Method of optical analysis of a fluid including an optical waveguide

In a method of optical analysis of a test sample labelled with a light emitting fluorescent, phosphorescent or luminescent material, a part of said test sample being bound, directly or indirectly, to the surface of an optical waveguide in contact with sample liquid, said method comprising measuring light emitted from the bound test sample which has entered and propagated through said waveguide and emerged from an edge thereof at an angle less than .alpha. relative to the optical axis of said waveguide, wherein

• .alpha..apprxeq.arcsin(n.sub.2.sup.2 -n.sub.1.sup.2).sup.1/2,
• n.sub.2 is the refractive index of the waveguide and n.sub.1 is the refractive index of the sample liquid, the improvement comprising placing an optical structure close to the edge of said waveguide so as to collect light emerging therefrom prior to said measuring, said optical structure comprising a propagating layer of transparent material of refractive index n.sub.3 interposed between other layers of transparent material of refractive index n.sub.4, n.sub.3 and n.sub.4 being such that
• n.sub.3.sup.2 -n.sub.4.sup.2 .apprxeq.n.sub.2.sup.2 -n.sub.1.sup.2,
• the propagating layer being constructed so as to collect light emerging from aid waveguide and propagate that portion of light emerging from said waveguide at an angle less than .alpha., and the other layers being constructed so that the major part of the light passing into them from the propagating layer, which light is desired to be excluded from said measuring, does not pass back into the propagating layer, and measuring the light propagated through the propagating layer.