WO2011071701A1 - Commande d'éclairage dynamique pour écran de projection laser - Google Patents

Commande d'éclairage dynamique pour écran de projection laser Download PDF

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Publication number
WO2011071701A1
WO2011071701A1 PCT/US2010/057979 US2010057979W WO2011071701A1 WO 2011071701 A1 WO2011071701 A1 WO 2011071701A1 US 2010057979 W US2010057979 W US 2010057979W WO 2011071701 A1 WO2011071701 A1 WO 2011071701A1
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WIPO (PCT)
Prior art keywords
image
frame
image data
light
display apparatus
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PCT/US2010/057979
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English (en)
Inventor
Michael Alan Marcus
John Alphonse Agostinelli
Marek W. Kowarz
James G. Phalen
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Eastman Kodak Company
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Priority to EP10836424A priority Critical patent/EP2510477A1/fr
Publication of WO2011071701A1 publication Critical patent/WO2011071701A1/fr

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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N9/00—Details of colour television systems
    • H04N9/12—Picture reproducers
    • H04N9/31—Projection devices for colour picture display, e.g. using electronic spatial light modulators [ESLM]
    • H04N9/3102—Projection devices for colour picture display, e.g. using electronic spatial light modulators [ESLM] using two-dimensional electronic spatial light modulators
    • H04N9/3105—Projection devices for colour picture display, e.g. using electronic spatial light modulators [ESLM] using two-dimensional electronic spatial light modulators for displaying all colours simultaneously, e.g. by using two or more electronic spatial light modulators
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N9/00—Details of colour television systems
    • H04N9/12—Picture reproducers
    • H04N9/31—Projection devices for colour picture display, e.g. using electronic spatial light modulators [ESLM]
    • H04N9/3179—Video signal processing therefor
    • H04N9/3182—Colour adjustment, e.g. white balance, shading or gamut

Definitions

  • the present invention relates to electronic display apparatus using spatial light modulators and more particularly relates to apparatus and methods for improved contrast and dynamic range in an electronic projection system.
  • C/R contrast ratio
  • C/R white luminance/black luminance (1).
  • the black luminance level is a result of light that passes through the active display devices in the off state and stray light coming through the projection optics of the display. Stray light can come from unwanted reflections from optical components of the projection system.
  • 7,413,314 (Kim et al.) describes an optical system having an iris controlled in real time for reducing light from devices in the off state.
  • an iris controller senses luminance information in the light output and controls the projection iris according to the luminance information. With the opening range of the iris controlled in real time, the contrast ratio (C/R) is improved.
  • U.S. Patent No. 7,204,594 (Akiyama) describes a projector including an illumination device, an electro-optic modulator, and a projection optical system that includes a light shielding member provided with a stray light elimination member that reflects unwanted light away from the projection optics path.
  • Another approach has been to modify the display screen itself.
  • U.S. Patent No. 7,403,332 (Whitehead et al.) describes a display having a screen incorporating a light modulator which is illuminated by a light source composed of an array of controllable light emitters. The controllable emitters and elements of the light modulator may be controlled to adjust the intensity of light emanating from corresponding areas on the screen.
  • the mechanical iris of the '314 disclosure must be a high-speed device and can be relatively costly.
  • the light-shielding member of the '594 disclosure sends stray light out of the projection path and to other surfaces inside the projector, with the potential for some portion of this light to be projected onto the screen.
  • the specialized display screen taught in the '332 disclosure adds significantly to projection system cost and may not be a suitable solution where it is desirable to replace existing film projection equipment.
  • solutions that are appropriate for projection systems that use laser light sources can include, for example, systems that use spatial light imaging modulators such as liquid crystal devices (LCDs) or digital micromirror devices, such as the DLP device from Texas Instruments, Inc., Dallas, TX.
  • spatial light imaging modulators such as liquid crystal devices (LCDs) or digital micromirror devices, such as the DLP device from Texas Instruments, Inc., Dallas, TX.
  • Linear light modulators form images by a rapid, repeated sequence in which each single line of the image is separately formed and is directed to a screen or other display surface by reflection, or other type of redirection, from a scanning element.
