Reference beams

Figure Bl.18.7. Principle for the realization of interference microscopy. The illuminating beam is split by beamsplitter 1 before passing the object so that the reference beam is not affected by the object. The separated beams interfere behind beamsplitter 2.

With gas-filled detectors, a chopped light system is normally used in which one side of the detector sees the source through the analyzing beam and the other side the reference beam, alternating at a frequency of a few hertz. 

The deuterium arc continuum travels the same double-beam path as does the light from the resonance source (see Fig. 21.9). The background absorption affects both the sample and reference beams and so when the ratio of the intensities of the two beams is taken, the background effects are eliminated. 

There is supporting evidence in the literature for the validity of this method two cases in particular substantiate it. In one, tests were made on plastics heated in the pressure of air. Differential infrared spectroscopy was used to determine the chemical changes at three temperatures, in the functional groups of a TP acrylonitrile, and a variety of TS phenolic plastics. The technique uses a film of un-aged plastic in the reference beam and the aged sample in the sample beam. Thus, the difference between the reference and the aged sample is a measure of the chemical changes. 

The top and the bottom x-ray detector each contain a multiplier phototube coated with phosphor. This tube compares the intensity of the x-ray beam entering the detector with that of the light from the reference standard, a discharge lamp. The reference beam is part of a circuit that maintains the x-ray source at constant intensity. The deviation wedge comes to rest when the intensities of the transmitted x-ray beams stand in a predetermined ratio. At this point, the unbalance in the servo system has been compensated, and the position of the deviation wedge consequently indicates the thickness of the strip. In 1955, this application was made fully automatic that is, the unbalance (or error signal) just mentioned was used to readjust tandem cold reduction mills of the United States Steel Corporation. Automatic control proved significantly more effective than manual control. 

Reversible reactions. Consider an experiment on the reversible first-order reaction A =i P in which two identical solutions were prepared at different times. The age separation between the two is denoted as t. The older solution is placed in the reference beam of a... 

Two identical reaction solutions were prepared, one at T,(= 25.000 °C) in the sample compartment of a double-beam spectrophotometer, the other at T2( = 27.170 °C) in the reference beam. A direct recording of AAbs = Absi - Abs2 was made as a function of time while the difference in reaction temperature was maintained to 0.0001 °C. Evaluate kffk and AW1 for the run shown note this calculation is possible with an arbitrary time axis. 

The cancellation of gas phase spectral features using the "half plate design Is far superior to methods Involving a second gas cell placed In the reference beam. This Is because the gas density and Its rotational state population will differ In the two cells for different sample (and therefore gas) temperatures. For high sensitivity measurements, these effects can be difficult to handle using two cells. 

We are now ready to evaluate the expressions in equation 53-59 and substitute then into equation 53-5. We will use the same value of k for both sample and reference beams. By having k the same, the results will be independent of the transmittance of the sample, as discussed previously. It also eases our task, since we will not have to compute a family of curves, but only one curve representing the change in computed transmittance as k varies. Evaluating it this way also eliminates the need to perform a double integration we can simply keep the sample transmittance constant at unity, and plot the variation in computed transmittance. 

The reference beam. This is the other light path. It s not visible light but another part of the electromagnetic spectrum. Just remember that the reference beam is the one farthest away from you. 

Reference beam aperture. This is where nothing goes. Or, in extreme cases, you use a reference beam attenuator to cut down the amount of light reaching the detector. 

Gently slide reference beam attenuator and watch pen — stop when pen points to 80 90% T... 

Fig. 129 Using a reference beam attenuator with a KBr window.

Remove your sample, and replace it with the standard polystyrene film sample. You will have to remove any reference beam attenuator and turn the 100% control to set the pen at about 90%, when the chart is at 4000 cm-1. 

Figure 6.9. Schematic of optical arrangement (top) and experimental setup (bottom) of two reference beam holography for recording images of a deforming droplet at different stages of spreading. (Reprinted from Ref. 368.)...

Figure 3.1 Schematic diagram of an AAS spectrometer. A is the light source (hollow cathode lamp), B is the beam chopper (see Fig. 3.2), C is the burner, D the monochromator, E the photomultiplier detector, and F the computer for data analysis. In the single beam instrument, the beam from the lamp is modulated by the beam chopper (to reduce noise) and passes directly through the flame (solid light path). In a double beam instrument the beam chopper is angled and the rear surface reflective, so that part of the beam is passed along the reference beam path (dashed line), and is then recombined with the sample beam by a half-silvered mirror.

Figure 3.2 Beam chopper in AAS. In a single beam instrument it is mounted vertically off-centre, so that it chops the beam. In a dual-beam instrument it is angled and mirrored so that it alternately allows the sample beam through and reflects the reference beam along the secondary path.

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