Your plants look really good regardless of having only 200 watts in the tent.
A hang height increases, PPFD drops but the PPFD maps evens out. Check the PPFD map of any manufacturer/vendor and you'll see the pattern.
I tested Photone when I was named Korona and it is accurate for some iPhone models. I recommend it if you can calibrate it against a known good source and if you're using either the diffuser that they sell or if you're using a 22# strip of paper as a diffuser. If not, your numbers are going to be very far off.
A key point to remember with a light meter is that all it tells you is the PPFD of your light. That's good to know because it allows you to get your light levels in the right ballpark but only your plants can tell you how much light they can use. We know that yield increases in a linear/an almost linear manner as light levels increase so it pays to get as much light on your plants as they can handle, regardless of what your meter (or Photone) is telling you,
"1 gm/watt"- not sure about the thought behind that. Perhaps a rule of thumb? Before the days of LED, yield was estimated based on the input wattage for the HPS lamps. You can get a rough idea of the output of an LED from the input wattage but that concept is only valid for starting the process of buying a light because LED grow lights vary significantly from one model to another and, also, based on the spectrum of the light. Red diodes, for example, are more electrically efficient than blue. There is a difference in yield between the different wavelengths. At common PPFD levels, the difference falls into the "measurable but insignficant" category.
With the advent of LED's the yield estimate for cannabis has changed to 0.2 to 0.3 gm of flower/square meter per mol of light
that the grow has received over the course of the grow,
With $$ burning a hole in your pocket, you're tempted to get another light and the question you're asking is about the cost benefit. At this point, on a pure cost benefit basis, you're going to get only a very small increase in yield because it's so late in the grow.
Second, I wouldn't get another small light.
You have a pretty good sized tent and the SF100, like the other small light I discuss below, is designed for small tents. Two key elements in the design - they're board lights and they have a spectrum that's quite blue heavy.
The latter issue-most 2' x 2' tents aren't very tall and blue photons keep plants short, among other things. But lights for 2' tents have a lot of blue, one reason being that will stop plants from growing into the top of the tent. That's a great idea, for people how have small tents. That's
not you.
Another issue is that board lights tend to run hot. That's simply a function of how they're designed and no manufacturer can change the fact that ventilation in a board light.
Another issue with board lights is that they have a hot spot in the middle. You've run smack dab into this.
Finally, board lights are the lowest tech lights on the market. They're a very old design and while some companies are enhancing them, they're an inexpensive product that will not see any signficant innovation. From a product design perspective, they're so low cost that there's just very room to add features compared to lights for larger grow spaces.
My tuppence—save your $$ until you can get a
Spider Farmer® G4500. It's an excellent light. It's a bar style light so not only does it put out a lot of light, it also puts out a very even spread of light (the "PPFD map"), it runs cooler than a board light, the driver is detachable, and the spectrum has a much higher percentage of red than the SF100. The icing on the cake is that they just put out a new model that's controllable by Bluetooth and/or WiFi. I don't know of a better light for a 2' x 4' tent and I've spent
a lot of time researching that size light (the lights I use are discussed below).
If you get the larger light, you will be able to use your current lights as "fill lights" because you've got a tent that slightly larger than 2' x 4' and seeing that you're been astute enough to get a light meter, I have a hunch you'll find a way to use those extra lights.
I really think that Spider is the way to go but if you want to stay with a 2' light, I'd recommend the Vipar Spectra XS1500 Pro over the SF100. If you look at the PPFD maps below, from the manufacture web sites, you'll see a significant difference in light output between the two lights. At 12", the SF100 has good light output at center mass but the operative word for the corner readings is that the light levels "collapse". In contrast, the XS 1500 Pro is generating 1kµmol in the center and 810 in the corners.
Both readings center mass are BS. There's a tiny spot right under the light that hits that value so the manufacturer's d*ck wiggle to get a big number but, the reason why the XS 1500 Pro is so good becomes apparent when you disregard that number. The key point is that the PPFD map in the XS 1500 Pro is show a tiny amount of variance across the entire area. Vipar did a great job with this light.
Full disclosure-I'm not a Vipar fan boy. I have the Viper XS 1500 but I only use it for seedling stage and as a "fill light". I use a Growcraft X3 veg light, then switch to their flower light, and I use a Mars
SP3000 as a fill light, as well.
Your plants are very impressive and, with more light, your yield will increase. It's a little late for you to get a boost in yield for this grow but, regardless of which light you end up with, you will see a significant increase in yield with more light. That's just how cannabis works.
Going with the G4500 will give you a bit more light than 3 x 100 watt lights because the G4500 generates more photons than the smaller lights but you'll also get taller plants because the G4500 has less blue in the spectrum.
Forgot to mention this - blue photons inhibit cell expansion (that's why veg lights keep plants short) but there's a secondary impact in that blue photons tend to reduce yield. Sure, you get short compact plants but, if you're running a high % of blue, that reduces yield because "blue photons inhibit cell expansion".
gottem in where they fit in lol.