  • Types of linear light modulators that operate in this manner include devices such as grating light valves (GLV) from Silicon Light Machines and described in U.S. Patent No. 6,215,579 (Bloom et al), and elsewhere. Display systems based on GLV devices are disclosed, for example, in U.S. Patent No. 5,982,553 (Bloom et al).
  • linear light modulator is the piezoelectric based spatial light modulator (SOM) developed by Samsung and disclosed, for example, in U.S. Patent No. 7,133,184 (Shin et al).
  • SOM piezoelectric based spatial light modulator
  • An improved type of linear imaging modulator is the grating electro-mechanical system (GEMS) device, as disclosed in commonly-assigned U.S. Patent No. 6,307,663 (Kowarz), and elsewhere. Display systems based on a linear array of conformal GEMS devices are described in commonly-assigned U.S. Patent Nos. 6,411,425, 6,678,085, and 6,476,848 (all to Kowarz et al).
  • GEMS device architecture and operation is given in a number of commonly-assigned U.S. patents and published applications, including U.S. Patent Nos. 6,663,788 (Kowarz et al), and 6,802,613 (Agostinelli et al).
  • light is modulated by diffraction.
  • a GEMS chip for example, a linear array of conformal electromechanical ribbon elements, formed on a single substrate, is actuable to provide one or more diffracted orders of light to form each line of pixels for line-scanned projection display.
  • Color display system architectures using LCD, DLP, GLV, SOM, and GEMS devices generally employ three separate color paths, red, green, and blue (RGB), each color path provided with a separate spatial light modulator and laser source.
  • the light imaging modulator modulates its component red, green, or blue laser light to form the image, a single frame of pixels or line of pixels at a time.
  • the resulting modulated frames of pixels or lines of pixels for each color are then combined onto the same output axis to provide a full-color image that is then directed onto the display screen.
  • Linear light imaging modulator arrays have exhibited some advantages over their area array spatial light modulator (SLM) counterparts by virtue of higher resolution, reduced cost, and simplified illumination optics.
  • GLV and GEMS devices are actuable to operate at fast switching speeds for modulating laser light.
  • GLV and GEMS devices have advantages for high resolution, high native bit depth, variable aspect ratio, and relative freedom from motion artifacts when compared against other types of spatial light modulators.
  • galvanometrically actuated scanning mirror that is conventionally used to scan modulated light across the display surface rotates over a short angular range to form each 2-D (two-dimensional) frame of the image. Following each scan, mirror position must then be reset into the starting position for the next scan. During this reset interval, image content is not projected, when using the standard scanning sequence. Thus, light output is not available during about 15-25 % of the operating cycle, since the mirror requires some amount of time to stop, reverse direction, and return back into position for the next scan. This inherent reduction of the available light output limits the light efficiencies that can be obtained. Due to this scanning mirror reset time and to acceleration and deceleration times of the mirror, the effective duty cycle for providing modulated light with such systems, the so-called "pixel on" time, is typically no more than about 72-85%.
  • Another problem related resulting from the scanning sequence relates to the need to minimize the effects of stored charge as the ribbon elements are repeatedly switched between positions. Electrostatic energy is used to actuate the ribbons. Maintaining the same charge polarity for the integrated circuit (chip) substrate from one scan to the next quickly builds up a residual charge in the device that must be compensated for or dissipated in some way.
  • U.S. Patent No. 6,144,481 discloses a method for correcting for charge accumulation in the spatial light modulator device.
  • This method applies, to the dielectric ribbon elements, a modulated bipolar voltage signal whose time average is equal to the time average of a bias voltage applied to the bottom conductive layer of the modulator device.
  • the resulting alternating waveform switches the polarity of the substrate bias voltage effectively canceling the charge build-up during operation of the device.
  • Area spatial light imaging modulators such as DLP devices do not exhibit the same switching effects as linear GEMS, SOM, and GLV devices.
  • both area and linear light-modulating devices have a refresh cycle, during which unmodulated light can be inadvertently directed to the display surface. While the laser itself could be momentarily turned off to eliminate stray light during the refresh cycle, such a mode of operation is not optimal for existing semiconductor laser devices, compromising wavelength and thermal stability and potentially shortening laser lifetimes.
  • improving image contrast relates not only to methods that help to reduce stray light, but also to methods that can help to make details more visible within darker areas and other local areas of an image.
