Densitometer method and system for identifying and analyzing printed targets

A hand-held battery operated densitometer for identifying and analyzing printed targets is provided which automatically determines whether an unprinted substrate, a solid black, a muddy magenta solid, an overprint, a solid color, or a halftone has been detected. Referenced density values measured through red, green, blue and visual optical filters for an overprint and the first and second down colors are used to determine and display percent trap. Referenced density values for halftone and corresponding solid targets are used to determine and display percent dot area. Referenced density values for solid targets may also be displayed. The printing process is adjusted based upon the displayed solid densities, percent trap and percent dot area.

TECHNICAL FIELD 
The present invention relates to a densitometer method and system and, more 
particularly, to a densitometer useful for printing process control. 
BACKGROUND AND OBJECTS OF THE INVENTION 
An optical densitometer is a device used to determine the optical density 
of printed ink films. A densitometer normally contains four optical 
filters each matched to one of the four standard printing ink colors, 
cyan, magenta, yellow and black. 
The densitometer is an indispensable tool to the pressmen who operate four 
color lithographic presses. It provides accurate, repeatable and objective 
readings of optical density of solid patches of ink, halftone patches and 
overprinted patches. Density readings taken on these patches are used to 
adjust inking levels on the press and to adjust other printing parameters 
such as dampening solution levels and roller nip pressures. 
The densitometer user normally performs a referencing procedure before 
measuring the density of printed ink. The optical density of a patch of 
the unprinted substrate is read through each of the four filters. These 
densities are designated as reference densities by the user and stored as 
such in the densitometer's memory. If the readings are made but not 
designated as reference readings, they will not be stored as such and the 
user will subsequently read densities erroneously believed to be 
referenced to the unprinted substrate. Actual density readings require the 
user to first identify the color of the target patch and decide whether 
the target is a solid, halftone or overprinted patch. The user must also 
correctly match the target patch color to the filter chosen to read the 
optical density. An error at any point in this procedure will, of course, 
yield a useless density reading since it is misidentified. By way of 
example, it is possible to confuse cyan and blue or red and magenta. 
While taking readings during a press run, the pressman must concentrate on 
performing the correct sequence of events. He is necessarily distracted 
from the more important business of monitoring the behavior of the press. 
Also, if the densitometer is not easily portable, the pressman's movement 
is restricted by the length of the densitometer's line cord. A 
densitometer that cannot be held in one hand when operated is less 
convenient to use than a hand-held version of the same device. 
It is desirable to eliminate user error to the extent possible, since user 
error results in wasted printing material and lost time. Thus, it would be 
highly desirable to provide a densitometer capable of automatically 
performing decision-making tasks. Such tasks include (i) the ability to 
recognize unprinted substrate and read and store all four filter position 
densities as reference densities, (ii) the ability to identify the color 
of the target being read, match the color to the appropriate filter and 
display the appropriate density, (iii) the ability to distinguish among a 
group of solid patches, (iv) the ability to distinguish between solids and 
halftones, solids and overprints and halftones and overprints, (v) the 
ability to determine and display percent trap of an overprint and (vi) the 
ability to determine and display percent dot area of a halftone. 
A densitometer capable of performing the above mentioned decision-making 
tasks in an unaided manner would advantageously reduce user error and free 
the pressman to concentrate on operating the press rather than the 
densitometer. A battery operated, hand-held densitometer which is capable 
of performing the above tasks would be even more desirable since it could 
be taken anywhere in the pressroom area to read optical density. 
Gretag introduced a hand-held battery operated densitometer, the Gretag 
D-1, in 1966. This densitometer does not read all four filter positions 
automatically for each sample and does not perform the decision-making 
tasks described above. 
