Distributed duty-cycle operation of digital light-modulators

• US 5,751,264 A
• Filed: 06/27/1995
• Issued: 05/12/1998
• Est. Priority Date: 06/27/1995
• Status: Expired due to Fees

• Current Assignee: Philips Electronics North America Corporation (Koninklijke Philips N.V.)
• Original Assignee: Philips Electronics North America Corporation (Koninklijke Philips N.V.)
• Inventors: Cavallerano, Alan, Ciacci, Claudio
• Primary Examiner(s): BRIER, JEFFERY A
• Application Number:
  US08/495,290
• Time in Patent Office: 1,050 Days
• Field of Search: 345/63, 345/77, 345/85, 345/89, 345/147, 345/148, 348/771
• US Class Current: 345/85
• CPC Class Codes: G09G 2300/0814 used for selection purposes, e.g. logical AND for partial update
• CPC Class Codes: G09G 2300/0842 forming a memory circuit, e.g. a dynamic memory with one capacitor
• CPC Class Codes: G09G 2300/0857 Static memory circuit, e.g. flip-flop
• CPC Class Codes: G09G 2320/0266 Reduction of sub-frame artefacts
• CPC Class Codes: G09G 3/2018 by time modulation using two or more time intervals
• CPC Class Codes: G09G 3/2022 using sub-frames
• CPC Class Codes: G09G 3/2033 with splitting one or more sub-frames corresponding to the most significant bits into two or more sub-frames
• CPC Class Codes: G09G 3/34 by control of light from an independent source
• CPC Class Codes: G09G 3/346 based on modulation of the reflection angle, e.g. micromirrors micromirrors devices per se G02B26/0833
• CPC Class Codes: H04N 3/12 by switched stationary formation of lamps, photocells or light relays

Abstract

In a matrix display of light reflecting elements, each element is duty cycle modulated into one of two states represented by successive bits of a digital display code to produce a luminance level represented by the code. In order to correct a visual artifact which occurs when the human eye views the element during only part of the modulation cycle for the code, the element is repeatedly sequentially activated for short periods into optical states represented by the most significant bits of the code.

Claims

1. A method of operating a display apparatus comprising a light source, a screen for displaying successive images during respective frame periods, an array of digital light modulating elements interposed in an optical path between the light source and the screen, and means for activating each of the digital light modulating elements into either a first state, in which said element enables the light to illuminate a corresponding pixel of an image area of the display screen, or a second state, in which said element impedes the light from illuminating said pixel, said method comprising, for each of said frame periods:

a. storing, in association with each of the digital light modulating elements, a multi-bit code representing the luminance of the corresponding pixel during said frame period, each bit having a value representing either the first state or the second state and having a respective weight corresponding to a duration equal to a predefined percentage of the frame period during which the associated digital light modulating element is to be in the state represented by said value, a first one of said bits having a weight which is substantially greater than the weight of a second one of said bits; and

b. activating each digital light modulating element, into the states represented by the bits of the associated code, for the respective durations corresponding to the weights of said bits, said activation of said element into the first state represented by the first bit being interrupted at least once while said element is activated into the second state represented by a different one of the bits in the code, wherein the first and second states collectively represent the luminance of the corresponding pixel.

15. The method as in claim 1, 2, 3, 7 or 12 wherein the digital light modulating elements comprise deformable mirrors.

2. A method of operating a display apparatus comprising a light source, a screen for displaying successive images during respective frame periods, an array of digital light modulating elements interposed in an optical path between the light source and the screen, and means for activating each of the digital light modulating elements into either an ON state, in which said element enables the light to illuminate a corresponding pixel of an image area of the display screen, or an OFF state, in which said element impedes the light from illuminating said pixel, said method comprising, for each of said frame periods:

a. storing, in association with each of the digital light modulating elements, a group of bits forming a binary code representing the luminance of the corresponding pixel during said frame period, each bit having a value representing either the ON state or the OFF state and having a respective weight corresponding to an activation duration equal to a predefined percentage of the frame period during which the associated digital light modulating element is to be in the state represented by said value, a sub-group of said bits having a collective weight corresponding to a collective activation duration which is greater than one-half of the frame period; and

b. activating each digital light modulating element, into the states represented by the bits of the associated code, for the respective activation durations corresponding to the weights of said bits, said element being repeatedly sequentially activated into the states represented by the bits in the sub-group to cumulatively achieve the collective activation duration.

