BACKGROUND OF THE INVENTION Field of the Invention; The present invention relates to the field of densitometry, and more specifically, to a densitometer which automatically aligns itself with a sample to be measured.

Description of the Prior Art; A reflection densitometer is a well known optical measurement instrument that is used for quality control in color printing production processes. The most common use of a reflection densitometer is to measure ink on paper to control the printing process, although densitometers are also used to measure photographic prints and other images on various substrates such as paper, plastic and metal, in order to monitor and control the production of the images or to test the production equipment. Basically, a reflection densitometer directs a beam of light having a known color quality, i.e., a balance of red, green and blue light in conformance with ANSI/ISO standards, at a printed sample. By using a filtration system, the densitometer then measures the amount of light which is reflected from the surface of the sample and generates an output signal which is indicative of the color density of the sample. In practice, it is most common that the densitometer output signal is actually the reflectance of the sample, but a conversion to density is straightforward since density is the inverse logarithm of reflectance. Unless otherwise indicated, it will be assumed herein that a

densitometer generates a reflectance signal as its output.

Illumination and measurement systems are referenced to an axis which is perpendicular to the sample. According to accepted standards, the illumination angle is either 45° or 0° degrees, and the measurement angle is then 0° or 45°, respectively. It is then assumed that the light absorbed by the sample is the difference between the incident light and the reflected light. The sample color density can then be calculated according to an accepted logarithmic relationship.

Automatic densitometers, such as the AutoSmart ^w Densitometer made by Graphic Microsystems, Inc., have been developed which use a computer to control the position of the densitometer with respect to a measurement table, thus enabling the densitometer to be automatically driven to measure a predefined pattern of measurement points on the printed sample. Using motor driven horizontal and vertical positioning, automatic densitometers can read up to 250 points on a printed sample in less than one minute.

However, it is difficult to place a printed sample sheet in an identical x-y orientation relative to the measurement table every time a sheet is placed on the table for measurement. Thus, the position of each measurement point on a sample sheet may be slightly offset relative to the table for each successive sheet which is measured. This causes a conventional automatic densitometer measurement scheme to take readings at close, but not exact locations. Therefore, it is desirable to have a method of locating the pattern of measurement points

with a high degree of accuracy each time a new sheet is put on the table for measurement.

The AutoSmart™ Densitometer requires an operator to manually align an optical target in the densitometer with two or three selected "alignment points" on the printed sample. Once the positions of the three alignment points are known, the locations of all points to be measured can be calculated by the computer. Some automatic position indicators use auxiliary targets and sensors to locate the targets. For example, the Hammamatsu quadrant detector can be used to locate targets such as light spots. Bar code scanners can be used to sense the widths of target lines. Other more elaborate schemes use "machine vision" techniques that locate targets by using video images. The disadvantage of all of these methods is that they require complex and costly additional equipment, including a light source, a detector, and electronic processing circuitry.

If the task of locating the alignment points could be automated, the measurement of a sample sheet would be easier and faster. Also, the accuracy of locating the measurement points, which was previously limited by the operator's ability to visually position the target on an alignment point, can be improved.

SUMMARY OF THE INVENTION The present invention provides a system and method for automatically aligning a densitometer relative to a printed sample. The sample is placed on a measurement table to which the densitometer is movably affixed. The densitometer then scans a predefined target on the sample. The densitometer

output, i.e., the reflectance, undergoes a step change as the target is scanned. By analyzing the step change in the reflectance signal, the position of an alignment target relative to the table is established. All other points on the sample have a known position relative to the target position. By using two or three alignment points, a predefined pattern of points on the sample can be measured by the densitometer system using the target positions as references.

One advantage of the present invention is that it uses the conventional measurement optics of the densitometer to automatically locate a pattern of points to be measured on a given printed sample. In addition, a novel target can be incorporated into the sample to permit confirmation that the proper target has been correctly located. Preferably, vertical and horizontal edges are scanned to define a target location. Advantageously, different targets may be used for operator location and automatic machine location, but these targets have a known position relative to each other so that the operator targets and the automatic machine targets can satisfy different requirements. Other features and advantages of the present invention will be more fully understood and appreciated upon consideration of the following detailed description of the invention and accompanying drawings which set forth an illustrative embodiment in which the concepts of the invention are utilized.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a perspective view illustrating a densitometer system in accordance with the present invention.

Figure 2 is a plan view illustrating a typical printed sample.

Figures 3a and 3b are plan views illustrating the preferred alignment target in accordance with the present invention.

Figures 4a-4d are waveforms illustrating the reflectance signal from a densitometer scan of the target of Figure 3 in accordance with the present invention.

