Abstract:
A color density measuring device ( 10 ) for determining the color density of an ink layer applied to a printing material ( 22 ), having a light source ( 24 ) for illuminating the printing material ( 22 ) and a sensor for receiving the light remitted from the printing material ( 22 ), is characterized according to the invention in that the sensor is fashioned as a multicolor image sensor ( 12 ), and a device ( 14, 24, 26; 30 ) is provided with the aid of which the light incident at the multicolor image sensor ( 12 ) is restricted to at least one predetermined wavelength band.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention.  
         [0002]     The invention relates to a color density measuring device for determining the color density of an ink layer applied to a printing material or subject, the color density measuring device having a light source for illuminating the printing material, and a sensor for receiving the light remitted from the printing material. The invention further relates to the use of such a color density measuring device on a printing apparatus for multicolor printing. Moreover, the invention also relates to a method for determining the color density of at least one of the three colors cyan, magenta or yellow, or black on a colored material.  
         [0003]     2. Description of the Related Art  
         [0004]     Particularly for the purpose of controlling the ink management in the case of printing units for multicolor printing, it is customary to determine the color density of an ink layer applied to the printing material. So-called reflected colored light densitometers are currently used for this purpose. This type of measurement is based on a linearity between a printed quantity of ink and optical density. The density measurement is performed using the known principle of remission measurement of body colors. An emitted measuring light beam penetrates the ink layer and is reflected at the interface between ink layer and printing material. The reflected light penetrates the ink layer once again and is received by a photoelement of the reflected colored light densitometer. Upon two-fold penetration of the ink layer, the originally white measuring light is colored and attenuated by absorption. The remission of the light is reduced in a typical wavelength region. An electric signal proportional to the light intensity is generated at the photoelement. This electric signal contains the information, resulting from the modulated reflected light, relating to type and quantity of the printed color that is used to determine the density value.  
         [0005]     Densitometers having so-called status filters or color filters are known which can be used for separately detecting the printing inks cyan, magenta, yellow and also black. In the case of these densitometers, individual photoelements for white light are arranged downstream of a total of four individual filters each having different transmission ranges. The photocells form individual measuring channels for each of the printing inks cyan, magenta and yellow. The density of these colored inks is determined in each case in the low remission range because variations in remission take place here that are prominent and can be effectively measured. The density of cyan is therefore measured with the aid of a red filter at a wavelength of approximately 600 nm. Similarly, magenta is measured with a green filter at approximately 530 nm, and yellow is measured with a blue filter at approximately 430 nm.  
         [0006]     The characteristic of the filters is laid down in the standards DIN 16 536-2 and ISO 5-3. Filters of status DIN E are used in Europe. With regard to the SWOP color standard, ISO T filters are customary in the USA and, to some extent, in England.  
         [0007]     This procedure is illustrated in  FIG. 1  using the example of remission curves of cyan for various color densities. The density is measured with the aid of a red filter (transmission range at a wavelength of approximately 600 nm) in the region of the largest variations.  
         [0008]     It is an object of the invention to specify a color density measuring device for determining the color density and the use thereof, as well as a method for determining the color density which together lead to a substantial reduction in cost by comparison with known measuring devices and methods.  
       SUMMARY OF THE INVENTION  
       [0009]     The invention provides for the sensor of a color density measuring device to be fashioned as a multicolor image sensor that is directed to the printing material. Furthermore, the invention provides a device with the aid of which the light incident at the multicolor image sensor is restricted to at least one predetermined wavelength band. In the case of the inventive method for determining the color density of at least one of the colors cyan, magenta or yellow on a colored material, this material is illuminated with light and a pictorial recording of the light remitted by the material is produced with the aid of a multicolor image sensor. The light incident at the multicolor image sensor is restricted to at least one predetermined wavelength band. According to the invention, the color density is determined by evaluating the pictorial recording produced with the aid of the multicolor image sensor by taking account of the determined light intensity at at least one of the pixels of the multicolor image sensor.  
