The image fidelity achievable from a machine vision camera is only as good as the optics you use! Many of the machine vision cameras used today utilize very small pixels, down to 1.25um. The crispness of the images are a result of the resolution of your machine vision lens, so matching the right lens to the camera sensor is extremely important. We classify the lens resolution in terms of line pairs per millimeter (lp/mm). So, what lenses can help resolve these small pixels?
First, the relationship of pixel pitch can be put in terms of lp/mm as seen in the chart to the right. Machine vision lens manufacturers today typically provide the resolving power on their data sheets to help in the lens selection and ensure your matching the lens to the image sensor. If not matched properly, image contrast will suffer.
For machine vision cameras with small pixels, the Moritex ML-M-UR “Ultra high” resolution lenses are an excellent choice with the capability of resolving pixel pitches down to 2.2um with good contrast.
The comparison images below show the Moritex ML-M-UR series compared to a conventional lens with less resolution. As you can see in the cutout of the corner, the contrast is much higher with lenses with high resolving power.
Lens resolution typically has fall off to the edges of a lens. To maintain high contrast, Moritex has optimized the ML-M-URseries to have good resolving power to the edges of the lens.
The charts below represent the contrast (MTF) corresponding to image height (x-axis ) showing the contrast from lens center to lens edge. The X-axis represents the center starting at zero millimeters and mapping MTF to the edge (furthest right point) The ML-M-UR shown in the left chart demonstrates very good performance across the lens. (A relatively flat line is good!) As a comparison to another lens (right chart), contrast is degraded across the lens from center to edge.
Aside from high resolution, The Moritex ML-M-UR series are a compact, 29mm diameter design and well suited for typical 29mm cube cameras. Additionally the lenses have an anti-vibration design with maximum acceleration to 10G’s.
IDS Imaginghas released its new contrast-based autofocus features in the popular LE board level cameras. These additions take advantage of standard liquid lenses from Varioptic with resolutions up to 18 megapixels. The uEye software now comes with an intuitive GUI with adjustable regions of interest and various image sharpness measurement algorithms.
As much as “Auto focus” seems like it would be the flip of a switch, its important to understand the various methods used in the image analysis. In order to focus an image, algorithms are needed to measure image sharpness which is relayed to the liquid lens to make adjustments. These methods as based on principles in measuring edge sharpness to analyzing histogram values of the pixel grey scales.
Measuring image sharpness additionally has various algorithms which which can be run providing more exact methods versus basic analysis. It is important to understand these methods as additional processing power is required, effecting the overall camera frame rate.
IDS Imaging has a “Tech Tip”which covers various auto focus methods, defines the characteristics of search algorithms and how they effect speed and provides application examples. Click the icon below to download.
A great question and maybe you can, but what ARE the differences?
Basically, there are just a few questions you need to answer to see if you should use a webcam for you machine vision application which are as follows:
Do you need to program to integrate the video into an application with processing or control?
Do you need consistent image quality?
Are you doing computer vision (the computer is making decisions based on the images) or are you just viewing the images visually?
Do you care if the camera specifications change over your product’s life cycle?
Is the object under inspection moving?
Do you need to control when you take the picture or interface to a trigger or strobe?
Do you need to be able to choose what lens you will need?
If the answer to any of the above are YES, then a webcam will NOT work well or at all for your application. If the answers are NO, then by all means, you might be able to save money and just use a webcam. (You can stop reading here if you want, or continue for more details below).
Machine Vision Camera Software
Webcams do NOT come with a SDK as they are made to show video only. They normally provide a universal video driver, and also an application for viewing video.
Webcams have rolling shutter sensors which mean they cannot acquire images of moving objects without ‘smearing’ them. Industrial machine vision cameras use sensors with global shutters providing the ability to freeze the image to produce non smeared images of moving objects.
