Hi there,

I'm trying to render the most realistic diamond there can be. To do so I need
realistic IOR value and dispersion value. At first I found them in consts.inc
shipped with POV-Ray, namely IOR value = 2.47 (or sth) and dispersion value =
1.035. Then I got across some scientific page about diamonds. As of 2003, the
most accurate value for diamond's IOR is 2.4175, and dispersion value of 0.044.

So my question is: how does 1.035 from POV-Ray relate to 0.044 from the web?
The 0.044 value is said to be "disspersion strength":

  Index of
  Colour    Refraction  Source Line    Wavelength

  Red       2.407       Solar  B-line   687   nm
  Yellow    2.417       Sodium D-line   589.3 nm
  Violet    2.452       Solar  G-line   431   nm

The difference between the indexes of refraction (2.452 - 2.407) is the
dispersion (0.044). (There is a small rounding error.)

http://www.folds.net/diamond_design/index.html#ed_note_04

I'd appreciate any help.

Best regards,
Opal

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Opal nous apporta ses lumieres en ce 2008/04/06 21:05:
> Hi there,
> 
> I'm trying to render the most realistic diamond there can be. To do so I need
> realistic IOR value and dispersion value. At first I found them in consts.inc
> shipped with POV-Ray, namely IOR value = 2.47 (or sth) and dispersion value =
> 1.035. Then I got across some scientific page about diamonds. As of 2003, the
> most accurate value for diamond's IOR is 2.4175, and dispersion value of 0.044.
> 
> So my question is: how does 1.035 from POV-Ray relate to 0.044 from the web?
> The 0.044 value is said to be "disspersion strength":
> 
>   Index of
>   Colour    Refraction  Source Line    Wavelength
> 
>   Red       2.407       Solar  B-line   687   nm
>   Yellow    2.417       Sodium D-line   589.3 nm
>   Violet    2.452       Solar  G-line   431   nm
> 
> The difference between the indexes of refraction (2.452 - 2.407) is the
> dispersion (0.044). (There is a small rounding error.)
The dispersion is not a difference but a quotiant.
> 
> http://www.folds.net/diamond_design/index.html#ed_note_04
> 
> I'd appreciate any help.
> 
> Best regards,
> Opal
> 
> 
The only value you need to compute is the dispersion.
The dispersion value is the quotiant of the violet ior divided by the red ior: 
2.452/2.407 = 1.0186955
But, the value you have are not from the extreme ends of the visible spectrum. 
The resulting dispersion is thus slightly smaller than the real dispersion value.

To render the most realistic diamond possible, you'll need to take into acount 
the birefringeance of the diamond, whitch is not possible at present. You'd need 
a syntaxe like:
ior{ior1, ior2, neutral_axis}
where
ior1 is tha base ior, sphericaly constant.
ior2 is the secondary ior perpendicular to the neutral_axis, not constant.
neutral_axis is the axis where the 2 iors have the same value.

-- 
Alain
-------------------------------------------------
I find the affluence of incahol to be totally, whatever he said

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Opal nous apporta ses lumieres en ce 2008/04/06 21:05:

> 
Forgot to mention that the dispersion is also variable in a birifringeant 
material. You need 2 dispersions values, each associated with it's respective ior.

-- 
Alain
-------------------------------------------------
Monday is an awful way to spend 1/7 of your week.

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"To render the most realistic diamond possible, you'll need to take into acount
the birefringeance of the diamond, whitch is not possible at present."

Hmmm. Nevertheless, the statement is still true - the most realistic diamond
possible - and since simulating birefringent materials is impossible I can
cross out this option (for now).

BTW: Is birefringence (and trirefringence) planned in future versions of
POV-Ray? And isn't there any trick to achieve similar effect to birefringence?

Best regards,
Opal

Post a reply to this message

Opal nous apporta ses lumieres en ce 2008/04/07 13:23:

> BTW: Is birefringence (and trirefringence) planned in future versions of
> POV-Ray? And isn't there any trick to achieve similar effect to birefringence?
> 
> Best regards,
> Opal
> 
> 
> 
Not to my knowlege.

It may be possible to fake it to a point using an averaged material. Need to 
experiment.

-- 
Alain
-------------------------------------------------
The tree of liberty must be refreshed from time to time with the blood of
patriots and tyrants.
Thomas Jefferson

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Opal wrote:
> BTW: Is birefringence (and trirefringence) planned in future versions of
> POV-Ray? And isn't there any trick to achieve similar effect to birefringence?

Like Alain said, you can average two textures together. Each texture 
would have a slightly different surface normal, but this is a hack and 
will not be very accurate (to the eye, maybe...)

If you didn't know about this already, I'll tell you now of a useful 
include file containing different ior, dispersion and birefringence 
values for many different minerals:

http://www.seasoft.tecbox.com/Pov/NewIOR.html

The dispersion values are based on MegaPOV's implementation of said 
effect, but they should also work with the official versions.

I hope this helps!

Sam

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Wow!

The thing's great.
Thank's alot for that.

There's also a gem cutting library, but very basic.

I also work on a gem cutting library - more complex one - that would contain not
only modern cutting techniques, but also historical ones. Tho, these techniques
don't give yet same effect as real-life jewels. Everything would be put in
parameterized macros (not containing hundreds of single facets).

Thanks all for help, I really do appreciate it.

Regards
Opal

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