Why not use all the colours in colorimetry?

Colorimetry (and it's cousin visible-UV spectroscopy) is a form of chemical analysis that works by sending visible light of one wavelength through the sample. It then analyses how much of the light was absorbed in that process so you can determine concentration. 


What I don't understand is why only one wavelength is used. Apparently, even for Infrared Spectroscopy, only one wavelength is analysed at one time yet the whole spectrum is needed. Why not send in white light (with all the wavelengths at once)? The output light could be split through dispersion and every wavelength could be analysed for its absorbance. Perhaps some wavelengths of light aren't worth measuring or too many frequencies at once cause interference? 





> 
> Why not send in white light (with all the wavelengths at once)?
> 
> 
> 


This is certainly and routinely done today. The main thing is how much price are we willing to pay? In a rigorous sense, colorimetry refers to the fact that we use optical filters to isolate wavelengths rather than monochromators. When the latter are used, you call them spectrophotometers. Basically, the question is referring to scanning instruments versus diode array or multiplexing instruments. Both exist today and both have their advantages and disadvantages.


[![http://www.chromatographyonline.com/important-aspects-uv-detection-hplc](https://i.stack.imgur.com/N0iLr.png)](https://i.stack.imgur.com/N0iLr.png)


In scanning spectrophotometers, one wavelength passes through the sample at a time. In diode array instruments, all light passes through the sample at once and it is dispersed later. The dispersed light falls on an array of detectors (thousands of tiny detectors, usually 1024). The detectors do not know what wavelength is falling on them, however, they are calibrated. 


Similarly, if you were working in the 1970s-80s you would use a scanning infrared spectrophotometer (one wavelength at a time). One needed a prism made of salt or a very special diffraction grating to disperse the infrared radiation. This made life very difficult. Imagine working with a salt prism!


In Fourier transform infrared spectroscopy, all wavelengths are passed at once, but there is no array of detectors. Unfortunately, there is no detector which can differentiate wavelengths on the basis of their frequency and separate them. We need to modulate them in such a way that an ordinary detector can "follow" the frequency of wavelengths. Here comes in the Michelson interferometer. The raw output does not look like a spectrum at all, but it has all the information "hidden" there. You need Fourier transform to recover a typical infrared spectrum.


Now one may ask why don't we use FT for UV-Vis? The answer is that the required precision of a moving mirror is so high that it is rather difficult to make one.


[![A typical interferogram](https://i.stack.imgur.com/jgWWT.png)](https://i.stack.imgur.com/jgWWT.png)


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The previous answers by @Buck Thorn and @M. Farooq are very good, but not quite complete. The main advantages of FT UV-Vis spectroscopy would be 1) accuracy of wavelength (or wavenumber) determination and 2) better spectral resolution of bands, relative to conventional UV Vis spectroscopy. But it is very hard to do well, and very expensive.


The disadvantage is that source shot noise, which is white noise, and source 'flicker' noise, which is low frequency noise with power spectral density proportional to $1/f^\alpha$, get modulated by the moving mirror. The modulation frequency for any given wavenumber of light is directly proportional to the light's wavenumber value. But *the source noises also get modulated*.


The absorbing specimen absorbs the modulated light, as per the absorber's absorption bands, resulting in an interferogram like the one shown in M. Farooq's answer. Then the FT does the demodulation, i.e., 'decoding', resulting in the spectrum of the absorbing specimen. But *the modulated source noises also get decoded and cause a reduction in the signal-to-noise ratio (S/N or SNR)*. The noises on the large spectal bands are the problem: the white noise spreads throughout the spectrum, thereby *lowering the SNR and making it harder to resolve the small bands*. The source flicker noise also causes trouble by making 'band shoulders', as it were. Thus, the hoped for advantage of high wavelength accuracy (and improved spectral resolution) gets very significantly compromised by the decreased SNR.


For more details, see E.Voigtman and J.D. Winefordner, “The Multiplex Disadvantage and Excess Low Frequency Noise”, *Applied Spectroscopy*, **41**, 1182-1184 (1987). Also see the comments posted after Buck Thorn's answer.


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An explanation is provided in this abstract ([1](https://www.osapublishing.org/as/abstract.cfm?uri=as-43-2-342)):



> 
> Fourier transform spectrometry in the UV-Vis region (FT/UV-Vis),
>  because it is source shot-noise limited, has a signal-to-noise ratio
>  (S/N) disadvantage in comparison to dispersive spectrometry,
>  especially with dense spectra. At the expense of poorer S/N, FT/UV-Vis
>  can be satisfactory for high-resolution measurements. However,
>  low-resolution spectroscopic studies, such as molecular absorption
>  measurements, are not expected to be performed advantageously by
>  FT/UV-Vis. The broad, dense spectra with high intensity throughout a
>  wide spectral range should show a significant S/N degradation, in
>  comparison to results with dispersive spectrometry.
> 
> 
> 


Further explanation of the subject of noise and why FT does not provide an advantage here can be found in course notes [made available](http://www1.udel.edu/chem/sneal/sln_tchng/CHEM620/CHEM620/Chi_8._DFT_&_Noise.html) by U. Delaware Prof. S. L. Neal: 



> 
> Filters do not always improve the quality of spectral data. 
>  Measurements that are dominated by noise from the source, e.g.,
>  fluorescence, are often distorted by flicker noise. Whereas white
>  noise is frequency independent, flicker noise is larger at low
>  frequencies. In fact, it is sometimes called pink noise (since low
>  frequencies are associated with red rather than blue light). 
>  Filtering techniques are based on the idea that the noise does not, in
>  general, consist of the same frequency components as the signal. In
>  the case of flicker noise, this condition is not met. This is one
>  reason that FT-UV-VIS spectrometers are not widely used. The
>  improvement in signal to noise that comes from measuring the entire
>  spectrum many times using an interferometer during the time a single
>  spectrum can be measured on a dispersive (monochromator based)
>  spectrometer is the square root of the number of times the spectrum
>  was measured for shot noise limited (FT-IR) measurements. In other
>  words, a ten point spectrum measured by the interferometer has a
>  signal to noise ratio that is about three times ($\sqrt{10}$ ) larger than the
>  same spectrum measured using a dispersive device because signal
>  averaging reduces random noise by the square root of the number of
>  acquisitions. (Remember this is called the multiplex or Fellgett
>  advantage.) This improvement is always smaller in the presence of
>  flicker noise, and in very crowded spectra, a multiplex disadvantage
>  in which the signal-to-noise ratio in the data acquired by the
>  interferometer is lower than in the dispersive instrument can be
>  observed.
> 
> 
> 


**Reference**


1. Mark R. Glick, Bradley T. Jones, Ben W. Smith, and James D. Winefordner 
Applied Spectroscopy Vol. 43, Issue 2, pp. 342-344 (1989)
2. V. Saptari, Fourier-Transform Spectroscopy Instrumentation Engineering, SPIE Press, Bellingham, WA (2003).


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