WO2004111985A1

WO2004111985A1 - Lcd display panel including segmented illumination scheme by scrolling illumination of the corresponding panel segments

Info

Publication number: WO2004111985A1
Application number: PCT/IB2004/050876
Authority: WO
Inventors: Duncan J. Anderson; Michel J. Zwanenburg
Original assignee: Koninklijke Philips Electronics, N.V.
Priority date: 2003-06-13
Filing date: 2004-06-09
Publication date: 2004-12-23
Also published as: CN1806272A; EP1636782A1; KR20060015644A; JP2006527413A; TW200511194A

Abstract

A step -wise scrolling of an LC display device is achieved using an array of modulatable/controllable light sources. The LC display is segmented, and each segment is addressed and subsequently illuminated simultaneously with each of the other segments. In addition to other benefits, this affords performance advantages, while substantially eliminating mechanically moving parts.

Description

LCD DISPLAY PANEL INCLUDING SEGMENTED ILLUMINATION SCHEME BY SCROLLING ILLUMINATION OF THE CORRESPONDING PANEL SEGMENTS

Liquid crystal technology has been applied in projection displays for use in projection televisions, computer monitors, point of sale displays, and electronic cinema, to mention a few applications.

A more recent application of LC devices is the reflective LC display on a silicon substrate, commonly referred to as liquid crystal-on silicon (LCOS). Often, the liquid crystal is a twisted nematic liquid crystal (TNLC) device. Silicon-based reflective LC displays often include an active matrix of complementary metal-oxide-semiconductor (CMOS) transistors/switches that are used to selectively rotate the axes of the liquid crystal molecules.

In many LC display devices, an image is constructed on the display first by selectively altering the orientation of the LC molecules in the LC device (panel) and then illuminating the panel selectively with light. This selective orientation of the LC molecules may be referred to as addressing or 'preparing' the panel. To this end, the panel is prepared with the particular image so that it is synchronized with the illumination by applying suitable voltages to the individual picture elements (pixels). This process is often effected via Field Sequential Color Illumination (FSC), in which each row of the LC device is prepared for illumination in a sequential fashion. The process requires the time to completely address each pixel, as well as any lag time for the last row to reach a steady state prior to illumination.

After the pixels of each row of the LC have been addressed (prepared), the illumination occurs, and the process repeats for the creation of images in a scrolling process. In many applications, the illumination is effected using the primary colors, red (R), blue (B) and green (B). The illumination time relative to the full color frame time (i.e., R, G & B) is termed the duty cycle. As can be appreciated, in order to provide suitable brightness to the viewer, it is useful to maximize the illumination duty cycle. Accordingly, it is beneficial to minimize the sequential preparation time of the pixel rows of the panel, and the lag time for the last row to reach steady state. One known technique to increase the illumination duty cycle involves the use of two transistor-based memory elements per pixel. According to this known technique, each pixel includes an operating transistor-capacitor pair that provides crystal orientation for the current image, and another pair, which is charged with the suitable voltage for the next image/FSC. As such, when the next image is to be displayed, there is no significant delay/lag as all pixels are prepared (addressed) simultaneously, and not sequentially.

While the referenced method provides an improved duty-cycle, it is often not practical. In particular, the requirements of smaller and less costly panels cannot be achieved if each pixel requires redundancy of elements.

Accordingly, what is needed is a method and apparatus of illuminating displays, which use LC devices, that has improved duty cycles and that overcomes at least the deficiencies of known methods and apparati.

In accordance with an exemplary embodiment, a method of illuminating a display in an LCD includes preparing a segment of an LC panel for illumination of light of at least one color, illuminating the segment, and substantially simultaneously preparing at least one other segment of the LC panel for subsequent illumination. Illustratively, each segment comprises at least two rows of pixels.

In accordance with another exemplary embodiment an LCD apparatus includes a first array of discrete light-emitting devices, which illuminates a first segment of a liquid crystal (LC) panel with light of a first color; a second array of discrete light-emitting devices, which illuminates a second segment of the LC panel with a second color; and at least one additional array of discrete light-emitting devices, wherein each of the at least one additional arrays illumates a respective segment of the LC panel with light of a distinct color, wherein each respective segment is illuminated only with light from its respective one of the at least one additional array, and wherein each of the first, second and the respective segments is illuminated simultaneously for a first particular time period.

The invention is best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion. Fig. 1 is a schematic view of a step-wise scrolling light emitting device color bar illumination apparatus in accordance with an exemplary embodiment.

