Ocrad cannot recognize text in png format file

Hi,

I'm beginner user of ocrad.

Ocrad cannot recognize text in png format file.

The text in png file is as follows:

        Filesystem           1K-blocks      Used Available Use% Mounted on

The result of invoking command:

        pngtopnm filename.png | ocrad

is the following:
 
        |_n__|_n_ O___oO -_ o_


Can you tell me how I need to "force" ocrad to proper recognize text which is "coded" in my png file.

Jerzy
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