Why FM screen has larger gamut than AM screen?

[Bloodsaler]

This question has confused me for a long time.
I`ve recently done a FM screen test,under the same ink&paper&press,but FM icc got a larger gamut~?
Why cause this happen?For the dot shape or dot area?
Can sb explain it?Thanks a lot~

 

[TheProcessIStheproduct]

was the dot gain the same? maybe the FM test printed fuller???

 

[Bloodsaler]

even under the same dot gain condition(I mean after compensation),the FM one still have a larger gamut~

 

[bewilliams]

In FM, you print a lot of small dots. As the dots are smaller, also the ink thickness is less than in conventional AM. This might sound strange, but the thinner the ink layer, the more 'pure' the ink is transferred onto the substrate.
A good example is: take a glass of water out of the ocean and hold it against the light. It will look clear and transparant. However, if you look at the ocean as a whole, it looks grayish and dirty (around here it does ). That's because the small particles influence the fraction of the light and the more volume, the more particles. This influence of fraction causes desaturation of ink.
In other words: by printing a thicker ink layer, you will get a less saturated result.
FM dots and special AM screens such as Concentric Screening from EskoArtwork reduce the ink layer thickness, therefor improving the overall gamut.

 

[meddington]

Quality In Print: AM and FM gamuts compared - part 1 of 2

 

[gordo]

Technically speaking, FM - or more properly microdot screening - does not actually increase the gamut. Instead, it is more accurate to say that FM reduces the potential gamut less than the larger dots of an AM screen does. The function of ink in printing is to filter light, when that happens you see color according to what part of the spectrum is filtered by the ink.

If, for example, the dot areas (not dot gains) of a 175 lpi AM/XM screen and a 20 micron FM screen are the same then the FM will have a larger gamut.

The main reasons for this effect are:

1- For the same tone value the FM screen covers more of the paper compared with an AM/XM screen. That means more light is filtered by the ink rather than reflecting off of uninked paper. Light that reflects off of uninked paper dirties the color we see.

2- For the same tone value the FM screens have a thinner and more homogenous (even) film of ink on the dots compared with an AM/XM screen. This enables the ink to filter light more efficiently. (somewhat as bewilliams described it with the ocean analogy)

3- Because of their small dot size and thinner film of ink FM screening dries faster which improves overprint trapping efficiency.

It's important to note that it is not the fact that the screening is FM - i.e. how the dots are organized - that results in the gamut difference. It is the fact that the dots are smaller - i.e. the lpi is effectively higher.

If you run an AM/XM screen up to about 400 lpi it will have a similar gamut to a 20 micron FM screen.

In the case of Esko Concentric screening, because it is at heart an AM screen, if you compare it at 175 lpi to a conventional AM/XM screen also at 175 lpi then, in my tests, its gamut will be less, not more, than the AM/XM screen gamut.

The reasons are explained here: Quality In Print: Esko Concentric screening - some observations

best, gordo

 

[Deleted member 16349]

1- For the same tone value the FM screen covers more of the paper compared with an AM/XM screen. That means more light is filtered by the ink rather than reflecting off of uninked paper. Light that reflects off of uninked paper dirties the color we see.

best, gordo

Gordon,

This does not sound right.

Since FM screens have a higher dot gain, at the same tone value, I have thought that FM actually covered less of the paper.

One of the factor with FM is that some of the light that hits the non image area of the paper will diffuse under the dot and come up through it. Having a thinner ink layer on the dot will mean that more light comes up through it than with AM. This will increase the L value of the dot itself. Kind of like a back lit effect.

Also normally much of the light that goes to the dot travels down to the paper and reflects back up. It travels twice through the ink film. When light comes from below the paper, it only travel once. This adds to the affect of filtration by thinner films. The same is happening to the light that goes into the dot and is diffused in the paper and comes out in the non image area.

Thinner ink films can have different colour than thicker ink films for the same ink and paper combination. My view is that the lighter appearance of the FM print due to the diffusion effects and the change in colour of the thinner ink films contributes to the changes in gamut. But that is only my view and that is why I thought it would be interesting to apply vectors to show the shift direction from AM to FM for similar tone conditions or similar dot size conditions.

It is interesting to think about.

