RDWorks Learning Lab 114 Can We Dither Greyscale images

If you own a Chinese laser cutter this little series of videos about me learning how to use the free software provided, may solve the problem of trying to learn from a virtually unreadable manual.
I am nothing to do with RD Works, I am not an instructor and I am no expert. This series will document the essential bits of many hours of trial and error

[johnrevill]

I've been working on greyscale engraving for a while and there are a lot of variables to take into account to get good greyscale images. Moisture content of the wood, Temperature of the wood. But think of this when it comes to greyscale. The Greyscale pattern on something like wood is not linear. As the wood starts the burn it gets darker. As it gets darker it absorbs more of the laser energy quicker and gets even darker quicker. So you can not use a linear power curve to get nice greyscales as the image will look very dark and the dark end of the scale, but by reducing the power, the image becomes faint and washed out. If you want a 10% grey, you might need 15% power, to get a 20% grey, you might need 20% power, and to get a 30% grey you might need 22% power. I have plotted a chart to workout the power curve that produces the best results on different materials. I'd like to send you some photos of my greyscale work so you can see what I have achieved.. Greyscales are not easy to achieve, but when tweeked right do look very good.

[Runner]

Great observations Ross, I much appreciate everything you do.

[PeterEduard]

Hi Russ.

I use a voltage divider to test the output of the high power PSU. I don't think a simple divider has any latency. It would make it easy for your scope to measure what happens at the tube.

(Voltage divider quite literally does what it says. The ones I use divide the voltage by a factor of 1000. This makes it possible to test the output on standard equipment).

I have got a spare, so let me know if I should mail it to you.

Peter

[TheBruster]

Hi Russ
Love the videos, keep em coming, learned a bunch so far. My Laser, when i got it, was so far out of wack i ended up gutting it and rebuilding it and found your videos extremely helpful.
On the laser signals, if its pulse width modulated, it would be like the old AM radio signals. The frequency of each of the pixels would be a different frequency depending on its value determined by the 0 to 255 value of the picture. By varying the frequency, would change the power output of the laser. While going from say 10 to 20 would probably have little to no change in the picture since the frequency being sent would only change marginally. If you were to maybe break the levels of the image into more radical steps, you may be able to see the frequency change sent to each of the pixels. The blue signal you were showing is just to turn on the laser as you mentioned in the other section of your video, shows it turning on and off to make the dots instead of the PWM used to change the power sent to the laser. I may have to set up a 0 to 255 gray scale to see where the actual crossover points occur in the power supply's ability to produce the actual scale. Will have to do it in both dithered and direct to see the change.
As for the dithering issue.
I took this out of one of my old manuals: A dither pattern is a pattern of dots used to simulate a color or shade of an image. If the pattern is changed, the characteristics of the image will also change. You can select a pattern suitable for the image to be printed.
Dithering is a technique where tiny dots are printed in patterns in order to create the illusion of tones and colors that cannot be physically reproduced.
Error Diffusion is a type of dithering that produces halftones by printing dots densely for dark colors and diffusely for light colors. Select the appropriate pattern for the image you want depending on the operating system or application, error diffusion may not be printed correctly..
Looks like i have a lot of playing about to do, good thing i am retired now!!
Thanks again.
Bruce

[jimmybleron]

Dear Russ,

Just today while doing some tests on my companies lightblade machine (90/60 60W) I managed to do a, quiet decent looking, grayscale engraving with 500mm/s (min 10% Max 22%). The source was a 120 dpi picture ( yes the horse:) ), but after seeing your video now I am not so sure if in fact it was a dotted picture with no gray and that RD works was actually fooling me. If you want I can send you the file with a picture of the result.

Best regards
Jimmy
PS:
"Power is gained by sharing knowledge not hoarding it"'
Thank you and as always learning a lot from you!

[MrArjenD]

Hi,

I also have the weird problem of sending 'large' files through the LAN-port of the laser. The head of the laser moves all kind of ways it shouldn't.
The only solution is to first 'download' the file to the internal memory of the laser ( which however doesn't also work 100% of the time ).


For measuring the hi-voltage output, wouldn't it be possible to measure the current of the tube with your picoscope, that's just a max of 25mA. Does the picoscope have a current input, just like a multimeter has?

Regards,
Arjen.

[notsofastener]

Just a curious thought here, but what about taking a new measurement with the picoscope where you measure the variance in the light output of the plasma in the tube to see what kind of wave you get there? I don't know if you have a third channel on the little scope or not but it would be interesting to see what a photodiode or phototransistor makes of the light intensity of the pink plasma compared to the other two signals.

[jeffaley]

Your scope is not really telling you what the controller is outputting. Well, it is for the on/off signal (blue), but it is not for the red. As you stated in your earlier video, the output of the controller is a PWM (pulse-width modulated) signal, and you've put it through an RC filter to turn it into a voltage that you can observe. I believe that the "limit" of resolution (in milliseconds) you're seeing is actually a result of your filter and not of what the controller outputs. In fact, the limit should be equal to the period of the PWM pulses. So if you really want to know what the controller can do, measure the period of the raw PWM signal.

Of course, this only tells you about ONE component in the system; it does not tell you anything about what the HV supply does with that signal, nor how the laser responds. At the very end of the chain, I think you want to see what is the slew rate of the laser (minimum time between off and on). I think you may already have some of this data in Lab 105 or one of the others where you fired a checkerboard at a piece of acrylic.

Regards,

-Jeff

[svip]

Hi Russ ，I am studying the high frequency ionization engraving photo, I have got a RECI 90W laser tube, but I want to use my old 60w tube till it's life ended, So I have a question : Is there any chance that I can use a 100W power supply with a 60W laser tube? Has anyone tried higher power wattage supply for lower wattage laser tube ? If so, Will it stable for work if I limit it's power out put for like 20mA ? Thank you.