A.T. Robertson Commentary Luke 11:36

"If therefore thy whole body be full of light, having no part dark, it shall be wholly full of light, as when the lamp with its bright shining doth give thee light." — Luke 11:36 (ASV)

With its bright shining (τη αστραπη). Instrumental case, as if by a flash of lightning the light is revealed in him. See on 10:18.