The speed of line scan cameras has greatly increased in the last years. Modern line scan cameras operate with integration times in the range of 15 µs. In order to achieve excellent image quality, in some cases illuminance levels of over 1 million Lux are required. One of the most important criteria for assessing image quality is noise disturbance (white noise). There are various noise sources in image processing systems and the most dominant one is called “shot noise”.
Shot noise has a physical cause and this has nothing to do with the quality of the camera. The noise is caused by the special essence of light, by photons. The image quality depends on the number of photons which hit the object and ultimately on the number of photons which reach the camera sensor.
In a set-up with a defined signal transmission there are three parameters which influence the 'shot noise' when capturing an image:
integration time (scanning speed)
aperture (depth of focus and maximum definition)
amount of light on the scanned object
The choice of lens aperture greatly determines the required light intensity. If, for instance, the aperture is changed from 4 to 5.6, twice the amount of light is required in order to maintain the same signal to noise ratio (SNR) – see fig. 01). By using a greater aperture, more depth of focus is achieved and the image quality is improved due to reduced vignetting effects with the majority of lenses.
LIGHT FOR ALL APPLICATIONS CURRENTLY
LEDs are available in various shades of color. You can get them in red, green, blue, yellow or amber. Even UV LEDs and IR LEDs are obtainable. The choice of a specific color and thus a specific wave length can determine how object properties on surfaces with diverse spectral response are made visible.
In the past, red light was often used wherever high intensity was required. However, relevant performance increase in LED technology today usually occurs with white LEDs. These high-performance LEDs are used for example in car headlights and street lamps. The core of a white LED actually consists of a blue LED. Using fluorescent substances, part of the light from the blue LED is converted into other visible spectral ranges in order to produce a 'white' light.
UV-LEDs are frequently used to make fluorescent effects visible. In many cases a wavelength of approx. 400nm is sufficient. UV-LEDs with shorter wavelengths may be suitable for hardening paint, adhesives or varnishes. In comparison to blue or white LEDs, UV-LEDs are less efficient. By focusing through a reflector however this can be improved. IR lighting is implemented for food inspection. Wavelengths of 850nm or 940nm are used. When sorting recyclable material, wavelengths from 1,200nm to 1,700nm are used to identify different types. Here however, IR-LEDs in this range are not as adequate as classic halogen lamps with appropriate filters where beam output is concerned.
The small design enables a very short warm-up phase. This circumstance presupposes good thermal dissipation, in order to maintain appropriate working conditions, i.e. temperatures. As a rule: the better the cooling, the longer the LED durability. Apart from durability, LED temperatures also influence spectral behavior (possible color shifting) and general output (luminance).
In systems where precise color reproduction is required, it is recommended to keep the lighting’s temperature steady at a predetermined value. At present, efficient control systems can regulate the LED temperature to within a spectrum range of less than 2°C.
Modern lighting systems, such as the Corona II lighting system developed by CHROMASENS,provide numerous cooling options. This includes passive cooling with thermal dissipation via convection, compressed air cooling, water cooling and ventilation. Active ventilation, compressed air or water cooling are good cooling methods for measuring applications situated in surroundings with high temperatures. By monitoring the temperature of the LEDs and regulating the cooling system, shifts in color reproduction can be completely avoided or at least greatly reduced.
FOCUS ON THE ESSENTIAL
If a flat object at a known and fixed distance is to be illuminated, selecting the adequate focus is relatively simple. Selecting the right lighting is more complicated, if the object is not at a predetermined distance from the light or has no flat surface. In such a case, assuring a permanently sufficient image brightness is a challenge. Here the use of reflector technology facilitates the accumulation of light from a LED (greater coverage angle of the reflected light) and a better light distribution from the depth.
In contrast to background or bright field lighting, focused lighting is normally used for top lighting. Customary lighting systems use rod lenses or Fresnel lenses in order to achieve the necessary lighting intensity. CHROMASENS adopts a novel and completely unique approach. While the use of rod lenses causes color deviations due to refraction, the mirror (reflector) principle developed and patented by Chromasens has no such trouble.
