Re: Instrument Illuminants
Re: Instrument Illuminants
- Subject: Re: Instrument Illuminants
- From: Graeme Gill <email@hidden>
- Date: Tue, 06 Jan 2009 23:07:01 +1100
Mike Eddington wrote:
Question on instrumentation illuminants: Do typical spectrophotometers
(i1 for example) typically assume the illuminants spectral output is
illuminant A ( and correlate that to D50) or is the correction to D50
tuned to the illuminants custom curve (either as a family of devices or
each specific device having its own custom correction). Also wondering
Typically none of the above. Calibration means that few assumptions
need be made about the instrument illuminant, apart from it being of
sufficient intensity across the desired wavelengths. The calibration
tablet is of a known reflectivity, so a calibration curve is created
such that the instrument measures the target reflectivity for the tablet.
In this manner the instrument illuminant is "divided out" of the
system. The reflectivity is then used to compute the XYZ under
D50.
if the LED source used in newer spectrophotometers (Isis, Munki) are
close to illuminant A or has it's own spectral output that is then
correlated to D50 (I'm guessing the latter)?
LED is simply another illuminant, and typical white LEDs have even
less resemblance to D50 than A.
Since there is no true D50
illuminant in a spectrophotometer, would anyone have an opinion as to
whether LED technology could in the future be used to more closely
emulate D50 both in the visible and UV portions of the spectrum?
White LEDs are typically worse in regard to UV, since they rely on
a blue LED + phosphor, and given the blue is relatively narrow,
there is little output at shorter wavelengths (ie. they are
by default "UV Cut").
Of course it's possible someone could make a sophisticated instrument
with many LEDs covering the whole spectrum, and then be able to select
an illuminant that roughly matches a particular target.
Other more exotic approaches are in development (ie. "Quantum dots"),
but it seems unlikely they will be used in cheap instruments any time soon.
Graeme Gill.
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