An optical instrument uses light to form an image, measure a property or control a beam. An optical telescope collects and redirects light from distant objects; a spectrometer separates wavelengths; a refractometer infers refractive index from a measured direction or boundary.
Its usefulness depends on more than enlargement or brightness. An image requires controlled optical aberrations and adequate optical resolution, while a measurement requires a known relation between its readout and the desired quantity. Blocking the input beam, measuring a reference sample and changing one optical setting at a time help distinguish specimen signals from stray light and photodetector offsets.
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