Alright so, let's start off with providing context to everyone who isn't as privy to Underworld's lore as others are, this is going to be a cliff notes version, but it will discuss the basis of how it works.
How Underworld Works
Underworld is a simulation created by RATH to simulate real human life by recreating Fluctlights, or the soul. How this is achieved is through putting Fluctlights in a lightcube and then putting those lightcubes in a lightcube cluster. Underworld is not a game as a game that runs on polygons and 1s and 0s would not be able to handle the amount of data or computing power required for this, so instead Underworld runs off the Mnemonic visual data of all its fluctlights, essentially the shared memories of the souls. This data is sent to the Main Visualizer, which manifests the world and lets everyone exist in a shared memory or dream, essentially.
Due to this though, an unintended side effect is the existence of Incarnation, an ability that processes your willpower or perception of the world in order to warp it and change it to your own liking. This happens because when you have immense willpower, the Main Visualizer is forced to process it, and this forces it to essentially make what you thought was real or happening real.
It'd be too tedious to get every single scan of this, but here is a
blog Xmark made which shows Incarnation's abilities.
This is important to understand because after War of Underworld and at this point in the series, Kirito is connected to the Main Visualizer itself after Higa did so to help Kirito get out of his comatose state. His Incarnation and understanding of the world are IMMENSE at this point.
Underworld Mechanics
Underworld has 3 very important stats that all things possess, and you can view through a Stacia Window. Life/Durability, Object Control Authority, and System Control Authority.
Life is simple, it's the amount of Life you or an object has, damaging something removes life, or even using weapon abilities.
Object Control Authority determines what weapons you can hold, like a strength stat. If I have a OCA of 40 and a sword has one of 30 I can lift it easily, the reverse is also true.
System Control Authority determines what type of Sacred Arts and System Commands you can perform. The higher it gets, the more crazy and ludicrous stuff you can do.
For example, Quinella had the highest SCA which gave her access to commands that let her perform a ritual that hides your memories, resurrect people from the dead, even make Divine Weapons.
OCA and SCA is important, and it can be raised by killing things such as animals or other Underworld denizens.
Once again, important because Kirito has since killed Quinella and gained A LOT of that from his fight with her.
The final mechanic which is important is Divine Objects. There are special items in Underworld that reach such a high level of System Priority (usually above 40) they are divine in the system. These objects can get turned into weapons. Bercouli's was made out of a system clock, Kirito's was made out of the Gigas Cedar, Eugeo's was made out of a Blue Rose that became the Blue Rose Sword
These can manifest a special ability associated with the weapon's memories. In order to manifest these abilities, you usually say Enhance Armament, or Release Recollection.
Enhance Armament enhances the memories within the weapon, partially manifesting its memories and producing an offensive ability, Bercouli's sword lets him create slashes in the future, Kirito's lets him make a big beam, Eugeo's lets him freeze people in blocks of ice.
Release Recollection releases all the memories within, granting an extremely potent ability, Bercouli's lets him slash the past, Eugeo's lets him insta freeze people in a large circle.
Now that you understand this, let's get to Sacred Arts.
Sacred Arts
Sacred Arts are essentially magic in Underworld. In order to use them, you use a voice command and your imagination to use the Spatial Resources/Sacred Power around you to make the command execute.
On the topic of the syntax, I'll make a quick example of how this works.
"System Call: Generate Luminous Element! Lighting Shape. Discharge"
It is a process of a couple of things. System Call (Begins the process, you start visualizing what you want to create) > Generate Luminous Element (You are defining and imagining the element you wish you produce for the command), Lighting Shape (Visualize lightning itself and Luminous elements becoming that), Discharge (Fire off the lightning and release the Sacred Art)
This is a long process, that in every step requires you to use your imagination/Incarnation to actually make it happen.
Now that I've established this, I can finally get to the final part of the arguments and hopefully settle this.
Is your point here that 'magic' in the verse operates by controlling specific properties of objects, rather than simply adjusting the objects themselves, and that this feat should therefore be taken as 'taking the property of luminosity of the sun into Kirito's blade' rather than 'taking a set amount of photons into Kirito's blade'? If so, can you substantiate this with scans? And if it's not what you meant, can you explain what you intended to say?
To explain this, this is 100% how it works. As I explained above, Sacred Arts is a long and involved process requiring oneself to visualize and imagine the properties of things or whatever you wish to command rather than just... adjust it. Throughout the entire process, if you break concentration, the Sacred Power goes away and the command fails. Here is a scan of such.
Once you produce the elements, you have to imagine and reshape them in the form of the thing you want, if you break concentration then it disappears. But as shown with other commands in the series, you have to imagine it taking on new properties rather than just going "Oop I actually want a sword now, go back!" if you were trying to generate a shield out of cryogenic elements. A couple examples of these are.
In a couple of these examples, there's more than just manifesting an object or element. Eldrire creates ice birds, specifically meant to counter Thermal Objects, Kirito has to form cryogenic element into a shield which he shoots forward, and in the final he makes fire arrows that he specifies to fly straight and discharge.
So yes, Kirito would actually have to absorb all of Solus' light, or well the "property" of it as you say, it’s what he was targeting so he can’t just absorb a tiny bit of it, or reshape Solus itself, if he targeted Solus’ luminosity it’d get absorbed wholesale. It also just makes more sense, why go through the tedious process of absorbing light out of this specific area when he could just imagine his sword taking in all of Solus' light, something inherit to the sword's memories and abilities. Why risk not having enough Sacred Power when he can just absorb all of Solus' light. He has the ability to, he stands as the uncontested strongest in the verse at that moment.
Now, onto the Gigas Cedar part about it this, Kirito's sword is made of the Gigas Cedar, an object that had absorbed the most spatial resources from Solus and Terraria (the earth). This lets it absorb Spatial Resources as its Release Recollection, mainly Sacred Power from Solus. BUT ALONGSIDE THIS it had another memory which was Eugeo, Kirito and the other woodcutters looking up at the stars in the sky with it. When Eugeo died, he named the sword the Night Sky Sword, based off how dark it was and how it reflected the pitch black night sky with stars and all. It's a divine object, the point is that it has immense capabilities and powers.
It being a divine object especially helps this because Divine Objects do the thing their memory is associated with. Blue Rose Sword was made from a frozen rose so it freezes things and has ice powers, Time Splitting Sword was made from a clock so it cuts the past and future, Gigas Cedar absorbed so much of Solus’ light and the woodcutters looked at the night sky with it, so it absorbs all of Solus’ light to make that.
The fundamental memories of the Night Sky Sword is absorbing Solus' Light, and the complete night sky filled with stars. There is no reason for us to think it absorbs light out of a specific area based on this. And as I mentioned earlier, it's kind of ludicrous to think that Kirito using the Release Recollection of the sword here is just a specific area when
1. This is the most powerful ability pertaining to the sword.
2. The memories associated are not just light out of a specific area.
3. It would be tedious and stupid to just absorb a bit of light out of this area.
Speaking of point 3, he makes tens of thousands of light elements. Absorbing the light of this specific area would certainly not give him enough to makes tens of thousands of light elements. Tiese says she as much that she doesn't have the ability to beat the Dark Minions because she can't make that many light elements, much fewer ones so potent that they almost blind you.
Conclusion
I'm hoping this is the final argument, I had to grab a lot of shit to do this, but I appreciate everyone's support and analysis. If anything, this has helped me further understand SAO... I feel as if I've awakened to something... I feel like I'm on a high after writing this much.