Introduction to Multi-Channel Spectroscopy

In spectroscopy, a scanning monochromator and a single element detector positioned at the exit slit are typically used. Such a scanning system generates a complete spectrum point by point through selection of each wavelength by moving the grating. Conversely, Multichannel Spectroscopy uses fixed grating and an array of detectors in place of the single detector and exit slit.

As each detector views a different wavelength, the entire spectrum can be captured within the time required for recording one wavelength point with a scanning system. This improvement is called the Multichannel Advantage. Array based spectrometers exhibit improved stability and reproducibility due to lack of moving components. Silicon photodiode arrays (PDAs), and charge coupled devices (CCDs) are the two key arrays used.

Silicon Photodiode Arrays

A PDA is a linear array of individual photodiodes built using integrated circuit technology (Figure 1). The electronic FET switches required to read from each photodiode are put up right onto the ‘chip,’ reading only one diode at a time but consecutively.

Layout of Photodiode Array Detector (PDA).

Figure 1. Layout of Photodiode Array Detector (PDA).

The analog signal read out by each FET is in correlation with the amount of incident light on the pixel. The PDAs employed in Oriel Instruments’ InstaSpec™ product line are specially engineered for spectroscopy. Each component is thin and tall to compete the slit geometry used by spectrographs. Most multichannel detectors are based on silicon and can be used between ~180 and 1100nm. Oriel Instruments also offers InGaAs PDAs for the NIR (800-1700nm).

Charge Coupled Devices

A CCD shown in Figure 2 is a 1 or 2D array of photosensors offered in a semiconductor ‘chip’ package like the PDA. However, there is a vast difference in the readout mechanism between PDAs and CCDs. Overlaying of each pixel with a low-voltage carrying element called an electrode is on a CCD, causing charge accumulation in the pixel when the chip is illuminated.

Layout of Spectroscopic Charge Coupled Device (CCD).

Figure 2. Layout of Spectroscopic Charge Coupled Device (CCD).

The data acquisition is performed by applying a sequence of voltages across the electrodes for charge shifting row by row down the vertical dimension of the chip and into a shift register at the array bottom (Figure 3). This charge is then shifted in the same manner throughout the shift register to the output node, where it is transformed into a digital form for further processing. This readout mechanism results in very low readout noise. The sensitivity of CCDs is the same as a photomultiplier tube, but a CCD can tolerate overexposure to bright lights.

Readout pattern of two dimensional CCDs

Figure 3. Readout pattern of two dimensional CCDs.

To improve sensitivity when full imaging is not needed, the special linear image sensor mode in the InstaSpec™ IV CCD system groups the vertical elements together (binned) to make a linear array of tall thin elements as in the case of PDAs. In this mode, the CCD acts like a PDA but exhibits a sensitivity similar to a linear array of small photomultiplier tubes. A shutter needs to be used when a CCD is used for any imaging application involving continuous light. Oriel Instruments offers two CCD families, namely the LineSpec™ family that uses linear arrays and the InstaSpec™ IV family that uses 2D arrays.

Comparison of PDAs and CCDs

With 100X more sensitivity than PDAs, CCDs are ideal detectors for low light levels, such as in Raman scattering and low quantum yield luminescence. The InstaSpec™ family of CCDs in conjunction with Imaging Spectrographs from Oriel Instruments can be employed for 2-D spectroscopy. The LineSpec™ family of CCDs is economical and can be used for spectroscopic applications involving plenty of signal and for applications like beam profiling and monitoring where in the beam profiling and monitoring crucial.

