# Line interval questions

Can someone please explain line interval a little please? I understand line interval is the distance between each linear burn line, but the numbers for dpi, are they set in stone? Let me explain a little… if i go in and prep a photo to 282, which is a scap gap of .09 and then literally go to the scan gap and change that to .08, the dpi jumps to 318. 318 seems to be a pretty popular number that gets thrown out there for image prep, but what about the dpi numbers between 282 and 318? Would it not work if it were a scan gap of something odd like .085?
Just trying to understand lasers a bit better and this situation seems to confuse me. I was told its 252 or 282 or 318, etc. Nothing in between.

The “native units” in LightBurn are millimeters, with a resolution of 0.01 mm. When you set a value in dpi (dots per inch), it gets converted to the metric equivalent (mm per line) and chopped to a multiple of 0.01 mm.

So those mysterious numbers look like this:

• 254 dpi → 0.10 mm/line (exactly)
• 282 dpi → 0.09 mm/line
• 318 dpi → 0.08 mm/line

You can’t use values between those dpi numbers, because there aren’t any “numbers” between the mm equivalents. Well, you can type them in, but the result is back-converted from the chopped metric equivalent.

Given that 0.01 mm is about one wavelength of the IR light coming out of a CO₂ laser, it’s close enough for most measurements.

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Ok, i can wrap my head around that. Thank you for this description. I was picturing some strange pi like number in the background of numbers between 282 and 318 dpi lol.

Sometimes it’s easier to think of this in terms of the machine abstractly.

All of these are a ‘tool path’ that is followed by the tool (laser). This is the guideline of cut. You adjust the offset from the tool path in two direction, this is a kerf adjustment. Your ‘tool’ will cut the tool path or on either side, if programmed.

All the line interval is, from the software’s perspective is how close the tool paths are from each other… As with math in general you can ‘compute’ this down much further than is practically useful.

If my math holds out and my memory for size, the pulley on mine is about 20mm X 3.14 is about 63 / 2000 steps/rotation is about 0.0315mm in one step… I’d think that’s the best resolution you could hope to achieve. There are u-step devices out there…

Seem correct to the rest of you?

An easier way to think of it without dragging in pi:
`(teeth around pulley × belt pitch) / (steps/rev)`

The controller keeps track of the intended position with high resolution, rounds it to the nearest possible stepper position, and drives the motor to that position. The internal position doesn’t accumulate errors due to stepper position truncation and the stepper always stops at the closest possible (micro)step.

But the real limit is the LightBurn numeric resolution of three decimal places (*) for the mm/line value, so the lines can’t have better accuracy. That isn’t the resolution limit for the motor step calculations, however: for my RuiDa controller, the position is figured with floating point numbers in units of microns.

You can step the mm/line value by 0.001 and watch the DPI equivalent jump in much larger steps.

(*) I mis-remembered it as two places; that’s true for the DPI value and other metric fields.

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