         [0010]     The inventive multicolor image sensor is understood as a sensor that is sensitive to light and has a multiplicity of individual sensor points for these different colors, in particular for the colors red, green and blue. The individual sensor points in this case have a different relative spectral sensitivity. In particular, the sensitivity to blue light has a maximum at a wavelength of approximately 460 nm, the sensitivity to green light has a maximum at a wavelength of approximately 520 to 530 nm, and the sensitivity of the sensor points to red light has a maximum at approximately 620 to 640 nm. Such image sensors can be bought at particular cost advantage because, for example, they are currently already being applied in comparatively high numbers of items for CCD cameras. It is therefore particularly preferred in accordance with the invention to form the multicolor image sensor with a customary multicolor camera, in particular a red, green, blue camera.  
         [0011]     The invention renders it possible to operate in the case of a color density measurement with a particularly cost-effective multicolor image sensor that, on the basis of the multiplicity of pixels or sensor points provided on it, also delivers a multiplicity of individual measuring points for the individual color density measurement. According to the invention, the light respectively incident at the individual sensor points of the multicolor image sensor is restricted in this case to at least one predetermined wavelength band. This restriction is preferably performed by means of a filter device that is, in particular, inserted in the beam path of the emitted light upstream of the multicolor image sensor. Consequently, only a subregion of their wavelength spectra is led through the provided filter device to the individual pixels of the multicolor image sensor. Thus, for example, the individual red sensor point provided for red light receives only that subregion of the wavelength spectrum for red light which is required for a color density measurement in the case of a cyan remission curve.  
         [0012]     The remaining sensor points of the multicolor image sensor can be used, if appropriate, to measure the color balance and secondary densities.  
         [0013]     It is possible, in particular by selective provision of filter devices having various transmission ranges, to address the individual sensor points of the multicolor image sensor with corresponding subregions of the associated wavelength spectra in such a way that color density measurements of yellow and/or magenta are also possible in a way similar to the color density measurement of cyan that has been explained.  
         [0014]     Particularly advantageous to this end is a restriction in a preferred form of a filter device having at least one second transmission range with the aid of which the incident light is restricted to a subregion of the wavelength spectrum of a second color of the colors of the multicolor image sensor, in particular of the three colors red, green or blue.  
         [0015]     Furthermore, the restricting device also particularly advantageously has a third transmission range with the aid of which the incident light is restricted to a subregion of the wavelength spectrum of a third color of the multicolor image sensor, in particular of the three colors red, green or blue.  
         [0016]     It is possible with the aid of such devices and, in particular, filter devices to make simultaneous color density measurements of at least and, in particular, three different colors, in the present case the colored inks cyan, magenta and yellow, doing so simultaneously and in the course of a pictorial recording that is also relatively large. The individual sensor points for red, green and blue light are respectively allocated a subregion of the associated wavelength spectrum with the aid of the filter device. In contrast, the transmission ranges of the filter device that do not match the sensitivity range of the individual sensor points pass light from the wavelength regions to which these “non-matching” sensor points are not sensitive, or are scarcely so.  
         [0017]     The above-named subregions, in particular the transmission ranges of the filter device should be restricted with particular advantage to a region between approximately 420 nm and approximately 460 nm, in particular between 430 nm and approximately 450 nm. With the aid of such a transmission range, the sensitivity of a sensor point that is sensitive to blue light can be restricted in such a way that said sensor point acts as a density measuring device or densitometer for the colored ink yellow.  
         [0018]     A further advantageous subregion of the light restricted according to the invention is the region between approximately 510 nm and approximately 550 nm, in particular between approximately 520 nm and approximately 540 nm. With the aid of such a subregion, the sensitivity of a sensor point that is sensitive to green light can be restricted to a wavelength region such that said point acts as density measuring device for the colored ink magenta.  
