Level and window
The level and window settings define the linear mapping of the gray values of the CT volume to the gray values of the computer screen, ranging from 0 to 255.
The level and window tool can be used on 2D viewers to change the level and window of the values used to convert the data into (CB)CT reslices. Interactively drag the cursor to adjust the contrast and brightness of the shown image or set the level and window according to specific presets.
Note
The action cannot be used on a 3D viewer.
In the Viewer menu in the title bar of the different 2D viewers, click 
Level/window. Alternatively, use the shortcut [Ctrl+L] or [Cmd+L]. A level/window mouse icon and a tooltip are shown.
Drag on a 2D viewer to change the level (brightness) or the window (contrast). While dragging, the level and window values are shown in the tooltip.
Change the level by vertical movement over the 2D viewer: drag downwards to lighten the image (decrease level) or drag upwards to darken the image (increase level).
Change the window by horizontal movement over the 2D viewer: drag from left to right to obtain a lower contrast (widen window) or drag from right to left to obtain a higher contrast (narrow window).
Right-click to stop the action.