The best of both worlds
Line scan mode represents a cost-effective alternative to the line scan camera
Line scan mode is indispensable for many applications in industrial machine vision, such as the analysis of curved surfaces or endless web inspection. If objects cannot be captured in their entirety in a single exposure due to their size or shape, they have to be "scanned". The camera captures information line by line to build up a picture of the object in its memory that is free of any perspective distortions or non-homogeneous lighting. When line scan mode is used, movement is essential – either the material to be inspected is moved, e.g. on a conveyor belt, or the camera itself moves, in which case a clock is required.
"In addition to endless web inspection, general inspection, measuring and sorting tasks involving fast moving objects are typical areas of application for conventional line scan cameras. Others include the inspection of round objects like wood or wine corks", explains Patrick Schick, Product Manager at IDS Imaging Development Systems GmbH. Line scan cameras are camera systems that use image sensors with a single light-sensitive line to scan objects in this way. In contrast, the sensors in area scan cameras (also known as matrix scan cameras) consist of a two-dimensional matrix of many lines arranged in parallel and designed to capture entire objects in a single exposure. The type of application involved determines which of these two camera variants is used in each case.
In no way can special line scan cameras be described as simply pared-down variants of area scan cameras because their single light-sensitive line usually has a much higher resolution. As a result, the quality of the analysis data for each line is considerably higher. In addition to this increased data volume per line, these cameras can also achieve very high line speeds, with the line being read at multiple points simultaneously by means of parallel data channels. This volume of data can normally only be transmitted to the host PC using a camera link interface. As a result, line scan applications with extreme requirements in terms of data volume and speed, e.g. endless web inspection of textiles, paper, wheel rims or silicon wafers, almost always necessitate the use of an optimized line scan camera of this kind.
However, depending on the application, this is often not essential. The line scan mode offered in IDS software transforms an area scan camera into a line scan camera, thereby offering a simple, cost-effective solution for many conventional line scan camera applications. "Customers who use area scan cameras are often unaware of the benefits offered by line scan mode", says Schick. "We show them that this option gives them the best of both worlds. We also support our customers by delivering the 2D images they expect from the information captured in line scan mode. Customers make their preferred settings, and our software delivers distortion-free images". This makes the image data as easy to use as in an area scan camera.
With the IDS Software Suite, users can activate line scan mode using the "Vertical AOI Merge Mode" function for certain GigE and USB 3.0 camera models with CMOSIS, e2v or Sony sensors. In other words, they can use the area scan camera as a line scan camera. To emulate the behavior of line scan cameras as closely as possible, an option for triggering each individual line is provided in addition to the standard trigger for image acquisition. This allows fluctuations in conveyor belt speed to be offset, for example.
The software naturally requires suitable hardware to run on. To achieve the full potential of line scan mode, IDS recommends the UI-3370CP industrial camera model, with its highly sensitive and fast 4.2 megapixel CMOS sensor CMV4000 from CMOSIS. This means that expensive line scan cameras can simply be replaced in many fields of application by a more affordable and extremely powerful area scan camera. In combination with the USB 3.0 interface, the sensor achieves 80 fps in full resolution. When operating in line scan mode, the UI-3370CP industrial camera captures images with 9.4 kHz.
The area scan camera also offers benefits when it comes to setting up the application. Whereas a precise adjustment of a line scan camera is not exactly straightforward due to the missing 2D image, the area scan camera's field of view is extremely easy to focus. With the user interface developed by IDS, line scan mode can be configured in just a few clicks. In the histogram, you can define the line with the most even distribution of brightness and switch to line scan mode. All parameters can then be saved in a camera configuration file. This avoids unnecessary programming and delivers reproducible results in every application. Depending on the application, users can therefore decide for themselves whether to use an expensive line scan camera or a simple, affordable solution from IDS.