         [0019]     Finally, a third advantageous subregion to be recommended is that between approximately 610 nm and approximately 650 nm, in particular between approximately 620 nm and approximately 640 nm. With the aid of this subregion, the sensitivity of a sensor point that is fundamentally sensitive to red light can be restricted to a wavelength region such that said sensor point acts, just as explained using the example in the introduction, as a density measuring device for the colored ink cyan.  
         [0020]     Alternatively or in addition to these subregions, it is preferred according to the invention to provide a device with the aid of which the light incident at the multicolor image sensor is restricted to a subregion of the wavelength spectrum of infrared light. This development is based on the finding that colored printing inks are transparent to infrared light, whereas the black printing inks based on carbon remit infrared light. Furthermore, the pixels of multicolor image sensors are also sensitive in the infrared region. Consequently, by restricting the incident light to infrared light as provided by the invention it is possible also to determine the density of the printing ink black independently of the colored inks virtually without additional outlay. It is particularly advantageous in this case that this measurement can even be made over a large range of image and/or subject because of the pictorial recording according to the invention.  
         [0021]     In order to be able to provide the above-named transmission ranges in a single filter device in a cost-effective fashion, the invention preferably provides for the use of a multiple bandpass filter for this purpose, in particular a so-called triple filter such as is customarily used for multifluorescence measurements. Such multiple bandpass filters can also be produced very cost-effectively.  
         [0022]     In addition to a restriction of the light incident on the multicolor image sensor by means of filtering, it is also possible alternatively or in addition already to restrict the wavelength of the light emanating from the light source to one or more predetermined wavelength bands. Such an illumination restricted to specific wavelength bands can be provided particularly advantageously by means of a number of light emitting diodes or lasers.  
         [0023]     As has been mentioned, color density measurements can be made in a pictorial recording with the aid of the color density measuring device according to the invention, the use thereof and the method according to the invention. During the recording, the individual sensor points or pixels of the multicolor image sensor used according to the invention can respectively serve to make individual measurements at various image regions within the pictorial recording. The interrelationships between these individual measurements can then be found, this being done according to the invention with the aid of appropriate evaluation software. Furthermore, it is possible according to the invention to carry out a measurement simultaneously for all three primary colors cyan, magenta and yellow. The time outlay required for the measurements is thereby substantially lowered. However, the time outlay is also reduced because a need otherwise arising to move the image sensor to a number of measuring ranges can be omitted owing to the comparatively large pictorial recording.  
         [0024]     Furthermore, the pictorial recording according to the invention enables a comparison to be undertaken between a colored surface and a white one. Imaging need be done in this case only at an image region where such a colored image section and also a white image section are present. It is then also possible, for example, to assess the quality of the printing paper itself with the aid of the measurement at the white image section. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]     Exemplary embodiments of an inventive color density measuring device are explained below in more detail with the aid of the attached schematics, in which:  
         [0026]      FIG. 1  shows a graph of remission curves of cyan for various color densities, and the associated density measurement with the aid of a red filter.  
         [0027]      FIG. 2  shows a graph of the spectral remission curves of the colored inks cyan, magenta and yellow and of the unprinted white paper.  
         [0028]      FIG. 3  shows a graph of the typical standardized sensitivity of a multicolor camera, a red, green blue camera in the present case.  
         [0029]      FIG. 4  shows a graph of the transmission of an inventive triple bandpass filter.  
         [0030]      FIG. 5  shows a greatly simplified perspective view of a first exemplary embodiment of an inventive color density measuring device.  
         [0031]      FIG. 6  shows a graph of the standardized sensitivity of the multicolor camera used according to the invention, together with the triple bandpass filter according to the invention.  
         [0032]      FIG. 7  shows a greatly simplified side view of a second exemplary embodiment of an inventive color density measuring device.  
         [0033]      FIG. 8  shows a greatly simplified side view of a third exemplary embodiment of an inventive color density measuring device.  
         [0034]      FIG. 9  shows a greatly simplified side view of a fourth exemplary embodiment of an inventive color density measuring device.  
