A line scan camera takes only one single line of the image at a time. As the material to be inspected is moving on, the next image taken will be the next line right close to the one before. As a result, a 2-D image taken with a line scan camera shows an image where every line of the image is taken at a different point of time. This is different to an image taken with an area sensor or the human way to perceive images and has implications for the machine vision software.

The decisive aspect in the use of line scan cameras is the ability to reconstruct the endless images. In a high frequency, many extremely narrow lines are combined into a single image. In the process of this recombination, it is important for the lines to be captured in sync with the object's speed as it passes by. In most cases, an impulse generator for the camera is integrated in the conveyor, generating the trigger signal for the camera. Like this, the camera's line frequency is adjusted in accordance with the speed of the object.

Below are three sets of lens specifications that would result in the desired system NA. There is an infinite number of specifications that will give the desired NA if one is allowed to change more than one spec.

F-number

If the medium is not air, as is common for some microscope objectives, the approximation above can be multiplied by the index of refraction of the medium as shown below.

Line scan cameras take only a single line as an image and send this to the computer. The host computer will assemble the lines. As this never stops, a virtually endless image is created. Line scan cameras are the perfect solution for inspection in print, textile production and with rotating or moving objects.

aperture中文

Human eyes or a std photographic area camera takes an image with all pixels of the image at the same time, no distinction is made between still objects and moving objects. In one moment, all the image data is gathered.

Single-line CCD sensors capable of detecting dark and light first entered the market with the advent of fax machines. Indeed, most customary document scanners are still based on this principle. In this case, the sensor bar is  located right beneath the moving object. With advances in sensor miniaturization and rising resolution, the sensors have increasingly moved further away from the object. Today, line scan cameras are integrated in customary camera bodies and can be operated with standardized lenses.

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Alternatively, 2 or 3 lines, each with a specific colour, might be used. This kind of setup challenging as the colours of the same pixel have to be taken with a object-related time difference. The benefit is a colour resolution with the sensor's full resolution.

To determine whether lens specifications are compatible, we need to find the resulting numerical aperture from the other three specifications. To do this we will first need to use the equation below to relate the image height h, focal length f and the half field of view Θ. This equation can be derived using simple geometry using the relationships shown in the red triangle in figure 3.2.

This is an exact equation relating the NA to the f/#, but it is often convenient to have an approximation for this. When n = 1 (medium is air) and if we use a small angle approximation (sin α ≈ tan α) then:

Aperturediaphragm

Line scan cameras with 2 or more lines make greater demands of synchronicity. They can increase the maximum possible speed of the objects or improve the dynamics of the image in an HDR mode.

Totally different to area cameras is the lighting setup. With the same line being taken (and the material driving by very fast) you need to illuminate only one line of the image. But, due to the speed of the material driving by (and the speed of the sequence of lines) exposure times are low, typically in the µs range. Consequently, line scan cameras very often require lighting setups with high or extreme light densities (up to 1000000 lux).

As is the case with large-area cameras, constraints are placed on the data rates of line scan cameras by the interface in industrial machine vision. Common interfaces i.e. are GigE Vision or Camera Link. This is calculated on the basis of the vertical resolution of the line (number of pixels in the line) and the number of lines per second, expressed as kHz / line per second.

Line scan cameras require either the camera or the object is moving. The size of the final image is determined by the camera's resolution in one axis, the other axis is determined by the number of lines the host computer is joining. There are two possibilities for a line camera setup:

aperture发音

Pupilaperture

As with large-area cameras, line scan cameras can be fitted with monochrome or color sensors. The filters for red, green and blue are placed over the pixels as Bayer patterns or distributed over the lines to which a particular color is allocated and which are generally behind a filter. The resultant raw image is transmitted as monochrome information and then converted into a color image on the grabber.

Shutter speed

It generates 70,000 lines with 8,000 pixels per second. With a conveyor speed of 100 km/h and an object width of one meter, this translates into possible resolution of 0.122 mm per pixel latitudinally and 0.514 mm per line longitudinally.

Numerical aperture (NA) refers to the cone of light that is made from a focusing lens and describes the light gathering capability of the lens (similar to f/# ). NA is defined by the following equation, where n is the index of refraction of the medium (often n=1 for air), and α is the half angle of the cone of light exiting the lens pupil.

Numericalaperture

In contrast to top-down document scanners, line scan cameras require optical systems. These are mostly identical to the lenses used with area sensor cameras. But, as the line scan cameras provide very high resolutions (up to 16k per line) the camera requires very high quality lenses with a decent MTF over the whole image.

So what if the customer needs a numerical aperture of 0.25? To get this, at least one of the other specifications need to change. To do this, lets start with the initial specification for NA=0.25 and find what the f/# would be using this spec.

Now that we have briefly explained what numerical aperture is, we can equate it to f/#. As explained here, f/# is also a measure of how much light can get through a lens. f/# of a simple lens is defined by the following equation, where f is the focal length of the lens and D is the diameter (or more specifically the entrance pupil diameter for more complex lens systems).

In order to equate NA and f/#, we can use simple geometric relationships. Figure 3.1 shows a simple lens focusing light rays (blue lines) from infinity to a point. This creates a cone of light that can be described by numerical aperture using the previous equation. The half angle, α, can now be defined by the following equation:

Aperture

As the sensor chip captures only a single line, it is easier to read the data held by a line scan camera compared with a large-area camera. It does not make any difference whether CCD or CMOS light-detection technology is used. As resolution is high and the objects are moving fast (thus joining lines is fast as well) the data rate typically is very high. You need a frame grabber to read the camera's data into the computer.

Often times when starting the design process one can inadvertently request conflicting specifications. This example will show how easy this is to do and how to avoid it when specifying a lens.

Now we can find the focal length of the lens by assuming that the customer wants entrance pupil diameter to stay at the specified 20mm.

NEopt also offers the possibility of configuring its two-line cameras for only two colors. In this case, red and blue do not so much constitute two colors as two camera operations.

Industrial Line scan cameras are the best solution for inspecting continuous materials or rolls by machine vision. Quite often they provide very high image resolutions and due to their speed demanding data rates.