Macbeth.sup..RTM. introduced the TDA-1000 densitometer in 1970. This 
device provided four sequential readings of each sample. It was not a 
portable device, however, and did not perform decision making tasks. In 
1977, Macbeth.sup..RTM. introduced the On-Press Color Monitor, an on-line 
densitometer which took four simultaneous readings of optical density on 
four successively positioned color patches. The color and relative 
position of each patch was preprogrammed into the memory of the on-line 
operating system. The densitometer consisted of four distinct sets of 
light sources and filtered detectors. This device was not portable and did 
not perform the type of decision-making tasks described above. In 1981, 
Macbeth.sup..RTM. introduced a version of the On-Press Color Monitor 
using a single light source and a fiber optic probe which was a single 
bundle at the point where light reflected from the sample was collected 
and which was then separated into four bundles (quadrifurcated). Each of 
these bundles passed a portion of the reflected light through a blue, 
green, red or visual filter, respectively, to one of the four detectors. 
This device was not portable and could not perform the types of 
decision-making tasks described above. 
U.S. Pat. No. 4,239,393 issued to Tobias in 1980 describes a densitometer 
containing a rotating filter wheel having red, green and blue filters 
which make it possible to read three densities automatically for each 
sample. This device is not portable, does not provide a separate filter 
matched to black ink, and is not capable of performing decision-making 
tasks. 
In 1986, X-Rite introduced the X-Rite 408 Color Reflection Densitometer. 
This device is battery operated and hand-held. It uses a rotating filter 
wheel to read all four filter positions for each sample. It displays the 
largest optical density reading of the four filters. This device does not 
distinguish between solids and halftones, solids and overprints or 
overprints and solids. Nor does it recognize an unprinted substrate as a 
reference without operator instruction. 
Therefore, it is an object of the present invention to provide a portable 
hand-held densitometer. 
Another object of the present invention is to provide a densitometer which 
automatically recognizes unprinted substrate, overprints, solids and 
halftones. 
Another object of the present invention is to provide a densitometer which 
automatically selects and displays appropriate density measurements. 
A further object of the present invention is to provide a densitometer 
which automatically selects the appropriate density measurements and 
calculates and displays the percent trap of an overprint. 
Yet a further object of the present invention is to provide a densitometer 
which automatically selects the appropriate density measurements and 
calculates and displays percent dot area. 
These and other highly desirable and remarkable results are accomplished by 
the present invention in a portable, hand-held densitometer which 
automatically determines type of the target being measured, stores 
pertinent measured parameters therefrom and, where appropriate, determines 
and displays vital information parameters such as percent trap and percent 
dot area for use by the pressman in adjusting and controlling the printing 
press. 
Objects and advantages of the invention are set forth in part herein and in 
part will be obvious herefrom, or may be learned by practice with the 
invention, the same being realized and attained by means of the 
instrumentalities and combinations pointed out in the appended claims. 
The invention consists in the novel parts, constructions, arrangements, 
combinations, steps and improvements herein shown and described. 
SUMMARY OF THE INVENTION 
In accordance with the present invention a densitometer method and system 
are provided which automatically recognize the type of target measured, 
updates appropriate memory registers with information from the particular 
target measured and, where appropriate, determines percent trap in an 
overprint and percent dot area in a halftone. Appropriate density, color, 
percent trap and percent dot area information is displayed to the 
pressman, for use in controlling the printing process. 
The densitometer according to the present invention includes a convenient 
measuring head which illuminates a sample target with annular illumination 
and collects the reflected light in a fiber optic bundle. The reflected 
light is transmitted to the densitometer body through the fiber optic 
bundle, where the bundle is divided into four separate bundles, i.e., 
quadrifurcated, each bundle being associated with a dedicated optical 
filter and detector. The filters used are red, green, blue and visual 
optical filters. The filtered light is detected by the dedicated detectors 
and the analog signals from all four detectors are multiplexed, converted 
to digital signals by an analog to digital converter, and sent to a 
microprocessor for analysis. Analyzed data from the microprocessor is 
displayed to the pressman for use in controlling the printing process. 
The digital data is analyzed by the microprocessor in order to determine 
whether an unprinted substrate, a solid black, a muddy magenta solid, an 
overprint, a solid color or a halftone has been detected. 
