lighting theory - very long post!

[jmhart]

You are all right. As I was lying awake in bed thinking about this stuff, I realized that the stated color temperature (K) of a fluorescent is a weighted average of the K emitted by each of its phosphors. That's why bulbs with the same K and lumens/watt could have different PAR---they have different mixes of phosphors. :wall:



Hmmm... if certain wavelengths are more scattered, wouldn't they "punch down" less? And wouldn't it then stand to reason that light emitted at a wavelength that doesn't scatter as much in water would "punch down" more? I know that blue gets more scattered, but some resources seem to indicate that red passes through and others indicate that it is absorbed by the water (and turned into heat). :help:

I really have to get some work done now. Waaaay too much loafing off at AC during the work day recently.

Yeah, there would be some difference, but I doubt it's significant over the wavelengths available to us in aquarium lighting.

Karl, if you were responding to cellodaisy's post above, then I want to note that the difference is extremely significant. If you were responding to something else, then ignore this.

Cellodaisy, kelvin ratings of bulbs are more-or-less an average of their spectrum. There's also a differences in how manufacturers rate bulbs. Some manufacturers are fairly reliable in indicating a percieved kelvin value for the light produced by their bulb.

To better understand, it's important to understand what a kelvin rating actually is. When carbon is heated to a certain temperature(say 6700 K) the light eminating(the wavelength) from that carbon is said to be 6700 K. Make sense?

So, how this applies to some manufacturers is that they say a bulb produces 6700k. Is that the entire wavelength produced by the bulb? No, but it's an approximatation of the color....meaning the color produced would be similar to if you heated carbon up to 6700K.

So, that system makes since.

Some other manufacturers, seemingly, just pick arbitrary values that really don't seem to mean anything.

Anyway, how this all relates to what Karl said, is that it does seem to be an average of the spectrum produced. I wish I could find a decent example(maybe somebody can help), but....There are dozens(if not hundreds) of 10,000k bulbs on the market. Next time you are around a good selection, pick up a couple of "10,000K" bulbs and compare the spectrum provided on the package. The spectrum is really quite different, despite all being 10,000K.

 

[jmhart]

I believe the longer wave length is scattered,absorbed more(red,yellow,orange,)
while shorter wave lengths can penetrate deeper(violet,blue,green)
much of the visible light is absorbed/scattered in the first 33' the surface turbulence also comes into play as light is also reflected off the surface.

(random scattered thoughts)

That's right. That's why a lot of dive lights appear very red when used on the surface, because down underwater there is still a lot of blue to be used.

Also relevant to a discussion about why SW need actinic lighting

 

[jmhart]

For an interesting read on PAR, check out:

http://www.efn.org/~k_mccree/Professional/PAR.html

 

[cellodaisy]

Kelvin ratings of bulbs are more-or-less an average of their spectrum. There's also a differences in how manufacturers rate bulbs. Some manufacturers are fairly reliable in indicating a percieved kelvin value for the light produced by their bulb.

To better understand, it's important to understand what a kelvin rating actually is.

I do understand Kelvin ratings---what I had failed to take into account initially is that fluorescents use different combinations of phosphors and that each one will emit its own little piece of the spectrum and have its own average Kelving rating, and that the reported Kelvin rating of a bulb is an average of the averages. To totally oversimplify, the average of 1 and 5 is the same as the average of 2 and 4, but if plants are only sensitive to 1 and 5 then then the bulb producing 2 and 4 is useless, even though it has the same average.

A lumen is a measurement of light adjusted for human perception. If you took a bulb's stated lumen output and unadjusted it, you would be left with something closer to its total light energy output. If you could then figure out how much of that light was coming from each type of phosphor, and therefore how much light was emitted at each K rating, you might have some useful information. Sounds awfully complicated, though. Much easier to just get PAR ratings from a meter or fellow hobbiests!

For an interesting read on PAR, check out:

http://www.efn.org/~k_mccree/Professional/PAR.html

Great link! Looking forward to reading it after work.

I'm still confused about the whole wavelength scattering thing. I found this site that explains (in very simple terms and cartoons) why the sky is blue---because the blue light gets scattered everywhere and the red light gets through: http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html. Wouldn't water be the same?

 

[SuBXeRo]

my head hurts too much speculation for me. here is what i dont get. everything below is me just using common sense and my basic knowledge of the subject.

plants need radiation (light) to photosynthesize to grow. Radiation outside of light would be harmful and useless to plants as those would include microwaves, radiowaves, gamma rays, etc. SO we are talking about the light spectrum specifically.

If we have two bulbs that are flurescents with exact same specs but different gas mixtures, you are essentially emitting the same color (kelvins) with the same efficiency. so that means that the gas mixtures differences would have to emit radiation in different wavelengths for their PAR's to be higher because plants need higher PAR values to propagate better. In emitting wavelengths in different quantities you would change the color of the bulb because of a greater intensity in that spectrum.

now i can see this being true if our eyes are not sensitive enough to notice the change. i assume that kelvins are based on what the average human can see but that would have to be based on mathematical equations for the reproduction of these tempewrature ratings on bulbs. with a mathematical background, differences in color would be detected by the math and not our eyes and bulbs should be labeled as such. The important waves used by plants can be seen by the human eye so we should not be worrying about the outlying wavelengths.

does this make any sense? there has to be a relation between color temp (k) and pars because once u change color you change the dominant wavelength which would affect PAR.

