CA1075034A - Optical property measurement and control system - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
In an illustrated embodiment, brightness, color, opacity and fluorescent contribution to brightness are measured by an on-line sensing head providing for simultaneous measurement of transmitted and reflected light. By measuring two independent optical parameters, paper optical properties of a partially translucent web are accurately characterized substantially independently of paper grade and weight.
The instrument is designed so as to be capable of transverse scanning of a moving paper web on the paper machine, and so as to monitor desired paper optical characteristics with sufficient accuracy to enable on line control of the optical characteristics of the paper being manu-factured.

Description

~07S034 The invention relates to the obtaining of a quantative measure of an optical property, specifically colour or brightness of a sheet material, particularly, but not exclusively, paper, either on or off a machine by which the material is manufactured or treated.
The invention thus provides apparatu for measuring colour or brightness of a sheet material, the apparatus comprising receiving means for receiving the sheet material, and an optical system so arranged that light characterizing the colour or brightness of sheet material at the receiving means is directed from sheet material so received along first and second paths to photometric sensor means, said optical system comprising means for separately transmitting respective narrow bands together covering the spec-trum required for characterizing colour or brightness.
The invention also provides a method of measuring colour or brightness of a sheet material, the method having the step of receiving light from the sheet material along first and second paths by photometric sensor means, the light being modified so that the light received character-izes the colour or brightness of the sheet material, in which the light is transmitted in separate narrow bands together covering the spectrum required for characterizing colour or brightness.
In a preferred embodiment, filter means are located in an incident path between a light source and the receiving means.

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Instead of using a single filter, use for example of a series of ~ narrow-band filters, provides signals from the sensor n~ans which per-mit a computer to simLlate brightness or colour spectral distribution, by means of weighted ordinate integration.
m e invention will be more readily appreciated from the following illustrative description and accompanying drawing, in which:
Fig. 1 is a schematic part-sectional side view of an apparatus embodying the invention; and Figs. 2 and 3 are schematic plan views of optical filter ~P~n.c included in the apparatus of Fig. 1.
Figs. 1-3 illustrate an instrument intended primarily but not exclusively for use independently of a paper machine. It is contem~
plated that in off-machine use this instrument will enable the develop-ment of the relationships between off-machine specification and the on-line instrument described in detail in the specification, and shown in Figs. 1-20, of British Specification 1 498 417 (Serial No. 216125 -Canada~. mese relationships would include the "grade-correction"
factors to be used in the on-line system of Figs. 1-2Q of British Specification 1 498 417 relative to off-machine optical specifications.
m e instrument of the present Figs. 1-3 is shown as including a specimen support 1000 having an aperture 1001 which may conform in ter with the ape~ture 130 shown in Fig. 3 of said specification 1 498 417. The web support lOG0 is of extended area so as to be capable of conveniently supporting a full-width web and for adjustment of such web to expose successive portions thereof at the aperture 1001.
At the s æ ti~, the support 1000 will accommodate a small size paper specimen such as indicated at 1002. Generally the hnusing for the ~ A~

