How to Make Your Own Vitamin D Sun Tanning Lamp

Derek Antosiek

Derek Antosiek / February 3, 2026

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Are you worried about Vitamin D deficiency or seasonal affective disorder? Are you looking for a cheaper alternative to the Sperti Vitamin D Lamp?

In this guide, I’ll show you how to build your own vitamin D lamp step by step!

  • I’ve officially tested this build, and it works! This was using 4x Arcadia 46″ 14% UVB Bulbs.
vitamin d test results

Here is the finished product:

Since this setup also emits a lot of UVA, it should also be able to function as a tanning lamp for those of you looking for that.

Why Did I Build This?

I live in Michigan, so the sun and temperature are too low for vitamin D production for several months of the year.

Why not use the Sperti lamp? I was curious to see if I could build something similar for a lower price.

Testing the DIY Light

I finally got my hands on a UV spectrometer, so let’s review its actual output and compare it to the Sperti.

Spectral Output

Measuring at around 15 inches from the lamp, similar to the distance you’d use the Sperti, here’s the result:

Spikey like a fluorescent! Let’s look more closely at the UV portion now:

Here, we can see that the DIY with the Arcadia bulbs puts out quite a bit more UVA than UVB. The DIY lamp I made is around 15% UVB and 85% UVA, pretty accurate to the advertised ratios. Meanwhile, the Sperti is closer to 42% UVB and 56% UVA, making it more targeted for vitamin D and less for tanning.

So the Sperti is a bit more “purpose” built than my DIY lamp for vitamin D. However, the DIY lamp is cheaper and clearly still works. The DIY UV ratio is also a bit more “sunlike” because it’s closer to the UVA:UVB ratio of natural sunlight.

Testing Flicker

This isn’t really a “visual” device, but for fun I checked the flicker. It looks like there is some 120 Hz flicker here, which is nothing I’d be too concerned about, especially given the use case.

Does UVA Affect Vitamin D Production?

Kind of.

In this study, in which 75 participants total finished. The researchers compared vitamin D production in 4 groups.

UVB only, UVA only, UVB+UVA, and a control.

It was observed that almost no differences were seen in the full-spectrum UVA+UVB group compared to the UVB-only group.

uva and uvb vitamin d study screenshot

The full spectrum group actually had a higher mean change in serum D3 than UVB only.

They do note that under longer duration times (≥9 min) UVA may decrease pre-vitamin D3 production. Which is fine; UVA does act as a D3 modulator in nature.

So I don’t think there’s anything to worry about here. It’s likely a good thing as UVA is helpful for nitric oxide production, blood flow, and increasing endorphins.

The Parts You’ll Need to Build It

First, review the parts you’ll want to pick to build our lamp.

Testing and Choosing Your Bulbs

For my original build, I purchased four 14% UVB Arcadia fluorescent lights, which, as you saw in the beginning, worked great! However, I’ve since tested several others, so let’s go over how they compare so you can maybe choose something different if you’d like.

I purchased several UVB bulbs for this test:

  • Arcadia 6%
  • Arcadia 12%
  • Arcadia 14%
  • Reptisun 5.0
  • Reptisun 10.0
  • AgroMax

All the fluorescent bulbs were 2 ft versions. I also put the data from the Sperti, the Mitolux, and the DIY build in for comparison. All measurements were taken at 12 inches.

Here is an interactive graph of the data: (disable the DIY lamp to see everything scale more)

You can see that the AgroMax is putting out some serious juice! Comparing it to the Sperti, it actually looks like the same spectrum, just way more power.

This makes sense since the AgroMax is a T5HO bulb, which is a much more recent fluorescent technology than the bulbs used in the Sperti.

At relative strengths, though, the AgroMax is still putting out more shorter wavelength UVB than the Sperti. This will make it likely more effective, but also perhaps a bit less safe as well.

  • CAUTION: Several users have been quite badly burned using large arrays of AgroMax; now we can see why. If you plan to use these, one or two 2′ bulbs should do it and stand at least 2 feet away!

