Optical property measurement system and method
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a division of application Ser. No. 543,902, filed Jan. 24, 1975. 
     The present application is a continuation-in-part of my copending application Ser. No. 429,637 filed Dec. 28, 1973, abandoned, and of my copending application Ser. No. 540,251 filed Jan. 10, 1975, and the written disclosure and drawings of each of these copending applications are incorporated herein by reference in their entireties. Said application Ser. No. 540,251 issued as U.S. Pat. No. 4,019,819 on Apr. 26, 1977. 
    
    
     BACKGROUND OF THE INVENTION 
     In the prior art it is known to obtain an indication of color and brightness characteristics of a paper web during manufacture by an on-line measurement of reflectance value (Rg), but this measurement is decidedly different from that necessary for actual color and brightness characterizations. Accordingly, such a measurement must be accompanied by very frequent off-line testing, so as to enable an adequate empirical calibration of the measuring instrument. Further, a separate set of calibration parameters is required for each grade and weight of paper. Off-line instruments which adequately measure these characteristics require that a pad of several thicknesses of paper be exposed to the light source aperture so that a different reflectance value (Roo) is obtained. Obviously this is impossible with an on-line instrument unless the far more inaccessible reel itself is tested. 
     Only where the on-line measured reflectance value (Rg) approaches the off-line value (Roo), as in instances of paper of extremely high opacity such as heavily coated or heavily dyed paper, can the above problems be minimized to the point where accuracy becomes sufficient for control purposes. 
     There is some possibility that the prior art includes the sequential measurement of two separate reflectance parameters such as one with a backing of near zero absolute reflectance and one with a white backing, but there appears to be no concept of a multiple property measurement system and method utilizing corresponding multiple sets of parameter measurements based on respective common spectral response functions and the prior art concept may be largely limited to a direct determination of Tappi opacity and/or to an abstract investigation of the feasibility of determining absolute reflectance (Roo) at a given wavelength such as 457 nanometers. 
     SUMMARY OF THE INVENTION 
     This invention relates to an optical device and method for sensing optical properties of a substantially homogeneous sheet material, and particularly to an on-the-paper-machine device and method for simultaneously sensing both transmitted and reflected light so as to obtain measurements from which the optical properties of interest can be calculated substantially independently of grade and weight of paper involved. 
     Accordingly it is an object of the present invention to provide an optical monitoring device and method for sensing optical properties based on measurements made on a single thickness of partially translucent substantially homogeneous sheet material and which measurements sufficiently characterize the actual properties of interest that a minimum of empirical calibration is required regardless of changes in grade and weight of paper. 
     Another object of the invention is to provide such an optical monitoring device and method capable of accurately sensing two or more optical properties such as brightness, color, opacity and/or fluorescent contribution to brightness preferably essentially continuously, or at least substantially simultaneously, and especially adapted for operation with a continuously moving web. 
     With such an optical monitoring device is useful off-line for sensing optical properties of a single thickness sample, it is a further important object of the present invention to provide such an optical monitoring device which is of sufficiently light weight and compact construction so as to be adapted for on-line monitoring of the desired optical properties. 
     Another and further object of the invention is to provide an on-the-paper-machine optical monitoring device of sufficient flexability and accuracy to enable control of desired optical properties during the paper making process. 
     A unique feature of the on-line optical monitoring device is its ability to essentially simultaneously measure both reflected and transmitted light. By measuring two independent optical parameters it is possible to thoroughly characterize the paper optical properties of a partially translucent web with a minimum of empirical correction for factors such as paper grade and weight. Preferably a plurality of sets of optical parameters are sensed continuously or at least substantially simultaneously. 
     Other objects, features and advantages of the present invention will become apparent from the following detailed description taken in connection with the accompanying drawings. 
     INCORPORATION BY REFERENCE 
     For a disclosure of a complete optical property measurement system relating to the present invention, reference is made to FIGS. one through twenty and the description thereof of U.S. Pat. No. 4,019,819 issued Apr. 26, 1977. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 through 20 are briefly described in U.S. Pat. No. 4,019,819 referenced above, at column 2, line 20 through line 64. 
