Cassegrain objective

Table 1 shows some approximate r values calculated for frequently used absorptions. NA values are assumed for customary Cassegrain objectives (for transmission and ATR, respectively). 

R is approximately between 4 and 30/mi in the mid-IR range (2.5-20//m) when NA = 0.70. Sometimes it is possible to obtain IR spectra of good quality for samples with dimensions below the theoretical limits. The high diffraction effects of Cassegrain objectives may require the use of two apertures, one below and the other above the sample, to minimize the diffraction in the transmission mode. 

Figure 2.20 illustrates the general layout of an FTIR imaging microspectrometer. The infrared beam from a Michelson interferometer is focused onto a sample with a reflective Cassegrain condenser. The light transmitted is collected by a Cassegrain objective and then focused onto an FPA detector. The imaging process... 

Figure 55. Beam path of a typical infrared microscope I = Infrared transmittance beam II = Infrared reflectance beam Ml =Condensing mirror M2 and M3= Cassegrain objectives M4 = Movable, semi-transparent mirror ...

The conventional mid-FTIR apparatus uses a standard FTTR bench coupled to a microscope equipped with reflective optics such as a Cassegrain objective and condenser, to focus the IR light onto the sample and collect the transmitted light for delivery to the detector (Fig. 6.4) [6]. 

Figure 6.4 The influence of spectral resolution (RES) and zero-filling factor (ZFF) on the detectability of IR spectral features of colon tissue. In this example, identical positions of a tissue sample mounted on a CaF2 window of a thickness of 1 mm were characterised by utilising a Bruker IR Scope II IR microscope. Transmission type IR spectra were recorded using a circular aperture of 900 pm diameter and a Cassegrain objective (36 x, NA 0.5, SR ca. 25 pm). A Happ-Genzel apodization function and a first derivative Savitzky-Golay filter with nine smoothing points were applied to the spectra.

Figure 6.5 Spectral features as a function of the apodisation function in mid-IR microspectroscopy of tissues (colon tissue cryo-section, same sample position as in Figure 6.4). Transmission type spectra were acquired using Bruker s IRScope II microscope and an IFS28/B spectrometer. Further measurement parameters aperture diameter 900 pm, Cassegrain objective (36 x, NA 0.5), 128 scans, optical substrate CaF2 pm of 1 mm thickness. Spectral resolution 6 cm zero-filling factor (ZFF) 4. Transmission spectra were processed with a first derivative Savitzky-Golay filter with nine smoothing points.

FT-IR spectrometers specifically manufactured for reflection microspectrometry are commercially available. Such spectrometers can be combined with most microscopes having infinity-corrected optics (in which the output from the Cassegrain objective mirror becomes parallel radiation). Since, in the infinity-corrected design, the output... 

Recent studies of micro ATR have defined the best experimental conditions for establishing optical contact between the ATR crystal and the sample [25]. This experimental approach has been applied to the analysis of the failure surfaces of adhesively bonded joints. ATR-microscopic measurements have been used for direct measurement and identification of raw materials in textiles coated and impregnated substances on paper [26]. An ATR microscopic probe has been developed which allows one to examine the sample optically through the probe in the microscope. The hemispheric ATR crystal is mounted at the focus of the Cassegrain objective, below the secondary mirror. One can position the crystal in contact with the sample, and run the spectra [27]. In the survey mode, visible light at nearly normal incidence is selected to locate the area of measurement. In the contact mode, low incident angle visible is used to detect contact of the sample to the ATR crystal surface. In the measurement mode, the ATR crystal is slid into position and the incident beam is optimized for total internal reflection. In the Spectra Tech version, all of the available crystals, i.e. ZnSe, Diamond, Silicon, and Germanium can be used. However, Ge and Si are opaque and cannot be used in the survey or contact mode. An optical contact sensor can be used. 

Figure 7.2 Optical block diagram of the wide field Kerr-gated microscope. Note the position of the sample (S), the sequence of three matched Cassegrain objectives (COi, CO2 and CO3), polarizers (Pi and P2), the Kerr medium (K) and blocking filters (F). A prism spectrometer (PR) can be inserted into the path of the gated light allowing monitoring of the collective spectral dynamics of objects within the field of view.

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