Author Topic: About gray scale "Printing"  (Read 971 times)

RobotEyes

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About gray scale "Printing"
« on: March 07, 2017, 04:41:03 AM »
Many people asked me how to make a gray scale engrave.
Follow some of my considerations and advice.

First some general observations about Laser engraving:
We must understand that we are not printing on a sheet of paper with a variable amount of ink,
but we are burning the surface of a material.

The key thing to understand is that we are providing some energy in a very small area ( 0.01 to 0.04 sq mm[size=78%]);[/size]
this energy is formed by a certain power (laser light) for a certain time (inverse to the speed of movement);

Each material reacts differently to application of energy:
at low energies simply absorbs without structural changes,
then, gradually the energy increases, it begins to change its structure.
In materials that we use normally it "burns", in other materials (metals, plastic etc.) blends, and so forth.
Another very important factor is the type of reflectivity of the material:
A kitchen aluminum sheet never fuse with a 5W laser, because it reflects almost all of energy,
with a 100 W laser can hope that 1-2 w being absorbed.
A sheet of white paper will need more energy in the same blackened sheet.
Moreover, absorption also depends on the wavelength.
The most "efficient" is the infrared, the violet laser that we use are born for
data transmission and point and are not very efficient
Further consideration to be made, with regard to the "burning" is the speed or time of application:

Although 1 Joules per 1 sec is the same energy of 1 MegaJoule for a microsecond,
the materials react differently, almost all require a certain time to "burn",
and the energy must exceed a certain threshold to begin the oxidation process called combustion.
Then (even for the same energy) the slower application (assuming that exceed this threshold)
Iwill result in a more "deep burn".
Finally comes in the  structure of the material:
for example beech wood  react oxidizing much more quickly than a sheet of MDF (compressed wood powder).

After all this preamble we begin to understand that there are so many parameters to take into account what to think of
"Print" a photo with our laser is more a matter of luck than technical ability.
(If you can, immediately play the lottery)

My advice to "print" through RobotLaser are as follows:
Get a good amount of material that we will use to make the tests;
the results obtained will be valid ONLY for that material.
Obtain an image with a gray scale to 8 levels.
Adjust the brightness and contrast parameters to 0, and gamma to 1 (that does not change the image scale)
Make a VERY ACCURATE focus of the laser .
Print with maximum power (255) and speed as to have parts not burned and parts completely burned ,
recommend print so that the laser scan do the full scale  0 - 255 , ie horizontal image, horizontal scan.
The ideal would be to have the point that changes from white to black in the image center.
Find the point where the material starts to blacken slightly.
Find the point in which the material and totally burned.
These two points will have a power deduced from the series: 0 32 64 96 128 160 192 224 255

In the power curve page assign :
the gray 0 (black value) of the complete burning power
the gray 255 (white value) a value slightly less than the power of the minimum blackening.
Distribute the remaining values gradually (on the graph below helps you to see the results)

At this point, try to regenerate the GCode and print again.
After a bit of trial and error should be possible to obtain an acceptable result.

Another solution is to use dithering (dots) as did the old linotype.
The results are decent only "big" images (more than 50 mm)
In this case the dominant factor is the writing speed.

Following are some of greyscale images made of solid board (Amazon envelope )
This is what I got in about one hour of tests.
The power curve is: 50 40 30 22 18 16 14 12 8

The first thing you may notice is that the material I chose supports 4 levels of gray.
Perhaps, by working more, still can improve and get to 6/8 levels, certainly not more.

Obviously, by changing the laser power, the curve is scaled

Hope to have clarified some unclear points.

For any clarification or ask are at your disposal

Carlo
« Last Edit: March 07, 2017, 04:42:48 AM by RobotEyes »

fvuichard

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Re: About gray scale "Printing"
« Reply #1 on: March 14, 2017, 09:52:17 AM »
Thanks,

I have tested on wood and the result is very good.  ;)

(Image size : 80*50mm)
« Last Edit: March 14, 2017, 09:53:42 AM by fvuichard »
Laser: 1600mw A3 eleks maker
OS: Windows 10 all in one pc
Software: T2, Robot Laser
Location: Switzerland

RobotEyes

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Re: About gray scale "Printing"
« Reply #2 on: March 15, 2017, 01:46:52 AM »

Good job,
however, as I said in the post, you can not expect to have more than 6/8 levels of darkening.
On your image I count 5 levels, and I think the result is of good quality