     FIG. 21 is a diagrammatic view illustrating the embodiment described in the section entitled &#34;Proposed Instrument Design&#34; of the aforementioned copending applications; 
     FIG. 22 illustrates further details of the embodiment of FIG. 21 as described in the copending applications; 
     FIG. 23 shows an embodiment similar to that of FIGS. 21 and 22, but with the interior of the transmittance cavity providing the backing for the reflectance measurements; and 
     FIG. 24 shows an embodiment similar to that of FIGS. 21 and 22, but utilizing multiple sets of reflectance and transmittance optical light pipes. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     DESCRIPTION OF FIGS. 21 AND 22 
     An instrument 1010 is shown in FIG. 21 made up of two scanning sensing heads 1011 and 1012, one above and one below the moving paper web 14, and a dedicated computer with appropriate couplers for input and output forms part of digital computer system 1996, FIG. 22. The bottom head 1012 receives light transmitted through the sheet and subsequently analyzed for its X, Y, and Z tristimulus components. It also contains a backing 1135 of some specified effective reflectance (for example a black body of zero, or near zero, reflectance) located just ahead or behind (machine direction) relative to entrance 1154 of the transmitted light receptor compartment indicated at 1145-T, FIG. 22. 
     The upper head 1011 contains light source means 1201 as well as a reflected light receptor indicated at 1145-R, FIG. 22. The latter receives light energy after reflection from the moving web at a point just above the backing 1135, on the bottom head 1012 and the reflected light energy would also be analyzed for its X, Y, and Z tristimulus components. Both light receivers 1145-R and 1145-T and, for that matter, the light source means 1201 itself may be integrating cavities of a type. This would be one way to insure the uniform distribution of emitted, transmitted, and reflected light in the X-direction in addition to providing identical samples of light going to each photoelectric cell installed with filters within the cavities themselves, this arrangement being represented in FIG. 22. The flux of the light source means 1201 could be monitored or controlled by a third partial, or full, set of filter-photocell combinations. The availability of both the transmitted (T) and reflected (R g ) light signals described above and as indicated in FIGS. 21 and 22 allows for precise computation of the reflectance with an infinite backing (R oo ). It is the latter, R oo  value, which is required to characterize color, brightness, and an index of fluorescence. In addition, it would eliminate the need for any grade corrections in measuring either printing or TAPPI opacity, both of which could be made available if desired. 
     A small, rather low-cost, dedicated computer such as that forming part of system 1996 with appropriate interface equipment such as indicated at 1997 could be used to receive all signals, compute all pertinent optical properties, and determine the signal for direct, closed loop control of: 
     a. 2-5 separate conventional dye additions; 
     b. fluorescent dye feed to the size press; and 
     c. PKT, TiO 2 , or other slurry flow; 
     so that brightness, opacity, color (L, a,b) and fluorescence could be maintained almost exactly as chosen by, perhaps even a master computer, if desired. 
     Kubelka-Munk equations, quantitative color descriptions, and their inter-relationships, recently acquired wet end mathematical models, along with existing control theory, are all presently available in some form or other to convert the input signals from the scanning heads 1011 and 1012 to optical measurements and flow feeds with which paper manufacturers are familiar. The combined mathematical technology above is also sufficient for adequate decoupling of this otherwise complicated information so that overlapped control is avoided. 
     Use of dedicated computer would eliminate most of the electronics now associated with optical measuring equipment. It could also be used to integrate results across the web and simplify and/or maintain calibration. The package would lend itself to rather universal application and minimize the time and effort on the part of the purchaser. 
     The key feature of the instrument 1010 which distinguishes it from existing on-line optical testers, is that it provides for the measurement of both transmitted and reflected light without undue complications. This, in turn, can cause a great deal of improvements regarding sensitivity, accuracy, flexibility, and thoroughness of a continuous optical property measuring device. 
     DESCRIPTION OF FIGS. 23 and 24 
     FIGS. 23 and 24 show further means by which the incorporated patent applications can be employed without the use of a mechanically operated filter wheel. 
     In FIG. 23, an ultra violet light absorbing filter is located at 2271, and the U.V. filtered incident light beam 2133 is directed to impinge upon the moving web 14. 
     Transmitted light is collected within the integrating cavity 2145-T below the web, just as in the embodiment of FIGS. 1-6. The resulting intensity and color of this light is analyzed in a manner comparable to such embodiment by employing, for example, six separate ports containing six different filter-photocell combinations including filters such as 2281-T, 2282-T and 2283-T (and as shown in FIG. 22) representing Brightness, X R , X B , Z, Y C , and Y A , respectively. The output of each photocell could be read continuously or intermittently by means of a computer equipped with appropriate interface electronics as in the previous embodiments. 
     The reflected light component is directed through an appropriate lens system 2273 into a second integrating cavity 2145-R located above the moving web. Its intensity and color is, likewise, analyzed by the use of six filter-photocell combinations including filters such as 2281-R, 2282-R and 2283-R (and as shown in FIG. 22) and connected eventually to the same computer. 
