Advantages of Foveon sensor Locked

Started Jun 23, 2013 | Discussions thread
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bobn2 Forum Pro • Posts: 58,551
Re: There is at least one absolute, and you refuse it.

xpatUSA wrote:

Kendall Helmstetter Gelner wrote:

Bobn2 wrote:


That's potentially the case, but in actuality Foveon sensors lose more photons than do Bayer.

In actuality they lose almost none, can you truly disagree that most of the photons hitting the sensor end up in the sensor at one depth or another?. It's the processing that matters.

We seem to be forgetting Quantum Efficiency (QE) in this thread. QE is the measure of how many photons get converted to electrons and are thereby detected in the sensitive parts (diffusion zones) of the sensor. Most of the photons are not detected and many pass all the way through the silicon and exit unimpeded from the back of the slab.

That would make the silicon transparent, which it is not. The conversion rate for photons to free electrons is in fact pretty good in most silicon sensors. The problem is how much of that electron charge makes its way to the gates of the pixel source follower transistors. That depends on the right electric field patterns to transport those electrons to the gates. The ones that don't find their way to the gates in the end find their way to a 'hole', releasing the original photon energy as heat.

One of the reason for the Foveon's low QE is the difficulty in engineering the electric fields to ensure the collection of all the electrons.

Early Foveon F7 sensors had quite a low QE - thirty something per cent max (which Foveon hid by taking the spectral average of each layer and adding them together - shades of 10MP versus 3.4MP).

The low QE was exacerbated by the broad spectral response of each layer which causes quite a bit of subtraction during interpolation. Small wonder that the SD9 was limited to ISO 400. For this very reason, Foveon's first step downward was taken in the F7-C9110 sensor by increasing the fill factor through the use of a micro-lens layer added to the sensor face.

By comparison, Bayer sensors - with their more separated and peaky CFA spectral responses - generally have higher QEs than later Foveons (is my guess). Does anyone have comparative figures for Merrills vs. Bayers?

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SD9, SD10, EF-500, GH1.

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