Fig. 2 is a perspective view of an array of light-emitting diodes (LED), each coupled to a collimation optical element in accordance with an exemplary embodiment.

Fig. 3 is a conceptual view of a of an LC panel comprising N rows of pixels in accordance with an exemplary embodiment.

Fig. 4 is a graphical view of the row drive voltage over time of the first and Nth row pixels of an LC panel in accordance with an exemplary embodiment.

Figs. 5a and 5b are conceptual views of the illumination time of the known full display addressing and illumination method and the sub-display (bar) addressing and illumination method of an exemplary embodiment.

In the following detailed description, for purposes of explanation and not limitation, exemplary embodiments disclosing specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure, that the present invention may be practiced in other embodiments that depart from the specific details disclosed herein. Moreover, descriptions of well-known devices, methods and materials may be omitted so as to not obscure the description of the present invention.

According to at least one exemplary embodiment, step -wise scrolling of an LC display device is achieved using arrays of modulatable/controllable light sources. This affords performance advantages described below, while substantially eliminates mechanically moving parts.

Fig. 1 shows an exemplary embodiment of an illumination apparatus 100 for creating an image on a display (not shown) using an LC device 101, which is illustratively an LCOS device. The apparatus 100 includes an array of light emitting elements 102, 103 and 104, which provide red light, blue light and green light, respectively. The light emitting devices are illustratively light emitting diodes (LED's), which have emission wavelengths at the respective colors referenced. These LED's may be one of a variety of types. Alternatively, other types of discrete, addressable light emitting devices may be used in this capacity. These include, but are not limited to known light sources such as incandescent bulbs, organic light emitting diodes (OLED's) and laser diodes, provided they meet certain requirements of addressability, modulatability, response time, and lumen output. As such, devices that meet these requirements may be used.

In the present exemplary embodiment, each array of LED' s is coupled to an optical collimator 105, each of which is coupled to its respective optical integrator 105. The outputs of the integrators 105 are input to a dichroic combiner 106, which combines the light of the individual colors (wavelengths) into an output color. These elements are useful in providing a more uniform light source to the LC device 101.

The light output of the dichroic combiner 106 is imaged onto LC device 101 by a lens 107 and polarization beamsplitter (PBS) 108. The PBS 108 provides polarization selectivity as well as dark-state light selectivity as is well known, and thereby, contrast in the image. The light reflected from the PBS 108 is incident on the LC device 101, and then selectively to the projection optics 109. Moreover, the uniformity of the illumination may be detected using a sensor disposed at 110, and a feedback loop to the LED array may provide corrections and adjustments as needed.

Fig. 2 shows an LED array 201 in accordance with an exemplary embodiment. The LED array 201 is an array for one of the primary colors, and may be any one of the arrays 102, 103 and 104 of Fig. 1 depending on the output wavelength of the LED's of the array 201. Each LED array 201 may be disposed on a circuit board or similar connection scheme to enable selective addressing of the individual LED's of the array 201. This is rather advantageous, allowing for one or more of the three primary colors to illuminate the LC device in a sequential/scrolling manner according to an exemplary embodiment described herein.

The collimation optical elements 202 are illustratively compound parabolic reflectors, which usefully preserve the etendue of the light. The light is homogenized using an integrator, such as a slab lightguide 203. The various colors from the arrays (e.g. arrays 102,103, 104), are combined using a dichroic combiner 204 or similar element.

It is noted that each of the glass sections of the slab lightguide 203 guides the light from its corresponding row of LEDs. The glass row sections are separated by a suitable glue, fluid or other suitable material (not shown), having a refractive index that supports total internal reflection (ΗR) 205 within the lightguide 203. Hence light is guided from a particular row of the LED array 201 to the corresponding exit row of the illumination unit. This beneficially substantially prevents optical cross-talk between the rows.

The LED's of the array 201 are selectively addressable by current source drivers such as field effect transistor (FET) circuit current supplies or similar device, mounted on the interconnect. These are particularly useful in effecting the step-wise scrolling by sequential illumination, which is described herein. It is noted that the LED array may be coupled to the video signal to accentuate certain images. For example, bright or dark regions may be produced on the display to improve contrast.

It is further noted that the various elements used in realizing the embodiments shown in Figs. 1 and 2 are merely illustrative. In fact other elements and arrangements may be used to provide arrays of addressable discrete optical sources, for step-wise scrolling color bar illumination in accordance with the exemplary embodiments.

