Epson 660 - Manual (Page 54)

In addition, if we take a look at the two images below:
A Histogram displays the distributing status of dark and bright pixels in
an image. The x-axis is divided into 256 equal portions, representing the
brightness level of 0 to 255. The y-axis is used to represent the quantity
of pixel in each brightness level. A comparison of two images and their
histograms follows:
Brighter images have
brighter pixels, therefore
the bars in the right are
higher than that in the
left.
Darker images have
darker pixels, there-
fore the bars in left are
higher than that in the
right.
Image 1 Image 2