For a 4x 4' area you need two 4ft. bulbs. Below is a test with an Arcadia 2ft bulb above a 2x 2' area and different reflectors. The stucco sheet reflectors seems to have more spread while polished aluminum has higher intensity.
There is no need to convert the daily dose as long as you take 4 times as much on a 4x 4'. Because the 4footers are 54w you get anyway a bit more like from 2 2ft bulbs.
A friend of mine sended me a few PureUV bulbs via USPS but shipping and custom fees has more than doubled the price. These bulbs are so intense that you need at least ~24"/60cm distance(∅170μW/cm²). Some users mounted them directly under the ceiling to get usable intensity. Intense reptile bulbs (10-14%) need at least 12"/30cm to reach this level and the softer 5-6% bulbs can literally run all day long without issues.
I still have some stucco reflector sheet of which I plan to make a kind of long wing reflector for better distribution and I will put a L-profile (10x 10mm) like a super-spreader in front of the bulb, so that the plants get only indirect UV light directly below the bulbs. This way I hope I can use them in 16-20" without burning their asses off, lol! Another way to reduce their intensity is a finely woven fabric, just like curtains. Reptilians use such fabrics to halve the high intensity level of new bulbs.
The dose princip is reciprocal, you can use a high powered bulb for a short time or are low powered bulb for a long time and get the same results. But here is a problem!
Each UVB lamp takes up to 5 minutes to reach the specified values. Right after power up the level can be much higher. 5 minute intervals are virtually impossible to control if you do not plan to buy a broadband UVB meter. But such devices currently cost around $ 250! The Solar-Meter 6.2 is currently the best one because it most weighted the 280-300nm range (important for vitamin D3 formation). For us, this area is most important too because it is completely absorbed or reflected by thc acid and flavonoids without causing any damage.
Wavelength below 280nm do not exsist but wavelength above exsist and this wavelength seems to harm the plant. Each UVB bulb produce unfortunately much more UVA than UVB and the output is almost stable over it's lifetime. I'm pretty sure it's the unnatural high amount of UVA what can harm the plant.
At the foot of Mount Kilimanjaro at an altitude of 1800m above 0 the UVB radiation reaches up to 480μW/cm² for 1h at noon and the average of 6h is over 300μW/cm² per hour. Such levels should not harm the plant but if you use so much indoors you'll see burnings within a few days.
I am quite happy that I first used a reptile bulb to gain some experience. Getting started with Agromax bulbs would definitely have destroyed one or more runs because I've used the bulb in the same height like my LED's.
Strips in CRI90 does not exsist, at least not with LM301b or 561c. Only Bridgelux has the Vesta series in CRI90, which is a two spectrum doulbe row strip. One side is 2700°k(129lm/w) and the other side is 5000°k(135lm/w). Each side is driven seperately on an own driver which allows to mix the spectrums. Unfortunately Bridgelux used their smd2835 gen.1 diodes and for this reason they only reach EBgen1 efficiency. You need to drive them realy soft to get comparable efficiency levels.
But you can add deepred to get a higher CRI. If you add 7-10% deep-red to a white CRI80 spectrum the spectrum shifts towards CRI90..
Here is a calculator screenshot with 6 Zeusboards and ~10% deep and far-red. The latter has less effect on CRI but it's added for other reasons.
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