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Exploring a cheapo diy spectrometry possibility.
The standard spectro meter uses eg. reflected light
bounced off a diff. grating. One modern twist is to put
this light onto a ccd, then diferent parts of ccd get
different colors, and the resultant intensity vs. color is
However another poss. I am trying to grok is doing this
without a grating, instead using eg rgb leds. It would
require de convoluting a signal that is due to known
of led spectra * unknown reflection spectrum. I cant
if the information is 'inthere' or hopelessly smeared out.
Take for example 2 leds and vary their percent
contribution to illumination, call this percent p, freq. is
w, and the individual led spectra are led1(w) and led2(w)
The combined illumination is then
L(w,p)= p led1(w) + (1-p) led2(w)
Assuming the sample is 'nice' and doesnt play any
frequency shifting tricks (i believe this is quite rare and
limited eg to some crystals that can do frequency
anyway it seems this must be an assumption of optical
spectroscopy ) then it has an unknown reflection
R(w) to be found.
Finally assume that the detector has flat spectrum - this
can be done by normalizing the actual response, i dont
think it helps anything to include the detector sensitivity
curve in this analysis. Use two or more detectors e.g.
values from a ccd if that helps matters, I dont think it
So the problem is to use the detected signal D(p) from
light detector/ccd, which is a weighted average of the
intensities at each freq., to find R.
======== ============= ========== ==============
D(p)= integ over w of (L(w,p)*R(w))
= integ over w of ( p led1(w) R(w) + (1-p) led2(w)
Knowing D(p) (and led1, led2) can R be found?
======== ============= =========== =============
Alternatively / maybe equivalently, is there only one
solution for R ( in which case numeric solution is possible
if nothing else). Use 3 leds if necessary, etc.
Differentiating wrt p does not seem to help matters. Any
maths whizzes able to shed some spectrum on this?
Take a gander at [see link] which does something close,
am not convinced he extracts a spectrum however.
led 'spectrometer' [spalpeen, Dec 23 2011]
Full spectrum tunable LED
Build it with one of these. [Wrongfellow, Sep 19 2012]
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||The linked article seems to use 5 different LED wavelengths to get a _very_ crude spectrograph - one with 5 frequency values.
||If you're suggesting that by illuminating a sample with mixtures of light from different LEDs you can get more information than by illuminating it with each LED in turn, then I fear you are mistaken.
||>>If you're suggesting that by illuminating a sample
with mixtures of light from different LEDs you can
get more information than by illuminating it with
each LED in turn, then I fear you are mistaken.
||Thats my suspicion as well - that the varying linear
comb. of two spectra is not 'variable enough'. But I
do not directly see it from the math.
If a single led with variable center frequency were
available then it almost surely could be use for this
purpose. If the center freq. can be varied slightly
with power/temp, then a spanning set of LEDs
||AFAIK spectral transmittance is strictly linear, except in extreme cases such as heating of the sample by the light such that its optical properties change. If so, your idea is mathematically useless.
||Your best bet might be to get a large number of LEDs of different frequencies. Even LEDs of a single type from a single manufacture have slightly varying frequencies, and are sometimes divided into different frequency bins for sale. You then have to calibrate for each LED, but that might end up being an almost trivial software problem.
||Altering the LED's frequency function using temperature or power can certainly be done, but it might not be very repeatable (read - reliable) if it tends to damage the LED, which I suspect it might.
||I was thinking about a prism. But you wouldn't need
to spread the frequencies across the CCD, just
physically move the CCD through the nicely expanded
spectrum in a sweep. Then you can simply integrate
the signal from the entire CCD to gain sensitivity.
However if you're cooling the CCD with single or
series Peltier units, it will be bulky. It might be
easier to sweep the light path across the CCD by
pivoting the prism. Still trivial though.
||I assumed sensitivity and noise were always
||[+] for a prism. KISS (keep It Simple S*****)
||//Keep It Simple Sirrah// Yebbut, then it becomes a Perfectly Ordinary Spectrophotometer. And you're just replacing one kind of complexity with another - you'll need a stable smooth- and wide-spectrum light source, a slit, a prism or grating, and a CCD array or physical scanning mechanism, all precisely aligned.