question

Pamela J Case avatar image
Pamela J Case asked ·

ABBE Numbers

Is this parameter an actual Abbe Number parameter? That is, does this parameter represent the actual Abbe Number range between 1 and 100? If I have a physically accurate Abbe Number value for a glass material, and I put this value into this slot, will the results be physically accurate? Or, ... does this parameter represent more of an approximate slider for some relative degree of dispersion?

10 |600 characters needed characters left characters exceeded

Up to 5 attachments (including images) can be used with a maximum of 2.0 MiB each and 9.8 MiB total.

Pamela J Case avatar image
Pamela J Case answered ·

Thank you Lee. I'll give that a try and return with the results. Am coming down with some flu, so it might be a day or two before I can reply to your kind answer.

Share
10 |600 characters needed characters left characters exceeded

Up to 5 attachments (including images) can be used with a maximum of 2.0 MiB each and 9.8 MiB total.

Pamela J Case avatar image
Pamela J Case answered ·

Yes, before posting this question, I experimented with it as systematically as I could (after reading the available documentation you've cited above). I set up a "measured" light stage affair with checkerboard plane behind a sphere - all with regular metric sizes and standard camera settings. Worked out the scale settings of everything to make the testing rig as sensitive to slight changes in the IOR values as possible. And then started a series of systematic experiments on the Abbe Numbers parameter, holding everything else in the Transmission settings constant, of course. But, the Abbe parameter seems relatively "insensitive" - that is, it takes rather large changes in the value to develop a discernible change in the amount of visible dispersion in the edges of the checkerboard blocks. (And, as you can imagine, the statement in the documentation above that "for glass and diamonds, this is typically in the range of 10 to 70," when the total scale range is only 1-100 to begin with, doesn't really tell me if we are dealing with the classical Abbe Number scale of 1-100.) I'm not meaning to be critical, rude or disparaging. Just trying to ask for a little bit more information.

Share
10 |600 characters needed characters left characters exceeded

Up to 5 attachments (including images) can be used with a maximum of 2.0 MiB each and 9.8 MiB total.

Lee Griggs avatar image
Lee Griggs answered ·

To get a match for a given material you would:

  1. have to find out what the refractive index is at 589.3nm, and set specular_IOR to that,
  2. set the correct Abbe number for the material.

There could be other issues at play here, like flipped surface normals, self intersecting geo, thin shells instead of closed mesh, etc.

Share
10 |600 characters needed characters left characters exceeded

Up to 5 attachments (including images) can be used with a maximum of 2.0 MiB each and 9.8 MiB total.

Lee Griggs avatar image
Lee Griggs answered ·

>If I have a physically accurate Abbe Number value for a glass material, and I put this value into this slot, will the results be physically accurate?

I would hope so. Have you tried it? Does it look right?

From the docs:

"Specifies the Abbe number of the material, which describes how much the index of refraction varies across wavelengths. For glass and diamonds, this is typically in the range of 10 to 70, with lower numbers giving more dispersion. "

Share
10 |600 characters needed characters left characters exceeded

Up to 5 attachments (including images) can be used with a maximum of 2.0 MiB each and 9.8 MiB total.

Write an Answer

Hint: Notify or tag a user in this post by typing @username.

Up to 5 attachments (including images) can be used with a maximum of 2.0 MiB each and 9.8 MiB total.

Welcome to the Arnold Answers community.

This is the place for Arnold renderer users everywhere to ask and answer rendering questions, and share knowledge about using Arnold, Arnold plugins, workflows and developing tools with Arnold.

If you are a new user to Arnold Answers, please first check out our FAQ and User Guide for more information.

When posting questions, please be sure to select the appropriate Space for your Arnold plugin and include the plugin version you are using.

Please include images, scene and log files whenever possible as this helps the community answer your questions.

Instructions for generating full verbosity log files are available for MtoA, MaxtoA, C4DtoA, HtoA, KtoA, and Kick.

If you are looking for Arnold Documentation and Support please visit the Arnold Support site.

To try Arnold please visit the Arnold Trial page.

Bottom No panel present for this section.