What Is a Spectroscope?
A spectroscope is a device that separates light components in order to measure only the intensity of light at the wavelength of interest from a composite of light of various wavelengths.
In recent times, many spectrometers have integrated a detector of the separated light, and the entire process from light separation to detection mechanism is sometimes collectively referred to as a spectroscope.
Uses of Spectroscopes
Spectroscopes are used in all industries and research fields because they can, in principle, spectrate light sources in various wavelength bands, from radio waves to radiation, regardless of whether the light is reflected or transmitted, and not only visible light.
In the field of analytical chemistry, it is used to measure the intensity of sunlight and plasma luminescence, and is also used to evaluate optical properties such as reflectance of materials.
They are also often incorporated without awareness into quality control lines that detect reflected light or other arbitrary wavelengths in product inspection lines using light sources such as lasers.
Principle of Spectroscopes
Generally, to spectrally analyze a light source, it is necessary to first shape the light.
After setting the light resolution by passing the light source through a gap called a slit, the light source is collimated by a collimator made of lenses and mirrors.
Spectroscopy can be performed by passing this collimated light into a spectrometer. There are two types of spectrographs: a diffraction grating-type that uses the diffraction phenomenon of light as well as a prism-type that uses the refraction phenomenon of light.
In the diffraction grating-type, the wavelength and resolution of light that can be detected can be changed by changing the diffraction pattern. This is because spectroscopy is performed using the reflection of light reflected by diffraction gratings engraved at regular intervals on the surface of the monochromator.
The principle of a diffraction grating-type spectroscope is explained here.
When collimated light from a light source (white light) containing light of various wavelengths is incident on a diffraction grating, multiple gratings, or grating-like structures (G1, G2, …), are formed at each position. Here, interference of light occurs, and monochromatic light, in which only a specific wavelength λ is intensified, is emitted in the angular direction (θ) where the optical path difference (dsinθ) of the reflected light originating from each grating satisfies a predetermined condition (integer multiple of wavelength λ).
In this way, different wavelengths are dispersed (separated in a rainbow-like pattern) at different angles by the diffraction grating.
By using a slit, only monochromatic light of a specific wavelength can be extracted from the dispersed reflected light. This is the principle of a grating-type spectroscope. By rotating the grating, it is possible to change the wavelength of the light to be extracted.
How to Choose a Spectroscope
When using a spectroscope with an integrated detector, it is necessary to select an appropriate one for the wavelength of the light source measured.
For example, if the light source is in the range from ultraviolet to near-infrared, a CCD is fine, but if the light source is longer wavelength than that, an InGaAs type detector is necessary.
As mentioned in the measurement principle, the wavelength of a diffraction grating-type spectroscope is determined by the diffraction pattern, so it is necessary to select a spectroscope suitable for the wavelength of interest.
The resolution of a prism-type spectrometer is determined by the nature of the prism, but it has the feature of no loss of light intensity, so it is advisable to select the right one depending on the application.
How to Use a Spectroscope
The general procedure for using an analytical instrument with a spectroscope is as follows:
- Decide on the substance to be measured and the wavelength range to be measured.
- Select a spectroscope corresponding to the wavelength you wish to measure.
- Shine a light on the substance and spectrate the desired wavelength.
- Put the desired light into the sensor to detect the signal.
- The obtained signal is converted to a spectrum.
If it is an expensive object used in a laboratory, a spectrometer called a Michelson interferometer automatically detects the wavelength of a specific light. Even a small, portable machine can detect the wavelength of interest by passing the light transmitted or reflected through the material through an interchangeable spectroscope.
The resulting wavelengths enter the sensor (detector) and are detected as a signal for each wavelength. This signal is converted into a waveform called a spectrum, and by analyzing this spectrum, the state of matter is analyzed.
Examples of Spectroscope Experiments
There are several examples of experiments using spectroscopes, depending on the wavelength to be measured.
For example, the following are examples of experiments in each wavelength range, starting from the short wavelength side.
- An x-ray spectroscope is used to identify the composition of a material’s surface by exposing the surface to x-rays and passing the reflected light through the spectroscope.
- Ultraviolet/visible spectroscope identifies the composition of the object and the amount of light it contains by passing the light through the material.
- Infrared spectroscopes reveal the structure of a substance by shining light on the bonds between molecules.
Thus, the information obtained depends on the wavelength range of the spectroscope.
Spectra Obtained From a Spectroscope
The purpose of using a spectroscope is to acquire information from an unknown or known substance and analyze it to identify the state of the substance. The final spectrum, or waveform diagram, obtained from a spectroscope is used for this analysis.
The spectrum obtained from a spectroscope includes the examples below. By first defining the information you want to know, it is important to select the appropriate spectroscope to acquire the spectrum.
- An x-ray spectroscope identifies atoms from the peaks of the characteristic x-rays being measured.
- UV/visible spectroscopes detect the energy difference between the electrons excited when light is transmitted through the sample as a spectrum.
- The infrared spectroscope detects the vibrational energy between the bonds connecting atoms as a spectrum.