GB2227308A

GB2227308A - Glossmeter

Info

Publication number: GB2227308A
Application number: GB8900749A
Authority: GB
Inventors: Martin Coulthard
Original assignee: Surface Inspection Ltd
Current assignee: Surface Inspection Ltd
Priority date: 1989-01-13
Filing date: 1989-01-13
Publication date: 1990-07-25
Anticipated expiration: 2009-01-13
Also published as: GB2227308B; GB8900749D0

Abstract

A glossmeter comprises a light source-and-detector assembly (15) for directing light towards a surface (19) the gloss of which is to be measured, and detecting light which is reflected back from such a source, and a retro-reflective screen (16) for directing light from the source which has been reflected by the surface (19) back along substantially the same path. The assembly (15) is fixed relative to the screen (16). The return beam may be split into two orthogonally polarised components and each component directed to a respective detector so that when angles of incidence close to Brewster's angle are used, compensation may be made for randomly polarised stray light reaching the detectors. <IMAGE>

Description

TITLE GLOSSMETER TECHNICAL FIELD The invention relates to a device that measures the gloss of a surface.

BACKGROUND The specular gloss, or reflectivity, of a surface can be loosely defined as the proportion of incident light that is specularly reflected by the surface. It varies with the angle that the incident beam makes with the surface, the width and divergence of the beam, the wavelength and polarisation of the incident light. the curvature of the surface, and the angular distribution of the reflected light that is considered to be the specular reflection, among other factors. More complete definitions of gloss therefore refer to the precise conditions under which the measurement is carried out and the apparatus to be used.

A number of such definitions exist, of which British Standard BS3988:PartD5:1980 (Measurement of Specular Gloss of Non-Metallic Paint Films at 26, 6 and 85 Degrees) is an example.

Gloss is a measure of surface finish quality which is important for a number of reasons. The aesthetic appeal of a surface finish is highly dependent on gloss. Gloss is a useful indicator of the micro-geometry, or roughness, of a surface, and can therefore be used to estimate the friction and wear properties of the surface, and to evaluate how well a subsequent surface coating will adhere.

Gloss is used as a quality control parameter in a wide range of industries. In the car manufacturing industry gloss is one of the parameters used to evaluate the quality of the paint finish on a car body. It has been shown that the shininess or gloss of the paintwork has a strong effect on a customer's assessment of the quality of a car. Many other examples of the use of gloss can be found in the steel, plastics, ceramics and paper industries.

PRIOR ART Conventional Glossmeters Glossmeters that are currently available are effectively contact devices, that is to say they need to be positioned against the surface to be inspected, (the 'test surface'), before a reading can be taken.

The principle of operation of a conventional glossmeter is shown in Figure 1. A light source 1 and collimating optics 2 produce a collimated beam of light that is directed at the test surface 3. Focussing optics 4 direct the specularly reflected light onto a photodetector 5.

The intensity of the specular reflection, and hence the gloss, is indicated by the signal from this photodetector.

Glossmeters of this type have a number of limitations.

Limitation 1 - The performance of this optical system is highly sensitive to the position and orientation of the test surface 3. If significant misposition occurs then some or all of the specularly reflected beam is no longer collected by the focussing optics 4, and does not impinge upon the detector 5. The value of gloss measured is therefore incorrect. For this reason it is vital to ensure that the position of the optical system relative to the test surface is correct. This is normally done by having the optical system enclosed in a housing 6. The housing has a single opening, which is presented to the surface as the glossmeter is held against it. The housing has the additional function of excluding ambient light, which might otherwise affect the reading.

Limitation 2 - Conventional glossmeters can only produce accurate readings when placed on a plane surface. When used on curved surfaces the housing no longer fits well against the surface, so the position of the optical system relative to the surface is not fixed, and some ambient light can enter the device. Also the collimated incident light beam is reflected by the test surface as a divergent or convergent beam, depending whether the surface is convex or concave. The focussing optics are unable to redirect all the specularly reflected light onto the photodetector, and the gloss reading given is incorrect.

Limitation 3 - The photodetector receives not only light which has been specularly reflected by the test surface, but also that which has been diffusely reflected in the specular direction. As the intensity of diffusely reflected light is dependent on the colour of the surface the gloss reading from a conventional glossmeter is affected by colour. True specular gloss, obtained by measuring Just the proportion of light which has been specularly reflected, is dependent only on the refractive index of the material and the micro-geometry of the surface, and is unaffected by colour.

