prism train

A prism train is a set of optical prisms arranged in series along a light path, each one refracting light to increase the total angular separation of different wavelengths. Instead of relying on a single prism to disperse white light into its spectrum, a prism train uses two, three, or more prisms positioned so that light exits one prism and enters the next, with each stage adding further dispersion. The cumulative effect produces much sharper spectral resolution than any single prism could achieve, which is essential when distinguishing closely spaced spectral lines.

The arrangement matters. In a typical configuration, prisms are oriented so their refracting angles work together; light enters the first prism at an angle chosen to minimize deviation while maximizing dispersion, and then the partially separated beam enters the second or subsequent prisms at similarly optimized angles. The prisms are often made of glass types with high refractive index and high dispersion, such as flint glass, to maximize the bending effect per stage. Spacing and alignment are critical: even small shifts degrade the spectral purity and resolution of the output.

Prism trains are most commonly found in spectroscopes and spectrographs, where they replace or supplement single-prism systems. A three-prism train was standard in many mid-20th-century laboratory and astronomical spectroscopes. The advantage over a single large prism is that a train of smaller prisms is easier to manufacture with high optical quality, can be mounted more rigidly, and allows finer control over dispersion by adjusting the orientation of individual prisms. A disadvantage is that each air-glass interface introduces some reflection loss, which reduces light transmission; anti-reflection coatings help mitigate this.

The term "train" reflects its mechanical structure: prisms are mounted in a linear or near-linear sequence, rather like cars on a track. In some designs, all prisms are housed in a single rotating unit so they can be adjusted together to scan across a spectrum. In others, they are fixed and the light source or detector is moved relative to them. Prism trains have become less dominant in modern spectroscopy, displaced by diffraction gratings and electronic array detectors, but remain in use in high-resolution UV and visible spectroscopy where their achromatic properties and low stray light are valued.