LUX Measurement & Ideal Ranges

Hi,
LUX, or Luminance is a measurement based on a standardized model of human brightness perception. Since humans are more sensitive to green wavelengths, light sources containing these wavelengths appear to be brighter. As a result of this subjectivity LUX is not a reliable indicator of light energy within the context of photosynthesis.

Photosynthesis is a process dependent on incident radiation within all wavelengths in the visible spectrum. The incident radiation carries with it photons which collide with chlorophyll molecules to liberate electrons. The measurement of this activity is referred to as Photosynthetically Active Radiation, or PAR. The relative growth rate and net carbon assimilation rate of plants depends in part on the plant's intercepted PAR also referred to as Photosynthetic Photon Flux Density (PPFD which can be thought of as the number of photons crossing a unit area during a unit time period) and the pathway the plant uses to assimilate the carbon.

Therefore, instead of a LUX meter it is suggested to use a PAR meter. A review of PAR meters can be found here: http://www.advancedaquarist.com/2005/7/review/view

A discussion of PAR measurements and suggested levels in planted tanks can be found in this thread: http://www.barrreport.com/articles/4265-light-intensity-aquarium.html?highlight=PAR+meter


Cheers,

 

Oops, double post.

 

Well, it's no secret that bulbs get dimmer as they age so the LUX readings simply confirm that. It follows that the photon flux that a bulb is able to produce will also diminish as the bulb deteriorates. Since you are comparing the same bulb over time this is logical, but since you have no direct correlation between LUX value and PAR value there is no way to determine absolute values of the change in PAR. In other words you have no way of knowing how many photons are striking 1 square centimeter of a plant surface at coordinates X-Y-Z distance from the centerline of the bulb. You simply know that the number is lower now than it was a year ago.

In this sense it's not completely meaningless but since you have no meaningful numbers as regards PAR you are simply confirming qualitatively what you already know.

Where it really becomes meaningless is if you were for example to compare the LUX values of several bulbs of different spectral characteristics. The LUX meter would give you a higher reading for the bulb containing the higher green content whereas higher photon flux is likely to emanate from the bulb with more blue content simply because a higher frequency wave will deliver more photons to a surface per unit time compared to a lower frequency wave. Also a bulb with a broader spectral curve (having more area under the spectral curve) will generate more photons even though it may not "look" more luminous. So there are many ways in which the LUX meter readings will simply be rubbish and will be no more meaningful than what you can see with your own eyes.

But even this is muddied by the fact that at a given X,Y,Z coordinate in the tank there is reflection, refraction, attenuation and a host of factors that affect exactly how many photons per second actually reach that X,Y,Z point, and only a PAR meter will tell you that. Even with the PAR meter it's necessary to build a 2D or more accurately a 3D photon flux profile across the dimensions of the tank to have an idea of the photosynthetic energy distribution of the tank using a specific bulb set.

As Tom says, it's better to sell the LUX meter to someone who hasn't read these threads and to get the PAR meter.

Cheers,

 

Tom,

Thanks for this info. Saves me a future thread and some research as I wanted to get some reasonably priced test equipment, but wasn't sure what to get.

Yes, the c02 meter is out of my reach, but the others are not.

I would like a PAR meter, good quality tests for N03, P04, and PH. What about a TDS meter? Will this be of any practical use?

If you have specific models that you recommend, go for it lol

What is an LDO meter?

I would be interested in knowing 'more' about my tank specifically in regards to these measurements.