Optics: measuring and testing – Of light reflection – With diffusion
Reexamination Certificate
2001-06-04
2003-12-30
Adams, Russell (Department: 2851)
Optics: measuring and testing
Of light reflection
With diffusion
C356S429000, C356S430000
Reexamination Certificate
active
06671052
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to densitometers for measuring optical density. In particular, the invention relates to optical density measurement of multiple spots or test patches for monitoring and quality control of printed output.
BACKGROUND OF THE INVENTION
In printing and copying apparatus, machine parameters are adjusted, either manually or automatically, to produce images having well regulated darkness or optical density. Printer process control strategies typically involve automatically measuring the transmissive or reflective optical density of printed areas (called “test patches”) as they are printed. Alternatively, printed samples may be measured manually using a “bench-top” densitometer. In either case, the density measurements are the basis for quality evaluation and control. Adjustments may then be determined to regulate the printed test patches to the desired density levels. The adjustments are often applied automatically, though some printer adjustments may require manual adjustment by an operator. For test and diagnostic purposes, large nominally uniform areas or multiple test patches may be printed to check for deviations from the desired uniformity.
In a large-format printer, uniformity may be difficult to maintain over the large printed areas, particularly in the direction perpendicular (cross-track) to the process direction. Nonuniformity can also be a problem in non-printed web production processes, such as plastic sheet, textiles, and paper. In these cases optical density measurements may be needed transversing the cross-track direction. U.S. Pat. No. 5,546,165, to Rushing et al uses a document scanner as a test print densitometer. Such a scanner may use an essentially continuous linear array of light sensors to collect measurements from several thousand picture elements or pixels spanning the cross-track dimension. These measurements provide the cross-track density “profile”. This approach requires uniform illumination across the wide printed area, imaging optics, and a shift register driven to provide an output voltage signal representing the pixel-by-pixel cross-track profile of transmittance or reflectance.
A simpler and less expensive approach is to obtain representative cross-track measurements at just a few spots. Three or four representative cross-track measurements are sufficient to guide basic adjustments and/or maintenance on the typical machine. The objective of such basic adjustments and/or maintenance is typically to balance the average density edge-to-edge, and avoid a “high” or “low” spot in the center.
Adjustments to the engagement or spacing of the various work stations, relative to the image-bearing medium, are often used. Independent adjustments are often available at the front and back ends of a work station. An independent center adjustment may also be available. In an electrophotographic process, such adjustments may be applied to corona charging devices, to illumination or exposure, and to toning stations. Image exposure adjustments, gradual from edge-to-center-to-edge, are disclosed in U.S. Pat. No. 5,933,682 to Rushing, for example. In paper manufacturing processes discussed by Gorinevsky et al, sets of identical independent actuators, distributed across the paper web, control cross-track uniformity of web attributes at several production stages.
The electrophotographic printer described in U.S. Pat. No. 5,983,044 to Kodama et al has a densitometers positioned before and after transfer of the toned image from the photoconductor drum to the receiver. One densitometer is positioned to read test patches on the drum before transfer. Two more densitometers provide post-transfer readings of the transferred toner patch and the residual toner remaining on the photoconductor drum, respectively. Transfer efficiency is determined from these readings. Deviations from the normal transfer efficiency are the basis for electrical adjustments applied to the multi-color developing units and to the transfer process.
In a typical multi-color printer, test patches of each process color are monitored for process control purposes. Ideally, each color has its own dedicated densitometer channel, with a light emitter color or peak emission wavelength selected for high sensitivity of the readings. Separate dedicated channels may also enable density readings to be taken farther upstream in the imaging process, in the individual color processing modules, before the separations are collected on a single web. The earlier upstream readings minimize the delay in obtaining measurements, and enable faster-responding feedback control loops for the process.
An economical single-channel densitometer for an electrophotographic color printer is disclosed in U.S. Pat. No. 5,075,725 to Rushing et al. The transmission densitometer has an infrared emitter and measures test patches covered with cyan, magenta, or yellow toner. Previously stored base density readings from untoned film are subtracted to yield net toner densities. The machine logic keeps track of the color of the passing patches, so that the measured net toner densities can be compared to target values for the respective colors.
This single-channel infrared-emitting densitometer outputs usable density signals for patches covered with the colored toners, and black toner. However, colorants in other applications, such as ink jet inks, do not sufficiently block infrared light. Even for unfused colored toners, better sensitivity is obtained using emitters of complementary color to the respective test patches, i.e., red, green, and blue emitters for cyan, magenta, and yellow toner, respectively. Finally, the single-channel configuration does not address the need in some printer configurations for density readings at multiple positions.
The approach described in U.S. Pat. No. 3,995,958 to Pfahl et al obtains good color sensitivity. Filters of complementary colors to the test patches automatically rotate into position in front of a white light emitter, so that each colored patch is read with light of the particular color for highest sensitivity. While using only one densitometer, this approach requires bulky, complicated, and expensive mechanisms to change filter positions at the appropriate times. Furthermore, all the variously colored patches must be on the same single track of the moving medium, and pass the densitometer one after another. Such a fixed-position single-channel configuration cannot address needs for density readings in multiple in-track or cross-track positions.
As an alternative to the white light source and multiple color filters used in U.S. Pat. No. 3,995,958 by Pfahl et al, multiple emitters of different colors can be used, e.g., red, green, and blue LEDs. These emitters are aimed such that transmitted or reflected light is collected by a single photodetector. To read cyan, magenta, and yellow test patches, a single complementary-colored LED is energized, according to the known color of the test patch to be read at that time. A test patch of mixed or layered colorants is read by rapidly and successively energizing the LEDs one at a time, to obtain a set of density readings characteristic of the overall test patch color while the test patch is in position. Such a configuration measures test patches of various colors without the complexity of mechanical motion in the densitometer, but otherwise is subject to the same limitations as other fixed-position single-channel configurations.
Using mechanical drives, a single-channel densitometer configuration can be adapted to obtain readings at different positions. In U.S. Pat. No. 4,003,660, Christie et al disclose a densitometer movably mounted on support members that extend across the width of a printed web. Such a mechanical drive is bulky, complicated, and expensive. Optical spacing and alignment tolerances are-more difficult to maintain, owing to the motion. At any given time, readings can be obtained at only one position, and time is required to move to the next position.
Multiple densitometers have been incorporated int
Adams Russell
Sever Andrew
LandOfFree
Multi-channel densitometer does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Multi-channel densitometer, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Multi-channel densitometer will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3178883