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telecentric fixed focal length resource guide 112 +44 (0) 1904 788600 | Edmund Optics® Section 7: Liquid Lenses in Machine Vision
Liquid lenses allow imaging systems to overcome depth of field
(DOF) limitations by allowing the focus to be electronically adjusted
without requiring any mechanical movement. This can be a
great solution for applications with varying object heights and WDs.
Traditional solutions for these types of applications include motorized
zoom lenses or repositioning the object to bring it into focus.
Another way to increase the DOF in a traditional lens is to increase
the f/# by reducing the aperture size of the imaging lens. However,
this also reduces the resolution and amount of light that gets
through the imaging system, thereby reducing acquisition rates
and image quality (see Section 2.1 for more information on f/#).
By integrating a liquid lens, an imaging system can change focus electronically
without compromising speed or image quality, regardless of
the object’s distance from the camera.
Like traditional optical lenses made from glass, liquid lenses are single
optical elements, but are composed of an optical liquid material that
can change its shape. The focal length of a glass lens is dependent
on the material it is made from and its radius of curvature. The same
basic principle applies to liquid lenses, though liquid lenses are unique
in that their focal length is alterable by changing the radius of curvature.
This change in radius is electronically controlled and rapidly
changed on the order of milliseconds. Manufacturers use electrowetting,
shape-changing polymers, and acusto-optical tuning methods to
control the liquid lens's radius of curvature and refractive index.
Most imaging lenses are multi-element assemblies because a single
optical lens provides insufficient imaging performance. For this same
reason, using a liquid lens by itself is not advised. However, integrating
a liquid lens with an imaging lens in a multi-element design, takes
advantage of the liquid lens's speed and flexibility. Having the ability
to focus both closely and to optical infinity in milliseconds makes
integrating liquid lenses an ideal choice for applications that require
focus at multiple distances where objects under inspection are different
sizes or are at different distances away from the lens; such as barcode
reading, package sorting, security, and rapid automation. Liquid
lenses can be used to maximize imaging system flexibility across a
wide variety of applications requiring rapid focusing.
Figure 7.1: The focal length depends on the index of refraction of
the glass and the shape of the surfaces of the lenses.
1
= (n – 1) × R1
1
The Lens Maker's Equation is used to calculate the focal length (f ) of
a lens and shows that focal length is dependent on index of refraction
(n), the radius of curvature of the left glass surface (R1), and the
radius of curvature of the right glass surface (R2). The focal length can
be shortened by increasing the index of refraction of the glass - view
top image or the focal length can be shortened by changing either of
the radii of curvature (R) – view bottom image. Liquid lenses work by
electronically controlling these properties of the lens.
Response Time
Liquid lenses have very fast response times, are electrically tunable
via voltage or current, and respond in a matter of milliseconds. Fixed
focal length and zoom lenses often rely on mechanical or manual adjustments
to change focus, which can slow the imaging system.
Versatility
Liquid lenses can be implemented in various locations throughout an
imaging system, such as within or threaded to the front or rear of an
imaging lens.
Size
Mechanics are eliminated in a liquid lens which enables their compact
design. In addition, liquid lenses are comparable in functionality to
many individual lenses within an assembly. By removing those lenses
and replacing them with a small liquid cell, the overall size and weight
of the full lens is reduced.
Sensor Coverage
Liquid lenses are limited by their small aperture sizes - the largest is
approximately 16 mm in diameter. The small apertures restrict liquid
lenses and only allow for use with sensors up to about 1/1,8” if the lens
is retrofitted onto the front of an existing machine vision lens. Liquid
lenses embedded inside systems do not reduce sensor coverage.
Focusing
Liquid lenses are designed to operate across a very wide range of optical
powers (focal lengths) at high speeds. By eliminating the moving
parts and mechanical adjustments typically found in fixed focal length
and zoom lenses, the focusing process is quicker.
Complexity of Integration
Depending on available equipment and the application, liquid lenses
can be difficult to integrate. Liquid lenses can be used in conjunction
with many accessories, such as filters and apertures, which are
required by many high-speed applications, such as distance sensors
or controllers.
Lifetime
Liquid lenses consume very little power. Typical mechanical lenses
are capable of ~100.000 cycles, whereas a liquid lens can withstand
approximately ~50.000.000 cycles (Berge, 2013).
Section 7.1: Liquid Lens Basics
Section 7.2: Liquid Lens Features
f
1 –
R2