When we seek the definitions of Brightness and Contrast, we get the following answers from Wikipedia:
“Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target. This is a subjective attribute/property of an object being observed.”
“Contrast is the difference in luminance or color that makes an object (or its representation in an image or display) distinguishable to the human eye. In terms of visual perception, contrast is determined by the difference in the color and brightness of the object and other objects within the same field of view.”
So, the phrase “brightness of an image” does not provide sufficient inference. The brightness of every pixel in the image is its intensity value. We can define the brightness of the image to be the mean intensity value but it can always be changed by adding or subtracting a fixed number to/from each pixel intensity.
Contrast is a construct of visual perception. We can always define the contrast of an image to be one of the several ways mentioned here. In fact, different books provide different definitions. But the problem is that each of these definitions makes sense only for very simple images. We can’t usually numerically define the contrast of a complex image. Sure, you can use the, say, RMS contrast to be the value but I am not sure whether it provides any useful information for complex images. However, we can easily change the contrast of an image by multiplying/dividing the intensity values of the image by a fixed number. (Eg: Dividing by the standard deviation of the pixel values will be interesting)
Anyway, if you have to know the brightness and contrast values of an image, take the mean intensity value as brightness and the RMS contrast as the contrast value. Brightness refers to the overall lightness or darkness of the image. Increasing the brightness of every pixel in the frame gets lighter. Contrast is the difference in brightness between objects in the image. Increasing the contrast makes light areas lighter and dark area in the frame becomes much darker. Hence, brightness gives an understanding of the mean of pixel intensities while the contrast gives that of the standard deviation of pixel intensities in an image.
Thanks to my dear friend Aruna for asking this probing question.