T. Enomae, Applications of desk-top flat-bed image scanner to analysis of paper properties, August 28, 2000

METHODOLOGY TO USE FLAT-BED IMAGE
SCANNER FOR FORMATION ANALYSIS OF PAPER

with a transparency unit has been used since 1995 as an input
device of light transmission images in place of a CCD video
camera for a formation analysis. Shinozaki6) built up an image

T. Enomae and S. Kuga

analysis system mainly for sheet formation, where an image
scanner is used as one of input devices. However, he did not

Department of Biomaterial Sciences
Graduate School of Agricultural and Life Sciences
The University of Tokyo
Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-8657, Japandiscuss how to configure the scanner for reliable images. The
purpose of this study is to examine output characteristics of ascanner and to analyze the output images from a viewpoint offormation. The fundamental methodology will be useful formany other possible applications of a scanner for examining
paper structure and properties.

ABSTRACT
MATERIALS

Versatility of a desk-top flat-bed image scanner with a

Apparatustransparency unit as an image input device for formation
analyses was exhibited and discussed. If the scanner output

Flat-bed image scanner GT-8500WIN and transparency

under satisfied prerequisite conditions is calibrated based on aunit GTA4FLU (Epson Inc., Japan) were used in combination.

standard film or by comparison to Elrepho type reflectometer,

The working mechanism of the scanner is as follows: RGB

the scanner was found to provide reasonably scaled gray levels.

xenon fluorescent lights irradiates a sample with three colors ofHowever, there remained some distortion in the output

rays. Transmitted (or reflected) light was collected by a lenscharacteristics even after the calibration. So, unlike Elrephoand detected by an in-line CCD sensor. The sensor is

type, optical density squared was almost proportional to theassembled with the fluorescent lights, so they move together inbasis weight of paper. Formation of handsheets wasworking. The direction of the CCD sensor alignment is called

quantitatively analyzed with the light transmission images

the main scan direction; and that of its movement the sub-scanobtained by the scanner. The formation index, the standard

direction. On the other hand, common CCD video camera

deviation of optical density squared correlated better with thesystems use a halogen lamp, but uniform illumination is not

subjective ranking than that of optical density or of gray level.

warranted because a relatively wide area is acquired at one time

Optical density squared divided by the basis weight showed theand external room light may interfere with the destined

highest Kendall's coefficient of correlation. Anyway, theillumination. A system of a pinpoint beam in combination withstandard deviation of gray level was far from propriety as aa scanning sample stage eliminates this defect, but specific lightformation index.

scattering coefficient depends on basis weight for parallel
incidence 7 ). This may create different images from those

INTRODUCTION

experienced commonly over diffuse light. Furthermore, it takes

A lot of work on sheet formation analysis of paper havelong time to scan the whole area in two dimensions and the

been performed for over sixty years. Through the period of

accuracy of positioning is lower. To interface the image

time, image input devices or scanning devices based on ascanner to image analysis software, image input drivertransmission of light had a wide variety. The representative(software) Epson ScanⅡ32, 2.00J, 1994 was used. The optical1)systems so far were the rotating disc of paper with a light spot ,resolution was 256 gray levels.
Image analyzer DA-5000S (Oji Scientific Instruments,

the Alinco tester using a cathode ray tube whose beam sweeps
2

2, 3)

, the rotating

Japan) and the original software the authors developed usingtransparent cylinder with a light spot 4 ) and video camera

Visual Basic 5.0 (Microsoft Japan) were used to calculate

over an area of up to 44×44 mm by raster5)

optical density of every pixel in the images for the formation

systems .

analysis.