 

[gordo]

Gordon,

This does not sound right.

Since FM screens have a higher dot gain, at the same tone value, I have thought that FM actually covered less of the paper.

Here's a photo comparing a 175 lpi AM/XM screen and a 20 micron FM. Their dot areas are the same - 40% - i.e. the measured tone value on the press sheet is the same (the dot gains are different).

You can see that there is "white" paper between the AM dots but with the FM that white paper is pink due to the greater optical gain of the FM dots. The white paper effectively greys the hue - i.e. reduces the chroma

One of the factor with FM is that some of the light that hits the non image area of the paper will diffuse under the dot and come up through it. Having a thinner ink layer on the dot will mean that more light comes up through it than with AM. This will increase the L value of the dot itself. Kind of like a back lit effect.

Any lightness caused by the thinner ink film with the FM screeen is factored out by the dot gain compensation curve applied to the plate. We're comparing tones that on press measure the same - i.e. L* value.

More important is the less homogeneous ink film thickness of the AM/XM dot compared with the FM. This was the subject of a TAGA paper a few years back.
You can see the difference in this image where the density of the dots have been plotted to (exaggerated) height:

The ink film on the AM/XM dots vary in density across the surface of the dot. Those areas of increased density across the surface of the dot are thought to reduce the light filtering efficiency of the ink on the AM/XM dot.

best, gordo

 

[Deleted member 16349]

Their dot areas are the same - 40% - i.e. the measured tone value on the press sheet is the same (the dot gains are different).



Any lightness caused by the thinner ink film with the FM screeen is factored out by the dot gain compensation curve applied to the plate. We're comparing tones that on press measure the same - i.e. L* value.

best, gordo

Gordon,

Maybe I am wrong but I understand that TVI or the tone includes the optical dot gain. Your statement based on that understanding does not add up.

If the areas of the dots are the same and the tone is the same, how can the optical dot gain be different?

Of course the optical dot gain is different and is greater for FM screen and therefore my comment was if the tone is the same, the area of the dots must be less for FM.

Also related to the L value comment. I stated that the L was lighter on the dot, not the total print. Your image seems to confirm that. The greater optical dot gain decreases the L value in that area between the dots. The total can be less as you stated.

And of course, compensating the imaging of the plate does not address the change in colour due to these small dots.

The work that has been done to micro-measure the reflectance on and around the dots should help confirm what actually happens but the test samples need to be carefully made so that other factors do not lead one to wrong conclusions. I would add that it is not so easy to do properly controlled tests.

There has also been a lot of modelling of the light interaction with dots of ink but I don't know how valid they are when compared with real inking conditions and if they cover the full spectrum of light.

 

[gordo]

Maybe I am wrong but I understand that TVI or the tone includes the optical dot gain.

Yes.

If the areas of the dots are the same and the tone is the same, how can the optical dot gain be different?

Because dot gain is a perimeter effect. Smaller dots have larger perimeter to area ratios therefore more dot gain and a greater portion of it is optical because the dot is not carrying as much ink.

Of course the optical dot gain is different and is greater for FM screen

Yes.

and therefore my comment was if the tone is the same, the area of the dots must be less for FM.

OK, I think we're using the term dot "area" differently. All I mean by dot area is the measured tone not the area of ink.

And of course, compensating the imaging of the plate does not address the change in colour due to these small dots.

Correct

The work that has been done to micro-measure the reflectance on and around the dots should help confirm what actually happens but the test samples need to be carefully made so that other factors do not lead one to wrong conclusions. I would add that it is not so easy to do properly controlled tests.

There has also been a lot of modelling of the light interaction with dots of ink but I don't know how valid they are when compared with real inking conditions and if they cover the full spectrum of light.

Correct. Interestingly, this difference in gamut as well as hue shift has also been observed in tests using laminate pigment-based proofing media. Using laminate proofing media factors out the variables associated with ink, ink/water and pressure.

best, gordo

 

[Deleted member 16349]

Y
OK, I think we're using the term dot "area" differently. All I mean by dot area is the measured tone not the area of ink.

OK, I think we are in agreement now. When I hear dot area I think of Mechanical Dot Area as opposed to Apparent Dot Area which I relate to tone. Its the engineer in me.