There are three main types of glossmeters available, all of which use the conventional optical system, or systems closely related to it. Hand-held meters are compact instruments which are held by hand against the surface to be inspected. Laboratory meters are larger and, usually, more accurate. On-line meters are for use on production lines to inspect a moving strip product. The position of the strip has to be tightly controlled, usually by precision rollers.

A Previously Proposed Retro-Reflective Glossmeter A simple retro-reflective glossmeter that uses an alternative optical system has been proposed. - This proposed glossmeter is illustrated in Figure 2.

Light from the source 7 passes through a beam-splitter 8 before being reflected by the test surface 9 onto a retroreflective screen lie. This screen retro-reflects the light back onto the test surface 9 where it is reflected again, back towards the source 7. This doubly specularly reflected light will be referred to as 'specular light' for brevity.

Some of the specular light is redirected by the beamsplitter 8 towards a detector 11. The light path is shown in the Figure as a dashed line, 12.

In this proposed glossmeter there is a rigid connection 13 between the test surface 9 and the retro-reflective screen le.

When the test surface/retro-reflective screen assembly is moved relative to the source/detector assembly, the position and direction of the specular beam after its second reflection from the test surface remains essentially the same as before the movement, and the value of gloss can still be measured. In this way one of the limitations of conventional glossmeters can be overcome, however there are several other significant limitations.

Limitation 1 - Since it is proposed that the retroreflective screen be rigidly connected to the test surface the glossmeter would be a contact instrument, and could not be used to inspect a moving surface. In addition, considerable time and effort would be required to attach the screen to the surface before each measurement of a different part of the surface could be taken.

Limitation 2 - Values of gloss measured by the proposed glossmeter on light coloured surfaces would be different from both the values that would be obtained by a conventional glossmeter and from the true specular gloss.

This is because the single photodetector measures the intensity of not only the specular light, but also that of light which has been diffusely reflected back towards it ('back-scattered') without reaching the retro-reflective screen.

This light path is shown in Figure 2 as a dotted line, 14.

Though this path is shown as being adjacent to the specular path 12, the two are in fact co-axial.

As an example, two tiles having the same true specular gloss can be considered. One is black. with a low diffuse reflectivity, while the other is white, with high diffuse reflectivity. When the black tile is under the glossmeter the photodetector receives Just specular light, as there is almost no back-scatter. When the white tile is used the intensity of the specular light falling on the detector is about the same as for the black tile (as it has the same level of gloss), but in addition it receives a significant level of back-scattered light, so the total signal is considerably larger than it was for the black tile. The gloss values given by the meter are therefore different, though the true specular gloss of the two tiles is the same.

Limitation 3 - A similar effect, also leading to an error in the gloss value measured1 is encountered when there is a high level of ambient lighting. Ambient light can be diffusely reflected by the test surface to fall on the photodetector. This increases the measured light intensity value, causing the gloss value calculated to be too high.

Limitation 4 - The proposed system would suffer from drift due to both temperature and light source intensity variations. The temperature of the meter would increase after power-up due to the heat output of the light source.

The signal from the photodetector, which is temperature sensitive, and the gloss value that is calculated would slowly change. A longer term effect would be seen due to variations in the intensity of light from the source, particularly towards the end of its life time.

THE INVENTION The present invention seeks to provide a glossmeter that is less subject to one or more of the foregoing limitations.

The present invention is accordingly directed to a noncontact retro-reflective glossmeter.

According to the present invention the retro-reflective screen is fixed relative to the source/detector assembly, rather than to the test surface.

Further the source/detector assembly has multiple detectors whose signals are used to analyse the state of polarisation of the light arriving at the assembly so that the intensity of the specular light can be found even when there is a significant level of back-scattered and ambient light.

Back-scattered and ambient light is randomly polarised as it has been diffusely reflected by the test surface.

Specularly reflected light, on the other hand, is polarised when reflected by non-metals. The degree of polarisation depends on how close the angle of incidence (the angle between the axis of the incident beam and the normal to the test surface) is to the Brewster's Angle for that material. If the angles are equal then the specularly reflected beam contains just s-polarised light, (light which has its electric field vector orthogonal to the plane of incidence), as all the p-polarised light (light which has its electric field vector parallel to the plane of incidence) is refracted or absorbed into the material. Brewster's Angle is dependent only on the refractive index of the material.

The intensity of the specular light can therefore be calculated from the intensities of the s and p-polarised components of light entering the source/detector assembly.

DESCRIPTION OF FIGURES Figure 1 shows a diagram of a conventional contact glossmeter.

Figure 2 shows a diagram of a previously proposed retroreflective glossmeter.

