Thresholding in luminance image

Question

I have an image that contains an illuminate. First I crop the area which I want to process then convert it into the binary image. I use Otsu's thresholding, but it gives a bad result for this problem. I have to try to use adaptive threshold, but this method dependent on block size and C parameter (opencv method). What should I do to get a good result in this problem?

Original image, but I crop the certain area :

Otsu Thresholding result

Answer 1

adaptive threshold in not suitable for your case. if you like to simply create a binary image with black background and white text (or vise-versa), and you have tight cropped area, you can simply do below steps:
1-convert image to gray scale
2-normalize your image (ignore 1% of darkest and lightest pixels)
3-use a fixed threshold (something between 0.3 to 0.7)
4-do some morphological enhancement like eroding, dilating, opening and closing for eliminating noise.
adaptive thresholding used in case of uneven luminance when you have a gradient light on board which is not present in you example.