How does Photograv process black and white

I have an Allen Datagraph monumental laser system which is used mainly to engrave black granite monuments. My laser's manual has a tutorial on how to set up a job using Photograv and says this...

 "In order for PhotoGraV to see a complete range of white to black we need to have a white border."

So, I am assuming that Photograv needs to see at least some white, and some black and then will fit everything else distributed in between those two extreme values.

I am usually lasering graphics or portraits sized about 8"x10" or smaller on stones approx. 12" x 24". I like to set up my jobs sized to fit the entire monument being engraved, rather than just the 8"x10" size (this makes it easier to center jobs) This results in a file with the 8"x10" image surrounded by the remaining empty area of the 12"x24" which I have set to be black.

Is all of that "empty" black area affecting Photograv's results? If it is true that you do have to have totally black and totally white values present, does the amount of black or white present change the way that Photograv will process the image?

I have noticed that I get different results from Photograv if I only process the 8"x10" graphic without all of the surrounding black.

Any help is greatly appreciated.


Edited by walmem - Jan 25 2010 at 3:15pm