Photo DPI

[Bruce Dorworth]

Ok, where does the true DPI number come from. I was using PhotoGrav to process an image. I load an image, select a material, then resize the image. Well when I go to resize the image was 130 DPI and some size. I changed the size and then changed the DPI to 300. When I engraved the image on a laser tile it looked great except every .5" I got a darker band about .125" wide. I then changed the DPI on the laser to 260DPI and it worked great no banding. Hey, I think I have this laser thing figured out.... I decide to do another image I look at the original DPI and it is 230 DPI, so I decide to laser engrave it at 460 DPI. I get banding, I the try 230 DPI I get banding. This image was processed same as the first at 300 DPI.

So I ask again, where is the correct place to get the DPI number. I can't keep burning up tile.

By the way the image on the laser tile looks really good.

Thanks for the help,
Bruce

[Ernie Balch]

Assuming that your laser is working properly. Banding is a common problem with halftones. As I recall, for a given printer resolution you can represent a certain contrast range in a photo. Making the screen resolution too high causes banding so you solve the problem by using a lower halftone screen resolution to cover the wider dynamic range. Notice that newspapers often use coarse halftones like 45 LPI.

Sometimes adding noise to a photo will minimize the apparent banding.

Other causes of banding can be a collision between grids. When the printer resolution and the halftone resolution are different you get a moire pattern. Put one window screen in front of another and you will see moire patterns.

[Dee Gallo]

I always thought it best to get your image to the final size BEFORE doing any processing, whether in PhotoShop or PhotoGrav or whatever.

[Martin Boekers]

I know this doesn't solve your issue, but do a test first on a cheaper substrate.