A desk-top flat-bed image scanner is an useful device toavailable to many computer users at low costs. In the

Brightness R∞ was measured on Elrepho typereflectometer, TD-1800, Tokyo Denshoku, co. ltd., Tokyo,

department of Biomaterial Sciences of the University of Tokyo,

Japan with D65 diffuse illuminant and normal observer.

analysis of paper sheet formation is one of the major subjects of

Opacity R0 was measured with the same configuration on thethe juniors’ experimental course. The image scanner combined

same refelctometer with a black cavity backing.

obtain image data to a computer. Nowadays, it is prevalently

1

T. Enomae, Applications of desk-top flat-bed image scanner to analysis of paper properties, August 28, 2000

This must be linear, namely, 1 for a quantitative analysis. The
values of highlight and shadow are a threshold value above

Samples
To ensure the accuracy of the image scanner output, awhich all the levels in input are set at 255 in output, or below

standard film designed for optical instruments was used. The

which all the levels are set at 0, respectively. So, care must be

film was a piece of Density Step Tablet, Fuji Photo Film Co.,taken in setting so that the third requirement in the last section

Ltd., manufactured to measure diffuse visual density. Thisis fulfilled. Then, the in-between values are divided into 256

tablet is a film consisting of portions at several different

levels at equal intervals.
With the three prerequisites satisfied, the samples were

densities calibrated preliminarily according to Japan Industrial

scanned at a resolution of 120 dpi, namely, at ca 0.2 mm

Standard (JIS K 7605).

intervals. The scanned area was 100 mm×100 mm each, but

To compare output characteristics between the scannerand the Elrepho reflectometer, a plastic film 199 µm thick of

the calculated area was 60 mm×60 mm (283 pixel×283 pixel)

178 g/m2 in basis weight, and tracing paper 41.0 µm thick ofeach. The standard film of Density Step Tablet was scanned at39.3 g/m2 in basis weight were used. The plastic film was

the same resolution and the calculated area was 12.7 mm×6.4

colored blue, but transparent. The tracing paper was of low

mm (60 pixel×30 pixel). For the plastic film and the tracing

light scattering for paper, but translucent. Some sheets of thempaper, almost the same area was measured.

were stacked and loaded slightly by a weight or a scanner coverCalculation of optical properties

in measurement.
For an application to formation analysis, handsheets madeOptical specification of a sheet, film or any material isof softwood or hardwood kraft pulp, loaded with 40 % calcium

likely to be achieved favorably by determining brightness andcarbonate to dry pulp, were prepared. Retention time before

specific light scattering coefficient. The Kubelka-Munkdrainage was 10 (standard-based) or 120 s. Cationic polymer,

equation 8) describes the relation between those parameters.

polyamideamine-epichlorohydrin resin (WS-570, Japan PMC

According to it, optical density OD (refereed also toCo., Japan) was added to a half of the samples at 0.4 % to dry

absorbance) is expressed by the equation:

pulp. Thus, four kinds (2×2 conditions) of handsheets eachb
OD = − log
 , ........
(1)
 a sinh bsW + b cosh bsW 
where s is the scattering coefficient, W is the basis weight, andwere prepared from softwood and hardwood pulps.
METHODSsinh and cosh are hyperbolic functions. Likewise, OD of

Configuration of scanner

scanner output, that is, gray levels was defined as follows:V 
OD = log 0  ,........
(2)
V 

It was found that there are some requirements of thescanner configuration. They are;(1) to cover the rest of the sample area on the document glass

where V and V0 are the gray level of the considered pixel andstage with an opaque sheet to prevent extra reflected lightthe gray level in full transmission with no material, respectively.

on the sample surface from coming into the detector,s can be calculated from R∞ , a and b as follows:
1 − aR0 ,1s=
Ar ctghbW
bR0(2) to make external lighting uniform in the direction of
sub-scan because no calibration is made in this direction
to compensate for its non-uniformity, and

.....

