WAY TO GO, KATHERINE!!
Fantastic detective work there! How long did it take you to figure that out? I had been looking for days and was stumped!
As far as the light levels, I don't fuss with footcandles either... I just move plants around until they're growing and flowering enthusiastically... then call that light "catt light," "phal light," etc. I currently have this one in "display light," which is basically where I put plants to preserve their flowers as long as possible, so that wouldn't help you at all for growing purposes.
However, while this was the only 'Sonia' the store had (I know it's not official, but I'm going to call it that for now), it did have a sibling-type that was on clearance and had finished its flowering cycle. The canes looked identical to my Sonia... same fat, squat canes... so I snatched that one up, too. I don't know if it will be an exact clone of the 'Sonia' when it flowers or if it will be a variation, but I'm hoping for something slightly different so I don't have two that are EXACTLY the same, but that are similar.
Anyway, I can tell you what light the sibling likes. I have it baking on my hot windowsill in full afternoon sun, next to my Vandas, and it is quite happy there. It can take quite a lot of heat and light without flinching. It's currently growing a new cane on that hot windowsill. My kingianum started drooping and wilting in that light, so I know it can comfortably take more than a kingianum, and my Vanda coerulea started to show some signs of sunburn a couple of days ago, yet this Den that was right next to it was fine. So these guys are pretty hardy monsters. But I imagine they probably don't NEED that much light. I just try to put plants on that windowsill if they can handle it because it's a huge windowsill and it conserves space in other areas of the house for me. So I put Dends there and then watch how they do to see if they can handle the light and heat. And this one can (all the Catts I've put there have been miserable, though, so I could safely say this plant likes more light and heat than Catts.)
But like I said, it probably doesn't NEED that much light, and I imagine if I put it outside in 90 degree heat with no air movement and sun beating on in for a full day, it would probably fry. Right now it gets direct sun from about 1:00 to 5:00 on the "hot" windowsill. There is no shade cloth involved, so it gets REALLY hot there. But I have air conditioning vents directly below that windowsill, so there's constant air blowing strongly through the area, which helps keep the plants from frying.
After the 'Sonia' drops its flowers I will slowly introduce it to the hot windowsill too and see how it does (although by that point I think I'll have my plants outside in the summer sun anyway). I have a feeling it will behave just like its sibling and start enthusiastically growing canes with lots of sun. I'll report back and let interested parties know how the 'Sonia' does in the sun once the flowers have dropped!
Okay, so I'm getting the impression here that 4" x 5" Dendrobium flowers are NOT the norm, huh? Probably why I haven't come across any until now. So mine is apparently just an incredibly gorgeous freak of nature? LOL It is absolutely the center of attention in my house... the flowers are larger than anything else blooming anywhere in my collection right now, and the color is so bold that it just commands attention! The flowers have a "sparkly" appearance when the light kisses them, too, which is just delightful.
I can't wait to see what this one's sibling does when it flowers. If it turns out to be an exact clone of the 4"x5" one I already have, I can list it in OrchidBids if anyone is interested in it. Obviously we can't OFFICIALLY call it a 'Sonia,' but it's gorgeous enough to be included in a collection even without an ID!
Again, way to go Katherine on finding that plant! I don't know how you managed it!