The MitoLux lamp is an LED-based system, so you can see that this peaks at around 295nm, which is optimal for the production of vitamin D without any “unnecessary” UVA. This device also features a dynamic infrared cycle to help with healing and protection from the UV, pretty cool!

But anyway, the Arcadia 14% that I originally bought still seems to be a good option if you don’t mind the extra UVA present in this type of light.

Grow Light Fixtures

The fixture I used in this project was a 4-foot 4-bulb version explicitly built for 54w fluorescent T5-HO bulbs. However, you can also get one-bulb or two-bulb versions if you want to try something less powerful.

You can also find a 2-foot version as well. Here are some to check out, but you might have to look on your own if these aren’t available, they’re all mostly the same thing.

4 Bulb 4-foot Fixture

vivosun 4 bulb fixture product photo

4 Bulb 2-foot Fixture

It comes with four bulbs we won’t need, so you can recycle them. I’ve been unable to find a fixture without lights.

The Protective Screen

This setup’s possible downside is that it doesn’t have a protective grate, which is nice because fluorescent UVB bulbs utilize mercury. You don’t want to break these bulbs.

If you live alone and aren’t worried about anyone breaking it, you can always skip this part.

The best option I’ve been able to come up with is attaching a metal wire mesh to the front.

mesh screen product photo

1/2 Inch Galvanized Welded Wire Mesh

This mesh is quite stiff and will work well to protect the lights.

zip tie mounts product photo

Adhesive Zip Tie Mounts

You’ll need something like this to attach the mesh to the light fixture.

Building the Vitamin D Lamp

Okay, now let’s put it all together!

Preparing the Light Fixture

Here’s what you’ll need to do with the fixture.

  1. Remove the blue protective film from the reflective trays; this is pretty easy.
  2. Fit the trays into each other. They come a bit loose, but once you do, you’ll figure out how they fit together.
  3. Clean up any smudges.

Installing the Bulbs

Now, carefully pull the bulbs out and install them one by one. These are bi-pin lights, so the connectors on either side are installed vertically into the fixture.

You may have to push the light fixture connecter out on one end to get the light pins to fit into the slot.

Once it’s in the slot on both ends and is pushed down all the way, you can spin it to the right to lock the light into place.

Now repeat for all four lights!

Mounting it on the Wall

To mount the light on your wall of choice, you’ll want to start by installing a hook or screw of some kind into a stud at around 70″ off the floor.

Next, you’ll hang the light using one of the hooks it came with.

closeup image showing hook attachment to screw in stud
Just like that! Don’t mind the crack in the wall, it’s not that heavy, that’s just my amazing drywall work.

I also attached the bottom to the wall using a couple of 3M sticky pads.

Here’s what the finished product will look like:

the diy vitamin d lamp fully installed on the wall

Not bad!

Installing the Protective Grate

Now for the protective grate, should you want it…

You’ll need tin snips for this part!

Cut off a 49″-long section of the mesh and flatten it so it’s easier to work with.

Next, place the mesh against the panel and bend the top portion over. It’s pretty stiff and rigid mesh, so this isn’t the most effortless process.

Once you have the top done, you can move on to the sides.

The trick here is to bend the sides just a bit near the top so that you know where to bend it down.

Once you have a reference point, remove the mesh and bring it to a flat surface with an edge (like a table or counter). Using that edge, bend the mesh down on both sides. This way’s much easier than trying to do it on the lamp.

  • Make your bends just a bit more than 90 degrees. This way, once it’s attached to the light, the metal won’t want to pull away from the zip-tie mounts.

Once it’s all bent to shape, you can cut out the corner sections that aren’t needed.

closeup view demonstrating the cut corners of the mesh
Example of how the corners will look once you’ve cut them to shape.

And that’s about it! It’s a little bit of a pain, but once it’s done, it’s done.

