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Glass Refractivity

With a transparent material such as glass there will be both reflection and refraction of light depending on the relative angle and position of both glazing and light. To create a focusng effect the glazing index of refraction for transmitting photons should be greater than 1.0.

In reality the Refraction index value for glass surrounded by air is generally taken to be 1.44. or a value between 1.50 and 1.89. The average estimate of daylight transmittance through clear 6mm sheet glass is around 0.8 and through clear double glazing with average dirt is aorund 0.5. Refractive objects with different shapes will refract diffrerently and the refraction will vary according to how concave or convex the shape is.

The default for this control is a value of 1. Increasing the refraction value above 1 (usually to values between 1.33 and 1.50) focuses the caustics whilst decrasing the value to less than one diffuses the effect. A value of 0 for the refractive index produces no caustics as no refraction through the glass is occuring and takes a long time to render whilst mental ray is searching for photons.

Index of Refraction Values
Further Information
Susan Windeatt