Dutone-Screen Angles-Color Flashing

SteveSuffRIT

Well-known member
Hi Industry, just want to share with you a lab project we did in academia. Students printed a Duotone with dot-on-dot 45/45 angles, Moire with 45 & 50 angles, and Rosette with 45 & 65 angles. Used trigonometry to calculate screen angles, arctan(rise/run). Attached is a .doc with photos and a video.
 

Attachments

  • Duotone_Dot-Dot_Color Flashing.pdf
    241.2 KB · Views: 221
  • Duotone_Color FLASHING.mp4
    75.9 MB · Views: 0
  • Photo Finished Project.jpg
    Photo Finished Project.jpg
    1.4 MB · Views: 233

Alois Senefelder

Well-known member
Hello Steve,

Great to see a Printing Project that involved your students, especially that there is a "Fundamental" lack

of technical education in the USA - throughout what was "Blue Collar" manufacturing skills. Like most of those skills even in the

UK/EU went to CHINA, apart from Germany.

Regards, Alois
 

gordo

Well-known member
There are, IMHO, some critical issues with this exercise.

In the post, it's stated that the screen angle rotation is 20 degrees. 20° is the angle written on the plate and is the angle stated in the video.

But the PDF states: "Using a 30 degree screen angle rotation for the color ink will eliminate color flashing, and create a rosette pattern, but not a moire pattern.

So, is it 20° or 30°?

But that's not the big issue.

When two identical screens, as in this exercise, are overlaid and angled you will have moiré. What changes as the screen angles change is the frequency of the moiré. It's the frequency of the resulting moiré that determines its visibility. As the angle of the screens relative to one another increases the frequency of the moiré also increases but its visibility decreases. This is easy to demonstrate. Just print two identical screens onto a transparent substrate, overlay and rotate them at different angles. You can also do this virtually in PhotoShop. You can see that effect here at 5° and 10° offset:
5 10 degrees.jpg


And now 15° and 25°
15 20 degrees.jpg


Smaller moiré structures are simply smaller - and smaller things are simply harder to see.

The least visible (highest frequency) moiré pattern will be at 45 degrees. So I don't think it's correct to say that a 30 degree angle rotation creates: "a rosette pattern, but not a moire pattern".

The "flashing" is just the higher visibility of a low frequency moiré created when two screens are overlaid at the same angle but misregistered.

The color shift (color flash) that results is caused by an entirely different mechanism.

Going down the rabbit hole on rosettes...

A rosettes has a frequency 1/2 that of the screen frequency that created it. So, a 150 lpi screen would create rosettes with an equivalent 75 lpi frequency. That, and the fact that the rosette that's formed at that frequency is a fairly large structure means that certain color screen builds - especially the purples - can appear grainy/noisy in large, flat, color areas as the eye can easily see the rosettes.
 
Last edited:

Macmann

Well-known member
Gordo the two identical screens caught my attention as well - no printer would ever do this. The truly scary thing is what happens to the decades of knowledge in our noggins when we achieve room temperature? Oh well, off to the scooter store. :)
 

gordo

Well-known member
Gordo the two identical screens caught my attention as well - no printer would ever do this. The truly scary thing is what happens to the decades of knowledge in our noggins when we achieve room temperature? Oh well, off to the scooter store. :)
Starting with dot on dot and then gradually shifting one screen's angle is a good way to demonstrate the rational behind screen angles. What's scary is if what happens with overlaid screens is misunderstood or simply wrong.
 

What About Profitability?

Canon
What about Profitability?
Offset yields new advantages

Read All About It

   
Top