Exegesis:

kai egeneto … to eidos tou prosōpou autou heteron ‘and the appearance of his face became different, or, changed,’ without indicating the nature of the change. But according to v. 32 the disciples saw Jesus’ doxa, i.e. ‘his bright splendour’ (cf. on 2.9). This is also consistent with what follows in v. 29. For eidos cf. on 3.21.

kai ho himatismos autou leukos exastraptōn scil. egeneto ‘and his clothing became white (and) gleaming.’ For himatismos cf. on 7.25. leukos exastraptōn without connective. Most translations render it as one phrase and treat exastraptōn as an adverb modifying leukos (cf. Revised Standard Version). leukos.

exastrapto ‘to flash, or, gleam like lightning.’

Translation:

The phrase as he was praying is more than a simple indication of time here; the verb is repeated to bring out the connexion between Jesus’ communion with God in prayer and his transfiguration. The aspect is durative, ‘during his prayer,’ ‘there he prayed for a time, then….’

The appearance of his countenance was altered, or, ‘the appearance of his face took another form, or, became different (lit. it one kind)’ (East Nyanja, Trukese), ‘the shadow/picture of his face was another one’ (Yao). To render ‘appearance’ one may have to shift to a verbal construction, ‘his face was seen to have become different’ (cf. Tae’ 1933), ‘it became visible that his face had changed,’ ‘his face appeared manner of otherness’ (Kituba). Some versions slightly simplify the expression, e.g. ‘his characteristic-appearance changed’ (Balinese), ‘his form/shape/appearance/face changed’ (South and Tae,’ Shona 1966, Sinhala, Ekari, Sranan Tongo). Tboli says ‘he became different,’ because ‘his face was changed’ is an idiom to say that a person is dying. The translation to be used will often have to coincide with, or come close to, that of “he was transfigured”.

Raiment or ‘clothing,’ cf. on 7.25. A more specific term may be used, where necessary, provided it refers to an outer garment.

Dazzling white, or, ‘white and shining/sparkling’; some versions render the phrase by one term expressing a high degree of splendour/brightness/whiteness.

Quoted with permission from Reiling, J. and Swellengrebel, J.L. A Handbook on the Gospel of Luke. (UBS Handbook Series). New York: UBS, 1971. For this and other handbooks for translators see here . Make sure to also consult the Handbook on the Gospel of Mark for parallel or similar verses.