From: pcdhSpamMeSenseless@electrooptical.net   
      
   On 1/26/2014 2:12 PM, Joe Gwinn wrote:   
   > In article , Phil Hobbs   
   >  wrote:   
   >   
   >> On 1/25/2014 1:31 PM, Joe Gwinn wrote:   
   >>> In article , Phil Hobbs   
   >>>  wrote:   
   >>>   
   >>>> On 1/24/2014 9:55 AM, Joe Gwinn wrote:   
   >>>>> In article , Phil Hobbs   
   >>>>>  wrote:   
   >>>>>   
   >>>>>> On 01/23/2014 09:50 AM, Joe Gwinn wrote:   
   >>>>>>> In article , Phil Hobbs   
   >>>>>>>  wrote:   
   >>>>>>>   
   >>>>>>>> Hi, all,   
   >>>>>>>>   
   >>>>>>>> I have a gig to design a microplate reader for a new bioassay system.   
   >>>>>>>> To match the reagent systems, it needs to work over a range of   
   >>>>>>>> wavelengths in the 340-500 nm region, none of which is particularly   
   >>>>>>>> well   
   >>>>>>>> matched to mercury emission lines.   
   >>>>>>>>   
   >>>>>>>> So, I'm casting about for a light source.  It really doesn't need much   
   >>>>>>>> power, maybe a few milliwatts per square cm in a 5-nm passband.   So   
   >>>>>>>> 5-10 W output would be fine for an arc lamp, much less for a LED.   
   >>>>>>>>   
   >>>>>>>> For particular purposes, I can get LEDs in almost any wavelength I   
   >>>>>>>> need.   
   >>>>>>>>       However, it would be very useful to have a broadband source.   
   >>>>>>>>   
   >>>>>>>> Most white LEDs appear to cut off very sharply below about 420 nm,   
   >>>>>>>> which   
   >>>>>>>> is pretty understandable given that that's the short wavelength tail   
   of   
   >>>>>>>> the blue LED chip.   
   >>>>>>>>   
   >>>>>>>> High pressure xenon lamps have nearly flat spectra in that region,   
   >>>>>>>> which   
   >>>>>>>> would be terrific if I could find one rated at less than a kilowatt.   
   >>>>>>>>   
   >>>>>>>> Any lamp- or LED-selection wisdom?   
   >>>>>>>   
   >>>>>>> How about a pulsed xenon flashlamp, ie, a stroboscope?  These are   
   >>>>>>> easily built.  (I built one to stop motion in a coil winder, so one   
   >>>>>>> could diagnose winding problems.  The flash is triggered from a axle   
   >>>>>>> position sensor, so the image stands still regardless of rotation   
   >>>>>>> speed.)  Pulsed at low power (relative to the capacity of the flashlamp   
   >>>>>>> in question), the life can be quite long.  The pulsed output fits into   
   >>>>>>> lock-in amplifier schemes nicely.   
   >>>>>>>   
   >>>>>>> Joe Gwinn   
   >>>>>>>   
   >>>>>> Thanks, Joe.   
   >>>>>>   
   >>>>>> That's the approach that a lot of existing microplate readers use.  The   
   >>>>>> problem is the pulse-to-pulse variation, which hurts the measurement   
   >>>>>> repeatability.  Something nice and stable like a LED would be my first   
   >>>>>> choice.  I may wind up with a white LED with a violet and a UV one for   
   >>>>>> fill-in, but that's a bit on the messy side and tends to waste light.   
   >>>>>>   
   >>>>>> The sample will be in rapid motion during the measurement, so an arc   
   >>>>>> lamp is a possibility.   
   >>>>>>   
   >>>>>> There's some specification creep happening at the moment.  Originally it   
   >>>>>> was a single-wavelength system, where a filtered LED would have been   
   >>>>>> just the ticket, but now it looks more like a fibre-coupled   
   spectrometer.   
   >>>>>   
   >>>>> If pulse-to-pulse variation is the problem, I'd be tempted to have a   
   >>>>> two-beam setup, using one beam as the reference and the other for   
   >>>>> measurements, and taking their ratio instant by instant.  This should   
   >>>>> largely cancel variation over pulse time, pulse amplitude, and pulse   
   >>>>> shape.   
   >>>>>   
   >>>>> The copious optical power available from a flashlamp makes up for a   
   >>>>> host of other sins.   
   >>>>>   
   >>>>> Joe Gwinn   
   >>>>>   
   >>>>   
   >>>> Getting that sort of system really right is hard, though.  A lot of the   
   >>>> pulse to pulse variation arises from the arc starting at different   
   >>>> locations, which makes the noise spatially variant.   The accuracy and   
   >>>> repeatability specs on this gizmo are very tight, so engineering the   
   >>>> illuminator to guarantee adequate performance over lamp lifetime would   
   >>>> be a big project.   
   >>>>   
   >>>> I may wind up having to do it anyway, but a half-dozen   
   >>>> different-coloured LEDs would sure be a lot easier.   
   >>>   
   >>> What I'm not understanding why pulse-by-pulse compensation would not   
   >>> work.   
   >>   
   >> It will, but because of the spatial variation, you have to do it   
   >> point-by-point in space.  For instance, a fibre-coupled system has to do   
   >> compensation sampling individually from each fibre, or at least each   
   >> bundle.  That makes system design a bit like kicking dead whales down   
   >> the beach.   
   >   
   > Hmm.  Two things occur to me.   
   >   
   > First, if one can get a 2D CCD in the right wavelength range,   
   > point-by-point could be practical.  Silicon could work.   
   >   
   > Second, little xenon flashlamps are pretty cheap, so one could use an   
   > array of individual flashlamp tubes in a common white-walled cavity   
   > behind a diffuser, presenting averaged uniform illumination to the   
   > plate.  Most of the money is in the power circuit, which can be common.   
   >   
   > What are the plate dimensions?   
   >   
   > And there would be no reason not to use the CCD with the array of   
   > little flashlamps.   
   >   
   > The pulse power will also vary over time, probably from pulse to pulse,   
   > and this would need compensation as well.   
   >   
   >   
   >>> I also recall that Edgerton solved a related problem (pulse time   
   >>> jitter) by keeping a trickle current through the flashtube.  I don't   
   >>> recall how he kept this from causing premature flashes; I'll have to   
   >>> look in his book (Flash, Strobe).   
   >>   
   >> That's called a "simmer circuit".  It reduces time jitter, since there   
   >> are always ions around, so you don't have to wait for the first   
   >> avalanche event to start the arc.  Simmering really helps lamp life at   
   >> low energy, apparently by reducing the amount of cathode material that   
   >> gets sputtered at each arc initiation.   
   >   
   > That's it.  But the word does not appear in the book index, so it'l   
   > take some searching.  Use of tiny currents and UV is mentioned on pages   
   > 14-15, but there was a full circuit somewhere.   
   >   
   >   
   >>> Radioactive materials on one electrode, or photoemission due to a UV   
   >>> tickle light have also been used.  Perhaps a UV LED?   
   >>>   
   >>> Arc welders use RF to strike and stabilize the arc.   
   >>   
   >> Arc lamps often use big magnetic fields for stabilization.  But the   
   >> problem is arc motion generically--a normal arc lamp has a cathode hot   
   >> spot that moves around, sometimes as fast as 50 m/s, and a flashtube's   
      
   [continued in next message]   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)