A method for sensing the color registration of a multi-color printing press having a printer for a reference color, a printer for a first process color, a printer for a second process color, and a printer for a third process color comprising the steps of:
- (a) printing with the reference color printer at predetermined locations in the non-image areas of a sheet to be printed at least one reference register mark corresponding to each of the first, second and third process colors, each of said reference register marks being comprised of a reference color area and a clear area;
- (b) overlaying each of the reference register marks corresponding to the first process color with a corresponding first process color register mark, each of said first process color register marks being comprised of a first process color area;
- (c) measuring a single value of average reflectance for each overlay of said reference and first process color register marks, said single value of average reflectance being measured at a single point in time over the entirety of a first region comprised of a plurality of subregions formed as the result of said overlaying of said reference and first process color register marks, said subregions being comprised of a clear subregion having an area equal to the clear area of the reference register mark corresponding to the first process color overlay to the extent said clear area is not overlayed by said first process color area, a reference color subregion having an area equal to a portion of said reference color area within said first region to the extent said portion is not overlayed by said first process color area, a first process color subregion having an area equal to said clear area to the extent said clear area is overlayed by said first process color area, and an overlay subregion having an area equal to said portion of said reference color area within said first region to the extent said portion is overlayed by said first process color area, said average reflectance being a monotonic function of the registration of the first process color, each of said measured single values of average reflectance being an area-weighted sum of the reflectance of said subregions and representative of the registration of said first process color;
- (d) overlaying each of the reference register marks corresponding to the second process color with a corresponding second process color register mark, each of said second process color register marks being comprised of a second process color area;
- (e) measuring a single value of average reflectance for each overlay of said reference and second process color register marks, said single value of average reflectance being measured at a single point in time over the entirety of a second region comprised of a plurality of subregions formed as the result of said overlaying of said reference and second process color register marks, said subregions being comprised of a clear subregion having an area equal to the clear area of the reference register mark corresponding to the second process color overlay to the extent said clear area is not overlayed by said second process color area, a reference color subregion having an area equal to a portion of said reference color area within said region to the extent said portion is not overlayed by said second process color area, a second process color subregion having an area equal to said clear area to the extent said clear area is overlayed by said second process color area, and an overlay subregion having an area equal to said portion of said reference color area within said region to the extent said portion is overlayed by said second process color area, said average reflectance being a monotonic function of the registration of the second process color printer, each of said measured single values of average reflectance being an area-weighted sum of the reflectance of said subregions and representative of the registration of said second process color;
- (f) overlaying each of the reflectance register marks corresponding to the third process color with a corresponding third process color register mark, each of said third process color register marks being comprised of a third process color area;
- (g) measuring a single value of average reflectance for each overlay of said reference and third process color register marks, said single value of average reflectance being measured at a single point in time over the entirety of a third region comprised of a plurality of subregions formed as the result of said overlaying of said reference and third process color register marks, said subregions being comprised of a clear subregion having an area equal to the clear area of the reference register mark corresponding to the third process color overlay to the extent said clear area is not overlayed by said third process color area, a reference color subregion having an area equal to a portion of said reference color area within said region to the extent said portion is not overlayed by said third process color area, a third process color subregion having an area equal to said clear area to the extent said clear area is overlayed by said third process color area, and an overlay subregion having an area equal to said portion of said reference color area within said region to the extent said portion is overlayed by said third process color area, said average reflectance being a monotonic function of the registration of the third process color, each of said measured single values of average reflectance being an area-weighted sum of the reflectance of said subregions and representative of the registration of said third process color;
- (h) converting each of the measured single values of average reflectance representative of the registration of the first, second and third process color printers to a first, second and third registration error signal, respectively, using a predefined equation;
- (i) inputting the first, second and third registration error signals into a closed-loop control system operatively connected to a printing press cylinder registration mechanism of each of the first, second and third process colors to adjust each of said mechanisms, and thereby automatically maintain the registration of the first, second and third process colors;
- (j) printing at predetermined locations in the non-image areas of the sheet at least one solid patch for each of the reference, first, second and third process colors;
- (k) measuring the reflectance of each of said solid patches, and thereby, of said reference, first, second and third process colors;
- l compensating for variations in the reflectance of said reference, first, second and third process colors in converting said single values of average reflectance into said first, second and third registration error signals;
- (m) printing at predetermined locations in the nonimage areas of the sheet for each of the reference and first, second and third process colors;
- (1) at least one first color bar pattern comprised of a plurality of linear bars parallel to the direction of motion of the paper, each of said bars having a width equal to the spacing between said bars, and
- (2) at least one second color bar pattern comprised of a plurality of linear bars perpendicular to the direction of motion of the paper, each of said bars having a width equal to the spacing between said bars,
- (n) measuring the reflectance of each of said first and second color bar patterns, said reflectance of said first color bar pattern being representative of fill-in, said reflectance of said second color bar pattern being representative of slur;
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- (o) compensating for variations in said reflectances, and thereby, said fill-in and slur in said reference, first, second and third process colors in converting said single values of average reflectance into said first, second and third registration error signals.