A histogram graph can be displayed in the Gamma Window to show the tonal movement throughout the design. This option can be used on separated layers and RGB or CMYK images, and can be a very helpful tool for seeing which tones are most used on a particular layer.

1. Go to Edit Menu > Gamma.
2. Choose Histogram from the action menu at the bottom of the Gamma Window. This will show a graph of the current tonal movement on the active layer in blue.
   
3. In the example below, the blue curve is mostly below the 50% density line (the middle vertical line on the graph). This means most of the tones on this layer are lower than 50% density and most coverage on the layer is around 10 – 15% density. This could be seen to be a waste of a cylinder and could make colouring this layer confusing, as the colour of the layer will always appear lighter than the colour chip for that layer in the Layers Palette.
   
4. If you adjust the curve, a new pink graph will appear. This displays where the tones will be if you apply the curve by pressing Do Gamma. The blue curve remains visible, so you can compare what the tonal movement was, and what it will be after applying the curve. The adjustment in the snapshot below shows more tones at higher percentages, and there are now some 100% tones on the  layer, indicated by the small vertical line at the 100% mark on the right hand side of the graph. The pink graph shows that around 35% of the layer has a density of 100%.
   

If you need the separation to look similar to the original effect (the snapshot in Step 3), you can recolour the spot layer lighter. In the snapshot below, the curve in the Gamma Window has been applied, and the layer has been recoloured. Notice the blue graph of the histogram is the same as the pink one above.

Using the histogram in the Gamma Window can be helpful to see if you have a nice balanced tonal range on your separated spot layers while ensuring that you have some areas of 100% tone on the layer, if that is a requirement for your production.