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Re: Lights



> Date: Sat, 22 Dec 2001 13:24:08 -0800 (PST)
> From: "S. Hieber" <shieber at yahoo_com>
> Subject: Re: Lights
> 
> Gary (MStark7789 at aol_com) wrote:
>> snipped...

> Maybe I misunderstood your question.  CRI (color Rendition Index) is
> measure of how well colors, when viewed by humans, under a given lamp
> will seem normal -- I think "normal" is standardized on tungsten bulbs.

It is how close the lamp's light is to rendering a "perfect" (100) white
illumination, without any color tinting to the standard human eye. Tungsten
lamps are close, because they are broadband (quasi-blackbody) sources but
usually not hot enough for really good white. Halogens are pretty close, I
think.

>  Plants don't prefer the same colors as humans anyway.  So a higher CRI
> doesn't mean the bulb will yield more light that your plants like.

CRI tells you only how it will *look* and has nothing to do with what the
plants need, true. Some ugly purple Plant and Aquarium lamps have lousy CRI,
look really nasty, but have pretty good ability to generate photosynthesis. We
usually want a compromise that both looks nice and grows well, too.

> 
> Lux is a measure of light output.  

Not at all. Watts is a measure of light output. Lux and lumens are only how
bright it looks to the so-called "standard observer," that is, its luminosity.
They are defined in terms of a standard spectrum that is rich in green and
very weak in red and blue. That's how humans *perceive* brightness. Your eye
is essentially ten times as sensitive to green light as to red or blue, so
lumens are weighed very heavily in favor of green. That happens to be right in
the middle of the big dip in most plants' action spectrum, so cutting red and
blue in favor of green (as in making "cool white" tubes to get high
lumens/Watt) is exactly what many plants do not need.

[That fails to explain why my Riccia always seems to like "cool white"
illumination, best. <g>]

> It's the International System unit
> of illumination, equal to one lumen per square meter.   Roughly
> speaking, a lumen is a unit of light of one candela on one square meter
> of surface, the illumination of which is uniform.  A candela is the
> unit of luminous intensity equal to 1/60 of the luminous intensity per
> square centimeter of a blackbody radiating at the temperature of
> solidification of platinum (2,046K).

"Luminous intensity" is just how it looks to a human eye, not how much actual
light is there.

Careful about swinging between physical measures and photometric ones.
 
> 
> More bulbs close together will add more light.  

Restrike is a major cause of light loss, so *thinner* tubes are an improvement
(T12 -> T8), but crowding thick tubes is generally not as worth while. Tubes
should not block most of the reflector, for example. The reflector should make
all sides of the tubes appear visible, looking up from the water. That does
not happen to crowded tubes (or with bad reflector designs).

> More lux will add more
> light but not necessarily in the frequency range that plants like.  

It will add more *appearance* of brightness, but may well have less useful
light for plants. It might even have less total light. Photometric units are
great for (and originally were developed for) designing store window displays.
Use with great caution in other areas, and ignore for most plant purposes.

> A
> good rule-of-thumb is to stick with bulbs designed for plants, like
> triphosphor flourescents or metal halides.

There are many good commercial tubes that are not the quick-death candidates
of some LFS tubes. Most retain 90% of rated output to the point they will no
longer fire. Tri-phosphors can be good (many smaller CFs are tri-phosphor), as
well as "daylight" and "sunshine" (Chroma 50, Schedule 50) tubes. Check the
Sylvania, Phillips and GE web sites for lots more than you ever wanted to
know. :-)

Any decent tube should last about 2 years in 12 hour daily use. Replacing
every 6 months is ludicrous, these days, IMHO. [I understand some folks are
still doing it, but they may be way richer than I am.]

> 
> RE your light meter, it might be sensitive to certain wavelenths, which
> might not be radiated as strongly at the center of your bulbs.  Your
> eyes may be less sensitive to those wavelengths.  For example, the
> meter might be sensitive to infra red or ultra violet, but your eyes,
> like mine do not see.  The light meter's manufacturer (owner's
> manual?)can probably tell you what frequencies it is sensitive too.

Fluorescent phosphors don't often have much spectral shift along the tube's
length. He was probably measuring true relative light output. [Badly-made
aquarium tubes might be an exception, as some have truly amateur phosphor
coatings.]

> 
> If you want a little tiny bit more light, get a more efficient bulb
> that emits more lumens (hopefully in the range plants like).  

Lumens *define* the range being measured, and it is one plants tend to not
need or want. You want more light (not more lumens), and probably over a
broader spectrum. More green will just reduce photosynthesis, all other things
being equal. More lumens/Watt usually means more green and less red and blue.


> To get a
> lot more light:
> 
> 1) add more bulbs in parallel (or otherwise positioned to shine on the
> same area of the tank.)
> 
> or 2) change to a different kind of lamp.  Change from normal
> flourescent to VHO flourescents or to Power Compacts, or change to
> metal halides.

Going to the inherently higher output and increased reflector efficiency of
slimmer tubes often is the best, easy way to improve illumination (T8 or even
T5, i.e., Power Compacts). The object is to get both good photosynthesis,
which requires a broader spectrum than your eye's response, and good-looking
light to your eye. The former calls for lots of wide-spectrum light, which is
not usefully measured by photometry (lumens, color temperature, CRI, etc.).
The latter calls for a high CRI and a color temperature (which is not the same
thing as a black-body temperature) in the 5000-6500K range. I even like it as
low as 3000-4000K as the extra red can make some fish colors (Rasboras and
Cardinals) look better. YMMV, as this is a very subjective thing.

Wright

-- 
Wright Huntley -- 650 856-4245 -- 879 Clara Dr. Palo Alto CA 94303

The San Francisco Bay Area Killifish Assn. (BAKA) has a new web
site at:  http://www.sfbaka.net.        [I'm the proud Web Poppa.]