(3)


1 1

1 1
− R∞  , ..... (4)+ R∞  and b = a = 
2R
2  R∞ ∞
where R0 is the reflectance of single sheet with a black cavity
backing, R∞ is the reflectance factor of infinite pile of paperand Ar ctgh is the inverse function of a hyperbolic function,
ctgh. But, actually the calculation was made following themodified equation introduced by Hamada9).
Radiation of such as β-ray or soft X-ray used commonly
for formation analyses tends to attenuate in relation to the local
basis weight of paper or distance of the medium through which
it is transmitted. Lambert’s law below describes therelationship.

(3) to set input parameters at proper values so that the whole
range of optical density for the sample including fulltransmission out of the sample will fall within the whole
input range.
Proper input parameters
In the scanner driver, users can set some input parameters
to their purposes. For a formation analysis, one need to set
values of γ, highlight and shadow at 1(100 % for this driver) ,255 or higher in some cases and 0, respectively. γ-value is acharacteristic curve to determine gain of output to input levels.

2

T. Enomae, Applications of desk-top flat-bed image scanner to analysis of paper properties, August 28, 2000

I log 0  = αd ,........
(5) I 
where I, I0, α and d are the intensity of the transmitted radiation,

thickness of the material. Lambert's law applies limitedly to thethe intensity of the incidental radiation, the linear absorption

paper, OD was not proportional to the number of sheets (opencoefficient and the distance of the medium, respectively. The

squares). This behavior is typical to paper having many

Kubelka-Munk equation reduces to this law if s equals to zerofiber-air interfaces that tend to scatter light (open triangles), as

in the derivative form.

taught by the Kubelka-Munk equation.

case that the scattering coefficient is negligible. Accordingly,
the coefficient was very low (filled triangles). For the tracing

Subjective ranking

OD by the scanner was supposed to provide the same

Subjective ranking was made by 6 panelists with the 8

relationship if the agreement found in Figure 1 is taken into

handsheets in terms of the degree of inhomogeneity in

2.0

look-through over diffuse white light. The judgement was madeMeasured optical density

by comparing every pair of the sheets. The better one was given
1 point and the total points per sample were calculated. The
figure of ranking represents the average point given to theindividual sheets. The correlation coefficient of each formationindex with the subjective ranking was determined according to
Kendall’s method. This method is as follow: the same order of
each pair with the subjective ranking gives 1 point and thereversed order gives –1 point; the correlation coefficient is

Scanner
ElrephoScanner - after 1 year

1.5

1.0

0.5

expressed in terms of the total points divided by the number of0.0

every pair, that is, 28 in this case. This correlation method is

0.0

advantageous in that it correlates to the ranking order, not tothe absolute ranking point that is not reasonably scaled.

0.5

1.0

1.5

2.0

Pre-calibrated optical density of standard film
Fig.1 Comparison in optical density of the standard film
measured between by scanner and by Elrepho-typereflectometer

RESULTS AND DISCUSSION
Scanner output characteristics
density OD was calculated. Figure 1 shows a good agreementof OD measured between by the scanner and by the Elrepho
reflectometer up to ca 1. This range roughly means that there is

1.8

9

1.6

8

1.4

7

1.2

6

1.0

5

0.8

4

0.6

0.2

Plastic film - Opt Dens 3
Tracing paper - Opt Dens
Plastic film - Scat Coef 2
Tracing paper - Scat Coef
1

0.0

0

a good linear relationship in gray levels ranging from 25 to 250,
Measured optical density

for example. The values are also consistent with the
pre-calibrated ones noted on the standard film. However, theoutput characteristics changes over time. After one year, thesame standard film was scanned and OD was calculated again.
The gradient representing the relationship became much lower,
maintaining a good linear relationship (broken line). This may
be because the light source has deteriorated. To avoid thispossible deterioration, determination of absolute OD values

0.4

needs calibration as based on a standard film. Scanner of othermanufacturers were also confirmed to show a linear