  • a method for improving image contrast for a display apparatus which includes obtaining a frame of image data that includes one or more code values for each pixel in the frame of image data; analyzing the frame of image data to identify a distribution of dark regions therein; adjusting at least one of the one or more code values for the frame of image data according to the distribution of dark regions in the frame of image data; and attenuating a brightness level available for the image frame according to the distribution of dark regions in the frame of image data.
  • linear and area light modulators With linear and area light modulators, light attenuation is combined with laser blanking, synchronized to the scan to minimize stray light on screen from light passing through the optics when the lasers are off the screen. For example, during this time period, when a GEMS device is used, the charge applied to the substrate may be flipped to eliminate hysteresis. Laser blanking minimizes the stray light which occurs during the substrate flipping.
  • the laser blanking can be accomplished by either switching the laser off or using an electrooptic modulation device.
  • an electro-optic modulation device such as an LC optical shutter
  • the contrast of the image can be enhanced by making the blacks appear blacker and boosting the delivered code values of the image proportionally by the amount of attenuation.
  • the modulation device can be segmented so that different regions on the screen can have different amounts of attenuation. This is useful for scenes in which bright areas exist such as sun-filled sky towards the upper parts of the image and dark details at the bottom of the image.
  • code values in the image can be adjusted by a 10X range and the attenuator can be set for lOx attenuation. This can increase the contrast significantly.
  • Figure 1 is a plan view of an image having a dark region that conventionally displays with poor contrast
  • Figure 2 is a schematic block diagram that shows components of a projection apparatus that uses a GEMS imaging modulator in each color channel;
  • Figure 3 A is a logic flow diagram that shows steps for image attenuation and display according to one embodiment
  • Figure 3B shows a set of gamma curves for representative gamma values
  • Figure 4 is a schematic block diagram that shows components of a projection apparatus that uses a GEMS imaging modulator in each color channel and that is capable of providing improved image contrast according to one embodiment
  • Figure 5 is a graph showing relative transmission for a given voltage signal for a typical EOM device
  • Figure 6 is a perspective schematic view showing the use of control logic processor for analyzing image data and controlling projection and brightness attenuation apparatus accordingly;
  • Figure 7A shows an example histogram as conventionally used with a full brightness curve
  • Figure 7B shows an example histogram with an increased contrast range and reduced brightness
  • Figure 8 shows imaging results for the image of Figure 1 using the method of the present invention
  • Figure 9A is an example original image having considerable dark pixel content
  • Figure 9B is a simulated image showing the results of applying the method of the present invention to the original image of Figure 9A;
  • Figure 9C is a histogram showing code values for Figure 9A
  • Figure 9D is a histogram showing code values for adjusted Figure
  • Figure 9E is a graph that shows normalized accumulated counts versus code value obtained from each of the three color channels of the original image of Figure 9A.
  • Figure 10A is an example original image having considerable dark pixel content
  • Figure 1 OB is a simulated image showing the results of applying the method of the present invention to the original image of Figure 10A;
  • Figure IOC is a histogram showing code values for Figure 10A
  • Figure 10D is a histogram showing code values for adjusted Figure
  • Figure 10E is a graph that shows normalized accumulated counts versus code values obtained from each of the three color channels of the original image of Figure 10A;
  • Figure 11 is a timing diagram showing a number of signals used to provide laser blanking between image frames.
  • Figure 12 is a schematic diagram that shows an alternate configuration for a display device according to one embodiment.
  • a "dark region" in an image frame is an area of the image frame that contains a substantial number of pixels below a given threshold code value for darkness within an image.
  • the definition of what constitutes a dark region in any particular case depends on a number of factors, including the type of imaging system and its data representation scheme, the number of pixels below some threshold code value that is identified as dark for that system, and user perception.
  • low data values such as values below 50 for an 8-bit system with a range of image code values from 0 to 255, are assigned as dark values in a projected image.
  • image frame data usually consists of 8, 12, or 16 bit integer data provided in the form of an image frame matrix of size R by C by 3 where R is the number of rows in the display, C is the number of columns in the display and 3 is the number of color planes used to display the image.
  • the location of each image pixel on the display is indicated by the row and column number of the image frame data.
  • the third dimension of the matrix is defined as the color plane with 1 corresponding to the red color plane, 2 to the green color plane, and 3 to the blue color plane.
