Bio-Rad Image Lab™ Software - Manual (Page 172)

6 | Analyzing Images
172 | ChemiDoc MP Imaging System with Image Lab Software
Volume Background Subtraction
When you draw a volume, some nondata background pixels might be included
inside the volume. These background pixels usually have an intensity value that you
do not want to include in your volume quantification. There are two methods of
calculating this background intensity for your volumes: local and global.
 Local — local background subtraction calculates a separate background
intensity for each unknown and standard volume you create. For each
volume, the intensities of the pixels in a 1-pixel border around the volume
are added together and divided by the total number of border pixels. This
gives an average intensity for the background around each, which is then
subtracted from the intensity of each pixel inside the volume. If the
background value is greater than the pixel value inside the volume, the
background-adjusted quantity of the volume could be <0. In this case,
redraw the border for this volume.
 Global — global background subtraction calculates a single background
intensity for the entire gel. This average background intensity is then
subtracted from all the volumes in the gel. The average intensity of the
pixels in the background volume is calculated and subtracted from each
pixel in all standard and unknown volumes. Therefore, it is not necessary
for the background volume area to be the same size as your unknown.
To calculate global background subtraction
1. Use one of the Volume Tools to create a volume in a representative background
region of your image (that is, a nondata region similar to the background
surrounding your data).
2. Double-click the volume. This opens the Volume Properties dialog box.
3. Select the Background option button.
Note:
 If you select Global in the Volume Tools toolbox but do not define a
background volume as described, no background subtraction is
performed.