Wide-bandpass

In comparison to infrared detectors, it is much more difficult for silicon-based optical detectors to achieve high QE over a wide bandpass. The main challenge is the tremendous variation of absorption depth shown in Fig. 8. In addition, the index of refraction varies significantly for A = 0.32-1.1 m, as shown in Fig. 10, making it difficult to optimize anti-reflection coatings for broad bandpass. 

Effect of bandpass and choice of wavelength on a Beer s law plot. Curve A represents a calibration curve using a narrow bandpass monochromator at k. Curve B represents a calibration curve using a wide bandpass filter at X or a narrow bandpass monochromator at. ... 

A slightly less accurate determination is possible using a colorimeter with a wide bandpass filter, e.g. a simple (non-interference type) purple-red filter but an interference filter in a quality instrument gives results comparable to a spectrophotometer. 

Tsuruta, H., Brennan, S., Rek, Z. U., Irving, T. C., Tompkins, W. H., and Hodgson, K. O. (1998). A wide-bandpass multilayer monochromator for biological small-angle scattering and fiber diffraction studies. J. Appl. Crystallogr. 31, 672—682. 

The type and number of fluorescent probes also plays a role in the optimizing of the filters. For a small number of probes with adequate spectral separation it is possible to use traditional wide bandpass filter sets. In protocols where 5 or 6 probes are being used, it is necessary to use dye-specific narrow band filter sets to reduce spectral bleedthrough. 

Figure 3-7 Spectral characteristics of a sharp-cutoff filter (a) and a wide-bandpass filter (b). The narrow-bandpass filter (c) is obtained by combining filters a and b. The spectral bandwidth of filter c (distance n-m) is defined as the width in nanometers of the spectral transmittance curve at a point equal to one half of maximum transmittance.

The irreversible loss of signal was noticed earlier by Jeanmaire and Van Duyne, who also reported that different modes had a different intensity dependence on the electric potential. However, all the modes gave a maximum signal at -0.55 to -0.85 V. An important experimental point to note is that such potential-depen-dent measurements should be, and were, carried out with a wide bandpass of the spectrometer, to allow for the shift of the band positions with potential. 

An alternative wide bandpass system is to replace the soap film with an attenuator such as aluminium foil which gives a low energy cut-off and is sometimes used in Laue protein crystallography (chapter 7). 

Another requirement of the monochromator is that it should be capable of using wide slits—i.e., wide bandpasses, for elements whose emission spectra are uncomplicated. Zinc, arsenic, selenium, calcium, lead, and the alkalis are examples of such elements. When the bandpass is wider, more energy can be passed through the monochromator and the obtainable precision and detection limits are better. 

Once the target is ascertained to be present, a wide bandpass filter can be gradually narrowed about 2 /( — /l or 2I/2 — /l1 and thereby used to obtain Doppler information, Alternatively one could, of course, switch to a conventional configuration. 

Fig. 4. Beer-Lambert law. The spectrum and the bandpasses used for measurement are shown on the left, and the relation of absorbance to concentration is shown on the right, (a) Measurements made with a narrow bandpass at an absorbance maximum (solid lines), (b) with a wide bandpass at an absorbance maximum, or with a narrow bandpass on the side of a peak (dashed lines), showing negative deviation.

Background in a Test Dewar - Wide Bandpass Filter 71... 

Spectral break wide bandpass into many narrow bands... 

Now calculate the background if we use the wide bandpass filter of Figure 2.21. 

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