Re: using a scanner as a densitometer?

Ryuji,

instead of Matlab (not cheap) you can use R which is free and loaded with
statistical package to do just the kind of regression you suggest. The
learning curve for both is relatively steep though. You could probably use a
Excel or similar as well and maybe this page could help others as well
(http://www.normankoren.com/color_management_4.html) for formulas amoung
others.

Regards
Yves

----- Original Message -----
From: "Ryuji Suzuki" <rs@silvergrain.org>
To: <alt-photo-process-l@usask.ca>
Sent: Tuesday, February 21, 2006 2:08 PM
Subject: Re: using a scanner as a densitometer?

> The best way is to use the RAW tiff image file (signals coming out of
> the scanner sensors with very minimal of processing--not even gamma
> correction) saved by VueScan. The reading aperture of VueScan's
> densitometer function is rather too small and this will lead to an
> unacceptable level of measurement noise in the read value. You would
> want to apply a certain reading area and average the values within the
> area on this raw tiff file.
>
> Then read a calibrated step tablet to find the mapping from the
> optical density to the scanner's value. Find a suitable polynomial to
> interpolate the measured values. (Or, use a linear interpolation
> between two adjacent points read off the calibrated step tablet, if
> the curve is reasonably linear, especially in low density range.) Find
> the inverse function of the interpolating function. Then apply this
> inverse function to the value read off the sample of unknown density
> to obtain the optical density reading.
>
> I use a custom MATLAB program to do this, and I don't know of a good
> software package that is designed to do the right things.
>
> Also, it's probably not very difficult to make a blue- and green-
> reading transmission densitometer using some of the preamplified
> photodiodes incorporating lens and filter. These are sold by
> TAOS. Combine these with a white LED. (I was told by TAOS that the
> ones with analog output may be discontinued, but they still make ones
> with digital output.) These sensors are also very useful in making
> enlarger meters as well.
>
>
> From: ryberg <cryberg@comcast.net>
> Subject: using a scanner as a densitometer?
> Date: Tue, 21 Feb 2006 09:27:53 -0800
>
> > I've read a couple of casual mentions of using a scanner as a
densitometer.
> > My Epson 3200 reads the density as my choice of 0 to 100% or 0 to 255
units.
> > The only way I can think of to get a log density reading is to scan a
step
> > tablet as the same time I scan a negative and make comparison of the
> > readouts. Is there another way?
> > Charles Portland OR
> >
Received on Tue Feb 21 14:13:52 2006