Contrast and Uniformity of Lenses
1. Contrast and Stray Light
Contrast: Contrast, as we commonly know, refers to the measurement of different brightness levels between the brightest white and the darkest black areas in an image. A larger difference range indicates higher contrast, while a smaller difference range indicates lower contrast. The impact of contrast on visual effects is crucial. Generally, higher contrast results in clearer, more prominent images with more vivid colors, while lower contrast can make the entire picture appear dull and murky. When discussing the contrast of lenses, it's essential to note that it differs somewhat from contrast in screens and images. To make images appear with clear outlines, black details need to be displayed as black and white details as white. Therefore, when talking about contrast in lenses, the focus is mainly on their ability to reproduce real shapes and contours. Simply put, lenses with higher contrast have fewer transition pixels at black-white boundaries, leading to higher accuracy in measuring samples. If a lens has low contrast, there will be many gray pixels of different depths at the black-white boundary of an object. These gray pixels often interfere with the accuracy of visual detection, resulting in inaccurate test results.
Stray Light: The contrast of a lens can be influenced by various factors, whether due to mechanical design issues or optical path design issues. One common problem is the influence of stray light. Stray light, also known as optical "noise," is a harmful light in optical systems that can affect normal imaging. As the detection requirements for optical systems continue to increase, stray light can affect the performance of optical systems, reduce imaging contrast and image quality, and even affect the signal. Therefore, the analysis and suppression of stray light requirements are gradually attracting attention and have become a necessary part of image quality inspection in optical systems.
Stray light may be caused by multiple reflections within the effective aperture, specular reflections, or light reflections outside the effective aperture, resulting in ghosting or glare. These stray lights may affect normal imaging, thus requiring accurate analysis and appropriate treatment.
Common methods for eliminating stray light include:
- Changing the optical type, optimizing the system structure, or optimizing the mechanical structure.
- Adding a baffle to eliminate stray light.
- Using thread baffles.
- Treating the inner walls of the lens barrel with non-reflective black oxidation, spraying non-reflective paint, or attaching non-reflective velvet.
2. Uniformity
Now let's talk about a parameter mentioned in both projectors and lenses — uniformity. In projection equipment, uniformity refers to the difference between the brightest and darkest parts of the projected image. Generally, the brightest part of a projector's image will be the center, while the darkest part will be the edge of the image. Therefore, higher uniformity means higher consistency from the center to the edge of the image, while lower uniformity results in more severe inconsistencies in brightness across the image. In lenses, uniformity generally refers to whether the focus at the center and the focus at the edge are basically the same, whether in telephoto or wide-angle shots. Image uniformity is mainly determined by the lens. A lens with good imaging uniformity will have basically the same focus at the center and the edge, without significant differences. However, a camera with quality issues may show brighter centers compared to the edges, resulting in vignetting, and the image may appear grainy, significantly affecting visual effects.
In addition to the quality of the lens itself, lighting can also affect the uniformity of lens imaging. The difference in shooting effects between a conventional telecentric lens and a coaxial telecentric lens can be seen in the following image.
Uniform lighting is an advantage of coaxial design, which can significantly reduce the difficulty of lighting. If non-coaxial light sources are used to shoot highly reflective parts, very expensive light sources may be needed. In contrast, using a coaxially designed light source is much simpler. In fact, with coaxially designed lenses, simply inserting a point light source into the lens can provide very uniform lighting.
In summary, understanding contrast, stray light, and uniformity is crucial for assessing the performance of lenses in optical systems. These parameters directly impact the clarity, accuracy, and overall quality of the images produced.
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.