Isaiah 6:1, "In the year of King Uzziah's death, I saw the Lord sitting on a throne, lofty and exalted with the train of His robe filling the temple."
This is confirmed by the Apostle John at John 12:41, "These things Isaiah said, because he spoke of Him." At John 12 Jesus is addressing the Jews and John says at John 12:36, "These things Jesus spoke, and He departed and hid Himself from them."
Vs37, But though He had performed so many signs before them, yet they were not believing Him; vs38, that the word of Isaiah the prophet might be fulfilled, which he spoke, "Lord, who has believed our report? And to has the arm of the Lord been revealed? vs39, For this cause they could not believe, for Isaiah said again,
Vs40, "He has blinded their eyes, and He hardened their heart; Lest they see with their eyes, and perceive with their heart, and be converted, and I heal them."
The verb Isaiah used for "saw" in Isaiah 6:1 is (ra'ah). In the qal, it refers to the act of seeing in the literal sense, to see with the eyes (as opposed to, for example "machazeh," which is the act or event of an ecstatic "vision.) In referring to this event, John uses the Greek word (eidon), also a verb referring to the act of seeing with the eyes in the natural sense.
We know that God the Father is invisible, "whom no man hath seen, nor can see" (1 Timothy 6:16). He is transcendent and lives in unapproachable light (1 Timothy 6:16). But the Son is "the image of the invisible God" (Colossians 1:15).
Thus the one whom Isaiah "saw" in the literal sense with his eyes is the one whom he explicitly identified as "YHWY", the same one whose glory he saw according to John at John 12:41. Jesus Himself makes this clear at John 12:45, "He who beholds Me beholds the One who sent Me."