In this window is shown a histogram of Preview or Stored image.
The histogram illustrates how pixels in the image are distributed by graphing the number of pixels at each color intensity level. This can show you whether the image contains enough detail in the shadows (shown in the left part of the histogram), midtones (shown in the middle), and highlights (shown in the right part) to make a good correction.
The histogram also gives a quick picture of the tonal range of the image, or the image key type.
A low-key image has detail concentrated in the shadows; a high-key image has detail concentrated in the highlights; and an average-key image has detail concentrated in the midtones. An image with full tonal range has a high number of pixels in all areas. Identifying the tonal range helps determine appropriate tonal corrections.
Current information in the Histogram window is dependent on similar conditions like in Zoom window - for digital still camera and 'Folder Monitor' source - if mix is equal 100% - histogram shows Preview image information, below 100% stored image information. Moreover for showing information of stored images Zoom window must be visible. If Zoom window is hidden histogram includes Preview image only.

For video source, histogram shows information from Preview as well.

- 'R', 'G', 'B' check boxes - turn on/off histogram of RGB components.