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re: CRI question

>Date: Tue, 09 Dec 1997 09:49:08 -0500
>From: "K. A. Bryant" <smskahj at netacc_net>
>
>In the 1997 Greinger catalogue (no.388) on page 1042 there is some
>information on Fluorescent Lamps.  The heading is GE Lighting, then there
is
>the subheading "Color Rendering Index (CRI)" under which it states:
>
>"The CRI of different sources can be compared only if the sources have the
>same color temperature."
>
>I'm not sure how to interpret "sources."  Does this mean that we can
compare
>the CRI of, for instance, a 6500K flourescent bulb with another 6500K
>flourescent bulb, but can't compare the CRI of, for instance, a 6500K
>flourescent bulb with a 5000K flourescent bulb? Or by "sources" do they
mean
>fluorescent, incandescent, HID, etc., in which case it would just mean
that
>we can't compare the CRI of a fluorescent bulb with the CRI of a HID bulb
>unless, for instance, they are both 5500K bulbs, but can compare the CRI
of,
>for instance, any fluorescent bulb to any other fluorescent bulb? Or are
>there other possible interpretations?

The comparison is between bulbs or tubes of the same color temperature,
regardless of technology.  

For instance, all incandescent lights have a CRI of roughly 100 unless
their output is filtered.  This is because the color rendition of the
incandescent filament heated to produce light in the color temp of say,
3000K is due to the 3000 Kelvin filament temperature.  Since the filament
is essentially a heated black body (the definition of color temperature
compares to the light output of a heated black body), the incandescent
light has a perfect CRI (beware that it also uses the majority of its
energy in heating the filament to the point where it's producing visible
light and is therefore not very efficient in producing visible light
compared with other sources).

Fluorescent and high-intensity discharge lamps produce lower CRI values
because they emit light in spectral spikes instead of an even curve.  A FL
lamp rated at 5500K can be directly compared with a MH lamp rated at 5500K
in terms of CRI.

As has been stated many times on this list, intensity is more important
than color temp or CRI in growing plants.  A high CRI can help, but it may
be better to increase CRI for the purpose of viewing pleasure, rather than
to help your plants grow faster.

--
David W. Webb           Texas Instruments
(972) 575-3443 (voice)  http://www.dallas.net/~dwebb
(214) 581-2380 (pager)  2145812380 at alphapage_airtouch.com