In the Image window, click 
to open the Image Scaling window.
The Image Scaling window includes a line with three nodes (or handles) that graphically represent the intensity scale. It also includes a histogram that shows the distribution of pixel intensities in the image file. The Y-axis shows the number of pixels for a given intensity. The X-axis represents intensity values.
In the Wave options, select which channel to scale.
To change the range of the image scale, use the mouse to click and drag the white handles on the line as follows:
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To change the minimum or maximum scale value, click and drag on the left or right handle.
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To slide the scale range to the left or right, click and drag on the center handle.
As you move the handles, the image that is displayed in the Image Viewer changes. Changing the minimum and maximum intensity values changes brightness and contrast by mapping all 256 color shade values to a larger or smaller range of data. Data values higher than the maximum value are assigned the brightest shade. Data values lower than the minimum value are assigned the dimmest shade.
To change the intensity scale distribution, change the shape of the curve by clicking anywhere on the histogram (except on one of the handles) and dragging the mouse up or down.
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To improve contrast at the low end of the intensity range, increase the slope of the curve at the left side of the graph.
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To improve contrast at the high end of the range, increase the slope of the curve at the right side of the graph.
Changing the scale distribution increases the contrast at one end of the data range and decreases it at the other end.