sutherla@qtp.ufl.edu (Scott Sutherland) (08/28/90)
I just finished listening to a member of our research group talk about an aspect of ray tracing that I was not very familiar with but which appears to be very powerful for optical spectroscopy. You can ignore the specifics of the application, but I include them here (generalized) so that you can see the types of applications to which I refer. Imagine a light source in the center of a narrow tube (for those of you interested, a specific example is a sample of molecules in a graphite furnace illuminated by shining a laser beam through the tube. The "light source" of interest is the fluorescence of the analyte). Now, the "source" emits light in ALL directions, but the "detector" is positioned at one end of the tube and will only collect a certain solid angle of the emitted light. This "diverging" light is then to be focused onto a slit (the entrance slit of a monochromator) and then will disperse to fill the face of an optic on the other side of the slit (usually a mirror inside the monochromator). A "crude" diagram is given below: ----- Mirror (M2) to be "filled" with image of ** \ / \ / ----- ----- The SLIT (S) . / \ Fluorescence focused to a "slit" | | |---|-|---| | | | | Focusing Lens (FL) |---|-|---| | | ^ ^ | | | | \ | | \ | | \ | | -------------------------- \|-------------\||/ Laser (L)-----|---------> -**- Tube (T) \|___________/||\ \ -------------------------- \ \ Mirror (M1) with a hole in the center to pass the laser beam. ** is the sample (e.g. light source for my purposes) and the lines all around it (\, |, /, etc.) are the light rays emitted (in all directions). Now to the problem which is to be solved. What is of interest is WHICH RAYS emitted from ** reach the mirror M2. The "shape" of the image, the "shadows", or occluded parts (from the tube, the slit, etc.), etc. are all of interest. Assume M2 is circular in cross section. What we want to do is to figure out which parts of the "image" at M2 eminate from the source, **, from the walls of the tube, T, and from other sources (e.g. in the real experiment there are quartz plates on either end of the tube which can act as additional "sources" of light by fluorescing. So why do we care about this??? If we can do these calculations, we can test different types of lenses (L) or lens combinations (for instance, in the talk I saw today, a pair of matched plano-convex lenses, with the convex sides facing one another and the flat sides facing the reflected light from L and passing the focused light onto the slit (S) worked much better at preventing any rays which eminated from the walls of the tube from reaching the mirror M2. This reduces the background "noise" in our experiments and makes the detection more efficient.). Also, by knowing the "patterns" of the light which reach M2 that are emitted by undesired sources as well as the sample (**), it is possible in some instances to "block" out the undesired light by means of a physical mask placed infront of M2. The talk I saw this morning displayed that large decreases in the amount of noise (signal, or light rays reaching M2, from unwanted sources) in the experiment. Now for the use of Turbo Silver. Well, the data that I was shown today was generated by a program written by another researcher explicitly for this purpose. They calculate only 200 rays to get an idea of what the projected image at M2 will look like and it takes them 2-4 hours on a PC AT with a math processor. Knowing that TS CAN handle refractions and reflections well, I was wondering if calculations of the type mentioned above could be made. Now, I realize that TS was NOT designed for this purpose, so it would not be easy, but TS should be much faster at calculating these data. Specific questions include: 1) Has anyone tried to use TS to calculate simple images from lenses?? 2) Does anyone have ideas as to how you would create lenses with given focal lengths (I can see lathing or extruding out the shapes, but getting the focal lengths right would be difficult)? 3) Any ideas on how I would get the "image" at M2?? (Would just positioning the camera so that M2 filled the view work??) 4) Am I grasping at straws here? Am I asking too much of TS?? Please email responses to me. If any of the problem description above is unclear, just ask. Oh, another point of interest (one not readily calculated by TS) is to calculate the "fraction" of all rays emitted which actually reach M2, a number referred to as the collection efficiency (other factors are involved in calculating this efficiency, but the "solid angle" viewed by M2 would be a good first approximation). Any ideas as to how to get this?? Thanks, Scott Sutherland sutherla@qtp.ufl.edu