SE441217B - PROCEDURE FOR SOIL-FREE Saturation of the ABSOLUTE CONTENT OF A SUBSTANCE IN A MULTIPLE SUBSTANCE - Google Patents

Description

7906518-1 in the infrared radiation region is explained in the molecular structure of such materials, which context within the framework of the present patent application does not need to be further discussed as it is previously generally known.

Furthermore, it is previously known that in water the strongest absorption of radiant energy takes place in the radiant region which is close to the infrared, namely in the so-called valence oscillation range at 7 \ = 2.92 / L. For absorption measurement on very small amounts of water or very thin water layers, this wavelength would therefore be very suitable. For measuring the water content in organic substances, such as paper, however, one must fall back on the significantly weaker combination oscillation via A = 1, 94; t. This is due to the fact that all carbohydrates have in the vicinity of λ = 2.9 / L a resonance site of the CH bond and that the measured size would thus be dependent on the carrier substance. Under practical conditions, however, it has been found that even when using this significantly weaker combination oscillation at Ä = 1.94 / h, conclusions can be drawn regarding the water content of a material from the absorption ratio of this material.

To measure the water content of paper and similar materials, in which the water is mixed in finer distribution with the other substances of the material in question, the spectral comparison method (two-beam measurement) has also been used so far, namely both as a transmission method and as a reflection method in accordance with DE-AS 20 55 708. In this measuring method, radiation pulses of the strongly absorbed measuring wavelength and the weakly absorbed comparative wavelength are produced alternately by means of an optical vibrating means. These radiation impulses penetrate the material in question, are captured by a detector and are converted into measurement or comparison signals. After joint amplification, the measurement signals are separated from the comparison signals. A quotient network then finds out the relationship between the signals and indicates this if necessary. - Provided that the comparative wavelengths in the spectrum are not too far from the measuring wavelengths, this measuring method achieves in principle the following advantages over a per se also possible measurement with measuring path length radiation only: Each of the measuring object's displacements relative to the measuring point the respective measuring object is eliminated with respect to the measuring result; any contaminants present in dispersed form in the water in the paper or similar material have an approximately uniform effect on the radiation losses and therefore have less effect on the produced quotas; any variations in the sensitivity of the detector due mainly to temperature fluctuations become smaller; and any variations in the gain factor in the measuring amplifier are also eliminated.

As a comparative wavelength when measuring the water content in paper, for example, a wavelength of Å = 1.8f ~ can be used. With this small distance to the measuring wavelength ÅÄ = 1.94} ~, namely in the order of about AI = 0, 14) ~, the measuring wavelength and the comparative wavelength are sufficiently close to each other to guarantee that the above-mentioned disturbances are reduced to at least a considerable degree. .

It is further known that cellulose, which is present in a considerable amount in paper, shows strong absorption within a wavelength range of Ä = 211ßf.

In one in MELLIAND, booklet 10/1958, p. 1157 - 1160, it can further be deduced that when it comes to measuring the water content in textiles, completely different measurement values can be obtained for the same water content, while as is known with transparent materials, in particular transparent cellulose (cell glass) uniform measurement values for the water content time is obtained. The variable measured values of textiles are based on a so-called "average penetration" or on variable "dispersion distribution" (Streuweglängenverteilung) of the infrared radiation.

It can also be deduced from the essay in question in MELLIAND that attempts have already been made in a mathematical expression to characterize the structurally complex features of opal cellulose (paper) or textiles in a mathematical expression in such a way that satisfactory measurement results can be obtained. is also obtained with such materials. However, these attempts have so far not led to success.

As hitherto, for example, when measuring the water content of paper, the problem remains that the measured value of the water content may be changed even with unchanged water content of the paper when physical or structural characteristics of other substances in the paper, especially cellulose but also fillers, such as titanium dioxide, changes during a manufacturing process.

The present invention has for its object that when measuring the content of a substance in a thin web of material or in a thin film of material, it is possible to use a measuring method based on infrared radiation. achieve satisfactory measurement results even when the physical or structural characteristics of the other substances of the material in question, in particular cellulose in the case of paper, change during the manufacturing process. This object is solved by the invention stated in the characterizing part of claim 1.

