for the moment i managed to partially solve the problem by changing the original image to black and white
factor 165 works best for me

Bitmap bM = (Bitmap)System.Drawing.Image.FromFile(picture_path);
bM = make_bw(bM);
oOCR.GetTextFromImage(bM, Square(x1, x2, y1, y2));

public Bitmap make_bw(Bitmap original) /*from https://stackoverflow.co...ure-black-and-white-in-c*/
{

Bitmap output = new Bitmap(original.Width, original.Height);

for (int i = 0; i < original.Width; i++)
{

for (int j = 0; j < original.Height; j++)
{

Color c = original.GetPixel(i, j);

int average = ((c.R + c.B + c.G) / 3);

if (average < 165)
output.SetPixel(i, j, Color.Black);

else
output.SetPixel(i, j, Color.White);

}
}

return output;

}