Moving optical element

The dispersive spectrometers suffer from greater wavenumber errors, of a less predictable form, owing to their general mechanical and thermal instability and can also be affected by non-uniform illumination across the monochromator entrance slit [26]. FT-spectrometers typically use a He-Ne laser as a reference beam to monitor the displacement of the moving optical element, so providing an active internal absolute wavelength calibration... 

Diode array instruments have no main moving optical elements therefore, no mechanical errors or drift arise. Consequently, the widely accepted rule in scanning spectrometry emphasizing that an accurate quantitation requires the use of the absorption maximum as analytical wavelength is no longer critical for diode array instruments. Therefore, such a choice, especially in the case of multicomponent analysis, should be focused only on reasons related to selectivity. 

Broadband instruments Broadband instruments measure solar irradiance in a specified wavelength range, typically 20 nm to 100 nm wide. This range is defined by the construction of the detector and it results from a combination of different optical elements such as filters and photoelectric sensors. The output signal of broadband instruments corresponds to the integral of the incident irradiance multiplied by the spectral response of the detector. Therefore, any information about the detailed spectral structure of the incident solar radiation is lost. On the other hand, the measurement is instantaneous and thus allows rapid changes in irradiance to be followed, due to fast moving clouds for example. 

Multivariate optical elements provide a number of advantages over other types of spectrometers the cost of creating the filters is low, and the design of the spectrophotometer is simple there is no moving part, thus reducing the need for instrument maintenance, and the instrument footprint is limited, with devices that can be reduced to hold in the hand. 

There are many other types of two-beam interferometers besides the one originally described by Michelson (see Chapter 5). Many of these interferometers do not vary the path difference between two beams by a single mirror moving at constant velocity. Except for stationary interferometers used for Fourier transform spectroscopy (Section 5.6), an optical element or combination of optical elements is moved so that the optical path difference is changed at a certain rate, known as the optical velocity or OPD velocity, V. For the Michelson interferometer, V = 2V. In general, the Fourier frequency for radiation of wavenumber v is given by... 

Some interferometers have been designed where the beam is dispersed across a linear array detector so that the interferogram is measured simply by reading the signal at each detector element without scanning any type of optical element. Such stationary interferometers are very attractive, as there are no moving parts and hence they are not susceptible to velocity errors. 

J (s) = Jl (s) dsi is the slit-smeared scattering intensity, P(t is the total primary beam intensity per slit-length element - a quantity determined by the moving slit device. R is the distance between sample and detector slit as measured on the optical axis of the camera. L is the (fixed and known) length of the detector slit in the registration plane. H is the (adjustable) height of the detector slit. exp(—jut) is the linear absorption factor of the sample19. 

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