Window/Level  
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Menu:   View > Window/Level  

Use this command to optimize the mapping of pixel values to the luminance scale of the display device without changing the actual content of an image. Window/Level tool can be used to increase contrast and enhance display in low-level situations. You can view the tonal range of an image using the historgram display
above the sliders. If you choose this command, the Levels dialog box will appear with the following options:

 
Levels
 
Let you set the darkest and lightest pixel in the input image to be mapped to the luminance scale of the display device. Two vertical markers on the histogram indicate the lower and upper limit of the intensity levels. All pixels whose value is equal or lower than the lower limit will be displayed in black. All pixels whose value is equal or higher than the upper limit will be displayed in white. You can change the lower or upper level by dragging a corresponding marker with the cursor, or you can move the window they form by placing the cursor between the markers. You can also indicate the desirid values of the levels in the respective boxes.  
 
+ --
 
Allow you to zoom in and out the window between the markers.  
 
Reset
 
Lets you reset the lower and upper limits to the high and low ends of the image dynamic range.  
 
Best Fit
 
Optimizes the position of the lower and upper limits to provide the maximum contrast between the brightest and darkest pixel in the image.  
 
 
Update   
 
Applies the changes to the image content by rescaling its pixel values. If you do not use this option, Window/Level only changes the appearance of the image without affecting pixel values.