According to an exemplary embodiment, step-wise scrolling of the image is achieved by sequentially illuminating the segments of the LC device 101. Illustratively, each segment comprises at least two rows of pixel elements. This and other embodiments are described more fully presently.

Fig. 3 shows a display area 300 of an LC device such as LC device 101, of an exemplary embodiment. The display area 300 includes N rows of pixels 301, where each row has M pixels; and N,M > 2. In accordance with an exemplary embodiment, the MxN array of pixels is subdivided into L segments, which form the entire LC device. The total number of rows in each segment equals N/L.

In accordance with an exemplary embodiment, this display area 300 is segmented into three bars (segments) with each bar having 240 (720/3) rows. The device may be implemented in a High Definition TV (HDTV) display has a total resolution of 1280(M columns) by 720(N rows). This is merely illustrative, and many other displays could benefit from the exemplary embodiment. Finally, it is noted that the individual pixels of the segments may be addressed/prepared row-by-row, and from top to bottom, or may be addressed column by column, or in groups of rows and columns simultaneously. In the extreme, the entire segment may be addressed simultaneously. As referenced previously, the pixels are prepared/addressed by selectively applying a voltage of a certain magnitude to orient the liquid crystal molecules so that when light traverses the LC device. An image is formed by modulating the polarization of a light across the pixel array, which is subsequently selectively separated by the PBS. In accordance with an exemplary embodiment, the full display area 300 of the LC panel modulates the light incident thereon according to the desired input video information for the colors impinging on its various portions. This modulated light is reflected from the display, and when all L-segments are scrolled through, the image is complete.

According to an exemplary embodiment, all three colors are incident on the display area 300, with each color incident on N/3 of the rows. To wit, each color is incident on a segment that is 1/3 of the total area of the display area 300. In a subsequent illumination, each segment is illuminated by a different color, providing the desired stepwise scrolling. This process is repeated continuously. Finally, it is noted that the use of three colors is merely illustrative, and additional colors, or other combinations of colors could be employed, with further segmentation as a result. For example if four colors were used, each color would be incident on N/4 rows (four segments).

After the video signal is applied to the entire segment, the light from the LED arrays is incident on the segments of the display 300, and each portion of the total image is projected on the display. The addressing of the display 300 is effected by the simultaneous addressing (i.e., in parallel) of each segment, followed by the simultaneous illumination of each segment.

As can be appreciated, therefore, unlike the full display illumination techniques previously described, the exemplary embodiments provide a segmented preparation and illumination that increases the illumination time of each row of pixels, and thereby improves the brightness of the display. Stated differently, rather than waiting for the video signal and alignment of all pixels of the LC device, the exemplary methods and apparati are delayed only by the address time of the sub-display (segment) and the LC response time. This increases the illumination duty cycle and hence display brightness.

Fig. 4 illustrates another advantageous aspect of an exemplary embodiment. The first curve 401 shows the row drive voltage for the pixels of row 1 versus time, and second curve 402 shows the row drive voltage for the Nth row of pixels. Since the voltages are applied sequentially, the initial application of the Nth row drive voltage is delayed. To ensure more uniform illumination, the row drive voltage of the Nth row pixels may be greater than that of the first row pixels to compensate for the finite line address and response speed of the liquid crystal. This results in the same area under the curve, and thus equal field flux for each row, which provides a more uniform projected image of the entire segment of rows. It is noted that this technique may be applied to each segment. In this case the last row is N/3, and the process is repeated in the next segments, with the applied voltage of the (N/3)*¹¹ row of the segment being greater than the first row of the segment.

The segmentation of the LC device provides the ability to illuminate each of the L-segments simultaneously and is limited only by the address time of the segment and LC response time. This is in stark contrast to known full field techniques, which require delaying the illumination until the entire LC device is prepared, and until the lag for the last rows to reach steady-state. As can be appreciated, the segmentation technique of the exemplary embodiments allow longer duty cycles and greater illumination time.

Figs. 5a and 5b show a comparison of a known full display addressing and illumination, and the segmentation display illumination and addressing of an exemplary embodiment, respectively.

In Fig. 5a, the LC device 501 requires that all rows are properly oriented and in a steady state, so the total illumination time is given by: It = T(Color)-T(LC)-LATxN where It is the illumination time, T(Color) is the color frame time, T(LC) is the LC response time, LAT is the line address time, and N is the number of rows.

In contrast, the segmented LC display 502 addresses and illuminates each segment 503, 504 and 505 in parallel as described above. In this arrangement, the total time for illumination I_t is given by:

It= T(Color)-T(LC)-LATxN/3

Accordingly, the total illumination time is increased. As such, the display address time is reduced by the present embodiment.

