Scanning the market
Line and area scan cameras are two very different beasts. While line scan cameras rely on the camera moving to produce an image built up line by line, similar to how a photocopier works, an area scan camera moves this up to the 2D and is, for all intents and purposes, a conventional camera.
A line scan camera works in 1D, scanning the object to be imaged using a line sensor and putting these lines together to build up a picture, or just scanning an object in 1D to check everything on that line appears as it is supposed to. The line itself is an array of pixels, with resolutions covering anything from 512 to 16,000 pixels.
For an area scan camera it, again, builds up an image using an array of sensitive pixels, but the area scan camera uses a square or rectangular shaped sensor to capture an image all in one go. Simon Hickman, OEM sales manager at Multipix Imaging, says: ‘The resulting image has a width and height corresponding to the number of pixels on the sensor – this gives different aspect ratios for differently-shaped sensors.’
The scanning boundaries of the two differ. Hickman says: ‘The main differences between area and line scan cameras are the shape of the sensor and how it is used to capture an image. A line scan camera can capture images of fixed width and infinite length. An area scan camera captures images of fixed resolution in both directions.’
Christophe Robinet, strategic marketing manager for machine vision at e2v, adds: ‘As a typical line scan sensor features pixels in one row, registers can be located on each side of the sensitive area, offering a 100 per cent fill factor [the photosensitive area divided by the pixel size]. Area scan sensors feature a frame of pixels, each of them embeds some shutter and possibly local circuitry, thus having a fill factor of 80 per cent or less.’
Exposure time between the two types of cameras differs too. Dr Joachim Linkemann, product manager at Basler Vision Technologies, explains line scan cameras: ‘The lines are taken one after the other. This means the exposure time is short. To take an image of 1,000 lines (at 1,024 pixels per line) within 100ms, will give an exposure time per line of only 100μm. Whereas an area scan camera with 1,000 x 1,024 pixels will have the 100ms exposure time for all pixels.
‘A further difference is that the exposure time for an area scan camera will expose all pixels, where a line scan camera will grab the lines sequentially to get a full image. Therefore, line scan cameras must be more sensitive than area scan cameras.’
This fixed resolution of an area scan camera has direct implications on its cost and applications, as Mark Williamson, sales and marketing director for Stemmer Imaging, explains: ‘The resolution of an area scan is limited by the number of pixels on a 2D sensor, which is typically up to 2k x 2k (4 Megapixel) and beyond this they get very expensive. As a line scan camera only has 1D pixels, an 8k line scan camera is cheaper than a 4-Megapixel area scan camera. This could acquire an 8k x 8k 64 Megapixel image. So line scan is good for very high resolution and continuous product inspection where the product is moving continuously.’
As Williamson hints, the application areas for both line and area scan cameras can vary due to the nature of how they take images. For example, line scan cameras are suited to very high resolution imaging at very high speeds. Hickman says: ‘Line scan cameras are best suited to so called “web” applications – this is a continuous process that has no discrete beginning or end such as paper, textile or glass manufacturing. Line scan cameras are also very suited to applications where very high resolution images are required – a single line of up to 12k pixels can be used to create huge images in one pass.’
And, for line scan cameras, Linkemann adds: ‘Other applications include glass inspection, railroad inspection, food sorting, wood inspection, and many more.’
The manoeuvrability of a line scan camera also adds to its application areas. Robinet says: ‘Surface inspection and metrology also benefit from the camera positioned at angles other than 90° to the scene.’
Mark Butler, product manager at Dalsa, also sees line scan cameras as having more dynamic applications than their area scan counterparts. He says: ‘Line scan cameras are best suited towards applications where the object you want to take images of is a continuously moving object. For example, to inspect mail, postal sorting machines use line scan cameras to take the image on an envelope that is moving at more than 3m/s. Other line scan applications are flat panel display inspection, parcel sorting, and web inspection.’
Despite line scan cameras’ advantages, it seems area scan cameras are more commonplace though. Linkemann says: ‘Area scan cameras can be used within nearly every application [and] are used in machine vision, intelligent traffic systems (ITS), medical applications, face recognition, movies, and much more.’
Butler adds: ‘Area scan camera are best suited towards applications where the object is stopped, thus you can take a full image of the object for further analysis. Key applications include medical imaging, security, and printed circuit board automated optical inspection.’
According to Williamson, area scan cameras are suited to a wide range of applications because of their ‘lower resolution and lower cost applications’ meaning that they are simpler to set up and subsequently cause fewer issues during operation. He adds: ‘Cameras can take the 2D image in a few tens of microseconds, whereas a line scan camera has to build the image up as the product moves.’
