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Hello, I'm trying to make a simulation of a glass cylinder heating up radially from the z axis and then comparing how light would go through it after thermal lensing and expansion of the glass. So far I have been able to heat up the lens succesfully so that it is hotter at then center and gradually colder as the radius increases. I've checked that the index of refraction is changing due to heat (although there seems to be weird noise going on with that) and I think the material is also expanding although it's not perceptible and im not sure how to check. My main issue is with the ray tracing, I currently have my rays released at the same time that the heating starts which means that the glass has barely heat up when the rays pass through, is there a way to delay the ray release until after the glass has been heating for a while and then comparing the path difference of the rays going through a heated vs unheated glass?

First picture is the temperature distribution on the cylinder, second is a graph showing the change in index of refraction as the radius increases.