Patent Application
20070030563
The invention relates to a multi-bandpass filter for use in color projection devices for efficient color correction.
So far it has not been possible to replace the gas discharge lamps used today as white light sources in color projection systems in terms of intensity and reliability. Yet, they have a series of undesirable radiation characteristics, which require action.
Gas discharge lamps like those used in projection displays, in addition to visible light, also emit high-intensity ultraviolet (UV) radiation and infrared (IR) radiation. In this specification, UV radiation is considered radiation with a wavelength less than 420 nm, but greater than 300 nm. IR radiation is radiation with a wavelength greater than 690 nm, but less than 2 μm. These UV and IR rays can cause significant damage to the optical components of typical projection display devices. When subjected to UV radiation, the component materials may decompose. This happens particularly to components containing organic materials. The IR rays may result in extraordinarily high and hence stressful temperatures and/or temperature gradients inside the optical components and, in extreme cases, may destroy them.
For this reason, both UV filters and IR filters are required in projection display applications. Filters of this kind are particularly necessary for projectors, which use liquid crystal components (LCD) as imaging elements. LCDs of this type are particularly sensitive to UV radiation and/or high temperatures.
The UV and IR filters are generally placed directly behind the light source in the optical path in order to filter out damaging UV and IR components of the radiation as early as possible.
In configurations based on 3 imaging elements, the light, which is generally white, is split into three beam paths. Typically such splitting is done using two dichroic color filters, which are placed, for example, at a 45° angle on the optical axis in the beam path. If the first filter is a blue reflector, for example, blue light B is reflected at an angle of 45°, i.e. deflected 90°, while green light G and red light R are transmitted through the filter. If the second filter is a green reflector, green light G is reflected and red light R is transmitted. This way the original beam of white light is split into three partial beams.
However, splitting it cleanly at wavelength intervals is difficult because generally the dichroic filters are not acted on by parallel beams of light, but instead a wide range of angles is represented in most cases. The reason for this is that lenses are installed in the projector in an attempt to minimize loss along the path of the beam. The consequences are non-parallel light beams, so-called conical intensity distributions with low F-numbers. Since the spectral characteristics of dichroic filters vary as a function of the angle of incidence (particularly in relation to the position of the filter edges), spectral splitting is limited, and the color of the beams inside the cone of incidence varies as a function of the angle of incidence.
This means that the blue light beam may have portions of its wavelength that may actually be associated with the green light beam, the green light beam also has blue and yellow-red portions, and the red light beam also has yellow components. These undesirable components make the color saturation that can be achieved with the projection device insufficient in many cases.
If UHP lamps are used, there are also pronounced interfering intensity peaks present in the emission spectrum. In particular, the intensive yellow peak produces a slightly red impression in the image.
It is therefore necessary to improve the color saturation. Typically, color filters are used for this purpose, i.e. so-called trim filters are placed in the individual partial beams. These trim filters are normally also composed of dichroic filters, but they are placed perpendicularly or almost perpendicularly in the path of the individual partial beams R, G and B. Since the angular dependence of the spectral characteristics of these dichroic filters is less prominent for small angles (with almost perpendicular incidence), the color saturation improves significantly.
However, one disadvantage is that the additional trim filters result in added cost. To produce these filters, additional substrates have to be vacuum-coated. These filters must also be placed in the housing, which requires other holders and/or assembly and adjustment steps. In addition, although the trim filters are used at a basically perpendicular angle of incidence, due to the cone of light and the angular distribution produced in it, a relatively broad spectrum of angles is produced. As a result, the color saturation cannot yet be optimally configured.
The object of this invention is therefore to solve, at least partially, the aforementioned problems in the state of the art. In particular, the object of this invention is to achieve good color saturation with no need for additional components, such as trim filters, in the paths of the partial beams.
The invention solves the problem by manipulating the spectrum by means of a modified UV-IR filter placed basically directly behind the lamp.
Typically, UV filters and IR filters are made on two substrates or on the two opposite sides of a transparent substrate.
In the first embodiment of this invention, the UV filter and the IR filter are built into a system of layers that is made on only one side of the substrate. On the opposite side of it, for example, a simple anti-reflective coating can then be provided.
