When purchasing any OEM spectrometer, several factors have to be considered. These factors include the spectral range, spectral resolution, Signal-to-Noise Ratio (SNR), and the type of measurement being made.
Other criteria include cost, weight, ease of use, maintenance, and operational considerations. Among these, the most difficult factor is to assess signal-to-noise comparison. Many spectrometers include specifications in which the SNR is usually cited by comparing the instrument’s digital dynamic range and noise floor, but this does not indicate the actual measurements where users must consider factors like the spectrum’s properties, detector noise and the amount of light gathered.
HES Spectrometer and Czerny Turner System
IS-Instrument has designed the HES series of spectrometers that enable users to perform transmission Raman measurements in a compact and cost-efficient package. The high throughput of the instrument offers an excellent SNR to be measured when utilizing these systems.
The amount of light obtained is a function of three main factors: the instrument's optical transmission, throughput, and the detector Quantum Efficiency.
SNR treatments usually consider the shot noise limited case and deduce the effects of detector noise; throughput remains the same for all spectrometer designs. Other treatments consider factors such as phase errors, instrument drift and other possible effects, which can make direct comparisons between systems.
In this technical brief, the SNR comparison is made between the HES spectrometer and a standard Czerny Turner system. The computation considers detector noise and signal strength, but does not consider drift, phase errors, and other secondary factors.
Dispersive and Interferometer Based Systems
Spectral measurement systems can be divided into two categories, such as interferometer based systems and dispersive systems. IS-Instruments HES spectrometer combines the benefits of both of these systems. The system is easy to operate, eliminates the need for expensive optical elements, and maintains the throughput advantage of a Michelson interferometer.
The instrument is compact and has no moving parts. The high throughput further enhances the flexibility of the HES OEM spectrometer, enabling it to be integrated into larger systems, especially when examining diffuse sources.
The amount of signal collected by a spectrometer from a diffuse source is shown in the following equation:
S = QE OE AΩ
Here, QE denotes the detector quantum efficiency, OE represents the optical transmission of the instrument and AΩ represents the spectrometer receiving an optical area solid angle product. For the HES spectrometer, the AΩ product is specified by the area of the dispersive element illuminated and the field of view (FOV) of the instrument.
For a Czerny Turner system, the AΩ product is incited by the instrument’s f/# number and the slit width. This system is proportional to the instrument’s spectral resolution.
FOVHES = 2π/R
The R stands for the system’s resolving power, which is equivalent to the throughput of a Michelson interferometer and usually results in the HES spectrometer having a 100 fold or more throughput over a typical Czerny Turner system of same spectra resolution and physical size.
The instrument’s QE is obviously a function of the detector being utilized. For the purpose of this study, it is assumed that both systems include the same detector.
When choosing any spectrometer, users must consider the detector in terms of its noise and sensitivity. In order to acquire the high throughput advantage, the HES spectrometer forfeits half of the light in terms of optical transmission. Hence, the signal observed by the HES instrument (SHES) in contrast to a Czerny Turner system (SCZ), in identical observing conditions is:
SHES ≥ 50 SCZ
In the HES spectrometer, the spectrum is recovered in Fourier space. The light returning from the signal source is recovered concurrently for all spectral lines and is uniformly dispersed across all the pixels. Therefore, when studying a single spectral line, the HES instrument is more sensitive to the detector’s noise effects.
In the shot noise limited case, the HES instrument will have a 7.07 improvement in the SNR when compared to the Czerny turner instrument when doing identical measurements.
Basic Models of HES Spectrometers
Three basic models of HES spectrometers are available. The detector used is the main difference between each system. The three types of detectors used in the HES spectrometer are an uncooled, CMOS device similar to that found in mobile phones; a high quality, cooled CCD, similar to the Andor IDUS 420 system; and cooled CCD similar to the IVaC camera (figures 1-3).
Table 1. Simulation cases.
||Signal photons/s observed by Czerny Turner spectrometer
||Detector noise (e/pix/sec)
||Read Noise (RMS)
||Cost effective, uncooled CMOS detector.
||Mid-range, cooled camera (-50 to -60 °C)
||Top range, cooled camera (- 80 °C)
Figure 1. SNR comparison for an uncooled CCD: LH image = 10 photons observed per second; RH image = 100 photons observed per second. Red line = HES spectrometer performance; Blue line = Czerny Turner performance.
Image Credit: IS-Instruments
Figure 2. SNR comparison for a cooled CCD (case 3 and 4): LH image = 10 photons observed per second; RH image = 100 photons observed per second. Red line = HES spectrometer performance; Blue line = Czerny Turner performance.
Image Credit: IS-Instruments
Figure 3. SNR comparison for a high quality cooled CCD (case 5 and 6): LH image = 10 photons observed per second; RH image = 100 photons observed per second. Red line = HES spectrometer performance; Blue line = Czerny Turner performance.
Image Credit: IS-Instruments
When the spectrum gets distributed over more pixels, the noise from each pixel must be taken into account when utilizing a Czerny Turner system. This reduces the overall SNR that is observed. On the other hand, the HES spectrometer offers a partial multiplex advantage. Since all the light is dispersed over the detector, there is no equivalent effect and thus provides an enhanced performance over that produced by a traditional dispersive spectrometer.
The results demonstrate that the HES spectrometer offers a major advantage, even when the detector’s noise dominates over the shot noise. This makes the instrument suitable for Raman measurements.
The HES spectrometer is a cost effective solution and proves ideal for a number of applications, including transmission Raman measurements. The system has a larger throughput when compared to a standard dispersive system. The HES instrument offers a multiplex advantage which further enhances its performance. It is also compact, easy to use, and provides significant enhancements in the SNR measured from a specific source.
This information has been sourced, reviewed and adapted from materials provided by IS-Instruments.
For more information on this source, please visit IS-Instruments.