Diffusing LED Light

[Phil Hobbs]

Try sanding the ends of the LEDs. Perhaps start by experimenting with
one, rather than messing with your whole array. By altering the shape
and surface polish of the epoxy "lens" you can do all sorts of things to
the pattern of the light out.

Any diffuser (such as matte drafting film, which would be a good choice
among that type of material) will cut down the light output as well as
diffusing it, so if you can get a good result without doing that, you'll
be better off.

Even if you make the LEDs perfect diffuse emitters (which is hard to do
at decent efficiency), you're still going to be fighting the infamous
cos**4 law, which makes the illumination of a surface from a given
source fall off very steeply with illumination angle (measured from the
surface normal). The four powers of the cosine come from three effects:

1. Even perfectly diffuse sources fall off with increasing obliquity,
basically because they look smaller seen edgeways. Flat surfaces
foreshorten as the cosine, concave ones faster, convex ones slower (and
spheres, not at all).

2. Whatever light you do receive gets spread out over a bigger
area--evening shadows lengthen as the secant of the angle, so the
intensity (power per unit area) goes as the cosine.

3. The distance to the source goes as the cosine, and the intensity goes
as 1/distance**2, so that's another two powers of the cosine.

Thus you get some pretty bad scalloping unless the light sources are
spaced close together. Using cosine**4 as an approximation will let you
calculate how bad it's going to be.

Note also that once the individual sources are frosted, putting other
scatterers in front won't help as much as you might think. You do win
some because the light spreading out sideways reduces the nonuniformity
from the cos**2 falloff due to radial distance, but you'll win a lot
faster by using more LEDs.

Cheers,

Phil Hobbs

 

[Phil Hobbs]

3. The distance to the source goes as the cosine, and the intensity goes
as 1/distance**2, so that's another two powers of the cosine.

Brain check--the distance goes as the secant, so the intensity goes as
cos**2.

Cheers,

Phil Hobbs