Medical Design Briefs - June 2022 - 15

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Chromatic Focus Shift
conventional design
Corresponding NIR
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400
Wavelength [nm]
Fig. 4 - Comparing chromatic focus shifts between Tamron and conventional lens designs across
visible and NIR ranges.
The Modulation
Transfer Function
Modulation transfer function (MTF)
is a reference value that enables
optical designers to quantify the
overall imaging performance of a
system in terms of its resolution
and contrast. Designers often refer
to MTF data in applications like
fluorescence endoscopy that depend
on imaging accuracy for success.
Although a helpful value to know, MTF
does not always reflect the real- world
performance of the lens. Nonetheless,
understanding the MTF curves of the
lens and other parts within an optical
system allows designers to select
and optimize the components for
a particular resolution.
a broad spectrum of wavelengths, all
while minimizing the focus shift between
the visible near infrared, and even shortwave
infrared ranges (see the sidebar,
" The Modulation Transfer Function " for
more details).
For example, during testing, one Tamron
lens eliminated the need to refocus
between the visible light range and the
NIR range (from 400 to 850 nm), as well
as between the visible light range and the
short-wave infrared range (from 400 to
1,700 nm).
This lens minimizes the focus shift
thanks to its optical design, which includes
a variety of materials and spherical
and aspherical lenses designed to
meet the demands of various markets
(see Figure 2).
Medical Design Briefs, June 2022
Cov
MDB Tamron Feature 0622-CC_1.indd 15
Extra-Low Dispersion Glass
In imaging, chromatic aberration occurs
when a lens element refracts different
wavelengths at slightly different angles,
causing color fringing and reducing
the sharpness of an image. Lowdispersion
(LD) lens elements, which
are made of a special optical glass with
extremely low dispersion indices, can
combat this effect.
In other words, for this type of
glass, the refraction of a ray of light
into its colors is extremely narrow. As
a result, these lens elements compensate
for chromatic aberrations at the
center of the field. They also compensate
for lateral chromatic aberrations
that can occur at short focal
lengths toward the edges of the field
(see Figure 3).
Similarly, anomalous dispersion
(AD) optical glass offers even greater,
more precise control of chromatic aberrations
to enhance overall imaging
performance. This type of glass provides
an abnormally large partial dispersion
ratio for specific wavelengths
within the visible spectrum. Therefore,
combining AD glass with normal glass
makes it possible to control the dispersion
factors of specific wavelengths.
Optical lenses used in fluorescence endoscopy
should avoid chromatic aber -
rations, maintaining focus shift consistency
across extended wavelengths (see
Figure 4).
This article was written by Kota Yamamoto,
Section Manager, Medical Device Development,
Tamron, Saitama, Japan. For more
information, visit http://info.hotims.com/
82322-340.
www.medicaldesignbriefs.com
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Medical Grade Crystals,
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* Used in implantable devices
* Excellent long term aging
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Free Info at http://info.hotims.com/82322-772
Focus Shift

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Medical Design Briefs - June 2022

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