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: Marco Ugolini <email@hidden>
- Date: Sat, 25 Oct 2008 18:02:30 -0700
- Thread-topic: CCFT versus LED [was: Spectros better than colorimeters for monitor profiling?]
In a message dated 10/25/08 5:34 PM, Robin Myers wrote:
> 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.
Wonderful, Robin.
Thank you very much for your very clear and detailed answer.
All the best.
Marco Ugolini
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