Autofocus Systems Information
Autofocus systems obtain focus through feedback from range-finding or other sensing methods. They measure a parameter, adjust the distance between the lens and the object’s surface, and then measure the autofocusing (AF) parameter again. The process is repeated until the object is in focus according to the specified autofocusing parameter. Shallow depths of field, poor contrast in the sample, and low illumination can result in low-quality autofocusing. Dynamic focusing autofocus systems are used to continuously maintain focus, or for tracking a moving target. Some specialized autofocus systems use a laser to project a series of dots or lines onto the sample for autofocusing.
There are two main types of autofocus systems and autofocus sensors: passive and active.
Passive autofocus systems and passive AF sensors use contrast measurement differences or phase detection to determine focus.
Active autofocus systems and active AF sensors send a signal to the surface to illuminate and/or determine the distance to the sample. The signal can be a laser, light, or sound.
Both active and passive AF technologies are used in applications such as:
- microscope automation
- digital cameras
- lithography systems for mask alignment
- laser engraving and machining
- machine vision systems
- imaging analysis systems
- optical comparators
Some autofocus systems use techniques such as optical triangulation sensing and time-of-flight (TOF) sensing. Optical triangulation position sensors use reflected waves to pinpoint position and displacement. The source of these waves may be a light emitting diode (LED), infrared (IR) light, or laser. Typically, the sensing component for an optical triangulation position sensor is either a charged coupled device (CCD) or a photodiode (PSD). Time of flight optical sensors, or TOF sensors, determine displacement and distance by measuring the time it takes light to travel from the instrument to a target and back. These position sensors use either lasers or LEDs as light sources. Light emitting diodes (LEDs) are less powerful than lasers and, therefore, have a shorter effective distance in autofocus systems. Autofocus systems may use capacitive positioning sensing and ultrasonic position sensing methods. Capacitive linear position sensors are designed primarily to take non-contact measurements of position, displacement, and vibration. These sensors can be used on conductive and nonconductive target materials, but may be sensitive to environmental parameters that change the dielectric constant of the medium between the sensor and the target (usually air). Ultrasonic linear position sensors and switches use ultrasonic waves in combination with light and frequency readings. These autofocus systems use surface acoustic waves (SAW) and Lamb waves that reflect off the target.