1. A lighting device comprising:a carrier having an upper surface and a recess comprising a gasket,
an optical device for imparting a shape to a light beam,
a light source located on the upper surface of the carrier for emitting light towards the optical device,
wherein the optical device has a base, the base comprising:
a first surface for determining a distance from the light source to an exit window of the optical device by engaging with the upper surface of the carrier, wherein the distance provides a predetermined light characteristic of the lighting device, and
a protrusion extending into the recess of the carrier, the protrusion terminating at a bottom face,
wherein the recess has a depth that is greater than the length of the protrusion, such that a gap exists between the bottom face of the protrusion and the bottom of the recess, wherein at least part of the gap is free of any part of the gasket, and
wherein the gasket directly acts upon a protrusion side wall to provide a seal between the carrier and the optical device.