With the measuring method according to the invention, it is possible, for example, when measuring the water content of paper, to obtain measurement results which are independent of physical or structural peculiarities or changes in such peculiarities of other substances in the paper. The design or changes in the design of cellulose fibers and / or filler particles no longer give false measured values. The method according to the invention is based on the experimentally established fact that the depth of the absorption bands of the material to be measured and of an additional substance, e.g. water and cellulose, changes in proportion to the basis weight of the material, respectively the cellulose, and in particular to the physical or structural peculiarities of the material parts of the material. This deviation can be explained by the theory that the length of the path that the infrared radiation has to travel between the entrance to and the exit from the material depends on the structure of the material and its basis weight. This path of radiation through a material corresponds in the least common cases approximately to the path of radiation through an equally thick, solid glass-clear medium, such as, for example, cellular glass. - The longer the path that infrared radiation has to travel at the same material thickness right through the material, the greater the absorption of infrared radiation energy within the absorption band range of the material's substances. As a result, the absorption band of a material with often repeated deflection of the radiation against small particles, such as fibers, pigment particles and the like, is considerably more pronounced (deeper) than in a material which admittedly contains the same amount of substance but as in optical In this respect, the radiation nevertheless passes unimpeded, as, for example, in the case of cell glass.

A more precise description of this effect is unlikely to be possible due to the complex and highly variable shape of the refractive and / or dispersing particles, for example in the case of paper, the shape of the cellulosic fibers surrounded by water.

Of course, the method according to the invention can be used to advantage not only in measuring the water content of paper but in general in all measurements on thin material films or thin material _ -..-_......_...._. ..__...> ._... M .__ »...___ .. _., _,, 5 7906518-1 albanas, in which the content of a substance in the material is to be measured, which substance is mixed with other substances in the material. whereby the additional substance that can give rise to misleading measured values, like the substance to be measured, has a pronounced absorption band.

~ In the case of the above-mentioned two-beam measurement procedure, the measured values at measuring wavelength are set in relation to a comparison value. This comparative value can be seen from a separate infrared radiation measurement »also this at measuring wavelength, with the material removed from the measuring gap between radiator and detector - ie without material. In continuous measuring procedures, however, it is not possible to remove the material to be measured from the measuring gap. Therefore, the comparative value is formed by a measured value which, using a comparative wavelength close to the spectrum, since such a measured value in many cases (eg measurement on oil glass) hardly deviates from the measured value which would have been obtained when the material was removed from the measuring gap.

With a problem solution according to claim 1, the two measuring wavelengths Ä-1 and Ä3 are also set on a previously known basis in relation to a comparative wavelength Ä 2 which is in a spectral proximity to the two measuring wavelengths 7 \ 1 and 7 \ 3.

However, using the present method in the manner set out in claim 1 leads to misleading results when measuring materials that exert a strong scattering of infrared radiation. The radiation losses caused by dispersion vary, as is known, with the fourth power of the wavelength. Spectrograms of such infrared radiation in dispersing materials are therefore, depending on the dispersion intensity of the radiation, more or less strongly inclined towards a spectrogram for a material of the same chemical and quantitative composition of matter, but nevertheless a material that optically practically does not affect radiation, such as glass. . Using the measured value obtained at the comparison wavelength as a constant reference value or to form a baseline dependent on the wavelength for the measurements at the measuring wavelength must in such cases lead to misleading measurement results.

In order to obtain satisfactory measurement results when measuring on materials which disperse infrared radiation, the method according to claim 1 has been further developed as stated in claim 2.

Using the problem solution according to claim 2, adapted reference values are calculated for the measuring wavelengths Ä 1 and) \ 3.

The calculation of these reference values takes place against the background of an extrapolation that the measurement wavelengths 7 \ 1 and Ä 3 are adjacent »the comparison wavelengths 7 \ 2 and. Ä 4 comprehensive range of the function I = f () Q within the wavelengths Ä 1 and. 7 \ 3 area. These reference values thus do not represent any measured values but serve to indicate exactly the depth of the absorption bands in question of materials which contain substances with dispersing properties.

By means of the method according to claims 1 and 2, a hitherto unattainable accuracy in the measurement of the substance parts in a material tap or material film is made possible, and above all a measurement which is independent of physical or structural variations of the substance in the material.

