Method and device for measuring a halftone dot area rate or a halftone picture density

A method for measuring a halftone dot area rate or a halftone picture density in a densitometer or half-tone dot area rate measuring means. A light beam is emitted by a light source and impinges upon an object to be measured. The light beam passes through or is reflected from the object to be measured and is received by a photoelectric element. A weighting of the light beam is performed by a weighting means so that the assigned weigth is substantially large at its central part and is reduced radially to its periphery, the reduction depending upon a transmittance or a reflectance of the object.

BACKGROUND OF THE INVENTION 
The present invention relates to a method for measuring a halftone dot area 
rate or a halftone picture density. 
In a conventional halftone dot area rate measuring means, since the 
gradation of a measured halftone picture image varies, the aperture 
diameter which will determine the measured area is preferably small. 
However, if it is too small, when the halftone picture image is measured, 
the value which is measured will vary depending on the relative position 
of the aperture and the halftone dot. 
Large variances do not arise when the number of scanning lines per distance 
increment is large. Large variances often occur when the number of 
scanning lines is small, e.g., a gauge having about ten lines per 
centimeter for printed matter. Such a gauge is printed in a relatively 
small area such as 5 mm.times.6 mm, and hence this problem can be resolved 
only by expanding the diameter of the aperture or the measuring area. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a method for measuring 
a halftone dot area rate or halftone picture density in a densitometer or 
halftone dot area rate measuring means free from the aforementioned 
inconveniences, which is stable and reliable, and which is capable of 
measuring an area which is small when compared with a screen pitch. 
It is another object of the present invention to provide a device for 
measuring a halftone dot area rate or halftone picture density in a 
densitometer or halftone dot area rate measuring means free from the 
aforementioned inconveniences, which is stable and reliable, and which is 
capable of measuring an area which is small compared with a screen pitch. 
According to the present invention there is provided a method for measuring 
a halftone dot area rate or halftone picture density in a densitometer or 
halftone dot area rate measuring means. Pursuant to the present invention, 
a light beam generated by a light source is incident to an object to be 
measured. The light beam is passed through or is reflected from the object 
and is received by a photoelectric element. In operation, the light beam 
passing through or reflected from the object is weighted so that the 
weight is large in its central part and is reduced radially to its 
periphery depending on the transmittance or the reflectance of the object. 
The present invention constitutes a device for measuring a halftone dot 
area rate or halftone picture density in a densitometer or halftone dot 
area rate measuring means. A light beam generated by a light source is 
incident to the object to be measured. The light beam passes through or is 
reflected from the object and is received by a photoelectric element. The 
light beam passing through or reflected from the object is weighted so 
that the weight is large in its central part and is reduced radially to 
its periphery depending on a transmittance or reflectance of the object.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1 and 2 illustrate optical systems employing conventional 
transmission densitometer or halftone dot area rate measuring means, and a 
transmission densitometer or halftone dot area rate measuring means 
according to the present invention, respectively. 
The apparatus in FIG. 1 comprises a light source 1 such as an incandescent 
lamp, a first condenser lens 2, an aperture plate 3 made of a 
light-shielding material having an aperture therethrough, a diffusion 
plate 6 surrounded by a guide 5 which prevents the ambient light from 
coming in, a second condenser lens 7, a filter for correcting spectral 
characteristics, and a photoelectric element 9 such as a photomultiplier. 
All are aligned along a light axis. The object 4 to be measured is 
disposed between guide 5 and aperture plate 3. The aperture edge of the 
aperture plate 3 is tapered away from the light source 1 and it is adapted 
to be formed so that a luminous energy distribution over a measured 
surface may be uniform regardless of the thickness of the aperture plate 
3. 
In this embodiment, the light beam generated by the light source 1 is 
converged by the first condenser lens 2 and then passes through the 
aperture of the aperture plate 3. The measuring area is determined by the 
aperture. The light beam emitted through the aperture then passes through 
the object 4. The luminous energy of the light beam is decreased depending 
on the transmittance of the object 4 when it passes through the object. 
