When you're building a spectrometer, the core of the device is the component that separates light into its constituent wavelengths. The two most common choices for a DIY project are a prism and a diffraction grating. We set up two different test rigs to see which one performs better.

First, we put together a simple setup using a slit, an LED light source, and a prism. The prism works by refraction: as the light passes through the glass, different wavelengths bend at slightly different angles.

While a prism is a great way to demonstrate the principle of light dispersion, its output isn't sharp enough for precise spectral analysis.

Next, we built a similar rig but swapped the prism for a diffraction grating. A grating works on the principle of diffraction, where light waves interfere with each other after passing through a series of tiny, closely spaced lines.

The difference in the output was immediately obvious. The spectrum produced by the grating is significantly sharper and more well-defined. You can see the distinct lines of the LED's spectrum with much greater clarity. You can also see the zero-order image of the slit 

For any serious spectrometer project, a diffraction grating is the clear winner. The sharper, more resolved spectrum it produces allows for much more accurate readings. Another key advantage is the sheer variety of gratings available, each with a different groove density (lines per millimeter). This allows you to select a grating that is perfectly suited for the resolution and wavelength range you need, giving you far more control over your instrument's performance.

For even more advanced setups, you might encounter a grism. This device is a clever combination of a grating and a prism. The grating is etched directly onto the face of a prism, allowing the light to be dispersed and refracted simultaneously. The key advantage of a grism is that it can be designed to keep the light path straight, which simplifies the optical alignment of a spectrometer. They're a fantastic option for high-performance applications where both high resolution and a compact, in-line design are needed