If you want a taller plant, you're better off with a different light and/or increasing the amount of light you're giving the plant. By reducing the amount of light, you will immediately slow growth and, over time, you will reduce the amount of inflorescence. That will, in turn, reduce plant yield and the quality of the final product.
This light has a blue heavy spectrum which will help ensure that plants don't outgrow a 2' x 2' space. If you compare that spectrum to other "white" LED's, you'll see that it has a lot of blue in the spectrum and it doesn't have the typical peak around 660nm. I haven't seen the data that breaks out the output by wavelength band so there could be more red than meets the eye but check it against other Vipar lights or Mars or Atreum etc and the spectrum on Pro the is quite different. With all that blue, it looks more like a traditional blue "veg" spectrum than it does a "normal" white LED.
Key point is that light
color drives plant shape; light
volume drives plant growth.
If you drastically reduce the amount of light a plant is getting, it will drastically cut back on the inflorescence and will grow taller to try to get more light but that's just growing an emaciated plant.
If you want a plant that's taller, use a light that has more red in the spectrum. Blue photons inhibit cell expansion, red photons encourage cell expansion.
At 14" and 75% power, you're getting about 700µmols on your plants (light output decreases at about the same rate as power input is reduce). That's a good amount of light because your plants are young (it would help if you put the age of the plant in your postings, BTW). The blue heavy spectrum is keeping the plants short and encouraging lots of inflorescence and short internodal space.
Per the image from Vipar, the light is generating 897µmols at some spot in the center but the readings the 1' square in the center only average, call it, 830 µmols. At 75% power, that's 623µmols. Vipar doesn't give PPFD readings at 16" but we can sorta get there from here.
The drop in PPFD when raising the light from 11" to 14" is about 170µmols. For ease of calculation, that's a loss of 60µmols for each 1" increase in height. By going from 14" to 16", the Pro will be generating about 830µmols-120µmols = 610µmols at 100% power (it's actually lower than that but go with 610). At 75% power, you're going to be getting 610 X 0.75 ≈ 450µmols.
If your first day above ground was 3/6/23, the plant is only 17 days old but it and it clearly is responding well to that the current light levels (and photoperiod). The blue-heavy spectrum will continue to do what blue light does (inhibit cell expansion, resulting in a compact plant) but, because the amount of light has been decreased, growth will slow and the amount of inflorescence will tend to be lower than it would be under higher amounts of light. That will have tend to result in reduced yield.
To grow a healthy plant, turn up the light until the light saturation point is reached (the LSP is considered to be 800-1000µmols in a non-CO2-enhanced environment) which will allow the plant to reach its genetic potential. To grow a tall plant, ditch the blue heavy light (which could well have been designed to keep plants small so they stay in the tent) and switch to a flower light.
The info above is based on research papers on how cannabis responds to light color and/or volume, as well as listening to and studying the transcripts of all videos on that topic on YouTube by Bugbee, de Bacco, and others.
The spectrum for this light per Vipar. Compare it to the veg and flower lights from another vendor and then check the images of the spectra for other lights.
The XS-1500 Pro spectrum, above, is the same as the XS-1500, shown below. I've tested a Uni-T lux meter against my Apogee MQ-500 and the conversion factor, to convert lux to PPFD, is 0.155
Veg and flower lights from another vendor
Dedicated veg light from a well known vendor