Figure 3 shows a diagram of a specific embodiment of a non-contact retro-reflective glossmeter, being an example only of a glossmeter made in accordance with the present invention.

Figure 4 diagrammatically shows in greater detail a source/detector assembly of the glossmeter shown in Figure 3.

A SPECIFIC EMBODIMENT A general view of a specific embodiment of a glossmeter, being an example only of a glossmeter made in accordance with the present invention, is shown in Figure 3.

A light source, photodetectors and other optical components are mounted in the source/detector assembly 15.

A retro-reflective screen 16 is fixed to the source/detector assembly by a rigid connection 17. The light path for the specular light is shown as a dashed line 18.

The source/detector assembly and the retro-reflective screen form the optical system for the non-contact retroreflective glossmeter. The optical system has the positions of all its parts fixed relative to the others.

The meter is mounted over or otherwise adjacent to the test surface 19.

Figure 4 shows the source/detector assembly of the specific embodiment of the invention in more detail.

Light is produced by the source 28, and shines on the first beam-splitter 21. Half the light is reflected towards the source detector 22, while the other half is transmitted through the beam-splitter to the lens 23, which is positioned so as to produce a beam of light that is essentially parallel, or collimated.

This collimated beam shines on the test surface. Some of the light is specularly reflected and returns to the lens after following path 18 (as shown in Figure 3), while some of it returns after being back-scattered by the test surface. Some ambient light, from sources outside the glossmeter, or from daylight, may also arrive at the lens.

The light passes through the lens onto the first beamsplitter 21 again. As before, half the light is transmitted, while half is reflected, though this time away from the source detector 22.

The reflected light falls on the second beam-splitter 2lit, where half is reflected, and passes through a polariser 25 onto detector A, 26, while the other half is transmitted to pass through polariser 27 onto detector 28. Polariser 25 has its polarisation axis (the direction of the electric field vector in preferred transmission) in the plane of the source/detector assembly, as it is shown in Figure 4, and so passes the s-polarised component of the beam. Polariser 27 has its axis perpendicular to this, and therefore passes the p-polarised component.

The beam-splitters are selected so that they produce transmitted and reflected beams which have states of polarisation similar to that of the incident beam. The two detectors, 26 and 28, can therefore be used to measure the intensity of the s- and p-polarised components respectively of the light arriving back at the source/detector assembly.

When the glossmeter is used with an angle of incidence equal to the Brewster's Angle for the material of the test surface, then the doubly specularly reflected light arriving at the optics assembly is completely s-polarised.

The back-scattered and ambient light, on the other hand, is randomly polarised, and so can be considered to consist of half s-polarised and half of p-polarised light.

Detector 26 therefore measures the intensity of the specular light plus that of the s-polarised component of the randomly polarised light, while detector 28 measures the intensity of the p-polarised component of the randomly polarised light. As the s and p-polarised components of randomly polarised light are equal, a measure of the intensity of the specular light alone can be obtained by subtracting the signal from detector 28 from that of detector 26.

The signal values obtained from these two photodetectors is normalised to compensate for variations in the intensity of the light source, and for changes in the performance of the photodetectors due to temperature, by dividing it by the value given by the source detector 22.

The result of this is further processed by electronic circuitry to give the correct specular gloss value for the test surface.

When the glossmeter is used with an angle of incidence different to Brewster's Angle the specular light is only partially polarised. If the glossmeter has been calibrated on a surface that has the same refractive index, and hence Brewster's Angle, as the test surface, the degree of polarisation of the specular light during that calibration is the same as the degree of polarisation of the specular light when the test surface is inspected.

The result is that the correct gloss value for the test surface is still calculated.

More commonly, however, the Brewster's Angle of the calibration surface is different to that of the test surface, and both are different from the angle of incidence. This means that the degrees of polarisation of the specular light from the two surfaces are different.

resulting in an error in the gloss value calculated. In practice this error has been found to be small as long as the Brewster's Angles for the two surfaces are reasonably close to the angle of incidence, as is the case for most common solids when an angle of incidence of sixty degrees is used.

POSSIBLE MODIFICATIONS 1. A polarising beam-splitter can be used instead of the non-polarising beam-splitter 24. This makes the two polarizing components 25 and 27 redundant, and they can be omitted. This modification gives greater light intensity at the detectors 26 and 28, and improves the signal to noise ratio.

