Patent Application
20060109088
Spatial light modulators (SLM's) are used in a variety of applications to control projected light, because they can be modulated at kilohertz rates and can handle relatively high levels of power. They can be used in transmission and/or reflection.
For high power applications, SLMs based on micro-electromechanical systems (MEMS) are typically used. These MEMS can be one dimensional or two dimensional arrays of elements. Examples include grating light valve (GLV) devices, which are based on diffractive optical MEMS.
GLV devices comprise a series of tiny ribbons on the surface of a silicon substrate that are typically electrostatically driven to cause the ribbons to move by a fraction of a wavelength of the relevant light. This creates a dynamic, tunable grating that precisely varies the amount of light that is diffracted or reflected.
Other examples include tilt mirror MEMS devices in which the movement and positioning of mirrors is performed in order to guide a beam of light. These are very common in fiber optic systems and display devices.
More recently SLM's have been used in commercial printing systems. Their high-speed modulation enables a substrate to be exposed very quickly with high resolution. Moreover, these MEMS SLMs can meet the high power handling requirements that are required to expose the printing substrates, or plates, at high speed.
For example, imagesetters and platesetters are used to expose the media that are used in many conventional offset printing systems. Imagesetters are typically used to expose film that is then used to make the plates for the printing system. Platesetters are used to directly expose the plates. Systems are being deployed that use a combination of a light source and a spatial light modulator (SLM). As a result, the speed of operation is no longer limited by the rate at which the laser can be modulated or the power that can be extracted from that single laser.
One currently deployed system uses a combination of a laser bar and a GLV SLM. The laser bar, in combination with projection optics, produces a rectangular cross-sectioned beam of light that is projected onto the GLV SLM. The GLV is then used to modulate transmitted light through the GLV onto the plate substrate. These systems enable very fast exposures of large substrates. Light from multiple semiconductor lasers in combination with the GLV allow for the scanning of multiple lines simultaneously on the media.
Calibration of these SLMs is very important especially in print applications. The human eye can be very sensitive to artifacts in the resulting image that is produced by the print or display imaging system. This is especially true if the artifacts result in lines or regions of different shading that extend across the image.
One example of this is banding in print media. It arises when elements of the imaging system expose the print media at different exposure levels. The result can be horizontal or diagonal lines that extend across the image, which, even if very faint, many times can be discerned by the human eye. This results in an unacceptable image.
This characteristic has been a barrier to the implementation of SLM devices in printing applications and especially commercial printing applications. As a result, many imaging systems used in printing applications still use a conventional modulated raster-scanned laser dot to expose the photo or thermally sensitive media.
One solution to avoid the generation of these artifacts in the generated image is to calibrate the SLM to achieve uniform exposure. This is typically done by equalizing the transmitted intensity across the width of the SLM.
One problem, however, that arises with these conventional calibration routines is that they are performed under static conditions. Specifically, the spatial light modulator is scanned in front of a slit detector. The light transmitted through the elements spatial light modulator, when the elements are transmissive and non-transmissive, are compared. Then, the drive or control levels of the digital-to-analog converters (DACS) are set so that the transmit levels for both the on-states and the off-states of the elements of the spatial light modulator are made uniform. In short, this system can be used to create relative uniformity across the length of the SLM when it is scanned in front of the slit detector during this calibration process.
The drawback associated with this procedure, however, concerns that the approach does not take into account the dynamic variation of the spatial light modulator and light source or LIM unit during operation. Specifically, changes in these on-DAC and off-DAC levels result in changes to the pulse width of the light that is transmitted through the GLV as it is modulated and scanned over the media on the drum. These changes in the pulse width result in changes to the spot size, since the period over which the GLV is transmissive during its modulation dictates the size of the spot in the fast axis scan direction. Thus, these dynamics associated with the modulation of the GLV result in changes to the spot size, and thus, the density of ink, for example, that the media retains, or not, during the off-set printing process. Conventional uniform calibration processes do not compensate for the dynamics and thus, can detrimentally impact the quality of the image on the media.
As a result, the present invention is directed to a method and system for calibrating a spatial light modulator, such as a GLV in a printing system. As in previous systems, it detects intensity levels of light provided by elements of the spatial light modulator. It determines control levels for the elements of the spatial light modulator that will compensate for spatial variation in the intensity of light across the spatial light modulator. These changes in intensity can be a result of the changes in properties of the GLV across its length. It can also be the result of changes in the intensity of light provided by the LIM across the spatial light modulator.
