An optical spectrometer is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. The variable measured is most often the irradiance of the light but could also, for instance, be the polarization state. The independent variable is usually the wavelength of the light or a closely derived physical quantity, such as the corresponding wavenumber or the photon energy, in units of measurement such as centimeters, reciprocal centimeters, or electron volts, respectively.

स्पेक्ट्रमलेखी या स्पेक्ट्रोग्राफ (spectrograph) एक उपकरण है जो प्रकाश या अन्य किसी विद्युतचुम्बकीय विकिरण में उपस्थित अलग-अलग तरंगदैर्घ्यों की मात्रा को मापकर उसे रिकॉर्ड करता है।