As a quantitative example consider a device with a display update frequency of 100 Hz, the color update is three times this, or 300 Hz, or a color frame time of 3.3 ms. Given the address line time T (LAT) of 1 μs and a liquid crystal response time of 1 ms, the LED illumination time for the two cases of Figs. 5a and 5b, respectively, in a 1024 row device are, 1.3 ms and 2.0 ms. Clearly, the illumination time of the device of the exemplary embodiment is over 50% greater than the known device of Fig. 5a. This is also shown graphically in curves 506 and 507 of Figs. 5a and 5b, respectively. Clearly, the illumination time of the embodiment of Fig. 5b is significantly greater than that of the known technique of Fig. 5a.

The step -wise scrolling an exemplary embodiment is also shown in Fig. 5b. Each segment 503-505 undergoes a first illumination with a respective first color simultaneously. Then in a subsequent illumination, each segment is illuminated with a respective second color, again simultaneously. This process is repeated to effect the scrolling, and is in contrast to the known technique where the full display is illuminated at a single time with a first color, and in sequence with other colors.

The invention being thus described, it would be obvious that the same may be varied in many ways by one of ordinary skill in the art having had the benefit of the present disclosure. Such variations are not regarded as a departure from the spirit and scope of the invention, and such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims and their legal equivalents.

Claims

CLAIMS:

1. A method of illuminating a display in an LCD, the method comprising: substantially simultaneously preparing at least two segments of an LC panel for illumination; and illuminating the segments substantially simultaneously each with light of a different color, wherein the segments each comprise at least two rows of pixels.

2. A method as recited in claim 1, wherein the at least two segments comprise three segments.

3. A method as recited in claim 1, wherein the process is repeated continually, and with each repetition, the color of the light illuminating each segment changes.

4. A method as recited in claim 2, wherein a first of the three segments is illuminated with red light, a second of the three segments is illuminated with blue light and a third of the three segments is illuminated with green light.

5. A method as recited in claim 4, wherein the process is repeated continually, and with each repetition, the color illuminating the first, second and third segments is different than a last illumination.

6. A method as recited in claim 1, wherein a number of the segments is equal to a number of colors of light illuminating the display.

7. A method as recited in claim 1, wherein an illumination time for each segment is given by:

I,= T(Color)-T(LC)-LATxN/C where I_t is the illumination time, T( Color) is the color frame time, T(LC) is the LC response time, LAT is the line address time, C is the number of colors illuminating the LC device and N is the number of rows.

8. A method as recited in claim 1, wherein the method includes applying a voltage to pixel elements in a first row that is less than a voltage applied to pixel elements of a last row of each of the at least two segments.

9. An LCD apparatus, comprising: a first array of discrete light-emitting devices, which illuminates a first segment of a liquid crystal (LC)panel with light of a first color; a second array of discrete light-emitting devices, which illuminates a second segment of the LC panel with a second color; and at least one additional array of discrete light-emitting devices, wherein each of the at least one additional arrays illumates a respective segment of the LC panel with light of a distinct color, wherein each respective segment is illuminated only with light from its respective one of the at least one additional array, and wherein each of the first, second, and the respective segments are illuminated simultaneously for a first particular time period.

10. An LCD apparatus as recited in claim 9, wherein the at least one additional array is a third array, which illuminates a third segment with light of a third color, and simultaneously with the first and second arrays.

11. An LCD apparatus as recited in claim 9, wherein after the first particular time period, each of the first, second and the at least one additional arrays illuminates for a second particular time period a respective other segment that is different than the segment illuminated during the first particular time period.

12. An LCD array as recited in claim 11, wherein after the second particular time period each of the first, second and the at least one additional arrays illuminates for a third particular time period a respective other segment that is different than the segment illuminated during the second particular time period.

13. An LCD apparatus as recited in claim 12, wherein a scrolling by each array from one segment to another segment is effected continuously, and simultaneous illumination is effected after each scrolling step.

14. An LCD apparatus as recited in claim 9, wherein, before the illuminating of the first, second and at least one additional segments, each of the segments is simultaneously prepared for illumination with selective voltages applied to each of a plurality of pixels of the segments.

15. An LCD apparatus as recited in claim 9, wherein each of the arrays is selectively addressable to emit light of a particular color that is different from the color of light from the other arrays.

16. An LCD apparatus as recited in claim 10, wherein the first color is red, the second color is green and the third color is blue.