As with all other cameras, lighting is crucial for the quality and precision of the images captured by line scan cameras.
Obtaining non-distorted and detailed images from the frames captured by line scan cameras is especially important for the cases when these specialized cameras are used as part of a machine vision system for automated quality assurance, process control and monitoring.
Being able to capture every single defect or problem on the moving production line is the sole purpose of most of these vision systems, so ensuring that the light is suitable and consistent is crucial for the effectivity of the system for monitoring and control.
What are line scan camera based machine vision systems
These are modern versions and much more efficient alternatives to the traditional human quality and production process control.
Line scan cameras are installed above, underneath or in front of the passing products, in order to capture every single part of the moving objects and to detect any non-conformities or defects during the production process.
Thanks to the speed of these cameras, as well as to the other elements of the machine vision system, such quality assurance and process control can be performed for every single part of the product or every single product on the conveyor line or sorting line without the need for stopping the process, and without relying on the traditional quality control of only certain samples of a batch of products.
Some of the other vital elements of a well configured and synchronized automated system for control and monitoring include encoders (linear or rotary) which measure the speed of the passing or rotating objects or products in order to synchronize the capture rate of the line scan cameras. Also, trigger pulses are used for calculating the start point and end point of each passing object, so that the line scan camera captures absolutely every part in a detailed image.
Frame grabber cards are used for synchronizing the light with the cameras.
In the end, specialized hardware and software are used to process the single slices of the image into one full and very concise image of the whole product.
But for the image to be detailed enough, in order for any small defect or other inconsistency to be noticed, it is necessary that the lighting is suitable, sufficient and consistent at all times.
This is often quite a challenging task due to the fluctuations in lighting which the fast-moving conveyor or other production process or action causes.
Setting up such a system for control and inspection is not an easy task, but thankfully most manufacturers of hardware and software for machine vision systems work in compliance with a common standard.
Choosing the proper lighting for the line scan cameras
The lighting is a key element of all machine vision systems. The images which are created and analyzed by these systems are all based on the light which is reflected on the object rather than examining the product or action instead. This is why it must be understood exactly how the light will be reflected from the objects being inspected. This depends on a number of variables including the size, material, color, shape, texture and the finish of the product.
By ensuring that the machine vision system is equipped with the most appropriate type of lighting, the manufacturers and the machine vision experts make certain that the images captured are with a consistent quality and allow for detailed and precise control and inspection.
The various light sources suitable for such systems for control differ in type, size, shape, illumination and wavelength.
Here are some of the most frequently used lights and lighting types for line scan cameras:
- Contrasting, colored lighting – this type is used when certain elements of the object being observed need to be better enhanced
- Infrared light – this is suitable for detecting surface defects and bruises
- Ultraviolet light – this type of lighting causes excess ink or glue to fluoresce for easy detection
- Parallel aligned (collimated) lighting – when extra sharp images of the product inspected are required
- Diffused light – this helps reduce and remove any glare or shadows which can interfere with the proper monitoring and inspection of the object or surface
- Backlighting – this type of lighting is very useful when dimensional measurements need to be made rather than an inspection of the surface of the objects because the backlighting obscures the object
- Direct lighting – it creates an excellent contrast but can cause glare or shadows depending on the texture and surface of the object. More than one direct lighting source can be used for minimizing these shadows and glare
- Structured lighting – it is provided by fiber optic or laser lights and allows for precise measurement of the depth and height of a continuously moving surface (such as plywood, metal, wood, textile, paper and many others)
- Off-axis/diffuse lighting – it is aligned right next to the line scan camera so that it doesn’t get in the way of the sensor. This is done with the help of a mirror through which the camera captures the image
Other features which are essential for choosing the right line scan camera and lighting for a certain job
When a line scan camera is picked for a particular task, the most important aspects to be taken into consideration are the size, the sensitivity, the line rate and as mentioned earlier – the lighting.
The sensitivity of the line scan camera is ensured by the larger pixels of the sensor and the 100% fill factor for accumulating sufficient amounts of photons.
The size of the sensor pixels of the line scan camera needs to be chosen so as to enable the camera to capture images at a resolution sufficient for minimum 3-4 pixels to cover the possible defects. So, the size of the line scan camera must be determined in accordance with the minimum defect size and the view.
The exposure time of a line scan camera needs to be precisely calculated so that the resulting images acquired by the camera are not distorted or blurred. The exposure is the actual illumination of the line scan sensor for a certain fixed period of time. Shorter exposure times can be used when the lighting of the objects is sufficient and consistent.
The line frequency of the line scan camera is calculated according to the resolution required and the speed of the moving object or conveyor line.
As you can see, although line scan cameras are rather simple devices, they need to be very carefully chosen and configured along with all of the other elements of the machine vision system for inspection and quality assurance, in order to make them efficient in spotting problems and defects, in measuring dimensions and for sorting or reading fast moving products and objects.
Apart from choosing the best type of line scan camera to use, it is crucial to pair it with the most suitable lighting type, for the best end results and for 100% efficiency of the automated system for production control and quality assessment.