         [0035]      FIG. 10  shows a greatly simplified side view of a fifth exemplary embodiment of an inventive color density measuring device.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0036]     The spectral remission curves of the printing inks cyan (C), magenta (M) and yellow (Y) and of an unprinted white paper (PW) are illustrated graphically in  FIG. 2 . The curves show for white paper a virtually uniformly high remission of above approximately 0.8 in the entire wavelength region above approximately 430 nm. By contrast therewith, the spectral remission curves of the colored inks cyan, magenta or yellow exhibit rising, and also falling, sections over the illustrated wavelength region. In order respectively to provide density measurements with a high information content in the case of the remission of printing inks that is illustrated in such a way, as already mentioned at the beginning the density measurements are respectively undertaken in the low remission range of the colored inks. The measurements are therefore made specifically in comparatively restricted wavelength regions of the spectral remission curves. As illustrated in  FIG. 1 , in the case of customary density measurements, an individual red filter is inserted upstream of a photoelement sensitive to white light and in each case filters out from the remission curves of cyan only a wavelength section around approximately 600 nm for the photoelement. Depending on color density, correspondingly strong deviations result in light intensity for the photoelement.  
         [0037]     According to the invention, by contrast, the use of a customary multicolor camera, in particular a red, green, blue camera is proposed which comprises on a flat image sensor a multiplicity of sensor points for different colors, in this case a multiplicity of blue, green and red sensor points or pixels. A diagram in the right-hand area of  FIG. 3  shows the two-dimensional arrangement of such blue (B), green (G) and red (R) pixels or sensor points on such a color camera. Furthermore, the left-hand area of  FIG. 3  illustrates the typical sensitivity of the individual pixels with the aid of their standardized sensitivity.  
         [0038]     It is clearly to be seen from  FIG. 3  that the blue pixels have a sensitivity maximum at approximately 460 nm, the green pixels have a sensitivity maximum at approximately 520 nm to 540 nm, and the red pixels have a sensitivity maximum at approximately 620 to 630 nm. The pixels of such type are preferably produced from silicon and therefore act as individual photosensitive elements whose sensor signal can correspondingly also be evaluated individually.  
         [0039]     Inserted upstream of the multicolor camera of such sensitivity is a filter device that is fashioned as a multiple bandpass filter, in particular as a triple bandpass filter or a so-called triple filter. Such a filter device has three respectively restricted transmission ranges. With as high a relative transmission as possible (ideally approximately 1), a first transmission range is restricted to a wavelength region between approximately 430 nm and approximately 450 nm. With as high a relative transmission as possible (ideally approximately 1), a second transmission range is restricted to the wavelength section between approximately 520 nm and approximately 540 nm. Finally, with as high a relative transmission as possible (ideally approximately 1), the third transmission range comprises only the wavelength region between approximately 620 nm and approximately 640 nm.  
         [0040]     The color density measuring device  10  fashioned in such a way is illustrated in  FIG. 5  once again with its camera  12  acting as multicolor image sensor, as well as the upstream triple filter  14  acting as filter device.  
         [0041]     During operation, the color density measuring device  10  is directed, for example inside a further printing apparatus (not illustrated) for multicolor printing, onto a printing material  22  that is provided with an ink layer and is, therefore, colored.  
         [0042]     The color density measuring device  10  is, moreover, coupled operationally to a control and evaluation circuit  18  via lines  16 . Furthermore, the color density measuring device  10  is supported such that it can be displaced on a cross member  20  by a motor.  
         [0043]     The multicolor camera  12  arranged in such a way is then used not, for example, to take customary color photographs, but color density measurements are carried out that have at least the same measurement quality as do measurements with the aid of customary densitometers.  