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-~ 107S034 optical components will conform with the housing 11 of the on-line instrument of said specification 1 498 417 from the standpoint of light proofing and interior finish.
The optical system as ~i~grammatically indicated includes a light source means 1010 and the lenses 1011-lQ16 generally having the charac-teristics of the lenses 202, 273 and 274 of the on-line instrument of said specification 1 4~8 417 and such that the spectral response of the system can duplicate that of that instrument. The illustrated optical system further includes a fixed lamp socket lQ20 and an iris diaphragm 1021 for attenuating the incident light beam.
Fig. 1 illustrates also a transmittance sensing head lQ25 which may be hingedly secured to the support lQOQ at a single corner so as to m;n;nn~e the obstruction provided to movement of a paper web over the support surface 1000. The transmittance sensing head lQ25 may include an optical window 1026 of the s æ diameter, thickness and physical composition and characteristics as the window 135 of the on-line instrument of said specification 1 4~8 417. As illustrated, the lower surface of window 1026 may directly contact the paper specimen `- 1002 which will be in smooth continuous contact therewith over the optical viewing area of the system which may he of the same d~mensions as that described with respect to the Prior on-line embod~ment. The sensing head 1025 may comprise a light integrating cavity 1028 and a transmittance sensing light photocell 103Q.
It will be understood that the reflectance and transmittance light paths have the incident path in ccmmon, and that in the illustra-ted embodiment the transmittance light path irto the integrating ' ' . '.
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cavity 1028 may confoDm with those described with respect to the on-line instrument of said specification 1 498 417. Also, the reflectance light path generally conforms with that of the on-line instrument and includes a reflectance photocell 1032. The photocell 1032 may have a plate 1033 with a 3/8-inch aperture and may conform with the pla_e of the on-line instrument. Thus, a piece of diffusing glass corresponding to the glass oE the on-line instrument may be located in the aperture so that the light distribution over the surEace of the photocell 1032 will conform to the light distributi~
with respect to the surface of photocell 203 in the on-line ,'nstrument.
The instrument of Fig. 1 further includes an incident light filter disk 1040 and a reflectance filter disk 1041. The disks may be provi~ed with iOw torque motors 1042 and 1043 which may operate essentially as described with respect to the motor 209 of the on-line instrument. Both filter wheels 1040 and 1041 are under constant torque from a motor and slip-clutch arrange~ent. Each is prevented from turning by a stop pin seated in a small hole in the w~.eel. Each of the twenty-one filters in a wheel has a corresponding hole. The wheel rotates whenever a solenoid pulls the pin clear of the wheel.
It stops again after the pin is dropped and a new hole ccms under the pin allowing it to seat. It will be apparent that the indexing of the filter wheels 1040 and 1041 may be controlled fram an on-litte camputer in the same manner as generally described with respect to the on-line instrument.
Figs. 2 and 3 diagrammatically indicate the respective filters 1101-1121 and 1201-1221 of the filter disks 1040 and 1041. The ' , . .

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.~ -filters 1101-1106 may conform identically to the incident heam filters of the on-line instrument while filter position 1107 may be open and free of a filter. In this case, the complete number seven filter of the on-line embodiment can be located at position 1207 in the reflected beam. The filters 1108-1121 and 1208-1221 comprise interference type narrow-band filters which ; together transmit the complete visible spectrum.
By way of example each of the filter diameters may be 3/4-inch. The reflectance and transmittance photocells 1032 and lQ30 may be of the Schottky Silicon Photodiode type. Amplifiers for each of the photodiodes can be the Analog 234 ~ and AD 741 C. Two digital volt ` meters can be used, one for reflectance and the other , 15 for transmittance and these may be 3 1/2 digit, 0.200 millivolt instruments with 8-4-2-1 ~CD positive logic . outp~t.
As indicated at 1040a, a portion of each of the filter wheels lQ4Q and 1041 is preferably exposed - 20 outside of the case so that the number of the filter in the optical train can be observed directly by the operator.
~ The filter wheel arrangement accommodates a manual place-:~ ment of both wheels to any position desired. Thus manual ~; means is provided for unlocking the solenoid operated pin for each of the wheels, whereupon the wheels may be ; manually manipulated at the exposed region such as 1040a.
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The ability for any operator to test a machine wide strip by moving it either left to right or right to left is desirable and is accommodated by the illustrated arrangement.
By providing at least one open position in the reflec- -tance filter wheel, such as a position 1207, it will be apparent that the filter wheels 1040 and 1041 may provide the seven reflectance measurements and the six transmit-tance measurements with respect to the paper specimen 1002 in precise conformity with the corresponaing measurements of the on-machine instrument. Reflectance and transmit-. tance values could be obtained by the on-line computer from simultaneous readings of the photocells 1030 and 1032, -~
or the readings could be taken separately. Since the sensing head 1025 is conveniently removable, the ir.strument of Fig. 1 can also measure thickpad reflectivities, Roo.
The basic design consideration is that the reflectivity value determined on a thickpad would be in agreement with ' the established scale and that the thickpad reflectivity ''f 20 calculated from a reflectance and transmittance measurement made on a single sheet would be in agreement with the direct--ly measured value. To accompiish this objective, the four-teen narrow-band filters 1108-1121 and 1208-1221 are employed - -to obtain data permitting calculation of the thickpad reflectivity through the weighted-ordinate integration approach. nlters of identical kind could be introduced in the incident and r~ected ,~ - .