Unprinted substrate is identified by comparing referenced optical 
densities, i.e. measured optical densities minus corresponding stored 
substrate optical densities, if any, to a first stored constant K1 and, 
where available, to stored substrate optical density values. Where all 
referenced optical densities values are less than K1 or the stored 
substrate density values it is assumed that an unprinted substrate has 
been measured, the substrate density values in memory are updated with 
unreferenced measured optical densities, and the fact that an unprinted 
substrate has been referenced is displayed to the pressman. The pressman 
then moves the measuring head to a different target. 
Where an unprinted substrate is not detected, the referenced densities 
D.sub.r, D.sub.g, D.sub.b are compared to a second stored constant K2. If 
all values of D.sub.r, D.sub.g, D.sub.b are greater than or equal to K2 it 
is assumed that a solid black target has been detected, the referenced 
visual density reading D.sub.v is stored in memory as the corresponding 
solid density S.sub.v, and the fact that a solid black patch has been read 
is displayed to the pressman, who then moves the measuring head to a 
different target. 
Where neither an unprinted substrate nor a solid black is detected, a 
determination is made whether an overprint has been detected. In order to 
make this decision the densitometer determines whether any two of D.sub.r, 
D.sub.g, D.sub.r are greater than or equal to a third stored constant K3 
and whether the difference between the largest and smallest referenced 
density values is greater than or equal to a fourth stored constant K4. If 
any two of the densities are greater than or equal to K3 and the 
difference is greater than or equal to K4, then a determination must be 
made whether an overprint or a "muddy magenta" solid has been detected. 
This determination is made by comparing D.sub.g to D.sub.r, D.sub.b to 
D.sub.r and/or D.sub.b to a fifth stored constant K5. If a muddy magenta 
has been detected the green density D.sub.g is stored as the green solid 
density S.sub.g and the density of the muddy magenta patch is displayed to 
the pressman, who then moves the measuring head to a different target. If, 
however, an overprint is detected, the densitometer prompts the pressman 
to provide readings from a sample of the first and second down colors and 
then calculates and displays percent trap to the pressman, who thereafter 
moves the measuring head to a different target. Percent trap is a measure 
of the ability of the first down color to serve as a substrate for the 
second down color. 
Where none of an unprinted substrate, a solid black, a muddy magenta or an 
overprint are detected, the densitometer determines whether a solid color 
patch of cyan, magenta or yellow has been read by determining whether any 
of D.sub.r, D.sub.g D.sub.b are greater than or equal to sixth stored 
constant K6. If so, the densitometer assumes it has detected a solid patch 
corresponding to the filter which provided the largest referenced density 
measurement, stores the largest measured density as the corresponding 
solid density, and displays the solid density and, optionally, color to 
the pressman. The pressman then moves the measuring head to a different 
target. 
Where no unprinted substrate, solid black, muddy magenta solid, overprint 
or color solid is detected, the densitometer assumes it has detected a 
halftone target. The densitometer then determines whether a black halftone 
has been detected by comparing the difference between the largest and 
smallest referenced density readings to a seventh stored constant K7. If a 
black halftone is indicated the densitometer memory is check for a stored 
value of the corresponding black solid density, S.sub.v. If the stored 
value of S.sub.v is greater than the second stored constant K2 the visual 
density reading D.sub.v is stored as D.sub.h and the percent dot area is 
calculated using D.sub.h and S in the Yule-Nielsen equation. The percent 
dot area is displayed to the pressman, who thereafter moves the measuring 
head to a different target. If the halftone is not black, however, the 
largest measured density is stored as D.sub.h and the densitometer checks 
to determine whether the corresponding solid density S for that color has 
been stored. If the corresponding stored S value is greater than or equal 
to the sixth stored constant K6 percent dot area for the halftone is 
determined using D.sub.h and the corresponding solid color density S in 
the Yule-Nielsen equation. The percent dot area and, optionally, color are 
displayed to the pressman and another target may then be measured. 
