Means of enhancing the sensitivity of a gloss sensor

A gloss sensor for optically measuring the gloss or reflectance of a surface. The gloss sensor includes a light source of emitting a light beam toward a surface to be measured. The gloss sensor also includes a light detecting device arranged to preferentially detect the intensity of the diffusely reflected component of the reflected light beam relative to the specularly reflected component.

BACKGROUND OF THE INVENTION 
The present invention relates to a method and device for optically 
measuring the gloss or reflectance of a surface, and more particularly, to 
a gloss sensor having increased sensitivity to changes in the gloss of a 
surface and to a method for increasing the sensitivity of gloss sensors. 
One of the parameters used in determining the quality of a particular 
surface is the gloss of the surface. For example, in the papermaking 
industry, various grades of paper having different surface gloss qualities 
are produced to suit various applications. During the production of paper 
sheet, it is often desirable to continuously or periodically measure the 
gloss of the surface of the sheet to ensure that the surface of the sheet 
being produced has the desired gloss. Typically, the gloss of paper is 
measured with a gloss sensor during the last step of paper production just 
before the produced sheet is packaged in the form of rolls. 
Known devices for determining the gloss of surfaces include, for example, 
an optical system which measures the intensity of a beam of light 
reflected from the surface under test. Typically, the gloss of such a 
surface is determined by comparing its total reflectance (specular and 
diffuse) to the reflectance of a gloss standard, such as, for example, a 
glass tile having a polished surface of a known gloss. "Specular 
reflectance" refers to reflections of the type produced from a polished, 
perfectly flat surface or mirror. "Diffuse reflectance", on the other 
hand, refers to scattered reflections produced by irregular surfaces. 
Because no actual surface is perfectly specularly reflective, reflections 
from any real surface will have both a specular component and a diffuse 
component. 
Specifically, in measuring the reflectance of a surface, an incident beam 
of light is first projected onto the surface from a light source and a 
resulting reflected beam, having both specular and diffuse reflection 
components, is reflected from the surface. A light intensity detector is 
positioned to receive the reflected beam and measure its intensity. To 
determine the gloss of the surface, the intensity of the reflected beam is 
compared with a known intensity value, such as the intensity of a 
reflected beam which results when the incident beam from the light source 
is reflected from a polished glass tile having a known gloss. This 
comparison can be accomplished with known electronic devices. 
Typically, a gloss sensor viewing angle is fixed. That is, the angle which 
the incident and reflected beams make with the normal to the surface plane 
remains constant during the reflectance measurement of that surface. 
Different angles, with respect to the normal to the surface plane, are 
used for different sheet reflectivity measurements, with smaller angles 
being employed for more reflective surfaces. For reflectance measurements 
of paper, the standard angles, with respect to the normal to the plane of 
the paper, are 20.degree. for paper having a high reflectivity surface and 
75.degree. for paper with a low reflectivity surface. 
SUMMARY OF THE INVENTION 
The present invention is directed to a gloss sensor having a relatively 
high sensitivity to changes in the gloss of a surface being examined by 
the sensor. The invention includes a gloss sensor having a reflected light 
intensity detector constructed such that the detector is preferentially 
sensitive to diffuse reflection from the surface, as opposed to specular 
reflection from the surface. 
In one embodiment of the present invention, preferential sensitivity to 
diffuse reflection is obtained by placing the light intensity detector in 
the path of the reflected beam and then positioning a light attenuating 
device, such as a filter, in the path of the reflected beam between the 
surface and intensity detector. The light attenuating device is disposed 
such that it preferentially attenuates only the central portion of the 
reflected beam containing the specularly reflected light. The diffusely 
reflected light toward the outer edges of the reflected beam is allowed to 
pass by the attenuating device, and falls, essentially unattenuated, upon 
the intensity detector. 
By the attenuation of the relatively intense specular reflection, the 
intensity detector of the gloss sensor exhibits an increase in sensitivity 
to changes in the intensity of the relatively less intense diffuse 
reflection. That is, the change in the detected intensity of reflected 
light is proportionately greater, for a given change in gloss, when the 
detected reflection is primarily the relatively low intensity diffuse 
reflection rather than an overall measurement of reflection intensity 
resulting from an unbiased combination of the diffuse reflection and the 
relatively high intensity specular reflection. The above construction 
thereby affects an increase in the overall sensitivity of the gloss sensor 
to changes in the gloss of the surface being examined.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The following description is of the best presently contemplated mode of 
carrying out the invention. This description is made for the purpose of 
illustrating the general principles of the invention and should not be 
taken in a limiting sense. The scope of the invention should be determined 
by reference to the accompanying claims. 
