Sensitivity of Spectrometer
The sensitivity of a spectrometer relates to the devices ability to detect weak or low-intensity signals. For spectrometers this is important for the detection of low light level signals and in applications where the line spectra may have relatively weak intensity. The sensitivity of a spectrometer can be defined by it’s most limiting factor. In most UV-Vis spectrometers, sensitivity is defined by the signal to noise ratio (SNR). In more sophisticated systems, it can be defined by other things such as the quantum efficiency of the detector in the spectrometer.
Different components of a spectrometer will determine how it interacts with and processes incoming light, affecting the sensitivity of the device. The primary components that will affect the sensitivity are the slit width, the diffraction grating and the detector. It is also important to note that by enhancing the sensitivity, you will often decrease the spectral resolution. If you require a highly sensitive spectrometer, then some resolution sacrifice may be necessary.
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There are several applications that require a sensitive spectrometer, and these include.
- Low light applications
- Some fluorescence spectroscopy
- Trace analysis where you may be quality control checking for elements at low concentrations for example: 1 part per trillion.
- Astrophysics where you require the accumulation of as much light as possible from far away sources.
- DNA sequencing
Typically sensitivity is valued in areas of research where the signals you expect are relatively weak or low intensity. This could be due to a complex background spectrum or from attempting to study events that are relatively rare or low in concentration.
While sensitivity is a priority in certain areas, for more basic measurements such as photoluminescence or absorbance, it is important to find a balance between sensitivity and resolution. As focussing in on sensitivity typically costs the resolution (and vice versa) for basic measurements there needs to be a balance. The Ossila Optical Spectrometer is built to strike a balance between high resolution and high sensitivity in the UV-Vis spectrum range.
How is Spectrometer Sensitivity Measured?
It is hard to perfectly define sensitivity as there are many different definitions. Generally the limiting factor for sensitivity will depend on what spectrometer you use and for what task. For most UV-Vis spectrometers the limiting factor is the signal to noise ratio.
For a signal to be useful it must be more intense than the surrounding noise, thus sensitivity is typically measured as the signal to noise ratio. For spectrometers that require extreme sensitivities the quantum efficiency is considered but for basic measurements this is not required. To calculate the SNR you simply take the ratio of the maximum signal intensity to the noise.
Where Sm is the maximum intensity of the signal and Nrms is the root mean square of the background noise.
This is the most general measure of SNR with different companies and researchers using their own definition. The SNR is typically expressed as either a ratio or in decibels with a signal greater than 1:1 or non-zero decibels being a signal that is greater than the noise and is thus detectable. The Ossila Optical Spectrometer for example has a SNR >500:1.
Sensitivity of Your Instrument
Factors that can affect the sensitivity of the spectrometer include:
- The slit width
- The diffraction grating
- The detector pixel array density
|Effect on Sensitivity
|Number of gratings per unit length
|Density of pixel array
The slit width of a spectrometer affects the sensitivity (and correspondingly the resolution) due to the way it diffracts light. When the light signal enters the slit, it diffracts outwards with the curvature of the diffraction being dependent on the slit width. The slit width also limits how much light can enter. A wider slit allows for a greater light collection resulting in a larger signal and thus a greater signal to noise ratio
The diffraction grating in a spectrometer is used to separate light into its corresponding wavelengths. A grating consists of a repeating pattern of grooves that cause light to diffract. By increasing the density of grooves light will diffract at a larger angle. This means there is larger spatial separation between light at different wavelengths, allowing for an increase in the signal resolution. The resolving power of a diffraction grating can be calculated simply by
Where N is the number of gratings per unit length and n is the order of diffraction. To increase the sensitivity of the system however, the number of gratings must be minimized which lowers the resolution. By lowering the grating density, the split light becomes less spread and thus the signal intensity is increased.
Most UV-Vis spectrometers use a detector consisting of a discrete array of pixels (such as the CCD detector used in the Ossila Optical Spectrometer). These pixels measure light intensity individually. After the different wavelengths of light have been spatially separated by the diffraction grating, this array of pixels can be used to measure the light intensity as a function of wavelength. To improve the sensitivity of a detector, you can lower the density of the pixels so that for the same amount of light, there is more light per pixel. You can also improve the quantum efficiency of your detector. As the quantum efficiency is the conversion rate of photon to electronic signal this is often not the limiting factor and is usually the last improvement to consider.
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How to Decrease Noise in Your System
Other than altering the components of a spectrometer, there are a few external factors that can affect the noise in a spectrometer system:
- The noise of the system/signal
- The quantum efficiency of the detector
- Using optical fibers
- Data processing techniques
By reducing background noise, the intended signal becomes more prominent and is easier to distinguish thus improving the SNR. There are many ways to reduce noise with a large amount of research on low noise electronics. However, there are some easier things you can consider when taking a measurement yourself, such as background light, high temperature and vibrational isolation.
By reducing the background light by either turning off surrounding lights or by isolating the system from the environments light there is less interference with the samples signal. The temperature of the room may also affect electronics as certain detectors such as CCD’s work better at temperatures as low as liquid nitrogen levels. Finally, vibrations in the housing of samples can affect the experiments through phonon effects and thus these can be reduced using a suspension table.
Optical fibers can drastically improve the sensitivity in your measurements by removing a large portion of background noise in air. When taking measurements in air, there are many additional factors that can cause signal loss which include loss due to interaction of airborne particulates as well as additional background light. Thus, an optical fibre can be used to isolate the signal from the environment and prevent any further loss in sensitivity.
Another way to increase the sensitivity of measurements is by average data or utilising accumulated data taking. When using the Ossila Spectrometer software you can utilise the accumulate function to sum the signal from multiple measurements. which giving a larger signal intensity. This will also increase the noise but as the signal intensity is likely to be larger than the noise it should help to distinguish the peak of the signal from the background noise. Additionally, statistical noise can be reduced by averaging data. With both techniques you can further increase the sensitivity of your measurements.