White light detector

Schematic optical layout of FT-IR spectrometer. A = Source, B = Off-axis paraboloid, C = Beamsplitter, D = Fixed mirror, E = Movable mirror, F = Off-axis paraboloid, G = Sample point, H = Off-axis ellipsoid, I = Infrared detector, J = Visible beamsplitter, K = Fixed mirror, L = He Ne and white light sources, M = He Ne and white light detectors.

In conventional chip experiments, fluorescence scanners are used for chip read-out. In the case of laser scanners, HeNe lasers are used as excitation sources and photomultiplier tubes as detectors, whereas CCD-based scanners use white light sources. The optical system can be confocal or non-confocal. Standard biochip experiments are performed using two fluorescent labels as... 

Here is an extremely sensitive method for measuring nitrite (NOj) down to 1 nM in natural waters. The water sample is treated with sulfanilamide and N-( 1 -naphthylethylenediamine) in acid solution to produce a colored product with a molar absorptivity of 4.5 X ItHM" 1 cm- 1 at 540 nm. The colored solution is pumped into a 4.5-meter-long, coiled Teflon tube whose fluorocarbon wall has a refractive index of 1.29. The aqueous solution inside the tube has a refractive index near 1.33. The colored solution is pumped through the coiled tube. An optical fiber delivers white light into one end of the tube, and an optical fiber at the other end leads to a polychromator and detector. 

Analogous to the principal concept of multiplex CARS microspectroscopy (cf. Sect. 6.3.5), in multiplex SRS detection a pair of a broad-bandwidth pulse, eg., white-light femtosecond pulse, and a narrow-bandwidth picosecond pulse that determine the spectral width of the SRS spectrum and its inherent spectral resolution, respectively, is used to simultaneously excite multiple Raman resonances in the sample. Due to SRS, modulations appear in the spectrum of the transmitted broad-bandwidth pulse, which are read out using a photodiode array detector. Unlike SRS imaging, it is difficult to integrate phase-sensitive lock-in detection with a multiplex detector in order to directly retrieve the Raman spectrum from these modulations. Instead, two consecutive spectra, i.e., one with the narrow-bandwidth picosecond beam present and one with that beam blocked, are recorded. Their ratio allows the computation of the linear Raman spectrum that can readily be interpreted in a quantitative manner [49]. Unlike the spectral analysis of a multiplex CARS spectrum, no retrieval of hidden phase information is required to obtain the spontaneous Raman response in multiplex SRS microspectroscopy. 

Several other types of filters are discussed by Chase (5, 7). Filters are also necessary to remove the optical output of the He-Ne laser (used for referencing) because the laser has optical axes colinear with the main laser source. Since the detectors used in FT-Raman are sensitive to the He-Ne wavelength, the laser is a source of interference. Here, plasma emission filters can be used. The white light of the instrument is filtered with a near-IR cutoff filter. A final filter may be used in front of the detector. 

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