Stick your zip tie mounts onto the light and start zipping the mesh into place!

UVB LED Options

Several studies have shown that UVB between 293-295nm is the most effective at triggering the production of Vitamin D. However, natural sunlight has almost no light in this frequency range, so I have concerns about trying to emulate this.

It’s not easy to find UVB LED products on the market just yet, but here are some of the options I’ve found so far:

MitoLux Vitamin D Lamp

Pros

  • Dynamic UVB + infrared cycle is very innovative
  • Doubles as a red light therapy panel
  • The proximity sensor is a nice touch

Cons

  • The only real con here is the price
  • I was unable to raise my vitamin D levels significantly, I’m unsure as to the reason

Get 10% off with the link above!

Chroma D-Light

chroma vitamin d light

It is by far the most expensive Vitamin D light on the market, but it is probably the most legitimate, safe, and effective way to get vitamin D.

The combination of red and Near-IR combined with narrowband 297nm UVB LEDs make this a powerhouse.

Use code OYB10 for 10% off!

How to Use Your Vitamin D Lamp

Okay, so it’s built and mounted, and you’re ready to stand naked in front of it.

But how far away? And for how long?

The Warm-up Period

When first turned on, the lamps aren’t evenly lit; they take a few minutes to warm up fully.

So, wait until they are uniformly lit to use the lamp.

How Far Away Should You Stand

I currently use my lamp at a distance of around 14-16 inches.

I’d start at two feet for safety.

How Long Should You Use the Light

How you react to ultraviolet radiation depends on your situation.

Getting my vitamin D from 23 ng/dL to 39.2 took about a month, and on average, I spent about 10 minutes per day in front of my lamp 5-6 days a week.

The time it takes to develop erythema will vary depending on your skin type, diet, genetics, and other factors.

For those who don’t know, erythema is just a reddening of the skin. If your skin is getting very red, you’ve been standing too close for too long.

It’s as simple as that.

Didn’t get red at all? Feel free to increase the time or stand a little closer. Only change one variable at a time so you know what is and isn’t working for you.

The Best Time for UV Exposure

The best time to use this would be in the middle of the day, preferably after some infrared exposure, as infrared is protective against UV damage.

For most, the best routine would be to use infrared light therapy in the morning, followed by a UV light session when they get home.

You don’t really want to expose your skin to UV first thing in the morning because it’s very unnatural. This might harm your skin and potentially cause problems with circadian timing and hormone rhythmicity.

Alright, that’s it!

Conclusion

I’ve personally used the Sperti, and I can confidently say that my DIY sun lamp feels much better.

It feels warm and powerful, like the sunlight. I’m so fond of standing underneath when I can.

Hopefully, this has inspired you to build your own!

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Dr Carl

As a Seattle area northern latitude resident, physician, and avid researcher, I like to keep my D3 levels between 70-90 ng/ml by blood test (after a month of supplementation). Over 100 is okay by me, even if it freaks out the mainstream allopaths.

Lauri S

Have you compared your diy lamp to the Sperti tanning lamp 75% uva 25% uvb? I cannot find any information on testing of their tanning lamp and increase in vit d levels.

Mia

HI! What do you think about having two arcadia bulbs instead of 4? How do you think it affects output and effect?

Bird

Hi,
Is it possible to use only 2 bulbs in the fixture for 4 bulbs? Will it still work?
In Norway we can only get the Arcadia 12%. Will that still be ok to use?
Thank you for your work!

Kelly Milk

Hey! So, how often would you say you use the “Derek DIY Grow Sun Lamp?” I am going to attempt this, or have someone help me with it as winter is approaching here on Hellinois. 😀 would you say you made it all for less than $300 total? Thanks!

radi0star

I’m stupid. I have only now figured out how to use the UVC card. It will shine green in the moment when the UVC reflects off it into your eyes; it will not darken up like the general UV detector section.

My Agromax UVB does shine green at the UVC detector. Oof. I hope someone will measure it properly for UVC.

