"that tender light" in Byron's poetry

0 To what does "that tender light" refer to in line 5 of Lord Byron's 01u00She Walks in Beauty:02br
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00She walks in beauty, like the night 02br
00 Of cloudless climes and starry skies;02br
00 And all that's best of dark and bright02br
00 Meet in her aspect and her eyes:02br
00 Thus mellow'd to that tender light02br
00 Which heaven to gaudy day denies.02br
00One shade the more, one ray the less,02br
00 Had half impair'd the nameless grace02br
00 Which waves in every raven tress,02br
00 Or softly lightens o'er her face;02br
00 Where thoughts serenely sweet express02br
00 How pure, how dear their dwelling place.02br
00And on that cheek, and o'er that brow,02br
00 So soft, so calm, yet eloquent,02br
00 The smiles that win, the tints that glow,02br
00 But tell of days in goodness spent,02br
00 A mind at peace with all below,02br
00 A heart whose love is innocent!0-