[filmscanners] Re: Newish Digital Tech

["Karl Schulmeisters" <karlsch@earthlink.net>]

Yes you can distinguish without a filter. I don't know the exact numbers on
the physics properties of silicon, so I'm going to use water as the example,
but the basic idea is the same.

Clear silicon transmits different frequencies of light to a different
extent.  Red has the least transmissivity, Blue the greatest.  This is very
similar to water - which is why everything below about 50' starts to look
green and below that it rapidly shifts to blue.  So imagine if you put a
full spectrum photocell at 15', 50' and 100',  you'd then be able to
calculate the blue component from the 100' sensor multiplied by Blue
Frequency attenuation.  Green would be calculated by subtracting the Blue
Freq component from the50' sensor (again normalized for both blue and green
attentuation), and Red would be calculated by subtracting Blue and Green
from Red.

The key to the speed is that the attenuation rate stays fixed, so you could
essentially have a calculation that looks something like

B = 100'sensor *8
G = (50'sensor * 4) - B == (50'sensor - (2x100'sensor))x4  (this latter
equation being pretty easy to gin up with analog electronic components)
R = (15'sensor *2) - B - G == (15'sensor - (2x50'sensor))x2

But the gotcha is that the sensor in the Foveon case is not at a single
depth, its a 'region' of doped silicon.  And so you are going to get an
averaged value, and you are also going to get some red propogating all the
way to the Blue and Green sensors quantum mechanically - ie
probablistically.

So you can probablistically compensate for it, but its going to take time
for them to tune the compensations.
----- Original Message -----
From: "Austin Franklin" <austin@darkroom.com>
To: <karlsch@earthlink.net>
Sent: Tuesday, January 14, 2003 12:09 PM
Subject: [filmscanners] RE: Newish Digital Tech


Karl,

> That's not how the Foveon chip works.  There are no filters. They
> are taking
> advantage of the fact that different light frequencies have
> different depth
> penetrations into silicon.

Well, yes and no...but anyway, filtering HAS to take place, or you could not
distinguish between RGB.

> Essentially there is not going to be as clean a
> differentiation between the amount of light at the R,G and B
> sites,

What EXACTLY is the sensing mechanism?  Do you know, and if so, can you
describe it?

> and they
> are relying on subractive calculation to compute the R and G
> values.

Speculation, or do you have a resource for this information?

Regards,

Austin

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