Thanks Frank / Rabbit Laser and Dean @ the-lasersedge.com

[ArturoV]

Originally Posted by SkipW I think the scan gap is the distance between passes, or resolution. The higher the number, the greater distance between passes of the beam. It should probably be named "span gap". Hope that made sense. SW

Does that mean the higher the number the lower the resolution is? Any idea what "expand scale" do?

thanks,

[SkipW]

Yes, the higher the number, the lower the resolution. (why didn't I say that?)

I don't have a clue as to what the expand scale does, sorry.

Skip

[glintid]

The scan gap is the distance the laser moves vertically (the Y axis) between passes. I think the Rabbit laser software expresses the distance as a fraction of a millimeter, ie. 0.1 is 1/10 mm etc. Because I am used to resolution expressed as standard numbers DPI (dots per inch), I converted mine and keep a chart near my computer. For example 300dpi is 300/25.4 dp mm which equals 11.811 dpmm. To convert it to distance between dots (passes) in mm you invert the number. So 1/11.811 = a scan gap of 0.084mm. I did this conversion for all the common usable resolutions which are 600, 500, 300, 250, 200, 150, 100, 75, 50, 40. Easier for me to remember than .084. When engraving photos, you want to use a scan frequency that is a multiple of the dot screen applied to the photo to convert it to a 2 bit .bmp file. This avoids a moire which is interference which is resultant when two lined patterns are superimposed and are out of phase. Like placing two pieces of screen material together at a slight angle.

If that seems simple, hold on to your hat. There is another variable affected by Scan Gap, power. It's actually logical. The laser spot on any material is not a crisp circle. The burn microscopically varies from material to material. It's similar to the pattern of a flashlight on the wall with an intense centre fading out to the edges. When engraving an image made of dots, the resolution is maintained by definition between the "black" zones and the unburned "white" zones. Some marerials .are inverted.

Each material is different. When the scan gap is close, the edges overlap which doubles the power at that point on the material. As the centre spots get closer, as you can imagine, the effect can be quite a dramatic increase in power. This sound efficient but in reality negatively impacts the image resolution, tending to blur it. So dependng on the burn rate of the material, you may have to increase the scan gap to get the optimum resolution. Wood for example requires a much wider scan gap than anodized aluminum.

Some brittle materials like glass, granite and marble don't burn but tend to microscopically fracture from the heat of the laser. (I think marble and granite fracture, open to correction). Anyway, if the scan gap is too close and the laser overlaps too much, the glass heats up and fractures through the area between dots resulting in a blurred, low res image. Some acrylic too tends to melt the whole surface when the dots are dense in the shadow areas and you loose resolution completely in the shadows resulting in a throw away.

These are just a few things I have learned by myself with trial and error and reading some scientific stuff on lasers and how they interact with materials. From this one realizes that laser focus and optics can make a huge difference. I have been thinking of getting an optically better (more expensive) lens for my laser to see if this in fact makes a difference to the resultant dot pattern. By the way, I experiment on plain, cheap brown card for the basic settings. It engraves quickly and holds a photographic image quite well.

Anyone suggest a source in Canada (or the US) for the lenses?

[MacGyver]

I've searched a bit, but haven't come across anything vastly cheaper for optics. You can always go with the ol' standby of Edmunds, but also consider using Thor Labs... they're optics seem reasonably priced (comparatively speaking, of course).

[SkipW]

Originally Posted by glintid The scan gap is the distance the laser moves vertically (the Y axis) between passes. I think the Rabbit laser software expresses the distance as a fraction of a millimeter, ie. 0.1 is 1/10 mm etc. Because I am used to resolution expressed as standard numbers DPI (dots per inch), I converted mine and keep a chart near my computer. For example 300dpi is 300/25.4 dp mm which equals 11.811 dpmm. To convert it to distance between dots (passes) in mm you invert the number. So 1/11.811 = a scan gap of 0.084mm. I did this conversion for all the common usable resolutions which are 600, 500, 300, 250, 200, 150, 100, 75, 50, 40. Easier for me to remember than .084. When engraving photos, you want to use a scan frequency that is a multiple of the dot screen applied to the photo to convert it to a 2 bit .bmp file. This avoids a moire which is interference which is resultant when two lined patterns are superimposed and are out of phase. Like placing two pieces of screen material together at a slight angle. If that seems simple, hold on to your hat. There is another variable affected by Scan Gap, power. It's actually logical. The laser spot on any material is not a crisp circle. The burn microscopically varies from material to material. It's similar to the pattern of a flashlight on the wall with an intense centre fading out to the edges. When engraving an image made of dots, the resolution is maintained by definition between the "black" zones and the unburned "white" zones. Some marerials .are inverted. Each material is different. When the scan gap is close, the edges overlap which doubles the power at that point on the material. As the centre spots get closer, as you can imagine, the effect can be quite a dramatic increase in power. This sound efficient but in reality negatively impacts the image resolution, tending to blur it. So dependng on the burn rate of the material, you may have to increase the scan gap to get the optimum resolution. Wood for example requires a much wider scan gap than anodized aluminum. Some brittle materials like glass, granite and marble don't burn but tend to microscopically fracture from the heat of the laser. (I think marble and granite fracture, open to correction). Anyway, if the scan gap is too close and the laser overlaps too much, the glass heats up and fractures through the area between dots resulting in a blurred, low res image. Some acrylic too tends to melt the whole surface when the dots are dense in the shadow areas and you loose resolution completely in the shadows resulting in a throw away. These are just a few things I have learned by myself with trial and error and reading some scientific stuff on lasers and how they interact with materials. From this one realizes that laser focus and optics can make a huge difference. I have been thinking of getting an optically better (more expensive) lens for my laser to see if this in fact makes a difference to the resultant dot pattern. By the way, I experiment on plain, cheap brown card for the basic settings. It engraves quickly and holds a photographic image quite well. Anyone suggest a source in Canada (or the US) for the lenses?

glintid you are the man !!! Thanks for that awesome explanation and for the formula for converting scan gap into resolution. You've saved me many hours of work.

Skip

[glintid]

Originally Posted by SkipW glintid you are the man !!! Thanks for that awesome explanation and for the formula for converting scan gap into resolution. You've saved me many hours of work. Skip

Hey no problem Skip, I was asking the same questions not so long ago.