PDF (adobe.com)

Refresh the histogram display

When a histogram is read from a cache instead of the current state of the document, the Cached Data Warning icon  appears in the Histogram panel. Histograms based on the image cache are displayed faster and are based on a representative sampling of pixels in the image. You can set the maximum cache level (from 2 to 8) in the Performance preference.
Note: A higher cache level setting will increase the redraw speed for large, multi-layer files, but requires additional usage of system RAM. If RAM is limited or you work mainly with smaller images, use lower cache level settings
 To refresh the histogram so that it displays all of the pixels of the original image in its current state, do one of the following:
  • Double-click anywhere in the histogram.

  • Click the Cached Data Warning icon .

  • Click the Uncached Refresh button .

  • Choose Uncached Refresh from the Histogram panel menu.

For information about cache level, see Histogram panel overview.