Mel

Forgive me if I missed this bit of info, but how hot does your DIY lamp get? Does it make the whole room hot and the user sweaty? Thank you for your help.

Charles

Is the 75% UVB safe or will I get rekt if I use that for a couple minutes 3 days out of a week?

Zalman

How is your DIY lamp putting out so much more UV rays than the Arcadia 14% if that’s the bulbs that you used?

Chris Masters

Dude, this is awesome! I live in Michigan, too. I’ve wanted something like this for the winter to add to my MitoRed red-light panel. I want the 4-foot panel for full-body coverage, and the only 4-foot lamps I can find are the ArgoMax bulbs. If I went this route, should I install just 2 bulbs (leaving 2 slots empty) and maintain a 2-foot distance? If I use just 2 bulbs, which slots should I use, or doesn’t it matter?
Thanks for your help, man, I appreciate it.
Chris Masters

Chris Masters

Thanks, Derek. I ordered the 22″ Arcadia 14% bulbs and fixture.

gb

Anyone have any experience with reptile lamps. For example, this one:
https://www.amazon.com/REPTIZOO-Spectrum-Self-Ballasted-Mercury-Amphibian/dp/B07CXJMRMP?th=1

a mercury vapor lamp with UVA, UVB & infrared heat?

Would something like this work?

Thanks

gb

Derek, thank for your response.

I thinking that the AgroMax might be the best alternative. Would one 2′ bulb be enough (note: the REPTIZOO bulb is ~ 4″ long, so the AgroMax would be 3x the length)? I’m a little concerned about the 75/25 uvb/uva ratio.
Also, would this fixture work for the ArgroMax: https://www.amazon.com/MouMouPurr-24W-T5-Reptile-Fixture/dp/B0FH74RRVR/ref=sr_1_1_sspa?crid=37Y6LSLXH64LR&dib=eyJ2IjoiMSJ9.J6TB6QOmRK6V4HPYpHGNrmsqKAwzP1YHj_4MP1xk1jcOurQKhIQBYm1vdf2W6iC0BzByah_03U9AhIyQGhSh4AhET_DG8MnFU-O8Kto9mAur0eWFbdfWOZA9l8y2apEd.yRY6__FAuiIPFnFglAY-_iN4SnogWtEp1jL6rsoGDdA&dib_tag=se&keywords=htg+t5+fixture&qid=1761767284&s=lawn-garden&sprefix=htg+t5+%2Clawngarden%2C67&sr=1-1-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&psc=1

Thank You

Zalman

Hi, I just created a 2 foot vitamin D lamp with 12% vitamin D bulbs based off of your recommendations here. My vitamin D came in this morning on my blood test at 26. I’m very curious where it’s gonna be in a couple months after I use the lamp often.

Should we be going until we are a little red?

B P

reading the research of others, erythema (reddening) is not a recommended practice. It means you have been damaged. It is recommended with the professional version (Sperti) that you do 5 minutes every other day. That is a very minimal amount of time and should not produce erythema.

David

Hey Derek,
Phenomenal article. You’ve inspired me to make this.

I am looking at the spectrum comparison and wondering if a 1-bulb agromax plus a 3-bulb Arcadia 14% would yield better results than Arcadia 14% alone. Or perhaps a 50/50 split of the two types. Agromax seems to mimic the sperti but with higher power, which provides the higher UVB (vitamin D) stimulus, while the Arcadias give more of the UVA. Can you mix and match in the same fixture? Do you see any downsides? Would 2 agromax + 2 Arcadia be too much power? I am looking to get the most efficient (shortest time/day) yet safest dose.

Would appreciate your thoughts on that. Thanks!!

Dave

Kay

Very cool DIY!

I’m thinking of DIYing a 2FT 2-Lite AgroMax with *this* box. Could it be used?
Thanks!

