Why CMYK 100% density values are different?

[drummerpaco]

1. Does anybody can explain why the 100% printed C, M, Y, K density values are different?
I noticed that each C M Y K 100% print density values are different in Offset, also as Flexo/Gravure.

2. Why 100% Black and 100% White opacity % (density) is different when you print on film?
Is this because of Pigment, viscosity or chemical that make the ink?
I am measuring density in White and Black in Transparency and it just came to my thoughts why makes them different. I know that physically you will see less light thought the black on film compares with White on Film but I think there should be some sort of knowledge. Maybe I am missing something.

3. One more thing that is any body know what "White Hold Back" is?
I am assuming that white hold back is relating with "White opacity" but never heard of the terminology.

 

[BeauchampT]

Could you clarify your intial question please? Are you asking why the difference due to the printing processes, or the materials used?

Also, what sort of densitometer are you using? That will make some difference

 

[drummerpaco]

I am simply asking 100% of Black ink and 100% White ink density value, not Linearization process. My question is why opacity % is much higher when you print with 100% pure black ink than 100% white ink, in another words, Why light can transmit more in 100% white ink on film.

I have Xrite 939, Xrite 341C for reading opacity

 

[BeauchampT]

If those are transmission-densitometers, it probably has to do with the light scattering effects of the particular black ink compared to the white. With another white ink, you may get a measurement similar to black.

In a lot of ways, it sounds like the differences in measurement you get when measuring on a product on top of white backing versus black backing.