Trigger and Strobe Control
Webcams only have an interface to the USB data, whereas industrial machine vision cameras have hardware and software inputs and outputs. These allow for exact timing for a trigger to take a picture and a strobe to illuminate the object.
Camera Specs Changing over time
Webcams just need to show you video! In turn the manufacturers are not concerned if the sensors inside the camera change every six months. Whether the sensitivity changes by 10% makes no difference when you are just video conferencing with Grandma.
Industrial machine vision cameras are made with image sensors that don’t go obsolete every 6 months, but rather companies hope for 10 year life spans. It makes a huge difference if you are doing a computer vision algorithm that you have 5 man years of software development and the sensor’s sensitivity changes by even 1%.
Furthermore, the form factor of webcams change frequently as well. This doesn’t make a difference when it is just on your desk. It makes a huge difference when your camera and lens is fixtured in a machine that has 500 hours of CAD work to design, much less build. Moving the camera and lens 10cm might not be possible!
Do you need to choose your lens?
Webcams come with an integrated lens that is suitable for general viewing, and this lens is integrated with the camera and not changeable. Industrial machine vision cameras come with no lenses as not only do lenses come in a variety of focal lengths for different magnification, but also lenses coming in a variety of resolutions. Choosing a lens requires you to know the size of the sensor, your working distance, your field of view, and the pixel size. (See related educational blogs on lenses at end of this post)
What are your options for a low cost camera solution?
If you need industrial machine vision camera solutions with a solid SDK, long life cycles, at a low price, there several solutions to consider. Rolling shutter imagers are always lower price which are always a place to start along with USB2 interfaces. Read our previous blog HERE which outlines some specific models which are low cost. There is also a great new platform coming providing 5 Megapixel resolution with a rolling shutter imager, but with great performance for $280! Contact us for more details.
Liquid lens technology, with its ability to change focus within the order of milliseconds is opening up a host of new applications in both machine vision and the life sciences. It is gaining growing interest from a wide cross section of applications and easily adapts to standard machine vision lenses.
Liquid lens technology alone provides nice solutions, but when combined with advanced controls, many more applications can be solved.
In this blog, we will highlight several case application areas for liquid lens technology.
Case 1: Applications requiring various focus points and extended depth of field: This does cover many applications, such as logistics, packaging and code reading in packaging. Liquid lenses provide the ability to have pre-set focus points, auto-focus or utilize distance sensors for feedback to the lens. In the example below, 2 presets can be programmed and toggled to read 2D codes at various heights essentially extending the depth of field.
Case 2: 3D imagery of transparent materials / Hyperfocal (Extended DOF Images: When using an Optotune liquid lens in conjunction with a Gardasoft TR-CL180 controller, sequence of images can be taken with the focus point stepped between each image. This technique is known as focus stacking. This will build up a 3D image of transparent environments such as cell tissue or liquid for analysis. This can also be used to find particles suspended in liquids.
A Z-stack of images can also be used to extract 3D data (depth of focus) and compute a hyper-focus or extended depth of field (EFOF) image.
The EDOF technique requires tacking a stack of individual well focused images which have preferably been synchronized with one flash per image. An example is show below with the rendered hyper focus image shown at right.
Case 3: Lens inspection: Liquid lenses can be used to inspect lenses, such as those in cell phones for dust and scratches looking through the lens stack.
Case 5: Large surface inspections with variation in height: Items ranging from PCB’s to LCD’s are not flat, have various component heights and need to be inspected at high magnification (typically using lenses with minimal DOF). Optotune Liquid lenses are a perfect solution using preset focus points.
Machine Vision applications using Optotune Liquid lenses and controller are endless!
These applications are just the tip of the iceberg and many more exist, but this will give you a good idea of capabilities. Gardasoft TR-CL controllers are fully GigE Vision compliant, so any compatible GigE Vision client image processing software such as Cognex VisionPro, Teledyne Dalsa Sherlock or National Instruments LABVIEW can be used easily.