Lumiastar 2Ft 2Bulbs T5 Grow Lights Panel, 6500K Full Sun Light, 5000 Lumen Outputlumiastar 2Ft 2Bulbs T5 Grow Lights Panel, 6500K Full Sun Light, 5000 Lumen Output
https://www.amazon.com/gp/product/B0DFHTSH3T/ref=ox_sc_act_title_3?smid=A2PESJVQC6LAFZ&psc=1

Scott

What wavelengths are the bulbs that come with the Vivosun fixture putting out? Did you measure the output on those to see exactly what they are outputting? If they have any reasonable output maybe an individual could use two of them and two of the Arcadia to save a little money.

Kayla Rogers

Hi Derek!

I ordered the Agromax 4 foot bulbs and waiting to figure out if I should just use a 1 bulb or 2 in a fixture. They don’t sell the 2 foot bulbs anywhere they are out of stock. I can’t take vitamin d pills super reactive to them. And how far away should I stand?

Michal

I want to say thank you!!

I built the 14% Arcadia version (even got my UVA+B dosimeter to double check I’m not doing something dumb). Have separate RLT+near infrared panel I use beforehand. I had normal D3 vitamin levels before starting this thanks to 5k IU D3 supplements. I feel _much_ better after using DIY UV lamp. Almost like I went on a beach walk every day (even though the closest sunny beach from me is thousands of km away in Spain/Italy). There is research published in Cell about UVB affecting endogenous opioid production/release. It also appears to impact HPG axis (and IIRC even testosterone levels). These appear to be D3-independent, UVB-mediated pathways. Might partly explain why D3 supplements don’t solve SAD.

I read the research a while ago, but never had the courage to build this at home. Your article + detailed spectrograms made the difference! Thank you!

john

Thanks, Michal, for this enlighning comment about D3 supplement not solving everything.

If I may, I`d add 2 things about that:
1. I remember listening to a video from a female “Youtube Dr” (I think it was Dr Boz, a real MD), in which she explained how she failed to improve her dad’s vitamin D level by prescribing every supplement available under the sun, and that UV ligh solved the problem decisively;

2. In another youtube video, which i listened in its original non-english version, a european biochemist with an impressive track record (I checked) explained that sun-made vitamin D is the only form of vitamin D usable by numerous biochemical processes (i.e. vitamin D taken by the oral route don’t feed thoses processes).

David

can you share the links please

John

“Have separate RLT+near infrared panel I use beforehand. ”

Hi Michal,
Can you please give more details about the equipment you use fort that?

Thanks!

mo

Hello Derek
I too live under that mostly gray Michigan wintertime blanket so am fortunate to have found you and the research you’ve done.
The wall-mount photo reveals a 4-foot fixture as does the Amazon link. But then the websites for the bulbs (and also your testing?) was with the 2-foot bulbs?
I’ll be ordering the 4-footers now that you’ve removed the hesitation.
Thanks in advance for the clarification.
Be well !
Mo

John

Addition to my previous comment about UV Index computation:

On a petshop website selling the 3 lenghts of the Arcadia 14% tubes (22.5, 35 and 46 inch), the UV index at 15 inch is said to be 7.00 (as measured with a Solarmeter 6.5 device – the classic equipment used by repiles owners). Quite a drift from the computed 4.0!

More investigation needed (scaling from 1 to 4 tubes, reflector caracteristics, presence/absence of a protective grid, measurement distance taken from tube vs fixture edge, response curve of the Solarmeter 6.5 device).

h t t p s : / / w w w .roonami.com/arcadia-prot5-uvb-14-dragon-edmonton-alberta-canad.html?id=79441892

John

Please erase my comment on the computation of the UV index. I made a mistake.
I will write back with the corrected value, or questions for your readership, about the computation.
Regards.

john

The formating of my last comment was messed up when I transfered copied & pasted it in the comment form: all the carriage returns were deleted. Please ease it, This is my second attempt – hope it goes fine…
——————————————–
Hi Derek,
I’ve redone my homework concerning the UV Index computations (the first time I was a little carried with excitement, but now I`m pretty sure I got it right):

At 12 inches:
– Single Arcadia 14% tube, UV Index = 7.27 (1);
– Single Agromax tube, UV Index = 122.07 (2);
– Sperti, UV Index = 45.05 (3);
– DIY-Build: UV Index = 23.99 (4).