From the drawing, Figs. 1 - 9, the method according to the invention is further clarified. Fig. 1 shows in schematic representation a measuring device possible for carrying out the method, Fig. 2 shows a diagram with radiation pulses as these are consecutively received by the measuring device according to Fig. 1, Fig. 3 shows a diagram (spectrogram) in which idealized for a certain material the measuring signal I emitted by a detector is indicated depending on the wavelength, Fig. 4 shows the diagram according to Fig. 3 together with further curves for further material on a larger scale, Figs. , the infrared radiation sequence in three separate materials adapted to the spectrograms according to Fig. 4. Fig. 8 shows, roughly schematic, the infrared radiation course in a further material and Fig. 9 shows spectrograms for the materials according to Figs. 5, 6 and 8. Fig. 1 shows a transmitter 1 for infrared radiation in front of whose radiation emission 2 a filter wheel 3 rotates on a previously known way. In the filter wheel 3, the filters 4, 5, 6 and 7 are arranged distributed around the circumference of the filter wheel 3. In the radiation direction of the transmitter 1, a detector 8 is arranged at a certain distance from the transmitter. The detector 8 converts the radiation (radiation pulses) obtained from the transmitter 1 into electrical measuring signals I. Between the filter wheel 3 and the detector 8 there is a relatively thin film-like response web-shaped material 9.

The radiation pulses 11 are corresponding to different wavelengths corresponding to the successive filters 4 to 7 corresponding to the rotation of the filter wheel 3.

After the influence of the material 9, more or less attenuated radiation pulses 12 reach the detector 8. The radiation pulses 12 received by the detector 8, adapted to the wavelength used, are converted into electrical measuring signals I (see also Fig. 2) and transmitted for a further 7 7906518 -1 re machining to an electrical evaluation means not shown, known per se. The measuring signals I emitted by the detector 8 can then have, for example, the same shape and size as the measuring signals 14, 15, 16 or 17 produced in the diagram according to Fig. 2.

Fig. 3 shows a diagram (spectrogram), in which for a material A the measuring signal I emitted by the detector 8 is produced in dimension [Ä] depending on the wavelength of the infrared radiation dimension4 [; ¶]. In terms of the effect on the infrared radiation, the material A corresponds to the previously known cell glass and affects the infrared radiation as roughly schematically shown in Fig. 5. Thereafter, the infrared radiation which penetrates the material A on its way through this material to the detector 8 is neither deflected nor dispersed in practice.

The production of the measurement signal I depending on the wavelength THAN in Fig. 3 is idealized. In reality, the spectrograms of most materials do not show such elongated rectilinear areas as the curve according to Fig. 3, but these rectilinearly produced areas have in reality mostly more or less strongly curved curve shape. By selecting a suitable scale, for example such a scale in which on the abscissa the wavelength l is not applied but linear h ", however, in a satisfactory approximation, approximately straightness is obtained in the specified areas of such spectrograms. E It appears from Fig. 3 that in addition to a substantially straight and approximately horizontal part of the curve shown, there are areas with relatively deep incisions.These areas correspond to different absorption bands of the material A. At the wavelength Ä = 2.92 F 'possesses free water so-called valence oscillation. The absorption band resulting from this combination oscillation is used to measure the water content of the material A, namely by means of a hitherto - as is generally known per se - on the basis of a two-beam measurement. method with empirically determined correction of the adjustment by the basis weight and structure of the material in question of the e the measured signals.

The spectral program according to Fig. 3 further shows an additional absorption band which lies at a wavelength of Ä = 2.10 fß and which is coordinated with the cellulose in the material A.

Fig. 4 shows a spectrogram, in which the spectrogram according to Fig. 3 is in the stripe-dotted outlined area represented on a larger scale. Furthermore, Fig. 4 shows spectrographic curves for further "ION QUALH", 7906518-1 g material B and CW, while material B affects infrared radiation to a degree roughly sohematised in Fig. 6, material C affects infrared radiation approximately corresponding to the representation in Figs. Compared with the radiation course of the infrared radiation shown in Fig. 5, the length of the path which the infrared radiation has to travel through the material up to the detector 8 increases considerably with materials B and C, despite the fact that in the case of materials B and C those which, in terms of parts by weight of water and cellulose per unit area (basis weight), correspond to material A.

From the spectrograms in Fig. 4 it can be seen that the depth of the absorption bands at the wavelength λ \ 1 and 3 3 for the materials A, B and C is very variable and is all the more pronounced the farther the infrared radiation has to pass when it penetrates the materials A, B and C to reach the detector 8. Correspondingly, the measurement signals I Å 1 and I Ä 3 also vary at the measurement wavelengths, Ä 1 and) l3.

