For spectrum detection with the use of integral detection system, the detector is installed directly behind the exit slit of a spectral device. The width of exit slit determines the width of a spectral band of light which passes through a monochromator. Width of a spectral band is calculated as a product of the slit width value and the reciprocal linear dispersion of a monochromator.
Spectra detection with the use of integral detection systems is carried out point-by-point. The monochromator is tuned step by step according to wavelengths (spectrum scanning) at the preset step of scanning with simultaneous detection of electrical signal on a detector in each point of a spectrum . Scanning on a spectrum is performed by rotation of a grating around its axis. Here the wavelength interval in the focal plane and thus the spectral band of the light that passes through exit slit of a monochromator are changing, while the detector is detecting the integral light energy at the selected spectral bandwidth.
The process of spectra scanning with the use of a monochromator and the detection process should be synchronized in order to obtain the correct spectrum graph. Two types of such synchronization are possible: synchronization in time and step-by-step synchronization. With time synchronization, monochromator’s scan rate should be constant across the scan range (it is set in nm/sec). When using a detector or a signal recorder as a detection system, the scanning rate should be correlated with the recorder’s tape rate. In this case, the spectrum image is visualized with the help of the recorder. Some other ways of time synchronization are possible, for example, with the sync pulses that are generated by a monochromator in certain wavelength intervals in the process of scanning.
However, the systems “scanning monochromator- detection system” with time synchronization are not convenient for practical applications. Scanning monochromators, produced by SOL instruments, make use of more advanced principle of synchronization – step-by-step synchronization of a spectrum scanning and spectrum detection. An embedded analog-to-digital converter (ADC) is one of the main part of such detection system, which is offered as an option with some spectral devices. The ADC board receives electrical signals from photodetectors (photodiodes, PMT, etc.) to process them using a 16bits ADC. The ADC board is built into the spectral instrument. Both a spectral device and ADC board are controlled through PC. Such ADC board allows to build a measuring system on basis of a spectral device. That is to say, the spectrum can be scanned along a specified spectral range at a set scan step with simultaneous detection of a photodetector signal. The obtained spectrum is displayed on monitor as a graph. Our specially developed spectroscopic software SpectraSP is used to control the system operation via a PC.
А spectrum detection is performed point-to-point. The algorithm of a spectrum scanning is as follows: the spectral scan range, the scan step and the average measurement time of a detector signal in each spectrum point should be set. Once the scanning process is initiated, the monochromator starts tuning to the initial wavelength, then stops and the signal with the set measurement time is measured. After that, the monochromator tunes to the next wavelength, which differs from the previous one by one scan step, stops again and the signal at this wavelength is measured, and so forth. The entire scanning range is covered with such point-to-point measurements. Thus, the scan process consists of two sequential cycles: setting of monochromator to a needed wavelength (spectrum point) and measuring the photodetector signal at this wavelength. The spectrum is visualized simultaneously with the scan process.
During a measurement cycle the analoge-to-digital converter executes more than one readout in each spectrum point. The readouts are averaged that allows to improve the measurement accuracy. The time period between ADC readouts is 82 µsec approximately. The minimal measurement time in each point of a spectrum is 1 µsec. Thus, even with the shortest measurement time, the ADC executes 12 readouts in each point of spectrum. The longer time of measurements, the greater number of ADC readouts (linear dependence), the closer the averaged value of a detector signal to an actual value, i.e. the higher accuracy of measurement . It is possible to change the measurement accuracy by changing time of measurement. In order to obtain the desired accuracy of measurement, the set measurement time for detection of low signals has to be longer that that for the detection of large signals. The absolute value of signals, detected at different measurement time, will not change but accuracy of measurement will be increased. Taking into account this specific feature of measurement algorithm, the time of measurement in each point of a spectrum is named as “averaging time”.
With such a measurement algorithm, the scan rate (speed) is not specified. The scan rate is generally set in nm/sec and is defined as a ration of the scan range (in nm) to the scan time (in sec) of the set range. The scan time for the same spectral range depends on the number of spectral points and the averaging time in each point, because a scan process consists of two sequential cycles, a cycle of monochromator tuning to a specified spectrum point and a cycle of measurement at this point. The minimum averaging time in each point of a spectrum is 1 msec, the maximum one is 5000 msec.
A minimal scan step is equal to that of a wavelength tuning step and depends on the grating parameters (number of lines per mm). For example, the minimum scan step of monochromator-spectrograph MS3501 is 0.01 nm (when using a diffraction grating with 1200 lines/mm).
As SOL instruments scanning monochromators utilizes two output ports to hold two detectors, the ADC board also has two inputs to get signals from two detectors. Each detector can be coupled with one of the ADC input ports.
Switching over the output ports of scanning monochromator is performed with the computer controlled output mirror. The output port which gets light from the output mirror is called active one. The detector mounted on the active port will be active too. If two detectors are in use, only one of them is active at the moment. The ADC board can receive a signal only from one of the detectors coupled with its active input port. The active input port of ADC can be selected (depending on the output mirror position) either automatically or manually.
Photomultiplier tubes (PMT) and photodiodes are integral detectors which are mostly used in the detection systems.
When using an integral detection system, full light that passes through the exit slit should reach the active area of photodetector. This is very important. The light diverges from the slit of monochromator at a definite spatial angle and it is not a parallel beam. Therefore, it would be best to put the detector directly behind the exit slit, where there is the smallest aperture of a beam. Unfortunately it can not be always realized in practice. Some of light detectors (e.g. PTA-928 or a photodiode with active area of 10х10 mm) have a large size of active area. The detectors based on such sensors can be mounted directly on exit slit of monochromator.
A special conjunction unit should be used to couple detectors with active area less than 5 mm. A toroidal mirror inside this unit transfers an image of the exit slit to the photosensitive area of detector. In this case, full light from the output of monochromator will fall to the active area of the detector without any losses.
We can calculate and produce a conjunction unit with a detector on request.