2. A hot mirror can be mounted between the source 2 and the beam-splitter 21. This component reflects most of the infra-red content of the light from the source back past the source, and so reduces the intensity of the infra-red light that enters the source/detector assembly. In this way heating of the source/detector assembly is reduced, the beam of light shining on the test surface is cool (it has a low infra-red content), and the light that is measured by the detectors is mainly of visible wavelengths.

3. Semiconductor photodetectors have, in general, a higher sensitivity to light with wavelengths in the red and infra-red region of the spectrum than to blue or green light. A blue/green filter mounted between the two beam-splitters 21 and 24 helps to correct for this effect, so that the spectral response of the glossmeter is closer to that of the human eye.

4. The glossmeter can be designed and built without the source detector 22. This gives a somewhat simpler instrument, but problems of drift due to source intensity variation and temperature would be experienced.

5. The intensity of the light falling on the source photodetector is considerably greater than that falling on the other two photodetectors, and can cause this photodetector to saturate. The problem can be solved by positioning one or more neutral density filters between the source photodetector 22 and the beam-splitter.21.

.6. When a beam of light is reflected by a bare metal surface it is not polarised as it is by a non-metallic one, so the improved glossmeter, as described, does not function correctly on this type of surface. (It does, however, function correctly on metals that are coated with a non-metallic substance, such as paint).

Nevertheless, valid gloss readings can still be obtained on bare metals by changing the signal processing so that the gloss is calculated from the sum of the readings from detectors 26 and 28, rather than the difference.

Gloss readings calculated in this way are not corrected for back-scattered or ambient light, but are accurate when the metal surface has a high gloss value.

7. In optical systems that use a retro-reflective screen, the specular light arriving at the detectors is reflected twice by the test surface. As the specular gloss of a surface is the proportion of incident light that is specularly reflected by the surface, the intensity of specular light is proportional to the square of the gloss. A square root transformation of the signal corresponding to this intensity is therefore required for the output of the meter to have a linear relationship with gloss.

8. When the intensity of the ambient light arriving at the lens in its axial direction is low, as is normally the case, the signal from detector 28 gives a measure of the intensity of the back-scattered, or diffusely reflected, light. It can be used to calculate a measure of the diffuse reflectance of the test surface. In addition to this, the difference between the specular gloss and the diffuse reflectance gives an indicator of contrast gloss. A total of three surface reflectance values can therefore be found - specular gloss, diffuse reflectance, and contrast gloss.

9. The rigid connection between the source/detector assembly and the retro-reflective screen can conveniently be made by mounting both in a rigid housing. The housing can be box shaped, with a window through which the specular light passes in the face which is positioned adjacent to the test surface. The housing also contains the electronic and other components which are required. The housing protects the contents from damage, and helps exclude dust, moisture and ambient light.

18. The gloss value calculated by the meter can be displayed on a digital panel meter mounted in one wall of the housing.

11. The processing electronics can be digital in nature.

The outputs from the detectors are converted by analog to-digital converters.

12. Digital processing and analysis can be carried out by a microprocessor.

13. The meter can be designed to acquire new readings either continuously or on reception of a trigger signal.

14. The processing electronics can automatically detect when the measured gloss value goes out of preset ranges, and activate alarm lights and digital outputs when this occurs.

15. When they are acquired gloss values can be stored in random access memory (RAM) to be accessed later for review or analysis.

16. Stored gloss values can be statistically analysed by the processing circuits.

17. The meter can produce an output for a printer or chart recorder.

18. The meter can communicate with a remote digital computer, such as a Supervisory Computer or a Programmable Logic Controller (a PLC). The remote computer can request stored gloss values and can setup and control all the other functions of the meter.

19. More than one glossmeter can communicate with a single remote computer to allow the gloss to be simultaneously monitored at several points on a production line, or on different lines.

ADVANTAGES OF THE SPECIFIC EMBODIMENT The non-contact retro-reflective glossmeter overcomes the limitations, as described in the Prior Art section, of both conventional glossmeters and of the previously proposed retro-reflective glossmeter.

1. The meter is tolerant to limited variations in the position, orientation of the test surface.

2. The meter is tolerant to curvature of the test surface.

3. The meter measures true specular gloss, so its readings are not affected by the colour of the test surface.

4. The meter's optical system forms a single unit which does not have to be fixed to or precisely positioned by the test surface. This means that the meter can be used as a non-contact, on-line instrument to measure the gloss of a moving product, even if the position of the product is not tightly controlled.

5. The meter is insensitive to variations in ambient lighting.

6. Drift of the measured gloss value due to temperature and source intensity variations is minimised by the use of a source detector.

7. As well as the specular gloss, two other optical properties of the test surface can be measured - diffuse reflectance, and contrast gloss.

