Optics: image projectors – Composite projected image – Multicolor picture
Reexamination Certificate
2003-06-03
2004-09-28
Gray, David (Department: 2851)
Optics: image projectors
Composite projected image
Multicolor picture
C353S032000, C353S084000
Reexamination Certificate
active
06796658
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed towards the optics of luminance-chrominance systems for projection displays.
BACKGROUND OF THE INVENTION
Optical projection systems in which the image is generated by light modulated by one or more “lightvalves” are becoming increasingly common. Devices such as televisions, presentation projectors and computer monitors have utilized such lightvalve based projection systems. Typically, in a single lightvalve system, a color image is produced by projecting red (R), green (G) and blue (B) (collectively referred to as “primary” hereinafter) image fields in a time sequential manner with sufficient rapidity that flicker is not apparent. The overall frame rate desired for color images is typically 60 Hertz or greater. Thus, the corresponding interval between each color image field is {fraction (1/180)}th of a second or less.
Single lightvalve systems such as those used in LCD (Liquid Crystal Display) projection systems are relatively inexpensive and the resulting performance is satisfactory. However, an inherent drawback of time sequential lightvalve and other systems is an effect known as ‘color breakup artifact’ or ‘field sequential color artifact’. Color breakup artifact manifests itself to a viewer as a transient rainbow-like fringing effect when rapid eye movements of several degrees are made. The effect is an inherent property of the human visual system but sensitivity to the effect varies greatly from person to person. Moreover, the seriousness of the effect depends strongly on the nature of the image being viewed.
An important issue in evaluating projection system methods is the total number of pixels required, because the cost and complexity of the display normally increase as the number of pixels required increases. In describing various projection systems it is convenient to express the number of pixels as a multiple of the number, N, which is the required number of pixels in the field sequential projection system. In addition to the number of pixels, the final screen luminance should be high enough so that it matches or exceeds the luminance provided by other display systems under commercialization. The final screen luminance of the whites of a projection system, can be expressed as a function of L, the final screen luminance of a field sequential projection system using a single reflecting lightvalve array with a polarizing cube.
A number of methods which have been traditionally used include the use of a separate projection system for each of red, green and blue lamps, or a single lamp with dichroic mirrors providing three separate red, green and blue beams. Another set of methods uses a large mosaic filter with a reflecting lightvalve array and polarizing beamsplitter.
FIGS. 1-4
show some of these methods that may be employed in projection systems. Projection system
100
of
FIG. 1
uses three lamps (e.g. projection cathode-ray tubes)
110
to achieve the projection of red, green, and blue images in register on a reflecting or on a translucent screen. This has achieved commercial success, but the cost is high. The light from lamps
110
are filtered by interference filters
120
, three in number, which will each increase the luminance of the red, green and blue beams as compared to when using dye-based filters. The red, green and blue light beams resulting from the filtering are each passed thru condensers
130
. Condensers
130
are a series of lensing elements that make the light beams spatially uniform (i.e. even out the power of light across the area of the beam). In projection system
100
, three lightvalve arrays
140
are used, each within its own beam area, to project red, green, and blue images in register on a translucent screen
160
. With the aid of projection lenses
150
, the light from the arrays is focused and thus combined onto a single RGB point on screen
160
. Registration problems can be reduced by using reflecting arrays since their size can be small compared to the size of the final image. The number of required pixels is 3N.
Alternatively, as shown in
FIG. 2
, the light from a single lamp is
210
split into red, green, and blue components by three dichroic mirrors
215
. The light from dichroic mirrors
215
are filtered by interference filters
220
, three in number, which will each increase the luminance of the red, green and blue beams as compared to when using dye-based filters. The red, green and blue light beams resulting from the filtering are each passed thru condensers
230
which make the beams of light spatially uniform. In projection system
200
, three lightvalve arrays
240
are also used, each within its own beam area, to project red, green, and blue images in register on a translucent screen
260
. With the aid of projection lenses
250
, the light from the arrays are focused onto a single RGB lumen on screen
160
.
Both projection systems
100
and
200
suffer the cost disadvantages of having three lightvalve arrays and three lenses, of maintaining the registration of the three projected images. The number of total pixels for projection system
200
is again 3N as with the three lamp system
100
of FIG.
1
. Compared with the prism assembly method (shown below in FIG.
4
), potential advantages are that, if three lamps are used as in system
100
, the light output is tripled; or if one lamp is used with beam-splitting dichroic mirrors as in system
200
, they are less costly to assemble and manufacture than the prism assembly methods.
An alternative approach is to abandon time sequential imaging while still using only one lightvalve by presenting the primary colors to the viewer in the space domain, rather than in the time domain. One way of constructing such a field sequential system would be to arrange the R, G and B pixels in a mosaic pattern, like the arrangement of phosphor spots in a Cathode Ray Tube device. The lightvalve would be illuminated using white light, and each R pixel would be covered with a red filter, each G pixel with a green filter and each B pixel with a blue filter. The requisite filter array would contain about 10
6
or more filters. Furthermore, in the case of a micro-display lightvalve array, each filter would measure only 10×10 &mgr;m
2
. Though conceptually easy, implementing such large filter arrays and such small individual filters could be prohibitively expensive. Disadvantageously, mosaic filter arrays need about three times as many pixels.
FIG. 3
shows one such mosaic filter
300
that can be employed in a projection system. Mosaic filter
300
contains a pattern of red, green and blue pixel locations. In light-sensitive arrays (such as CCD arrays) more green cells are usually incorporated than red or blue cells. Where required mosaic filter
300
may instead be composed of cyan, magenta, yellow, and green cells or any combination of colors. For most displays, the relative numbers of red, green, and blue cells have to be chosen to maximize the luminance of the display and to keep the total number of cells to a minimum. To meet these requirements an equal number of red, green, and blue cells are used as in the case of mosaic filter
300
.
Single transmitting LCD arrays, covered with a mosaic of red, green, and blue filters, such as filter mosaic
300
, provide either panel displays illuminated by fluorescent lamps or projected displays by means of overhead projectors. In projection system
400
of
FIG. 4
, a reflecting LCD array
430
is used with mosaic filters
440
to provide a projected image based on light originating from a single lamp
410
. Reflecting array
440
is used because its small size makes a compact projector possible. Condenser
420
makes the light from lamp
410
to be spatially uniform. Field lens
450
transmits the image onto the appropriate position on projection lens
490
. The image rays are first passed through a polarizing cube
470
which allows any light arriving in a given direction X to pass through its hypotenuse while reflecting back any light perpendicular to that direction X. The projection
Hubel Paul M.
Hunt Robert W. G.
Gray David
Sever Andrew
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