Velocity Reviews > Logarithmic RAW images

# Logarithmic RAW images

Alfred Molon
Guest
Posts: n/a

 03-05-2004
In article <M512c.9282\$(E-Mail Removed)>, david-
http://www.velocityreviews.com/forums/(E-Mail Removed)-this-bit says...

> > Interestingly enough, a photo-diode used in photo-voltaic mode rather

> than
> > photo-current mode is inherently logarithmic. The catch is that existing

> imaging
> > devices integrate the photo-current over the duration of the exposure by

> using
> > it to charge a capacitor. The corresponding operation on a photo-voltage

> would
> > be incredibly difficult by comparison.

>
> Too much of a sensitivity loss - throwing away that integration!

Well, you'd have to integrate a number of measurements over time. Don't
know how sensitivity would be affected, but such a logarithmic
photodiode sensor would have a HUGE dynamic range - in the region of 120
dB (10^6:1 or 1000000:1).
--

Alfred Molon
------------------------------
http://groups.yahoo.com/group/Olympus_405060/
Olympus 5050 resource - http://www.molon.de/5050.html
Olympus 5060 resource - http://www.molon.de/5060.html

John Navas
Guest
Posts: n/a

 03-05-2004

In <(E-Mail Removed) .com> on Fri, 05 Mar 2004
19:30:40 GMT, Alfred Molon <(E-Mail Removed)> wrote:

>In article <M512c.9282\$(E-Mail Removed)>, david-
>(E-Mail Removed)-this-bit says...
>
>> > Interestingly enough, a photo-diode used in photo-voltaic mode rather than
>> > photo-current mode is inherently logarithmic. The catch is that existing imaging
>> > devices integrate the photo-current over the duration of the exposure by using
>> > it to charge a capacitor. The corresponding operation on a photo-voltage would
>> > be incredibly difficult by comparison.

>>
>> Too much of a sensitivity loss - throwing away that integration!

>
>Well, you'd have to integrate a number of measurements over time. Don't
>know how sensitivity would be affected, but such a logarithmic
>photodiode sensor would have a HUGE dynamic range - in the region of 120
>dB (10^6:1 or 1000000:1).

You would still be limited by the fundamental physics of the device. Current
devices have excellent quantum efficiency, so there's not much room for
improvement.

--
Best regards,
John Navas
<http://bobatkins.photo.net/info/charter.htm> <http://rpdfaq.50megs.com/>]

Alfred Molon
Guest
Posts: n/a

 03-05-2004
In article <8952c.6104\$(E-Mail Removed)>, spamfilter0
@navasgroup.com says...

> You would still be limited by the fundamental physics of the device. Current
> devices have excellent quantum efficiency, so there's not much room for
> improvement.

Not so excellent quantum efficiencies - in the region of 20-30% (only
back illuminated CCDs have QEs of 80-90%).
But what I'm talking about is the dynamic range, not the quantum
efficiency.
--

Alfred Molon
------------------------------
http://groups.yahoo.com/group/Olympus_405060/
Olympus 5050 resource - http://www.molon.de/5050.html
Olympus 5060 resource - http://www.molon.de/5060.html
Olympus 8080 resource - http://www.molon.de/8080.html

Paul H.
Guest
Posts: n/a

 03-05-2004

"David J Taylor" <(E-Mail Removed)-this-bit> wrote in
message news:57M1c.8674\$(E-Mail Removed)...
> "Paul H." <(E-Mail Removed)> wrote in message
> news:(E-Mail Removed)...
> >

> []
> > The output of the photodiodes comprising the CCD is linear, so why would

> you
> > want an ADC with a square-root or logarithmic transfer function? Far

> from
> > extending the dynamic range, such a scheme would introduce even more
> > quantization errors into an already noisy and futzed-up signal,

> particularly
> > so at the low end of the scale.

>
> So to make the most of using logs, you would want a sensor with a log and
> not a linear response. Just think of the dynamic range in the shadows!
>
> David

Exactly right. A *native* log-response sensor would be the digital
photographers dream!

