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From: liz@poppyrecords.invalid.invalid (Liz Tuddenham)
Newsgroups: sci.electronics.design
Subject: Re: "Colorimeter"
Date: Wed, 21 May 2025 12:05:28 +0100
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Don Y <blockedofcourse@foo.invalid> wrote:

> On 5/18/2025 2:15 PM, Liz Tuddenham wrote:
> > Don Y <blockedofcourse@foo.invalid> wrote:
> > 
> >> On 5/17/2025 2:03 PM, Liz Tuddenham wrote:
> >>> Don Y <blockedofcourse@foo.invalid> wrote:
> >>>
> >>>
> >>>> How can I determine the spectrum of incident light on a sensor,
> >>>> in general?  Then, how many corners can I cut to sacrifice resolution
> >>>> and accuracy?
> >>>
> >>> Spinning or oscillating prism?
> >>
> >> That might be better than a varied filter.  But, probably require finer
> >> control (or sensing) of its current orientation.
> > 
> > If it is spinning steadily, all you need is a synchronising pulse at
> > some point once per revolution and a wide spectrum photocell with an
> > optical slit and a lens.  Software can work out the wavelength from the
> > rotational speed and the known characteristics of the prism.  The
> 
> Of course.  But, if spinning faster than your integration interval,
> I suspect any jitter in your angular resolution might be difficult
> to factor out of the mix.

Mount the prism on a a flywheel and spin it rapidly.  The only jitter
might come from errors in the timing pulse (or knackered bearings!).

One way of obtaining a jitter-free timing pulse would be to reflect a
known pattern of light off the faces of the prism into the photocell;
use the software to recognise it and make corrections for any long-term
speed drift.


> This would, instead, suggest a slower rotation so the prism feeds
> the detector a single wavelength for a longer (continuous) period.
> 
> That means the time to get a sampling of the spectrum is multiplied
> by the integration interval.  If, instead, you could get "quick peeks"
> at each wavelength "quickly", and the more precise integration "later",
> you have more data to work with, sooner.

If it spins faster you can simply integrate multiple 'passes' for as
long as you want until the noise is negligible.  The frequency response
of the photocell and head amplifier is likely to be far wider than any
mechanical system needs, so the physical narrowness of the slit and the
distance from the prism will set the resolution limit.  .A narrow and
distant slit will give higher resolution at the expense of a worse S/N
ratio, which can be overcome with a longer integration time.  

> 
> [This is the approach I have historically taken with data acquisition
> as it lets me trade response time for resolution, dynamically]

Yes, it has many advantages.

[...] 
> > The same hardware could be used for an expensive high-resolution device
> > or a cheap and cheerful version - the software and the time to reach a
> 
> "cheerful"?

"Cheap and cheerful" is a slang [UK English] expression meaning  a quick
rough estimate or goods that aren't intended for serious long-term use.


-- 
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk