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Units of PAR


  1. [Fwd: PAR units]
    by eworobe/cc.UManitoba.CA (Wed, 7 Oct 1998)
  2. PAR conversions
    by Petemohan/ (Sun, 18 Oct 1998)
  3. Is this enough light?
    by "Michael Dubinovsky" <mikluha/> (Tue, 1 Feb 2000)
  4. (No Title)
    by ()
  5. Lumens per liter
    by Wright Huntley <huntley1/> (Thu, 07 Dec 2000)

[Fwd: PAR units]

by eworobe/cc.UManitoba.CA
Date: Wed, 7 Oct 1998
To: Steve Pushak <teban/>

1 mole = 6.023 x 10^23. This number is called Avogadro's number... 
probably after the guy who thought this would be a good number to use 
for talking about atomic particles. This is like saying 1 dozen = 12.

Light of course, in some mystical way, is both a particle and a wave... 
kind of like the Holy Trinity in Christianity. Anyways, being a particle, 
you can measure the amount of light in terms of the number of photons 
that are present.

Thats how PAR is measured. The units used to be called 'einsteins' but 
then somebody decided it was more appropriate to use the standard term, 
which is 'moles'. They are one and the same thing.

The interesting thing, of course, is that light of different wavelengths 
has different amounts of energy per photon. Even more interesting is the 
fact that the difference in energy makes NO difference to photosynthetic 
plants... Ps is a stoichiometric relationship... 1 photon yields 1 excited
electron... yields 'x' number of atp atoms (I forget exactly how many).


PAR conversions

by Petemohan/
Date: Sun, 18 Oct 1998

	Steve Pushak recently asked me if I would take some measurements of metal
halide lamps with my PAR and foot-candle meters.  Suspecting this had already
been done by someone in the coral hobby, I contacted Dana Riddle who shares my
interest in the subject and who has written much about lighting and presented
papers at the Western Marine Conference and MACNA.  Dana has also been bitten
by the plant bug, and expects to have an article on lighting for planted
aquaria published in one of the major magazines in the coming year.
	Dana asks only that when this information is reproduced or forwarded that his
business, Riddle Aquatic Laboratories, be credited as the source, and that
their email address ( be listed.
	Some additional data of my own (conversions for a number of fluorescent
lamps) will appear in an upcoming Aquatic Gardener.  Together these two sets
of data should cover most of the lamps in use for planted aquaria.
	Here is Dana's data:  Multiply Lux readings by the following conversion
factors to obtain PAR values.
> Sunlight - 0.02000
> Metal Halide Lamps
> 	AB 150w 6,800 K- 0.02000
> 	Coralife 175w 10,000K - 0.02128
> 	Coralife 175w 20,000K - 0.02128
> 	Coralife 250w 10,000K - 0.01887
> 	Coralife 400w 10,000K - 0.02041
> 	Hamilton 175w "True 10K" - 0.01852
> 	Iwasaki 400w "Daylight" - 0.01754 (using CWA ballast)
> 	Osram 150w 5,600K PowerStar - 0.01818
> 	Radium 400w "Blue" - 0.02083
> Fluorescent Lamps
> 	Hamilton Compact Fluorescents (4x55w, 2 Daylight / 2 Actinic Combo) -
> 		0.02000
> 	Sylvania PowerCompacts (4x96w, 2 daylight/ 2 actinic combination) - 0.01852
> 	URI VHO Fluorescent Lamps (4x110w, 2 daylight / 2 actinic combination) -
> 		0.02083

	Many thanks to Dana!
	Pete Mohan

Is this enough light?

by "Michael Dubinovsky" <mikluha/>
Date: Tue, 1 Feb 2000

- ------------------------------
> Date: Tue, 1 Feb 2000 09:54:05 -0500 (est)
> From: "Roger S. Miller" <>
> 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 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

(No Title)


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,'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

Lumens per liter

by Wright Huntley <huntley1/>
Date: Thu, 07 Dec 2000

George Booth wrote:

> Gosh, once you have a lux meter, why go back to goofy lumens? 

Well, both are *equally* goofy for measuring plant growth activity, so what
difference does it really make? They are both just based on the CIE's
"Standard Observer" which is the *human eye*, and not on a photosynthesizing

Actually, when looking at lamps (not at finished tanks), lumens is quite a
good guide for how much they have emphasized the (somewhat less useful)
green and neglected the really active red or even blue ends of the spectrum.
I tend to go for the *lowest* lumens/per/Watt, all other things being equal.
That gives the most photosynthetically-active spectrum *if* the phosphors
are otherwise optimized and long-lived (which leaves out 80% of all the
higher-priced lamps at the LFS, BTW).

Then I look for the highest CRI I can make match with that rough choice, for
I do like to look at tanks in something approaching natural light. [My
brain, and most folk's, will do the rest, for we can accurately see colors
in surprisingly different illumination.]

> That's sort
> of like figuring out how many gallons of gas you put in your fuel tank by
> dividing miles traveled by what your car thought it did in miles per
> gallon. Why not just read the gas pump meter? Lux is what you really want -
> the actual intensity as produced by the lumens coming form the bulb and
> bouncing off the reflector.  Hey you can even tell how bright it is at the
> bottom after all the shading and light piping takes place. Cool!

Not worth a damn, IMHO, unless you have megabucks to try them *all* in a
live situation. In the real world of engineering design, we can't always
measure the final result until after it's built. Engineering economy
dictates that we avoid using the wrong components, right off the bat, if we
have any clue as to how to do better. No lamp manufacturer dares to
speculate on lux in all applications. They do know lumens, tho. They even
publish them!

I still prefer W/G as a rough guide, but really good reflectors coming on
line (e.g., AH Supply's and 3M's Silverlux silver) can give those rules of
thumb a 50-60% boost, easily.

For final measurements and to compare results, as George suggests, I plan to
use a filter over a standard cell (as found in a Lux meter) that is designed
to allow photography in a fluorescent-lighted room with daylight film. That
pinkish/purple filter will give comparison numbers a bit more meaningful for
plant growth, I think. 

For that, lux sucks, as does lumens. PAR is actually only somewhat better.
My filter will basically measure relative PAR with a lux meter, even with
different lamp spectra, by de-emphasizing the green part, which most plants
reflect away, anyway.


- -- 

           Wright Huntley, Fremont CA, USA, 510 612-1467 
     "Government, in its very essence, is opposed to all increase in
knowledge.  Its tendency is always towards  permanence and against
change...[T]he progress of humanity, far from being the result of
government, has been made entirely without its aid and in the face if its
constant and bitter opposition."
  -- H.L. Mencken

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