In another embodiment of this invention, in the area where otherwise only UV radiation and IR radiation are blocked, now the crossover areas between blue and green and between green and red are blocked, at least partially, and so color trimming is already achieved shortly after the beams are produced. This can be done with an additional filter in front of the UV-IR filter or behind it. However, it is a special advantage, and therefore also an inventive step, to build a trim filter of this type directly into the system of layers of the UV-IR filter. This obviates the need for one or more additional substrates, which otherwise would be required for the trim filter.
This invention will be explained in greater detail below with reference to the examples given in the figures.
This means that modified white light, which contains at least roughly three separate wavelength ranges RGB and to a large extent no longer contains any UV and IR components, is transmitted by the multi-bandpass filter. In the drawing, this light has been marked as RGB light.
The PCA 7 and, if need be, a first lens system 11 are now downstream from the multi-bandpass filter 9. Further downstream in this example is a first dichroic mirror 13, which reflects blue light B and transmits red light R and green light G. Further downstream from the red and green partial beams, there is a second dichroic mirror 15. It reflects green light G, while it basically transmits red light R. As a result, the original unpolarized beam of white light is split into three colored, basically polarized partial beams.
The reflected blue light B is reflected via a deflecting mirror 17 in the direction of the transmissive liquid crystal component tLCD blue 19 provided for blue light. There, its polarization is modulated spectrally resolved. Typically, in the state of the art, a trim filter would be placed upstream from the tLCD. But because of the multi-bandpass filter 9 in the invention, this is not necessary. A polarization filter connected downstream from the tLCD transforms the spectrally resolved, polarization modulation into spectrally resolved, intensity modulation.
The green light G accordingly shines on a tLCD green 21 and is polarization-modulated there. The polarization modulation is transformed to intensity modulation by means of a polarization filter (not shown).
The transmitted red light R is reflected via deflecting mirrors 23, 23′ in the direction of the transmissive liquid crystal component tLCD red 25 provided for the red light. There, its polarization is modulated spectrally resolved. A polarization filter connected downstream transforms the spectrally resolved polarization modulation into spectrally resolved intensity modulation.
In the example, the spatially intensity-modulated partial beams are combined downstream by means of a color cube 27.
The color cube is followed by a projection lens system 29, which contains at least one lens and reproduces the image defined by spatial modulation of the tLCDs on a projection plane.
In the state of the art, trim filters would be connected directly in front of the tLCDs. The inventive multi-bandpass filter provided in this invention directly behind the light source, however, eliminates the need for this. In essence, the trim filter can be eliminated.
For further fine trimming, however, it is certainly conceivable to provide additional trim filters without running counter to the purpose of the invention.
As
The transmission difference between 415 nm and 435 nm is preferably at least 90%, and/or the transmission difference between 675 nm and 700 nm is preferably at least 90%.
The transmission in the crossover areas between blue and green and between green and red is preferably at least less than 10%.
The system of layers used to configure the UV-IR filter preferably contains an interference layer system. By varying the refractive index of the layers in the system, interference effects of the light occur inside the layer system, resulting in wavelength-dependent reflection and/or transmission. Interference layer systems may contain an alternating system of layers made of materials with a high refractive index and a low refractive index. Materials with an index more than 1.70 at a wavelength of 550 nm are considered materials with a high refractive index. Examples are TiO2 and Ta2O5. Materials with a refractive index less than 1.55 at a wavelength of 550 nm are considered materials with a low refractive index. Examples are SiO2 and MgF2. Materials with a refractive index greater than or equal to 1.55 and less than or equal to 1.70 at a wavelength of 550 nm are considered materials with an average refractive index. An example is Al2O3. Optical interference layer systems suitable for this invention may contain materials from only one of these three groups, only two of these three groups or all three groups or mixtures thereof. Preferably, however, an optical interference layer system is made of a system of alternating layers of materials from the groups of materials with high and low refractive indices.
| Number | Date | Country | |
|---|---|---|---|
| 60688230 | Jun 2005 | US |
