Leica DVM User Manual

Leica DVM - 3D Visualisation

Vertical resolution in the balance between numerical aperture and
depth of field

Living up to Life

Vertical resolution in the balance between

numerical aperture and depth of field

Digital microscopy offers clear advantages for a wide variety of industrial quality inspections, particularly surface
analyses. Fracture analyses, analyses of inclined or vertical surfaces or onsite inspections of large parts such as turbine
rotors are just a few examples in which the strengths of digital microscopes make the biggest difference. But what are
the key criteria for successful use of digital microscopes and which parameters affect the three-dimensional imaging to
be expected for these specimens?

One of the main features of a digital microscope is the speed and ease with which it enables surface models to be created
of macroscopic and microscopic structures. In a qualitative evaluation, these provide better understanding and documen­tation of the specimen. In addition, quantification of the surface provides valuable information about the composition of
the surface and its wear. Which specimens are suitable for use with a Leica digital microscope, and what are the limita­tions of the method used?

The three-dimensional imaging of the Leica DVM series is based on the principle of focus variation. The limited depth of
field of the optics is utilized to determine depth information for the specimen. Vertical movement of the specimen relative
to the objective determines the focus information along with the distance to the optics. For each vertical position, the area
of the image that is in sharp focus is separated from the blurry area, and both are processed by the software to create a
surface model. One of the advantages of this method is that in addition to the height information, the texture of the speci­men is also documented.

Which influencing factors are determinative for successful creation of a 3-D surface model and how do these
variables influence lateral and vertical resolution?

2

Optics and Surfaces

Depth of field

The author of the first publication on
the subject of visually perceived depth
of field was Max Berek, who published
the results of his extensive experi­ments as early as 1927. Berek's formula
gives practical values for visual depth
of field and is therefore still used today.
In simplified form, it is as follows:

T

= n·[l/(2·NA2) + 340 μm/(NA·M

VIS

T

: Visually perceived depth of field

VIS

n: : Refractive index of the medium
in which the specimen is situated. If
the specimen is moved, the refractive
index of the medium that forms the
changing working distance is entered
in the equation.

1. Optics – in the balance between numerical aperture and depth of field

In microscopy, depth of field is in many cases an empirically understood
metric. In practice, the correlations between the numerical aperture, reso­lution and magnification determine this parameter. With their adjustment
options, today's microscopes create a balance between depth of field and
resolution that is optimal for the visual impression – two parameters that in
theory are inversely correlated. In DIN/ISO standards, the specimen-side
depth of field is defined as the "axial depth of the space on both sides of the
specimen plane in which the specimen can be moved without detectable
loss of sharpness in the image focus, while the positions of the image plane
and objective are maintained."

)]

TOT VIS

However, the standard does not give any clues on how to measure the
detection threshold of the deterioration of focus. Particularly at low mag­nifications, the depth of field can be significantly increased by stopping
down, i.e. reducing the numerical aperture. This is usually done using
the aperture diaphragm or a diaphragm that is on a conjugated plane to
the aperture diaphragm. However, the smaller the numerical aperture,
the lower the lateral resolution. Thus it is a matter of finding the optimum
balance of resolution and depth of field depending on the structure of the
specimen.

l: Wavelength of the light used; for

white light, lambda = 0.55 μm

NA: Specimen-side numerical aperture

M
microscope

If in the equation above, we replace
the visual total magnification with the
relationship of the useful magnification
(M
clear that in a first approximation, the
depth of field is inversely proportional
to the square of the numerical aper­ture.

: Visual total magnification of the

TOT VIS

= 500 bis 1000·NA), it becomes

TOT VIS

4

2. Texture of the specimen

The texture of the specimen surface, also known as its composition,
encompasses all of its features and characteristics. These include color
and brightness characteristics of the surface. As described above, the
principle of focus variation is based on the methodical approach of a lack
of specimen sharpness. The better the specimen can be divided into sharp
and out-of-focus areas, the better the results of the surface model will be.
This method is particularly well suited to textures that have a good cont­rast. As in many application areas of microscopy, the illumination is given
an especially important status, as it frequently determines success or fai­lure. Selecting a suitable illumination makes it possible to document even
a specimen with little texture. For example, you can select an oblique inci­dent illumination that makes even hidden structures visible.

Mechanics and Illumination

3. Mechanical resolution in the vertical direction

The third influencing factor in this equation is the mecha­nical resolution in the vertical direction. This term means
the smallest possible steps in the z-direction of the focusing
drive, which is usually motorized. To make full use of the
performance capacity of the optics, the smallest possible
step must be smaller than the currently used depth of field,
as otherwise image data are lost. A motorized focus drive
with a resolution of 10μm, for example, is suitable at a depth
of field of 15μm.

