Yup - it has lots of energy well up into the blue end of the spectrum,
which pushes the average colour temperature up.
(the colour temperature being just the temperature you would need to
heat a perfect black body radiator to so as to get a similar light
> From the spectrum of G, how could you work out the colour temp?
There is a thing called Wien's "displacement" law (an approximation of
what was later refined by Planck) that relates the colour temp to a peak
wavelength. It's given by :
lambda = b / T
where lambda is the wavelength, and b is a constant ~ 2.9 x -10^6 m.K,
and T is the colour temp in Kelvins.
So a T of 6500K gives you a peak wavelength of 2,900,000 / 6500 = 446 nm
It basically shows how the whole spectra shifts in peak energy toward
the blue at ever higher black body temperatures.
(it fits in with our normal expectation of how things heat first to a
dull read, then orange, then white, right through to the blue of a plasma)
There is a nice graph on the page her:
> And if you take a high CRI LED lamp, say 98, the spectrum also looks
> similar to G, but the colour temp is given as 3000K. How does that work?
Usually there will be less short wavelength content, see the A filament
spectra. However you can also "fake it" to an extent by boosting the red
/ yellow content of light that does also contain more blue - as is often
the case with "while" LEDs that use a blue LED light source to excite a
> If you have a dimmable lamp, does the colour temp vary with brightness?
With a tungsten lamp, yes - very much so. With a normal dimmable LED,
then no - it stays the same at all brightnesses and looks a bit odd
because of it (our expectation is that higher colour temperatures are
usually experienced at much higher intensities - so daylight looks
natural when very bright, but starts to feel "cold" when experienced at
lower intensities (hence why the light looks "colder" on an overcast
day, or why a single LED daylight bulb can look very cold or blue (but
lots of them together seems quite "sunny")).
Some posher dimmable LEDs have more than one chip - one having a very
yellow filter, and they blend the output of both in varying amounts
depending on brightness. These do dim in a way similar to an
incandescent lamp, with lower levels appearing more "warm" in tone (i.e.
lower colour temperatures)
Also keep in mind that the term colour temperature works in the opposite
direction to the terms we normally use to classify light qualities. i.e.
we say "warm white" meaning a *lower* colour temp, and might describe a
very high colour temperature lamp as being very "cold"!