A lid for a multiwell plates which allows improved light scattering measurement of liquid samples within the wells of a multiwell plate, and which at the same time mitigates evaporation from said samples is disclosed. A surface element protrudes from the bottom of the lid into the fluid in a well. The protruding element may be hollow or solid, and the beam of light, directed into the element may act to capture or direct the beam while preventing backscatter from reaching the light scattering detector or detectors. The protruding element may thus direct the beam from the well without the beam having to pass through a fluid/air interface. The angle and shape of the lid surfaces may be optimized to minimize or eliminate back-reflection. Light absorbing and/or light blocking colorization may also be employed to minimize or eliminate back reflection. Evaporation is controlled by physically capping the well with the lid, either sealing against the face at the top of the well or the inside surface of the well.