Apparatus and method for using light scattering to determine the size of particles virtually independent of refractive index

In an apparatus for determining the size of particles suspended in a medium, said apparatus being of the type having a means for producing a beam of light;

• a means for exposing the particles to the beam of light, whereby the light which contacts the particles is scattered;
• and a means for detecting the light over a given collection angle after it has passed through the means for exposing the particles to the beam of light, the improvement comprising:
• a spatial filter for reducing the collection angle to less than about three degrees wherein the means for producing a beam of light is capable of producing a collimated beam of light such that the rays of light are all generally parallel to an optical axis, wherein the spatial filter is used in conjunction with a lens placed on the optical axis of the collimated light, the lens being located between the detecting means and the means for exposing the particles to the collimated light, wherein the lens will focus all rays of light which are parallel to the optic axis at a focal point and wherein the spatial filter comprises a fiber optic cable having a diameter equal to the diameter of the beam of light rays at the focal point, said fiber optic cable being located at the focal point of the lens, such that only the rays of light which pass through the focal point will be directed into the fiber optic cable.