Figures 5, 6a, 7a-7d, 8a-8d, and 9 illustrate alternative alignment targets in accordance with the present invention.

Figure 6b is a waveform illustrating the reflectance signal from a densitometer scan of the target of Figure 6a in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In describing the present invention, the following definitions will prove helpful:

Measurement Point; the center of a spot to be measured with the densitometer. Its location is defined by the x,y coordinates of the center of the spot.

Measurement Pattern: An array of measurement points each with a known location relative to the other measurement points. Alignment Point: A point used to define the location of all points in the measurement pattern. Alignment Target: An image used to locate an alignment point. For example, a crosshair could be used to locate the center of the crosshair as an

alignment point; a dot could be used to locate the center of the dot as an alignment point; a corner of a printed rectangle could be used to locate the position of the corner as an alignment point. NOTE: One of the novel concepts of the present invention is that the alignment point need not be at any particular location with respect to the alignment target. For example, the alignment point could be defined as 0.050" to the left and 0.100" down from the center of a crosshair target, or 0.2" to the left and 0.1" upward from a specified corner of the sample used as an alignment target.

Projected Target: A target that moves with the measurement head of an automatic densitometer and which may be positioned on an alignment target for visually locating the alignment point.

Operator Alignment Target: A target which is easy for the operator to locate by positioning the projected target in alignment with it. Automatic Alignment Target: A target which is easy to locate automatically.

NOTE: One of the novel concepts of this invention is that the operator alignment target and the automatic alignment target for locating the same alignment point can be different. Therefore each can be optimized to facilitate use of either manual or automatic alignment.

Target feature: A feature of an alignment target such as the edges defining the corner of a rectangle or the lines of a crosshair.

An automatic densitometer system 2 is illustrated in Figure 1. The system 2 includes a measurement table 10 that has an arm 12 to which a densitometer head 14 is attached. The arm 12 is movable in the horizontal or x-direction along tracks

16a, 16b on the top and bottom, respectively, of the table 10, and the densitometer head 14 is movable along the arm 12 in the vertical or y-direction. The arm 12 and head 14 are motor driven under control of a conventional personal computer system (not shown) , such as an IBM DOS-compatible PC, such that the arm 12 and head 14 can traverse the full layout of the table 10. In this way, the computer can drive the arm 12 and head 14 to any predefined point, preferably referenced as an x,y coordinate, relative to the measurement table 10.

In operation, the operator places a sample sheet 18 of printed material on the table 10. While the present invention is described with reference to a rectangular flat printed sample, it should be recognized that other sample shapes and configurations, for example, a printed two piece can, could be used with equal success by modifying the way in which the sample is held and the way that the densitometer traverses the sample.

As illustrated in Figure 2, a typical printed sheet 18 comprises an unprinted substrate, such as paper, to which a design is applied by coating portions of the substrate with one or more colored inks by a variety of methods, such as lithography. The printed matter 20 is located on the substrate such that it leaves a border or trim area 22 of unprinted substrate around the outside of the printed matter 20 into which a color bar 24 or other reference information such as an alignment target 26 may be provided. The trim area 22 will usually be present on all four sides of the printed matter, and while preferred, it is not required.

As noted above, the illumination and measurement geometry for the densitometer is precisely defined.

See, for example, the following standards: ANSI Ph2.17, ISO 5/4, and DIN 16536. Thus, according to industry standards, a circular spot on the sample is uniformly illuminated by a beam of light having a known color quality, and the reflected light is measured by the densitometer in conformance with accepted standards.

The Autosmart™ Densitometer, described in the Background section above, used automatic scanning of a spot approximately the size of the measurement aperture to determine the precise relative position of the projected target and the center of the measurement spot. The projected target and the measurement spot are separated by two inches, but due to manufacturing tolerances, the relative locations may vary by up to 0.050". By making one x and one y scan across a black spot which the operator manually aligns with the projected target and determining the location of the minimum reflectance signal for each scan, the x and y distance between the center of the projected target and the center of the measurement spot can be measured by the densitometer to within +/- 0.005". This method is not adequate for automatic alignment of sheets, however, because the difference between the estimated position of the target before alignment and the actual position of the target must be very small or the alignment will fail. For example, with a two millimeter (.080") measurement spot, the difference can only be about +/- 0.050" to provide effective alignment.