According to the invention, however, the compensation comprehends the pulse width changes of the elements of the spatial light modulator, due to changes in the control levels due to the spatial variation compensation. This second level of compensation ensures that changes to the spot size that may result from conventional compensation are comprehended in generating a total compensation scheme that accounts for the dynamics in the operation of the spatial light modulator.
Specifically, in one implementation, control levels for the elements of the spatial light modulator are selected to under compensate for the spatial variations to account for changes in the control levels due to the spatial variation compensation.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.
In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings:
This imaging engine 10 can be deployed in a platesetter in which the media 12 is a photosensitive plate. In another implementation, it is deployed in an imagesetter in which the media 12 is film. Typically, in imagesetters, however, an internal drum configuration is used.
The imaging engine 10 comprises a media drum 110. The drum 110 revolves on an axis-of-rotation 112 that is co-axial with the drum 110. In the illustrated example, the media 12 is held against the outside of the drum 110. This configuration is typically termed an external drum configuration.
In an alternative implementation, the media 12 is held along an inner side of the drum 110 to provide an internal drum configuration.
A carriage 120 is disposed adjacent to the drum 110. It is controlled by a controller 131 to move along track 140 that extends parallel to the rotational axis 112 of the drum 110.
In the internal drum configuration, the carriage 120 moves within the drum 110 and is typically supported on a cantilever-like track, generally extending down through the center axis of the drum 110.
In either case, the carriage 120 supports a GLV SLM system 121 that includes a light source or LIM 122. In the present implementation, this light source 122 comprises an array of laser diodes in a bar. The beams from these laser diodes are combined into a single output and coupled into an integrating optical system 124.
Generally, because of the multi-source nature and because individual laser diodes have spatial intensity profiles that are somewhat Gaussian, the integrator 124 is typically required to generate a beam 126 with a rectangular cross section and with a uniform or improved spatial intensity profile.
The spatially homogeneous beam 126 is coupled to projection optics 128, which ensure that the beam has a rectangular cross-section and a planar phase front. This rectangular beam is then coupled through a spatial light modulator 130 to the media 12 held on the drum 110. A Hall-effect focus motor 129 is used to adjust the focus position provided by the projection optics 128 under control of the controller 131.
In the present implementation, the spatial light modulator 130 comprises a linear array of grating light valves (GLV). The elements of the grating light valve array function as shutters that control the level of transmission to the media 12. Generally, each grating light valve comprises an optical cavity that will propagate light through the grating light valve to the media in response to the optical size of the cavity and the wavelength of light generated by the light source 122.
In other implementations, different spatial light modulators are used. For example, in some examples, the spatial light modulator 130 comprises a two-dimensional array of elements. Different types of spatial light modulators can also be used, such as spatial light modulators based on liquid crystal or tilt mirror technology.
In the present implementation, the operation of the spatial light modulator elements is controlled by an ON DAC system 132 and an OFF DAC system 134. These devices dictate the modulation level of the elements of the spatial light modulator 130.
The elements of the spatial light modulator 130 are controlled in a binary fashion such that, during operation, they are either in an ON or transmissive state to expose the corresponding pixel or spot on the media 12, or an OFF state or dark, non-transmissive state to leave the corresponding pixel on the media 12 unexposed. Whether the elements of the spatial light modulator 130 are in a transmissive or non-transmissive state depends on the size of their respective optical cavities. The ON DAC system 132 and the OFF DAC system 134 are loaded with ON and OFF control level data that dictate the drive voltages used to control the elements during the on and off states. These ON and OFF control level data are loaded into the ON DACS 132 and the OFF DACS 134 by the controller 131.
In other implementations, the elements are modulated to multiple levels, such as 256, to provide gray-scaling, for example.
A calibration sensor 150 is provided. In the present embodiment, this calibration sensor 150 comprises a photodiode 152 and a slit aperture 154. The combination of the photodiode 152 and the slit aperture 154 enable the controller 131 to monitor the operation of individual elements of the spatial light modulator 130 when the carriage is moved to the calibration position 156, such that it is opposite the calibration sensor 150.
In an alternative embodiment, the calibration sensor comprises a one dimensional or a two dimension sensor such as a CCD array. These linear or planar sensor arrays are useful to increase the speed of calibration and/or improve the uniformity of the illumination on a pixel by pixel basis.
