What the imager has

Started Feb 11, 2014 | Discussions thread
Laurence Matson
OP Laurence Matson Forum Pro • Posts: 11,913
Re: Once more with feeling

DickLyon wrote:

Laurence Matson wrote:

As with all previous Foveon imagers there are layers where colors are detected. The "filtering" done in the silicon and the filtering process is really just counting electrons at each discrete location. The electrons are the "corpses" from the expired photons. Since photons carry energy in proportion to their frequency, the stronger ones will penetrate furthest and the weaker one will penetrate least. So the "blue" layer is merely a device to count how many dead photon bodies are lying around; the same goes for the "red" layer and the "green" layer.

Yes, this is a simple explanations. Perhaps some engineers can parse it better. I just studied acting.


Checking dpreview after hearing the news, I was amused to see that not much has changed in recent years.

Your Shakespearean description is not bad. The collected electrons are exactly the corpses of absorbed photons, which is the point that most people who talk about the filtering miss: absorption and filtering and detection being the same event.

But the "stronger" and "weaker" is not quite right. The high-frequency blue photons are stronger (highest energy); but the way they interact with silicon makes them get absorbed soonest, near the surface. The lowest energy photons, in the infrared, penetrate deep. Low frequencies, at wavelengths greater than 1100 nm, where the photons are quite weak, are not able to kick electrons out of the silicon lattice at all, so the silicon is completely transparent to them. In between, there's a nicely graded rate of absorption.

Understanding the spectral responses of the layers starts with understanding that at any wavelength, the light intensity decreases exponentially with depth in the silicon. I think I've written about this some place... Anyway, the top layer is not white, not luminance, not blue, but a sort of panchromatic blueish that turns out to work well enough for getting a high-frequency luminance signal. We did a lot of experiments and amazed ourselves how "well enough" the 1:1:4 worked; it was not obviously going to be a good thing, but turned out awesome.

We had 1:1:4 sensors working, including very efficient and effective hardware and software processing pipelines, before I left Foveon back in '06, but the sensors didn't yet have the low-noise fully-depleted top-layer photodiodes of the "Merrill" cameras, and we were only targeting these for cell phones at the time. I expect it will be a killer combination: fully-depleted blue plus 1:1:4. I don't think the red and green are fully depleted, too; that was thought to be somewhere between hard and impossible, which is why they don't have the same low read noise, and one reason why aggregating red and green quads this way is a big win.

But understanding how it compares to the classic Foveon and to Bayer will keep many people busy for a long time. Something for dpreviewers to do while waiting for cameras, and something to keep the marketing people tied up... Should be fun.


Well, I guess you should know too, 'cause you are also an engineer. But Merrill was just a janitor who liked trains, so he was a wannabee engineer. Sort of.

I was just looking for that stuff. I guess you just looked harder. But of course it makes sense, like the low-frequency signals whales use and that stuff.

Google really isn't much of a friend when you don't know what you're talking about.

Welcome back. More stories please.

And there is nothing more fun than tying up marketing people. Duct taping them too.

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laurence at appledore-farm dot com
"I thought: I read something in a book, I dream, I imagine, and it comes true. And it is exactly like this in life.
"You can dream, and it comes true, as long as you can get out of the certitudes. As long as you can get a pioneering spirit, as long as you can explore, as long as you can think off the grid. So much time we spend in our education, in our lives is spent learning certitudes, learning habits, trying to fight against the unknown, to avoid the doubts and question marks. As soon as you start to love the unknown, to love the doubts, to love the question marks, life becomes an absolutely fabulous adventure."
Bertrand Piccard, a Swiss person

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