IOR of hot air (engine exhaust)?

[dmcgrath]

A chart I have says that, "Air=1.020". So I really don't know if it helps. Refractions can be very tough to calculate realistically. It might be easier to do it in post. If that isn't an option for you, using your eye is sometimes the best thing for this.

[wedge]

Originally posted by Gremlin
:lightbulb Apparently... the IOR of water @ 20ºC is 1.333; the IOR of ice (0ºC) is 1.309... does this mean a 20ºC change in temperature will cause a 0.024 change in IOR; so 20ºC hotter air would have an IOR of 1.0243?? This could help me calculate the IOR of engine exhaust!

play with this. you might find the relationship is linear, you might find it's exponential. just play with IORs until you find one that looks right. remember, professionals fake stuff all the time :thumbsup:

[Gremlin]

Originally posted by dmcgrath
A chart I have says that, "Air=1.020". So I really don't know if it helps. Refractions can be very tough to calculate realistically. It might be easier to do it in post. If that isn't an option for you, using your eye is sometimes the best thing for this.
only problem with this is that with such suttle changes, its hard to see the difference unless you animate... and animating takes way too much time, thus I'm trying to find the real value so I dont have to go by what looks best :D

:lightbulb the highest IOR is 3.5, which I guess is what angle? 90º?

Cheers,
BoxxNine Studios :D
seuldieu@hotmail.com

[colt]

Apparently... the IOR of water @ 20ºC is 1.333; the IOR of ice (0ºC) is 1.309... does this mean a 20ºC change in temperature will cause a 0.024 change in IOR; so 20ºC hotter air would have an IOR of 1.0243?? This could help me calculate the IOR of engine exhaust!

Don't use that relation. That would mean air at 500 °C would have a ior of 2.2 which is way too much distortion. Water is a strange thing ... expanding while getting colder.
Air gets thinner while getting hotter - so the IOR sinks - it's getting close to vaccum with IOR = 1.0. Since you won't render all the cold air in you scene with a physically correct IOR of 1.0003 and refraction turned on to get the effect of your hot air with a IOR of about 1.0001 your only option is to experiment with really low IORs for the hot air.
I'd experiment with settings around 1.0002 which should be subtle enough.

[Waters]

Im interested in how your result with turn out, seems like a very cool idea to animate the index of refraction within your 3d program, instead of doing a fake in post.

One thought would be to use the ideal gas law to calculate the density changes for air for a given temperature change. There are equations for calculating the change in index of refraction for a given change in density. It turns out that the changes are proportional. With that information it should be easy enough to get your values for index of refraction. But another problem is that this change needs to be properly animated for the movement of air, and this change wont be a 2-dimensional change, but a 3-dimensional change, so making an animated bitmap wont give you the effect your looking for.

[trthing]

For more in-depth analysis of the subject (including a simplified formula to calculate IOR for air):

http://patapsco.nist.gov/mel/div821/Wavelength/Documentation.asp [Appendix B might be useful]

http://www-sci.lib.uci.edu/HSG/RefCalculators4.html [Other sci data source]

http://users.wpi.edu/~ierardi/PDF/air_density_plot.PDF

Curious about how are you guys going to implement this...:eek:

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