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Author Topic: Quantum Efficiency  (Read 5522 times)

dreed

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Quantum Efficiency
« on: February 15, 2012, 11:24:41 pm »

With all of the technical talk about the merits of sensors, people might be interested in this web page:

http://www.sensorgen.info/

It presents a % (higher is better) representing how efficient the various sensors are.

And before I forget, the most efficient sensor that they currently have on display is Canon's Powershot G11 at 60%, almost double that of the 5D Mark II (33%). It will be interesting to see how that page fills out if they can get more MFDBs there.
« Last Edit: February 15, 2012, 11:30:05 pm by dreed »
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Jeremy Roussak

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Re: Quantum Efficiency
« Reply #1 on: February 16, 2012, 03:42:41 am »

With all of the technical talk about the merits of sensors, people might be interested in this web page:

http://www.sensorgen.info/

It presents a % (higher is better) representing how efficient the various sensors are.

And before I forget, the most efficient sensor that they currently have on display is Canon's Powershot G11 at 60%, almost double that of the 5D Mark II (33%). It will be interesting to see how that page fills out if they can get more MFDBs there.
Well, I've certainly learned something. For some time, I've thought that the image quality of a 5dII was better than that of the G11. Now I know its sensor is so much less efficient, I'll have to reconsider.

Jeremy
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hjulenissen

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Re: Quantum Efficiency
« Reply #2 on: February 16, 2012, 03:56:58 am »

Well, I've certainly learned something. For some time, I've thought that the image quality of a 5dII was better than that of the G11. Now I know its sensor is so much less efficient, I'll have to reconsider.

Jeremy
I don't believe that QE=IQ.

-h
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dreed

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Re: Quantum Efficiency
« Reply #3 on: February 16, 2012, 06:21:03 am »

Well, I've certainly learned something. For some time, I've thought that the image quality of a 5dII was better than that of the G11. Now I know its sensor is so much less efficient, I'll have to reconsider.

That's not the right way to interpret the numbers.

Rather, I believe that if the 5dII used the same sensor design for pixels as the G11 but kept its current pixel size then its sensor would be close to a full stop better right across its ISO range.
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BernardLanguillier

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Re: Quantum Efficiency
« Reply #4 on: February 16, 2012, 06:43:56 am »

the most efficient sensor that they currently have on display is Canon's Powershot G11 at 60%, almost double that of the 5D Mark II (33%).

There is probably a near perfect inverse correlation between the age of a sensor and its efficiency.

The 5DII probably has one of the oldest sensor still in production next to that of the 1ds3.

Cheers,
Bernard

PierreVandevenne

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Re: Quantum Efficiency
« Reply #5 on: February 16, 2012, 07:23:31 am »

Dynamic range remains proportional to FWC and read noise. This with resolution are the two essential parameters. At least at the start, a bad camera design can spoil good incoming data.

QE is basically how efficient you are at capturing photons. A sensor with a very high QE but a small FWC or high read noise would be much worse than a sensor with lower QE but higher FWC or lower read noise. All other things being equal, a lower QE means that you have expose a bit more or use a bigger collector (lens) to record the same number of photons.

Also note that R. Clarck numbers are often more specifically defined than QE in the generic sense of the term. He'll say, for example, that the green sensels have a QE of 30% for a  specific wavelength. But since the green pixels occupy 50% of the sensing area, it would actually be 15% compared to an unfiltered KAF sensor.

Assuming a camera given with 60% QE as a whole, it would mean 30% in green, 15% in red, 15% in blue compared to a bare sensor. Also QE varies a lot with wavelength. See http://www.starsensesci.com/product/product_astro/QHY9/images/KAF8300QE.jpg for example.

In that case, the 60% (30% effective at 550nm) is just a peak in green. At 700nm, in red, you are at 40%, 10% effective.

But it's true that better micro-lenses have enabled us to do more with the same flux of incoming photons, or do the same with smaller optics.
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hjulenissen

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Re: Quantum Efficiency
« Reply #6 on: February 16, 2012, 08:15:07 am »

But it's true that better micro-lenses have enabled us to do more with the same flux of incoming photons, or do the same with smaller optics.
If one could combine the "light-directing" properties of micro-lenses with the "wavelength-selectivity" of the CFA, that would be neat. I.e. "send the green light in this area to the green sensel, the red light to the red sensel" etc. Surely, this must be hard to do economically on a large scale, or we would have seen it.
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bjanes

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Re: Quantum Efficiency
« Reply #7 on: February 16, 2012, 09:29:22 am »

I don't believe that QE=IQ.

