Archived posting to the Leica Users Group, 1999/12/31
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]John Collier wrote: > > The Canadian factory made glass for many specialty cameras, x-ray machines > and what have you. They were quite often marked ELCAN for E. Leitz CANada. > Generally these lenses are now offered without the cameras or devices for > which they were intended. I would suspect that the 90f1.0 is optimized for > the transmission of x-rays (what a rocket scientist I am) and > would not be a > good visible light lens; but, you could test to find out. That > darn factory > is still making this kind of stuff but was sold by Leica a while ago. I > wouldn't pay much for such a curiousity. > Asking an x-ray to bend in a glass lens is akin to expecting a plummeting meteor to obey a right turn only sign on a country road! There are many x-ray digital imaging devices. The x-rays are transmitted though part of a person. The person, in this case, is like a negative which limits the transmitted density of x-rays (high energy photons), depending on the density of the tissue. The photons strike an imaging plate which fluoresces, emmitting visible light, at the points where the x-rays strike. The fluorescent light is focussed by the lens on the much smaller digital detector plate. Hence the higher the speed of the lens the less x-rays which must be used. High speed in this case is a *good thing*. The problem with these lenses as you might expect is that they are not easily corrected for distortion. We have measured the positional error at the edges of a 22cm field to be in the range of 1mm once digitized. Using a 33 cm field the error rises to 1.5mm. When we employ a more laborious technique of using an conventional film x-ray plate and then scanning in the exposed film plate (i.e. 8x10 or 11x14) the linearity is perfect (<.2mm error). My assumption is that the error is introduced as part of the fast optics in the detector. I've considered mounting one of these to a Visoflex through a Bellows II. Anyone tried this? Jonathan Borden