The present disclosure relates to light arrangement for an optical device for measurement of an index of refraction, having a light source, a fiber bundle arrangement for transmitting light from the light source, a diffusing member, and imaging optics for transmitting the light to a measuring window. In order to provide for an arrangement which is durable and accurate even when used for measuring hot specimens where the light source is positioned far from the measuring window, the fiber bundle arrangement includes a novel combination of a first fiber bundle and a second fiber bundle.