher contrast values and in particular, a deeper black can be achieved.
- compact design
- sharp images
- low loss of brightness when using high-resolution LCDs
- Black levels similar to tube projectors
- Due to the fixed resolution of the LCD in the device, the best quality is only given with a certain resolution of the input signal. Otherwise a quality-reducing scaling is necessary.
- slight tracing of the image
- LCD memory effect
- Decreasing image quality with increasing operating time (reduction in color saturation, contrast and homogeneity)
A laser can also be used as the illuminant, which excites a fluorescent material in order to generate polychromatic light. This phosphor is often applied to a rotating disk in order to enable or simplify cooling and thus to prevent thermal destruction of the phosphor. The point of the fluorescent material that is stationary in the beam path and usually shines white can then be used as a non-thermal light source in the illuminating beam path of a projector. Such devices have also been available for private users since the 2010s, and they are characterized by a comparatively long service life and low maintenance requirements.
The devices with the brightness modulated and rapidly deflected laser beam, which have so far only been developed for the professional market, are available in two versions: the laser display technology (see below) and the GLV technology ( Grating Light Valve ). Strictly speaking, it is not a matter of a projection - lenses are used solely to expand the beam and not for optical imaging.
With the Laser Display Technology (LDT) developed in Gera and Jena , the image is written line by line on the projection surface. The distraction is done by a special scanner with a facet mirror (line structure) and a tilting mirror (line feed). The laser beam is modulated beforehand, which defines the brightness and color of each pixel. In contrast to the GLV technology, the speckle effect of the laser is almost completely suppressed with the LDT. For this purpose, the laser beam is pulsed in the picosecond range. Hazards from the laser beam are eliminated through the combination of various safety technologies. Laser display technology projection systems are used for flight simulators. Here an entire hemisphere is illuminated with four projectors (360 ° horizontal × 90 ° vertical).
- projection surfaces of almost any shape
- no focusing necessary
- very high contrast
- large gamut due to the mixture of pure basic colors (RGB), i.e. monochromatic light
- limited light intensity
- The laser projectors are classified as laser class 3R (GLV) or 4 (LDT), so operation is only permitted by appropriately trained personnel.
- actual danger to eye health when looking into the beam current or from beams directed directly into the eye
Alternatively, there are also approaches in which the laser beam is fanned out and combined with a conventional image generator (e.g. LCoS).
field of use
Depending on the application, some of the projection techniques mentioned above are more suitable than others.
For the projection of films it makes sense to use projectors with widescreen format (e.g. 16: 9, 16:10 or 21: 9), since black bars at the top and bottom of the image can be reduced. Home cinema projectors of this type assume a well-darkened projection room and are not designed for excessive brightness, but rather for natural color representation. Often they have large fans that rotate more slowly for the same air mass movement and thus cause less noise.
Due to a very good black level, many DLP projectors have a very high contrast ratio of up to 4000: 1 compared to other projection technologies . LCD projectors optimized for home theaters achieve dynamic peak contrast values of 10,000: 1, primarily through the use of a mechanical iris diaphragm , which reduces the luminous flux in dark images. The in-picture contrast of LCD projectors lags behind that of DLP projectors, since LCD technology allows residual light to pass through even with black pixels. This worsens the black level and reduces the possible contrast.
For historical reasons, presentations are still often made in the 4: 3 format. The 16: 9 or 16:10 image format is becoming more and more popular in the office environment, because both laptops and desktop PC displays are delivered in this format. The availability of office video projectors with this aspect ratio is therefore increasing continuously. Video projectors of this type are often designed for high projection brightness in order to be able to generate a visible image even in not particularly darkened office spaces. The color fidelity (accuracy of color representation) often suffers.
An essential criterion for the usability of video projectors with a larger projection distance or large projection area is the luminous flux .
Commercially available video projectors have luminous fluxes between 1000 and 4500 lumens , large models for conference rooms or movie theaters also have up to 30,000 lumens. The actual usable luminous flux depends on the settings of the projector with regard to contrast and color gradient. With a balanced, natural mixture of colors, it is occasionally well below the nominally stated values. The manufacturer's brightness specifications contain comparatively large tolerances. Usually the deviation is 10 to 20 percent. In principle, the aging of the lamp leads to a loss of brightness. After reaching the service life specified by the manufacturer, the brightness should still be 50 percent.
Since the lighting means used usually convert a substantial part of the electrical energy supplied into heat, they have to be cooled, usually by fans . Particularly compact and particularly powerful video projectors therefore stand out acoustically.
In addition to usually larger fans, home theater models sometimes have an economy mode ( Eco mode ) in which the brightness and fan speed are reduced. In addition to reducing noise, this extends the life of the lamp and reduces the burn-in of liquid crystal displays.
Projection ratio (also called projection distance ratio)
The projection ratio can be used to calculate the possible image size (or size range) for a given projection distance or the necessary projection distance for a desired image size.
The projection ratio indicates the ratio of the projection distance to the image width and depends on the lens used . Since the devices are usually equipped with a zoom lens , a range is specified for the projection ratio. At a projection distance of 6 meters, a zoom lens with a projection ratio of 1.8: 1 to 2.3: 1 leads to an image that has a minimum width of 2.61 meters and a maximum width of 3.33 meters. So the smaller the ratio, the larger the image width.
The image width is calculated as follows , the image height can be determined from the aspect ratio, i.e. H. for projectors with a classic 4: 3 aspect ratio or for devices with 16: 9 widescreen format.
