Re: CCFT versus LED [was: Spectros better than colorimeters for monitor profiling?]
Re: CCFT versus LED [was: Spectros better than colorimeters for monitor profiling?]
- Subject: Re: CCFT versus LED [was: Spectros better than colorimeters for monitor profiling?]
- From: Robin Myers <email@hidden>
- Date: Sat, 25 Oct 2008 17:34:43 -0700
For a variety of reasons LEDs (not LED lasers) do not have really
narrow spectral emissions. Their peaks are generally from 20 to 50 nm
wide, sometimes wider. One measure of how wide the peak is the Full
Width Half Max (FWHM) value which is taken as the distance between the
point on the left side of the peak at half the peak's maximum and the
corresponding point on the right side of the peak. The FWHM is often
reported in the LED manufacturer's specification sheets.
A spectrometer such as the i1 Pro, which measures the spectrum in
approximately 3.5 nm bands will get at least 5 or more samples in a 20
nm peak. It combines these 3.5 nm measurements into 10 nm values
reported to the user. Since Nyquist showed that a signal must be
sampled at twice the frequency of the signal one is looking for, even
the 10 nm reporting interval of the i1 Pro is adequate for LED
measurement (even more so when considering the 3.5 nm measurements
physically measured by the instrument's sensor).
Perhaps the literature nomenclature is partly at fault here for the
confusion. Most of the LED literature describes LED emissions as
"narrowband" which can lead one to think of almost monochromatic
emission. However, the "narrowband" is a relative term, usually used
when comparing LED output to incandescent sources which are often
described as "broadband".
By these definitions a white LED is an example of a narrowband light
source being converted into a broadband source. White LEDs are usually
a blue or violet narrowband LED coated with one or more phosphors. The
most common white LEDs use a yellow phosphor which emits light in the
green and red portion of the spectrum. Along with unconverted blue
light the result is a very broadband spectral emission, usually almost
across the entire visible spectrum.
One advantage of the diffraction grating spectrometer (e.g. i1 Pro) is
that it can measure the backlights and filters of monitors as
technology changes. Filters for colorimeters must be matched to
specific technologies, thus rendering a given colorimeter filter
design less accurate as the technology changes. The spectrometer is
therefore more versatile, removing the difficulty and expense of
changing colorimeters for every monitor technology.
One disadvantage of the diffraction grating spectrometer is that it
can be sensitive to polarization angle. This affects LCD monitor
measurements since LCDs produce images using polarized light.
Depending on the spectrometer design, rotating the instrument in the
plane of the LCD monitor's surface will produce slightly different
results for the same point on the surface.
Robin Myers
On Oct 25, 2008, at 15:59 , Marco Ugolini wrote:
In a message dated 10/25/08 2:53 PM, edmund ronald wrote:
ROTFL :)
LEDs are quantum devices. The simples ones just emit one single
spectral ray. That's why we even have LED lasers.
It's harder to be spikier than that.
Some LEDs have a fluorescent shell encapsulating the actual diode, I
think this can turn them into white light LEDs, but the light here is
re-emitted.
Edmund,
If the following graph accurately reflects the typical emissions of
phosphor-based white LEDs, their spectral output is nowhere as spiky
as that
of CCFTs:
<http://en.wikipedia.org/wiki/Image:White_LED.png>
More on the subject (with emphasis on the ending statement in the
quote):
<http://en.wikipedia.org/wiki/LED>
Light quality: Most white LEDs have spectra that differ
significantly from a
black body radiator like the sun or an incandescent light. The
spike at 460 nm
and dip at 500 nm can cause the color of objects to be perceived
differently
under LED illumination than sunlight or incandescent sources, due to
metamerism, red surfaces being rendered particularly badly by typical
phosphor based LEDs white LEDs. However, the color rendering
properties of
common fluorescent lamps are often inferior to what is now
available in
state-of-art white LEDs.
A quick review of spectral graphs shown at the following URL makes
the point
quite well about the superior smoothness of LED light when compared
to CCFTs
(yes, there is some spikiness in LEDs, but *comparatively* they seem
to
represent a marked step towards improved smoothness):
<http://ledmuseum.home.att.net/spectra7.htm>
Also review the fluorescent spectra on this page (scroll halfway
down):
<http://en.wikipedia.org/wiki/Fluorescent_lamp>
Marco Ugolini
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