(1): I’d like to know if you ran this test with a 48″ Arcadia 14% tube, or if you purchased the 24″ version just for the sake of the comparison with the Agromax tube, which I understood was 24″ in length. Furthermore, the reflector type makes a huge difference, and the one in the DIY build seems better than than the one shown in the article photo (which is further impeded by a plastic cover).

(2) I understood this was a 24 inch tube, but please clarify. Same remark as above concerning the reflector quality, and presence/absence of a plastic cover.

(3) I understood you refered to a complete Sperti lamp (i.e. 4 tubes, each one 14.25″ in length),

(4) I understood this to be 4 x 48″ Arcadia 14% tubes, in a fixture.

At 15 inches:
– Sperti UV Index = 31.84;
– DIY-Build: UV Index = 20.01.

Key takeaways:

1. In order to compute the UV index I had to write down the data, point by point from, the graphs, in an Excel sheet. Since I had this data I also ploted severa graphs, and noticed that :

1.1 : when going from 12″ to 15″, at each wavelength, the irradiance value of the DIY build is reduced by a factor of 0.834 factor, and the irradiance value of the Sperti lamp is reduced by a factor of 0.707. The factors are so constant that the graphs completely overlap.

1.2 The values from the Arcadia 14% tube @ 12″ needs to be multipled by 3.3 to get the same UV Index as the DIY build @ 12″ (made of 4 tubes), but using this factor does not align the 2 spectrums perfectly. The match is very good up 305 nm, but begining at 306 nm the DIY build remains higher (by a factor that varies a bit). Luckily, at 306 nm the Erythemal Action spectrum (EAS) value is 0.177 (and going down exponentially fast), so the impact on the UV index is negligible in the part of the spectrumn when the irradiance drift is significant.

2. In order to get the same UV index from the single Agromax tube you tested than from a complete Sperti lamp, you need to stand away from the Agromax tube enough to attenuate its power 2.71 times (i.e get its power at 12″ reduc fed by a factor 0.369). An attenuation by a factor of 0.630 would align the two peaks (which occur at 313 nm), but doing so reveals that the Agromax is richer below 309 nm, and weaker after 313 nm, but the wavelenght that count most for the UV Index are the ones below 313 nm (at that wavelength, the EAS value is 0.039 (i.e. 3.9 %)), giving rise to a total UV Index of 76.91.

4. Predicting the irradiance values at 15″ from the irradiance values 12″ (or the opposite) can be done with good precision from the formula « Irradiance x Distance = constant » in the case of the DIY build, because the sources are cylinder shaped, and we are close to it. When averaging the 2 constants, this gives rise to a prediction carrying a 2.08% error. The further we move away from the tube, the more the model degrades, and at one point the system should be modeled as a point-like source (a sphere, or point-like source, requires using the formula « Irradiance x Distance^2 = constant»). The problem is that the degradation of the model occurs precisely at distances which are of interests for the DIY build).

5. I decided to build the DIY build, and I’m using it, but since I neither have access to an instrument like the Spectrophotometer used in the article, nor access to the proper Solarmeter model, my desire to stay on the safe side limits my capacity to quickly find the sweet spot I’m looking for. It would therefore be of great service to the community if the article’s author could take more measurements, in order to diminish the guesswork required.