  • Other sets of color planes can also be used for encoding the data, dependent on the image processing that is done at the projector, but assume RGB encoding in the discussions that follow.
  • an alternate data representation scheme could have dark pixels having a value above a certain threshold.
  • the apparatus and methods of the present invention adapt equally to either of these possible data arrangements, as well as to pixel data representations that use fewer than or more than 8 data bits per pixel.
  • the distribution of dark code values is used to determine, for one or more image frames, both how image data is adjusted and how output brightness is attenuated.
  • the apparatus and method of the present invention can be used with a display imaging apparatus that employs any of a number of different types of spatial light modulator.
  • a display imaging apparatus that employs any of a number of different types of spatial light modulator.
  • the description that follows is directed primarily to a display apparatus that uses GEMS devices.
  • similar approaches and solutions can also be used for display apparatus that use other linear spatial light modulators or that use area spatial light modulators such LCD devices or DLP modulators that use arrays of digital micromirror devices.
  • the term "chip” is used as it is familiarly used by those skilled in the micro-electromechanical device arts.
  • the term chip refers to the one-piece electromechanical circuit package that includes one or more light modulator arrays formed on a single substrate, such as the conformal grating devices described in detail in commonly-assigned U.S. Patent No. 6,411,425 (Kowarz et al.), mentioned earlier.
  • the GEMS chip not only includes the elongated ribbon elements that form the light-modulating grating for light reflection and diffraction, but may also include the underlying circuitry that applies the electrostatic force that is used to actuate these ribbon elements.
  • the tiny electronic and mechanical components that form the chip are fabricated onto a single substrate.
  • the chip package also includes signal leads for interconnection and mounting onto a circuit board or other suitable surface.
  • Methods of the present invention are particularly well-suited to take advantage of the high brightness levels of polarized light available from laser and other solid-state light sources. These methods can also be applied where xenon arc lighting or other light sources are used, where the modulated light output is substantially polarized.
  • a projection display apparatus 10 using a GEMS device as linear light imaging modulator in each of three color channels, a red color channel 20r, a green color channel 20g, and a blue color channel 20b.
  • a red light source 70r typically a laser or laser array, provides illumination that is conditioned through a spherical lens 72r and a cylindrical lens 74r and directed towards a turning mirror 82r.
  • Light reflected from turning mirror 82r is modulated by diffraction at a linear light imaging modulator 85r, shown and described herein as an electromechanical grating light modulator.
  • Modulated diffracted light from linear light imaging modulator 85r is diffracted past turning mirror 82r and to a color combiner 100, such as an X-cube or other dichroic combiner.
  • the modulated line of light from color combiner 100 is then directed by a lens 75, through an optional cross-order filter (not shown), to a scanning element 77 for projection onto a display surface 90.
  • Scanning element 77 can be a scanning mirror commonly referred to as a galvanometer or a galvo or other suitable light-redirecting scanning element, such as a rotating prism or polygon or an apparatus having one or more coupled reflective surfaces, which apparatus, in turn, directs the incident modulated lines of light for forming 2D images toward display surface 90.
  • Green color modulation uses a similar set of components for providing light to color combiner 100, with a green light source 70g, typically a laser or laser array, providing illumination through a spherical lens 72g and a lens cylindrical 74g and directed towards a turning mirror 82g.
  • a green light source 70g typically a laser or laser array
  • Light reflected from turning mirror 82g is modulated by diffraction at an electromechanical grating light modulator that serves as a linear light imaging modulator 85g. Modulated diffracted light from linear light imaging modulator 85g is diffracted past turning mirror 82g and to color combiner 100.
  • blue light source 70b typically a laser or laser array, provides illumination through a spherical lens 72b and a cylindrical lens 74b and directs light towards a turning mirror 82b.
  • Light reflected from turning mirror 82b is modulated by diffraction at an electromechanical grating light modulator that serves as a linear light imaging modulator 85b, is diffracted past turning mirror 82b, and is sent as a line of light to color combiner 100.
  • Embodiments of the present invention provide improved image contrast by combining a code value adjustment with brightness attenuation of the modulated light.
  • Brightness attenuation is achieved by interposing one or more electro-optical light modulators into the path of modulated light and selectively controlling the electro-optical modulator(s) to attenuate some percentage, but not all, of the light. Attenuation decreases the range of available light, but allows the same number of data values for light intensity to be used.