Commercially available models have a projection ratio between 0.6: 1 and 3.0: 1, depending on the lens. Most conventional devices have possible projection ratios between 1.5: 1 and 2.2: 1, with great differences in the flexibility of the zoom or the projection ratio. Many inexpensive video projectors do not have a zoom; To change the image size, you only have the option of moving the projector towards or away from the projection surface. Some professional video projectors have the option of changing the lens.
If the device does not project with its projection axis perpendicular to the projection plane, the image will be trapezoidal . The edge of the image farther from the projector will be enlarged, while the edge of the image closest to the projector will be reduced. The distortion can be compensated by lens shift or digitally by keystone correction (the English term keystone stands for a trapezoidal keystone in brick arches).
Possibilities for lens adjustment perpendicular to the optical axis are usually only found in higher quality models for professional use or in home cinema models. The lens is moved mechanically and does not affect the resolution of the projector. Devices with this option are not tilted in order to display the image asymmetrically to the projection axis without distortion.
Devices without adjustment options for the lens are usually tilted if they are set up away from the central normal to the image, but still aim at the same projection area. Many devices use keystone correction to correct the resulting trapezoidal distortion, which digitally distorts the image before it is projected. In the best case scenario, this computational distortion compensates for the distortion caused by the oblique projection, so that the expected parallel and vertical image edges appear on the screen. Disadvantages of this procedure are the reduced resolution, blurring and staircase artifacts in the image caused by the interpolation, the reduced effective light output and the black areas outside the actual image that are visible in well-darkened rooms. The last effect occurs mainly in projectors with liquid crystal screens.
Short throw projection
If it is not possible or not desired to set up a projector several meters away from the projection screen, short-throw or even ultra-short-throw projectors can be used, in which the distance from the lens to the projection plane is only a (small) fraction of the image diagonal .
The projectors use special lens and mirror systems in order to be able to project undistorted and high-resolution images despite the very short image widths . The adjustment is comparatively complex, and with movable or uneven projection walls, disturbances in the image geometry occur during playback.
Volume and noise development
The light source in a video projector generates waste heat , which is usually removed by one or more fans . Depending on the location and purpose, the resulting size of the projector and the light-generating technology, fans of different sizes with speeds that depend on this size are used. Fan-based cooling concepts cause acoustic noise. To reduce this, video projector providers offer a so-called eco mode, especially in the home theater sector, in which the luminance is reduced. This makes it possible to reduce the fan speed.
Home cinema projectors are usually more voluminous, so that large fans can be built in that achieve a specified air flow rate at a lower speed than with smaller fans. This enables quiet operation close to the seat.
Office projectors are designed for brightness so that they can project an image that is easy to read even in daylight. This leads to greater heating of the illuminant, which in turn requires greater cooling. Consequently, the fans must be large and / or spinning quickly. The greater noise development compared to home cinema projectors is tolerated due to the already noisy office environment.
Professional projectors such as those used in digital cinema projection or conference projectors for large halls are sometimes quite loud due to the high volume of the fan. They usually project from a greater distance to the audience or, in cinemas, from separate rooms.
The volume can be specified in dB . Quiet video projectors produce less than 25 dB, loud more than 30 dB. Very quiet projectors generate noises in the range of 19 decibels, very loud projectors even reach almost 50 decibels.
Video projectors process analog and digital image signals (from e.g. DVD players , PCs , video recorders , DV camcorders , TV tuners , etc.) since 2005 at the latest (price-dependent ). There are analog connections such as YPbPr - component video inputs (three cinch plugs ), SCART , S-Video , composite video , RGBHV or VGA . Since most current video projectors work internally digitally (LCD, DLP, etc.), analog input signals are first digitized, which can lead to quality losses.
Occasionally there are other special connections. In home theater projectors, for example, there are so-called triggers that can trigger the unrolling or rolling up of a motorized screen. Fixed-mount projectors are sometimes equipped with a serial interface through which the menu can be controlled remotely. There are also devices with sockets for a cable connection with a remote control.
The handling and the quality of the video projection are influenced not only by the performance characteristics of the projectors themselves, but also by important accessory components: projector brackets and screens. For mobile or temporary playback, for example, projector tripods or carts are available, while ceiling or wall brackets are intended for permanent installation. Similar criteria apply to the projection surfaces. Where a permanent projection surface is required, specially coated surfaces or fixed frame screens are a good choice. Tension screens are available for mobile projection. For example, where there is no fixed projection surface available in the home cinema area due to lack of space, roller screens with or without a motor drive have proven themselves.
In 2004, the publication of computer hardware website became Tom's Hardware Guide greater prominence was given in the building instructions how to get out a hand (used) flat screen and a overhead projector a XGA Own Projector created. The instructions assumed that the necessary hardware would be purchased from eBay , which led to the named material costs of 250 euros.
There was a need for alternatives to conventional video projectors, partly due to the high new prices for video projectors with resolutions over 800 × 600 pixels and the expensive, short-lived light sources - daylight projectors, on the other hand, are sometimes operated with conventional incandescent lamps, which are cheaper.
However, the quality of conventional video projectors could not be achieved: The illumination setting options of an overhead projector are minimal. The flat screen panel is not designed for the luminance and illumination of a projector, which is why the image quality suffers. Color distortion is a typical feature. In addition, daylight projectors with incandescent lamps consume a relatively large amount of energy.
A video projection can also be achieved with a smartphone: in a darkened room, the cell phone screen is displayed e.g. B. projected on the wall through a magnifying glass. Unfortunately, the light intensity of such a projector is low. The resolution of professional equipment cannot be achieved.
The name Beamer, which is common in German, Dutch and Latvian, is a separation of the brand name of the first tube projector, the Advent VideoBeam 1000 ( beam = beam / to beam = strahlen ), from the generic term .
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