NAKEDGARDENER
Guest
When it comes to lighting a grow area most of us just buy a light we like the look of and which says it delivers a lot of light energy in such measurements as lux or lumens or wattage. But how do you truly measure how much of the right kind of light your plants are getting from your light fixtures?
DANGERDAN Well fortunately for us we have a sitting expert on this subject right here on GWE.. Mr Dan or Dr Dan as I like to refer to him, however I still say he should change his nic to Illuminate due to his brilliance (pun intended) on this entire subject.
I asked him a very noob question since I know little about the science of the subject, and before doing so I made sure I had read as much as I could from his articles which are published here. I republish his very detailed and informative reply here for us all to digest and take onboard.
Measuring light sources is quite complex, with a lot of physics and mathematics involved. To measure a light fixtures total output capacity, it is usually done with what is called an integrating sphere. Which is like a hollow round sphere that captures all light from a light source and reflects it to a sump where a measuring sensor is placed. This is how systems such as photon flux, or radiant flux or lumens are quantified. These three different metric systems are all done by the same measurement equipment, but they measure or quantify light differently. So if you remember, photometry is about light that the human eye can see, using a metric value that is weighted. Radiometry is about anything related to light (electeromagnetic radiation) but measuring light unweighted, full energy. And Photon counting measures the spectrum of light plants use for photosysnthesis, but as a counter system which is neither weighted or unweighted. Its just a counter for photons.
So to answer your question, the measurement bandwidth of 400-700nm is exactly the range that plants use for photosynthesis. So no, wavelengths outside this range are not of concern. However, you can not reliably measure a light sources total output with quantum PAR meters. Because they were designed for outdoor applications where light is uniform in all directions. Artificial light sources are not, so the umols per square meter value you meaure, is not actually a true represnetation of what the light source actually is putting out per square meter. As the light intensity drastically changes as you move the sensor. However it has been traditionally used as a spot sensor for quantifying the immediate intensity where the sensor is placed.
With this, you can measure a light sources approximate output by using what is commonly called PPFD maps. Where you carefully measure the light sources footprint in many locations (higher sample rates in uniform locations improves accuracy) and then taking all the values and average them out to get what is known as averaged PPFD maps. Which is like an approximate PPF measurement. But this is still not the total light out put.
You do not however have to use integrating spheres to work out what your light source is producing. Many light sources have been measured and quantified into a efficiency value which can be used to calculate what an individual light source is (excluding LED sources as they are all different). For example.
Your CMH has han efficiency value of around 1.5umols per watt. Knowing your wattage you can calculate by multiplying the efficiency per watt with the total watts used by the fixture (at the wall not the lamp wattage).
So a 315 uses about 340 watts so 1.5umols x 340 watts is 510umols. A 630 CMH is the same to calculate, so 1.5umols x 650 is 975umols.
Now to be clear, your primary use with a PAR meter is for appropriate height and light intensities for your plants. Using your meter for improving the light utilization by your plants, not for quantifying light sources, because as previously said, this cannot be idealy done.
You might want to check out my article on using light efficiently where i provide the ideal light intensities for different stages of growth.
Happy growing
DANGERDAN (reprinted, and thanks to you.)
DANGERDAN Well fortunately for us we have a sitting expert on this subject right here on GWE.. Mr Dan or Dr Dan as I like to refer to him, however I still say he should change his nic to Illuminate due to his brilliance (pun intended) on this entire subject.
I asked him a very noob question since I know little about the science of the subject, and before doing so I made sure I had read as much as I could from his articles which are published here. I republish his very detailed and informative reply here for us all to digest and take onboard.
Measuring light sources is quite complex, with a lot of physics and mathematics involved. To measure a light fixtures total output capacity, it is usually done with what is called an integrating sphere. Which is like a hollow round sphere that captures all light from a light source and reflects it to a sump where a measuring sensor is placed. This is how systems such as photon flux, or radiant flux or lumens are quantified. These three different metric systems are all done by the same measurement equipment, but they measure or quantify light differently. So if you remember, photometry is about light that the human eye can see, using a metric value that is weighted. Radiometry is about anything related to light (electeromagnetic radiation) but measuring light unweighted, full energy. And Photon counting measures the spectrum of light plants use for photosysnthesis, but as a counter system which is neither weighted or unweighted. Its just a counter for photons.
So to answer your question, the measurement bandwidth of 400-700nm is exactly the range that plants use for photosynthesis. So no, wavelengths outside this range are not of concern. However, you can not reliably measure a light sources total output with quantum PAR meters. Because they were designed for outdoor applications where light is uniform in all directions. Artificial light sources are not, so the umols per square meter value you meaure, is not actually a true represnetation of what the light source actually is putting out per square meter. As the light intensity drastically changes as you move the sensor. However it has been traditionally used as a spot sensor for quantifying the immediate intensity where the sensor is placed.
With this, you can measure a light sources approximate output by using what is commonly called PPFD maps. Where you carefully measure the light sources footprint in many locations (higher sample rates in uniform locations improves accuracy) and then taking all the values and average them out to get what is known as averaged PPFD maps. Which is like an approximate PPF measurement. But this is still not the total light out put.
You do not however have to use integrating spheres to work out what your light source is producing. Many light sources have been measured and quantified into a efficiency value which can be used to calculate what an individual light source is (excluding LED sources as they are all different). For example.
Your CMH has han efficiency value of around 1.5umols per watt. Knowing your wattage you can calculate by multiplying the efficiency per watt with the total watts used by the fixture (at the wall not the lamp wattage).
So a 315 uses about 340 watts so 1.5umols x 340 watts is 510umols. A 630 CMH is the same to calculate, so 1.5umols x 650 is 975umols.
Now to be clear, your primary use with a PAR meter is for appropriate height and light intensities for your plants. Using your meter for improving the light utilization by your plants, not for quantifying light sources, because as previously said, this cannot be idealy done.
You might want to check out my article on using light efficiently where i provide the ideal light intensities for different stages of growth.
Happy growing
DANGERDAN (reprinted, and thanks to you.)
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