Try Canny filtering first to detect edges
Dim img As New Image(Of Bgr, Byte)("img.jpg")
Dim imgCANNY As Image(Of Gray, Byte) = img.Canny(90, 70)
Then make Binarization
Dim img_BINARY As Image(Of Bgr, Byte) = img.ThresholdBinary(New Bgr(100, 100, 0), New Bgr(0, 100, 100))
Convert it to C# and play with numbers to get best result, also u can try different filtering to get better result
(Sobel, Laplace, Sub, EqualizeHist, SmoothBilatral)
Additional information here:
www.emgu.com/wiki/
Best,
Nick