With a maximum S/N ratio of 10X times more than that of a CCD, PDAs are ideally suited to take radiometric, absorbance, transmittance, and reflectance measurements involving high light levels. Specifications of PDAs and CCDs are compared in the following table:

Table 1. Comparison of PDA and CCD Specifications

  InstaSpec™ II Silicon PDAs InstaSpec™ VI InGaAs PDAs InstaSpec™ IV Spectroscopy CCDs LineSpec™ CCDs (12 x 14 µm) LineSpec™ CCDs (14 x 200 µm)
Spectral response 180 to 1100 nm 800 to 1700 nm 180 to 1100 nm 200 to 1000 nm 200 to 1000 nm
Pixel size (binned) 25 pm x 2500 pm 50 pm x 200 pm 26 pm x 26 pm 12 x 14 14 x 200 pm
Saturation level (binned) 125 x 106 e- 44 x 106 e- 250 x 103 e-
(625 x 103 e-)
95 x 103 e- 140 x 103 e-
Saturation exposure >107 nJ cm-2@600 nm >145 nJ106 e-@1.55 pm >250 pJ cm-2@600 nm 8 nJ cm-2 @ 600 nm 0.6 nJ cm-2 @ 600 nm
Dark Current (25 °C) <0.1 pA/pixel
<160 pA cm-2
9.6 pA/pixel
0.21 pA cm-2
<0.1 fA/pixel
<16 pA cm-2
0.021 fA/pixel
12 pA cm-2
0.03 fA/pixel
1.2 pA cm-2
Detection limit <3.3 N cm-2@600 nm
<3750e-
<3.3 pJ cm-2@1.55 pm
<3750e-
<3.8 fJ cm-2@600 nm
<10e-
14 pJ cm-2 @600 nm
<160e-
0.5 pJ cm-2 @600 nm
<122 e-
Max S/N ratio 10,000:1 6,600:1 900:1 308:1 374:1
Dynamic range 32,786:1
(@62 kHz)
12,500:1
(@62 kHz)
65,536:1
(@62 kHz)
595:1
(@1.25 MHz)
1150:1
(@1.25 MHz)
Electrons/count 1900 666 10 35 55
Usable for linear spectroscopy/ imaging Yes Yes Yes Yes Yes
Usable for two dimensional spectroscopy and imaging (shutter required) No No Yes No No

The spectral response range of 180-1100nm for a silicon PDA is wider than most of PMT tubes. In case of InGaAs PDAs, the multichannel advantage is extended into the NIR, with a responsive range of 800-1700nm. The CCDs from Oriel Instruments are silicon based. The InstaSpec™ CCDs are offered in Front Illuminated (FI) with UV phosphor or Open Electrode (OE) to encompass the same range as silicon PDAs. The LineSpec™ CCDs are responsive from 200 to 1000nm. Typical quantum efficiency curves for Oriel PDAs and CCDs are presented in Figure 4.

Typical quantum efficiency curves for Oriel PDAs and CCDs

Figure 4. Typical quantum efficiency curves for Oriel PDAs and CCDs.

Dark Current and Cooling

The leakage current of a detector is termed as the dark current, which adds to the signal and contributes to the noise. The exposure time for measurement is limited by the dark current mainly due to the eventual occurrence of charge saturation. As it is temperature sensitive, the dark current can lead to background changes over time.

Since the dark current varies exponentially with temperature, these problems can only be alleviated through cooling. Cooled systems allow for maximum useful exposure time by decreasing charge build-up caused by the dark current. Oriel Instruments’ cooled systems employ efficient thermoelectric coolers within an evacuated housing, eliminating the requirement for liquid nitrogen, water cooling, or external power supplies.

About Oriel Instruments

Oriel Instruments, a Newport Corporation brand, was founded in 1969 and quickly gained a reputation as an innovative supplier of products for the making and measuring of light. Today, the Oriel brand represents leading instruments, such as light sources covering a broad range from UV to IR, pulsed or continuous, and low to high power.

Oriel also offers monochromators and spectrographs as well as flexible FT-IR spectrometers, which make it easy for users across many industries to build instruments for specific applications. Oriel is also a leader in the area of Photovoltaics with its offering of solar simulators that allow you to simulate hours of solar radiation in minutes. Oriel continues to bring innovative products and solutions to Newport customers around the world.

This information has been sourced, reviewed and adapted from materials provided by Oriel Instruments.

For more information on this source, please visit Oriel Instruments.

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