You can use the Otsu threshold

In matlab you can use following code to find Otsu Global thresholding for an image (I).

ThreholdValue = graythresh(I);

Then you can binarize the image like this:

binaryImage = im2bw(I,ThreholdValue);

If you work with matlab

u can calculate it with mean of pixel on the image. but first to do is make ur image become gray first. rgb to gray -> (0.7,0.2,0.1) = rgb. the total must be 1. after become gray image u can use many logic to determine the threshold of image.