How can I detect if the backgroud is brighter than the coins or it's darker?

For example:

It is important because when I make a binary image out of them, than I have to invert the image to be able to count it. (I have to invert the one with a brighter background) What is the best solution for this problem?

Answer 1

[Walter Roberson]

Examine the 4 corners. rgb2gray them, find out how many are > 128. If at least 3 are > 128 then you have a bright background.

This is not fool-proof. A solid bright red or bright blue background would have a brightness less than 128 after conversion to grayscale, but a solid bright green background would, so that tells you that you need to pin down more closely what you mean by "bright background"