Winsor,
It should be very approximately 3 time better for Foveon,
because the normal Beyer pattern reduces the area per colored pixel
by approximately 3, while the Foveon chip does not. (stacked
vertically rather than horitontally) This means if the depth of the
pixel diffusion layers in the case of Foveon, are the same as the
filter layer of a conventional chip the shading effect is greatly
reduced. The lateral angle of acceptance would be much greater
allowing wider angle lenses. It could be even better than this,
since the sensor pixel area is reduced still further by metalization
and interconnect between pixels on th chip surface. This added
overhead is probably greater for smaller pixels putting them even
deeper realtive to Foveon. One might guess the diffusions in
Foveon's chip are also thinner than an on-chip filter, improving
things further. In fact for the first layer there should be little
to no lat! eral effect. This means the sensitivity drop off with
lateral light rays is probably different for the three colors
leading to yet another variation in color sensitivity with extreme
wide angle lenses! This may be a negative for Foveon but given the
chip should work with much wider angle lenses to start with it is
probably still much better.
Regards,
Tim Hughes
TimHughes@xxxxxxxx
That is interesting and I am sure you are right, but I am not
entirely convinced. If you put a recessed sensor in a chip and then
create one 3 times as wide and 3 times as deep I am not sure that you
have gained anything. Since the bigger sensor elements are probably
thicker it is probably more than 3 times as deep.
I looked at the Foveon site again and I really do not understand it.
It seems to me that one would have to still filter the elements, but
if you did the bottom two elements would get no light. They say they
are using the silicon substrate essentially as a filter since
different colors penetrate to different depths. Perhaps it is
digital math. The top layer measures all the light. The second
layer measures the red and the green, and the bottom layer measures
just the red. Some subtraction and you get the values. So I guess the
question is whether three transparent Foveon sensors and two silicon
layers of the required filtering thickness is thicker than a single
mosaic sensor and filter.
--
Winsor Crosby
Long Beach, California
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