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Magnifiers: a closer look (XVI)
NIR spectroscopy of loupe materials

In the July issue we had presented the theory behind an advanced focal length measurement method (making up an almost universal micro-optics magnification measurement method!).

The respective practical procedure had been explained in August.

Some measurement results had been presented in September, October and November.


In December we had discussed a magnifier with plastics lenses. The respective resin type of those lenses (acrylics) had been identified by means of Raman spectroscopy.

In addition we had indicated that a very similar result might might have been achieved by an other spectroscopic method, faster and at a much lower cost. Here, just as a recap, once more an image of the discussed loupe:


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Fig. 1: EschenbachTM 8x magnifier with - quite respectable quality - plastics lenses.

The alternative method, the so-called NIR (near infrared) spectroscopy can be illustrated as follows (not the theory behind it, just the practical procedure):


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Fig. 2: A SCiOTM NIR spectrometer (on the left) in action. This instrument is merely matchbox sized. It weighs only 35 g. This spectrometer is sending a NIR flash through the round yellow window at the object under investigation. A light detector (situated behind the red square window) is analyzing the object's spectral response and passes it on to a smartphone with the dedicated SCiO software.

Besides, this instrument is not restricted to plastics materials. It is even able to discern painkiller tablets, to analyse chocolate and TupperwareTM containers! In the case of the mentioned Eschenbach magnifier we received the following result:


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Fig. 3: NIR spectrum of the Eschenbach plastics lenses (red line) in comparison with an acrylic resin reference spectrum (red line).

In order to verify this result we are going to show the NIR spectrum of a typical ordinary optical glass. There is a clear difference:


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Fig. 4: The blue overlay line is depicting the NIR spectrum of an ordinary optical glass.

Furthermore we should like to point out that many other types of artificial resins (plastics) can be discerned and recognized easily:


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Fig. 5: NIR spectra of some common artificial resins: CA: Cellulose acetate, CN: Cellulose nitrate, PC: Polycarbonate, PMMA: Poly (methyl methacrylate), PS: Polystyrene.


The SCiO is a quite remarkable instrument, capable to yield reliable results within seconds. Nevertheless its minimalistic solid state technology is resulting in NIR spectra which are not strictly comparable to other NIR spectra in literature. The reason behind is that the SCiO is internally working with 12 (filter) detectors. These are covering a wavelength range of ca.  800 nm to ca. 1050 nm. As a consequence the resulting spectra probably have to be understood as an overlay of 12 specific wavelength points whereas other spectrometers might work with a multiple quantity of sensor elements.
Nevertheless, when understood correctly as explained above, the instrument is well able to discern ca. 20 classic artificial resins! One might think of other spectroscopic methods for this task but all of them are much more complicated and much more expensive.

Let's have a look of a further NIR analysis example:


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Fig. 6: An incredibly poor quality triple lens folding magnifier marked "5x 10x 15x". We had already discussed the visually wobbly lens surface and the indicators of hopeless production problems in a previous issue of our magazine,


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Abb. 7: The NIR spectrum of this magnifier is indicating the use of polystyrene as a lens material. The figure is showing the NIR spectrum of the magnifier in fig. 6 (red line) in comparison to a polystyrene reference spectrum (green line).

This result is in accordance with our feeling that polystyrene shouldn't be regarded as the primary choice material for magnifier lenses. Even the most modest simple cylinder lens made of ordinary glass will deliver a much better performance!




Literature and further technical informations

A poster with a short characterisation of the SCiO spectrometer can be found here:
Tiny, "quick & dirty" NIR spectroscopy for the identification of plastics

Susanne Brunner and Martin Mach (2021): Tiny, 'quick and dirty'. NIR spectroscopy for the identification of plastics. In: Bechthold, T. (Ed.): Future Talks 2019. Surfaces. Lectures and workshops on the technology and conservation of the modern. Munich, p. 216-225.

The instrument used here is the basic version of the SCiO spectrometer which was available at a bargain price from the respective kickstarter project a few years ago.
Sadly, it appears that the low cost model is no more available (as the Consumer Physics company switched to a much more expensive licensing model based on yearly fees).



© Text, images and video clips by  Martin Mach  (webmaster@baertierchen.de).
The Water Bear web base is a licensed and revised version of the German language monthly magazine  Bärtierchen-Journal . Style and grammar amendments by native speakers are warmly welcomed.


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