Quite true! Other things being equal, one merely has to give more exposure with a camera having a lower QE and image quality wouldn't be affected, except for limitations imposed by shutter speed/aperture considerations. The color filters of the Bayer sensor absorb light, and if one makes the bandpass wider, more light will reach the sensel elements and QE would improve at the possible expense of color rendering. An example is given in the DXO White Paper comparing the Nikon D5000 and the Canon 500D.

For a low light camera such as the Nikon D3s, QE is important, but for a MFDB that will usually be used on a tripod and with better lighting, QE is less important. Indeed, a lower QE would require more exposure and lead to a lower ISO rating. People often complain that a camera does not have ISO 50. By lowering QE, the camera could have ISO 50 and could be used at longer exposures for shots of moving water, etc. However, one should not take the rated ISO as a true measure of sensitivity to light, since current ISO standards (the REI (recommended exposure index, see Wikipedia) allow arbitrary assignment of ISO at whatever the manufacturer thinks best to allow for highlight headroom, etc.

Regards,

Bill
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clkirksey

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Re: Quantum Efficiency
« Reply #8 on: February 16, 2012, 11:16:55 am »

The cited web site uses data from DXOMark to calculate QE. This site is being referenced continually asa  valid source for these QE numbers. Think about. If Nikon had a sensor with a QE twice that of Canon don't you think it would show up some were in their lit? What about DXOMark? They show all kinds of data why not just include QE if it was important. After all it is their data thatr the site is using.
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A bird in the viewfinder...

dreed

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Re: Quantum Efficiency
« Reply #9 on: February 16, 2012, 12:19:31 pm »

The cited web site uses data from DXOMark to calculate QE. This site is being referenced continually asa  valid source for these QE numbers. Think about. If Nikon had a sensor with a QE twice that of Canon don't you think it would show up some were in their lit? What about DXOMark? They show all kinds of data why not just include QE if it was important. After all it is their data thatr the site is using.

"Quantum Efficiency" is too scientific and incomprehensible to the average person buying a digital camera - unlike those pursuing photographic excellence and the narrow end of the curve here. Thus for it to appear in "marchitecture" would not be at all useful.
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BJL

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Quantum Efficiency of 60%! and what to means
« Reply #10 on: February 16, 2012, 01:54:26 pm »

The way that QE for a color sensor is usually measured and stated is a triple of numbers, one for each of the filter colors, and the measure is the detection percentage for the single wavelength that gives highest detect rate. My guess is that the single number cited here is either the one for green (which is usually the best and most important) or some average, not a sum. [Update: it is for green, I see now following the links to the source, which is a forum post at DPReview: http://forums.dpreview.com/forums/read.asp?forum=1019&message=34526220 ] But the figure at sensorgen for the H3D II 50 is 16%, whereas Kodak reports 22%, 22%, 16% for B, G, R for its sensor at
http://www.kodak.com/ek/uploadedFiles/Content/Small_Business/Images_Sensor_Solutions/Datasheets(pdfs)/KAF-50100ProductSummary.pdf

You can see lots of QE data for sensors from Kodak (sorry, Truesense Imaging) at
http://www.kodak.com/ek/US/en/Image_Sensor_Solutions/Products/Full_Frame_CCD.htm
Just click on the product name at left for the short spec sheet.

The only sensors I have seen with QE comparable to that 60% for the G11 [Edit] are monochrome, with no CFA. is a few Kodak ones with huge photosites, like this one with 40%, 55%, 65% for B, G, R:
http://www.kodak.com/ek/uploadedFiles/Content/Small_Business/Images_Sensor_Solutions/Datasheets(pdfs)/KAF-1001ProductSummary.pdf


I have to wonder if color discrimination is sacrificed to get better low light performance in those small sensors. [End Edit]


Anyway, we area mostly looking at a range of less than 1/2 stop, apart from a few outliers like the H3D II 50 and Phase One P65+, which have the big QE disadvantage of lacking micro-lenses.
« Last Edit: February 18, 2012, 07:10:55 pm by BJL »
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