Characterization techniques - Micromixers Fundamentals Design and Fabrication - page 306

digitizing device. A bit depth of n can generate 2 n different gray scales. Most common CCD and
CMOS sensors provide bit depths of 8, 10, or 12. An 8-bit, 10-bit, and 12-bit depth can describe 256,
1024, and 4096 different gray levels, respectively. If the image shown in Fig. 8.9 has a bit depth of 8,
the value of the white pixel (1,1) is 255, while the value of the black pixel (6,6) is 0. The human eye can
better discriminate between different shades of color than between varying shades of gray. Therefore,
gray-scale images are often rendered in pseudocolor by assigning specific gray-level ranges to
particular color values. This technique is useful for highlighting regions of interest, especially
concentration fields in micromixers. Pseudocolor imaging is widely employed in fluorescence
microscopy to display merged monochrome images obtained at different wavelengths utilizing
multiple stained specimens. Often, the color assigned to individual fluorophore images in a collage
assembly is close in color to that naturally emitted by the fluorescent dye.
The intensity of the pixels in a digital image can be graphically depicted in a gray-scale histogram,
which maps the number of pixels at each gray level present in the image. The histogram can be used
directly for evaluating the extent of mixing. Gray-scale histogram of a well-mixed concentration field
shows a single peak, while the histogram of a not mixed concentration field shows two intensity peaks
of the two liquids.
A digital image can be stored in a single file. Intensity values are stored in a single vector. The
coordinate of the pixel does not need to be stored because it can be derived from the known image's
spatial resolution and incremental counting of the pixels. The horizontal and vertical dimensions of an
image are often stored in the header of the image file. Reading the files with the known dimension can
restore the two-dimensional matrix for further processing. The size of a digital image file depends on
the pixel dimensions, formats (compression algorithms), and bit depth of the image. Uncompressed file
formats, such as tagged image file format (TIFF) and Windows image bitmaps (BMP), result in the
largest file size. Using compression algorithms, such as the popular Joint Photographic Experts Group
(JPEG) technique, can reduce the file size significantly, while the image quality is reasonably
maintained.
Digital images of optical sections obtained through confocal laser scanning microscopy can be
rendered to make a three-dimensional image. Two popular rendering techniques of an image set for
display in three-dimensions include volume rendering and surface rendering. In volume rendering, the
two-dimensional pixel geometry and intensity information are combined with the known focal
displacements to generate volume elements, termed voxels. The resulting voxels are then appropriately
shaded and projected to produce a view of the specimen volume, with associated perspective and
lighting to produce a three-dimensional representation. In surface rendering, only the surface pixels are
utilized, representing the outside surface of the specimen, and the interior structure is not visible
because of the surface opacity. Lighting, perspective, and depth cues subsequently help in generating
a three-dimensional representation of the image set.
8.2 MEASUREMENT USING OPTICAL MICROSCOPY
8.2.1 Measurement of velocity field
The most common technique for measuring the velocity field inside a micromixer is micro-
particle image velocimetry (micro-PIV). PIV was originally developed for the measurement of