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How to calculate light positions?

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I'm hoping to use the RTI method to image some fossils. The fossils are largely two dimensional with a very shallow relief of at most a few millimetres, and are generally approximately 5 to 10 centimetres long. I was considering doing this by constructing a dome, 25 cm in diameter with a central hole in the top for the camera and then 30-40 holes for light to be shined through. The light would be moved manually between photos. As I understand there is a method of doing an RTI without using reflective spheres, but this requires a file containing the different light positions. How are these light positions calculated?

 

Any other advice on how best to photograph these specimens would be more than welcome. Thank you. 

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Probably the simplest way is to use a small reflective sphere in conjunction with the RTIBuilder software to generate an lp file. If you include the sphere in the photo along with the fossil, you could use RTIBuilder every time to generate both the lp file and the ptm/rti datasets. However, if you always take the photos in the exact same lighting sequence, you could generate a single lp file, and then use that as a template for future photosets. Check the RTIBuilder manual for more info.

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Using a sphere to get light positions for a dome will work.  It is important that the lights will always be in the same position relative to the camera and taken in the same sequence if you want to use a pre-calculated LP file and the dome path in RTIBuilder.

 

For our light arrays, we generate the light positions from a CAD drawing, and then shoot a sphere as a secondary check.  If you generate them from the CAD drawing you have to do some work to normalize the data to be between -1 and 1.

 

Carla

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