Specific light scattering coefficient, m2/kg

The standard film was scanned by the scanner and optical

relationship.
Figure 2 compares OD measured by the Elrepho0

reflectometer for the two types of materials; the plastic film and

10

20

30

40

the tracing paper. The OD values of the plastic film (denoted

Number of sheets stacked

by filled squares) was exactly proportional to the number ofFig.2 Comparison in optical density measured by Elrepho-typereflectometer between plastic film and tracing paper
stacks.

sheets. This relationship can be explained by Lambert's law,equation (5), if the number of sheets is assumed to be the

3

1.8

1.6

1.6
Measrured optical density

Measured optical density

T. Enomae, Applications of desk-top flat-bed image scanner to analysis of paper properties, August 28, 2000

1.8

1.4
1.2
1.0
0.8
0.6

Plastic film - Scanner
Tracing paper - Scanner
Plastic film - Elrepho
Tracing paper - Elrepho

0.4
0.2

Scanner
Elrepho

1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0

0.0
0

10
20
30
Number of sheets stacked

0

40

10
20
30
Square root of basis weight, (g/m2)-1/2

40

Fig.3 Distortion of scanner output. For the same material
stacks, scanner optical density curves upward.

Fig.4 Comparison in optical density of tracing paper stack.
The scanner output is proportional to a square root of thetotal basis weight.

account. Figure 3, however, shows distortion in scanner output.

basis weight of the tracing paper stacks, which was easily

The OD is already corrected by the calibration based on thecalculated from the basis weight per single sheet times thestandard film. The gray square plots for the Elrepho output are

number of sheets. The relationship for the scanner is almost

exactly the same data with in the previous figure. The filled

proportional to the basis weight, while that for the Elrepho havetriangle plots for the scanner output curves upward in thea linear portion but does not pass through the origin. The

middle range. The reason for this is unknown for the present.

proportional relationship for the scanner may be coincidental or
may be designed so.

Figure 4 shows OD as a function of a root of the total

Formation analysis
Table 3

Properties and formation analysis results of the handsheets. r is Kendall’s coefficient ofcorrelation with subjective ranking

Key
HN10 HN120 HP10 HP120
FurnishHardwood
Cationic polymer
None None Added Added
Retention time, s
10
120
10
120
Basis weight (W), g/m2
63.8 62.4 73.6 76.6
7.99 7.90 8.58 8.75
W , (g/m2)1/2
Brightness, %75.5 75.8 74.9 71.6
Specific scattering coefficient, m2/kg 45
44
44
42
Gray level (V)Average(Vave)
45.3 45.6 34.2 32.2
Standard deviation(Vstd)
2.1
2.3
1.7
3.0
Optical density (OD)
Average(ODave)
0.752 0.750 0.875 0.904
Standard deviation(ODstd)
0.020 0.022 0.022 0.040
2
OD
Average(OD2ave)
0.567 0.565 0.769 0.822
Standard deviation (OD2std)
0.030 0.033 0.038 0.072
Coefficient of variation
5.3
5.9
5.0
9.0
-3
2
ODstd/B, 10 m /g
0.31 0.35 0.30 0.52
(ODstd)/ W , 10-3 (m2/g)1/2
2.5
2.8
2.5
4.6
(OD2std)/B, 10-3 m2/g
47
53
52
95
3.7
4.2
4.4
8.3
(OD2std)/ W , 10-3 (m2/g)1/2
Subjective ranking of formation
0.0
3.6
1.4
6.4
(the lower, the better)

SN10 SN120 SP10 SP120
Softwood
None None Added Added
10
120
10
120
56.4 60.3 68.1 69.1
7.51 7.77 8.25 8.31
77.5 77.0 75.8 75.1
32
27
37
36

Figure5isthescanner images of the 4rkinds of handsheets eachregarding retention timeandcationic

addition,polymer

made

from

hardwood and softwood
pulp, respectively. Table
3 lists several physicalproperties of the samples.