However, since the measurement signals I Ä 2 at the comparison wavelength, Ä2 for the three mentioned materials A, B and 0 practically do not change (for easier understanding of this, the rectilinear curve areas have been drawn with greater mutual distance than the actual one), a measurement must thus be made. of the water content in materials A, B and C on the basis of the previously known two-beam measurement lead to different measurement results despite the same water content in the materials in question. This will become even clearer from the subsequent clarification.

When determining the water content of paper as well as, for example, when determining a certain filler or additive in a plastic film with its own absorption band in the infrared radiation range, the following mathematical correlation is in principle assumed in šo = e "aM (1) In this equation 1 means "I" the strength of the measurement signal emitted by the detector 8 in, for example, the dimension Zlile after the influence of the infrared radiation used with measuring wavelength (thus within an absorption band range) by the substance in the material to be measured.

"Io" means the measurement signal emitted by the detector 8 in the same dimension at M = 0; "a" means the absorption coefficient of the substance in question at the measured wavelength selected for the ultra-red radiation and "K" means the basis weight of the substance in the material.

Since in measuring the water content of paper in accordance with the spectrograms according to Fig. 3 and Fig. 4, infrared radiation with 7906518-1 (measuring) wavelength AÄ 1 is used, below to form a comparative value of material present in the measuring gap. (transfer) wavelength) \ 2 is used, below equation 1 is completed by entering the corresponding index for the wavelengths in question. so that the equation will read š-š-zl = e-am (2) In the above equation 2 the measuring signal I 7 \ 2, which at the comparison wavelength Ä 2 is received by the detector 8, in practice corresponds to the measuring signal lo in equation 1. . Furthermore, it should be noted that the absorption coefficient "a" is of course coordinated with the measurement wavelength) \ 1. In another also possible measurement using a different suitable measuring wavelength, another absorption coefficient, namely one which is coordinated with this second measuring wavelength, would have to be used as a basis for equations 1 and 2, respectively.

After conversion of equation 2, for the basis weight of the substance in question in the material to be measured, the following mathematical relation is obtained:: vïï fl lnï 1 I; '- 2 a When the physical expression of the "natural" extinction E: E = -ln% O or E = -ln I) * 1 (4) ïïz is introduced, equation 3 reads as follows: H = EW g: ( s) “W l the above equation 5 denotes A the natural extinction of the absorption band of the water in question, MW is the basis weight of the water (water content) and aw is the absorption coefficient of water. since in the case of material A a true measurement result can be obtained on the basis of equations 3 and 5, respectively, the measurement result in the cases of material B and material C nevertheless does not correspond to the actual values. Namely, while the measurement result HW for the material A (cell glass) corresponds to a measurement result produced in the laboratory gravimetrically and can thus be regarded as accurate, the measurement result MW for the materials B and C will be more or less high above the measurement result for the material A although in fact in all three cases equal amount of water was soon omm; 7906518-1 10 in advance. This is quite clear from the spectrograms for materials A, B and G in Fig. 4.

By changing equation 5, the following formula applies to materials of type B, C and the like! r11 = E1 'WW (6), aW In the above formula 6, MW1 means the measurement result - which is incorrect - for the water content (water basis weight), E1w denotes the extinction and aw absorption coefficient increased by the physical or structural peculiarities of the substance in the material. water.

The mentioned incorrect measurement results MW1 for the water content of the materials B, C and similar materials ~ are due to the measurement signal I? \ 1 changing at the measuring wavelength Ä n. This change is a consequence of the conditions for the infrared changed towards the material A according to Fig. 5. the passage of radiation into or through materials of type B and C, respectively.

Through experiments it has been found that the altered natural extinction E does not only relate to the absorption band for water but also applies, and this proportionally, to the absorption bands for oellulose which in each case are found in materials B, C and the like.

Focused on measuring and calculating the cellulose content in the material in question (cellulose weight), equation 5 and equation 6, respectively, after the corresponding index has been introduced, have the following wording: Me = & (H aC resp 1_ 1 Mc _ ni (8) aC where Le the correct and MC1 means the incorrect basis weight of the cellulose content of the material determined in each case, EC denotes the normal and E01 the natural extinction changed by a special physical or structural structure of the material, and ac denotes the absorption coefficient of cellulose in the measure used - wavelength Ä 3.

