Exegesis:
ho luchnos tou sōmatos estin ho ophthalmos sou ‘the lamp of the body is your eye,’ ho ophthalmos is predicate. The discourse shifts from the ordinary lamp, which gives light from the outside, to a figurative extension of the word ‘lamp,’ here called ‘the lamp of the body,’ which gives light from within.
hotan ho ophthalmos sou haplous ē ‘when your eye is sound.’
haplous lit. ‘single,’ ‘simple,’ hence ‘sincere,’ ‘sound.’ Its opposite is ponēros ‘evil,’ here ‘unsound.’ Probably both ideas (sincerity and soundness) are present here.
kai holon to sōma sou phōteinon estin ‘your whole body too is full of light.’ phōteinos also v. 36.
kai to sōma sou skoteinon ‘your body too is dark.’
Translation:
Your eye is the lamp of your body, though worded as if addressed to Jesus’ interlocutors, holds true of men in general. Accordingly the two possessive pronouns have a generic function, which may have to be expressed otherwise, e.g. by using pronouns of the first person plural inclusive (Huixtec in Mt. 6.22), or by substituting an article or equivalent form at one occurrence (e.g. “your eyes are like a lamp for the body”, Good News Translation), or at both (as in Javanese, Sundanese, Sranan Tongo). The preposition of may have to be rendered by a verb, e.g. ‘the lamp that lights your/the body.’ For body it may be necessary to shift to a term for ‘inner being.’
Sound and unsound or, “sound” and “bad” (New English Bible), ‘healthy’ and ‘not healthy,’ ‘clear’ and ‘turbid/dim’ (Willibrord, Balinese, Sundanese).
Your whole body is full of light, or, changing the syntactic structure, ‘your whole body has light, or, is bright,’ ‘there is light in your whole body.’ Parallel changes will then be necessary in your body is full of darkness.
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.