The relationship between the speckle contrast of scattered light in near field and longitudinal distance that is perpendicular to scattering surface is investigated. The experiment indicates that when using the laser illumination source with proper time coherence length, the curve of speckle contrast vs. longitudinal distance appears minimum turning point. The position and value of minimum point are decided by the coherence of light and scattering matter. It is easy to obtain the correlation area of scattered light by measuring the minimum point position and the illuminated area. Comparing to traditional scattering technique, this method can simultaneously measure the roughness parameters of surface height variance and surface height correlation area.
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Near field scattering (NFS) techniques recently was introduced by M. Giglio group [1–8]. In contrast to traditional small angle scattering techniques, the scattered light is collected in the near field of the sample by CCD camera. The NFS technique can be realized by three different conceptual schemes, they are homodyne NFS [1,2], heterodyne NFS  and Schlieren NFS . For the NFS technique, the scattered intensity distribution can be derived by Fourier analysis of the speckle images in the near field. It has been widely used in nonequilibrium systems [5,6], complex fluids  and dynamics measurements .
In this letter, we will describe an interesting phenomenon about speckle contrast in NFS. When the illumination source with proper time coherence length is used, the curve of speckle contrast vs. longitudinal distance that is perpendicular to scattering surface appears minimum turning point. A proper analysis is presented based on optical coherence and the characteristic of NFS. Using this speckle contrast phenomenon, the correlation area of scattered light is easy gained which can be used in measuring the roughness of surface. The Asakura group introduced a method to acquire the roughness parameters of surface height variance and surface height correlation area [9,10], but their method requires to know either of roughness parameters in advance. Now, both parameters can be acquired by doing an extra measurement of the correlation area of scattered light in the same experiment. Furthermore, this phenomenon of speckle contrast is related with the time coherence of scattered light in our analysis, which is possible to be applied in biomedical system because of the decoherence in bio-tissue with multi-scattering.
In order to study the longitudinal distance change rule of the speckle contrast in the near scattering field, the measurement setup is described in Fig. 1 . The 488nm CW TEM00 Ar + laser (U.S., Spectra-Physics Inc., Model 177-G12), the 532nm CW TEM00 green diode laser (China, Photon Technologies, Inc., GDL8100) and the 520nm green LED (China, HongKe Optoelectronic Co.,LTD, H10- A21GHC5-G10-3P) were used respectively, their coherence lengths are about 50mm, 2mm and 0.01mm. A variable optical expander was used before the scattering screen. The incident angle was set as 60° in order to avoid the influence of mirror reflecting into CCD camera. The length of major axis of the ellipse illumination spot was 2cm and the minor axis was 1cm, and white painted board was used as scattering screen in our experiment. The microscope (Japan, Nikon Inc., ST100) is reconstructed and laid perpendicularly to the scattering screen. Its micro objective magnification is 10X and N.A. is 0.25. The CCD camera (Japan, Nikon Inc., DS-U2) take the place of microscopic ocular and its active area is adjusted to image plane. The time of CCD exposure is 1/60s and the digital gain is 1.0X.
Increasing the distance between the microscope and the scattering screen, the CCD camera acquired the speckle pattern with proportional spacing. Then the patterns are processed by computer to work out speckle contrast. When illumination sources with different time coherence length are adopted, the speckle contrast vs. longitudinal distance curves show the result as in Fig. 2 . The curves show an interesting phenomenon, which a minimum turning point appears while using the 532nm green diode laser as illumination source. But other illumination source of LED and Ar+ laser doesn’t represent this character. When the 488nm Ar + laser illuminates the scattering screen, the speckle contrast curve keeps steadily at a close distance and increases slowly with increasing distance. While taking the LED as illumination source, the speckle contrast keeps nearly constant.
In order to further study this character of the 532nm green diode laser, different scattering screens (white painted board and white cardboard) are used in our experiments. Figure 3 shows their surface micrographs, the white painted board is rougher than the white cardboard. Figure 4 is the experimental results of different boards, the speckle contrast curve of the rough surface lies on top of the smooth surface’s speckle contrast curve, but the minimum point position of the rough surface is less than the smooth one.