Shiny or reflective materials are a challenge for lighting. Unwished for reflections often appear in the image. In combination with a polarizing filter rotated 90 degrees in front of the camera, these unwanted light reflections can be prevented. When using such filters, certain factors have to be considered. The temperature stability of the filter is one point. In this respect, many polarizing filters can only be used to a certain extent. Another criterion is effectiveness: with such settings, only about 18-20 % of the original amount of light reaches the sensor. The amount of light provided by the lighting must therefore be great enough to minimize noise and yet achieve sufficiently good image quality.
When selecting the correct lighting for line scan camera applications, following factors ought to be considered:
The lense aperture and the light amount significantly influence the signal noise ratio
LED systems offer definite advantages compared to traditional lighting technologies such as halogen or fluorescent lamps + Good cooling ensures long durability, consistent spectral behavior and a high level of brightness
The use of reflectors assures optimal lighting, even from different distances
Color LEDs, UV- and IR-LEDs are extremely versatile
Polarizing filters prevent unwanted light reflection on shiny surfaces. The amount of light provided by the lighting must still be sufficient
Choosing the right illumination for the application is critical for acquiring the high quality images needed for calculating 3D data. We compare the imaging results of a directional coaxial brightfield illumination with a Corona tube light in terms of color image quality and height map for different samples. It could be shown that for material that exhibit considerable amounts of subsurface scattering, coaxial lighting geometry benefits the 3D measurement using 3DPIXA.In practice, it has to be kept in mind that introducing the beam splitter in the light path results in a shift of the working distance of the camera system, and a slight reduction of image quality.
An illumination scheme where the source rays are reflected from a flat sample directly into the camera is called a brightfield. With line scan cameras there are two possible ways to realize such a setup: either by tilting the camera and light source such that the angle with respect to the surface normal is the same but opposite direction, or by using a beam splitter. The first method is not recommended as it can lead to occlusion and keystone effects. Thus we want to discuss the brightfield setup using a beam splitter.
Figure 1 shows the principle of this setup in comparison to a setup with a tubelight. The tubelight is the superior illumination choice for a wide array of possible applications. It reduces the intensity of specular reflections and evenly illuminates curved glossy materials. Most of the time the tubelight should be your first choice and only some materials require the use of a coaxial brightfield illumination.
An example as such is material that exhibits strong subsurface scattering, which means that light beams partially penetrate a material in a certain direction, are scattered multiple times, and then exit at a different location with possibly different direction. Resulting from that is a material appearance that is translucent. Examples of such materials are marble, skin, wax or some plastics.
Using tube light on such materials results in a very homogeneous appearance with little texture, which is problematic for 3D reconstruction. Using coaxial brightfield illumination results in relatively more direct reflection from the surface to the camera, as compared to a tube light illumination. This first surface reflection contributes to the image texture; the relative amount of sub-surface scattered light entering the camera is thereby reduced.
There are some specific properties that have to be taken into consideration when using a coaxial setup with a 3DPIXA. Firstly, only a maximum of 25% of the source intensity can reach the camera as the rest is directed elsewhere in the two transits of the beam splitter. Secondly, the glass is an active optical element that influences the imaging and 3D calculation quality. In chapter 3 we have a closer look at these factors and offer some guidelines for mechanical system design to account for resulting effects. Prior to that, we discuss the effects of the brightfield illumination on a selection of a few samples to give an idea when this type of illumination setup should be used.
2.COMPARING BRIGHTFIELD AND TUBELIGHT ILLUMINATION
In this chapter we want to give you some impressions of the differences between using a coaxial illumination in comparison to a tubelight using different samples. As a tubelight we used the CHROMASENS CORONA II Tube light (CP000200-xxxT) and for the brightfield we used a CORONA II Top light (CP000200-xxxB) with diffusor glass together with a beam splitter made from 1.1 mm “borofloat” glass.
