Compensation of light source intensity pulse in scrolling colour type projection system

A scrolling colour projection system comprising a lamp (4) with a pulsed drive current (20) and a colour scanner (6, 8a, 8b, 8c, 9) for generating a light beam (5b) with a plurality of scrolling colour fields, which is arranged to illuminate a display device (3) to produce a projection of an image generated by the display device. The system further includes a filtering element (31) arranged in the light path between the lamp and the projected image, and the transmission of the filtering element (31) is synchronized with the lamp current so as to cancel an intensity peak in the lamp flux. The filtering element thus effectively removes the impact of the current pulse on the light intensity, resulting in a constant intensity in the projected image, and avoiding any interference patterns.

The present invention relates to a scrolling colour projection system comprising a pulsed lamp and a colour scanner for generating a light beam with a plurality of scrolling colour fields, arranged to illuminate a display device to produce a projection of an image generated by the display device.

Such projection systems are particular in that light from a light source is divided into a plurality of beams, which are sequentially scrolled over a display device, e.g. a reflective LCD, and then projected by means of a lens. Normally, the three beams (R, G, B) are arranged to form three horizontal bars with a total height which is large enough to cover the reflective display. The bars are scrolled, e.g. from top to bottom, and are synchronized with the display so that they complete a scrolling sequence within one picture frame.

In such projector systems, it is advantageous to use a light source, e.g. a UHP (ultra high performance) lamp from Philips, having a superposed current pulse to stabilize the arc position. In a scrolling colour type of projection system, such a current pulse may interfere with the colour scanner and result in visible interference patterns in the projected image. In principle, the pulse acts as a stroboscope, highlighting a momentary image of the scanner, and may make interference patterns in the form of colour bars or the intermediate fields (spokes) visible on the screen. If the pulse frequency is a sub-frequency of the frame rate, the interference pattern will be fixed, and if the lamp frequency is out of phase with the frame rate, the bars will roll across the screen.

It is an object of the present invention to provide a scrolling colour projector system having a reduced image interference.

This object is achieved with a projector according to the invention as specified in claim1.

The filtering element thus effectively removes the impact of the current pulse on the light intensity, resulting in a constant intensity in the projected image. Therefore, the mentioned interference patterns do not occur.

The filtering element can be located anywhere along the light path, for example behind the projection lens, or in front of the colour scanner. A filtering element located near the lamp must have a good resistivity in order to withstand the higher light intensity. A filtering element located close to the display must be free from distortion (e.g. particles on the surface) as such distortions may be visible in the projected image. Another factor is the size of the filtering element, which varies along the light path.

According to one embodiment, the filtering element is a liquid crystal (LC) cell. The transmission of the cell is then controlled so as to be synchronized with the lamp current and adjusted in accordance with the size of the peak in light intensity, so as to accomplish the constant intensity. In order to secure a long life-time, the LC cell is preferably placed behind the projection lens, where the light intensity is lower.

According to another embodiment, the filtering element is a rotating disc having a field with reduced transmission. The frequency of said wheel is synchronized with the lamp current, so that the field crosses the light path when the peak in intensity occurs. By adapting the transmission of the field in accordance with the size of the peak, a constant light intensity can be obtained.

A projection system with scrolling colour scanning according to the prior art, also referred to as Scrolling Colour Sequential (SCS) system, is illustrated inFIG. 1. The system comprises a display driver1, arranged to receive a data input stream2, from e.g. a personal computer or a video cassette recorder (not shown), and to drive a display device3, such as a reflective LCD. A light source4, preferably a UHP lamp followed by an integrator, is controlled by a lamp driver12to generate a light beam5a, which passes through a colour scanner (6,8a,8b,8c,9). The colour scanner converts the light beam5afrom the lamp4into a beam5bhaving a plurality of differently coloured fields, typically three colour bars (R, G, B), continuously scrolling from top to bottom (seeFIG. 2).

