Opto mechanical design overview and definitions
Opto mechanical design is an engineering specialty related to the positioning of optical elements such as lenses, filters, beamsplitters, reflectors, and diffractive elements in mechanical structures that will allow the optical system to perform correctly.
A typical opto-mechanical design package would include:
drawings of all lens elements. Said drawings define part geometries and should be drafted according to ISO standards (read more about lens drawings here)
layout drawings, such as the one below, showing the full assembly of all the mechanical and optical elements and their individual positional tolerances.
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Part drawings, for each mechanical element including spacers, lens barrels, and adjustment mechanics each individual part. One example is a part drawing below; it includes nominal measurements as well as required tolerances. All the part drawings include all the dimensions, tolerances, and symbols needed to produce an object.
Assembly drawings describe the complete opto-mechanical system and provide the information necessary for assembling it. Besides outer dimensions, they include lists of parts the assembly contains and defines the connections between them; even simple mechanical parts in an optical system can require extensive documentation and notations for coatings, materials, and tolerances.
In complex parts, part drawings become much more detailed. One more example of a complex part drawing is shown below. The development of the opto-mechanical design involves extensive input from the lens designer. Tightening lens element tolerances can help relax mechanical and assembly tolerances, while the inverse is also true. Reaching the desired performance with the most robust and easiest to produce optical system is a balancing act.
Example: Zoom lens design
Opto mechanical design is also essential for zoom and focusing systems that have moving elements which require precision positioning. One typical system is a cam zoom system which includes three main components: a front lens group, a rear lens group and a cam mechanism.
The front lens group is attached into a front carriage and the rear lens group is attached into a rear carriage. Both carriages move along rods fitted into a lens housing parallel to the optical axis.
A typical distance for slider bearings and guide is just 6 to 10 microns (0.00025 to 0.0004 in.). The reason is that with larger clearance, the image can jump causing it to go out of focus when the zoom motion reverses.
The smaller the angle of ascent of cam slots the bigger will be the magnitude of the backlash of the focus ring when the zoom motion reverses.
The advantage of the cam system is that it allows you to move more than one lens group by a nonlinear law of a motion at the same time. A front lens group is moved according to one law of motion and a rear lens group is moved on another one.
The disadvantage of such a design is the high manufacturing precision required because of manufacturing errors in the helical cam and backlashes in conjugations between cam tracks and cam followers.
Opto-mechanical Lens Focusing With Threaded Assembly
One of the most common mechanisms used in zoom and focusing systems is to move, relative to the focal plane, all the lens elements in an assembly.
In the image below, changing distance “A” from focal plane to the first lens element is accomplished by turning a focusing ring.
All lens elements are mounted into a barrel. Lenses are held in place relative to each other with a threaded retaining ring. This maintains the elements in the assembly distance, centration, and tilt constant.
In the example below, we use a multiple-lead thread ( 4-8 individual coarse threads working in parallel) When the focusing ring is turned, the threads act together to axially move the lens sub-assembly.
The yellow retaining ring is screwed into the lens barrel to prevent unscrewing of the barrel during focusing.
If the lenses also rotate about the axis, the image will shift laterally while focusing. This can happen as a result of errors of lens and mechanical parts manufacturing or poor design.
To avoid this problem, the construction is made so that, with rotation of a focusing ring, the cell with lenses is moved only linearly.
Please see our next blog posts for a description of other common zoom/focusing mechanism.
(The images below were prepared by Optics for Hire Engineer Viacheslav Lossik and may be used with attribution.)
Autofocus Opto- Mechanical Systems-One Approach
Autofocus camera technology is so well established in consumer, industrial, and medical applications, that it is easy to forget the complex mechanical and electronics system required to make it work well. One widely used method is described below.
The basic idea is to change the distance from the lens to the light-sensitive surface (matrix or film) providing focus. This can be achieved by moving either the lens or the light-sensitive surface. Depending on the application, that movement is powered by an ultrasonic piezoelectric,stepper, or VCM motor.
The images below show lens movement controlled by an actuator. The actuator creates linear movement of a rod and a return spring ensures a force-closure. This eliminates any gap between the rod and carriage. Two guides are used in this system to minimize dynamic and static lens carriage position errors. The support electronics for the actuator provide the signal analysis required to determine the correct position.