Ideally. For each inch from 15 to 36 inches :
5.1. Two UVB Index measurements: one in front of the center of the fixture, and one in front of one of the 2 inner tubes, in the middle of the tube. The room’s ceiling, floor and walls should be covered with a non-glossy black fabric (see why below);
5.2. In a small bathroom covered with white tiles, white walls, white bathtub, toiled, and white everything, and with its miroir covered with a white cloth, take the same measurements, plus another measurement, taken 10 inch away from the one in the center, looking in the direction opposite to the fixture. I thought this was overkill, until I played with the FastRT UV index calculator (what a wonderfull tool), and noticed that by variying the conditions from « lawn » to « fresh snow », the UV Index could be increased by a factor of 1.523 (52.3% increase). Reflexions therefore matters, and usage of the DIY build in such a bathroom is a plausible use case (disclamer: it’s my situation…).
5.3 One last detail : a quality ballast should be used : one ensuring a 1.0 ballast factor when 4 tubes are used (see why below).

In an iteration 2 of a modified DIY-Buid, I would very much like to replace the 4 Arcadia 14% 48 inch tubes with 2 Agromax tubes 48 inch tubes, in order to benefit from the 293 nm wavelenght (the best ones for vitamin D production, but the Arcadia 14% tube produces none), and because finding the right distance, with more powerfull tubes, results in a setup less sensitive to small distance variations (those with huge bellies would find it more convenient!). Before making such a move I would definiterly want to remove any guesswork, because the Agromax tube is one hell of a beast (side note : I noticed that some 4 tubes ballast go from a ballast factor of 1.0 to 1.2, when powering 2 tubes instead of 4. That’s 20% more light than one would expect. This is another important detail to be taken care of, before playing with such a powerful beast)

Thanks 1000 times ! (I’m already noticing health benefits…).

gb

Your contention is that the SINGLE 2 FT AGROMAX is 3x as powerful as the Sperti and 6x the DIY at 12″, with a RIDICULOUSLY HIGH UV INDEX (122 at 12″ and 76 at 15″?) . This would then be a very dangerous bulb. Note: per epa.gov ‘The UV Index predicts ultraviolet radiation levels on a 1–11+ scale’
Yet, the site originator has stated on numerous occasions that this bulb is COMPARABLE in strength to the Sperti (others have stated 1.5x more powerful).
I wonder if there’s anyone on this site that might be able to POST UV index levels for the 2ft Agromax (and some of the other bulbs) at 15″, 18″ and 24″. Personally, I’ve been using the Agromax single bulb 2 ft. IMO, 15″ is too close (though I doubt the UV index is 122).
Thank You

John

“Your contention is that …”

Yup. These numbers are the result of my computations!

I’d be happy if someone else could crunch the number, to see if we obtain the same result. The longuest part is taking the numbers out of Derek`s graph, and type them all into an Excel spreadsheet. The next step is to get the formula for the erethemal action curve. At first I took the GNU plot formula from the wikipedia page, but there’s a (small) error in it. I found that by comparing the formla’s values with the ones cited in the wikipedia article. I few google searches later I found the proper formula. The difference is not dramatic (the formla has undergone a few changes since it’s inception in 1987). Then, it`s integration time. Summing up rectangles is enough (I also did the integration with trapezes, and the disserence is insignificant).

Happy numerical analysis to everyone!

John

My reply to GB was erased ?!?
Can you please tell me what you considered out of place in my post?

John

Sorry Derek, the formatting is still messed up. I`ll try to find what went wrong.

John

Hi Derek, Thanks for your reply.

I gave more thought this week to the reflection problem, and took the decision to change my setup.

Standard white paint has a reflectance of 80% (imagine that of a mirror !), so the 1.523x factor I got with the FastRT UV index calculator might be surpassed, and there is no point trying to compute an UV index multiplier that depends on so many variables (room size, obstacles, mirrors, …).

I was suggesting you take measurements pointing in the direction opposite to the fixture, but now I think just one should suffice, has a cautionary tale (especially for those contemplating using the Agromax tube).

I`ll move the lamp from my white bathroom (with a big mirror), to a place where I can easily hang black curtains, so as to control the conditions.

So, to recap, measurments in controlled conditions (black curtains everywhere), from 16 to 36 inches, would be a huge helper.

Regards.