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' '~, ', '' ' '' ' ' 1075~34 beam. Transmittance and reflectance measurements would be performed with the open hole position such as 1207 in the reflected beam and the filter disk 1040 located through its various positions in the incident beam. The open hole 1107 in 5 the incident beam could then be used with filter disk rotation - in the reflected beam. In this way, fluorescence appearing in any part of the spectrum could be handled properly.
The scope of the program presently under way includes construction of the instrument as shown in Figs. 1-3 and testing of its operation to insure that it performs in accordance with the basic objective predicting thickpad reflectivity via the fourteen narrow-band filter and weighted ordinate integration approach.
The results of a feasibility study conducted at The Institute of Paper Chemistry in which an Automatic Colour-Brightness Tester equipped with sixteen narrow-band filters was ` employed to obtain Ro and Roo values and the General Electric Recording Spectrophotc1meter was employed to obtain transmittance data indicate the success of this approach in calculating the thickpad reflectivity compared to the directly measured values. This work was conducted in connection with the related Application 55 170/74 (Canadian Serial No. 229,679~ and is set forth in the Appendix hereto.
Instead of transmitting light from the light source through a series of narrow band filters such as the filters 1108-1121 and 1208-1221, it is possible to employ monochromatic or approxi-mately monochromatic light, the frequency of which is varied stepwise or is swept continuously between desired limits for :1~';'5034 example those of the visible spectrum. Thus the light source 1010 can be replaced by a source of substantially monochromatic light, the frequency of which can be varied over the desired range in discrete steps or which can be swept over this range without discontinuity, the variation being effected in either case either manually or in a predetermined manner on initiation of a preset programme. The cell output signal may be treated - - --in the manner previously described.
Although the illustrated embodiment of the invention employs two photocells, it is instead possible to employ only a single cell to which both reflected and transmitted light from the sheet materiaL is directed. The single cell can be arranged to respond to light on the transmittance and reflectance light paths, either or both of which can include light fibre optics, in alternation. Instead of locating the cell receiving reflected light on the same side of the sheet material as the light source, both cells, or a single cell receiving both transmitted and reflected light either alternately or in combination, can be located on the other side from the light 20 source. Transmitted light is then made to impinge on a semi- -~
;reflecting window, so that the light reflected from the sheet material is transmitted light reflected from the window and re-reflected from the material.
It will thus be evident that the invention can be embodied 25 in a variety of ways other than as specifically described and illustrated -within the scope, which is defined by the following claims.

~075034 APPENDIX
FEASIBILITY STUDY FOR THE DESIGN AND CONSTRUCTION OF
A LABORATORY INSTRU~ENT BASED OM TI~E PRINCIPLE OF OMOD

The ACBT equipped with the sixteen narrow-band filters was used to obtain Ro and Roo values for six of Nekoosa Edwards paper samples. Transmittance data for the same specimens were obtained using the conventional GERS. Roo values were calculated using the following formulas.

a = (1 + Ro2 - T2)/Ro Roo = (a/2) -J(a/2)2 - 1 The values for R, Ro, Roo measured, Roo calculated values are given in Table I. The data show reasonable agreement between the measured and calculated Roo values. There are several factors which contribute to the differences.
Fluorescence was not properly accounted for and some of the samples do fluoresce, particularly Samples 18 and 29. The samples were illuminated with a collimated beam whereas the theoretical relationship is based on diffuse illumination and diffuse viewing. The samples do change somewhat with handling as a large number of readings must/be taken on each specimen.-The same specimens were evaluated on filters No.- 6 and 21 after all the data were collected. The data given-in Table III
show that some changes occurred as a result of handling during the many tests.
Tristimulus values were calculated from the Roo values obtained from the T and Ro values using the weighting factors given by the CIE system. These tristimulus values were then . . ~ .

compared with the directly measured tristimulus values obtained on the ACBT using the "tristimulus filters". The data, given in Table II show good agreement for most of the samples. Here again the same factors discussed earlier are responsible for the differences. In addition, the broad-band tristimulus functions of the ACBT no doubt differ slightly from the theoretical functions. It appears that sample 29 (cherry bond) shows the largest discrepancy.
It appears feasible to design and construct an instrument similar to OMOD but also equipped with narrow-band filters which would give very nearly the correct tristimulus values in either mode. Perhaps the reasons for the discrepancies noted could be determined and further improvements made.

