Reference detector

The detection of a specific gas (10) is accompHshed by comparing the signal of the detector that is constrained to the preselected spectral band pass with a reference detector having all conditions the same except that its preselected spectral band is not affected by the presence of the gas to be detected. Possible interference by other gases must be taken into account. It may be necessary to have multiple channels or spectral discrimination over an extended Spectral region to make identification highly probable. Except for covert surveillance most detection scenarios are highly controlled and identification is not too difficult. 

Figure 12. C02 Detection expected variation of SNR as a function of optical filter centre wavelength and bandwidth, assuming a mean optical power incident on the reference detector of 10 nW nm 1. Reference and measurement cells were assumed to be 1 m long and to contain 100% C02 gas at 1 Bar/20 °C.

Now a sample in the carrier gas goes by one detector. This sample has a thermal conductivity different from that of pure carrier gas. So the sample detector loses heat at a different rate from the reference detector. (Remember, the reference is the detector that NEVER sees samples — only carrier gas.) The detectors are in different surroundings. They are not really equal any more. So the bridge circuit becomes unbalanced and a signal goes to the chart recorder, giving a GC peak. 

A reference channel (quantum counter or photodiode) has two advantages (i) it compensates for the time fluctuations of the lamp via a ratiometric measurement (ratio of the output signals of the photomultiplier detecting the fluorescence of the sample to the output signal of the reference detector) (ii) it permits correction of excitation spectra (see below). 

Source compensation Pulse-to-pulse intensity variations and intensity fluctuations in the spectrometric excitation source are often the dominant noise source affecting the performance of the detection system. However, since OIDs are parallel multichannel detectors, these intensityvariations do equally and simultaneously affect the entire spectral distribution as a whole. Thus, with the aid of a single-channel reference detector, monitoring a portion of the source s light flux, it is possible to accurately normalize for spectrum-to-spectrum variations and practically eliminate these and any other source flicker noise related effects. 

At the end of the 3 sec averaging period the signal on the reference detector D2 is checked. If the reference channel indicates that the line is not exactly at the center of the 128 step ramp the computer adjusts the laser temperature to bring the line back to the center. 

Luminescence spectra were recorded on a double Czerny-Turner scanning monochromator Model 1902 Fluorolog Spex spectrof1uoro-meter. Variations in the excitation radiation are automatically corrected by a reference detector equipped with a Rhodamine B quantum counter. Emission spectra were recorded with the right angle mode. Further details are available elsewhere (31). 

The thermal conductivity detector (TCD) is based on changes in the thermal conductivity of the gas stream brought about by the presence of separated sample molecules. The detector elements are two electrically heated platinum wires, one in a chamber through which only the carrier gas flows (the reference detector cell), and the other in a chamber that takes the gas flow from the column (the sample detector cell). In the presence of a constant gas flow, the temperature of the wires (and therefore their electrical resistance) is dependent on the thermal conductivity of the gas. Analytes in the gas stream are detected by temperature-dependent changes in resistance based on the thermal conductivity of each separated molecule the size of the signal is directly related to concentration of the analyte. 

Fig. 4-6. Detector circuits for vapor-phase chromatography, (a) Thermistor detector Di, D=, Victory Eng. Corp. 32A12 thermistors Ri, Ri, 1,0000 wire wound resistors Rz, 1,0000 Helipot Rt, 10,0000 1% carbon film resistor Rs, 5,0000 1 % carbon film resistor Rt, Rj, 2,5000 1 % carbon film resistor Sw, single-pole four-position switch, (b) Hot-wire detector Rz, filament current control, to adjust filament current between 150—300 ma ( 20 ohm 5w) Ri, R4, reference detectors Rs, R, sample detectors Re, zero control 20 Re, 600 1 % carbon film resistor R, 300 1 /, carbon film resistor R, 150 1% carbon film resistor Rse, 7.5Q 1 % carbon film resistor Rs, 7.SCI 1 % carbon film resistor M, 300 ma meter,- S, single-pole six-position sv/itch.

The rays reach the sample detector via an interference filter. A reference detector receives the rays reflected by the wall of the Ulbricht s sphere. The concentration of the analyte can be inferred from the difference between the two reflectance values. In addition, by this formation of difference values the optical system equalizes fluctuations of the xenon flashlight. A sapphire window placed before the sample and reference detector minimises excessive evaporation of surface moisture from the test area of the test strip. The interference filter, which is also situated before the detectors, is method-dependent and is inserted into the instrument with the plug-in module (Fig. 47). 

Figure 31 -9 Schematic of (a) a thermal conductivity detector cell and (b) an arrangement of two sample detector cells and two reference detector cells (From J. Hinshaw, LC-GC, 1990,8, 298. With permission.)...

Now a sample in the carrier gas goes by one detector. This sample has a thermal conductivity different from that of pure carrier gas. So the sample detector loses heat at a different rate from the reference detector. 

T he luminescence instruments shown in I igures 15-10 and 15-11 both monitor the source intensity via a ret-crence photomultiplier. Most commonly, the ratio of the sample luminescence signal lo the signal from the reference detector is continuously obtained. This can compensate for source intensity fluctuations and drift. Both doubic-bcam-in space and doiihlc beam-in time designs are employed. 

The radiant sensitivity of a detector - or its quantum efficiency - is one of the most important parameters for TCSPC application. Unfortunately absolute measurements of the radiant sensitivity or the quantum efficiency are extremely difficult. The problem is not only that a calibrated light source or a calibrated reference detector are required but also that extremely low light intensities have to be used. However, accurate attenuation of light by many orders of magnitude is difficult. 

The Cathode Radiant Sensitivity" is the current of the photocathode divided by the power of the incident light at a given wavelength. Measuring the Cathode Radiant Sensitivity requires a lamp, a monochromator and a reference detector, e.g. a calibrated photodiode. The setup is difficult to calibrate due to the various error sources. 

A technique for measuring the quantum efficiency of a photon counting detector without a calibrated reference detector is described in [301, 356, 357, 358, 423, 536]. The technique is based on the generation of photon pairs - or entangled photons" - by parametric down-conversion. The principle is shown in Fig. 6.28. 

A matched photovoltaic reference cell is typically used as the reference detector and a solar simulator is used as the light source to minimize the deviation... 

Typically, two photodetectors are used in optical instrumentation because it is often necessary to include a separate reference detector to track fluctuations in source intensity and temperature. By taking a ratio between the two detector readings, whereby a part of the light that is not affected by the measurement variable is used for correcting any optical variations in the measurement system, a more accurate and stable measurement can be obtained. 

Given the low sensitivities of photodiodes, one may question their use as the start detector in the TCSPC instrument schematically represented in Figure 4.7. In this case the light source was a laser, which could be readily focused onto the small active area of a photodiode. Photodiodes are not used as the reference detector with flash-lamps because of their low sensitivity. When the light source is a flashlamp, the detector is either a PMT or a wire which acts as an antenna to detect the RF leakage during the lamp pulse. 

C-arm CT systems with automatic exposure control (AEG) adjust X-ray exposure parameters such that the detector entrance dose remains constant. Detector entrance dose is the X-ray dose measured behind the antiscatter grid. System dose is the detector entrance dose evaluated at a reference detector zoom format. System dose is an important set-up parameter for C-arm CT imaging protocols on Artis zee systems (Siemens AG, Healthcare Sector, Forchheim, Germany). Due to internal adjustments, the detector entrance dose for C-arm CT is about half the system dose. 

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