In the example illustrated inFIG. 1, a first set of mirrors6divides the beam5ainto three beams7a,7b,7c. These beams are guided through three scanning prisms8a,8b,8c(red, green and blue), and a second set of mirrors9recombines the beams to one beam5b, as described above. The mirrors6,9and the prisms8a,8b,8ctogether form the colour scanner.

The beam5bwith scrolling colour bars23is directed onto the display device3, and an image generated by the display device3is reflected back into a polarizing beam splitter (PBS)10. The PBS10directs the reflected image to a projection lens11, for projection on a suitable screen (not shown).

The scanning performed by the colour scanner8a,8b,8cis synchronized with the frame rate of the video data2, so that the colour bars23of the beam5bcomplete a scrolling sequence (return to original position) in one frame period TF. This is illustrated inFIG. 2.

The diagram inFIG. 3shows a typical current waveform20with period TLin the UHP lamp4, including a pulse21to stabilize the arc position. The diagram inFIG. 4shows the corresponding lamp flux22from the projection lamp4, which is essentially the rectified waveform20inFIG. 3. As is clear fromFIG. 4, the lamp flux22comprises a DC flux with a superimposed AC light flux, resulting from the stabilizing pulse21. As a consequence of the rectification, the period TACof the AC component is only half of TL, i.e. the pulse frequency is twice the lamp frequency.

As mentioned above, the AC light flux resulting from the stabilization pulse acts as a fictitious light source, and causes a stroboscopic effect on the colour scanner8a,8b,8c. When the frequency of this AC component of the light flux (referred to as the pulse frequency) is a sub-harmonic of the display frame rate frequency, the colour bars23can be ‘captured’ by the stroboscopic effect, resulting in visible colour bars in the projected image. When lamp frequency and frame rate frequency are locked, the visible bars are fixed in one position. If they are not locked (i.e. a-synchronic), the visible bars will be scrolling over the screen because lamp and scanner are a-synchronic. The phase between lamp frequency and frame rate frequency determines the position of the colour bars on the screen.

A first embodiment is illustrated inFIG. 5. A filtering element in the form of a liquid crystal (LC) cell31is arranged in the light path between the lamp and the projection of the image, in this case behind the projection lens11. The LC cell is controlled by a drive voltage32, which is synchronized with the lamp current20by means of a synchronization unit33. In other words, the synchronization unit33is connected to the lamp driver12and to the LC cell31.

The synchronization unit33is adapted to control the LC cell in such a way that the variable transmission, which is a function of the voltage32, is decreased when a stabilization pulse21occurs in the lamp current20, and a peak occurs in the lamp flux22. The decrease in transmission is adapted to compensate for the increase in intensity present during the peak, and, as a result, the peak is cancelled and not visible in the projected image. This leads to a constant intensity in the projected image, and interference patterns are avoided.

A second embodiment is illustrated inFIGS. 6 and 7. In this case, the filtering element is a rotating disc41, made of a transparent material and having a neutral density grey filter42patterned onto its surface. Here, the disc41precedes the colour scanner mirrors6, but another location may also be found to be advantageous. As is shown inFIG. 7, the filter field42may be a sector of the disc41, but may also comprise two or more sectors and/or have a different shape.

A synchronization unit43is connected to the lamp driver12and to a disc controller44. The synchronization unit43is adapted to control the disc controller44to rotate the disc42with a phase and frequency such that the shaded filter field42passes the light path when a peak occurs in the lamp flux22. Furthermore, the transmission of the shaded field43is adapted to compensate the increase in light intensity present during the peak, and, as a result, the peak is cancelled and not visible in the projected image.

It is clear that the detailed description above is related to two specific embodiments of the invention, and that these embodiments do not limit the scope of the appended claims. More specifically, the scrolling projection system can be modified by the skilled person without departing from the inventive concept. For example, the colour scanner may be of a different type than the one-schematically illustrated in the drawings. Also, other filtering elements may be envisaged, and placed at other locations along the light path.