Digital BW, The Print

Thanks all for this great discussion. As regards scanning neutral density
images (B & W) on a color scanner, would it make a difference if  instead of
changing the sensors, the light source was another color? I presume the best
solution would be a sensor array that was not sensitive to color at all but
just light intensity.

 

Tony

 

-----Original Message-----
From: Austin Franklin [mailto:darkroom@...] 
Sent: Thursday, May 22, 2003 12:15 AM
To: DigitalBlackandWhiteThePrint@yahoogroups.com
Subject: RE: [Digital BW] Why is ND B&W scan better -- was Digital, film,
scanning comparisons

 

Truman,

> Quantum efficiently  has nothing to do with it.

Well, yes it does.  Quantum efficiency is the ratio  of the number of
photogenerated electrons captured by a sensor to the number of photons
incident upon the sensor during a period of time.  What that basically means
is that red, because of it's %80 quantum efficiency over a VERY broad
spectrum will fill the well much faster than the other two colors, which
means red will smear and bloom first, given the same amount of light energy
of each color.

> Red is at the low end
> or the visible frequency spectrum. Normally the higher the frequency the
> wider the bandwidth of filters at that frequency, unless you use a local
> frequency reference to down convert the signal. Why is this not the case
> for red light which is a lower frequency than blue light? Different
> materials for CCD's have different frequency reposes. I guess I don't
> understand why "red" produces more energy output.

Because the CCD is most sensitive to it, which is what quantum efficiency
is!

> The energy of a photon is Planks constant X it frequency. A photon of
> blue has more energy than a photon of red.

That doesn't matter.  What matters is how the CCD sees it, which is quantum
efficiency...

> Different scanners use
> different light sources. I am still at a loss - not that I am disputing
> it - with your statement. I just don't understand.

Light sources have nothing to do with it.  The light is filtered by either
the RG or B filter, and the CCD reacts to that color light only.  I don't
understand why you think that matters?  All the light sources cover the
spectrum that is being detected.

Austin






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