PhotoGrav DPI Question

[Jim Watkins]

I have read a lot of things but I have no definitive answer on this. Maybe all of you can lend your experience and thoughts...

At what DPI tends to work well on different materials?

I was doing a 100 dollar bill on a piece of baltic birch ply this weekend @ 600 DPI and I think I may have been lasering over the detail and may have needed to use 300 DPI or less.

What is your experience and on what material has this experience been with?

Thanks all...

[Mike Null]

You may well have discovered something. The dpi setting on your laser may well do the opposite of what logic would lead you to believe. There are many times and materials where a high dpi setting produces less detail than a lower dpi setting would produce.

[Jim Watkins]

I would encourage all to share their experience for all to benefit. Thanks Mike, I expected that more had experienced this phenomena.

[Rodne Gold]

At the MOST the laser can resolve 300 dpi in terms of discrete dots , anything over that = dot overlap and dot gain. so in the first place , 300 dpi should always be as good as it gets (unless you WANT overlap for depth or are using a 1.5" lens which has a small spot size)
Apart from that , one has to look at heat affected zones (essentially dot gain) in terms of how far a "dot" extends. Wood has large heat affected zones , ie zones of "burning" that extend far beyond the actual laser spot size. Photograv does not work with grey scales , it converts the image to a 1 bit black or white image relying on the spacing of the black dots to simulate shades. Thus if the heat affected zone extends beyond the black dots , you get less definition. Another easier to understand definition of dot gain is the difference between using an oil based vs a water based paint on blotting paper , the oil based paint droplet will not tend to spread and leak colour as badly as the water based paint will , so the oil based paint wont have big overlaps between its dots.
So for materials that exhibit dot gain , ie more overlap than the spot size , you need to reduce the DPI. Obviously reducing dpi loses detail but will produce a better visual effect. Essentially photograv "fools" the eye into thinking that the image is shaded.

[Mike Null]

Rodney

You have done a good job of explaining this now maybe you can take it a step further and explain why photo resolution and laser resolution are not the same.

[Rodne Gold]

Well , laser resolution is a single colour , either on or off , and what I mean by that is a laser can only print one colour in that if it fires , it creates a single dot of "black" (or whatever colour the material has when laserd)
However a photo exisits in shades of grey and each pixel of the photo (dot in the photo) assumes MORE than one colour. So for the laser to represent light grey using black , it has to use what is called 1/2 toning. That is it has a matrix of dots it can print , lets say a 3 x 3 dot matrix consiting of 9 cells. If all 9 cels are printed , the dot in the picture its trying to represent would be black , if it prints only the middle dot of the matrix , the 9 cell matric would appear a LOT lighter than black cos of the white cells adjacent to it.
XXX
XXX
XXX
= black
000
0X0
000
= light grey

So in essence , for the laser to represent 1 dot in the pic , it has to use a matrix of dots to fool they eye there is shading.
The dot in the picture is a pixel , the dot the laser lays down in this case is part of that pixel.
What this means , is that if you ask the laser to represent shades of grey , it has to use more than one dot that it fires to do so.
So if we know a laser can only resolve 300 discrete dots per inch and we are using a 3x3 matrix , it can ONLY possibly represent 100 pixels in the picture for that inch.

[Stephen Beckham]

Yea... What Rodne said...

Seriously - great layout...

The only thing I would add is that above 300 DPI setting on my laser does better than the 300 setting. SO the mix between the two is to use a 300 DPI setting and laser it at 600 DPI or 1200 DPI for the better quality. The only things I use 1200 for is the Aluminum and I use the 600 mostly on glass/crystal. If I'm using 300 on wood and there is no text, I use 300 in the graphic and burn at 300 setting.

The other common setting is 400 DPI for lettering. Anything less and you start to see horizontal strips across the etching. Normally not serious enough to call foul, but enough to cause me to use 400 dpi.

That brings up other issues - when I'm doing text and graphic on the side of a glass - I usually run both at a 400 DPI - you may get the cross over on the graphic, but the letters look better. I normally try to work my image down to four colors of grey scale if possible. I can't distinguish more than four-five colors with my eye before they start looking the same on glass - even with photograv. The other options are to make two passes - one at 300 for the graphic and re-print at 400 for the text or like I mentioned earlier - running it at 600DPI which the text and picture both will look good....

[Mike Null]

Rodney

Thank you.

Steve,

If I understand you correctly you and I go about this in just opposite ways.

I'm assuming you mean anodized aluminum when you say aluminum and there I find that 1000 dpi and even too much power will destroy detail in an image.

On the other hand I frequently use 1000 dpi when doing light woods in order to effect a darker burn.

[Stephen Beckham]

WOW - I'll have to try it... I played with the higher DPI on scrap maple and never felt it was working right - that was when I first got the laser... It could have been me:eek:

On the Annodized Aluminum - yes I use the 1200 DPI, but crank the power way down. Depending on how much detail of the photo - almost half of the recommended power. I too have found that too much power has no where to go when hitting Aluminum - therefore - it goes sideways.

The other thing I do is consider the power/speed as a ratio. When doing dogtags, I slow the speed down and reduce the power by that much more. It seems to give a better result versus just using the settings in the book.

[Jim Watkins]

Great responses. Just what I was looking for. I think so many here will benefit.

To the issue Stephan and Mike are going over. I by my sales rep. that it was his experience that when you use that high of a DPI on Annodized Aluminum, your almost whitewash the immage due to exactly what Rodne was talking about, dot overlap. The higher the resolution, the more the overlap and that tends to washout the image.

I have not experimented with Annodized Aluminum as of yet, kind of next on my list. However what he said seemed to make sense.

Thoughts?

[Mike Null]

That has been my experience and I do a considerable amount of anodized aluminum.