=Paper=
{{Paper
|id=Vol-2744/short49
|storemode=property
|title=Optical Design of a Dual Channel Microscope (short paper)
|pdfUrl=https://ceur-ws.org/Vol-2744/short49.pdf
|volume=Vol-2744
|authors=Anastasiya Kozhina,Evgeniia Soshnicova,Alla Uvarova
}}
==Optical Design of a Dual Channel Microscope (short paper)==
https://ceur-ws.org/Vol-2744/short49.pdf
Optical Design of a Dual Channel Microscope 1
Anastasiya Kozhina1[0000-0003-3547-9771], Evgeniia Soshnicova1 [0000-0003-4847-5825] and Alla
Uvarova1[0000-0002-5075-8376]
1ITMO University, St. Petersburg, Russia
crensta@mail.ru
Abstract. The article presents a new scheme of an optical microscope. The
proposed scheme contains two channels: one works with a wide field of view,
the other forms a high-resolution image. The simultaneous fulfillment of these
conditions significantly increases the information capacity of the system. The
proposed scheme overcomes the limitations imposed by the Lagrange invariant.
In addition, the presented work solves the relevant task of optical instrumenta-
tion: using one device,it becomes possible to simultaneously obtain an image of
an object in two scales, one of which gives a detailed image of microstructures,
and the other shows the state of the area around the part of the object being
studied in detail. This time synchronization is especially important for studying
biological objects. The design features of the optical scheme make it possible to
abandon the refocusing of the objective, which is necessary in biological micro-
scopes when the magnification is changed. Thus, the proposed system solves
not only scientific and technical tasks, but also improves the ergonomics of
work in microscopy.
Keywords: Microscope, Optical design, Light microscopy.
1 Introduction
Microscope is one of traditional types of optical devices used to research of micro-
structures. Working with a microscope is closely related to the peculiarities of how
operator perceives the object under research. It is known that before the formation of
a sensory image in a person's perception, there is a detection, distinction and identifi-
cation of an object (in whole or in its details). It should be noted that systems of ma-
chine vision try to implement a similar algorithm of working with objects [1, 2]. In
the case of observation with unaided eye, all stages of shaped vision (from isolating
the object of research from the general background to identifying its main features)
occur sequentially [3] with the help of eye accommodation and refocusing of opera-
tor’s attention.
The purpose of using optical devices is empowerment of human eye. Each of the
mentioned stages of perception assume the discrimination of smaller elements; there-
1 Copyright © 2020 for this paper by its authors. Use permitted under Creative Commons Li-
cense Attribution 4.0 International (CC BY 4.0).
2 A.Kozhina, E.Soshnicova, A.Uvarova
fore, the optical device should help to increase the resolving power of the operator's
eye. In visual instruments, this is associated with an increase in magnification. The
traditional solution of this purpose in microscopy is using of a revolver with several
micro-objectives in the device.
Nowadays, high requirements on optical devices in terms of the quality of the re-
sulting image are imposed. Also, consumers often want to observe as many objects as
possible at the same time. However, the ability of an optical system to provide a high-
resolution image in a wide field is impossible on the basis of the Lagrange invariant
[4], which expresses the law of conservation of information in geometric optics. The
Lagrange invariant connects the aperture and size of the object with the aperture and
size of the image, thereby imposing a restriction on the transformation of beams.
Changing the magnifications in the microscope takes away a significant part of the
information about the object: the operator is deprived of the opportunity to observe
what is locating around the research field, which is always subject of great interest in
the case of living biological samples.
In addition, the change in magnifications disrupts the continuity of the observation
process, therefore, does not allow use operator's psychophysical abilities effectively.
Also, this action takes time to refocus the system, which can be critical when studying
samples that change their appearance over time: they can be lost from the field of
view or, in the case of living cells that are sensitive to light, irradiate them in vain,
wasting their life resource only to configure the system [5].
As a result, there is a contradiction: the operator is either limited in resolution but
retains the possibility of indirect observation of the rest of the field, or acquires the
ability to see details with high resolution, but only in a very small area. Average mag-
nification with an average field of view is a compromise solution that includes the
disadvantages of both options rather than their advantages.
The purpose of this work is to design an optical scheme of a microscope that al-
lows observing objects simultaneously in a wide field and with a high resolution. The
use of such systems will make it possible to eliminate the need of lens refocusing
arising from a sharp decrease in the depth of field and will also provide the possibility
of detecting and identifying the structures under research without reconfiguring of
visual instrument.
2 Modern Microscopes
As stated in the Abbe diffraction theory, the resolution of a microscope is directly
related to its numerical aperture and wavelength [6].
𝜆
𝜎 = 2𝑁𝐴 (1)
In the case of work in the visible range, the resolution of the microscope entirely de-
pends on its numerical aperture. However, as mentioned above, observation of an
image at once in a wide field and with a high aperture is limited according to the La-
grange invariant.
Optical Design of a Dual Channel Microscope 3
In order to overcome this restriction, it was decided to develop a dual-channel opti-
cal system. Such systems are widely used in optical instrumentation and in microsco-
py in particular. Examples of instruments with observation in two channels are com-
parative microscopes and forensic microscopes. For instance, the LEICA FS CB [7]
microscope shown in Figure 1. Microscopes of this type have two channels and are
designed to compare two objects. Although such instruments have the theoretical
possibility of observing objects in one channel with a wide field of view and in the
other with a high aperture, using these types of microscopes experts research two
different objects.
