Hi Jonasz,

Unfortunately, the documentation for the Gaussian scattering model is deficient (to say the least), and none of the online articles I have seen help. Hopefully someone from Zemax see this thread and update the documentation (if my explanation below is correct, that could be just a matter of adding 1 or 2 sentences).

Most diffusers communicate their scattering angle in degree (sometimes FWHM, sometimes 1/e, ...), however Zemax’ Gaussian scattering model is not a Gaussian distribution in angle (unlike LightTools for example). The scattering formula is applied in cosine directors space (that is closer to the physics). As a result, the scattered cone becomes elliptic when the incidence is not normal. However, the value at normal incidence is the one you want to use for the definition.

So the sigma in Zemax is most likely following the sine of the angle, which for 0.27° would be 0.0047 (sigma is dimensionless according to the documentation, so that looks consistent. There may be some factor 2 or sqrt(2) to account for.

Of course, that is contradictory with the documented value of 5 for Lambertian (a sine of 5 is not really defined for an angle). A long time ago, I ended up computing tabulating values from 0 to 5 and their resulting sigma in ° in Excel and interpolating the value. The sine formula matched this tabulated way within 1% up to 10°, 5% to 25°, 10% to 40°.

Dear Ray,

Indeed, the manual entries on scatering models is lacking.

I´ve tried to fit the formula using the cosine directors but still did not manage to find an agreement between the nomimal sigma and the fitted sigma. In the end I followed the black box approach, similar to the your tabulated data approach.

Kind regards,

Hi Jonasz,

Did you get a value close to 0.0047 to work for your 0.27° ?

Best regards,