A device (10) is provided for measuring at least one characteristic (12) related to presence of particles. The device (10) includes a light source unit (16) for emitting light into a region (18) containing particles (20), with the light source being configured to emit light from a plurality of locations in a manner so that the emitted light follows a desired intensity distribution (22), the desired intensity distribution (22) being desired for the measurement of the at least one characteristic (12). In preferred embodiments the intensity distribution is Lambertian.