Gerber Viewer (gerbv) is a viewer for Gerber files. Gerber files are
generated from PCB CAD system and sent to PCB manufacturers as basis
for the manufacturing process.

The different layers of the PCB are separated into different
files. gerbv can load all files at the same time, though it can not
show them at the same time. You have to browse through the different
layers with the radio buttons on the right side.
   
gerbv has also a batch feature. By writing a "backend" in Scheme you
can for example generate other file formats or do design rule
check. gerbv parses the Gerber file and calls an external function
written in Scheme. Since the Scheme implementation is based on Guile,
there are more functions available than the ordinary R4RS.