Path: news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail
From: Martin Brown <'''newspam'''@nonad.co.uk>
Newsgroups: sci.electronics.design
Subject: Re: "Colorimeter"
Date: Fri, 23 May 2025 13:10:41 +0100
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On 23/05/2025 10:04, Liz Tuddenham wrote:
> Don Y <blockedofcourse@foo.invalid> wrote:
> 
>> On 5/21/2025 6:19 AM, Liz Tuddenham wrote:
>>> Don Y <blockedofcourse@foo.invalid> wrote:
>>>
>>>> Sampling jitter within a window corresponds to spectral resolution;
>>>> the more jitter, the wider the range of wavelengths potentially
>>>> involved in the sample (over time).  As sampling the detector
>>>> is a discrete time event (the interval between samples being the
>>>> width of the window), how frequently you do this further defines
>>>> the spectral resolution.
>>>
>>> I was assuming very fast sampling so that the presentation of each line
>>> was captured by many samples, that way the software could sort it out
>>> over a large number of repeated passes.  Keep the hardware simple and
>>> let the software deal with the errors if it can be given enough data to
>>> start with.
>>
>> Are you expecting to frequently sample the entire spectrum in each
>> "pass" ("revolution")?  Or, walk the sampling window up/down the spectrum
>> in stages?
> 
> I was expecting to sweep the whole spectrum at high speed many times,
> then analyse the captured data.  Television-type technology could easily
> cope with that data rate from a single photocell.

That was how the early Hardy spectrophotometers did it back in the day 
when photomultiplier tubes were large rare expensive beasts surrounded 
by insanely high voltages and lots of precision megohm resistors.

I still have a few small mirror bits from taking one apart long ago.

Today with ultra cheap LCDs and some with piezo shift facility the trend 
is towards making a 2D spectrum on a standard rectangular CCD sensor 
with high dispersion on one axis from a grating and low dispersion in 
the other from a prism. Mapping the entire spectrum into a 2D pattern.

Echelle spectrograph is the keyword you want.  Oxford instruments make 
rather high end ones but you don't need anything so fancy for this.

https://andor.oxinst.com/learning/view/article/echelle-spectrographs-a-flexible-tool-for-spectroscopy

Solar spectrum measured conventionaly but displayed in that style 
because it allows it to fit more easily on a page here:

https://noirlab.edu/public/images/noao-sun/



> A small gas-filled discharge tube, pulsed by an ignition transformer,
> would suffice for non-critical calibration.

A neon lamp or a mercury vapour one will do for a wavelength reference.
It's hard to get a sodium lamp small enough and at a good price.

-- 
Martin Brown