Paleonet: Orthochromatic images

Hans Kerp kerp at uni-muenster.de
Wed Mar 23 18:20:14 GMT 2011


Dear Enrico,

the use of polarized light might be very helpful. As far as I know this 
method was first described by Friedemann Schaarschmidt (1973: 
Pflanzenfossilien in ungewöhnlichem Licht. Natur und Museum, 103, 
247-253). A more recent account is by Stefan Bengtson (2000: Teasing 
fossils out of shales with cameras and computers, 
http://palaeo-electronica.org/2000_1/toc.htm), who also discusses 
digital methods to enhance the contrast. If you use polarized light be 
sure to adjust the white balance prior to exposure, because images often 
become to greenish. Do not use the maximum contrast setting of the 
crossed nicols, because black colours may come out as blue. Moreover, 
most digital cameras have an option to vary the contrast. Polarized 
light can also be used in combination with fluid immersion, but normally 
this is not necessary. Most fluids used for immersion are not good for 
your health.

Best regards & success,

Hans Kerp, Münster
http://www.uni-muenster.de/GeoPalaeontologie/Palaeo/Palbot/ebot.html

Am 23.03.2011 12:35, schrieb enrico bonino:
> Dear all
>
> I'm searching for a method that allow to acquire images with a digital 
> camera (Fujifilm filmpix S2Pro) that can emulate the old 
> Orthochromatic films.
> Orthochromatic films block the red wavelength and are sensitive to 
> blue and green only, giving a great contrast when red colors are 
> present in the image.
>
> These films are used in Xiangguang & Bergström  (1997), Arthropods of 
> the Lower Cambrian Chengjiang fauna, southwest China, in Fossils & 
> Strata n°45, to improve the contrast of some lower Cambrian Chengjiang 
> arthropods.
> This methods don't need the immersion of the sample in Toluene or 
> Xylene alcohols, and it is very useful when the matrix (and this is 
> the case) is soft, like marls and clays.
>
> Chengjiang fossils are generally covered by a thin Iron oxide layer, 
> and this technique allow to increase the contrast between the 
> gray-yellow matrix and the reddish fossil.
>
> Looking at some filters that have the same spectral  behavior, I've 
> found that the cyan Kodak Wratten gelatine filters 44 and 44A 
> (http://en.wikipedia.org/wiki/Wratten_number and 
> http://www.physics.uc.edu/~sitko/LightColor/4-ColorMix/ColorMixing_files/image026.png 
> <http://www.physics.uc.edu/%7Esitko/LightColor/4-ColorMix/ColorMixing_files/image026.png> 
> for the transmittance curve) are able to give the nearly same result 
> of the old and practically unavailable orthochromatic films.
>
> There is someone that has developed similar techniques and digital 
> image processing methods to increase the contrast using peculiar filters?
>
> Thank you,
>
> Enrico Bonino
>
> *********************
> Enrico Bonino Dr.
> GIS Specialist - Geologist at KEYOBS SA, Liège, Belgium
> Museum Curator at the Back to the Past Museum, Cancun, Mexico
> skype: enrico_bonino
> http://www.backtothepast.com.mx
> http://www.keyobs.com/fr/ebonino
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