After passing through object 4, the attenuated light beam is directed to 
the receiving section. In the light receiving section, the ambient light 
is prevented from coming therein by the guide 5 and thus only the light 
beam through the aperture is incident. The light beam is diffused in the 
diffusion plate 6 and a surrounding light shield tube (not shown). Then, 
the diffused light beam is converged by the second condenser lens 7 and 
its spectral characteristics are corrected by the filter 8. Finally, the 
corrected light beam is incident to the photoelectric element 9. 
The photoelectric current which is proportional to the transmittance of the 
object 4, is obtained in the photoelectric element 9. When the 
photoelectric current is fixed, the photoelectric voltage obtained in the 
photoelectric element 9 is proportional to the transmission density of the 
object 4. Thus the photoelectric current or voltage which is obtained is 
then converted into a halftone dot area or a halftone picture density. 
An optical system employing a conventional reflection densitometer or 
halftone dot area rate measuring means is shown in FIG. 3, and a system 
employing reflection densitometers or halftone dot area rate measuring 
means according to the present invention are shown in FIGS. 4 and 5. 
Referring to FIG. 3, object 17 is aligned with: an aperture plate 16 having 
an aperture therethrough, having same shape and functions as that shown in 
FIG. 1; the condenser lens 2 and the light source 1; a light shield tube 
15 for shielding the light generated by the light source 1; a frustum 
tubular mirror 18; the correction filter 8; and the photoelectric element 
9. All are aligned along the light axis so that the frustum tubular mirror 
18 may reflect to the photoelectric element 9 via the filter 8 the light 
beam which is incident from the light source 1 to the object 17, and is 
then reflected by the object 17. 
In this embodiment, when the light beam is reflected by the object 17, the 
luminous energy of the light beam declines depending on the reflectance of 
the object. 
As described above, the photoelectric current obtained in the photoelectric 
element 9 is proportional to the reflectance of the object 17. When the 
photoelectric current is fixed, the photoelectric voltage obtained in the 
photoelectric element 9 is proportional to the reflection density of the 
object 17. Thus the obtained photoelectric current or voltage is then 
converted into the halftone dot area rate or the halftone picture density, 
in the same manner as described above. 
FIG. 6 illustrates the density value curves of halftone dots having a 
screen pitch of one millimeter and a halftone dot area rate of 50%. The 
curves are obtained by varying the diameter of the aperture 3 or 16. The 
curve represented by a solid line is obtained when the center of the 
aperture is coincident with the center of a light part of the halftone 
dot, as shown in FIGS. 7 and 8, respectively. 
From FIG. 6, it is readily understood that the measured error of the 
halftone dot density becomes zero periodically. A point A for the aperture 
diameter, at which the measured error is zero, can be expressed as a 
function of the screen pitch P by the following formula (1), wherein n is 
a positive integral number: 
EQU A=P.times.(n+0.25) (1) 
The values for the aperture diameter A and screen pitch P are measured in 
millimeters. This means that when the halftone dots having the halftone 
dot area rate of 50% are reproduced, a group of aperture diameter values 
exist of which the ratio of the dark and the light parts of the halftone 
dot pattern is unity. 
Further, when the aperture 3 or 16 employs one of such diameter values, it 
is ascertained that, even when the center of the aperture is not 
coincident with the dark or the light part of the halftone dot, the 
dispersion of the measured values is very small. 
According to the present invention, since the measured error of the 
halftone dot density values varies periodically to the positive and the 
negative sides, as shown in FIG. 3, in order to improve the accuracy of 
the measurement, a weighting of the light beam passing through or 
reflected from the object to be measured is performed so that the weight 
will be substantially large in its central part and is reduced radially to 
its periphery depending on the density, transmittance or the reflectance 
of the object. 
A plurality of weight characteristic curves are shown in FIGS. 9-15. The 
diameter of the light beam is taken in the horizontal axis, and the 
diameter of the central circle part is assigned the largest weight. The 
diameter of the light beam is hereinafter referred to as "an inner 
diameter" of "an inner circle" and "an outer diameter." 
In the weight characteristics curve of FIG. 9, the assigned weight is 
largest in the inner diameter of the inner circle and linerally decreases 
to the outer diameter which has a value of zero. 
In the weight characteristics curve of FIG. 10, the assigned weight is 
largest in the inner diameter of the inner circle and decreases 
nonlinearly (linearly with respect to its density value) to the outer 
diameter which has a value of zero. 