Refering to FIG. 1, a light source 1 is shown emitting a beam of light 2 
along an optical axis 3. The light source 1 may comprise, for example, a 
tungsten halogen lamp, or any other suitable means for emitting light. 
Also shown in FIG. 1 is a light intensity detector 5. This light detector 
5 may comprise, for example, a photodiode, or any other suitable means for 
detecting light and producing a signal indicative of the intensity of the 
detected light. The light source 1 and the light intensity detector 5 
comprise elements of a fixed angle gloss sensor (having fixed angles 
.alpha.) which may be used to measure the gloss of a continuous paper 
sheet 14 being manufactured by a papermaking machine (not shown) in a 
paper mill. In FIG. 1 the paper sheet 14 is moving in the direction of 
arrow 20. 
The light detector 5 is operatively connected along line 13 to a circuit 
14, which may include a suitably programmed computer. The circuit 14 
receives the signal from the light detector 5, evaluates the signal by 
comparing the intensity of the signal to the intensity of a signal which 
would be received from a known gloss standard, and then provides an output 
indicative of the gloss of the paper surface 4 then being evaluated. In 
certain situations, the circuit 14 may include a process control computer 
which can control the process producing the paper sheet so as to obtain a 
desired sheet gloss. Circuits and devices for performing the functions of 
evaluating signal intensities and controlling the sheet gloss are known in 
the paper manufacturing industry and, therefore, in an effort to simplify 
the present disclosure, will not be described in further detail herein. 
The light source 1 and the light detector 5 are arranged to be at equal 
angles .alpha. with respect to the normal N to the surface 4. That is, the 
light source 1 is arranged such that the optical axis 3 thereof forms an 
angle .alpha. with the normal N, while the light detector 5 is arranged 
such that the optical axis 6 thereof also forms the same angle .alpha. 
with the normal N. Furthermore, the optical axis 3 of the light source 1 
and the optical axis 6 of the light detector 5 are positioned in a common 
plane with the normal N to the surface 4. In this manner, a light beam 2 
emitted from the light source 1 is reflected from the surface 4 and 
received by the light detector 5. 
The emitted beam 2 upon reflection from the surface 4, forms a reflected 
beam 7. Due to the above described arrangement of the light source 1 and 
the light detector 5, the incident beam 2, projected along the optical 
axis 3, is reflected from the surface 4 and forms a reflected beam 7 along 
the optical axis 6. As shown in FIG. 1, the reflected beam 7 is composed 
of a central specularly reflected beam component 8 and an outer diffusely 
reflected beam component 9, both of which share the common optical axis 6. 
In the embodiment of FIG. 1, a light attenuating device 10 is arranged 
between the surface 4 and the light detector 5. The light attenuating 
device 10 is arranged to be substantially centered with respect to the 
optical axis 6. In this way, part or all of the central specularly 
reflected beam 8 can be attenuated before encountering the light detector 
5. In the FIG. 1 embodiment, the light attenuating device 10 is shaped and 
arranged only to attenuate the specularly reflected central beam 8. In 
this regard, the diffusely reflected outer beam 9 is received 
substantially unattenuated by the light detector 5, while the specularly 
reflected beam 8 is attenuated before reaching the light detector 5. 
The light attenuating device 10 may comprise an opaque member which 
completely blocks light, or may comprise a partially opaque member which 
blocks only a portion of the light passing therethrough. Other embodiments 
of the light attenuating device 10 will be described below. 
In operation, the gloss sensor which employs the light source 1 and the 
light detector 5 of FIG. 1 operates in a somewhat similar manner to the 
known devices described above. However, to the best of applicant's 
knowledge, the inclusion of the light attentuating device 10, according to 
the above-described embodiment, is completely unknown in the prior art. 
The inclusion of this light attenuating device provides a greater 
sensitivity to changes in the gloss of a surface than that which is 
provided by previously known gloss sensors. 
