A 600w HPS has a quantum efficiency of ~1.5umols per watt in comparison your LED barlight doubles that.
Then, there's 3 other major differences, the HPS light has a much better penetration or leaf transmittance and its light recipe additionally causes longer internodes and that altogether makes the plant gather more light as a whole. Also the whole plant's oxygen evolution (a direct measurement of PS2 photochemistry) will be higher.
It's because the HPS delivers a complete package of yellow/orange light complimented by very little blue, farred and much heat (IR).
In stark contrast that LED has much blue, very high red but only very little farred and no IR, all things considered creates a stocky short plant with leaves upon leaves shadowing one another. If you ever think about improving that spectrum think about a farred extension kit on a separate time that will cut down your veg-time by weeks, and give a sleep initiation, too.
But the highest priority right now I see is to dial in the temperature right. An IR heater is best because it will heat up the leaves and not the air (which is mostly vented out anyway). Or you can manage it somehow to heat up the air before the intake.
Another method oftentimes used is to blow down the air from above the LED and have both a cooling effect (= diodes more efficient) and a plant benefit. Some LED growers even reverse the airflow and vent out from below to get that effect
Yes shade avoidance and the emerson effect. But i feel differently about the penetration and stretch being more beneficial under HPS.
Green provides great penetration and leaf transmittance and LED provides a lot lot more in that spectrum.
The stretch under HPS while having some benefit i feel as though it provides a negative to photosynthesis unless in natural sunlight because we are having to compensate intensity from a single stationary source above the plants for the tops and that means the lowers are getting far less. So while the slope of intensity fades slower under HPS (i attribute to the focus or angle of the source eg. 120 degrees under most LED and that’s only my thoughts as best as i can describe it at the moment not facts) the more compact structure under a more blue spectrum more than makes up for that and in turn imo increaseing penetration in that sense because the lower height we dont need to compensate for the intensity at the growing tips as much and overall the plant receives a higher intensity to more leaves… i hope that makes sense as its hard for me to sometimes put my thoughts into understandable text.
Add the large increase in green spectrum and i feel LED provides more penetration in a round about way.
I may be missing something and i know you may have an explanation for me so i wanted to take the opportunity for the discussion on it if you dont mind.
I also feel that IR is an iption but some newer studies have me retginking its benefit.
Also for the record if we are getting into the ravbit hole my opinion is blue leaning veg transitioned to a slightly red leaning flower followed by a very blue leaning finish. Which is where i seen tuneable LED coming more into the grow industry in the near future as we gain more knowledge in this area of all the impacts of spectral difference can make in growing