  • adjustments to the display gamma curve are also made, to allow for non-linearities in display output and in viewer response. As a result of these combined adjustments, contrast within the more limited light range can be enhanced, increasing the visibility of details that were not previously perceptible when considering the full range of available light.
  • FIG. 3 A shows the sequence of steps used in order to obtain improved contrast for an image frame according to one embodiment of the present invention.
  • An obtain image frame step S200 begins the process by obtaining the data for a single frame of the image.
  • a histogram analysis step S210 then generates and analyzes a histogram of the image frame in order to determine whether or not the image meets the necessary criteria for contrast improvement using this method. Criteria for suitability are empirically determined but are chiefly based upon the distribution of dark image code values. In one embodiment, this is determined by whether or not dark code value bands or portions of significant size can be detected within the image. Histogram analysis provides one type of pixel value distribution data that can be a particularly useful tool for making this determination.
  • an accumulated count of pixels below a specific threshold value can also be used for determining whether or not the image frame has significant dark content and for quantifying that content in some way.
  • histogram analysis needs to be performed on all three color planes in a color image, and the accumulated count of pixels below a specific threshold value criteria must apply to all three color planes in order to assess that the image frame has significant dark content.
  • More sophisticated utilities such as image processing algorithms that check for groupings of dark pixels within a region, can also be used to provide information about the distribution of dark pixel values.
  • a test step S220 then evaluates the image histogram analyzed in step S210 to determine whether or not the image meets the criteria for contrast adjustment along one of the processing paths shown collectively as processing step S230, or is displayed without attenuation and with the standard gamma curve applied.
  • the gamma setting is applied in a gamma application step S240. If the criteria are met, a discrete level of brightness attenuation is identified, shown by way of example as either 2X, 5X, or 10X in Figure 3 A. As the scene content gets darker, it is desirable to use greater pixel intensity shifting at lower image pixel code values relative to that employed at higher image pixel code values.
  • a display step S250 then displays the processed image. It can be noted that display step S250 can be executed immediately, so that the process shown in Figure 3A executes as a part of the standard image processing chain for a projector or other apparatus, for example. Alternately, the computed attenuation and gamma information for an image frame can be stored for subsequent use and for display at a later time, using any of a number of display types.
  • Gamma correction is an operation that is well known in image display processing, used to encode and present image data in a form that is well suited to human perception and display characteristics.
  • gamma is an adjustment applied to an input code value.
  • FIG 3B there is shown a set of gamma curves for representative gamma values. For this example, 8-bit code values (0- 255) are used.
  • a gamma of 1.0 is linear, imparting no change to an input code value.
  • the conventional display gamma for a typical DCI (Digital Cinema Initiatives) projector for digital cinema is 2.6, shown in bold in Figure 3B.
  • This gamma correction is applied by the projector or other display device to all input image data.
  • brightness attenuation is provided when the gamma value is less than 1 , as shown in the logic flow diagram of Figure 3 A.
  • MCV is the maximum code value
  • OCV is the original code value
  • ⁇ is the value of gamma.
  • MCV is the maximum code value
  • OCV is the original code value
  • ⁇ is the value of gamma.
  • Histogram analysis can be particularly efficient when a value range in the image histogram can be spatially correlated with a horizontal or vertical band or other specific area of the image.
  • a value range in the image histogram can be spatially correlated with a horizontal or vertical band or other specific area of the image.
  • the dark value range in the image histogram clearly maps to the buildings and other features along a lower band of the image.
  • FIG. 4 The schematic block diagram of Figure 4 shows how brightness is attenuated by an attenuation apparatus 38 in one embodiment of a display apparatus 120 of the present invention.
  • Attenuation apparatus 38 has two types of components: (i) a phase-modulating electro-optical modulator 64 that is a light polarization modulator disposed in the path of the combined modulated light, and (ii) an analyzer 66.
  • electro-optical modulator (EOM) 64 changes the phase of the incident light, effectively rotating the polarization of the incident light by a corresponding number of degrees.
  • Analyzer 66 has its transmission axis in parallel with light sources 70r, 70g, and 70b, so that modulated light passes through to display surface 90. Thus, analyzer 66 blocks a portion of the light that is modulated when EOM 64 is actuated.
