This is the same symbol we use in mechanical engineering when referring to a diameter. We don't use a symbol for radius however, just a lowercase letter "r".
Being a scientist in the optics industry for some time this causes no end of confusion between designers and engineers.
The symbol IS diameter. However a lens, (or anything with a curve) say as being part of a sphere, has a surface radius and a physical radius.
So radius is to be avoided and stick to diameter for the physical size and only use radius for surface curvatures.
this symbol represents diameter, not radius. so it is the diameter of the circle.
diameter. the line is going all the way across the circle. radius is only half way across
