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From: Don Y <blockedofcourse@foo.invalid>
Newsgroups: sci.electronics.design
Subject: Re: "Colorimeter"
Date: Sat, 17 May 2025 19:29:54 -0700
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On 5/17/2025 2:03 PM, Martin Brown wrote:
> On 17/05/2025 20:30, Don Y wrote:
>> Not quite, but, close enough...
>>
>> 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?
> 
> Short answer is you can't - at least without making some *very* questionable 
> assumptions. It is even worse now with narrowband LEDs.
> 
> If you are allowed to make the assumption of a radiant perfect black body 
> (something that doesn't exist) then it is much easier.

I'm not looking for a laboratory grade instrument.  (hence the
"corner cutting" caveat).

Rather, "how does the light falling on THIS body compare to the
light on this OTHER body" (using the same measuring instrument)

>> I've worked with true colorimeters (dual wavelength) in the past.
>> But, they were optimized to look for specific wavelengths.
> 
> True colorimeters were designed to match visible colours pretty much exactly 
> under *any* lighting conditions (extremely tough problem). The first that 
> actually worked well enough was the Imperial Match Predictor which ISTR was an 
> analogue computer made in the UK by ICI strictly for internal use only. I don't 
> think any documentation survives.

Ours controlled the color temperature of an incandescent lamp
"seen" through a pair of filters.  Then, compared the detected
signal from the sample under test (inserted between the emitter
and detector) in the same short time interval, looking for a
particular color shift (analyzing blood assays)

Again, you don't care WHAT "color" it is, just how the chemistry
altered the color within a band of expected results.

But, that system KNEW what to expect (expectations were dependent
on the actual assay being run)

> There was a US made spectrometer which formed a part of it whose manufacturers 
> name escapes me for the moment. Got it Hardy Spectrophotometer:
> 
> https://collection.sciencemuseumgroup.org.uk/objects/co11842/ge-hardy-spectrophotometer-c-1940
> 
> That model isn't quite the right one but it is close.
> 
> Now any suitable paint test chart and a mobile phone will do the job.

How durable are the CCDs used in phones?  Especially to high intensity light
sources?

>> I calibrate the light emitted by my monitors with a device,
>> but it controls the light source to do so.
> 
> If you are serious about doing this right then a 2D CCD sensor and a prism 
> hires grating combo at right angles will allow you to quantify the entire 
> visible spectrum at ultra high resolution. Be careful though Perkin-Elmer (and 
> others) have some very good lock out patents on this trick (may be about to 
> expire).

Again, not looking to make an "instrument".  The phone idea may work
if the CCDs don't freak out with high intensity sources.

> A few people can see longer wavelengths than most with an extra type of cone 
> cell. They were sought after in WWII (pre thermal IR band imaging) because they 
> could see the difference between live foliage still growing and cut down dying 
> foliage used as gun emplacement camouflage.

Also folks who are truly colorblind.  Camouflage looks different than
natural foliage when you are just looking at the values without the
hues to distract.

> Denatured chlorophyll looks much darker to them.
> 
>> With no knowledge of the actual (visible) spectrum impinging on
>> a sensor (and a bit of time to integrate results), how can I
>> do this short of swapping individual filters in front of the
>> sensor(s)?
> 
> Measure the intensity at all wavelengths in a single shot.

Or, leverage the fact that the spectrum won't be changing in
the short term (for some value of "short") and cycle a set
of filters (rotating disc?) between the detector and source.

Again, if you aren't looking for repeatability instrument to
instrument, this may be good enough to answer the question above.

> PE OES instrument in the early 1990's was the first with this.
> (I forget the model number) I was seriously impressed with it.
>