Slight horizontal banding on most dithered photos... on black granite, Problem

[jeffb1 (Jeff Bromagem)]

Hello All,
I am having an issue with some slight horizontal banding on most dithered photos.
Some photos have come out fine with no visible banding, so I am having trouble chasing down the problem. Line art comes out perfect, so I am pretty sure it is not the laser, power supply, belts, etc.

Here is what I have so far.

Hardware: Cam-Tech with 25 watt CO2 Synrad tube, no mirrors, upgraded to DSP Trocen 708c.

Substrate: Black Granite

I am new to photo engraving on this laser setup using the 708c and Lightburn. My Epilog lasers are a different story (easier).
I have tried a number of things, some of which are:

I have matched and re-sampled the photos dpi (250) to match the laser interval setting on the laser (tried .0039 and .004, and even .0047), I still get uniform banding. I have tried different dither patterns, stucki, jarvis, diffusion dither, still have banding. I have tried 300dpi, and others for the photo too.

I have exported the photo from Photoshop as a diffusion dither bitmap, still banding.

I have exported the photo from Photoshop as grayscale, imported into CorelDraw and converted it into bitmap with the dither options, still have banding.

Some photos were quite small (supplied by customer) and they were re-sampled from maybe 1.5" up to 13", using either 300dpi or 250dpi for final enlarged photo.

Is there a way to go about finding the best settings for photo DPI., laser Scan Interval, etc that will work?

Need to get a solution, as I have jobs waiting that I can’t engrave till I get this to work.

Photos will be added.

Thanks for any help,
Jeff

[jeffb1 (Jeff Bromagem)]

Hello,
The top image is good, no visible banding, but the second picture has banding across the shirts.
The bottom photo, please ignore the top 1" or so, I stopped the laser to try something different.
The rest of the image is a girl’s hair and forehead, with noticeable banding in the forehead area.
Sorry, the pictures don’t show the problem real well, due to lighting and reflections, etc.
The banding shows up even more when the image is filled with white ink/paint to give it a brighter, whiter look. These shots are not color filled.
Thanks again.
Jeff

![IMG_2245|375x500]

[Enno (Enno Groen)]

first of all your photo dpi should match your laser dpi settings in order to avoid such things. But it looks like something mechanical to me but it could be other machines kicking in like a chiller or such.

[jeffb1 (Jeff Bromagem)]

Enno, Thank you for your reply.

I don’t think it is mechanical, as the frequency of the banding changes with the photos and is not there at all on line art and solid logos (no dithering). It also appears in the same locations on the photo on another run. I believe it has to do with the dpi, and the dithering interacting and overlapping of the two. Very similar to Moiré patterns for those of you familiar with halftones and wave interface patterns.

There are no other machines or equipment or electrical loads running that should/would cause this. This particular laser is in a building with nothing else running, except for a few lights and a small Gast diaphragm air compressor for the stream of nozzle air, it runs continually until engraving is done. The building has 1600 amps of 3 phase power going to it, directly from transformers outside. No chiller. No one else in the building except me. Laser is 25 watt Synrad RF tube, running at 15-17% output, for granite. It does not produce much, if any heat that is even noticeable. I have a temp sensor and digital display on it, staying around 70-75 degrees F.

I think my main problem is finding the best DPI and scan interval for my laser to eliminate the interference.
Anyone have ideas on that?
Thanks again,
Jeff

[Enno (Enno Groen)]

create a picture in the right size at 300 dpi and burn it at 300 dpi, but my guess is you will still have the banding. If the banding was because of the dpi it would be more uniform. Turn the picture 90° and see how the banding is?

[Rick (Technical Evangelist)]

Are you doing any image processing / dither prior to bringing into LightBurn?

[The_Chaplain (Charlie Hawkins)]

I had a similar type of banding with an old Epilog Summit laser, I thought it was mechanical, but it may have been from the dpi settings. I used Corel graphics 12 and RDWORKS to engrave with, and the results were very good on marble and black granite tiles from Home Depot.

[jeffb1 (Jeff Bromagem)]

Thanks guys,
I think I have it under control now. I am used to the Epilog drivers and how they handle photos.
I’m fairly new to the Chinese controllers and Lightburn software and doing photos with them Little bit of a learning curve there.
It seemed to be an interaction with the DPI and the scan interval setting (which I was not used to setting separately if I changed DPI). I had great results with one of the above photos. I installed the newest version of Lightburn and it has simplified those settings for the dithering, etc.
Thanks for all the input, it is appreciated.
Jeff