In figure 2 we show a scanned image of a candle made of paraffin, which is a material that exhibits strong subsurface scattering. With coaxial illumination (right image) the surface texture is clearly visible and the height image shows the slightly curved shape of the candle. In comparison the tube light (left image) contains very low texture and height information could not be recovered for most of the heights (black false colored region). The texture is only visible with coaxial illumination because under this condition the light reflected from the surface is more dominant in the final image than the subsurface scattered light. However, the ratio between these two effects varies with different surface inclinations. The more deviated the surface normal is from the camera observation angle, the less direct light is reflected directly from the first surface. Therefore, texture in the image gets lower. For the candle sample, more than 15° deviation resulted in failure in recovering height information. This can be seen in the right image at the outer edges of the candle.
3Fehler! Verweisquelle konnte nicht gefunden werden.. The substrate area in the tube light image (left) shows low texture, resulting in partially low performance height reconstruction (black points in false-colored image overlay). With coaxial illumination (right image), the amount of source rays reflected back into the camera from the surface of the material is larger than the subsurface scattered light. The image texture is higher and height reconstruction performance improves.
However, if the height of the balls is the focus in the application rather than inspecting the substrate, the situation becomes more complex as the coaxial illumination results in specular reflection on the ball tops. If these areas are saturated, it negatively affects height measurements as well.
The best illumination therefore strongly depends on the measurement task and materials used and can often only be determined by testing. If you are unclear which light source is best for your application, please feel free to contact our sales personnel to discuss options and potentially arrange for initial testing with your samples at our lab.
The beam splitter essentially is a plan parallel glass plate which offsets each ray passing through without changing its direction. The size of this offset depends on the incidence angle, the thickness of the glass and its refractive index. The thickness of the beam splitter should therefore be only as small as is needed for stability reasons. In the following analysis we assume a thickness of the beam splitter of d=1.1 mm “borofloat” glass.
The result of the beam splitters influence is a movement of the point from where the sharpest image can be acquired in all three spatial coordinates. The change along the sensor direction (called x-direction) leads to a magnification change of the imaging system that is negligible small (<0.4%, with a small dependence on camera type).
The change along the scan direction (called y-direction) only offsets the starting point of the image. If the exact location of the scanline is important (e.g. when looking on a roll) the camera needs to be displaced relative to the intended scan line by
Δy = d*(0.30n – 0.12).
The equation is valid for all glass thicknesses d and is a linear approximation of the real dependency on n, where n is the refractive index of the glass material introduced into the light path. The approximation is valid in the interval of n= [1.4, 1.7] and for all types of 3DPIXAs. The direction of the displacement is towards the end of the beam splitter that is nearer to the sample, so in the scheme in figure 1 the camera has to be moved to the left.
The change of the working distance is different along the x- and y-axis of the system because of the 45° tilt of the beam splitter leading to astigmatism. In y-direction the working distance is increased by
zy = +d*(0.24n +0.23).
As above, the formula is valid for all d and n= [1.4, 1.7]. The change of the working direction along the x-direction is not constant but also changes depending on the position of the imaged point which leads to field curvature. Both astigmatism and field curvature slightly lower your image quality which influences the imaging of structures near the resolution limit. But they should not influence the 3D algorithm as generally only height structures that are several pixels in size can be computed.
Additionally to the optical effects discussed above the beam splitter also changes the absolute height values computed by the 3D algorithm (i. e. the absolute distance to the camera). The exact value of this height change is slightly different for each camera. Generally the measured distance between camera and sample decreases, so that structures appear nearer to the camera than they really are. This change is constant over the whole height range (simulations show 0.2% change) and also constant over the whole Field of View. In summary, relative height measurements are not influenced at all, and absolute measurements are shifted by a constant offset.
As the precise change of the calculated height is not known, the zero plane of the height map can’t be used to adjust the camera to the correct working distance. We advise you instead to set up your camera using the free working distance given in the data sheet and correcting it with Δzy from above.
On certain translucent materials (those exhibiting considerable subsurface scattering of light), using coaxial illumination can result in a significant increase in image texture which greatly benefits the 3D height reconstruction. However, the additional glass of the beam splitter in the optical path of the camera system when using coaxial illumination influences the optical quality negatively. Further, the working distance of the system changes slightly and the absolute measured distances are set off by a constant value. This does not affect relative measurements, which are generally recommended with the 3DPIXA.