According to the above-described embodiment, the light attenuating device 
10 operates to attenuate the generally stronger (more intense) specularly 
reflected beam 8, while allowing the generally weaker (less intense) 
diffusely reflected beam 9 to pass substantially unattenuated to the light 
detector 5. In this regard, the high intensity specular reflections can be 
completely removed or at least partially attenuated from the light beam 7 
received by the light detector 5. The light detector will, thereby, 
provide readings only, or preferentially, of the intensity of the 
diffusely reflected beam 9, and will not provide the high intensity 
readings characteristic of specular reflections. By employing means for 
attenuating the relatively intense specular reflections, the gloss sensor 
will exhibit an increase in sensitivity to changes in the diffusely 
reflected beam 9 and, thus, to changes in the gloss of the surface 4. The 
present invention is particularly useful for measuring the gloss of high 
gloss paper sheets at relatively large angles (e.g,, 
.alpha..gtoreq.75.degree.) with respect to the normal to the sheet. 
FIG. 2 is a view of the light attenuating device 10 and the light detector 
5 along the optical axis 6. As shown in the FIG. 2 embodiment, the light 
attenuating device 10 is substantially centered with respect to the 
optical axis 6 and the light detector 5. In this manner, only an outer 
annular portion 11 of the light detector 5 is left exposed to unattenuated 
diffusely reflected light. As described above, the light attenuating 
device 10 is shaped and positioned so as to attenuate only the central 
specular reflections, while allowing diffuse reflections to pass and 
impinge substantially unattenuated on the annular portion 11 of the light 
detector 5. 
The light attenuating device 10 shown in FIG. 2 is hatched to indicated 
that it is opaque and thus blocks all of the light impinging thereon. 
However, it is within the scope of the present invention to provide a 
light attenuating device 10 which blocks only a portion of the light 
impinging thereon. 
FIG. 3 shows another embodiment of the light attenuating device 10. In this 
embodiment, the light attenuating device 10 comprises a graded optical 
density filter. The graded optical density filter of FIG. 3 blocks only a 
portion of the light impinging thereon, while allowing a portion of the 
light to pass therethrough. The graded optical density filter has a 
greater concentration of filter elements (e.g., opaque dye or paint) 
toward the central region 12 than toward the outer region 13. Thus, the 
central region 12 blocks a greater portion of light impinging thereon than 
does outer region 13. The high intensity specular reflections impinging 
upon the central region 12 are blocked to a greater extent than the 
diffuse reflections impinging on the outer region 13. 
The graded optical density filter of FIG. 3 may be disposed in the path of 
the reflected beam as shown in FIG. 1. Alternatively, the graded optical 
density filter may be arranged so as to completely cover the light 
receiving portion of the light detector 5. In this embodiment, all of the 
light received by the light detector 5 will pass through the graded 
optical density filter. However, the light impinging on the central 
portion 12 of the filter 10 will be attenuated to a greater extent than 
the light impinging on the outer region 13 of the filter 10. Thus, the 
specular reflection component 8 of the reflected beam 7 will be attenuated 
to a greater extent than the diffusely reflected component 9 of the 
reflected beam 7. 
While the above-described embodiments provide a light attenuating device 10 
as a separate element with respect to the light detector 5, it is also 
within the scope of the present invention to incorporate the light 
attenuating device 10 on or within the light detector 5 itself. Thus, the 
light attenuating device 10 may include a completely or partially opaque 
dark spot (e.g., dye or paint) centrally located on the light receiving 
portion of the detector 5. 
In yet another embodiment shown in FIG. 4, the light receiving device 10, 5 
may comprise a photosensitive device, such as a photodiode, photodiodes, a 
photoresistive element or, plural photoresistive elements, shaped or 
arranged in a ring 11 so as to receive substantially only the diffuse 
reflections. 
While several presently preferred embodiments of the present invention have 
been described, it will be understood that various modifications may be 
made without departing from the spirit and scope of the invention. 
Furthermore, although the present invention has been described with 
reference to the gloss measurement of paper, the present invention is also 
suitable for measuring the gloss of surfaces other than paper. 
Accordingly, it is understood that the invention is not limited by the 
specific illustrated embodiments, but only by the scope of the appended 
claims and equivents thereof.