Specular reflectance of cold-rolled aluminum surfaces

Specular reflectance of cold-rolled aluminum surfaces

Optics and Lasers in Engineering 17 (1992) 103-109 Specular Reflectance of Cold-Rolled Aluminum Surfaces R. S i l v e n n o i n e n , ° K . - E . P e...

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Optics and Lasers in Engineering 17 (1992) 103-109

Specular Reflectance of Cold-Rolled Aluminum Surfaces R. S i l v e n n o i n e n , ° K . - E . P e i p o n e n , °* T. A s a k u r a , b Y a n - F a n g

Z h a n g , a C o n g G u , ° K. I k o n e n , c & E. J. M o r l e y '~ " V~iisfilfi Laboratory, Department of Physics, University of Joensuu, SF-80100 Joensuu, Finland Research Institute for Electronic Science, Hokkaido University, Sapporo 060, Japan c Department of Information Technology, Lappeenranta University of Technology, SF-55851 Lappeenranta, Finland a Hydro Aluminum a.s R&D Centre, KarmOy, N-4265 Hfivik, Norway (Received 26 November 1991; revised version received 14 January 1992; accepted 17 January 1992)

ABSTRACT The specular reflectance of cold-rolled aluminum surface is studied using a fiber optic system. An approximation for the determination of surface roughness is presented.

INTRODUCTION The measurement of surface roughness is important in many industrial processes. Some of the most rapidly developing techniques for surface roughness measurement are based on optical measurement. Nowadays there are many different schemes for the measurement ~-~ but they all share one common feature: the light propagation is affected by the surface roughness of the sample to be inspected. One well-known method for surface roughness inspection is the measurement of specular reflectance. It is possible, provided that the surface roughness is much smaller than the wavelength of the incident light beam, to extract information about the rms optical roughness of the material. The theoretical description of light interaction with rough surfaces was furnished by Beckmann. 5 Beckmann's theory has given satisfactory correlation between mechanical and optical measurements of roughnesses, especially for ground and flat lapped surfaces, whose * To whom correspondence should be addressed. 103

Optics and Lasers in Engineering 0143-8166/92/$05.00 (~ 1992 Elsevier Science Publishers Ltd, England. Printed in Northern Ireland

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roughness distribution can be described as having Gaussian or slightly distorted Gaussian distributions. 67 T h e r e has b e e n a considerable n u m b e r of studies using a H e - N e laser as a light s o u r c e 6"s-12 in the detection of specular reflectance from surfaces for the purpose of estimating the surface roughness. This paper reports on the specular m e a s u r e m e n t of cold-rolled aluminum sheets. The m e a s u r e m e n t device is based on the use of a fiber optic system.

EXPERIMENTAL

METHOD

AND RESULTS

The m e a s u r e m e n t s of specular reflectance were p e r f o r m e d using a semiconductor laser with a wavelength of 1.31/~m. The amplitude of the laser had a good stability. The laser b e a m was guided through a single m o d e fiber (core diameter 10 # m ) and there was a lens at the output end of the fiber in o r d e r to form a plane wave incident on the sample. The specularly reflected intensity was d e t e c t e d using an A n d o A Q - 1 1 3 5 E p o w e r m e t e r which is designed for the detection of w e a k signals in telecommunicational applications. The sample, the o u t p u t end of the fiber and the detecting sensor were installed into a stable goniometer (originally part of an X-ray diffractometer). A schematic diagram of the system is p r e s e n t e d in Fig. 1. For comparison, specular reflectance was also m e a s u r e d using a H e - N e laser as a light source in place of the laser diode and optical fiber. The results of the reflectance of cold-rolled aluminum sheets are as follows. The n u m b e r of the studied samples was 14 and their sizes were approximately 0.25 x 50 x 50 mm 3. The average roughness R~,, measured with a d i a m o n d stylus, ranged b e t w e e n 0-19/~m and 0-33 #m. Figure 2 shows an electron microscope micrograph of the surface texture of a sheet whose R,, = 0.27 ~m. The rolling direction is the SL

P

R

Fig. 1. S c h e m a t i c d i a g r a m o f the s e t - u p . S L = S e m i c o n d u c t o r laser, S M F = single m o d e fiber, G = g o n i o m e t e r , S = s e n s o r , L = lens, X = s a m p l e , P = p o w e r m e t e r a n d R = recorder.

Specular reflectance of cold-rolled aluminum surfaces

Fig.

2.

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A micrograph of a sheet whose R~ = 0-27/~m. The magnification is 200×.

vertical direction. Figure 3(a) and (b) present the reflection patterns using H e - N e illumination obtained in the rolling and perpendicular directions. The patterns were recorded from a screen placed on the sensor plane. We observe that the patterns are completely different. The best results for surface roughness estimation, using Beckmann's theory, were obtained when the laser beam was incident perpendicularly to the rolling direction. In the case where the beam was incident parallel to the rolling direction, the optical roughness values obtained from Beckmann's theory had relatively low values compared to the mechanically obtained ones. Figure 4(a) shows two typical curves of specular reflectance, I/Io, as a function of the angle of incidence, for a sheet that is not perfectly planar, i.e. it is slightly curved (a situation that may appear during a manufacturing process line). Figure 4(a) illustrates the situation where the sheet has two different curvatures. In other words the overall curvature of the metal sheets on a macroscopic scale is different. The reflectance is affected by the curvature and also by the roughness of the sample. With the present samples it was observed that, despite some slight random curvature of the sheets, the reflectance level (laser beam perpendicular to the rolling direction) usually stabilized to an almost constant value in the vicinity of O = 60 °. The reflectance was also measured using a H e - N e laser as a light source. Figure 4(b) shows such data measured for the same sheet and same curvatures as in Fig. 4(a).

