GOKConservator Posted August 16, 2013 Report Share Posted August 16, 2013 Carla, Mark, Marlin and Community – I’ve some questions about reflective spheres and optimal normal reflection vector precision (that is the repeatability) and accuracy (how closely measured normal reflection vector values are to the actual value). The CHI “Guide to Highlight Image Capture, 2.0 (http://culturalheritageimaging.org/What_We_Offer/Downloads/RTI_Hlt_Capture_Guide_v2_0.pdf) explains that: “Depending on the size and portability of the target object, you must compose the camera’s field of view so it can encompass both the object and two reflecting spheres of an appropriate size. The spheres should have a diameter of at least 250 pixels in the resulting photograph.” (Pg. 3, Target Object with Reflective Spheres.) THE SET-UP- As a practical example, let’s say I’m capturing a 36” wide x 24” high painting using a 50mm lens. With space on either side of the frame for the spheres to be mounted so that they do not cast a shadow onto the canvas during 15° flash positions, the total width of the frame area is roughly 45”. With my 5D Mark II and a 50mm lens, shooting the captures in RAW, I get photos with a total frame size of 5616 horizontal pixels x 3744 vertical pixels, a 21.026 megapixel file (21.0MP). That equates to roughly 125 pixels per inch on the canvas. Let’s assume that when I manufacture the assembly jpg’s, I first distortion correct for the aberrations around the outer areas of the lens frame so that every pixel is metrological. A one inch reflective sphere is 125 pixels in this set-up – HALF the recommended pixel diameter. In my experience, really takes about 20 pixels, MINIMUM, to resolve a condition I am interested in documenting so that I may track its changes accurately. So in reality the smallest features that will resolve clearly in this composition are about an 1/8th of an inch diameter, about 3.2 mm. Let’s assume from a qualitative standpoint, I’m happy with that resolution. THE QUESTIONS- Normal reflection vectors taken from HSH assembly code calculate the normal reflection data using the brightest-to darkest RGBL values where the light source is the inverse of the highlight position on the spherical surface. To what degree am I decreasing both the accuracy and the precision extra variability or noise into the processing of normal reflection vectors by having a reflective sphere only HALF the recommended size? Do the HSH algorithms require a nearly 250 pixel diameter hemisphere to accurately calculate the light sources and inverted reflection vectors? How much does variability (precision) depend upon having a minimum 250 pixel hemisphere? My guess is that the 250 pixel recommendation is based upon some optimization tests. But if nobody knows, perhaps I should gather that data?Thanks – Dale Kronkright (GOKConservator), Head of Conservation, Georgia O’Keeffe Museum Link to comment Share on other sites More sharing options...
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