Lux VS Par

[coralfragger101]

Noob question here:

What kind of relationship is there between LUX and PAR?

I'm hoping that as LUX increases that PAR increases as well.

[coralfragger101]

Bump for response. This isn't a difficult question - is it?

[coralfragger101]

I guess it is a difficult question.

[bluenose990]

I really wish someone would answer this question...

[areze]

lux and par are directly related for a same light source.

but for a recent example, you cant compare lumens of a MH to lumens of an LED and assume the higher lumen output will also be the higher par . since they dont use the same reflector system.

[renisel]

Lux are units for describing how bright a given light would appear to the human eye. PAR (photosynthetically available radiation) is typically measured in PPFD (photosynthetic photon flux density), which is a measure of the concentration of photons emitted by a light source that can be used for photosynthesis. Because lux measurements are weighted according to the sensitivity of the human eye to different wavelengths, there is no guarantee that an increase in illuminance (lux) or PAR will translate into an increase in the other. It is, in fact, possible to decrease illuminance while increasing PAR and vice-versa.

[hahnmeister]

He got it! He must have read Dr. Joshi's articles on lighting... that will tell you all you want to know here and more.

PAR/PPFD is based in the radiometric scale... which is not weighted... but a raw count of photons from 400-700nm, or the extended range which includes some UV and IR.

Lux/Lumens, etc, is based on the photometric scale, which is sort of like PAR, but because our eyes are most sensitive to green, then red, then blue (only about 10% of our cones are for blue), a heavily blue light source might seem dull to us (low lux/lumens), but have a very high PAR because our perception of blue sources is that they arent as bright as other colors. So a 20,000K bulb with a par at a given point of 200, and a 10,000K bulb with a par at a given point of 200 would have lux readings of say... 100 and 200 respective. As a consequence, you can not convert PAR to Lux easily without knowing the spectrums, or having a fixed spectrum source.

[coralfragger101]

So a LUX meter is pretty useless in this hobby since we really want to know what the PAR is?

[OneReef]

<a href=showthread.php?s=&postid=13262559#post13262559 target=_blank>Originally posted</a> by coralfragger101
So a LUX meter is pretty useless in this hobby since we really want to know what the PAR is?


I would like to know this as well. I had been thinking of getting a LUX meter from Drs. F and smith just to check out the different areas of my tank, and test old bulbs vs. new. I guess Lux may not tell me what I am wanting to know?

[coralfragger101]

At least you thought to investigate "first".

I've already purchased a LUX meter.

[rogergolf66]

you can still use a lux meter to know the intensity in different areas of the tank but other then that you are out of luck.

Roger

[renisel]

<a href=showthread.php?s=&postid=13262716#post13262716 target=_blank>Originally posted</a> by OneReef
I would like to know this as well. I had been thinking of getting a LUX meter from Drs. F and smith just to check out the different areas of my tank, and test old bulbs vs. new. I guess Lux may not tell me what I am wanting to know?

I could be wrong, but I think measurements in lux could still be useful for that purpose. For comparing areas of your tank, all of the light should be of more-or-less the same spectrum, so differing lux readings should correspond to proportional variations in PAR. Likewise, for testing old bulbs vs. new, the old bulbs should put out less light, which will translate into lower lux readings. The spectrum will also change in an older bulb, so a 25% decrease in the lux output doesn't necessarily mean a 25% decrease in PAR (it could be more or less, depending on exactly how the spectrum has been altered); but I think lux measurements should still be a decent way of determining roughly when a bulb needs to be changed. Where a lux meter would be much less useful is in comparing different bulbs to determine how much photosynthetic light they're putting out.

[hahnmeister]

Im not going to say its useless, but using a Lux meter isnt going to be accurate.

[OneReef]

They have LUX meters at Drs Foster and Smith for about $80. How much does a PAR meter cost and where would you get one at a decent price?

[hahnmeister]

$300