Quote - Well, trig tells us that:

cos(alpha) = E / S

Therefore, E  = S cos(alpha).  But since I made S = 1, that's just cos(alpha).

Now the opacity is the width of the obstructions divided by the width of the effective spacing.

So: opacity = D / E

Voila - effective opacity = D / cos(alpha)

BB, thanks for drawing the trigonometry diagrams and explaining how you came up with D / cos(alpha) as the effective opacity.  Your step by step explanation helped me understand the trig formulas involved, although on my own I would not have been able to derive from them the effective opacity.  You are simply brilliant BB.