In the weight characteristics curve of FIG. 11, the assigned weight is 
largest in the center and is linerally reduced to the outer diameter which 
has a value of zero. 
In the weight characteristics curve of FIG. 12, the assigned weight is 
largest in the center and decreases exponentially (the Gaussian 
distribution or normal distribution) to the outer diameter which has a 
value of zero. 
In the weight characteristics curve of FIG. 13, the assigned weight is 
largest in the center and is reduced along a sine wave form to the outer 
diameter which has a value of zero. 
In the weight characteristics curve of FIG. 14, the assigned weight is 
largest in the inner diameter of the inner circle and decreases in a step 
pattern to the outer diameter which has a small value. 
In the weight characteristics curve of FIG. 15, the assigned weight is 
largest in the inner diameter of the inner circle and is decreased by a 
small amount at the outer periphery of the inner circle, and then is 
linerally reduced to the outer diameter which has a small value. 
The weight characteristics curves are, of course, not restricted to these 
examples. Many other examples including combinations of the entire and the 
partial features of the examples described above and partial modifications 
thereof, can be practiced according to the present invention. 
In a densitometer or halftone dot area rate measuring device having an 
optical system of a scanning type, the weighting described above may be 
performed by changing the optical system, or by digitizing a photoelectric 
signal obtained and then performing the weighting of the digital signal 
according to a weight characteristics curve such as one shown in FIGS. 
9-15 by using a microcomputer, or the like, without changing the optical 
system, thereby obtaining the density or the area rate of the halftone 
dot. However, these methods may require high costs. 
According to the present invention the weighting is carried out, in 
principle, by using an optical system for a usual densitometer or halftone 
dot area rate measuring means. Some embodiments of the optical system for 
the transmission or the reflection densitometer or halftone dot area rate 
measuring means, according to the present invention are shown in FIG. 2 
and FIGS. 17 and 18 or FIGS. 4 and 5. 
FIG. 2 illustrates an optical system for the transmission densitometer, 
according to the present invention. With the exception of the aperture 
plate, the optical system is similar to that shown in FIG. 1. An aperture 
plate having an aperture therethrough consists of a laminated 
light-shielding plate 10 having a tapered opening 10a, and a translucent 
plate 11 made of a plastic or glass material in which a translucent 
material is dispersed. The translucent plate 11 has a tapered opening 11a 
whose large diameter is the same as that of the shielding plate 10. The 
tapered openings 10a and 11a constitute the aperture. The weight 
characteristics curve of this aperture is shown in FIG. 15. 
FIG. 16 illustrates an alternative embodiment of an aperture plate 12 
having an aperture, wherein a plurality of thin triangular cutoff parts 
12a are radially formed in its central circular opening periphery, the 
characteristics curve being shown in FIG. 9. This aperture is used instead 
of the aperture of FIG. 1. In practice, the cutoff parts 12a are thinner 
than those shown in FIG. 16. 
A film having a weight characteristics curve can be prepared by depositing 
a layer by evaporation, photographing it, or printing it so that the 
thickness of the film layer, the density of the film photographed, or the 
density of the film printed may be gradually thickened or increased from 
the center to the periphery. 
FIG. 17 illustrates another alternative embodiment of an optical system for 
the transmission densitometer in accordance with the present invention. 
This system possesses a similar construction to that of FIG. 2. In this 
embodiment the aperture of aperture plate 25 is spaced from the surface of 
the object 4 so that the luminous energy on the object illuminated by the 
light beam is largest in its opening circular part and is reduced 
gradually to its periphery. 
FIG. 18 illustrates still another embodiment of an optical system for the 
transmission densitometer in accordance with the present invention. This 
system is also similar in construction to that of FIG. 2. In this 
embodiment an additional aperture plate 26 having substantially the same 
construction and functions as those of the aperture plate 25 of FIG. 17 is 
positioned in front of and spaced from the diffusion plate 6 and is 
connected integrally to the guide 5. Thus, the aperture of the aperture 
plate 26 acts in the same manner as that of the aperture plate 25. 
