What is the difference between PAR and PPFD?

Z

Zoos

New member
Just like the title says, what is the real difference between PAR and PPFD, from what I've heard they are both basically the same, but I would like to know if t hey is a formula to converting one into another. I am no lighting expert, so this may seem like a silly question to some of you.

Example

When I look at the XM 250w 10k bulb ran on an Icecap ballast at cnidarian reef (JBNY) the PAR is 530
http://www.cnidarianreef.com/lamps.cfm


When I look at the same bulb and same ballast on on Sanjays site it has a PPFD of 115
http://www.reeflightinginfo.arvixe.com/selectperformancedata.php

I would like to know if there is a way to convert the par into PPFD and vice versa. in the case i listed about the PPFD is roughly 21% of the PAR, but in different bulbs that percentage changes.
Thanks in advance
Felix
 
The preferred measurement for PAR is Photosynthetic Photon Flux Density (PPFD). This is the number of photons
in the 400 to 700 nm waveband incident per unit time on a unit surface. PAR, provides photosynthetic photon flux density (PPFD) in units of Einstein per m**2 per second
 
A microeinstein per second per square meter (mE/s/m2 or mE/s*/m2) is a unit of light energy per second landing on (or passing through) a given area (Figure 2). Photosynthetically active radiation (that is, light in the 400 nm â€"œ 700 nm wavelength range; PAR) can be given in energy terms as PAR irradiance, which is the total energy in the 400-700 nm range. In that case, the units of PAR are watt/m2. It can also be measured in terms of photons. In that case, PAR is called Photosynthetic Photon Flux Density (PPFD). PPFD is a measure of the actual number of photons in the 400 nm to 700 nm wavelength range that are incident per time unit on a surface unit. When expressed in PPFD, all photons are equivalent regardless of their energy and consequently, PPFD is measured in microEinstein/m2/second.
 
ditto :)

PAR measures the number of moles of photons emitted between 400 and 700nm. PPFD is the density, i.e. moles per square-meter per second.
 
the actual measurements depend on the test setup, mostly distance from bulb. PAR by itself is meaningless. PAR@12" (for example) is what you need to think of big picture if comparing numbers from different sources. sanjay posted some conversion factors in that big long thread so people could compare his and joe's number. they also accounted for errors due to the apogee meter which doesn't really measure varying spectrums correctly. (which is why there is a different correction per bulb).
 
PAR is the measurement of the number of light photons striking an object in a given time. PPFD is just a different way of expressing that.
 
Zoos I've been thinking about that question also.

I understand what each means, but what are the steps in converting PAR to PPFD and vise-versa ?
 
Ok I may be way off here.
"units of PAR are watt/m2"
"PPFD is measured in microEinstein/m2/second."

watt/m2 = J/s*m2
a microEinstein/m2/second would equal a microEinstein * second/m2
both joules and microeinsteins are units of energy, there should be a conversion factor and you are able to convert PAR to PPFD and vise-versa.

(again, I could be way off, i'm only in highschool and we are just starting to study light now so you gotta cut me some slack! =) )
 
Does a bulb design, for example PAR36, have anything to do with the PAR you guys are speaking of? I dont think it does, but I want to get that cleared up.
 
<a href=showthread.php?s=&postid=9613382#post9613382 target=_blank>Originally posted</a> by invincible569
Does a bulb design, for example PAR36, have anything to do with the PAR you guys are speaking of? I dont think it does, but I want to get that cleared up.
Click to expand...

No.

Those lights you are using might have the light more focused but as far as PAR, it makes no difference. The only thing is what the bulbs are normally intended for, that might change the spectrum of the bulbs. But if you put a PAR meter under it you can compair the PAR from that bulb to any other light.
 
Thank you guys it becomes a little clearer now.

Manderx - Do you have a link to those comparisons Sanjay posted? BTW to to see you on a different thread (and not arguing with you on the CFL thread)

Grim - I kind of figured that, but it just gets complicated for most people i think after a certain point.

The0wn4g3 - I'm sure we weren't the only ones wondering this.
 
<a href=showthread.php?s=&postid=9611498#post9611498 target=_blank>Originally posted</a> by Zoos
:D can you try explaining that again in English?
Click to expand...

PAR, photosynthetically available radiation can be measured 2 ways. One is PPFD which tells now many photon particles are striking the surface. PAR can also be expressed in terms of how much energy those same photons are depositing and in that case it is Watts/m2.

When someone refers to the "PAR" output of a bulb they could be referring to either measurement. The preferred measurement however is PPFD.

I believe both those studies used the same units of measurement, but the experimental setup was different with the bulbs at different distances from the detector which will alter the light measurement considerably. That is why comparisons of output should only be made bewtween bulbs tested the exact same way.

Allen
 
<a href=showthread.php?s=&postid=9614213#post9614213 target=_blank>Originally posted</a> by ReeferAl
PAR, photosynthetically available radiation can be measured 2 ways. One is PPFD which tells now many photon particles are striking the surface. PAR can also be expressed in terms of how much energy those same photons are depositing and in that case it is Watts/m2.

When someone refers to the "PAR" output of a bulb they could be referring to either measurement. The preferred measurement however is PPFD.

I believe both those studies used the same units of measurement, but the experimental setup was different with the bulbs at different distances from the detector which will alter the light measurement considerably. That is why comparisons of output should only be made bewtween bulbs tested the exact same way.

Allen
Click to expand...

From what I've read PAR and PPFD have different units.
The reason I want to know how to convert the two measurement is because I can't find the comparison of bulbs I want with the same measurement. One bulb is always measured in PAR and the other ends up being measured with PPFD.
 
For our purposes they are the same. PPFD is just a way of measuring the intensity of the wavelengths of light. PAR is the PPFD of wavelengths 400-700.

Why you say PAR, is that way the person knows you are looking at the ppfd of wavelength 400-700. If you just say ppfd it could be more than those wavelengths or less. PAR narrows it down to a certain area.
 
<a href=showthread.php?s=&postid=9616621#post9616621 target=_blank>Originally posted</a> by JB NY
For our purposes they are the same. PPFD is just a way of measuring the intensity of the wavelengths of light. PAR is the PPFD of wavelengths 400-700.

Why you say PAR, is that way the person knows you are looking at the ppfd of wavelength 400-700. If you just say ppfd it could be more than those wavelengths or less. PAR narrows it down to a certain area.
Click to expand...

So PPFD is more general, and PAR is really what i am interested in? that is how i am interpreting this.
 
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