Apparatus for detecting spectra of light emanating from Polymerase Chain Reaction, PCR, reactions occurring in a plurality of sealed reaction vessels, the apparatus comprising: a 48, 96 or 384 well plate accommodating the plurality of sealed reaction vessels, each sealed reaction vessel comprising a receptacle portion having an emitting area from which light can emanate; a thermal cycler having a block heater; a heated lid arranged to heat and apply pressure to a seal of each reaction vessel; a masking element having an array of apertures through which light can pass, each aperture being smaller than the emitting area of the receptacle portion of the sealed reaction vessel, there being one or more apertures arranged adjacent each of the sealed reaction vessels; a plurality of light waveguides, each light waveguide arranged to guide light from a respective aperture in the masking element to a light detecting device comprising a CCD or CMOS detector, wherein the well plate is held in the block heater between the block heater and the masking element, the light detecting device being configured for simultaneously detecting the spectra in the light emanating from the PCR reactions via the array of apertures; and a further light waveguide for each reaction vessel arranged with a first end arranged in or at a further aperture in the masking element adjacent each reaction vessel and an excitation light accepting end adjacent an excitation light source (32, 33) for guiding excitation light from the excitation light source (32, 33) to each of the reaction vessels, the further light waveguides being joined together at their excitation light accepting ends to receive the excitation light, and wherein the excitation light source (32, 33) comprises a Light-Emitting Diode, LED. a 48, 96 or 384 well plate accommodating the plurality of sealed reaction vessels, each sealed reaction vessel comprising a receptacle portion having an emitting area from which light can emanate; a thermal cycler having a block heater; a heated lid arranged to heat and apply pressure to a seal of each reaction vessel; a masking element having an array of apertures through which light can pass, each aperture being smaller than the emitting area of the receptacle portion of the sealed reaction vessel, there being one or more apertures arranged adjacent each of the sealed reaction vessels; a plurality of light waveguides, each light waveguide arranged to guide light from a respective aperture in the masking element to a light detecting device comprising a CCD or CMOS detector, wherein the well plate is held in the block heater between the block heater and the masking element, the light detecting device being configured for simultaneously detecting the spectra in the light emanating from the PCR reactions via the array of apertures; and a further light waveguide for each reaction vessel arranged with a first end arranged in or at a further aperture in the masking element adjacent each reaction vessel and an excitation light accepting end adjacent an excitation light source (32, 33) for guiding excitation light from the excitation light source (32, 33) to each of the reaction vessels, the further light waveguides being joined together at their excitation light accepting ends to receive the excitation light, and wherein the excitation light source (32, 33) comprises a Light-Emitting Diode, LED.