GENERAL: Nd in relation to opaque or transparent objects
Posted: Fri Jul 07, 2006 3:57 am
Hi all,
This seems to come up often.
Here are a couple of pointers in regards to this aspect.
In reference to some physics concepts illustrated here:
http://www.maxwellrender.com/forum/view ... hp?t=17142
INDEX OF REFRACTION IN GENERAL:
Index of refraction is used both on Snell's law and in Fresnel equations.
This means that index of refraction determines:
The setting that switches a material from opeque to dielectric is the transmittance color (not the Nd). If the transmittance color is other than zero (0) then the material is technically a dielectric; at which point the lightness of the transmittance color and attenuation distance will define how dark it appears.
Each dielectric in nature has an index of refraction (at 489nm) associated to it. Most dielectrics range from 1.0 to 3.0, but the Snell and Fresnel equations can be used to predict imaginary dielectrics with higher Nd values as can be seen here:
These might not all exist in nature, but they might be constructed in a lab (material science). For example some chemical elements do not occur in nature but they are manufactured in a lab by means of breaking atomic nuclea and recombining them into new elements.
The above graph illustrates the fresnel behavior as the Nd is increased for a dielectric. In Maxwell we can turn off the transparency of the material (by switching transmittance to RGB[0,0,0] ) and then we are left to keep the fresnel reflection characteristics for that particular Nd value.
Real life materials have a specific reflectance which can be measured in a lab. Also, the complex IOR files (n,k data) contain lab measured reflectance values.
We can take the actual experimental data for reflection and then work backwords to find what Nd value of the material editor would give us approximately the same reflectance percentage.
When we do such corrolation then the result is something like this for some common metals:
For example, to emulate silver (which has a reflectance of 97% - 98.6 accross the spectrum) without using complex IOR data we can set:
SUMMARY:
-Index of refraction controls bending of rays as well as reflection percentage
-The Snell and Fresnel equations can predict high Nd dielectrics.
-We can turn off the transparency of a material but keep its reflection properties inherited by the Nd number
-We can use these high Nd numbers to emulate highly reflective materials such as metals.
-We do all this so that we do not have to always use complex IOR files which are hard to find.
-Therefore the material editor works in two modes
a) using complex IOR files
b) manual mode where we can emulate the look of real life materials
______________________________________________________
A word of caution:
Do not use the IOR values often listed on the web for *non* dielectrics in the "custom IOR" slot of the material editor (manual mode) ...
This seems to come up often.
Here are a couple of pointers in regards to this aspect.
In reference to some physics concepts illustrated here:
http://www.maxwellrender.com/forum/view ... hp?t=17142
INDEX OF REFRACTION IN GENERAL:
Index of refraction is used both on Snell's law and in Fresnel equations.
This means that index of refraction determines:
- The angle that the incident rays will bend to; when entering a dielectric (This is from Snell)
- The proportions of energy (the intensity) that goes into reflection versus refraction. It is this aspect of Nd that is responsible for the mirror-look (reflections) in objects. (This is from Fresnel)
The setting that switches a material from opeque to dielectric is the transmittance color (not the Nd). If the transmittance color is other than zero (0) then the material is technically a dielectric; at which point the lightness of the transmittance color and attenuation distance will define how dark it appears.
Each dielectric in nature has an index of refraction (at 489nm) associated to it. Most dielectrics range from 1.0 to 3.0, but the Snell and Fresnel equations can be used to predict imaginary dielectrics with higher Nd values as can be seen here:
These might not all exist in nature, but they might be constructed in a lab (material science). For example some chemical elements do not occur in nature but they are manufactured in a lab by means of breaking atomic nuclea and recombining them into new elements.
The above graph illustrates the fresnel behavior as the Nd is increased for a dielectric. In Maxwell we can turn off the transparency of the material (by switching transmittance to RGB[0,0,0] ) and then we are left to keep the fresnel reflection characteristics for that particular Nd value.
- An Nd of 1.5 corresponds to a reflectance-0 of 4%
An Nd of 3.0 corresponds to a reflectance-0 of 25%
An Nd of 5.0 corresponds to a reflectance-0 of 44%
An Nd of 10.0 corresponds to a reflectance-0 of 67%
An Nd of 20.0 corresponds to a reflectance-0 of 82%
An Nd of 50.0 corresponds to a reflectance-0 of 92%
An Nd of 100.0 corresponds to a reflectance-0 of 96%
An Nd of 200.0 corresponds to a reflectance-0 of 98%
An Nd of 500.0 corresponds to a reflectance-0 of 99.2%
An Nd of 1000.0 corresponds to a reflectance-0 of 99.6%
Real life materials have a specific reflectance which can be measured in a lab. Also, the complex IOR files (n,k data) contain lab measured reflectance values.
We can take the actual experimental data for reflection and then work backwords to find what Nd value of the material editor would give us approximately the same reflectance percentage.
When we do such corrolation then the result is something like this for some common metals:
For example, to emulate silver (which has a reflectance of 97% - 98.6 accross the spectrum) without using complex IOR data we can set:
- Reflectance-0 at RGB(248,248,248)
- Reflectance-90 at RGB (255,255,255)
- Transmittance RGB(0,0,0) (opeque object)
- Nd=140 (or up to 200)
- Roughness=3.0
SUMMARY:
-Index of refraction controls bending of rays as well as reflection percentage
-The Snell and Fresnel equations can predict high Nd dielectrics.
-We can turn off the transparency of a material but keep its reflection properties inherited by the Nd number
-We can use these high Nd numbers to emulate highly reflective materials such as metals.
-We do all this so that we do not have to always use complex IOR files which are hard to find.
-Therefore the material editor works in two modes
a) using complex IOR files
b) manual mode where we can emulate the look of real life materials
______________________________________________________
A word of caution:
Do not use the IOR values often listed on the web for *non* dielectrics in the "custom IOR" slot of the material editor (manual mode) ...
- These values are incompatible with the manual mode of the material editor; since they relate to complex IOR values. Actually they are substantially simplified / incomplete versions of complex IOR data and cannot be used without a corresponding extinction coefficient(s).
For example silver(Ag) is often listed as having IOR=0.18 which is only the 'n' value of the n,k file corresponding to about 400nm
This can be checked here: http://www.luxpop.com/
(Enter: Ag, 400nm, 25 C)
)
