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<!-- received="Sat Jul 31 02:23:08 1999 PDT" -->
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<!-- sent="Sat, 31 Jul 1999 11:22:07 +0200" -->
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<!-- name="Ewald R. de Wit" -->
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<!-- subject="Re: SANE & exposure times" -->
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<title>sane-devel: Re: SANE & exposure times</title>
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<h1>Re: SANE & exposure times</h1>
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<b>Ewald R. de Wit</b> (<a href="mailto:ewald@pobox.com"><i>ewald@pobox.com</i></a>)<br>
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<i>Sat, 31 Jul 1999 11:22:07 +0200</i>
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<p>
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<ul>
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<li> <b>Messages sorted by:</b> <a href="date.html#224">[ date ]</a><a href="index.html#224">[ thread ]</a><a href="subject.html#224">[ subject ]</a><a href="author.html#224">[ author ]</a>
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<li> <b>Next message:</b> <a href="0225.html">Oliver Rauch: "Re: SANE & exposure times"</a>
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<li> <b>Previous message:</b> <a href="0223.html">Ewald R. de Wit: "Re: SANE & exposure times"</a>
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<li> <b>In reply to:</b> <a href="0222.html">Stephen Williams: "Re: SANE & exposure times"</a>
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<li> <b>Next in thread:</b> <a href="0223.html">Ewald R. de Wit: "Re: SANE & exposure times"</a>
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Stephen Williams (<a href="mailto:steve@icarus.com">steve@icarus.com</a>) wrote:<br>
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<i>> <a href="mailto:rickand@gemse.fr">rickand@gemse.fr</a> said:</i><br>
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<i>> > When scanning color images, optical density allone is not enough, we</i><br>
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<i>> > need to know what color (wavelength) was used to do the scanning and</i><br>
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<i>> > what is the detector efficiency as a function of wavelength.</i><br>
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<i>> </i><br>
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<i>> Not so simple. A red sensor, for example, is not just sensitive to a</i><br>
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<i>> single color that it defines as red. Instead it is typically sensitive</i><br>
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<i>> to colors ranging from ultraviolet to infrared. And if that were not</i><br>
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<i>> enough, the source of light has its own spectrum as well.</i><br>
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<p>
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Don't forget about the peculiarities of the human eye too. Even when<br>
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scanning with monochromatic light, what Nikon Coolscanner do with<br>
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their RGB LEDs, the colors sometimes don't look 'right' at all.<br>
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The main issue when doing photographic scans is not to capture<br>
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the EM spectrum as accurately as possible, it's to capture<br>
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as well as possible the scene as it appears to a human.<br>
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<br>
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<i>> A red signal is generated when the frequencies that the sensor respond</i><br>
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<i>> to, the frequencies that the illuminator emit, and the frequencies that</i><br>
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<i>> subject reflect (or transmit, for transparencies) all match. It gets even</i><br>
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<i>> more complicated in some scanners, that have color filters in the optical</i><br>
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<i>> path. These are sometimes used to compensate for issues wrt illumination</i><br>
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<i>> or CCD sensitivity.</i><br>
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<i>></i><br>
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<i>> The closest thing there is to a calibration table for scanners is</i><br>
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<i>> an ICC color profile.</i><br>
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<p>
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Luckily scanners are linear devices so we don't need LU tables; A 3x3<br>
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color transform matrix is all that is needed. This color transform<br>
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happens in the scanner right now but I would like to have the matrix<br>
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exported as a pseudo well known SANE option as well. It would have to<br>
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transfrom from scanner RGB to a well defined linear colorspace,<br>
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preferably to the space defined in Rec. 709 for HDTV, only not with a<br>
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gamma of 2.2 but with 1.0 (i.e. only using the Rec. 709 primaries and<br>
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white point).<br>
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<p>
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<pre>
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--
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-- Ewald
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<p>
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<p>
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<p>
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<pre>
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--
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Source code, list archive, and docs: <a href="http://www.mostang.com/sane/">http://www.mostang.com/sane/</a>
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</pre>
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<p>
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<ul>
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<!-- next="start" -->
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<li> <b>Next message:</b> <a href="0225.html">Oliver Rauch: "Re: SANE & exposure times"</a>
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<li> <b>Previous message:</b> <a href="0223.html">Ewald R. de Wit: "Re: SANE & exposure times"</a>
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<li> <b>In reply to:</b> <a href="0222.html">Stephen Williams: "Re: SANE & exposure times"</a>
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<!-- nextthread="start" -->
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<li> <b>Next in thread:</b> <a href="0223.html">Ewald R. de Wit: "Re: SANE & exposure times"</a>
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<!-- reply="end" -->
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</ul>
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