[argyllcms] Re: About Hi-Res spectral mode

On 25 October 2010 23:09, Gerhard Fuernkranz <nospam456@xxxxxx> wrote:

> Am 25.10.2010 22:22, schrieb Graeme Gill:
> > Whether that is of any significance, depends on your context. If it is
> > of no consequence to you, then I guess purchasing an instrument with 1
> > nm resolution is not on your agenda :-) [Note that I currently have no
> > means of verifying whether the above differences represent an
> > improvement in accuracy of not.]
>
> Hi Graeme,
>
> looking at your laser pointer spectral plots it seems that the FWHM of
> the instrument's optical system is still the limiting factor [hard to
> tell exactly from the diagram - but I'd guess about 20nm ? - assuming of
> course that the laser is really monochromatic] The benefit of higher
> sampling resolution is of course limited if the bandwidth cannot be
> reduced as well. Theoretically the higher sampling resolution should
> offer an opportunity do deconvolve to a smaller effective bandwidth, but
> in practice a stronger deconvolution is limited either, by a too bad S/N
> ratio. Certainly, as we see, high-res mode does reduce the effective
> FWHM, but obviously it can't reduce the effective BW by the same factor
> as the sampling resolution is increased. So I'd still expect a
> noticeable improvement from an instrument which reports not just with
> 1nm resolution, but also *with 1nm FWHM*. According to the literature,
> 20nm BW is too much for accurate measurement of narrow-band light
> spectra, the literature rather suggests to use a BW of 5nm or less (and
> a corresponding resolution too, of course). To get a feeling for the
> impact, it's quite easy to simulate how the effective spectrum locus
> moves in chromaticity space, when monochrome spectra are "broadened" to
> a triangle or Gaussian with say 5nm, 10nm, 20nm, etc. FWHM [see
> http://tinyurl.com/ykrms2], and it should be easy as well to compute the
> corresponding colorimetric errors.
>
> Regards,
> Gerhard
>

That's a very interesting diagram you linked to there! I do wonder how much
processing instruments like the i1 do before reporting the spectral data.
Unfortunately I don't have a line source sufficiently close to 520nm to tell
properly how close a result it's giving.

Incidentally, I have re-assessed the LED spectra I measured, and I was
wrong. There is indeed a difference between hi-res and standard mode at
650nm. The difference appears as a shift along the spectral locus towards
green when switched to hi-res mode. I calculate it at about 9dE (1.7 dE94).
Who knows which is the real answer?

The sp files if anyone is interested: (intensity was not controlled between
measurements)
http://www.satsumatree.co.uk/redlaser_s.sp
http://www.satsumatree.co.uk/redlaser_h.sp
http://www.satsumatree.co.uk/greenlaser_s.sp
http://www.satsumatree.co.uk/greenlaser_h.sp

Sam Berry
www.satsumatree.co.uk

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