Threshold Image Segmentation with different contrasts

Hey there,

I am trying to make some easy image segmentation for a lot of images like the test_image above. The resolution of the image is quite bad. Now my question: I am trying to separate it with an easy threshold segmentation, but there are too many parameters that I have to adjust for images with different contrasts. Is there a more easy and accurate way to separate the black parts within the white circle?

Thanks a lot for your help!

Here is my code:

[filelist, pathname] = uigetfile({'*.jpg';'*.tif';'*.bmp';'*.png'}, ...
    'Select Folder', 'MultiSelect', 'on');

data_in = imread([pathname,filename]);
images(:,:,1) = rgb2gray(data_in);

[x,y,z] = size(images);

bw = (30 <= images(:,:,1)) & (images(:,:,1) <= 190);
bw2 = bwpropfilt(bw,'Area',[20,Inf]);
bw2 = ~bwareaopen(~bw2, 80);

mask = imfill(bw2, 'holes');
se = strel('disk', 20, 0); % Adjust to erase outer ring
mask = imerode(mask, se);
bw2(~mask) = false;

figure, imshow(bw2);