70.8
4.5

71.7
6.7

49.9
3.1

51.3
5.1

0.36

0.559 0.554 0.711 0.700
0.028 0.041 0.027 0.043 0.64

The result of Figure 4
suggested that the averageofOD2aveproportional

would

be

to

Basis

weight W. But, actually
0.315 0.310 0.508 0.492
0.031 0.045 0.038 0.061 0.79
9.9
14.7
7.6
12.4 0.43
0.49 0.68
40
63 0.50
3.7
5.3
3.3
5.2 0.57
55
75
57
88 0.86
4.1
5.9
4.7
7.3 0.79

they

2.3

cationic polymer retained

4

5.5

3.0

5.8

were

proportion
because
has

the

not

in

presumably

softwood

pulp

higherlightscattering

coefficientsandadditionoftheT. Enomae, Applications of desk-top flat-bed image scanner to analysis of paper properties, August 28, 2000

more calcium carbonate, which increased s, as both found inparticularly, Vstd were far from propriety as a formation index.

Table 3. The proportionality seems to apply limitedly to the

Vstd is sometimes used to estimate formation, but it must be

same furnish.
There are many formation “inhomogeneity”
indices that have been proposed from various
standpoints. Apart from frequency analyses10) andfloc structure analyses 11 ), formation has been
generally analyzed in terms of the coefficient ofvariation CV of local basis weight12,13). In this study,
a variety of formation indices were calculated fromthe scanner light transmission images. Firstly, OD
and OD2 were calculated every pixel. Secondly,
standard deviation std of Gray level Vstd, OD and
OD2, and the coefficient of variation CV (a ratio of

HN1 0

HN1 2 0HP 1 0HP 1 2 0

SN10

SN120

SP10

SP120

the average to std) of OD2 were calculated. Table 3
summarizes the results as well as the subjective
rankings. The eight images in Figure 5 weremodified to appear distinctively contrasted at thetime of press. So, it may be difficult to connect theimages to the statistic results.
Figure 6 shows the propriety of the formation
indices to relate to the subjective ranking. The
formation indices was normalized so that the
average of each index would be 3.5. Among thethree standard deviations, that is, Vstd, ODstd andOD2std, OD2std was found to distinguish the samples
with the fewest reversed pairs. This finding implies
that human perception is the most sensitive to theamount convertible to basis weight W. To be moreexact, this conversion applies limitedly to paper of
uniform structure in terms of fiber source, density,
filler content and so on. So, the conversion does
not apply to those samples. In spite of the actual
discordance, human perception has a scale basedon OD2. However, CV of OD2 showed a low
Kendall's coefficient r. So, it may be difficult tosense the amount of relative variation accuraately.
The ratio of OD2std to W, an index where theaverage of OD2 was replaced for W in CV of OD2,shows high propriety with r of 0.86. This index
distinguished well the eight sheets except HN120.
The sheet of HN120 was observed to have many
flocs with the order of ca. 5 mm. This order offlocs seems to be striking to visual perception, as
Shinozaki14) showed that the variation in the rangeof 5.7 to 6.2 mm in wavelength had a highcorrelation with subjective ranking. ODstd and,Fig.5 Light transmission images of handsheets made from bleachedhardwood kraft pulp. Details are in Table 3.

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T. Enomae, Applications of desk-top flat-bed image scanner to analysis of paper properties, August 28, 2000

confined to papers with close gray levels. Kajanto15) proposed

REFERENCES

that normalized Std (Std divided by a root of average both on1) Davis M N, Roehr W W and Malmstroem H E (1935) An

Relative value of formation index

8

Vstd
ODstd
CV of OD2
OD2std2
OD2std/W

7
6
5

instrument for formation measurement. Paper Trade J
101(July 25): 35-40.

0.36
0.64
0.43
0.79
0.86

2) Anonymous author (1956) Fast-operating formation tester.
Paper Trade J 140(April 16): 61.
3) Jamison W (1956) New approach to paper formation
measurement. Paper Trade J 140(October 8): 30-35.

