As I've never actually put the differences into words before, I'm going to defer the broader answer and give that some consideration and get back to you (if I may). However, when I see those last two words (in use), I realize I can nail that right now, from the hip. Since I use them both every day, I know exactly how I'm using them, even if explaining why may require more thought.Between the 519A 4500 and the SST-20 4000, can you describe the difference between them, in use?
First, I'll emphasize that I could be happy using either, 24 hours, and in most situations, and have been. That said, the 519A is generally in my pocket in daytime hours / when ambient daylight is involved. In the later afternoon hours, I generally switch that and put the SST-20 in my pocket for use the duration of the night. When I arise the next day in daylight, I generally switch that back to the 519A. I'll speculate that most of that could be the influence of the difference in color temperature, which may not be large numerically, but is distinctly noticeable and therefore significant in use, especially when mixed with typical daylight CTs, vs my typical indoor ambient CTs, and perhaps also the lower indoor ambient levels my eyes are functioning in particularly in the evening. I've also noted that the SST-20 renders my 1940s wood furniture pieces marginally more accurately. This becomes important outside.
Outside, at night, there is a distinction I can make that is fairly small, but fairly important. Last week I went out twice to just to evaluate several lights in our local area terrain (something I don't often do), including examples of ostensibly these same two emitters. Again, either can do this job well. I find the SST-20 4000K literally provides me with, at least marginally, but distinctly, higher resolution / accuracy vision of those surroundings than the 519A 4500K. The SST pulls ahead just a bit in how it distinguishes, and renders all the very subtle, as well as large, differences in coloration / contrast / shading, and the borders of them and the details revealed by them. As you know, resolution is P1 out in our local desert surroundings at night, so that's a case where I although could do fine with either, I do have a clear choice.
There's also a somewhat distinct difference in the emission / 'beam' pattern w/ these 2 LEDs in these lights (the Cs) with the same reflector, and you might have a preference on that as well. I'll try to A-B them some this weekend and comment more on that aspect. I need to get them both outdoors together. The central concentration / hot spot of the 519A is generally larger, and the SST-20 generally smaller. I need to see these in more space than my small LR to describe that better in terms of near field illumination coverage vs reach, transition, etc.
BTW, the box label lumen ratings on the 25C w/519A are 565/395 (LED and ANSI FL-1 [aka OTF] respectively). Those ratings on the 25C w/SST-20 are 475/335.
EDIT: One typo
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