John Navas
Guest
Posts: n/a

 03-05-2004

In <(E-Mail Removed) .com> on Fri, 05 Mar 2004
20:39:05 GMT, Alfred Molon <(E-Mail Removed)> wrote:

>In article <8952c.6104\$(E-Mail Removed)>, spamfilter0
>@navasgroup.com says...
>
>> You would still be limited by the fundamental physics of the device. Current
>> devices have excellent quantum efficiency, so there's not much room for
>> improvement.

>
>Not so excellent quantum efficiencies - in the region of 20-30% (only
>back illuminated CCDs have QEs of 80-90%).

Quantum efficiency of current conventional sensors is in the range of 70-90%.

>But what I'm talking about is the dynamic range, not the quantum
>efficiency.

That's limited by practical issues like quantum efficiency.

--
Best regards,
John Navas
<http://bobatkins.photo.net/info/charter.htm> <http://rpdfaq.50megs.com/>]

Alfred Molon
Guest
Posts: n/a

 03-07-2004
In article <9M62c.6122\$(E-Mail Removed)>, spamfilter0
@navasgroup.com says...

> >Not so excellent quantum efficiencies - in the region of 20-30% (only
> >back illuminated CCDs have QEs of 80-90%).

>
> Quantum efficiency of current conventional sensors is in the range of 70-90%.

That would be the quantum efficiency of back-illuminated CCDs, i.e. CCDs
which have been thinned to about 10 micrometers and which are exposed to
light from the back side.
Standard CCDs used in digital cameras are not thinned and receive the
light from the front side (front-illuminated). There the light is
obstructed by gates and other silicon, which reduce the quantum
efficiency.

I did a web search for a page which would clearly explain this and
found:
http://www.ccd.com/ccd101.html

Note that in standard front-illuminated CCDs the quantum efficiency (QE)
varies between a 5% and 40% in the 400-700 nm range of visible light. QE
is much higher for back-illuminated devices and varies between 65-85% in
the 400-700 nm range.
--

Alfred Molon
------------------------------
http://groups.yahoo.com/group/Olympus_405060/
Olympus 5050 resource - http://www.molon.de/5050.html
Olympus 5060 resource - http://www.molon.de/5060.html
Olympus 8080 resource - http://www.molon.de/8080.html

John Navas
Guest
Posts: n/a

 03-07-2004

In <(E-Mail Removed) .com> on Sun, 07 Mar 2004
10:00:42 GMT, Alfred Molon <(E-Mail Removed)> wrote:

>In article <9M62c.6122\$(E-Mail Removed)>, spamfilter0
>@navasgroup.com says...
>
>> >Not so excellent quantum efficiencies - in the region of 20-30% (only
>> >back illuminated CCDs have QEs of 80-90%).

>>
>> Quantum efficiency of current conventional sensors is in the range of 70-90%.

>
>That would be the quantum efficiency of back-illuminated CCDs, i.e. CCDs
>which have been thinned to about 10 micrometers and which are exposed to
>light from the back side.
>Standard CCDs used in digital cameras are not thinned and receive the
>light from the front side (front-illuminated). There the light is
>obstructed by gates and other silicon, which reduce the quantum
>efficiency.
>
>I did a web search for a page which would clearly explain this and
>found:
>http://www.ccd.com/ccd101.html
>
>Note that in standard front-illuminated CCDs the quantum efficiency (QE)
>varies between a 5% and 40% in the 400-700 nm range of visible light. QE
>is much higher for back-illuminated devices and varies between 65-85% in
>the 400-700 nm range.

See <http://space.mit.edu/ACIS/cal_report/node83.html> and
<http://www.src.le.ac.uk/instrumentation/solid_state/ccd/efficiency.html>,
which support what I wrote.

"... peak quantum efficiency reaches 85 percent for full-frame,
front-illuminated CCD imagers that combine an ITO gate material with
microlens technology ..."

--
Best regards,
John Navas
<http://bobatkins.photo.net/info/charter.htm> <http://rpdfaq.50megs.com/>]