434) Burkhard G, Wrist P E and Mounce G R (1960) A
formation tester which graphically records paper structure.
Pulp Paper Mag Can 61(6): T319-334.

215) Bergh P and Luner P (1983) The application of image
analysis to evaluate small scale basis weight variations inpaper. Trans. of 7th Intl. Fundamental Research Symp (Vol2), Mech Eng Publ, London, pp 707-722.

0
0

1
2
Good

3

4

5
Poor

6

7

Subjective ranking

6) Shinozaki M, Tajima Y and Miyamoto S (1996) Paper‘formation’ image analysis (in Japanese). Japanese J of
Paper Technology (Kamipa Gitai) 36(4): 24-28

Fig.6 Formation indices and their Kendall's coefficient ofcorrelation. OD2-based indices shows better correlation.

7)

the basis of grammage by β-radiography) fitted the appearance
better than the coefficient of variation. The normalized std,
(OD2ave)/ W correlated well but did not function better than(OD2ave)/W in this work, as the other possible indices did not.

Yuhara T, Nakayama S, Arie Y, Kodaka I and Okushima
S (1989) Equation of light transmittance of paper forparallel incidence. Proceedings of Japan Tappi pulp andpaper research conference 14-17.

8) Kubelka P (1947) New contribution to the potics ofintensly light-scattering materials. Part I. J Optical Soc ofAmerica 38(5): 448-457.

CONCLUSIONS
The desk-top flat-bed image scanner with a transparency

9) Hamada H, Enomae T, Onabe F and Saito Y (1999)
Colloidal properties of hollow latices and their roles incontrolling colorimetric parameters of coated paper surface.
Pre-symposium of the 10th ISWPC in press.

unit was found to function properly as an image input device
for formation analysis under satisfied prerequisite conditions if
the output is calibrated based on a standard film or bycomparison to Elrepho type reflectometer. However, there

10) Cresson T and Luner P (1990) The characterization of
paper formation analysis. Tappi 73(12): 175-185.

remained some distortion in the output characteristics evenafter the calibration. So, unlike Elrepho type, optical density11) Dodson C T J and Schaffnit C (1990) Flocculation andorientation effects on paper-formation statistics. Tappi
75(1): 167-171.

squared was almost proportional to the basis weight of paper.
The formation index, the standard deviation of optical density
squared correlated better with the subjective ranking than that

12) Norman B and Wahren D (1976) Mass distribution andsheet properties of paper. Trans. of 5th Intl. Fundamental
Research Symp (Vol 1), Tech Sec, Brit Paper Board Ind
Fed, London, pp 7-70.

of optical density or of gray level. Optical density squared
divided by the basis weight showed the highest Kendall's
coefficient of correlation. Anyway, the standard deviation ofgray level was far from propriety as a formation index.

13) Norman B and Wahren D (1972) A comprehensive method
for the description of mass distribution in sheets andflocculation and turbulence in suspensions. Svensk
Papperstidn 75(11): 807-818.

ACKNOWLEDGMENT
The authors wish to thank the junior students of14) Shinozaki M, Tajima Y and Miyamoto S (1998) Formation
analysis through multiple regression of light transmission
distribution and Fourier transformation analysis (inJapanese). Sen'i Gakkai Preprints 1998(G), Soc Fiber Sci
Tech, Japan: G-107

Biomaterial Sci. Dept. of The Univ. of Tokyo for preparing thehandsheets for the formation analysis. The graduate students on
Paper science laboratory of The Univ. of Tokyo are thanked fortheir contribution to the subjective test.

6

T. Enomae, Applications of desk-top flat-bed image scanner to analysis of paper properties, August 28, 2000

15) Kajanto I M, Komppa A and Ritala R K (1989) How
formation should be measured and characterized. Nordic
Pulp and Paper Res J 4(3): 219-228.

7