bright, luminous, glorious, clear
昞 centers on radiant light and clarity: something bright, luminous, glorious, or clear. The meanings all flow from the idea of shining visibility.
昞 combines 日 (sun) with 丙, which likely contributes the sound and may have originally depicted a bright, spreading light. The exact historical development is uncertain.
The sun 日 on the left shines beside 丙, a shape that suggests spreading radiance. Together they make everything bright and clear.
For ヘイ, imagine a bright hay bale glowing in the sun: hay -> ヘイ, and its luminous golden color makes the field glorious.