Introduction to Liquid Lenses
This is Section 8.1 of the Imaging Resource Guide
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 working distances. Traditional solutions for these types of applications include motorized zoom lenses or physically 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 can also reduce the resolution and amount of light that gets through the imaging system, thereby reducing acquisition rates and image quality. 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 they can change their focal length by altering their radius of curvature. This change in radius is electronically controlled and rapidly changed on the order of milliseconds. Manufacturers use technologies ranging from electrowetting to shape changing polymers to acusto-optic tuning to control the radius of curvature and index of refraction of the liquid lens.
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 ill advised. However, by integrating a liquid lens with an imaging lens in a multi-element design, the speed and flexibility of a liquid lens can be taken advantage of. Having the ability to focus both up close and to optical infinity in milliseconds makes integrating liquid lenses an ideal choice for applications that require focusing at multiple distances where the 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 1: Lens Maker’s Equation can be used to calculate the focal length of a lens. The equation shows that focal length is dependent on index of refraction (n) and radius of curvature (R). The focal length (f) can be shortened by increasing using a higher index of refraction glass - view top image. Or the focal length (f) can be shortened by changing the radius of curvature (R) – view bottom image. Liquid lenses work by electronically controlling these properties of the lens.
Integrating Variable Focus Liquid Lenses with Fixed Focal Length Imaging Lenses
TECHSPEC® Cx Lenses are fixed focal length imaging lenses with a unique, modular 3-piece design. The front and rear lens elements are separately housed allowing easy access to the center aperture stop of the lens. This allows users to easily integrate a Variable Focus Liquid Lenses into the center of the Cx Lenses. Available in various focal lengths, TECHSPEC® Cx Lenses provide the electronic autofocus working distance adjustment of a liquid lens with the image quality performance of a multi-element imaging assembly. The step by step diagram bellows explains how a Variable Focus Liquid Lenses can be installed in the Cx Lenses. For more information on our Cx Lenses, view the product family page.
|Variable Focus Liquid Lenses|
|Clear Aperture (mm):||2.5||3.9|
|Focus Range (diopters):||-5 to +13||-5 to +15|
|Liquid Lens Stock No./Price||#34-282||$95||#34-283||$125|
|Development Kit* Stock No./Price||#34-284||$795||#34-285||$795|
Integrating Optotune Focus-Tunable Liquid Lenses with Fixed Focal Length Imaging Lenses
Optotune Focus-Tunable Liquid Lenses are available with C-Mount housing. By using the appropriate thread adapter, the Optotune Focus-Tunable Liquid Lenses can be easily attached to the front of a TECHSPEC® C Series or Ci Series Fixed Focal Length Imaging Lens. Attaching a liquid lens to existing fixed focal length lenses can be useful in applications requiring a large depth of field. The liquid lens allows electronic focus throughout the focus range of the imaging lens. The step by step table below shows the appropriate imaging lens, adapter, and liquid lens combination required for various sensor sizes. Since the liquid lens is placed in front of the fixed focal length lens, it can become the limiting aperture and vignetting should be considered. To learn more about vignetting, see Sensor Relative Illumination, Roll Off and Vignetting.