【Optical Knowledge】Automatic Focus Technology
What is autofocus?
Autofocus refers to the ability to automatically adjust the focal length of a lens to maintain image sharpness based on the distance between the object and the vision system. It utilizes the principle of light reflection from the object, where sensors on the camera receive the reflected light and process it through a computer to drive the autofocus mechanism. Autofocus technology is typically divided into two types: active and passive.
Active:
This involves the use of infrared or ultrasonic generators on the camera to emit infrared light or ultrasonic waves to the object being measured. Receivers on the camera then detect the reflected infrared light or ultrasonic waves for focusing, similar to the principle of triangulation autofocus.
Passive:
This method directly receives and analyzes the reflected light from the object itself for autofocus. The advantage of this autofocus method is that it does not require a separate emission system, resulting in lower energy consumption and ideal autofocus for objects with sufficient brightness. This is typically known as contrast detection autofocus. In industrial applications, passive autofocus is more commonly used.
How is autofocus achieved?
Cameras equipped with autofocus capabilities typically use software combined with liquid lenses to adjust the focal length for achieving focus. Autofocus action occurs only when the optimal focus point in the area requiring focus is reached, after which autofocus stops. This process involves receiving reflected information from the object, analyzing image data, and controlling the liquid lens to change the focal length.
How is the optimal focus point determined?
There are different methods to measure image clarity based on two basic principles. The first principle is edge sharpness, which involves searching for significant jumps in grayscale values between adjacent pixels in the acquired image to highlight edges or contours. The contrast of these edge contours determines the sharpness of the image. The clearer the edge image, the higher the original image's clarity.
The second principle is based on the analysis of the histogram values of the image. It can be used to calculate the variance of pixel values in the average grayscale of the image. A larger variance indicates higher contrast in existing grayscale edge contours. If the image becomes unfocused, the previous significant jumps in grayscale values at the edges will exhibit a slope-like gradient, resulting in decreased image contrast. In other words, the clearer the image, the higher the grayscale diffusion, the higher the contrast in the image, and the clearer the image.
During autofocus, as the focal length changes, the image sharpness and variance will change in real-time. Image sharpness and variance analysis and processing through software result in significantly clearer images.
Advantages of autofocus technology:
Autofocus technology utilizes liquid lenses to achieve focus, which differs from traditional lenses made of glass. Liquid lenses are optical elements made of one or more liquid materials without mechanical connections, allowing internal parameters of the optical element to be changed externally. They offer performance that traditional optical lenses cannot match. In simple terms, the lens medium changes from glass to liquid. More precisely, it is a new type of optical element that dynamically adjusts the lens refractive index or changes its surface shape to alter the focal length.
Similar to lenses made of glass, liquid lenses are single optical elements, but their material can change shape. The focal length of a glass lens depends on its material and curvature radius. Liquid lenses follow the same basic principle, but their uniqueness lies in the ability to change the curvature radius, thus changing the focal length. This radius change is controlled by electrical means and can achieve changes in milliseconds. Manufacturers utilize various technologies, from electric wetting to deformable polymers to acousto-optic modulation, to control the curvature radius and refractive index of liquid lenses.
Most imaging lenses are multi-element lenses, and the imaging performance of a single optical lens is insufficient. Therefore, it is not wise to use only liquid lenses. However, by combining liquid lenses with imaging lenses in multi-element designs, the speed and flexibility advantages of liquid lenses can be leveraged. Liquid lenses can autofocus at near distances or optically infinite distances in milliseconds, making them an ideal choice for applications such as barcode reading, packaging sorting, security, and rapid automation, where autofocus is required at multiple positions with objects of different sizes or distances from the lens. In various applications requiring rapid autofocus, liquid lenses can enhance the flexibility of imaging systems.
FALenses Technology specializes in providing machine vision core hardware. You can go to the official website of FALenses Technology at https://www.falenses.com/ for more information.