According to the present invention, the densitometer head 14 is caused to automatically scan alignment targets 26 prior to making its measurements of the predefined measurement pattern. By establishing the exact position of the alignment

targets relative to the measurement table, the position of each predefined measurement point on the sample, previously known relative to the sample, is now known relative to the table. It is preferred to have three alignment targets

26, since scanning three points can cause the computer to account for movement of the sample by translation, rotation, or stretch. Thus, by locating three targets in a triangular relationship, the correction required by each target can be generally expressed as:

X = ax + bx + c (1)

Y = dx + ey + f (2 ) where X, Y are the true coordinates of target points or measurement points on the sheet, and x,y are the original coordinate of the target points or measurement points when they were initially selected, and a, _f C, d, e, £ are constants. The location of the three target points requires simultaneous solution of six equations in six unknowns to find the constants a,b, c, d, e, f. Then all measurement points can be located by reference to the equations. However, in many practical applications, stretch is not really a factor, thus two alignment points would suffice. Referring now to Figure 3a, an example of a preferred alignment target 26a is illustrated. Alignment target 26a provides high reliability and accuracy for both manual and automatic alignment with a shape that can fit in the trim area 22 of most printed sheets. The preferred target features are the vertical or horizontal "edges" of the printed bar 26a, i.e., the boundaries between the printed and unprinted substrate. Typically, the corner 29 of the bar 26a is defined as the alignment point. The

alignment target 26a is shown in a vertical orientation, but may also be in a horizontal or any other orientation by using a different scan pattern. It is an important concept of this invention that by scanning a vertical edge or line in the x direction, the x coordinate of the target can be determined without knowledge of the precise y coordinate. For example, if the vertical edge of target 26a is one-half inch long, it is practical to find the precise edge location if the estimated location is within about +/- one-quarter inch of the true location of the target. Once the x location is known, the edge to be located by the y scan can be much shorter, since the x position of the edge is known.

Figures 4 -4c show typical densitometer reflectance signals of a left to right scan of the alignment target 26a when it is placed in the left trim area 22a of the sheet 18. In each case, the boundary edge is located at the midpoint of the step change from low to high signal. The smaller transitions and the large transition from high to low signal may or may not be present depending on whether the scan starts to the left of the target but on the paper (Figure 4a) , on the target (Figure 4b) , or to the left of the target on the gray measurement table (Figure 4c) . All of the extraneous transitions are ignored by identifying the large transition from low to high signal and by prior knowledge of the approximate values of the low signal to be expected on the target and the high signal to be expected on the unprinted substrate. The expected signal levels are determined during set up of the measurement pattern.

In a variation of the above. Figure 3b includes a round dot 30 which may be used to confirm that the correct alignment point has been located.

Figure 4d shows a typical curve of signal as the alignment target 28 of Figure 3b is scanned downward in the y direction beginning at the center of the target in the x dimension. The center of the target in the x dimension is known from the first scan. The y scan may begin anywhere in the y dimension of the target depending on the relationship of the estimated position of the alignment point and the actual alignment point. The y dimension of the bottom horizontal edge of the target is located at the midpoint of the first large transition from low signal to high signal.

The second valley in Figure 4d following the transition which located the bottom edge of the target is caused by scanning the dot 30. The second valley, by its location and depth, can be used to confirm that the proper alignment target location was found and is a measure of the degree of accuracy of the location.

Other target examples are shown in Figures 5-9. In Figure 5, a crosshair 32 is used for the operator alignment target instead of spot 30. The crosshair 32 is a good target for operator alignment, but does not provide the verification of automatic alignment that the dot provides.

In Figure 6a, the intersection of lines 34, 36 is the operator alignment target. Lines 34, 36 are also the automatic alignment target. Figure 6b shows the output signal for an x scan of a line 34. The transition is not as large as that for bar 28, thus, is not as immune to detection of false targets.

In Figures 7a-7d, any one of the four corners 38a, 38b, 38c, 38d of a printed region 20 of a sample 18 could be selected as the operator alignment target. The computer uses the two edges associated with the corner to locate the x,y coordinates of the corner. Corners are less desirable for operator alignment targets, and are not as reliable for automatic alignment, but they do have the advantage that they are already present in many printed sheets. In Figures 8a-8d, the corners 40a, 40b, 40c, 40d are created by lines 42 and 44, 46 and 48, 50 and 52, 54 and 56, respectively. A corner is selected as the operator alignment target and the lines are used by the machine to locate the target. These types of corners are better for operator alignment, but not as effective for automatic alignment as the solid corners.

Figure 9 is an example to show that the horizontal edge 60 and the vertical edge 58 used by the machine do not need to be in any special arrangement with respect to each other, or with respect to the operator target 62. The only requirement is that these targets be in a known relative position with respect to each other. It should be understood that the present invention is not intended to be limited by the specifics of the above-described embodiments. The scope of the invention is defined by the accompanying claims.