  • electro-optical modulator EOM 64 is an electrooptic polarization rotator, such as an LF Series Optical Shutter from BNS, Sweden. This device is conventionally driven with a 2-5 kHz square wave of up to ⁇ 20 V. When there is no drive signal applied, the liquid crystal molecules in this EOM device rotate the polarization of the input light by 90°. When an AC signal with a high amplitude is supplied to the polarization rotator, the liquid crystal molecules realign and no longer rotate the polarization of the incoming light.
  • the mode of operation of the phase-modulating electro-optic modulator 64 can be reversed, so that, when not actuated by a control signal, it changes the phase of the incident light by 90 degrees and, when actuated, it causes 0 degree change to the polarization.
  • care is taken to provide a zero net DC bias to the liquid crystal layer. This is accomplished by applying a high frequency square wave during the closed state, typically between 2 - 5 kHz, as noted.
  • EOM devices of this type typically have different rise and fall response times.
  • Rise time is defined as the amount of time for the rotator to switch from the energized state (0° rotation) to the de-energized state (90° rotation), measured from 10% to 90% of full modulation.
  • Fall time is the amount of time for the rotator to switch from the de-energized state (90° rotation) to the energized state (0° rotation), measured from 90% to full 10% of full modulation.
  • the rise time is generally fixed by the design of the polarization rotator and varies somewhat as a function of temperature.
  • the fall time varies as a function of temperature as well, but it is also controlled by altering the amplitude of the AC drive signal. Higher amplitudes generally provide faster fall times.
  • the relative amount of rotation of the incoming light is also controlled by the amplitude of the applied AC drive signal.
  • a characteristic curve showing percent light output as a function of applied AC drive voltage is provided in Figure 5.
  • the active region of the EOM device enables control of the level of light transmission throughput.
  • a drive voltage of 4 volts provides a 2x brightness attenuation
  • a drive voltage of 5 volts provides a 5x attenuation
  • a drive voltage of 5.9 volts provides a lOx attenuation
  • a drive voltage greater than 12 volts provides a near 100% attenuation with an on/off ratio greater than 500 to 1.
  • Embodiments of the present invention allow use of this capability to provide a dynamically controllable level of modulated light, controlled according to image content.
  • FIG 6 shows how display apparatus 120 of the present invention handles the evening cityscape image of Figure 1 in one embodiment.
  • two EOMs 64a and 64b are shown being used for image display, each corresponding to a horizontal band or region 42 or 44 of the displayed image.
  • a single EOM could be used for the entire image frame, adaptive to the arrangement of image content.
  • local control of brightness attenuation could even be applied to any portion or region of the image, even including brightness attenuation applied over one or more individual pixels or clusters of pixels.
  • One consideration, however, in applying different attenuation levels to different portions of the image frame relates to transitions between portions, in order to minimize imaging anomalies.
  • a control logic processor 56 accepts and analyzes and conditions the input image data that goes to the light modulators in display apparatus 120 and performs the attenuation control and image data manipulation required for embodiments of the present invention. Control logic processor 56 controls the operation of EOMs 64a and 64b to provide the needed brightness attenuation, according to the analysis of the image frame.
  • image analysis proceeds by obtaining a histogram or other type of representative distribution for data values in each defined horizontal or vertical region of the image or in the complete image frame (step S210).
  • the schematic diagram of Figure 7A shows a representative histogram for the skyline content of the image shown as region 44.
  • the bulk of values from the histogram are grouped within an interval A that corresponds to a small portion of the brightness values available.
  • most of the histogram values for region 44 are within brightness levels not exceeding 0.4 normalized brightness. This means that a significant number of code values are unused or minimally populated, as shown.
  • a significant portion of the darker pixels lie within the "dark noise" range, and thus provide relatively poor contrast over darker portions of the image. As a result of this distribution, very little contrast is achievable in the darker areas of the image.
  • Figure 7B shows schematically how the method of the present invention improves image contrast.
  • the brightness level available is attenuated. In the example shown, only 0.6 of the brightness is now available due to modulation of EOM 64b ( Figure 6). However, this brightness attenuation allows previously little-used or unused code value space to be employed for contrast improvement. In effect, this expands interval A to provide interval A' as shown in Figure 7B.
