Page 300 - Fundamentals of Light Microscopy and Electronic Imaging

CHAPTER
15

DIGITAL IMAGE PROCESSING

OVERVIEW

Taking pictures with a video or digital camera requires training and experience, but this
is just the first step in obtaining a properly acquired and corrected image. Image pro-
cessing is used for two purposes: (1) It is required to establish photometric accuracy so
that pixel values in the image display the true values of light intensity. (2) It is also an
essential tool for optimizing an image for scientific publication. In preparing an image
for display or printing, it is essential that the image represent the specimen as objec-
tively as possible, and with few exceptions, this means including all of the information
(intensity values) contained in the object image. Because it is a matter of utmost impor-
tance, we discuss basic image processing operations in this chapter and guidelines for
preparing images for scientific publication in Chapter 16. You should distinguish
between the following terms: image processing, which refers to the digital manipulation
of pixel values in the image, and image analysis, which encompasses counting and
measuring operations performed on objects in the image. Many commercial software
programs for camera control and image acquisition perform these functions. Image
analysis, including measurements of morphological features, the use of grids in stereol-
ogy, image segmentation, and other important topics, are not covered in this book. Inter-
ested readers should refer to excellent texts by Russ (1998) and Russ and Dehoff (2000).
In this chapter we review four operations that are essential for understanding and using
image processing, namely:

1. Adjusting the image histogram to regulate brightness and contrast
2. Performing flat-field correction to establish photometric accuracy
3. Applying spatial filters for image enhancement
4. Using S/N ratio calculations to determine confidence limits of observations

In processing images, proceed through the following steps, performing them only
as necessary:

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