However, where the black solid density S.sub.v is less than K2, where the 
appropriate color solid density is less than K6 or where the appropriate S 
value has not been measured and stored the densitometer displays an error 
code indicating to the pressman that no usable solid target has been 
measured and, consequently, percent dot area cannot be calculated. 
Based upon the various information displays the pressman adjusts the 
printing process to maintain print quality. 
It will be readily appreciated that the densitometer according to the 
present invention can be provided in a hand-held, battery-operated 
portable unit which may be conveniently carried to various points in the 
pressroom. It will also be appreciated that the densitometer system and 
method according to the present invention, by automatically distinguishing 
among various solid and halftone and color and black targets, 
substantially reduces operator error resulting from misidentified targets 
and use of improper values for calculations of percent trap and percent 
dot area. 
Advantageously, all of the intelligent determinations made by the 
densitometer using the system and method of the present invention free the 
pressman from having to make similar determinations based upon his 
experienced observations. Thus, the present densitometer system and method 
remarkably enable the pressman to devote his attention primarily to 
operating the press and adjusting the printing process rather than to 
using his valuable experience and knowledge to recognize print targets and 
ensure that properly referenced measurements from appropriate targets are 
used to determine percent trap and percent dot area. 
It will be understood that the foregoing general description and the 
following detailed description as well are exemplary and explanatory of 
the invention but are not restrictive thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring more particularly to FIG. 1, there is shown a densitometer system 
10 in accordance with the present invention. As shown, a sample 12 is 
annularly illuminated by light rays 14 and reflected light rays 16 are 
collected by one end of a fiber optic bundle 18. Preferably, the sample is 
illuminated at a 45.degree. angle and the reflected light rays are 
collected at 0.degree. to the sample normal as shown. The other end of the 
fiber optic bundle is quadrifurcated so that the reflected light is 
divided among four separate bundles 18a, 18b, 18c, 18d and directed 
through red, green, blue and visual optical filters 20a, 20b, 20c, 20d, 
respectively, to a set of detectors 22. Analog signals from each of the 
detectors are sent to a multiplexer 24 which sequentially passes the 
analog signals to an analog to digital signal converter 26. The digital 
signals are then sent to a microprocessor 28 which is programmed to 
analyze to digital signals so as to determine what has been detected and 
what pertinent information should be displayed on display 30 to the 
pressman so that appropriate process control adjustments may be made based 
upon the displayed information. Advantageously, the entire densitometer 
system may be provided in a hand-held, battery-operated unit with a 
convenient measuring probe head attached to the densitometer system via 
the fiber optic bundle. 
The logic system utilized in the present invention makes use of the fact 
that the lithographic printing process is a subtractive color system. That 
is, the printing inks commonly used are chosen for their ability to 
selectively absorb rather than reflect particular colors. Thus, the three 
colored inks cyan, magenta and yellow absorb red, green and blue light, 
respectively. Consequently, a red optical filter passes a relatively small 
fraction of light reflected from a cyan ink patch, a green filter passes a 
relatively small fraction of light reflected from a magenta ink patch, and 
a blue filter passes a relatively small fraction of light reflected from a 
yellow ink patch. In order for the subtractive color system as a whole to 
be effective, however, it is necessary that the printing inks be 
"transparent", that is, able to pass, unscattered, the light wavelengths 
not absorbed. It is therefore clear that an overprint of cyan and magenta 
inks reflects essentially only blue light. Likewise, an overprint of cyan 
and yellow inks reflects essentially only green light, and an overprint of 
magenta and yellow inks reflects only red light. Black ink is also used in 
lithographic printing to add contrast and darken printed images. Black ink 
is much more neutral than cyan, magenta or yellow ink in that it absorbs 
light more uniformly across the visible spectrum. For detection purposes, 
an optical filter for black ink is one which mimics the sensitivity of the 
normal human eye across the spectrum of visible light. Such a filter is 
simply called a "visual" filter. In printing, the absorbance of each ink 
and, hence, the final image created by the combined absorbance of all 
inks, is primarily controlled by varying the fraction of the image area on 
a printing plate that is receptive to ink rather than by varying the 
thickness of the ink film. This ink receptivity is controlled by using ink 
sensitive dots of various sizes on the printing plate. All partially 
sensitized image areas of a printing plate are called "halftones" although 
the sensitized fractions do not always comprise half the total plate area. 