  • the range of brightness in the image is the same, but the number of brightness levels that are now available within this range is dramatically increased. The proportion of pixels that are now considered to be "dark noise" is greatly reduced. This leads to improved image contrast over the dark areas of this example, as shown in Figure 8.
  • a combination of both brightness attenuation and corresponding image data adjustment is provided by control logic processor 56 in order to achieve the resulting contrast improvement on an image-by-image basis.
  • Darker regions can be in horizontal bands, vertical bands, or distributed with other geometries.
  • the flexibility of this method can be affected by how regions of the display surface are assigned relative to EOM devices.
  • Histogram manipulation expands the number of code values available for darker portions of the image, but this is at the cost of reducing the number of values available for brighter portions.
  • the relative proportion of dark code values and the overall distribution of code values for an image frame can be factors in determining how to apply the method of the present invention.
  • Control logic processor 56 may be a computer workstation, microprocessor, or other type of computer device that performs image analysis and display control.
  • a separate microprocessor or other control logic device can be used specifically for EOM device control.
  • Gamma adjustment curves are generally specific to a particular imaging system type.
  • Figures 9A through 9D show an example image to which the method of the present invention is applied.
  • Figure 9A shows the original outdoor image, taken near dusk. Features below the horizon are barely perceptible in the original figure.
  • Figure 9B shows the results of a combination of attenuated brightness level and adjusted code values for the same image. Features below the horizon are now more clearly perceptible, as can be seen from the enlarged portion along the right side of each figure.
  • Figures 9C and 9D show the original and adjusted histograms, respectively, for Figures 9 A and 9B.
  • a brightness scale appears at the bottom of each histogram.
  • Figure 9D expands the relative range of the code values allotted to the darker code values.
  • a gamma value of 0.8 is applied in this example.
  • the graph of Figure 9E shows, in normalized form, the accumulated counts versus code values obtained from each of the three color channels of the original image of Figure 9A.
  • the vast majority of pixel code values in all three color channels for the original image of Figure 10A are below about 160; with less than 1% of the pixel code values above this level. Also, more than 50 % of the pixel code values are below 80 in all three color channels.
  • Figures 10A through 10D show the method of the present invention applied to an image of the same scene as in Figure 9 A, but taken further toward nightfall.
  • Figure 10A shows the original image, in which the sky is barely discernable from the land and features of the landscape are not perceptible.
  • Figure 10B shows the resulting image after applying the combination of reduced brightness, here attenuated to 10% of the full range, plus adjustment of code values for the darker pixels, here with a gamma of 0.5.
  • Figure IOC shows a portion of the green channel, here, the bottom half, for the original image.
  • Figure 10D then shows the histogram for the same data with the 10% brightness attenuation and 0.5 gamma applied.
  • the graph of Figure 10E shows, in normalized form, the accumulated counts for code values obtained from the original image of Figure 10A.
  • the vast majority of pixel code values for the original image of Figure 10A are below about 36 in all three color channels; very few code values above this level are in this dark image.
  • the method of the present invention can be used in conjunction with techniques that provide laser blanking between image frames.
  • performance of GLV or GEMS ribbons and other electromechanical modulators can be degraded both by long-term actuation and by charge deposition in repeated actuation, which causes "stiction" and other negative performance effects. Therefore, in practice, the GEMS device is neither driven with too many pulses of the same polarity nor are the pixels driven continuously within an image frame. Because of this, various timing schemes continually reverse the actuation voltage polarity using a grounded substrate and a bipolar high voltage driver for each pixel or, alternately, repeatedly switch the substrate bias voltage itself. After each image frame is displayed, the substrate is driven to the opposite voltage polarity. The effect of either of these solutions on transient light is the same; there is some transient effect that can result in the unintended leakage of light during the frame refresh cycle.
  • the timing diagram Figure 11 shows the temporal relationship of galvo drive signal 30 and a switched substrate bias voltage 50 and shows the effect of these switching signals as it relates to light delivery and light leakage for a GEMS or GLV projector.
  • a light timing signal 60 (dotted curve in light to screen graph) shows the light ON or enabled, during writing portion A of the scanning cycle as the image is being scanned to the display. However, during retrace portion B of the scanning cycle, an output light transient is detected due to unintended ribbon element movement. As shown in Figure 11, light transient repeats at each transition of substrate bias voltage 50, that is, once during each retrace portion B of the galvo mirror.
