Broadband illumination

Several approaches can be taken to minimize the effect of standing waves. Since the phenomenon results from coherent interference of monochromatic photons it can be reduced by employing broadband illumination. 

Figure 8.4 ATR-FTIR spectra of H—Si(lll) after exposure to air (a) in the dark, (b) with 450 nm (broadband) illumination, (c) with 350 nm (broadband) illumination, and (d) with Hg lamp illumination. Reprinted with permission from Reference [48], Copyright 2000 American Chemical Society.

An approach to full spectroscopic near-field imaging in the IR range was presented by Michaels et al. [62], whose illumination-mode a-SNOM set-up was based on a Ti sapphire-pumped optical parametric amplifier (OPA) coupled into a tapered fluoride glass optical fiber. The OPA system provides tunable broadband IR radiation with output powers in the miUi Watt range. After passing the thin film sample, the transmitted fight is collected by a Cap2 lens and coupled into a monochromator with a detector array. Broadband illumination with a bandwidth... 

The inverted geometry is of little consequence in most cases, but it should be noted that reactions of photosensitive samples can be induced by the fairly strong, broadband illumination (Fig. 2). This can be a problem in (micro)preparative work where the material needs to be recovered from the eluent. It can also lead to spectral distortions if sufficient photoproduct is formed while the material is passing through the detector cell. We have observed such reactions with bacteriochlorophyll b in acetone-containing solvent systems, where a ready photooxidation and/or isomerization to chlorophyll derivatives is possible. An illustration of bleaching of a photo-reactive protein is shown in Fig. 2. In absorption measurements, care also... 

The second building block is the illumination unit. Depending on the application, broadband light sources ranging from LEDs to specially modified kW-floodlights may be used. The light sources must provide an even, stable illumination of the sample, as shadows or other inhomogeneities in illumination may lead, for example, to erroneous classification results. 

It is important to understand that the differences in irradiance or illuminance intensities observed during chamber mapping are not simply artifacts of broadband radiometric measurements that is, the differences measured should translate into significant corresponding differences in the rates of drug photodegradation as a function of location in the chamber. This topic has been studied by both Baertschi (16) and Clapham and Sanderson (17). 

Figure 8.33. Temperature dependence of quantum efficiency for front-illuminated silicon CCD. Spectra of a broadband source with the CCD at the indicated temperatures were divided by a spectrum of the same source taken at 25°C. The observed Q at 750 nm was nearly independent of temperature. (Adapted from Andor Technologies product literature.)...

GWS in the bottoms of 384-well microtiter plates. When illuminated with broadband light, the optical sensors inside each well reflect only a specific wavelength that is a sensitive function of the index of refraction close to the sensor surface. The platform has a sensitivity of 5 pg/mm, which enables the detection of the binding of a 300-Da molecule to a 70-kDa immobilized molecule [29-31]. 

Fig.2. Current-density versus voltage (IV) characteristics of an Al/p-Si/PCBM/Al heterojunction at 295 K (top graph) and at 80 K (bottom graph). The inset shows an Arrhenius plot of the reverse current-density at -2 V bias. In the bottom graph the IV-characteristics at 80 K under broadband IR illumination (red triangles) is shown [2],...

This is the intensity of the exposure radiation in the plane of the wafer. The extended source method, or Hopkins method,is often used to predict the aerial image of a partially coherent, diffraction-limited, low-numerical-aperture-aherrated projection system based on scalar diffraction theory. For very high NA, vector calculations involving the complete solution of Maxwell s equation are used. The illumination may be of a single wavelength or it may be broadband. The illumination source may be a conventional disk shape or other more complicated shapes as in off-axis illumination. ... 

Fig. 9. DIS2 fluorescence image of a single petri dish. The plate is identical to the one in Fig. 8, but the instrument has been reconfigured for fluorescence imaging. Broadband blue-green light has been used for illumination, and the camera lens has been covered with an 865 nm long-pass filter. LHII is normally fluorescent in this bacterial strain because the reaction center (photochemical trap) has been genetically deleted. The same five colonies have been expanded below note that the colony on the right is highly fluorescent.

To avoid confusion between IPCE and solar-to-hydrogen conversion efficiency (STH), we should note three critical differences. First, in contrast to IPCE (electrons out/photons in), STH describes efficiency in terms of power (power out/ power in). Secondly, whereas IPCE measurements can be conducted with any calibrated and monochromated illumination source (i.e. it need not be AM 1.5 G illumination that is monochromated as long as the number of impinging photons at each wavelength are counted), STH requires the use of broadband solar-simulated illumination. The reader is referred elsewhere [5] for a discussion of solar simulation using laboratory illumination sources. The integration of IPCE over the entire solar spectrum can provide an estimation of the maximum possible STH, if (and only if) no applied bias is used in the IPCE measurement. Lastly, conducting IPCE experiments with an applied bias is allowable whereas STH requires true zero-bias conditions. Of course, the authors of a publication must be explicit as to... 

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