spectrometer

History

The first prototype of a spectrometer was a glass prism used by Isaac Newton in 1666 to decompose sunlight. Later, Joseph von Fraunhofer added a diffraction grating and saw dark lines in the Sun's spectrum.

How it works

Light passes through a narrow slit and hits an element that separates it by wavelength—this could be a prism (refracts violet more strongly) or a diffraction grating (reflects different colors at different angles). The detector records how much light arrives at each microscopic section. This constructs the spectrum—a graph of brightness versus wavelength.

History

The first prototype of a spectrometer was a glass prism used by Isaac Newton in 1666 to decompose sunlight. Later, Joseph von Fraunhofer added a diffraction grating and saw dark lines in the Sun's spectrum.

How it works

Light passes through a narrow slit and hits an element that separates it by wavelength—this could be a prism (refracts violet more strongly) or a diffraction grating (reflects different colors at different angles). The detector records how much light arrives at each microscopic section. This constructs the spectrum—a graph of brightness versus wavelength.

History

The history of the spectrometer began with the experiments of Isaac Newton in 1666, who decomposed sunlight with a prism. Newton introduced the term “spectrum,” calling the colored stripe a “ghost” (spectrum). In 1814, Joseph von Fraunhofer, replacing the prism with a diffraction grating of his own making, discovered dark lines in the solar spectrum (Fraunhofer lines) and measured their wavelengths. This laid the foundation for astrospectroscopy. In 1859, Gustav Kirchhoff and Robert Bunsen created the first spectroscope with a gas burner and prism, linking spectral lines to chemical elements. In the 20th century, the development of electronics led to photoelectric registration. Today, space observatories are equipped with spectrometers operating from X-ray to infrared ranges (for example, the STIS instrument on the Hubble telescope).

How it works

The principle of an optical spectrometer: the radiation under study passes through an entrance slit to a collimator, which forms a parallel beam. Then the beam hits a dispersing element—for example, a reflective diffraction grating with rules. According to the diffraction condition d(sin α ± sin β) = mλ, where d is the grating period, α and β are the angles of incidence and diffraction, m is the order, light of different wavelengths is deflected at different angles. Then a focusing mirror forms images of the slit in monochromatic light on the detector (CCD line or array). The signal intensity from each pixel is proportional to the photon flux of the corresponding wavelength. Calibration converts pixel number into wavelength. An alternative type is the Fourier spectrometer: it uses a Michelson interferometer, where the interferogram (intensity dependence on path difference) is measured, and the spectrum is recovered by Fourier transformation. The advantage is high resolution and light throughput.

Open problems & fun facts

Reducing the size of spectrometers to chip format (microspectrometers) while maintaining high resolution for portable devices.; Noise suppression and increasing dynamic range for detecting exoplanet atmospheres via transit spectroscopy.; Creating spectrometers for far ultraviolet and X-rays with angular resolution sufficient for spectroscopy of individual sources in clusters.

💡 The spectrometer on the Cassini probe discovered water geysers on Enceladus by analyzing scattered light; its resolution allowed distinguishing water vapor lines from ammonia and methane impurities.