From: pcdhSpamMeSenseless@electrooptical.net   
      
   On 1/11/2014 12:19 PM, haiticare2011@gmail.com wrote:   
   > On Saturday, January 11, 2014 12:17:23 PM UTC-5, haitic...@gmail.com   
   > wrote:   
   >> I am working on a medical research project with the following   
   >> constraints: arteriole model with dia. of ca. 1.5 mm, freedom to   
   >> surround arteriole with any cavity, led wavelengths. The aim is to   
   >> measure hemoglobin concentration. Scattering by blood cells is NOT   
   >> an issue in this experiment, just the absorption by the Hb, due to   
   >> the particular aim of the experiment.   
   >>   
   >>   
   >>   
   >> I have tentatively selected a 5 mm blue led and a generic Si pin   
   >> diode arranged in several geometries. The geometries have included   
   >> aiming the led strait through the arteriole model and out the other   
   >> side onto the detector (A), jacketing the arteriole model with a 2   
   >> mm channel and letting light bounce down it for a short distance to   
   >> an offset detector,(B),  or (C) creating a reflective chamber say 7   
   >> mm dia around the tube and fed by a blue led, with watching   
   >> detector on perimeter.   
   >>   
   >> This would be painted flat white inside.   
   >>   
   >>   
   >>   
   >> The first question is whether a blue led is best. I picked the 460   
   >> nm led because, obviously, anything absorbing in the blue looks   
   >> red. (ie Hb)  But of course a white or red emitting led is going to   
   >> be brighter.   
   >>   
   >>   
   >>   
   >> But here is my question: The apparent red color of hemoglobin may   
   >> be there just as a differential in the extinction coefficients   
   >> between red and blue. So a white led may still absorb in the red   
   >> enough to give a useful signal. I should mention in this model,   
   >> just as in vivo, the only absorption is from the Hb - there are no   
   >> other strong absorbers present.   
   >>   
   >>   
   >>   
   >> At this point I like the reflective cavity the best. (C). I tried   
   >> the constrained channel (B), but the signal was not very strong.   
   >> (It could be boosted, so is still a contender.) (A) attracted   
   >> because the source and sensor are in line, but is proving to be   
   >> tricky to align without the light going around the red tube.   
   >>   
   >>   
   >>   
   >> So I am left with (C0, a proven detection scheme for constrained   
   >> targets.   
   >>   
   >>   
   >>   
   >> As well, I had the idea that (B0 is just a narrow version of (C).   
   >> The light is bouncing down the tube, in equilibrium with absorption   
   >> by the target. Probably the advantage of a proper reflective cavity   
   >> is the collection gives a better photon budget.   
   >>   
   >>   
   >>   
   >> Any ideas welcome.   
   >   
   > B0, C0 = (B), (C)   
   >   
      
   All kinds of stuff absorbs at 460 nm.  You might try the 578 nm   
   haemoglobin absorption peak, and compare it against something nearby,   
   like 600 nm.   
      
   Cheers   
      
   Phil Hobbs   
      
      
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   Dr Philip C D Hobbs   
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