 

[jameshilljr]

:crazy: I guess I'm just one of the followers-I'm just glad my plants are growing fine with the lighting I have... Ya'll have put alot of research and thought into this one...very interesting read

 

[Nolapete]

Ashk, just because you don't understand Rex's theory doesn't make it garbage. It actually makes a lot of sense if you read it in the context outside of the WPG "rule" which so many people are duped by.

WPG is total idiocy just like inch per gallon for stocking is. There are far too many variables involved to have one set way of gauging how many watts of what temperature light you need for every aquarium.

The best practice is to either go with 1.5 wpg for low-mid light and 2.5 wpg for mid-high light plants and watch how they grow. Let the plants be the guide for the amount of light. All things equal, if they aren't growing the way you expect them to then you need to increase the amount of light that actually gets to the plants.

If you've read any of what Rex has written, that statement is the core of what he says about lighting. You could have 5 wpg and have horrible growth if the light is out in the room and not getting to your plants. 2.5 wpg with quality reflectors that direct the light into the tank is going to be more effective than more light without them.

 

[Nolapete]

I'm still confused about the whole wavelength scattering thing. I found this site that explains (in very simple terms and cartoons) why the sky is blue---because the blue light gets scattered everywhere and the red light gets through: http://math.ucr.edu/home/baez/physic.../blue_sky.html. Wouldn't water be the same?

Water acts as a prism, so not really the same. You can see it working the same way the sky does in the ocean though. The deeper you go the less the spectrum is able to penetrate. The blue scattered light is all that can make it through.

 

[AshK]

Ashk, just because you don't understand Rex's theory doesn't make it garbage. It actually makes a lot of sense if you read it in the context outside of the WPG "rule" which so many people are duped by.

Wow, please don't bring your perceptions of my intellegence in to this conversation. It is clear that I--and almost all the people in this thread excluding you--find his logic, math, and general theory to be fuzzy and in need of serious reformation. If all the people on this forum did 3 seconds of wikipedia research and then read Rex's LIS theory, they would surely consider it garbage as well. I challenge you to follow his exact method and post your results here. Try to apply his rules to your tanks.

 

[cellodaisy]

If we have two bulbs that are flurescents with exact same specs but different gas mixtures, you are essentially emitting the same color (kelvins) with the same efficiency. so that means that the gas mixtures differences would have to emit radiation in different wavelengths for their PAR's to be higher because plants need higher PAR values to propagate better. In emitting wavelengths in different quantities you would change the color of the bulb because of a greater intensity in that spectrum.

Fluorescents don't get their colors from gases; they get them from phosphors. Phosphors are materials that coat the inside of the tube. Without phosphors, fluorescent bulbs would emit UV radiation. Phosphors absorb that UV and emit light in the visible spectrum.

Two bulbs could have the same average K rating, but have different phosphors. One analogy would be colored sand. If you mixed red sand and blue sand, it would look purple, but you don't actually have purple colored sand. So I could take one bucket and mix 50% red sand and 50% blue sand, and in another bucket mix 70% reddish-purple sand and 30% blue sand, and both could look straight purple, but they actually have different colors of sand.

i assume that kelvins are based on what the average human can see

I don't think Kelvin ratings are based on human perception, but I could be wrong.

The important waves used by plants can be seen by the human eye so we should not be worrying about the outlying wavelengths.

Yes, the light plants need is in the visible spectrum, but plants are more sensitive to red and blue light, while human eyes are more sensitive to green. This is why lumens are a problematic measurement of light output, because lumens are weighted according to human eye sensitivity (i.e. a green light would have a higher lumen rating than a red light putting out exactly the same amount of energy).

there has to be a relation between color temp (k) and pars because once u change color you change the dominant wavelength which would affect PAR.

There is a relationship, but in the case of fluorescents we only have the average K rating and we don't know what components when into that average. In our sand bucket analogy, both buckets would have a "K rating" of "purple," but neither bucket actually has any purple sand in it and if we didn't know what colors of sand went into it we would have no way of finding out just from the K rating.

Water acts as a prism, so not really the same. You can see it working the same way the sky does in the ocean though. The deeper you go the less the spectrum is able to penetrate. The blue scattered light is all that can make it through.

I found a fantastic explanation at http://www.serc.si.edu/labs/phytoplankton/primer/hydrops.jsp. About halfway down the page, under "Absorbtion," it says:

The intensity with which a substance absorbs light depends on the wavelength of the light. It is the wavelengths that penetrate deepest that determine the color of the water. For example, water molecules selectively absorb energy from lower energy wavebands in the reds, yellows and greens, leaving only blue light. Therefore very pure and deep waters appear blue.

So some wavelengths (specifically blue) do "punch down" more than others, but it is more because of absorbtion than scattering.

I can't seem to let go of this Kelvin/PAR thing... If we knew what phosphors a fluorescent used, and what peak frequencies they emitted, could we at least estimate PAR? In theory?