The tejas in aditya — the sun — that lights the entire world; the tejas in chandra and agni — moon and fire. Viddhi — know — that tejas is mamakam — Mine.
Krishna re-locates the cosmic luminaries' light. The sun does not light by itself; the moon does not light by itself; fire does not light by itself. The tejas they apparently radiate is Krishna's, lent to them.
Shankara reads tejas as chaitanya-atmaka jyotis — consciousness-light. The same awareness that shines equally through all beings appears more manifestly in the sun and moon only because their sattva-guna is denser. The light is not three different lights from three different sources; it is one light, manifesting through three sattva-saturated apparatus. The bhakta who recognizes this stops attributing primary luminosity to the heavenly bodies.
Madhusudana reads 15.12 as the explication of the second half of the Mundaka shruti — tasya bhasa sarvam idam vibhati: by His light all this shines. Every tejas — solar, lunar, fire — is chaitanya-jyotis, the one brahma-consciousness shining through sattvic mediums of varying purity. Yet this is also Krishna's own vibhuti — the lordly manifestation by which the bhakta recognizes Bhagavan in cosmic phenomena. Both registers hold: chaitanya for jnana, vibhuti for bhakti.
Ramanuja reads the tejas of sun, moon, fire that illuminates the entire world as Krishna's own radiance, given by Him to those luminaries after they have worshipped Him.
Bhagavan's real and direct gift of power to dependent realities: the sun and moon are not independent sources but recipients of Ishvara's grace-radiation, which He confers as their inherent capacity. Aditya, chandra, agni are devata-bodies of Bhagavan; their light is His light through them.
Madhva reads the verse as prapanchayati — unfolding in detail — the jnana stated before. The tejo in sun, moon, fire that illuminates the whole jagat is mamakam — Hari's own, belonging solely to svatantra.
Jayatirtha presses: since Krishna is one and the lights are many, how is the tejas attributed to Him alone? Answer: the lights are paratantra-bodies; their luminous-capacity is Hari's. The bheda is preserved; the dependence is named.
Vallabha reads the radiance in sun, moon, fire as a portion of Krishna, the one sat-ananda-chid-rupa-dhama. Even the insentient transformation of matter that produces light is His vibhuti. Even the achit side of light — the dispersal of darkness that enables the senses to function — is Bhagavan's prasada-gift. The pushti-marga reading: the bhakta sees Krishna's lila in every photon.
Shridhara reads after establishing in the prior verses that Bhagavan's supreme dhama is beyond the light of sun, moon, fire, the Lord now reveals His infinite-power form. The very tejas by which sun, moon, fire illuminate the world is nothing other than the Lord's own radiance.
Shridhara reads the four verses 15.12-15 as a connected demonstration of ananta-shaktitva: the bhakta who sees the world's light correctly sees Krishna in it, which is itself a step toward the chapter's recognition.