How does a spectrometer measure a spectrum?
How does a spectrometer measure a spectrum?
Spectrometer Principle
A spectrometer is a device that breaks light down into all its different colors—really into their precise wavelengths—so we can measure and understand what the light is made of. Scientists and engineers use it in many areas: to check the quality of metals, identify rocks and minerals, track chemicals in water, test medicines and fuels, monitor air pollution, and even study the light from distant stars and planets.
Most modern spectrometers follow what’s called a Czerny-Turner (C-T) design. Inside, two curved mirrors guide the light. The first mirror straightens the incoming beam and sends it to a special grating—a finely ruled surface that spreads the light into separate wavelengths, like a very precise rainbow. The second mirror focuses that separated light onto a detector, such as a photodiode array or a camera chip, which records how bright each wavelength is. The C-T design is popular because it’s efficient, reliable, and easy to build in different sizes. Extra parts like filter wheels can be added to block unwanted reflections and improve accuracy. Thanks to this flexible design, spectrometers can be made small enough for handheld use, powerful enough for advanced laboratory research, or robust enough for a wide range of industrial applications.
C-T Optical Path
The C-T optical layout was invented in 1930 by Martin Czerny and Alfred Turner, and people still use it in most spectrometers today. Even though that was nearly a century ago, people still use this design in most spectrometers today because it gives very sharp results and works for many different jobs. This design is now the standard in instruments that measure ultraviolet (UV), visible (VIS), and near-infrared (NIR) light. There are two main ways to arrange the optics in a C-T spectrometer:
The traditional C-T setup keeps the mirrors and grating lined up so the light travels in a simple, straight path. The first curved mirror catches and straightens the incoming light, the diffraction grating spreads it into its separate colors, and the second mirror focuses those colors onto the detector. Because the path is straightforward, this design corrects optical errors like coma very well and delivers excellent resolution, giving you a clean, sharp spectrum. The trade-off is size: the mirrors have to be spaced farther apart, so the instrument ends up bulkier and heavier, which is fine for a lab bench but may less convenient for portable use.
Traditional C-T Design
The crossed C-T layout bends the light path by tilting the mirrors so they sit at an angle to each other. This makes the beam “cross” inside the instrument and shortens the overall optical path, allowing the whole spectrometer to be much smaller and easier to carry or fit into tight spaces. The compact design is perfect for handheld or field instruments where space and weight matter. However, because the light path is more complex, it doesn’t correct optical distortions quite as well as the traditional layout, and the ultimate resolution is a little lower. For most everyday measurements, though, the gain in portability and efficiency outweighs the slight loss in image quality, which is why many modern portable spectrometers use the crossed design.
Crossed C-T Design
Components in a spectrometer
In a practical spectrometer, the following components are typically included:
1. SMA905 Connector: Couples light into the spectrometer via an SMA905 fiber optic connector, compatible with a wide range of optical accessories.
2. Fixed Entrance Slit: Controls the width of the incoming light, directly affecting spectral resolution. A narrower slit increases resolution but reduces light throughput and can introduce noise, while a wider slit allows more light and improves the signal-to-noise ratio, at the cost of broader spectral features and lower wavelength resolution.
3.Collimating Mirror: Collimates the light from the slit, turning it into a parallel beam directed onto the diffraction grating.
4.Diffraction Grating: Disperses light into its spectral components. Multi-slit interference determines the positions of spectral lines, while single-slit diffraction influences the intensity distribution.
5. Focusing Mirror: Focuses the first-order spectrum from the grating onto the detector plane.
6.Detector: Typically a CCD or similar sensor, the detector is central to performance. Its material and design determine the spectral range, sensitivity, resolution, and signal-to-noise ratio. Light falling on the detector generates charges that are converted via an analog-to-digital (A/D) process to produce measurable spectral data.
7.Higher-Order Diffraction Filter: Removes unwanted higher-order diffraction by filtering out low-frequency signals, ensuring the measured spectrum is accurate and clean.
Based on the standard optical design, the light travel path in a spectrometer is typically as follows: SMA905 connector → Entrance slit → Collimating mirror → Grating → Focusing mirror → Detector → A/D conversion → FPGA → USB → PC interface.