Which lateral and vertical resolutions are possible with a
Leica DVM system? As described above, these parameters
depend on various influencing factors, such as the sur­face structure or illuminator, and thus must be determined
depending on the application. Interpolation attains a verti­cal resolution of one-half of the applied depth of field. The
lateral resolution is determined by the numerical aperture
of the magnification used.

Illumination

Selecting the suitable illumination is critical to
the success of the examination. The modular
structure of the Leica DVM product concept
enables you to combine the selected optics
with the optimal illumination for the applica­tion. The following methods are available for
selection:

1) Variable oblique incident illumination: This
method changes the illumination direction
from vertical to lateral. This approach is par­ticularly suitable for visualizing scratches or
small recesses.

2) Diffuser: For shiny surfaces, the dynamic
range of the camera is insufficient in many
cases and many areas of the specimen are
overexposed. A diffuser provides reliable
reduction of the overexposed area.

3) Coaxial illuminator: A coaxial illuminator
is used for very shiny or reflective surfaces,
such as wafers or metal sections.

4) Polarized light: is used to supress the reflec­tions or for documentation of plastic materials.

5) Coaxial illuminator with directed light: In
the applications described above, the directed
light creates a three-dimensional impression
of the specimen. This is helpful in many cases
for determining the surface with greater accu­racy.

6

u

Coin with incident illumination Coin with oblique incident illumination

v

Soder joint without diffusor Soder joint with diffusor

w

Semiconductor with coaxial illumination Semiconductor with coaxial illumination

x

Plastic with polarised light Watch with polarised light

y

Semiconductor with coaxial illumination Semiconductor with directed coaxial illumination

Maximum vertical resolution of Leica DVM systems

Zoom Depth of field at Vmax Vertical resolution

Leica VZ75 C @160x 250 μm 125 μm
Leica VZ80 C / Leica VZ80 RC @ 400x 80 μm 40 μm
Leica VZ100 @ 350x (10450392) 420 μm 210 μm
Leica VZ100 @ 700x (10450393) 110 μm 55 μm
Leica VZ100 @ 1400x (10450394) 4 μm 2 μm
Leica VZ100 @ 1400x (10450395) 3 μm 1.5 μm
Leica VZ100 @ 3500x (10450411) 1 μm 500 nm
Leica VZ100 @ 7000x (10450412) 700 nm 350 nm

Example from a real-world application

To illustrate this, let us provide an example from a real-word
application. The fracture plane of steel in a tensile test is
to be determined for quality inspection. The desired results
are a qualitative representation as well as a quantitative
statement about the surface. The desired vertical resolu­tion is to be at least 120μm.

Here are the key optical and mechanical data of the equip­ment used:

• Optics used: Leica VZ80 RC, 8:1 zoom with magnification
range 50 – 400x, at a maximum magnification of 400x

• Depth of field at maximum magnification of 400x is 80μm

• Resolution of the motorized focusing drive is 500 nm

In the example provided here, a theoretical vertical reso­lution of 40μm (depth of field / 2) is attained. The texture of
the specimen to be examined has a high contrast range and
can be distinguished easily by the software into sharp and
out-of-focus areas. Therefore, the Leica DVM equipment
configuration selected is outstandingly well suited for this
application area and the required vertical resolution can be
attained.

Resolution and application limits of Leica DVM systems

Based on the requirements of the specimen, it is easy to
determine the suitable zoom optics. Usually, the criteria for
the decision are the field of view and the lateral and verti­cal resolution. Generally speaking, the applications of Leica
digital microscopy are in the microscopic and macroscopic
area. For example, you want to examine an object field of

0.2 mm with a vertical resolution of 10 μm. From the tech­nical data provided, you can tell that the Leica VZ100 with
a 140x objective is suitable for this application. Additional
help is provided by the Leica SmartTouch control unit based
on a touch panel, which shows the specific depth of field
being used on the display.

A challenge is presented by specimens that consist of
transparent plastic, for example, and are to be captured in
the microscopic range, i.e. with a digital magnification of
greater than 1400x. The material provides only a little tex­ture and the results will be unsatisfactory, even though the
Leica DVM system is optically and mechanically suited to
the task.

What alternatives do you have? For specimens with little
texture, you can switch to another imaging method, such
as confocal microscopy or interferometry. Leica DCM 3D,
for example, combines both technologies in one instrument
and is exceptionally well suited to specimens with little tex­ture.

The table on the left provides an overview of the perfor­mance capabilities and application areas of the Leica DVM
systems.

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