The division of each of the on-DAC 132 and the off-DAC 134 into four separate drive devices enable for effective distribution of the electronic drive signals to the various shutters 216 of the GLV 130. In more detail, the left even DAC driver 224 drives the even shutters on the left side of the GLV 130, the left odd shutter DAC driver 226 drives the odd shutters on the left side of the GLV 130, the right even shutter DAC driver 228 drives the even shutters on the right side of the GLV 130, and the right odd shutter DAC driver 230 drives the odd shutters on the right side of the GLV 130. In a similar vein, a separate set of four off-DAC drivers are provided in the off-DAC system 134.
Moreover, the high spatial frequency variation 250 between successive shutters is generally due to the alternating even and odd DAC drivers. Specifically, since every other shutter 216 is driven by a different driver, there is shutter to shutter variation between the transmitted power through the GLV 130.
Similarly, there is spatial variation in the intensity of light 245 generated by the LIM 122. Specifically, the LIM exhibits spatial oscillation 244 that is somewhat coarser than the variation in the shutters 216 of the GLV 130. Moreover, even if these high frequency changes are ignored, there is also some longer period variation as illustrated by line 246.
In order to compensate for the variations in both the LIM's spatial intensity profile and the GLV's variation of transmitted light illustrated in
According to the invention, the calibration of the GLV is accomplished using the slit detector 150. It is done at several desired test power levels. The process begins with a calibration scan of the lens/GLV at a known power for the LIM and at a zero DAC level on the GLV. This calibration scan is recorded as the LIM signature as a function of LIM power and GLV shutter. Subsequent calibrations start at some flat DAC level, such as 70 or 90, and then altered on a pixel by pixel basis to match the LIM signature or an extinction factor that the DAC level represents, a function such as scaled by a constant K. This requires the DAC to yield a constant extinction or scaling for K for a current I in position X. In short, calibration for more than one DAC level is used to characterize the GLV non-linear response. In different embodiments, GLV at strategic beam positions is monitored and used to minimize the number of calibration processes. The current slit size is 5 to 10 micrometers, but large slit sizes can be used. This can be used to compensate for known changes and pulse width variation.
Specifically,
This process of loading 1-3 shutter patterns and then scanning the SLM GLV in front of the slit detector 150 enables the detection of shutter-to-shutter variation in transmitted intensity and specifically, allows for the detection of the high spatial frequency component shown in the plot of
In the typical implementation, the on-DAC level is set to a static level, such as a DAC drive level of 90 in step 252. Thus, the performance of the GLVs at similar drive levels is determined.
As discussed previously, gathering this static calibration information produced by the combination of the LIM and the GLV, however, does not take into effect the dynamic characteristics that arise during the scanning of the media 12 on the drum 110 during operation.
This is largely due to the fact that the pixel or spot that is generated on the media 12 when a given shutter 216 of the SLM GLV 130 is transmissive, is determined in part by the transition from the off to the on and the on to the off state. This dictates the size of the pixel or spot in the fast scan axis or in the direction Y, as illustrated in
In contrast, the size of the spot in the Y-axis or fast scan axis direction is dictated by the transition of each shutter 216 between the off and on states. Thus, the length of the spot 530 in the Y-axis direction would tend to be increased, for example, in response to higher speed rotation of the drum 110, or slower modulation of the respective shutter 216 of the SLM GLV 130.
Specifically, changes to the DAC level from a nominal level of 90, for example, to 80 result in a change in the pulse width from W90 to W80. This results in a change in the size of the spot 530 in the Y-axis direction as illustrated WA. These changes in the pulse width are the typical result of the process of calibration in which the on-DAC control levels are set in an effort to flatten the relative power transmitted through the shutters of the GLV.
Specifically, with reference to
This change in the drive level, however, from the nominal 90, to 80, for example, results in a change in the width of the spot 530 illustrated relative to
As a result, according to the present invention, the control levels of the elements of the spatial light modulator are set to compensate for spatial variation in the intensity across the spatial light modulator, but also the concomitant pulse width changes for the elements of the spatial light modulator due to changes in the control levels due to the spatial variation compensation.
In other words, the control levels for the elements of the spatial light modulator are set to under compensate for the spatial variation as illustrated with reference to
While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.
