1.  Field of the Invention
The present invention relates generally to an apparatus and method for accurately measuring and examining dot position deviation and dot density fluctuation in a sub scanning direction of a printed image in order to evaluate printer performance.  Particularly, the present invention relates to a technique for detecting cyclic fluctuation of a variation in dot position and cyclic fluctuation of a variation in dot size generated in a sub scanning operation performed by an imaging apparatus such as an electrophotographic printer that forms a two-dimensional image by generating a one-dimensional dot line extending in a horizontal (main scanning) direction and successively repeating the dot line formation in a vertical (sub scanning) direction. 
2.  Description of the Related Art
Prior art publications relating to the present invention are disclosed in Japanese Patent Laid-Open Publication No.7-175933 (“Image Evaluation Method and Apparatus”), Japanese Patent Laid-Open No.9-233235 (“Image Evaluation Method and Imaging Apparatus Using the Same”), and Japanese Patent Laid-Open Publication No.2001-74602 (“Image Evaluation Apparatus and Image Evaluation Method”), for example.  In the printed image evaluation performed according to these publications, a dot position (deviation) of the image is evaluated using either adjacent dot patterns or line patterns. 
Also, in Japanese Patent Laid-Open Publication No.11-039486 (“Image Quality Evaluation Method”), a technique for detecting dot density fluctuation with respect to the sub scanning direction (banding) in a printed image based on a scanned image of the printed image obtained by a scanner, for example, is disclosed.  According to this publication, the print pattern used in the image evaluation may be a 2-by-2 pattern (dot pattern in which dots are laid out at 2-dot intervals both in the horizontal and vertical direction), for example, as shown in FIG.  1.  It is noted that a maximum dot density can be achieved by using this print pattern.  Alternatively, the density fluctuation may be detected by calculating the brightness within a region having a predetermined width in the main scanning direction and a height of one dot in the sub scanning direction, this process being successively performed in the sub scanning direction. 
In this prior art method, the following problems exist. 
The first problem is that it cannot be discerned whether the detected banding is due to dot size fluctuation or dot position deviation. 
In the imaging apparatus being evaluated, the unevenness in dot density with respect to the sub scanning direction (banding) in the printed image can be caused by either dot size fluctuation or dot position deviation.  The differentiation of these two factors will be important in specifying the cause of the banding.  However, it is difficult to differentiate the two factors when the space between the dots is too narrow; that is, owing to the characteristics of the imaging apparatus being evaluated, when the dots are arranged too close to each other, the dots tend to influence each other, and as a result, the dot size increases (dot size fluctuation) and the dot space is narrowed as well (dot position deviation). 
FIGS.  2A and 2B are diagrams illustrating the relationship between the dot space and the dot size obtained from an image produced according to a laser electrophotographic method.  In a case where the laser beam irradiation pitch is 169 μm (FIG.  2A), the space between the imaged dots is 169 μm, which is the same as the laser beam irradiation pitch (dot pitch designated by the dot pattern).  However, when the laser beam irradiation pitch is 160 μm (FIG.  2B), the space between the imaged dots is 150 μm, and the dot size increases as well.  In FIG.  2B, the dot space is 19 μm narrower than that of FIG.  2A. This is because the electrical charges of the dots influence each other when the dot pitch for imaging the dots with the laser beam is so narrow, resulting in the dots of the printed image being enlarged.  Further, the dots expand in the directions that narrow the space between the imaged dots, resulting in dot position deviation. 
Another problem is that when the dots of the printed image are formed by a 2-by-2 pattern configuration (see FIG.  1), the detection of the dot density fluctuation cycle in the sub scanning direction in the image evaluation can only be detected in units no smaller than four imaging units.  This is because the dots are arranged into patterns each with a density of four imaging units in the sub scanning direction. 
If the dots are formed with a pattern density of one imaging unit in the sub scanning direction, a more precise analysis of the imaging apparatus can be realized since the density fluctuation cycle can be detected with respect to the smallest imaging unit and all the high-frequency position deviation oscillation components can be detected.  However, if such a pattern is used, the dots will be continuous (there will be no space between the dots), and it will be impossible to detect the dots as one single dot. 
A second problem is that in measuring the dots of a printed image formed by numerous dots laid out in a predetermined pattern, if each dot is detected and measured individually, a significant amount of time is needed in detecting all of the dots, and further, dust particles or stains, for example, on the reproduced image may be mistakenly detected as a dot, resulting in a measurement error. 
A third problem is that there will be some position deviation in the reproduced image with respect to the print pattern.  For example, the image magnification may be varied, or the reproduced image may be skewed due to the slanting of the image upon being printed on the support medium (paper) Also, the support medium may expand or contract after the image is printed out.  Thus, the dot spacing of the printed image may not necessarily correspond exactly to the print pattern (dot pitch).  This may cause errors in the image measurement, especially in the dot position measurement. 
A fourth problem is that upon inputting the reproduced image into an image input apparatus, especially in the case of using a scanner, unevenness in density caused by an inconsistency in the distribution of irradiation light for reading the reproduced image may be a factor influencing the image evaluation, and the dot size of the imaged dot may be prevented from being accurately measured.