Due to the stated proportionality between the variations in the measurement results MW and MW1 in water and Mc and M¿1 in cellulose, the following equation applies: ÉEI = E21 (9) Ew E 0 11 7906518-1 Decoration of the above equation 9 gives the following formula: a = E Ew1 (10) W (IE-T C By substitution with equations 5 and 7 one obtains from the above equation 10 the following relation nllw 3: - Åu 0 C 0 WI instead of the above formula 11 one can also write the following: -ln I7 \ 1: JHH _ see A3 O e IH> 1 I \) “” 12 aw ïšïšïïí <, J 7 \ 2 While in the above equation 11 the extinctions EW1 and E81 are previously known due to the obtained the measurement signals I Ä 1, I) \ 3 and I Ä 2, and in addition the absorption coefficient aw for water and ac for cellulose are previously known, the basis weight Mc of the cellulose is initially unknown. The basis weight Mc of the cellulose is determined - as initially stated already - by means of a separately performed measurement, in particular by means of a per se known β-radiation measurement. In such a β-radiation measurement, in addition to the basis weight of the cellulose, the content of water in the material in question is admittedly included, but due to the water's small percentage of the total basis weight of the material, the resulting error can still be neglected. With the use of mathematical iteration, especially with the help of a computer, this in itself insignificant error can be made no matter how small, practically to zero value. In other respects, this measurement is not affected by special peculiarities in the physical or structural structure of the material in question.

The basis weight of the water calculated in Equation 11 in a material to be measured is thus independent of the structural structure of the material in question, or changes in it, and is also independent of the basis weight of the material, or of its basis weight variations. The projected water basis weight therefore corresponds to the actual basis weight of the water.

On the basis of the above equation 11, it is now also possible POOR QUALITÜ 7906M84 12 to calculate the actual water content of material B, C or similar materials, since infrared radiation measurement has also been performed on materials such as materials B, C and the like in the field. for the absorption bands for water (7 \ 1) and for cellulose (7 \ 3), even at a relational wavelength (7 \ 2) on the basis of the measurement signals I Ä 1, I 7 \ 3 obtained in each particular case or I Ä 2.

Fibers, filler pigments and the like contained in the material intended for measurement entail, in addition to a greater or lesser extension of the path of the radiation through the material, also a more or less strong scattering of the infrared radiation used for the measurements.

The resulting radiation losses create further problems in the measurement. Fig. 8 shows roughly diagrammatically the course of the infrared radiation in a material D. The course of the radiation through the material D to the detector 8 corresponds approximately to the course of the radiation through the material C according to Fig. 7. In addition, a relatively strong scattering of the infrared radiation against fibers pigments and the like, with concomitant radiation losses due to dispersion.

Fig. 9 shows, in addition to a spectrogram for the material A according to Fig. 5, also a spectrogram for the material D according to Fig. 8. It can be seen from this that in the spectrograms as a rectilinearly presented spectrogram area next to the absorption bands at quantitative point of view equal to materials A and D is in case D bent downwards in relation to the corresponding area of material A. The bending of this spectrogram area of material D is due to the above-described losses of infrared radiation by dispersion in material D (see Fig. 8 ) and similar materials. As mentioned, these radiation losses are proportional to the fourth power of the wavelength.

From the spectrogram of the material D in Fig. 8 it appears without further ado that the calculation of the actual basis weight M of the water (water content) in the material D on the basis of Equation 11 with the measurement signals obtained during the measurement I 7 \ 1 »I 7 \ 3 and I4Ä 2 must lead to incorrect results. This is caused by the fact that due to radiation losses due to wavelength due to dispersion the measuring signal I D.2 at comparative wavelength can no longer be equated with the measurement signal lo mentioned in the introduction. This means that the measuring signal IAÃ 2 obtained at the comparative wavelength 7 \ 2 can no longer be used to convey the depth of the absorption bands at the wavelength 7 \ 1 and the wavelength 13 7906518-1; The expressions 3 I 7 \ I 7 \ -ln 1 and -ln 3 I7 \ 2 7 \ 2 thus no longer correspond to the actual extinction Ew and EW1 and EC and E01, respectively.

To solve the above problem when determining the actual values for the water content of the material with highly dispersing intrinsic axes of the substances in the material in question, using infrared radiation with an additional comparison wavelength, reference values are adapted which are adapted to the measurement wavelengths 7 \ 1. and 7 \ 3 which serve to determine the actual depth of the absorption bands at the measuring wavelengths Ä 1 and Ä 3.