It has been found that cyan, magenta and yellow ink films are 
inter-distinguishable since each film provides a relatively high 
absorbance when read through a particular filter. Black ink is 
distinguishable from the color inks due to its neutral optical behavior. 
In addition, solid ink films of a single ink are distinguishable from 
halftones of the same color by the relatively low overall density of the 
halftones. Overprints are distinguishable from single layer ink films by 
the fact that overprints provide two relatively high absorbance readings 
instead of one, as with single ink films. Finally, an unprinted substrate 
is distinguished by the very low density read through all filters. 
The following definitions will be useful in understanding the logic system 
of the densitometer according to the invention. 
O.D.=optical density 
r=red filter reading 
g=green filter reading 
b=blue filter reading 
v=visual filter reading 
W=unprinted substrate or "white" O.D., i.e. W.sub.r is the unprinted 
substrate O.D. read through a red filter. 
R=unreferenced target O.D. 
D=referenced O.D. reading of a given target, i.e. R - W. 
S=referenced solid O.D., i.e. R - W for a solid reading. 
T=referenced trap or overprint O.D., i.e. R - W for an overprint. 
T.sub.1 =referenced solid O.D. corresponding to the first-down ink in a 
given overprint, i.e. R - W for a given solid. 
T.sub.2 =referenced solid O.D. corresponding to the second-down ink in a 
given overprint, i.e. R - W for a given solid. 
K1=upper O.D. limit for D of unprinted substrate (typically 0.1 O.D.). 
K2=lower O.D. limit for D.sub.b, D.sub.g, D.sub.r, D.sub.v of a black solid 
(typically 1.0 O.D.). 
K3=lower O.D. limit for D of each of the largest two O.D. readings of an 
overprint (typically 0.8 O.D.). 
K4=lower limit of O.D. difference between D's of largest O.D. filter 
reading and smallest O.D. filter reading of black halftone (typically 0.3 
O.D.). 
K5=lower O.D. limit for D.sub.b of a red overprint (typically 1.0 0.D). 
K6=lower O.D. limit for D of solid patches read through the filter of 
largest O.D. (typically 1.0 O.D. for cyan, magenta and black inks and 0.8 
O.D. for yellow ink). 
K7=upper limit of O.D. difference between D's of largest O.D. filter 
reading and smallest O.D. filter reading of black halftone (typically 0.05 
O.D.). 
Using the above definitions, the logic system and method according to the 
present invention can be described as follows with reference to the flow 
chart shown in FIGS. 2A, 2B and 2C. 
The densitometer reading head is engaged and the densitometer determines 
whether there are W values in memory for the unprinted substrate optical 
densities. See FIG. 2A, steps 100, 102, respectively. 
If there are no W values in memory all W values are set equal to 0 (FIG. 
2A, 104) and the unreferenced optical densities R.sub.r, R.sub.g, R.sub.b, 
R.sub.v of the unprinted substrate are stored as W.sub.r, W.sub.g, 
W.sub.b, W.sub.v, respectively. FIG. 2A, 106. The measured density 
readings are then converted to referenced density values by subtracting 
the newly or previously stored W values from the unreferenced density 
readings. For example, the referenced red density D.sub.r =R.sub.r 
-W.sub.r. FIG. 2A, 108, 110. 
If D.sub.r, D.sub.g, D.sub.b are all less than stored constant K1 (FIG. 2A, 
112) or if all D values are less than corresponding stored W values (FIG. 
2A, 120) then it is assumed that unprinted substrate has been read and the 
W values in memory are updated with unreferenced optical density R values. 