         [0044]     Such color density measurements are possible because a particular standardized sensitivity of the individual pixels of the multicolor camera  12  results (see left-hand area of  FIG. 6 ) over the entire wavelength region owing to the inventive combination of the triple bandpass filter  14  (see  FIG. 4 ) with a multicolor camera  12  (see  FIG. 3 ). A high standardized sensitivity thus remains for the blue pixel only in the wavelength region between approximately 430 nm and approximately 450 nm. For the green pixel, a high standardized sensitivity is restricted to the wavelength region between approximately 520 nm and 540 nm. The red pixel is particularly sensitive only in the wavelength region between approximately 620 nm and approximately 640 nm.  
         [0045]     If this sensitivity resulting in the case of the multicolor camera  12  provided with the triple bandpass filter  14  (see left-hand area of  FIG. 6 ) is covered with spectral remission curves of the colored inks cyan, magenta and yellow as well as of the unprinted white paper (see  FIG. 2 ), it is to be seen that a densitometer for cyan (C) has been provided with the aid of the filtered “red” pixel (cf. also  FIG. 1 ), a densitometer has been provided for magenta (M) with the aid of the “green” pixel, and a densitometer has been provided for yellow (Y) with the aid of the “blue” pixel. This functionality of the multicolor camera  12  used in accordance with the invention is illustrated once more in the right-hand area of  FIG. 6  with the sensor points correspondingly designated (see, in particular, by comparison with the right-hand area of  FIG. 3 ).  
         [0046]     It is therefore possible to use the color density measuring device  10  to undertake a total of three density measurements in only a single recording and, furthermore, to carry out a comparison with an unprinted white paper surface, if appropriate.  
         [0047]     A second exemplary embodiment of a color density measuring device  10  is illustrated in  FIG. 7 ; it is fashioned like the example illustrated in  FIG. 5  with regard to the multicolor camera  12 , the triple bandpass filter  14  and the printing material  22 . In the exemplary embodiment in accordance with  FIG. 7 , however, the multicolor camera  12  cannot be moved, but is fastened on a stand (not illustrated) in a stationary fashion. A light source  24  in the form of a luminaire emitting light over the entire visible wavelength spectrum is, moreover, provided on the color density measuring device  10  in accordance with  FIG. 7 . This luminaire is arranged at a principal irradiation angle of approximately 45° to the plane of the printing material  22 . In the case of the exemplary embodiment in accordance with  FIG. 7 , the triple bandpass filter  14  is likewise inserted into the beam path of the light remitted by the printing material  22  directly upstream of a lens of the multicolor camera  12 .  
         [0048]      FIG. 8  shows an exemplary embodiment of a color density measuring device  10  in which the triple bandpass filter  14  is inserted not upstream of the multicolor camera  12 , but is inserted, directly downstream of the light source  24 , into the beam path of the light emitted by the light source  24 . Such an arrangement of the triple bandpass filter  14  already restricts the light directed onto the printing material  22  to the desired wavelength bands named above.  
         [0049]      FIG. 9  illustrates an exemplary embodiment in which instead of a single light source  24  and a triple bandpass filter  14  a total of three light sources  24  are provided upstream of which a single bandpass filter  26  is inserted in each case. Furthermore, the illumination means of such a type are assigned a light mixing system  28  in the form of color interference filters, said light mixing system being used to unite the beam paths of the three light sources  24  to form one beam path.  
         [0050]     Finally,  FIG. 10  illustrates an exemplary embodiment in which a total of four light sources  30  are provided. These light sources  30  are arranged next to one another and directed individually in each case onto the printing material  22 , a principal irradiation angle of approximately 45° to the plane of the printing material again being observed in each case (by contrast with the illustration, which is purely schematic). Of these four light sources  30 , three are fashioned as light emitting diodes or laser luminaries with specific wavelength spectra, restricted in accordance with the invention, in the region of blue, green and red light. The fourth light source  30  is an infrared luminaire and its wavelength band is likewise restricted. As explained above, this light source  30  from the infrared region can then be used in cooperation with the multicolor camera  12  to measure the density of the printing ink black.