But despite the differences, there is some overlap in the application areas between the two technologies. Hickman says: ‘There are many applications where either camera type can be used – main decision factors include: how easy it will be to achieve the best illumination; whether a scanning movement can be sensibly implemented; and the level of expertise available, as line scan systems are generally more difficult to set up and implement.’
Dalsa’s Spyder 3 (Camera Link or GigE) line scan camera.
More specifically, for an application such as surface inspection, both types of camera could fit the bill. Linkemann says: ‘For glass inspection, the whole substrate is scanned with at least one line scan camera. If there is a microscopic defect [on the substrate] an area scan camera (with a microscopic lens) could be moved to the location to get images with higher resolution and more detailed information.’
Butler also sees a potential applications overlap: ‘In most cases, the choice between line scan and area scan is fairly obvious or over time the best choice has already been determined by system designers. However, both line scan and area scan may be used in a production line. For example, in printed circuit board inspection a bank of line scan cameras may be used for inspection of the bare board (i.e. unpopulated boards) and then the populated board is inspected by area scan cameras.’
Over the years, the scanning camera industry has altered with both line and area seeing technical improvements. Linkemann says: ‘The quantum efficiency (how many electrons are created from 100 photons) is getting much better. Microlenses are attached to the pixels to increase the fill factor and colour filters have also been attached.’
Linkemann adds: ‘The maximum in sensitivity has also moved from the red to green to have a very similar behaviour to the human eye. Besides that cameras are getting much more intelligent; pre-processing is often done inside the camera too, with synchronised cameras used for sports analysis and 3D reconstruction.’
Protocols have altered the camera technology too, as Williamson says: ‘[For line scan] the use of Camera Link has made interfacing to these cameras a lot easier and the frame grabbers used have had a lot more control support to make implementation easier.’
The marketplace will continue to shift and Butler is optimistic that the application areas for both cameras will continue to grow: ‘There are a number of potential new applications that are emerging. Solar panel inspection and intelligent traffic systems are two areas that hold promise.’
Williamson agrees that solar panel production will continue to drive the market: ‘Moving forward the introduction of solar panel production will continue to drive the line scan market. But area scan sees the lion’s share of the new applications.’
Area scan cameras are becoming increasingly popular, say the experts. Hickman continues: ‘Area scan cameras are definitely encroaching on some applications where only line scan could be used, for example, area scan cameras are now available with very high resolution sensors that have higher and higher frame rates.’
And Linkemann adds: ‘Area scan and line scan cameras will be used in future. Due to the fact that area scan cameras are easier to align, area scan cameras will be the preference.’
Williamson says: ‘The challenge for line scan is synchronising the line rate to the movement of the product. If the two are not tied together the image will stretch or compress. The use of quadature encoders to tie the product movement to the line rate solves this but it involves more engineering. Also, on line scan at the higher speeds, the exposure time can be very short which often means the speed is limited to the amount of light that can be delivered to the object.’
But area scan can also throw up a few illumination problems, as Williamson continues: ‘Area scan cameras have fewer issues, as a single trigger to the camera will grab the whole image. The key for higher resolutions is getting even illumination over a large area and the resolution of the lenses, as they tend to have smaller pixels.’
Butler adds: ‘Feedback from system designers indicates that the biggest challenges tend to lie in the optics and lighting that are used as part of the inspection system. Many experienced system designers recommend you determine your lighting first, which tends to be counterintuitive to how a less experienced system designer may approach the problem.’
Linkemann thinks alignment can pose a problem for line scan cameras. ‘Once a camera is connected to the system, either with a frame grabber or by FireWire or Gigabit Ethernet, there is no big difference,’ he says. ‘It is harder to align a line scan camera, because the full image is not visible. The alignment of the line scan camera is challenging because it is not only the camera and the lens, it is also the light that has to match perfectly to the whole system.’
Hickman, too, can see the pluses and minuses of both technologies and says: ‘It is more difficult to achieve correct illumination with area scan cameras when compared to line scan, as area scan cameras generally have more limited resolution. Line scan cameras rely on a scanning movement – most often with an encoder set-up for controlling the line acquisition. It is also generally more difficult to set up a line scan camera – alignment and focus are more critical and dealing with the data can be more complex.’
Williamson summarises the decision-making process succinctly: ‘Area scan: simple, easy to understand and implement; line scan: great for higher speeds and continuous inspection.’