The invention provides a technique for increasing the illumination intensity of probe light in a diffusely scattering sample without increasing the power of the probe beam. Generally, an optical filter is used which permits a collimated probe beam of light to pass through to the sample, but which reflects back towards the sample much of the backscattered scattered probe light emerging at a wider range of angles. In particular embodiments a collimated laser beam is delivered to the sample through a multi-layer dielectric filter covering a portion of the sample. The filter is transmissive to the laser light at normal incidence, but reflective at shallower angles of incidence characteristic of the backscattered light.