This invention concerns a method and apparatus of using the relatively thin holographic material of a transmission holographic optical element in such a manner whereby a narrow spectral bandwidth may be achieved. The interference fringe planes oriented through the depth of the material are interrogated by way of the edge of the holographic optical element whereby the holographic optical element is caused to respond, in use, in the manner of a relatively thick reflection holographic optical element of considerable effective thickness. The apparatus takes the form of a narrow bandwidth optical filter which is interrogated by a tightly focussed interrogating beam.