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`- ` 107so34 Table I
Sample T Ro RINFM RINFC RINFM-RINFC
6-3 0.0940 0.61800.6250 0.6270 -0.0020 618 0.1070 0.47300.4740 0.4801' -0.0061 620 0.1150 0.55900.5700 0.5701 -0.0001 623 0.0420 0.44400.4450 0.4450 0.0000 629 0.0250 0.29400.2940 0.2942 -0.0002 :~ 630 0.0110 0.23300.2340 0.2330 0.0010 7-3 ' 0.1120 0.69200.7120 0.7095 0.0025 718 0.1270 0.76100.7880 0.7933 -0.0053 ' 720 0.1350 0.59300.6100 0.6104 -0.0004 ,. 723 0.0420 0.44400.4460 0.4450 0.0010 .~ 729 0.0280 0.31100.3110 0.3113 -0.0003 " 730 0.0090 0.219'00.2210 0.2190 0.0020 8-3 0.1350 0.72700.7580 0.7578 0.0002 ' 818 0.1460 0.8200. 0.8740 0.8903 -0.0163 820 0.1500 0.6160 '0.6400 0.6398 0.0002 823 0.0490 0.46400.4660 0.4654 0.0006 ' 829 0.0350 0.33000.3300 0.3305 -0.0005 , 20 830 0.'0120 0.23900.2400 0.2390 0.0010 9-3 0.1480 0.73700.7760 0.7768 -0.0008 918 0.1550 0.80500.8660 0.8765 -0.0105 920 0.1610 0.62400.6530 0.6525 0.0005 ' 923 0.0630 0.49100.4940 0.4936 Ø0004 929 0.0240 0.2810 '0.2810 0.2812 -0.0002 : 930 0.0170 0.27000.2710 0.2701 0.00~9 , 10-3 0.1600 0.74600.7980 0.7962-0.0018 ' 1018 0.1650 0.79800.8700 0.8778-0.0078 !
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~075~34 1020 0.1740 0.6380 0.6740 0.6738 0.0002 1023 0.1090 0.5820 0.5930 0.5928 0.0002 1029 0.0130 0.2090 0.2090 0.2090 -0.0000 1030 0.0430 0.3650 0.3690 0.3658 0.0032 511-3 0.1660 0.7520 0.8110 0.8089 0.0021 1118 0.1680 0.7950 0.8740 0.8766 -0.0026 ` 1120 0.1750 0.6390 0.6750 0.6754 -0.0004 1123 0.1310 0.6260 0.6440 0.6445 -0.0005 1129 0.0090 0.1590 0.1590 0.1590 -0.0000 101130 0.0860 0.4700 0.4760 0.4745 0.0015 12-3 0.1680 0.7480 0.8090 0.8051 0.0039 1218 0.1730 0.7910 0.8760 0.8766 -Q.0006 1220 0.1670 0.6210 0.6510 0.6515 -0.0005 1223 0.1300 0.6170 0.6350 0.6346 0.0004 151229 0.0100 0.1400 0.1400 0.1400 -0.0000 1230 0.1220 0.5400 0.5530 0.5517 0.0013 ` 13-3 0.1680 0.7420 0.8000 0.7971 0.0029 ~ 1318 0.1740 0.7890 0.8760 0.8744 0.0016 -~ 1320 0.1520 0.5950 0.6170 0.6176 -Q.0006 ~ 201323 0.1170 0.5920 0.6040 0.6049 -0.0009 j 1329 0.0110 0.1360 0.1360 0.1360 -0.0000 1330 0.1520 0.5960 0.6200 0.6186 0.0014 - 14-3 0.1700 0.7350 0.7900 0.7893 0.0007 .i 1418 0.1770 0.7880 0.8760 0.8769 -Q. OOQ0 251420 0.1280 0.5480 0.5600 0.5613 -0.0013 1423 0.0910- 0.5400 0.5470 0.5464 0.0006 1429 0.0160 0.1510 0.1510 0.1510 -0.0000 1430 0.1860 0.6450 0.6900 0.6878 0.0022 .
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1~75~34 Table I continued Sample T Ro RINFM RINFC RINFM-RINFC
15-3 0.1680 0.7320 0.7870 0.78390.0031 1518 0.1800 0.7870 0.8780 0.8796--0.0016 1520 0.1000 0.5000 0.5060 0.5068-0.0008 1523 0.0690 0.4870 0.4900 0.4901-0.0001 1529 0.0470 0.3230 0.3250 0.32380.0012 1530 0.2290 0.7010 0.8000 0.79520.0048 16-3 0.1720 0.7330 0.7910 0.78820.0028 1618 0.1810 0.7880 0.8840 0.88320.0008 1620 0.0820 0.4560 0.4600 0.45990.0001 1623 0.0530 0.4370 0.4380 0.4385-0.0005 1629 0.1680 0.6070 0.6400 0.63630.0037 1630 0.2430 0.7220 0.8610 0.85320.0078 17-3 0.1770 0.7400 0.8080 0.80180.0062 1718 0.1840 0.7880 0.8920 0.88820.0038 1720 0.0680 0.4220 0.4240 0.4244-0.0004 1723 0.0410 0.3960 0.3960 0.3968-0.0008 1729 0.2400 0.7140 0.8240 0.8321-0.0081 1730 0.2470 0.7240 0.8790 0.86540.0136 18-3 0.1870 0.7480 0.8290 0.82280.0062 1818 0.1880 0.7900 0.9020 0.90010.0019 1820 0.0660 0.4170 0.4190 0.4192-0.0002 1823 0.0380 0.3880 0.3880 0.3887-0.0007 1829 0.2520 0.7300 0.8780 0.8928-0.0148 1830 0.2510 0.7240 0.8850 0.87390.0111 19-3 0.1950 0.7560 0.8530 0.84500.0080 ;

