PAR and Lumens

Many LED sellers give a measurement of their light's PAR, lumens or lux, or compare their lights to other lights in terms of PAR per watt or PAR per dollar. There are so many problems with this that I believe it is useless as a way to compare grow lights, so much so that I'm not even going to compare the various sellers' PAR claims. Let me explain.

Ads by Google
Why are there ads on this site?

Lumens aren't for Plants

Let me start off with lumens and lux. 1 lux is 1 lumen per square meter, so lux and lumens are basically the same thing. Lumens measure the amount of visible light (400-700nm, excluding UV and IR) and give extra weighting to yellow light, as that's what human eyes see best. The best way to get a good lumen score is to produce only yellow light, and indeed the most efficient light commercially available in terms of lumens per watt may still be the low pressure sodium light, which can make up to 200 lumens per watt of pure yellow light. There's a reason you've never heard of anyone growing with low pressure sodium lights though–they won't even keep a plant alive, and demonstrate that plants can't live on yellow light alone. Lumens are a measurement of light intensity intended for human use, not plants, and are worthless for comparing grow lights.

If you come from the HPS world, please understand that lumens are a decent way of comparing 2 different HPS lights' intensity, as all HPS bulbs have the same basic yellow-heavy spectrum. Actually, for any two lights that have the exact same spectrum, comparing lumens would allow you to compare the lights' intensity. But for comparing grow lights in general they are worthless, as differences in the percentage of yellow light in the spectrum will skew the lumen score dramatically–and plants really don't need much yellow light.

If an LED grow light vendor is talking about how many lumens their grow light puts out, either they don't understand what plants really need for light, they are just trying to trick you with impressive but meaningless numbers, or both; either way I would be leery of buying from them.

Companies that brag about the lumens their lights put out are:

  • Apache Tech: We are using the most efficient LED chips on the market that produce 120 lumens per watt.
  • Lighthouse Hydro: the Chrome series of LED grow lights represents the most lumens per watt of any LED currently in production.
  • Lush Lighting: Provides 70,000 lumens and 1,200 μmoles when 2' away from LED panel!
  • Spectrum King bases most of their entire sales pitch on their high lumen numbers; there are dozens of places on their website where they brag about their lumen / lux output. They've even made highly deceptive videos falsely showing lumen and PAR readings follow the same trend for different color LEDs.
  • Truth Lighting provides light intensity measurements for all of their lights only in lumens.

To their credit, many of the other LED grow light companies have a discussion somewhere on their site that explains why lumens aren't even worth talking about for plant lights, and they are absolutely correct.

Why it's Best to Get Over PAR

PAR as a measurement of light intensity for plants is vastly better than lumens, but choosing a light based on a PAR number or PAR per watt is not smart for a lot of reasons. Put simply, it is possible to have a light with a lower PAR number that will grow plants better than a light with a higher PAR number. It isn't just one trick either; many different things will give lower PAR scores but still give you happier plants.

First, PAR only includes light in the visible spectrum (400-700nm) and does not include UV or IR that a light may put off, even though this light is beneficial to plants. If lights are being compared in terms of PAR per watt, the lights which include UV and IR will be unfairly penalized, even though these lights will probably grow plants much better!

Second, unlike lumens, PAR doesn't weight any color of light more than another. 1 photon of yellow light has the same PAR value as 1 photon of red light, even though a plant would prefer the red photon. A low-pressure sodium bulb still has a good PAR reading, even though it won't grow plants. Unless the light is putting out the right spectrum to grow plants, a PAR number is meaningless.

Third, because red photons are lower-energy than blue photons, you can make more red photons per watt than blue photons, so an all-red LED light would have a higher PAR per watt than a light that has a balanced spectrum with some blue in it that will actually grow plants better.

Fourth, a single PAR number for a light is meaningless because the PAR number will vary depending on where under the light you measure it. If you have a PAR meter or even a lumen meter you can try this out yourself with any light; as you move further from the light, the reading will go down. Right under the center of an LED light, the measurement will be highest at any given distance from the light, but if you move left or right, the measurement will go down. Having the light evenly spread out over the entire footprint is more important than just having a single high PAR measurement somewhere in the footprint.

One of the LED lights I've tried growing with at one point had very narrow secondary lenses that focused the light into a very bright spot under the center of the light. The portion of a plant that got to grow there was very happy, but even close side-branches on the same plant were showing signs of not getting enough light. The PAR number advertised with the light was very impressive, but that didn't mean it could keep even a small plant happy.

