From: pcdhSpamMeSenseless@electrooptical.net
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.
>
> 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.
>
> 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
cathode can initiate the arc in different places.
>
> At the low power levels required, one can control the flash current
> with a MOSFET bypassing the trickle current resistor, only triggering
> the flashtube at the beginning of a run. In other words, the flashtube
> no longer does the switching.
If I do have to use a flashtube, it'll certainly be actively switched.
Getting fast pulses at low energy often requires using a small cap
charged to a voltage significantly above the flashover voltage of the tube.
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net
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