~ -12-lV75034 1918 0.1930 0.79000.9140 0.9111 0.0029 1920 0.0710 0.42300.4250 0.4256 -0.0006 1923 0.0410 0.3930-0.3940 0.3938 0.0002 1929 0.2580 0.73000.8980 0.9111 -0.0131 ~; 5 1930 0.2550 0.72300.8900 0.8806 0.0094 20-3 0.2030 0.76400.8790 0.8715 0.0075 `` 2018 0.19~30 0.7930O.g270 0.9345 -0.0075 2020 0.0660 0.41100.4130 0.4132 -0.0002 2023 0.0400 0.38400.3840 0.3847 -0.0007 2029 0.2610 0.72800.9080 0.9142 -0.0062 2030 0.259~ 0.72400.8960 0.8940 0.0020 `~ 21-3 0.2110 0.76300.8960 0.8838 0.0122 , , ` 2118 0.2020 0.78900.9340 0.9337 0.0003 `~ 2120 0.0900 0.46500.4770 0.4699 0.0071 2123 0.0560 0.44100.4480 0.4427 0.0053 `~.i 2129 0.2610 0.72400.9110 0.8999 0.0111 ...~
,~ 2130 0.2600 0.71800.8980 0.8792 0.0188 ~'`y T Transmittance measured with GERS
'r~ Ro Reflectance with black backing measured on the ACBT
~ 20 RINFM Reflectance of opaque pad measured on the ACBT
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RINFC Reflectance of opaque pad as calculated from Ro and T
RINFM-RINFC Difference between the measured and calculated ,, Roo values ;:~ Sample The first number (6 through 21) designates the filter ~':-?
number. The last two characters designate the ' sample number.

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1~75034 Table II
X Y Z
Sample C M C M C M
3 17.7 77.9 79.6 79.3 90.4 90.6 18 86.5 86.3 88.0 87.6 102.4 102.5 49.3 50.0 55.1 52.2 76.4 76.3 23 45.0 45.3 52.8 52.9 59.7 59.9 29 52.2 50.9 34.2 31.5 33.0 31.8 71.1 69.9 68.5 68.2 34.0 33.9 C Values calculated from narrow-band filter data.
M Values determined using the "tristimulus filters".
Sample Description 3 Advantage offset wave 50 lb.
18 S-20 Nekoosa Bond 20S-20 Nekoosa Bond Blue 23 S-20 Nekoosa Bond Green 29 S-20 Nekoosa Bond Cherry 30S-20 Nekoosa Bond Buff -:
Table III

Change in the Measured Roo Values with Handling for No. 6 and 21 Filters on the ACBT
No. 6 Filter (401 nm) 3 18 20 23 29_ 30 Start of test 0.625 0.474 0.570 0.445 0.294 0.234 End of test0.623 0.465 0.570 0.445 0.293 0.234 ; No. 21 Filter (697 nm) Start of test 0.896 0.934 0.477 0.448 0.911 0.898 End of test0.889 0.927 0.474 0.446 0.895 0.898 ... . - . - , -, - .