1(i)6

Fig. 3.

R. Silvennoinen et al.

Reflection patterns recorded from a screen in the sensor plane: (a) perpendicular to the roiling direction; (b) parallel to the rolling direction.

107

Specular reflectance of cold-rolled aluminum surfaces

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0.0 50* 6'0" 7'0* 8'0" 9'0* 0"050* 6'0" 7'0" 8'0" 9'0* Fig. 4. (a) Specular reflectance (as a function of the angle of incidence) of slightly curved aluminum sheet ( R a = 0.27 ~um) when a semiconductor laser was used; (b) same as in (a) but using a H e - N e laser.

It is observed that under H e - N e laser light the method cannot distinguish the curvature compared to the use of the semiconductor laser. One reason for this behavior is that a higher proportion of light is reflected diffusely when the shorter wavelength of the H e - N e laser is used. When using a semiconductor laser the authors believe that the adjacent lines of the discrete emission spectra are interfering due to the curvature of the sheet and thus affecting to the reflectance. Evidently, using the present fiber optic system we can get information about the curvature of the sheets• Although the Beckmann's model is not expected to apply to cold-rolled surfaces, because the surface roughness distribution is not expected to have a perfect Gaussian shape, it was observed that in the case of semiconductor laser the formula 5

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was applicable as follows. The authors used five values of the angle of incidence in the vicinity of O = 60 ° and t o o k an average for the Rovt calculated with the aid of eqn (1). T h e r e was a relatively good correlation b e t w e e n R,,pt and R,, (as shown in Fig. 5). This was true despite the slight random curvature of the sheets. The authors wish to emphasize that when the sheets were perfectly planar s m o o t h curves were obtained as a function of ® when using the semiconductor laser. Finally it is pointed out that the calculation of Ro~,, with the aid of eqn (1) is usually accomplished when 8 - - > 9 0 ° whereas for the present samples the best results were obtained when 8--> 60 ° .

DISCUSSION It is not so surprising that the reflectance level is stabilized when the angle of incidence is decreasing, Such behavior can be shown with the aid of Fresnel equations. On the other hand, the spot size on the surface is decreased resulting, in the fact that the reflection of light is not so sensitive for slight curvature of the surface. Using the apparatus described it is possible to inspect the curvature and obtain information about the roughness of surfaces. The correlation b e t w e e n mechanical and optical roughnesses was found only when the samples were inspected in the lay direction.

ACKNOWLEDGEMENTS The authors wish to thank Dr O. Salminen and Mr J. Luostarinen for technical assistance.

REFERENCES 1. Asakura, T., Surface roughness measurement. In Speckle Metrology, ed. R. K. Eft. Academic Press, Orlando, 1978. 2. Kohno, T., Ozawa, N., Miyamoto, K. & Musha, T.~ High precision optical surface sensor. Appl. Opt., 27 (1988) 1{)3. 3. Quercioli, F., Tiribilli, B. & Molesini, G., Optical surface profile transducer. Opt. Eng., 27 (1988) 135. 4. Standard Test Method for Measuring the Effective Surface Roughness of Optical Components by Total Integrated Scattering, ASTM (American Society for Testing and Materials) Doc. F 1048-87 (1987). 5. Beckmann, P. & Spizzichino, A., The Scattering of Electromagnetic Waves from Rough Surfaces. Pergamon Press, Oxford, 1963.

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6. Tanner, L. H., The use of laser light in the study of metal surfaces. Opt. Laser Technol., 8 (1976) 112. 7. Peiponen, K.-E., Vartiainen, E. M. & Asakura, T., The Hilbert transfor in the description of surface roughness. Optik, 90 (1992) 45. 8. Tanner, L. H. & Fahoum, M., A study of the surface parameters of ground and lapped metal surfaces using specular and diffuse reflection of laser light. Wear, 36 (1976) 299. 9. Tanner, L. H., A comparison between talysurf 10 and optical measurements of roughness and surface slope. Wear, 57 (1979) 81. 10. Vorburger, T. V., Teague, E. C., Scire, F. E., McLay, M. J. & Gilsesin, D. E., Surface roughness studies with DALLAS-detector array for laser light angular scattering. Res. Nat. Bur. Stand., 89 (1984) 3. 11. Whiteley, J. Q., Kusy, R. P., Mayhew, M. J. & Bucktal, J. E., Surface roughness of stainless steel and electroformed nickel standards using a H e - N e laser. Opt. Laser Technol., 19 (1987) 189. 12. Peiponen, K.-E. & Tsuboi, T., Metal surface roughness and optical reflectance. Opt. Laser Technol., 22 (1990) 127.