     FIG. 24 is similar to FIG. 23 except that fiber optics (light conveying tubes) are utilized in place of the integrating cavities. A representative portion of the transmitted light is directed into each of six such tubes (five of the tubes being indicated at 3001-3005 in FIG. 24) which are ended by means of specific spectral response filters (Br, X R , X B , Z, Y C , Y A ,) and corresponding photocells, five of the filter-photocell combinations being indicated at 3011-3015. The same is done for the reflected light by means of filter-photocell combinations five being indicated at 3021-3025. All twelve outputs of this example are again read either continuously or intermittently by a computer. 
     The fluorescent component of light resulting from the use of &#34;optical brightners&#34; is not included in any of the above measurements. It can be measured, of course, by mechanically removing the U.V. incident beam filter and synchronizing such change with a status change to the computer. A short time later the U.V. filter is reinstated back into position. A rotating U.V. filter-chopper could be employed at 3271 providing proper synchronization of the computer data storage is also accomplished. 
     The set of twelve fiber optic light paths as shown in FIG. 24 may define six simultaneously operative reflectance light paths in upper sensing head 3011 and six simultaneously operative transmittance light paths in lower sensing head 3012. The six reflectance fiber optic paths include respective filters corresponding to filters 281-286 and respective individual photocells located to receive respective portions of the reflected light which is reflected generally along a path such as that indicated at 137 in FIG. 3. The six transmittance fiber optic paths would also include respective filters corresponding to filters 281-286 and respective individual photocells located to receive respective portions of the transmitted light which is transmitted generally along paths corresponding to paths 141-143 in FIG. 3. The filter means in the incident light path includes a filter 3271 similar to filters 271 and 272 for filtering out the ultraviolet component from the incident beam, so that the twelve simultaneous photocell readings corresponding to those designated RSD1 through RSD6, and TSD1 through TSD6 (when the device is off-sheet), and corresponding to those designated RSP1 through RSP6, and TSP1 through TSP6 (when the device is on-sheet), will exclude a fluorescent contribution. (See Table 3 where this notation is introduced in U.S. Pat. No. 4,019,819.) 
     If a reflectance reading corresponding to RSD7 (when the device is off-sheet) and corresponding to RSP7 (when the device is on-sheet) is desired so as to enable computation of fluorescent contribution to brightness, it is necessary to mechanically remove the ultraviolet filter 3271 from the incident light path, or otherwise introduce an ultraviolet component of proper magnitude, and obtain another brightness (Z), reading, for example from the number four reflectance photocell. 
     As an alternative to the above filter optic system with a common incident light path as shown in FIG. 24, seven fiber optical tubes incorporating filters corresponding to 281-287 of FIGS. 3 and 4, respectively, at say the light exit points of the tubes, could be used to supply the incident light to seven different points on the paper web. The reflected light from each of these seven points could be monitored by seven different systems, each involving lenses and a phtocell such as shown in FIG. 24, and the number seven reflected light path including also a filter corresponding to filter 288, FIG. 4. The transmitted light from the first six points would also need to be kept separately, and this could be accomplished by six integrating cavities and six photocells. 
     As a further alternative the seven fiber optical tubes defining the seven incident light paths could have a second set of seven filter optical tubes and photocells respectively disposed to receive reflected light from the respective illuminated points as in FIG. 24. Another set of six fiber optical tubes and photocells could be associated with the first six illuminated points for receiving transmitted light as in FIG. 24. This could eliminate the need for the light collecting lenses in the upper sensing head and the integrating cavities in the lower sensing head. 
     The last two mentioned alternatives with seven fiber optical tubes defining the incident light paths appear to be rather complicated systems, but they do offer means of eliminating both the mechanical filter wheel as well as any mechanical device to control the presence of ultraviolet light in the incident beam. 
     Still another alternative is to use &#34;screens&#34; in addition to the filters in the embodiments of FIGS. 21-24. The new photodiodes are considered sensitive enough to measure reduced light intensities so that screens with different transmittance values could be used with six of the incident beam filters or with the reflectance and transmittance filters so that the net photocell output for each reflectance light path, and for each transmittance light path, would be similar enough so that separate and individual pre-amplification for the respective reflectance outputs would not be necessary, and so that separate and individual preamplification for each transmittance output would not be necessary. This means that the feedback paths for the twelve amplifiers of components 1361-R and 1361-T in FIG. 22, for example, could have the same resistance values. 
     The term &#34;screen&#34; is understood in the art as referring to a network of completely opaque regions and intervening openings or completely translucent regions, such that light energy is uniformly attenuated over the entire spectrum by an amount dependent on the proportion of opaque to transmitting area.