RTL is a special raster format from Hewlett-Packard and not that well support in common graphics editors. You have to use specialized converters, such as SDI Convert if you are trying to transform it to some of the more common graphics formats, like PNG or JPEG.
Convert HP Raster Transfer Language bitmap image file to Portable Network Graphic.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert rtl to png on Windows
A CAD CGM converter
Convert from rtl file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.