3. A method of operating a display apparatus comprising a light source, a screen for displaying successive images during respective frame periods, an array of digital light modulating elements interposed in an optical path between the light source and the screen, and means for activating each of the digital light modulating elements into either an ON state, in which said element enables the light to illuminate a corresponding pixel of an image area of the display screen, or an OFF state, in which said element impedes the light from illuminating said pixel, said method comprising, for each of said frame periods:

a. storing, in association with each of the digital light modulating elements, a group of bits forming a binary code representing the luminance of the corresponding pixel during said frame period, each bit having a value representing either the ON state or the OFF state and having a respective weight corresponding to an activation duration equal to a predefined percentage of the frame period during which the associated digital light modulating element is to be in the state represented by said value, one of said bits having a weight corresponding to a minimum duration which is no greater than the weight of any of the other bits in the group and a sub-group of said bits each having a respective weight corresponding to a duration which is at least twice said minimum duration and having a collective weight corresponding to a collective activation duration; and

b. activating each digital light modulating element, into the states represented by the bits of the associated code, for the respective activation durations corresponding to the weights of said bits, said element being repeatedly sequentially activated into the states represented by the bits in the sub-group for respective periods which are at least equal to the minimum duration to cumulatively achieve the collective activation duration.

4. The method as in claim 3, wherein the sub-group includes a lesser significant bit having a first weight and includes a plurality of more significant bits having respective weights which are multiples of the first weight and wherein, during each of said repeated sequential activations of the digital light modulating element into the states represented by the bits in said sub-group, said element is activated into the states represented by the more significant bits for respective periods which are multiples of the period of activation of said lesser significant bit.

5. The method as in claim 4, wherein during each of said repeated sequential activations of the digital light modulating element into the states represented by the bits in said sub-group, said element is activated into the state represented by the lesser significant bit for a period equal to said minimum duration.

6. The method as in claim 4, wherein said element is activated into the states represented by the more significant bits for respective periods which are whole number multiples of the period of activation of said lesser significant bit.

7. A method of operating a display apparatus comprising a light source, a screen for displaying successive images during respective frame periods, an array of digital light modulating elements interposed in an optical path between the light source and the screen, and means for activating each of the digital light modulating elements into either an ON state, in which said element enables the light to illuminate a corresponding pixel of an image area of the display screen, or an OFF state, in which said element impedes the light from illuminating said pixel, said method comprising, for each of said frame periods:

a. storing, in association with each of the digital light modulating elements, a group of bits forming a binary code representing the luminance of the corresponding pixel during said frame period, each bit having a value representing either the ON state or the OFF state and having a respective weight corresponding to an activation duration equal to a predefined percentage of the frame period during which the associated digital light modulating element is to be in the state represented by said value, said group comprising;

i. a first sub-group of bits, each having a respective weight, one of said bits having a weight corresponding to a minimum duration which is no greater than the weight of any of the other bits in said first sub-group;

ii. a second sub-group of bits, each having a respective weight corresponding to a duration which is at least twice said minimum duration and having a collective weight corresponding to a collective activation duration;

b. activating each digital light modulating element, into the states represented by the bits of the associated code, for the respective activation durations corresponding to the weights of said bits, said element being repeatedly sequentially activated into the states represented by the bits in the second sub-group for respective periods which are at least equal to the minimum duration to cumulatively achieve the collective activation duration.