An example of two-channel devices, both channels of which are directed to the
same object, are microscopes using interference methods. They have found wide ap-
plication in optical coherence tomography [8-9]. However, only one of these channels
is for object observation. Another channel of these microscopes is for illumination.
Wide-field high-aperture microscopes are also known [10-11]. For example, elec-
tronic and fluorescence microscopes. Nevertheless, they use methods other than opti-
cal ones, therefore they cannot be analogues of this development.
Fig. 1.Comparative mechanical microscope for the research of micro-objects LEICA FS CB
3 Prototype of the Development Device
The prototype of the development is basic optical design of the research metallo-
graphic microscope MIM-7 [12] (see Fig. 2). It is classified as light microscope and
its frontal objective 12 provides two-channel operation.
The illumination channel includes a light source 1, the rays from which are di-
rected by the collector 2 to the mirror 3. Reflecting from the mirror, the light beam
sequentially encounters on its way a light filter 4, aperture diaphragm 5, lens 6 (and
4 A.Kozhina, E.Soshnicova, A.Uvarova
polarizer 21, if it is included into system), a photo shutter 7 and a field diaphragm 8.
The pentaprism 9 re-reflects the beam, which is then refracted by the lens 10 and falls
on the semitransparent reflective plate 11.
The scheme of the lighting channel is completed by microlens 12 and illuminated
research subject 13. At the same time, at this place the observation channel begins: in
a reverse beam path from sample through the microlens 12 and the beam-splitting
semitransparent plate 11. After the beam-splitting plate 11 the light passes through the
plug-in analyzer 20 and is refracted by achromatic lens 14. Depending on the selected
method of observation (visually or with a photodetector) operator can change the
position of the mirror 18. When the mirror 18 is included into the system, the beam of
rays enters the eyepiece. To photograph the object, the mirror is removed from the
system, and the beam of rays continues to move through the photo eyepiece 15, is
reflected by the mirror 16 and falls on the image receiver 17.
Fig. 2. Optical system of the metallographic microscope MIM-7
Optical Design of a Dual Channel Microscope 5
As mentioned above, the lens 12 provides two channels. It is designed as objective
with infinity tube, which allows the division of light without adding aberrations.
However, such a lens is not designed for use with a cover glass that is necessary for
the research of biological objects, which, at high apertures, will add large spherical
aberration. Furthermore, there is only one observation channel in the optical scheme
of a metallographic microscope. The illumination in this system is in reflected light,
while the research of biomaterial requires illumination in transmitted light. Based on
the foregoing, although the optical scheme of metallographic microscope MIM-7 is
the basis for the optical system under development, its basic optical design is far from
being a complete analogue.
4 Layout Description
The proposed scheme of a dual-channel microscope is shown on Figure 3 and consists
of a frontal, wide-field and high-aperture objectives, a beam splitter and optical image
receivers.
The front lens is designed for simultaneous wide field and high aperture operation.
Its presence removes the need for refocusing on the subject, which usually occurs
with changing the magnification in laboratory microscopes.
A beam splitter separates the light into two channels. To minimize aberrations, it
should have an aperture diaphragm. Moreover, a parallel beam of rays should fall on
the beam splitter, since in case an oblique beam falls on the beam splitter, intractable
coma and astigmatism will appear in the system [13-14]. For this reason, the front
lens must be designed as objective with infinity tube. Moreover, it must be designed
to work with a cover glass, which is necessary for the research of biological objects.
Fig. 3. Block diagram of a dual-channel microscope
The first channel will transmit information with a wide field, which is necessary for
observing large objects, as well as detecting anomalies in biomaterial. A wide-field
lens should provide the necessary correction of field aberrations: astigmatism, distor-
6 A.Kozhina, E.Soshnicova, A.Uvarova
tion, curvature. The second channel transmits information with high resolution, which
allows a detailed study of the object of research. In this channel, it will be necessary
to correct aberrations of wide beams of rays: coma, spherical aberration, spherochro-
matism.
Figure 4 shows the layout of dual-channel optical microscope in thin components.
The wide-field channel magnification is 4x, which corresponds to a number of stand-
ard microscope magnifications. The numerical aperture in this channel is 0.1. The
second channel is designed for 40x magnification with an aperture of 0.5. Working at
this aperture will allow viewing objects with a size of 0.55 μm, according to (1).
Fig. 4. Layout of dual-channel optical microscope
It is proposed to implement illumination in a dual-channel microscope according to
Koehler (see Fig. 5) [15]. This lighting system consists of collector, condenser and
two iris diaphragms: field and aperture. Source 1 is shown by collector 2 in the plane
of aperture diaphragm 3 located in front focal plane of the condenser 4. The field
diaphragm 7 is projected by the condenser in the plane of the studied biological object
5. Also, condenser projects the aperture diaphragm into the entrance pupil of the ob-
jective 6, that is, at infinity. In dual-channel microscope it is necessary to highlight
whole size of the object in the broadband channel with diaphragm 3, and the numeri-
cal aperture of the high-aperture channel with diaphragm 7.
Fig. 5. Lighting layout according to Koehler
5 Conclusion
Modern analysis of biological materials requires increasing of the capabilities existing
research instruments. The device based on the proposed scheme enables the operator
to receive and register information about the object simultaneously on two scales.
Optical Design of a Dual Channel Microscope 7
This demonstrates a new approach to solving the conflict between resolution rage of
the device and its field of view. Also proposed instrument significantly improves the
ergonomics of working with the microscope, eliminating the need for the operator to
configure the device too many times.
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