Claims

Claims

t. A glossmeter comprising a light source-and-detector assembly for directing light towards a surface the gloss of which is to be measured, and detecting light which is reflected back from such a surface, and a retro reflective screen for directing light from the source which has been reflected by such a surface, back along substantially the same path, in which the assembly is fixed relative to the screen.
2. A glossmeter according to claim 1, in which the assembly is provided with a collimator to collimate light from the light source.
3. A glossmeter according to claim 1 or claim 2, in which the glossmeter is so constructed that it provides an angle between the collimator light path and the normal to such a surface of substantially 6 degrees.
4. A glossmeter according to any previous claim1 in which the assembly is provided with at least one detector element and at least one linear polariser element to allow light which is polarised in a given preferred plane to pass through to the detector element in preference to light polarised in other planes.
5. A glossmeter according to claim b, in which the said given plane is the plane of the S-polarised light when the glossmeter is in use.
6. A glossmeter according to claim 4 or claim 5,, in which the assembly is provided with (a) two linear polariser elements oriented to allow through light which, in the reflected beam, is polarised in respective mutually orthogonal planes, and (b) respective detector elements downstream of those elements.
7. A glossmeter according to claim 6, in which the said two elements are placed downstream of a beam-splitter, to receive light from respective light paths from the beam-splitter.
8. A glossmeter according to claim 71 in which the said detector beam-splitter is such as to produce light of substantially the same states of polarisation in both split beams as the state of polarisation of the beam incident upon it.
9. A glossmeter according to claim 4 or claim 5, in which the linear polariser element comprises a polariser beam splitter, and in which respective detectors are positioned downstream thereof to detect light of respective light paths from the splitter, the splitter being such as to direct light of the received beam having substantially orthogonal planes into the respective paths.
10. A glossmeter according to any preceding claim, having source detector means which serve to provide a measure of the amount of light which leaves the light source.
11. A glossmeter according to any preceding claim, in which a beam-splitter is provided to direct some of the light returning from such a surface, to detector means of the assembly.
12. A glossmeter according to claim 11 read as appended to claim 1, in which the beam-splitter mentioned in Claim 16 serves to direct some of the light directly from the light source to the detector means mentioned in Claim le.
13. A glossmeter according to any preceding claim, further comprising calibration means to enable the glossmeter to be calibrated for a surface of given gloss value.
14. A glossmeter according to claim 16, in which a neutral-density filter is positioned in the path of light which is directed to the source detector means to reduce the overall intensity of light received thereby irrespective of its polarisation.
15. A glossmeter according to any preceding claim further provided with a digital display which serves to provide a visual indication of one or more signals provided by the glossmeter.
16. A glossmeter according to any preceding claim, in which a hot mirror is positioned in the light path to reduce the effect of infra-red radiation from the source on the signal provided by the glossmeter.
17. A glossmeter according to any preceding claim, in which the light source-and-detector assembly and the retro-reflective screen are rigidly connected to the inside of a rigid housing which has an opening to which light from the source is directed and from which light may be received by the detector means.
18. A glossmeter according to any preceding claim further comprising circuitry connected to the detector means of the assembly to process output signals therefrom and to provide a gloss measurement signal dependent upon that signal.
19. A glossmeter according to claim 18 when read as appended to claim 6 or claim 9, in which the circuitry is constructed to effect a subtraction of output signals from the said the said two detector elements.
20. A glossmeter according to claim 19, in which switch means are provided to enable the circuitry to be switched from a condition in which it effects a subtraction of the output signals from the said two detectors to a condition in which it effects addition of those output signals.
21. A glossmeter according to claim 19 or claim 20, in which the circuitry is constructed to effect square rooting of the difference between, or the sum of, the output signals of the said two detector elements, as the case may be.
22. A glossmeter according to any one of claims 18 to claim 21 read as appended to claims 16, in which the circuitry is constructed to effect normalisation of the output from the detector means of the assembly which receives light for such a surface, by means of the output from the source detector means.
23. A glossmeter according claims 18, in which the output signals from the detector or detectors are analogue signals, in which the circuitry is constructed for digital processing, and in which at least one analogue to-digital converter is connected between the detector or detectors and the circuitry.
24. A glossmeter according to claim 18 appended to any one of claims 6 to 9, in which the circuitry further provides a measure of the diffuse reflectance of such a surface.
25. A glossmeter according to claim 24, in which the circuitry is constructed to provide a measure of the difference between the specular gloss and the diffuse reflectance, to provide a measure of the contrast gloss.