Been a while since I’ve written, but I get a lot of questions about lamp life as it applies to lamps for cannabis, and felt an article might clear up a few misconceptions. It is different than how you figure lamp life for other UV uses, like curing violins or aging materials.
First of all, if you are a serious cannabis grower with at least 8 hoods, you need to invest in a UVB meter. Solarmeter makes a great one for around $250, you can find it on Amazon. You can’t really compare one lamp to a different model lamp (trust me, it has to do with spectral weighting, but they don’t compare cleanly). You can measure your new lights, then measure once a month so you know when you should change bulbs. I recommend changing once the UVB reaching 70% or of the original output.
To get a first reading, you should age the lamp for at least 10 hours by simply running it. then hold the meter to the lamp, at the glass, in the middle, and the highest possible reading you can. Do this for each of your lamps, and average your results. This is your reference reading. Once the lamps are down to 70% of this reading, when measured the same way, then it is time to swap lamps. If you are using pulse lamps (like the Flower Power) you can cheat a little and squeeze some extra life by simply increasing the time exposure per day, to maybe buy you another month or so. But this method will prevent you from buying new lamps too early, or too late, and it pays for itself in a year.
Now to the main topic, which is lamp life. Lamp life is determined solely by the UVB output of the lamp, it isn’t about about the lamp burning out. A typical UVB lamp will give 1000 hours of usable life, yet continue to burn for up to 3000 hours. It simply gets weaker and weaker until it is useless as a UV lamp once you get past 1000 hours. But lamps do not die an “even” death. As they age and get to their expected life, the spectrum of the lamp changes dramatically.
In order to produce UVA and UVB, you have to make a glass tube with at least two different phosphors in it. One for the UVA, one for the UVB. Our lamps typically use several phosphors, including some for buffering, to prolong the life. Each phosphor is a chemical that will produce a particular set of frequencies (the primary frequency and often, harmonics of that frequency). The glass tube is actually clear, the white stuff you see is the phosphors. So the difference in your average office light and a UVB light is mainly decided by the different chemicals we put in the tube. One phosphor might make the color “red” at 633nm, another phosphor might make the color “ultraviolet” at 365nm. All Solacure lamps (curing and horticulture) are 100% UV phosphor. We don’t use any visible spectrum phosphors. The visible light you see is a combination of harmonics and the fact that you are running plasma inside a tube, which always makes visible light. This is why they look dimmer than a regular light as well. All the real energy is in the UV band, where you can’t see it.
The problem with UV phosphors is they break down. Well, really, all phosphors can break down, but it takes a lot more energy to make a photon of UV light, which puts a strain on the phosphors. So UVB phosphors break down quickest, then UVA phosphors break down moderately fast, and visible spectrum phosphors break down slowly because they are lower energy.
When we rate the life of a lamp, we are really only rating the life of the UVB, because we know the UVA will still be fine. An an imaginary example, lets say you start with a 10% UVB / 90% UVA lamp. By the time you to 70% of the original power, it is likely a 7% UVB and 93% UVA lamp. We have shifted the spectrum towards UVA simply because the UVB is wearing out. if you keep going for 2000 hours, eventually you get a lamp that has 1% UVB, 99% UVA *but* the overall UV output is less than half.
These percentages do NOT tell you total power, they just tell you ratios. Just like how you can take two glass that are half filled, but if one is a 4 oz. glass and the other is a 8 oz. glass, they aren’t equal. They are both 50% full, but the total mass isn’t the same: One glass has 2 oz. of milk, the other has 4 oz. of milk. I say this because it is easy to get tripped up on percentage ratings of lamps. You can take two 20% UVB lamps, and find that one is actually twice as strong. Just like the glasses above, the ratio can be misleading. Ratios are handy for measuring the difference between otherwise identical lamps, like two lamps from the same manufacturer, but it isn’t for comparing two different brands of lamps.
So to recap, when we rate lamp lamp, we are rating how fast the UVB phosphors will break down. Using them past 70% of the original life can be done, but it is a problem because you have less output and a very different spectrum, so the results won’t be the same. This holds true for all UV lamps, regardless of brand.