Spectral Resolution of Spectrometer

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Resolution Definition

The resolution of a spectrometer is defined as the smallest distance between two peaks at which they can still be resolved as separate. This is referred to by some as the optical resolving power or the spectral resolution of an instrument. 

When two spectral lines exist very close to each other, the resolution is the minimum amount of space needed between the two peaks for a device to distinguish them as separate. If the resolving power of a spectrometer is insufficient, the spectral lines will overlap making it hard to differentiate between them.

The different components of a spectrometer will determine how it interacts with and processes the incoming light, affecting the resolution of the device. The primary components responsible for spectral resolution include the slit width, the diffraction grating and the detector pixel array. Resolution and sensitivity inversely depend on the same components. Therefore, by enhancing resolution you will often decrease the sensitivity of the spectrometer. In many high resolution spectrometer, you often need to sacrifice some instrument sensitivity.

Applications

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There are several applications that require a spectrometer to have high resolution, and these include:

• Laser characterisation
• Gas absorbance
• Emission line analysis
• Plasma monitoring

Typically resolution is valued in areas of research where incoming light is of a high intensity and the spectra being measured include peaks that are very close together. This could be due to monitoring highly energetic states of matter like plasma or observing the excitation and de-excitation spectra of hyperfine structures. 

In some fields, super high resolution is crucial. But for simpler measurements, like photoluminescence or absorbance, it's important to find a middle ground between high sensitivity and high resolution. The Ossila Optical Spectrometer, made for UV-Vis spectroscopy, is designed to give you both high resolution and high sensitivity within low price point.

How is Spectrometer Resolution Measured?

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The spectral resolution (SR) of a spectrometer is defined as the wavelength of the light being measured divided by the full width half maximum (FWHM) of the spectral peak being investigated. This measurement is typically performed using a monochromatic light source such as a low-pressure argon or mercury emission lamp, or a single mode laser.

Here, Δλ is the FWHM and λ is the wavelength at which the measurement was taken.

The FWHM is a measurement of a curve - you measure the width of the curve at half the maximum height or intensity. For a single wavelength light source, such as a single mode laser, the FWHM of the signal should be extremely low. Therefore, the measured FWHM of a monochromatic source will be a direct measure of your spectrometer's resolution. 

In addition to the measured spectral resolution (SR) the theoretical resolution of a spectrometer can be calculated by:

Where RF is a resolution factor, WR is the wavelength range, W_S is the spectrometers slit width, n is the number of pixels and WR is the wavelength range.

Pixel resolution and spectral resolution are not the same thing, as you require at least three pixels to measure spectral resolution. RF is a resolution factor that depends on the relationship between slit width and pixel width. It is worth noting that theoretical measurements of resolution are not the most accurate way of determining spectral resolution. These calculations assume perfect functionality of all optical components, which is unrealistic in real world conditions.

The spectral resolution of all Ossila spectrometers are checked before shipping to ensure reliable and consistent measurement that meets our high specification standards.

Resolution of Your Instrument

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Factors that can affect the resolution of the spectrometer include:

• The slit width
• The diffraction grating
• The detector pixel array density

Component	Component Property	Effect on Resolution
Slit width	Spacing
Diffraction grating	Number of gratings per unit length
Detector	Density of pixel array

Slit Width

The width of the slit in a spectrometer impacts its resolution (and, conversely, its sensitivity) because it affects how light spreads outward. As light passes through the entrance slit, it spreads outward, and the extent of this diffraction depends on the slit width. The slit width also controls the total amount of light that enters the spectrometer which will also affect the spectral resolution.

Diffraction Grating

A spectrometer's diffraction grating is responsible for separating light into its different colors or wavelengths. This grating contains a pattern of grooves that diffract light. Decreasing the spacing between the grooves will cause the light to be diffracted at a larger angle. In other words, light of different wavelengths will be more spatially seperated by the time they hit the detector, increasing spectral resolution. The resolving power of a diffraction grating can be calculated by

Where N is the number of gratings per unit length and n is the order of diffraction. To increase the resolution of the system the density of the grooves can be increased. By increasing the resolution however, the sensitivity of the spectrometer will decrease as a consequence. Thus, unless a very high spectral resolution is needed, most general-purpose spectrometers strike a balance between the spectral resolution and the sensitivity.

Detector

Many UV-Vis spectrometers use a detector made up of an array of photo-sensitive pixels (like the CCD detector in the Ossila Optical Spectrometer). Once the diffraction grating separates light into its various colors or wavelengths, these pixels can measure the intensity of light at different spatial points. This is how the CCD detectors measure light intensity as a function of wavelength.

Detectors with a larger density of pixels will have increased spectral resolution, as it can more easily distinguish between diffracted light. However, again you sacrifice spectrometer sensitivity, as each pixel will collect fewer photons per measurement.

Is Spectral Resolution Your Limiting Factor?

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The resolution of a spectrometer measurement depends on both the spectral resolution of the equipment, and the linewidth of the signal you are measuring. In most cases, linewidth is greater than spectral resolution. Therefore, the limiting factor of your measurement will be the linewidth of your peak. However, if the spectral resolution of your equipment is greater than the linewidth of your signal, then the FWHM of your peak will be limited by the spectral resolution of your spectrometer.

You should take this into account if you want to measure a single emission peak, such as that of a single-mode laser. Another time where this could be an issue is if you measure a sample where the characteristic peaks are close together. If you have many emission peaks with smaller distances between each one than the spectral resolution, you may struggle to resolve them as separate peaks.

If you require this level of resolution, you can purchase specialist high-resolution spectrometers. High resolution spectrometers, or spectrometers with adjustable entrance slits are more expensive than general spectrometers. Our spectrometer aims to provide a compact, low-cost optical spectrometer that optimises both resolution and sensitivity for a wide range of samples. The Ossila Optical Spectrometer has an entrance slit width of 25 µm and optimised diffraction grating spacing, which maximises both optical resolution and sensitivity for the largest amount of samples. For advice on how to choose an optical spectrometer that best suits your needs, read our guide on How to Choose a Spectrometer.

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