[Dragaera] Physics of fencer's masks [was: The Enclouding]

[Jerry Friedman]

--- On Mon, 10/13/08, Michael Wojcik <mwojcik at newsguy.com> wrote:

> From: Michael Wojcik <mwojcik at newsguy.com>
> Jerry Friedman wrote:
> > --- On Sat, 10/11/08, Michael Wojcik
> <mwojcik at newsguy.com> wrote:
> > 
> >> In a similar vein, consider someone wearing a
> >> fencing mask - they can
> >> see out quite clearly, but it's hard to see
> >> his or her face. That's
> >> partly because the layer diffracts both incident
> >> and reflected light,
> >> so the light the fencer sees by is diffracted
> >> once, but the light the
> >> audience sees reflected from the fencer's face
> >> is diffracted twice.
> >> But it's also because the fencer is much closer to the
> >> diffraction
> >> grating, so whatever the angle of diffraction, the
> >> rays reaching the
> >> fencer's eyes have diverged much less than
> >> those reaching the eyes of the audience.
> > 
> > I don't think there's much diffraction going
> > on in a fencing mask.  The spacing is too big.
> 
> Diffraction occurs any time a wave encounters an obstacle.

True, but not necessarily much.

> Wavelength
> separation, optical distortion, and similar effects - what
> you're
> probably thinking of as diffraction - would require spacing
> close to
> the wavelength, true; but shading and scattering are also
> diffraction effects.

Those are indeed what I'm thinking of, and apparently
we're just using different definitions.  I checked two
physics textbooks, and they both define diffraction as
a wave phenomenon, excluding things that a particle
picture would account for, such as shading and diffuse
reflection.  I could check more textbooks if you want.
But given the definition you're using, your statement
now makes sense to me.

> > What's going on is that the mask shades
> > the fencer's face while the rest of the room
> >(including the mask) is well lit,
> 
> Yes, that's a diffraction effect. Some of the light
> incident on the
> mask from outside is reflected back; some of it is
> reflected at
> various other angles (scattered). Some of the light that
> makes it
> through the mask and reflects off the fencer's face is
> scattered. The
> result of those various diffraction effects is
> "shading".

> > and the tiny visible bits of
> > the fencer's face are hard to resolve from the
> > bits of mesh next to them.
> 
> To some extent, but the human visual system is actually
> quite good at dealing with that sort of noise.

Okay, I'll buy that.

> I still believe that, possibly second to luminance,
> divergence is the
> major factor here. Just trace some rays reflected off
> facial features
> back through the mask and I think you'll see what I
> mean. If you're
> close to the scattering layer, there's still a decent
> probability of
> encountering rays reflected from various parts of the
> object. As you
> move further from the scattering layer, rays reflected off
> parts of the object's surface that aren't close to your
> normal to that surface
> have to reflect at a progressively greater angle to *their*
> normals.
> As that's less probable, you get fewer and fewer of
> those rays.

I'm not seeing this.  For something like a fencing
mask, I don't think your distance from the object
or the mask affects your probability of seeing rays
reflected from the object.  And for a diffuse
reflector such as skin, the probability of a ray
being reflected in a given direction is pretty
much uniform, not a function of the angle from the
skin's normal.  Try looking at your hand while
moving your head to see it from different angles.

One thing neither of us has mentioned is that the mask
is out of focus for the fencer, but nearly in focus for
someone trying to see the fencer's face.

Jerry Friedman