3. Theoretical analysis
Above experimental results can be explained based on optical coherence and the characteristic of speckle in the near field. The laser with TEM00 mode has good space coherence, so its coherence area is restricted by the time coherence length. To simplify the analysis without injuring reasonability of experimental results, we only take the center area of illumination region into consideration. As shown in Fig. 5(a) , the relation between the coherent length δ of illumination source and the linear dimension of the equivalent time coherence area (ETCA) can be expressed as:Fig. 5(b)):Eq. (1):
The shape of ETCA will become a circular for normal incidence or transmission.
On the other hand, according to the characteristic of speckle in the near field , when the distance between the plane of the scattering spot and the observation plane meets following equation:Fig. 6 . The effective scattering area can be expressed as :
The curves of andvs. z are described in Fig. 7 based on Eq. (2) and 5. When the time coherence of light is poor (LED, for example), the curve of ETCA remains nearly a small value constant line. So the number of the ETCA in the effective scattering area is large and the speckle contrast keeps in low level. When the time coherence length is so large(Ar + laser, for example) that its ETCA is larger than the area of illumination region, the effective scattering area is less than the ETCA in different distance and the speckle contrast keeps steadily. When the time coherence length is in proper range (532nm diode laser, for example), as the slope of the curve is less than that of the curve, the value of Nincreases according to Eq. (7) while the speckle contrast keeps decreasing. The effective scattering area remains unchanged after it equals to the area of illumination region, but the ETCA expands continuously with increasing distance. Then the value of N decreases and the speckle contrast starts to rise up. When the ETCA equals to the area of illumination region, the speckle contrast keeps steadily and rises slowly. They are concordant with experimental results (see Fig. 2).
Considering the significance of minimum point, we make further researches into the special phenomenon. When the coherence length is in proper range (approximately less than linear dimension of illumination region, see Fig. 8 ,), it will appear a minimum turning point. According to Eq. (5), the position of minimum point can be given by:
On the other hand, the minimum value of speckle contrast may be written formally based on Eq. (6). To simplify the qualitative analysis, we consider that the intensity of light is uniform and normally incidence. The speckle contrast can be expressed as:Fig. 9 ). For details, please refer to appendix.
Obviously, Eq. (8) and 10 show that the minimum of speckle contrast is decided by the coherence of light and scattering matter. If the scattering screen is rougher, the becomes smaller and the gets shorter according to Eq. (8) (see Fig. 8). According to Eq. (10), the minimum value of speckle contrast of the rough scattering surface is larger than the smooth one. Which in accord well with the experimental results seeing in Fig. 4.
The extremum of speckle contrast mentioned above provides a method to measure the roughness of surface, which is an extendable method introduced by the group of Asakura  . As the correlation area of scattered light can easily derived by the measurement of , it provides a method to get both parameters of and , which takes place of requiring the knowledge of two parameters in advance mentioned in reference 9 and 10. Suppose light incidence normally, the relationship between the correlation area of scattered light and roughness of surface can be expressed as :11]:Eq. (12) and expression, it supplies a measurement of the. Then can be acquired by uniting the other Eq. (11).
In addition, the value of minimum turning point of speckle contrast may estimate the time coherence of scattered light, which is likely to offer part information of volume scattering in biomedical system in terms of decoherence generated by multi-scattering.
In conclusion, the relationship between speckle contrast and longitudinal distance in the near scattering field is presented. The time coherence of light affects the changing rule of speckle contrast in the near field. When the laser with proper coherence length is used, the speckle contrast curve appears a minimum turning point. The position of minimum turning point is decided by the area of illumination region, wavelength and the correlation area of scattered light. So it is easy to gain the correlation area of scattered light in near field by measuring the position of extremum point and illumination region. By the relationship between the correlation area of scattered light and roughness of surface, it can be used in surface roughness measurement and has the potential applicative value in biomedical systems.
The minimum value of speckle contrast may be written formally based on Equation (6). To refine the qualitative analysis of speckle contrast of scattered light from different surfaces, we consider that the intensity of light is uniform and normally incidence. The intensity of scattered light can be written as (see Fig.9):Fig. 9, we have recursive relation on :
This work was supported by Region Major Program of Fujian Province Natural Science Foundation of China (No: 2009H4003), National Natural Science Foundation of China (Grant No. 61008062), Natural Science Foundation of Fujian Province of China (No:2009J01277 and 2010J01325) and Fujian Province Department of Education Fund of China (No: A10077).
References and links
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