# Opening angle of evalglare

Hi list,
I am puzzled about the opening angle of an specific area in the option of -t, -l, -L and -N of evalglare (@Jan_Wienold ) The size of the zone(s) is specified by the opening angle(s) in radians. How to calculate such angles? As for the -N option, in the following HDR image, if the pixel overflow area like the sun is not circular but a hemi-circle, is it still reasonable to select a circular zone?

P.S.This post is related to another one: https://discourse.radiance-online.org/t/invalid-exposure-entry/5857/8?u=lee
Any suggestions are appreciated!

Hi list,
I googled the definition of opening angle of light and the following image is from wiki:

So for evalglare, I think the calucation of the opening angle of specific task area is like this:

``````angle=2θ=2arcsin[r/d]
``````

where r is the radius of the circular task area, d is the distance between the center point of the circular view area and the one of the circular task area and the opening angle is in radius.
I hope someone can validate my guess.