FIGS. 4 and 5 illustrate essential parts of additional optical systems for 
a reflection densitometer or halftone dot area measuring means in 
accordance with the present invention. The construction is similar to that 
shown in FIG. 3. In the embodiment shown in FIG. 3, a film plate 19 having 
weight characteristics as shown in FIGS. 9-15, is positioned near the 
light source 1. In the embodiment shown in FIG. 3, another film plate 20 
of the same type as the film plate 19 is disposed substantially adjacent 
the aperture of the aperture plate 16. When these two embodiments are 
applied at the same time, the weight characteristics are a multiple of the 
individual characteristics. 
Some examples of the determination of the sizes of the inner and outer 
diameters will be described. 
First, with respect to the outer diameter it is possible to decrease it. 
Considering the typical gauge for the printed matter, it has been 
determined that a measurable size in a range 3-5 mm is preferable. Then, 
the inner diameter is determined under consideration of the difference 
between the inner and the outer diameters. 
EXAMPLE A 
A diameter obtained in accordance with equation (1) is approximately set to 
the middle of inner and outer diameters to be determined, and then 
relatively small positive and negative deviation values are properly 
selected, thereby determining the inner and outer diameters. For example, 
assuming that the maximum screen pitch is one millimeter (ten scanning 
lines per centimeter), when n equals four, using equation (1) the value 
for the quantity A is 4.25 millimeters. Then, the positive and the 
negative deviation values are selected to be 0.25 millimeters, thereby 
obtaining the inner diameter of 4 millimeters and the outer diameter of 
4.5 millimeters. It is not necessary that the positive and the negative 
deviation values are selected to be the same. 
EXAMPLE B 
First, the outer diameter is determined considering the mechanical 
restrictions such as described hereinabove. Then, the inner diameter is 
determined so that the difference between the inner and the outer 
diameters is equal to 2n where n is 1, 2, 3, . . . , so as to obliterate 
mutually the positive and the negative errors. For example, assuming that 
the maximum screen pitch is one millimeter (ten scanning lines per 
centimeter), the outer diameter is determined to 4.5 millimeters, and the 
inner diameter is determined to 2.5 millimeters so that the difference 
between the inner and the outer diameters may be twice as large as the 
screen pitch. 
EXAMPLE C 
First, the outer diameter is determined considering the mechanical 
restrictions described above. Then, the inner diameter is determined so 
that the difference between the inner and the outer diameters is more than 
five to six times as large as the screen pitch. This will reduce the 
influence of errors to a minimum even when the positive and the negative 
errors are not eliminated completely. For example, where the outer and the 
inner diameters are determined to 4.5 and 2.5 millimeters respectively, 
and assuming that the screen pitch is 0.391 millimeter (25.6 scanning 
lines per centimeter), a difference of 2 millimeters between the two 
diameters is more than five times as large as the screen pitch. Further 
assuming that the screen pitch is 0.254 millimeter (39.4 scanning lines 
per centimeter), a difference of 2 millimeters between the two diameters 
is slightly less than eight times the screen pitch. 
The result of measuring the density of the halftone dot having the halftone 
dot area rate of 50% according to the present invention are tabulated in 
the table set forth hereinbelow. 
As set forth in the table, using a conventional method the outer diameter 
is 4.5 millimeters. By using the present method, the weight 
characteristics curve shown in FIG. 9 is applied resulting in the values 
for the inner and the outer diameters of 4.0 and 4.5 millimeters 
respectively. 
TABLE 
______________________________________ 
1 mm 0.391 mm 
(10 lines/cm) 
(25.6 lines/cm) 
Screen Pitch Max. Min. Max. Min. 
______________________________________ 
Conventional 0.318 0.285 0.304 0.298 
Method 
Outer diameter 
4.5 mm 
Present Method 
0.302 0.300 0.303 0.299 
Inner diameter 
4.0 mm 
Outer diameter 
4.5 mm 
______________________________________ 
(The density of a halftone dot having a halftone dot area rate of 50% is 
0.301). 
It is readily understood from this table that the measured errors according 
to the present invention are smaller than those which would result 
according to the conventional methods. 
Although the present invention has been described with reference to 
preferred embodiments in connection with the accompanying drawings, 
however, various changes and modifications can be made by those skilled in 
the art without departing from the scope of the present invention.