Number plate recognition using tessract

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I am planning to perform ocr on Indian number plates.I used tessract 4.0 beta which uses LSTM engine for ocr. Although recognized characters are not coming out to be correct. I used cv2.Laplacian() while picking up images without blur and performed noise reduction using cv2.fastNlMeansDenoisingColored() on the image. For preprocessing I performed following steps

1)Sharpened image

2)performed otsu thresholding

3)Eliminated smaller noises/contours

4)Then inverted image

5)OCR using tesseract 4.0 --oem 1(Which uses lstm as detectio module )

Test images look like these.

Detected output: OL 1CT 5079 (Which seems ok)

Can you please suggest any other preprocessing required to improve image(Reduce noise)?

Also is there a way to restrict special characters in tesseract?(Could not find it in latest version)

(ps. I am using python)

Thanks in advance