A diagnostic instrument comprises a monochromatic light source, transmission means to transmit light from the light source to a test site, collection means to transmit scattered light from the test site, and spectral analysis apparatus to receive light from the collection means, the spectral analysis apparatus comprising a diffraction grating having a first grating element and a second grating element, wherein the first grating element diffracts light within a first wavelength range and the second grating element diffracts light within a second wavelength range, the spectral analysis apparatus further comprising a light-sensing apparatus, the first grating element arranged to diffract light onto a first area of the light-sensing apparatus and the second grating element arranged to diffract light onto a second area of the light-sensing apparatus.

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