Systems for the monitoring of bacterial levels in samples, using spectral analysis of the light diffracted from a substrate with an ordered array of pores having diameters enabling the targets to enter them. The trapping pore array is cyclically illuminated by light of different wavelengths, and the light diffracted from the pore array is imaged by a 2-dimensional detector array, with one pixel, or a small group of pixels receiving light from each associated pore. The temporal sequence of frames provides a series of images, each from the reflection of a different wavelength. A time sequenced readout of the signal from the pixel or pixels associated with each pore region, provides a spectral plot of the reflected light from that pore region. Spectral analysis of the light intensity from this series of different wavelength enables the effective optical thickness (EOT) of each pore to be extracted.