That is, each W memory is set as follows: W.sub.r =R.sub.r, W.sub.g 
=R.sub.g, W.sub.b =R.sub.b, W.sub.v =R.sub.v (FIG. 2A, 114, 116). At this 
point, "P.0.00" is displayed to the pressman to indicate that unprinted 
substrate, usually paper, has been read and that a reference reading has 
been taken. (FIG. 2A, 118). At this point the pressman moves the measuring 
head to another target. If, however, D.sub.r, D.sub.g, D.sub.b are not all 
less than stored constant K1, (FIG. 2A, 112) and all of D.sub.r, D.sub.g, 
D.sub.b, D.sub.v are not less than the corresponding W values (FIG. 2A, 
120), then it is assumed that unprinted substrate has not been detected 
and the next step of analysis is executed. 
Where unprinted substrate is not detected, the densitometer next determines 
whether D.sub.r, D.sub.g, D.sub.b are all greater than or equal to stored 
constant K2 (FIG. 2A, 122). If so, it is assumed that a solid black target 
has been read and D.sub.v is stored as S.sub.v (FIG. 2A, 124, 126) for 
later use in the calculation of dot area when a black halftone target is 
read. D.sub.v and, optionally, the black color of the target are displayed 
to the pressman (FIG. 2A, 128), who then moves the portable measuring head 
to a different target location. If, however, D.sub.r, D.sub.g, D.sub.b are 
not all greater than or equal to K2, then it is assumed that the target 
measured is not a solid black ink film and the next step of analysis is 
performed. 
Where neither an unprinted substrate nor a solid black target is detected, 
a check for an overprint is made. See FIG. 2B. If any two of D.sub.r, 
D.sub.g, D.sub.b are greater than or equal to stored constant K3 and if 
the difference between the largest and smallest densities (shown in FIG. 
2B as D.sub.L -D.sub.s) is greater than or equal to stored constant K4 
(FIG. 2B, 130), then the following overprint check is performed. If 
D.sub.g is less than D.sub.r (FIG. 2B, 132) then a green overprint is 
assumed. If, however, D.sub.g is greater than or equal to D.sub.r then a 
check is made of D.sub.b against D.sub.r. If D.sub.b is less than D.sub.r 
then a blue overprint is assumed (FIG. 2B, 134), whereas if D.sub.b is 
greater than or equal to D.sub.r, then D.sub.b is checked against stored 
constant K5 (FIG. 2B, 136). If D.sub.b is less than K5 then it is assumed 
that a "muddy magenta" solid has been read rather than an overprint. A 
muddy magenta is a magenta wherein D.sub.b and D.sub.r are relatively high 
with respect to D.sub.g. If a muddy magenta solid has been detected, 
D.sub.g is stored as S.sub.g (FIG. 2B, 138, 140) for later use in the 
calculation of dot area. D.sub.g and, optionally, the color of the target 
are displayed to the pressman (FIG. 2B, 142), who then moves the 
densitometer measuring probe head to measure a different target. If, on 
the other hand, D.sub.b is greater than or equal to K5 (FIG. 2B, 136) it 
is assumed that a red overprint has been detected. 
Where it is assumed that an overprint has been read, D.sub.r, D.sub.g, 
D.sub.b are stored as overprint density values T.sub.r, T.sub.g, T.sub.b 
(FIG. 2B, 144, 146), respectively. The densitometer then prompts the user, 
by displaying a predetermined code or other indicator, to provide a 
reading of a solid patch of the first-down color of the overprint (FIG. 
2B, 148). The measured values R.sub.r, R.sub.g, R.sub.b for the first down 
color are converted to referenced density values by subtracting the 
corresponding unprinted substrate densities and are respectively stored as 
T.sub.1r, T.sub.1g, T.sub.1b (See FIG. 2B, 150, 152, 154). The 
densitometer then prompts the user for a reading of a solid patch of the 
second-down overprint color (FIG. 2B, 156). The unreferenced second-down 
color density readings are converted to referenced densities T.sub.2r, 
T.sub.2g, T.sub.2b (FIG. 2B, 158, 160), respectively. The largest density 
reading of the second-down color selected from among T.sub.2r, T.sub.2g, 
T.sub.2b is stored as T.sub.2 (FIG. 2B, 162). The filter color designated 
as T.sub.2 is then used to designate the corresponding T and T.sub.1 (FIG. 
