Because the luminous intensity of the diffraction pattern on the phosphor screen is low, successive frames may need to be integrated in the CCD camera to improve the signal to noise ratio in the image.
Using too short an integration time on the CCD camera may give a poor image and in this case, integrating for longer on the CCD will improve the visibility of the diffraction pattern (Figure 1). The camera may be cooled to reduce electronic noise in the CCD when used in this way. The yield of backscattered electrons increases with atomic number, so low atomic number materials will require a longer integration time than higher atomic numbers.
(a) Effect of integration time and probe current on diffraction pattern. 36 ms, 2 nA (Probe currents are approximate).
(b) Effect of integration time and probe current on diffraction pattern. 36 ms, 200 pA (Probe currents are approximate).
The CCD camera resolution can also effect the integration time required to collect a diffraction pattern. Current CCD cameras can collect 12 bit images at a resolution of 1344 x 1024 pixels. Using “pixel binning”, neighbouring pixels in the CCD can be added together to form a single pixel in the image. When pixels are binned in this way, less integration time is required to achieve a given signal in the image pixel because the detecting pixel area is larger. For example, if a satisfactory diffraction pattern is obtained in 12 ms at 1344 x1024 resolution, a comparable pattern can be obtained in 3 ms at 672 x 512 resolution.
In addition, the transform works well on noisy images (Figure 2) so some experimentation is necessary to determine the optimum integration time required when collecting maps.