8. The method as in claim 7, wherein each of the bits in the second sub-group has a respective weight which is at least twice the weight of each bit in the first sub-group.

9. The method as in claim 7, wherein the first sub-group includes at least one more significant bit having a respective weight corresponding to at least twice the minimum duration and wherein the respective digital light modulating element is repeatedly sequentially activated into the state represented by said more significant bit.

10. The method as in claim 9, wherein periods during which each digital light modulating element is activated into the state represented by the at least one more significant bit in the first sub-group are distributed among periods during which said element is activated into the states represented by the bits in the second sub-group.

11. The method as in claim 10, wherein periods during which each digital light modulating element is activated into the state represented by the at least one more significant bit in the first sub-group are evenly distributed among periods during which said element is activated into the states represented by the bits in the second sub-group.

12. A method of operating a display apparatus comprising a light source, a screen for displaying successive images during respective frame periods, an array of digital light modulating elements interposed in an optical path between the light source and the screen, and means for activating each of the digital light modulating elements into either an ON state, in which said element enables the light to illuminate a corresponding pixel of an image area of the display screen, or an OFF state, in which said element impedes the light from illuminating said pixel, said method comprising, for each of said frame periods:

a. storing, in association with each of the digital light modulating elements, a group of bits forming a binary code representing the luminance of the corresponding pixel during said frame period, each bit having a value representing either the ON state or the OFF state and having a respective weight corresponding to an activation duration equal to a predefined percentage of the frame period during which the associated digital light modulating element is to be in the state represented by said value, said group of bits including;

i. a least-significant bit having a weight corresponding to a minimum duration which is smaller than the weight of any of the other bits in the group;

ii. a first sub-group of more-significant bits, each having a respective weight corresponding to a duration which is at least twice said minimum duration and having a collective weight corresponding to a first collective activation duration; and

iii. a second sub-group of most significant bits, each having a respective weight corresponding to a duration which is greater than that of any of the bits in the first sub-group and having a second collective activation duration; and

b. activating each digital light modulating element, into the states represented by the bits of the associated binary code, for the respective activation durations corresponding to the weights of said bits, said element;

i. being once activated for the minimum duration into the state represented by the least significant bit;

ii. being repeatedly sequentially activated into the states represented by the bits in the second sub-group to cumulatively achieve the second collective activation duration; and

iii. in time periods between said repeated sequential activations, said element being activated for respective time periods into the states represented by individual ones of the bits in the first sub-group to cumulatively achieve the first collective activation duration.

13. The method as in claim 12, wherein each of the respective time periods are substantially equal to the minimum duration.

14. The method as in claim 12, wherein the respective time periods are substantially evenly distributed in the time periods between the repeated sequential activations.

16. A method of operating a display apparatus including a light source, a screen for displaying successive images during respective frame periods, an array of digital light modulating elements interposed in an optical path between the light source and the screen, and circuitry which switches each of the digital light modulating elements into first and second states whereby light illuminates a corresponding pixel of an image area of the display screen or impedes the light from illuminating the pixel, respectively, said method comprising, for each of the frame periods:

a. storing, in association with each of the digital light modulating elements, a multi-bit code representing overall luminance of the corresponding pixel during a frame period, each bit having a value corresponding to one of said first and second states and having a respective weight corresponding to a duration equal to a predefined percentage of the frame period during which the associated digital light modulating element is to be in the state corresponding to said value, a first one of said bits having a weight which is substantially greater than the weight of a second one of said bits; and

b. repeatedly switching each digital light modulating element between said first and second states responsive to the bits of the associated code during a plurality of sub-periods of the frame period to generate a corresponding plurality of luminance sequences, at least one of which substantially represents the overall luminance of the corresponding pixel.

17. The method as in claim 16, wherein substantially all of the bits corresponding to the first state and substantially all of the bits corresponding to the second state are represented in said luminance sequences during the majority of said sub-periods.

18. The method as in claim 16, wherein the digital light modulating elements comprise liquid crystal cells.