2B, 164, 166). For example, if the largest second-down color reading is 
T.sub.2r, T.sub.2r is designated T.sub.2 and T=T.sub.r and T.sub.1 
=T.sub.1r. The densitometer system then calculates percent trap, an 
expression of the ability of the first-down ink in an overprint to serve 
as a printing substrate for the second-down color, using equation (1). 
##EQU1## 
Ideally, of course, T=T.sub.1 +T.sub.2 and the percent trap is 100%. The 
calculated percent trap is displayed to the pressman for use in 
controlling the printing process (See FIG. 2B, 168, 170), whereafter the 
pressman moves the measuring probe to another target. It is also 
contemplated that the color of the overprint, i.e. red, green or blue, 
could be displayed to the pressman as well. 
Where unprinted substrate, a black solid, a muddy magenta solid or an 
overprint is not detected, then each of D.sub.r, D.sub.g, D.sub.b is 
checked against stored constant K6 (FIG. 2C, 172). If any one of D.sub.r, 
D.sub.g, D.sub.b is greater than or equal to K6, then it is assumed that a 
color solid target has been read. The color of the target is assumed to be 
the color associated with the filter reading that yields the largest 
density value. Therefore, the largest density value among D.sub.r, 
D.sub.g, D.sub.b is assigned to a corresponding memory value S.sub.r, 
S.sub.g, S.sub.b and stored for later use in calculating dot area (FIG. 
2C, 174, 176). The density value and, optionally, the color of the target 
are displayed to the pressman (FIG. 2C, 178), who then moves the 
densitometer probe head to measure a different target. 
Where none of an unprinted substrate, a solid black, a muddy magenta, or a 
color solid has been detected, it is assumed that a halftone target has 
been measured and the densitometer checks for stored solid density values 
S.sub.r, S.sub.g, S.sub.b, S.sub.v (FIG. 2, 180). Where no S values exist, 
S.sub.r, S.sub.g, S.sub.b and S.sub.v are set equal to 0 (FIG. 2C, 182, 
184). 
The densitometer then determines whether the difference between the largest 
and smallest measured densities (shown in FIG. 2C as D.sub.L - D.sub.s) is 
less than or equal to stored constant K7 (FIG. 2C, 186). If so, the 
measured target is assumed to be a black halftone and the stored value of 
S.sub.v is checked to determine whether percent dot area can be calculated 
(FIG. 2C, 188). If S.sub.v is less than K2, no calculation of percent dot 
area can be made and an error signal is displayed to the pressman (FIG. 
2C, 190) indicating that no usable solid black target has been measured 
for use in calculating dot area. However, if S.sub.v is greater than or 
equal to K2, then D.sub.h is set equal to referenced density D.sub.v (FIG. 
2C, 192) and the dot area is calculated (FIG. 2, 194) by use of the 
Yule-Nielsen equation (2). 
##EQU2## 
where: 
D.sub.h =the largest measured density value D for the halftone 
S=the corresponding solid density 
n=Yule-Nielsen correction factor for paper scattering 
The calculated percent dot area is displayed to the pressman (FIG. 2C, 
196). In this case, where the halftone is assumed to be black D.sub.h 
=D.sub.v and S=S.sub.v, both read through the visual filter. The pressman 
uses the percent dot area data to control the printing process and moves 
the densitometer head to a different target location. 