Don't get me wrong; PAR is important, and a grow light with 0 PAR will not grow plants. But from all of the claims I see on LED sellers' sites, I believe the PAR numbers being thrown around are more often used to deceive you than to educate you about their lights. It's all too easy to be able to claim the highest PAR-per-watt or the biggest single PAR number, even though the light may not even keep plants alive. I highly recommend ignoring all of the PAR, PAR-per-Watt and even PAR-per-dollar numbers being presented to you on the seller's websites, as they really don't give you any meaningful information about how well the light will grow.

I believe there are much better ways to compare LED grow lights than with PAR numbers–that's what this whole site is about.

Dissecting a PAR Deception

For an example of how PAR is used to deceive you, consider the PAR chart helpfully provided by Kind LED on their website and frequently published in magazine ads, supposedly pitting one of their lights against a "UFO" LED light, an HPS light, and a LumiGrow LED light. 17 PAR measurements are given for each light at 24" distance from the light. The numbers in the diagram are a little hard to read so I'll replicate them here:

Some KIND LED Light
33
3434
158134158
3517758217735
158134158
3434
33
Some "UFO" LED Light
12
1212
444344
1346624613
444344
1212
12
Some HPS Light
100
9595
169171169
6316520916563
169171169
9595
100
Some LumiGrow Light
49
3838
152176152
3913829213839
152176152
3838
49

First, note that the Kind comparison doesn't say what model lights are actually compared in the chart; the picture implies it is their 270-watt K3-L450, versus some unknown "UFO" LED light of unknown wattage, an HPS of unknown wattage in pretty much the worst-imaginable reflector, and a LumiGrow light that looks like the 325-watt Pro 325. Without knowing at least the wattage of the HPS and UFO LED, the comparison to those lights is automatically somewhat meaningless; that could be a 100-watt HPS bulb and an 80-watt UFO. It isn't clear that they are comparing apples to apples; it's pretty safe to assume the 270-watt Kind light would have more PAR than an 80-watt UFO. The low-wattage UFO would also likely have a much smaller recommended coverage footprint, and likely less than a 24" recommended hanging height, so this is probably a completely unfair test for the UFO.

Second, note that while they provide the distance to the light for the measurements, they don't say how far apart the measurements were taken in the footprint, or even if the measurement distances were the same for each of the 4 compared lights! Is it even a fair test?

Third, notice the really high 582 PAR number in the center of the Kind light's footprint. This is what is supposed to impress you, but keep in mind that especially due to the secondary lenses they're using to focus the light into a narrow footprint, that number could drop to 134 PAR just an inch away from the center of the footprint. In fact, since we weren't given any information on the measurement distances within the footprint, perhaps this is what they are saying with their own diagram. Without more information we can't be sure.

Fourth, notice that if you add all 17 measurements up for the Kind LED light and the unknown-wattage HPS light, the HPS light has more total PAR (2263) in the footprint than the Kind light (2108), meaning that the HPS light is putting out more average PAR (7.4% more) over the whole footprint than the Kind light. Whatever wattage it is, the HPS is actually kicking the Kind light's ass when looking at the entire measurement area! The 582 PAR number dead-center in the Kind footprint distracts you from seeing the truth.

Finally, notice that the HPS, UFO LED and the LumiGrow LED have much more consistent PAR readings across the footprint than the Kind LED, meaning that they more-evenly cover their footprint and don't just have a "hot spot" dead-center under the light. If we compare the lowest PAR readings in each footprint with the highest, the Kind light has 33/582= 6% consistency; the UFO 12/62=19% consistency, the HPS 95/209=45%, and the LumiGrow 38/292=13%.

If all that matters is that the leaves dead-center under the light get the highest PAR, the Kind light is by far your best choice; if you'd rather have all of your plants or leaves within the footprint getting enough light without hot spots, the LumiGrow is twice as good and the UFO 3 times as good as the Kind light, with the HPS more than 7 times better!

Spectrum aside, based purely on the PAR numbers the HPS light is clearly the winner in this comparison–not the Kind LED, even though it looks that way at first.

Still think PAR numbers are a great way of comparing lights? They can be, but you have to be very careful to take a detailed look at the whole picture, including spectrum, total footprint coverage, and consistency of footprint coverage. Single PAR numbers are meaningless out of context, and most often misleading to you. As this example shows, even multiple PAR numbers can be misleading if they aren’t analyzed carefully.

If I could get PAR measurements for every square inch of each light's claimed footprint at the recommended hanging height for each light, I could compare lights' PAR for you somewhat fairly. Since I don't have this information, including PAR in my comparison of LED grow lights would be deceptive to you, and I'm all about exposing deception rather than creating more.


Comments

2 thoughts on “PAR and Lumens”

Leave a Reply

Your email address will not be published. Required fields are marked *