When measuring dispersing material, such as the material D according to Fig. 8, in addition to the mentioned comparison wavelength 'Ä 2, an extra comparison wavelength' Ä 4 is introduced. This extra comparison wavelength Ä 4 is, like the comparison wavelength Ä 2, within the straight line. - linearly represented spectrogram area close to the absorption band at Ä 1 and Ä 3; compare with Fig. 9.

At the extra comparison wavelength Ä 4 a measurement signal Ä 4 is obtained. Using the measurement signals I 7 \ 2 and I) \ 4 at the comparison wavelengths Ä 2 and 7 \ ¿one can calculate the reference values Io) \ 1 and Io 7 \ 3, which are adapted to the absorption bands at the measuring wavelengths Ä 1 and? 3, respectively, of the material D and which lie on a base line acceptable as a reference line, the deflection caused by dispersion of the radiation being included. The calculation of these reference values Io 7 \ 1 and lo 7 \ 3 takes place approximately according to the following equation: H 10%,: cA2 + x1 (I> \ 2-I7 \ 4) (13) I07 \ 3 ~ I; \ 2 + x2 (17 \ 2-I7 \ 4) (14) The quantities X1 and X2 in equations 13 and 14 can be calculated as follows: 7 \ -? \ Än -Än X1 = Ä; _7 \: e11erx1 = Ä _-- 7 \ -ní_ n; (15) POOR QUALITY 79065134 14 nä - i An - Än; X2 = Ä2_ or X2 = Än: _ Äng (15) Since the course of the baseline depends on the selected scale and is seldom completely straight, the reference values thus calculated are only approximate. By introducing certain mathematical corrections, a further approximation to the correct reference value in each particular case can be achieved.

Thus, in order to determine the basis weight (water content) of water 1 of a material with considerable dispersion of the radiation, as in material D, the equation 12 corrected by equations 13 to 16 is given the following wording: yw izw = 4n ï ° 7 \ 1. f; . n -ln I 7 \ 3 aw Io) \ 3 The terms -inš-OÃ-l and -lnï 7 \ 3 7 \ 1 Io7 \ 3 practically correspond to the actual extinction EW1 and E01 at the radiation wavelength in question.

A measurement and calculation of the water content in paper or of the proportion of filler with an absorption band in a plastic foil or of a similar measuring problem can be solved on the basis of the above equation 17 with an hitherto unattainable accuracy, eliminating all the difficulties encountered in measuring , or at least greatly reduced, which difficulties arise from the basis weight or changes in basis weight of the material in question, from the structure or structural variations of the material and from how the material behaves in terms of dispersion. ^

Claims (3)

15 7906518-1 Patent claims: A method for non-contact measurement of the absolute content of a substance (bee substance) in a mixture of several substances (main substance and bee substance) in the form of a thin film, in particular for measuring the absolute water content of paper, in which method infrared radiation is used with a measuring wavelength A1, which is absorbed by the sub-substance, eg free (chemically unbound) water in the paper, and a second measuring wavelength A2, which is substantially absorbed neither by the secondary substance nor the main substance, the measuring wavelengths' radiation intensities being compared with each other after being affected of the substance to be measured, characterized in that infrared radiation is used with a measuring wavelength A3 absorbed by the main substance (base material), the radiation intensity of which is compared with the radiation intensity of the comparison wavelength A2, and beta radiation or similar is used in the structural properties of the main substance and that on the one hand a correction factor, which corrects for the effect of e.g. the structural properties of respiration on the measurement result relating to the absolute content of the bee substance and obtained by means of. infrared radiation with measuring wavelength A1 and comparative wavelength A2, is formed by comparing measured values obtained by measuring the absolute content of the main substance by measuring wavelength A3 and comparative wavelength A2 on one side and by beta radiation on the other side. Method according to claim 1, characterized in that in combination with the "first comparison wavelength A2 a second comparison wavelength A4 is used, which is also not absorbed by either the substance to be determined, eg the free water in paper, or by other substances in the material. in question, which second comparative wavelength is used to determine a reference value corrected with respect to the scattering of the measuring radiation in the material for the measuring signals IA and AA obtained by the measuring wavelengths A1 and A3; 3. Poon QUALITY