Claims (22)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Apparatus for measuring colour or brightness of a sheet material, the apparatus comprising receiving means for receiving the sheet material, and an optical system so arranged that light characterizing the colour or brightness of sheet material at the receiving means is directed from sheet material so received along first and second paths to photometric sensor means, said optical system comprising means for separately transmitting respective narrow bands together covering the spectrum required for characterizing colour or brightness.

2. Apparatus as claimed in claim 1 having electro-mechanical means for rendering effective a selected one of a series of narrow band filters from a remote location.

3. Apparatus as claimed in claim 2 having means for manually overriding the electro-mechanical means.

4. Apparatus as claimed in claim 1 in which filter means are located in an incident path between a light source and the receiving means.

5. Apparatus as claimed in claim 1, with filter means comprising two series of narrow band filters cover-ing the complete visible spectrum, one series being selectively locatable in an incident path between a light source and the receiving means and the other series being selectively locatable in one of the first and second paths which one path conducts reflected light from the receiving means to a reflectance measuring photometric sensor means.

6. Apparatus as claimed in claim 5, the other of the first and second paths being operable to conduct transmit-ted light from the receiving means to a transmittance measuring photometric sensor means, and the one series of narrow band filters in the incident path serving to control the spectrum of the light incident on the sheet material at the receiving means which is then either reflected via the one path or transmitted via the other path.

7. Apparatus as claimed in claim 1 with the optical system being so arranged that approximately monochromatic light is directed from sheet material at the receiving means along first and second paths to photometric sensor means, the optical system including means for changing the wavelength of the substantially monochromatic light continuously or in steps through a predetermined range.

8. Apparatus as claimed in any one of claim 1 or 7 having computer means connected with the sensor means.

9. Apparatus as claimed in claim 1 in which a filter is provided such that light on the first and second light paths has a spectral response characteristic with an effective wavelength of substantially 457 nanometers.

10. Apparatus as claimed in claim 1 in which a filter is provided such that light on the first and second light paths has a spectral response characteristic substantially corresponding to the standard brightness spectral distribution of light energy.

11. Apparatus as claimed in claim 9 or 10 in which the filter is located in an incident path between a light source and the receiving means.

12. Apparatus as claimed in claim 9 or 10 having computer means connected to the sensor means and arranged to compute on the basis of the sensor means output a quantitative brightness indication.

13. Apparatus as claimed in claim 1 or 7 in which the first and second light paths are respectively reflectance and transmittance light paths.

14. Apparatus as claimed in claim 1 or 7 in which the sensor means comprises first and second photometric sensor devices receiving respectively the first and second light paths.

15. Apparatus as claimed in claim 1 or 7 in which the sensing means comprises a single photometric device.

16. Apparatus as claimed in claim 15 in which at least one of the light paths includes a fibre optic element.

17. Apparatus as claimed in claim 15 having means for rendering the output of the single photometric device responsive to light on the first and second light paths alternately.

18. A method of measuring colour or brightness of a sheet material, the method having the step of receiving light from the sheet material along first and second paths by photometric sensor means, the light being modified so that the light received characterizes the colour or brightness of the sheet material, in which the light is transmitted in separate narrow bands together covering the spectrum required for characterizing colour or brightness.

19. A method according to claim 18 in which the light is approximately monochromatic and is modified by stepping or sweeping the wavelength through a predeter-mined range.

20. A method as claimed in claim 18 or 19 in which the predetermined range corresponds substantially with the visible spectrum.

21. A method as claimed in claim 18 or 19 including the step of treating the sensor means output in a digital computer.

22. A method as claimed in claim 21 in which the computer computes thickpad reflectivity by weighted ordinate integration.