![]() An example of the result of such an approach can be seen in Figure 11. Because the CCD camera output is linear in intensity, the ratio of the two images is a true representation of the SPR-related reflectance variations over the imaged surface. The image with p-polarization contains the SPR microscopic image together with the contrast generated by spatial nonhomogeneities, whereas the image obtained with s-polarization only contains the same unwanted nonhomogeneities. In the SPR microscope the Pockels cell is employed to acquire two images using p- and s-polarized light, respectively. Such a cell can be configured such that the application of a voltage results in transmission of either p- or s-polarized light, without moving any part in the light beam. An appropriate method is the use of a Pockels cell with simultaneous digital image processing. This can be partially solved by spatial filtering as indicated in Figure 10 however, if there are any unwanted, spurious reflections in the light path between the spatial filter and CCD camera, then again a beam nonhomogeneity will occur owing to the relatively large coherence length of the laser light used. Another possible way to increase SNR is to increase laser power but this option is of limited value in view of the resulting destructive heating effects on the sample.Īnother experimental problem is lateral nonhomogeneity of the incoming laser beam intensity profile. Such an averaging will result in a signal-to-noise (SNR) improvement with a factor of n 1/2 where n is the number of added images. This can be particularly important if shot noise is present when working with low light levels. Integrating a number of images also results in averaging. To avoid this effect an option is to add a number of images with an effective dynamic range substantially larger than that of the frame-grabber used. With such relatively low contrasts the use of a digital frame-grabber (which has a typical dynamic range of 2 8) can result in quantization effects during image acquisition, which become apparent if a low-contrast image is digitally amplified. ![]() As already mentioned, the use of shorter wavelengths generally improves lateral resolution, but simultaneously results in lower intensity contrasts for a given height difference. Kooyman, in Encyclopedia of Spectroscopy and Spectrometry, 1999 Improvement of image qualityĪpart from the diffraction limit of the use of imaging lens there are a number of factors determining the eventual quality of a SPR microscopy image.
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