The main issue here is the directional lighting which illuminates mainly one side of the characters, so that a part of them disappears in the background. If you rotate the part, the appearance of the letters will change; this is bad for recognition. You may also face fragmentation of the characters (K for example).
I don't like this adaptive thresholding at all, as it reveals a lot of the texture and could very well cause the characters to touch surrounding blobs and be impossible to segment out. You should increase the adaptiveness scale or use global thresholding.
If possible, improve illumination using a diffuse source and let the background disappear.
No preprocessing step will restore what adverse illumination has lost.