> >Yeah, it _is_ white, after all. :-)
>
> I was going to add that to the line, but it seemed a bit much after
> mentioning "red spot" and "quite reflective".

Quite reflective, yes.  But you don't know the geometry of the froth ahead
of time.  The froth I am used to has peaks and valleys.  Could the dot get
lost among these?  How does the shape of the foam change how the light is
reflected? (is milk froth highly specular? lambertian?).  Does this
force you to use a larger spot?  Does "cold" foam have signficantly
different reflectance properties than "hot" foam (bubble size, etc).  Does
ambient light present a problem?  How can you guarantee that the sensor
is aligned properly to see a dot?  Do you use an array of sensors?  A
larger sensor?  Maybe you project a stripe instead of a spot?  How fast
does the froth level change?  How long do you need to see a spot before
you signal the machine to stop frothing?  A quick change in froth level
might put the spot past the sensor.  Larger detection area?  More than
one sensor?  Does the froth create steam that will attenuate the light?
What if the steam condenses on the emitter or sensor?

In my original message I was just trying to say that it involved more than
simple geometry.

-Steve

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