light, clear
烱 centers on brightness and clarity: light that illuminates and makes things clear. The two given meanings are closely linked, with 'clear' being a natural extension of 'light'.
烱 combines 火 (fire) with 冏, which likely contributes the sound and may relate to brightness or openness. The exact historical development of 冏 is uncertain.
Fire 火 on the left and the open, bright shape 冏 on the right: imagine a fire casting light into an open space, making everything clear and bright.
For ケイ, picture a bright light shining like a KEI (a made-up name pronounced 'kay') beam: KEI -> ケイ, and the fire 烱 makes everything clear.