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Re: Is this enough light?

> Date: Tue, 1 Feb 2000 09:54:05 -0500 (est)
> From: "Roger S. Miller" <rgrmill at rt66_com>
> Subject: Re: Is this enough light?
> On Tue, 1 Feb 2000 Wayne Jones wrote:
> > Did you actually read what I said? I was comparing lighting
> systems and not
> > lamp spectrums. How can there be any any significant variation in PAR
> > whatsoever if the spectrums are the same?
> Wayne,
> Yes, I read what you said, thanks.
> You claimed to be comparing two systems with similar spectra, but you
> actually compared two systems that used two very different tubes. The
> Sylvania 841-series T8, 32 watt tube is an 82% CRI, 4100K triphosphor
> lamp.  Sylvania doesn't seem to publish the spectrum for the F20T12
> coolwhite that you compared it too, but I'm pretty certain that it doesn't
> have the same spectrum as the triphosphor lamp. It wouldn't even have
> similar color temperatures or CRI.  In fact, there's no readily evident
> similarity between those lamps at all.

I put spectral curves of some lamps on my homepage


> Hence our cautions about using lumen ratings.
> In fact, it's been pointed out repeatedly on this list that lighting
> advice shouldn't be based on lumen ratings.  Lumens measure the brightness
> of a light as seen by the human eye, and (as Ivo shows) when comparing
> lamps of equal wattage there is little relation between the lumen rating
> of a lamp and the amount of photosynthetically active light that the lamp
> produces.

Not so long time ago, I put here a table (Gerald Deitzer, University of
Maryland) with Power/PAR and Lux/PAR conversion.

For most lamp to get energy in Wt/m^2 you need to multiply PAR (mkMol of
photons/m^2) by 0.2-0.22

To get Lux reading you need to multiply PAR by:
Sun - 55.2
Incandescent lamp  49.0
Cool white 78.8
Vita-Lite    62.8
Gro-Lux 37.0
Gro-Lux Wide Spectrum  55.1
HPS  83.3
MH Lamp 74.5

It's easy to understand, why same power lamps produce different number of
lumen and same number of PAR. To get lumen reading one needs to integrate
spectral power curve weighted with human eye responce curve, which is
relatively narrow. Thus, any spike in lamp spctral in green-yellow region
gives large variation in lumen reading.
From other side, to get PAR one needs to integrate energy wavelength
(=number of photons) weighted. This is relatively slow changing function.
Lamps has more or less similar spectrum (there is a chart on my homepage
shows lamps with different CCT normalazed to same value. They look similar)
and PAR value are almost the same.

> This argument is all somewhat pointless, as Erik Olson has already shown
> that the lighting requirements of planted tanks are easily described in
> system before we answer their question.  It isn't that complicated.
> Roger Miller

I put basic light calculation.at my homepage.

According Buger law, light intensity inside water equals to:

k-coefficinet of absobrtion and scattering.
k=1.5-2.5 for freshwater and k=0.5-1.5 for marine fishtank (much cleaner
I measured my water adn also took data from different lakes/rivers/ocean

From book (Barry James, Aquarium Plants)

500 Lx - low light (crypts, java fern)
1000 Lx - moderate light (Anubias, Echinodorus sp.)
1500 Lx - bright (Aponogeton sp., Ludwigia sp.
2500 Lx - very bright (Riccia fluitans, Limnophilia aquatica)

yeah, yeah....it's not ok to use lux, but it's easier

Baensch in his Aquarium atlas gives similar values.

Fishtank with water works similar to lightguid preventing light to escape
from aquarium due to total internal refraction. Light we see outside of
aquarium is scattered light.
Therefore we can find illumination at water surface:

Sure, light isn't parralel and so on, but we don't know k-value exactly and
this kills most of second-order effects (I've mader some simulation on

Light losses due to reflection and scattering from water surface - around
All calculation must be done in metric system. If you like footcandles feel
free to use them, but you need to change k-value.
For example, 50 cm (20", 35% transmission) deep tank and 1000Lx at the
bottom. At water surface:
E0=1000/(0.35*0.8)=3600 Lx

Aquarium length = 90 cm (36"), width = 30 cm (12"). Above gives lumen flux:

F=3600*0.9*0.3=970 Lm

Efficiency of two lamp with reflector (from computer simulation. it's also
at my homepage) is about 50%. Plus, lumen maintenance coefficient (1.2).

F=2400 Lm.

Two lamps, 20W each, give enough light. And this gives 0.3W/l (1.2 W/gal)

For deeper 60 cm fishtank fumen flux equals to 3300Lm (2x30W lamps or 3x20W
lamps, because 3 lamp reflector system efficiency is 40%)

SImilar result can be obtained using standard cavity method of room
illumination calculation (can be found in many lighting handbooks).
You can use PAR if you want. Results are very similar.
Baensch in his Marine Atlas uses "room cavity" method to calculated number
of lamps. He uses energy units and "grow factor" for different lamps
(similar to energy->PAR conversion)

Basically, all methods give 0.2-0.3 W/l  (0.8-1.2 W/gal) in the case of
low-moderate light level and 0.5-0.8W/l (2-4W/gal) in the case of deep tanks
and bright light.

oppps, I didn't mention that my site is in Russian :)  (hopefully, I can
find time to translate it to English in future) But charts have titles in

If you have any questions feel free to e-mail me.

Mike, who design crazy optics around crazy lamps