Where the difference between the largest and smallest density values is 
greater than stored constant K7 (FIG. 2C, 186), then it is assumed that 
the target measured is a cyan, magenta or yellow halftone patch. It is 
assumed that the color of the ink patch corresponds to the largest 
referenced density value measured through one of the red, green or blue 
optical filters, so the largest referenced density is stored as D.sub.h 
(FIG. 2C, 198). If the solid density S corresponding to the largest 
measured density, i.e. D.sub.h, is less than stored constant K6 (FIG. 2C, 
200), then no usable solid patch has been read and an error message is 
displayed to the pressman (FIG. 2C, 202) indicating that no usable 
corresponding solid patch has been read. If, on the other hand, the solid 
density S corresponding to the largest measured density is greater than or 
equal to K6, then percent dot area for the color halftone patch is 
calculated using equation (2), above, and displayed to the pressman (See 
FIG. 2C, 204, 206). Of course, it is contemplated that halftone color 
could also be displayed. 
All of the stored constant values referred to herein are set based upon 
experience with the properties of the particular type of target involved. 
The typical values set forth above in defining each constant are based 
upon the inventors' experience with the invention and are intended to 
provide working knowledge as to how the constant may be set in order to 
make use of the invention. Of course it is contemplated that variation of 
the constants may be desirable depending upon the particular paper and the 
types and combinations of inks used in a given process. 
Although the foregoing discussion has been directed to use of referenced 
density measurements, it is contemplated that unreferenced density 
measurements could be used, albeit less satisfactorily, with the present 
system. In addition, the foregoing discussion is merely intended to set 
forth the preferred logic system for the step by step analysis of measured 
data to determine what type of target has been measured and, where 
appropriate, percent trap or percent dot area and to display density, 
percent trap or percent dot area to the pressman for use in controlling 
the printing process. It is contemplated, of course, that it may be 
possible to develop variations of the particular logic sequence herein 
described and shown which are capable of making the necessary 
determinations to accomplish the automatic densitometer in accordance with 
the invention. 
In use, the pressman carries the hand-held, battery operated densitometer 
to any particular measuring location in the pressroom. The measuring probe 
head is placed against a printed target or unprinted substrate and a 
reading is taken. The densitometer automatically indicates the density 
and, optionally, color of the target measured. The pressman consecutively 
positions the measuring head on a number of targets, with the densitometer 
displaying density and perhaps color after each measurement and storing 
appropriate density information for later use in calculating percent trap 
and percent dot area. Where necessary, the densitometer prompts the 
pressman to provide measurements of the first and second down colors of an 
overprint so that percent trap can be calculated or indicates that an 
appropriate solid density reading is necessary before percent dot area can 
be calculated. Based upon the information displayed the pressman can 
determine what adjustments to the printing process, if any, are necessary 
and can provide measurements of particular targets indicated to be 
necessary by the densitometer. 
Since the densitometer automatically determines what target has been read, 
stores appropriate density information and calculates and displays percent 
trap and percent dot area, all in a portable hand-held device, the 
pressman can devote his attention and experience primarily to adjusting 
the press and printing process rather than to recognizing targets, 
selecting and storing appropriate density values, ensuring that those 
selected values are properly referenced, and calculating percent trap and 
percent dot area. 
Thus, the densitometer method and system according to the present invention 
advantageously permit accurate, reliable and convenient measurement of a 
variety of printed targets in a heretofore unknown fashion. Remarkably, 
all measuring electronics and display can be mounted in a relatively small 
and lightweight battery-operated densitometer body having a versatile 
measuring head connected to the densitometer by an electro-optic cable. In 
this manner it is possible to provide a hand-held portable densitometer 
which is capable of (i) determining what type of target is being examined, 
(ii) selecting and storing the appropriate density measurements and, (iii) 
calculating and displaying percent trap and percent dot area when 
sufficient information has been measured and/or stored. Of course the 
densitometer displays all referenced density measurements to the pressman 
as well for use in controlling the printing process. 
To the extent not already indicated, it will also be understood by those of 
ordinary skill in the art that any one of the specific embodiments and 
features of the invention herein described and illustrated may be further 
modified to incorporate features shown in other of the specific 
embodiments. 
The invention in its broader aspects therefore is not limited to the 
specific embodiments herein shown and described but departures may be made 
therefrom within the scope of the accompanying claims without departing 
from the principles of the invention and without sacrificing its chief 
advantages.