Cassegrain condenser

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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... 

The condenser goes by two names some call it a Cassegrain, while others call it a Schwarzschild objective. The present author has always been rather confused about the difference between a Cassegrain and a Schwarzschild objective but, aiming to resolve such confusion, contacted one of the designers of the first FT-IR microscopes. Bob Messerschmidt (now with Rare Light, Inc.), as to their difference. His reply to this question was as follows ... 

Reflectance slider b Objective Cassegrain Sample position Condenser Cassegrain View/IR mirror... 

Some of the new IR microscopes on the market are capable of minimising these problems. An IR-Plan IR microscope (obtained from Spectra-Tech, Stamford, CT) has been used to obtain IR spectra of Kevlar filaments and also to carry out orientation measurements. The microscope uses a 15x IR/visible reflecting lens with Cassegrain optics to focus the IR beam onto the sample (Refleachromat, Spectra-Tech). A lOx reflecting Cassegrain beam condenser is used to collect the transmitted beam after it passes through the sample. The beam condenser also has a sample compensator, which can be used to correct for optical aberrations caused by the fibre samples. The microscope uses its own narrow band liquid... 

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]. 

Cassegrain. The light reflects from the sample, is collected by the upper Cassegrain, and is then focused on the detector. The upper Cassegrain acts as both condenser and objective, and the lower half of the microscope is not involved in this measurement. We have now seen that an infrared microscope can be used in four different ways visible light transmission, visible light reflectance, infrared transmission, and infrared reflectance. The mode that will provide the best viewing and spectra will depend upon the sample. 

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