  • the time interval between times tl, t2, and t3 as shown is approximately 16 msec in one
  • the light to screen waveform 60' (solid curve in light to screen graph) is adapted to change the available brightness from frame to frame, as shown in the example of Figure 11.
  • the timing diagram of Figure 11 shows the timing of an EOM signal 68' (solid curve in EO modulator state graph) relative to galvo drive signal 30 and bias voltage 50, and shows how EOM actuation impacts light timing signal 60'.
  • the light transient between image frames is suppressed, eliminating this unwanted light from the output. Plus, the brightness attenuation and data manipulation of the present invention is performed, giving EOM devices 64a and 64b a dual purpose in display apparatus 120.
  • the timing diagram of Figure 11 also shows the EOM signal 68 (dotted curve in EO modulator state graph) which would be used when not attenuating during image frames, but includes blanking in between frames to suppress the light transient between image frames.
  • FIG. 12 shows an embodiment of display apparatus 120 with an EOM 64r, 64g, 64b in each color channel and a single analyzer 66 at the output.
  • each channel may have an analyzer and an EOM. It may be sufficient for some applications to provide light modulation and laser blanking to only some of the color channels, such as only to the green color channel, which has the most pronounced effect on luminance.
  • EOM 64a, 64b In order to switch at sufficiently high rates for laser blanking, EOM 64a, 64b must have a fast response time. Until recently, EOM devices were not able to respond quickly enough for the display apparatus timing described with reference to Figure 11. However, more recent improvements in EOM speed and overall performance are now making it possible to use these devices for the laser blanking required between image frames as well as for attenuation within the frames.
  • EOMs that can be used for laser blanking as described with reference to Figures 4 and 10 can be any suitable types of devices, such as VX series modulators from Boulder Nonlinear Systems, Inc., Lafayette, CO.
  • the apparatus and method of the present invention thus provide a dynamic illumination control for an image projection apparatus that can help to improve image contrast for individual image frames.
  • the contrast of the image can be enhanced by making the blacks appear blacker and boosting the delivered code values of the image proportionally by the amount of attenuation.
  • the modulation device can be segmented so that different regions on the screen can have different amounts of attenuation. This is useful for scenes in which bright areas exist such as sunny sky at the top and dark details at the bottom of the image. As an example, if the maximum intensity of a scene is only 10% of the maximum code values in the image can be adjusted by a 10X range and the attenuator can be set for lOx attenuation.
  • variable optical shutter such as an EOM
  • EOM can provide both brightness attenuation during projection of an image frame and laser blanking between frames, wherein the laser blanking is synchronized to the frame generation sequence to minimize stray light on screen from light passing through the optics, such as when the lasers are off the screen for scanning GEMS- and GLV-based systems.
  • the apparatus and methods of the present invention have been described primarily with reference to projection systems that use GEMS, GLV, or other scanned linear light modulators. It must be noted, however, that the apparatus and methods of the present invention are also applicable to projection apparatus that employ area spatial light modulators, such as LCD modulators or micromirror arrays, such as those used in DLP devices. It should also be noted that the method of the present invention can be applied to all composite colors of a color projector or to one or more color channels.

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  • Engineering & Computer Science (AREA)
  • Multimedia (AREA)
  • Signal Processing (AREA)
  • Control Of Indicators Other Than Cathode Ray Tubes (AREA)

Abstract

L'invention concerne un procédé permettant d'améliorer un contraste d'image pour un appareil d'affichage, ledit procédé consistant à obtenir une trame de données d'image comprenant une ou plusieurs valeurs de code pour chaque pixel dans la trame de données d'image, et à analyser la trame des données d'image pour identifier une distribution des zones foncées dans celle-ci. Le procédé consiste également à ajuster au moins une des valeurs de code pour la trame des données d'image conformément à la distribution des zones foncées dans la trame de données d'image et à atténuer un niveau de luminosité disponible pour la trame d'image conformément à la distribution des zones foncées dans la trame des données d'image.
PCT/US2010/057979 2009-12-08 2010-11-24 Commande d'éclairage dynamique pour écran de projection laser WO2011071701A1 (fr)

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US20130016287A1 (en) 2013-01-17
EP2510477A1 (fr) 2012-10-17

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