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: Wed, 21 May 2025 13:24:19 +0100
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On 20/05/2025 18:43, Don Y 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.
> 
> This would, instead, suggest a slower rotation so the prism feeds
> the detector a single wavelength for a longer (continuous) period.

I really wouldn't consider anything with moving parts. You can get 
reasonable grade replica grating for low resolution spectroscopy from 
the likes of Edmund scientific (intended for school labs).

https://www.edmundoptics.co.uk/c/gratings/621/#27766=27766_s%3AClear%2BPolyester%2BFilm

Or if you aren't too fussy about quality the photo filters sold to put 
rainbow stars on disco lights or on eBay. Astronomy magazines often have 
adverts for slightly better than average gratings for eyepieces.
> 
> 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.
> 
> [This is the approach I have historically taken with data acquisition
> as it lets me trade response time for resolution, dynamically]
> 
>> resolution can be as coarse or as fine as you like and algorithms can
>> work out the visual perception of line spectra (if that is what you
>> need).
>>
>> 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"?

UK alliterative saying.
I guess it doesn't translate into USAian too well.
> 
>> steady reading (longer integration period for lower 'noise') being the
>> only real differences.

Figure out how much resolution you need before starting out.

A shovelware DVD at glancing incidence can resolve the absorption lines 
in the suns spectrum if you do it just right. You look down onto the 
disk with the sun at a very shallow angle to the surface. Don't look at 
the reflection of the sun - only at the very dispersed spectrum.

The spectrum obtained with this simple kit is impressively high 
resolution. It will also have various funny organic dye lines in with a 
modern writeable one.

You really want aluminised media for this trick.


-- 
Martin Brown