light, clear
炯 centers on light that is bright and clear, often describing a penetrating clarity or a shining quality.
炯 combines 火 (fire) with 冋, which likely contributes the sound and may suggest a bright opening or distant light. The exact historical development of 冋 is uncertain.
Fire 火 beside 冋, which looks like a bright opening or a window frame, gives the image of a clear, shining light.
For ケイ, imagine a bright light shaped like the letter K: K -> ケイ, shining clearly through the 冋 opening.