Overview of difference between KINO’s contact angle meter and other manufactory’s
1, 2, 3 Control of sample stage along XY axis is used for measuring contact angle of different position on sample surface. And, control of sample stage along Z axis is used for measuring sample with different thickness. KINO is only manufactory of contact angle meter that adopts motorized linear stage or manual positioner for motion control system. By comparison, contact angle meter made by other factory always uses common adjustment mechanism such as “dovetail stage” or just one optical bench with holder unit and height adjustable rod stand. (Note: these is always used for quick and long travel range adjustment that need no accuracy.)
For more information of them, visit https://www.newport.com/Products/5465115/1033/nav.aspx or http://www.sigma-koki.com. KINO’s contact angle meter adopts such motion control system as this manufactory’s and accords with its design accuracy. You can find difference such as load capacity and travel accuracy between rack and pinion dovetail stage, translating optical post holder and crossed-roller guide positioner with micrometer. It is shown that crossed-roller guide positioner with micrometer is most suitable for vertical position. Motorized linear stage controlled by software with slide guide provide more stable and smooth movement and positioning of drop.
4, Level adjustment of sample stage except adjusting complete machine by four adjustable legs is most important. For example, after you adjusted levelness of sample stage at first, when you measure sample with poor level surface, it is more difficult to get good base line and obtain two contact points. As shown below:
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6, We suggest our customer to choose direct single syringe pump instead of syringe pump with tee-junction because latter is difficult to clean and easily lead to cross contamination e.g. water for measurement of contact angle is contaminated by oil.
direct single syringe pump
- Advantage: Easy to clean and remove syringe, High precision, pollution-free.
- Shortcoming: inconvenient to measure contact angle with multiple liquid.
syringe pump with 3 port value
- Advantage: Difficult to clean pump, value and tube, easily lead to cross contamination, cannot dosing liquid with viscosity
- Shortcoming: convenient to measure contact angle with multiple liquid when equipped with multi-channel pump.
7, usually, there are two drop transferring method- by move needle down and then up or by move sample stage upside and downside. We prefer first method due to we can easily control the base line of contact angle (it remains at same position) by first one. By second one, base line is bound to move because sample surface moved during this process. And for measuring contact angle of super-hydrophobic surface, movement of needle must be very carefully because it is difficult to transfer drop to this surface. So, we adopt positioner stage with micrometer or controlled by step motor to get high precision of control.
8, Interface of Camera that USA KINO adopted is USB2.0 or USB3.0, which is more mutually compatible with PC than IEEE1394a or IEEE1394b. Most suitable resolution of camera for contact angle meter is about 130M, and 40M is most common resolution that used. Visit http://www.uskino.com/news/50.html to find more information about this topic.
9, Telecentric Lens and Parallel light Background light are most suitable for contact angle measurement for the following reasons. With them, we can benefit highest precision measurement values at sub-pixel resolution.
(1) Magnification Constancy
Common lenses give different magnifications at different conjugates: as such, when the object is displaced, the size of its image changes almost proportionally with the object-to-lens distance. This is something anybody can easily experience in everyday life, for example when taking pictures with a camera equipped with a standard photographic lens.
Fig. 1: a standard lens generates different size images when changing the object-to-lens distance (indicated as ”s” in the drawing). On the other hand, objects of different sizes would look as if they had the same dimensions, provided they subtend the same viewing angle.
With telecentric lenses the image size is left unchanged with object displacement, provided the objectstays within a certain range often referred to as “depth of field” or “telecentric range”. This is due to the particular path of the rays within the optical system: only ray cones whose barycentric ray (or “principal ray”) is parallel to the opto-mechanical main axis are collected by the objective. For this reason, the front lens diameter must be at least as large as the object field diagonal. This optical behaviour is obtained by positioning the stop aperture exactly on the focal plane of the front optical group: the incoming rays aim at the entrance pupil which appears as being virtually placed at the infinity. The name “telecentric” derives from the words “tele” (which means “far” in ancient Greek) and “centre” which accounts for the pupil aperture, the actual centre of an optical system.
Fig. 2: in a telecentric system rays get into the optics only with an almost parallel-to-the-axis path.
(2) Low Distortion
Distortion is one of the worst problems limiting measurement accuracy: even the best performing optics are affected by some grade of distortion, while often even a single pixel of difference between the real image and the expected image could be critical.
Distortion is simply defined as the percentage difference between the distance of an image point from the image center and the same distance as it would be measured in a distortion-free image; it can be thought of as a deviation between the imaged and the real dimensions of an object. For instance, if a point of an image is 198 pixels distant from the center, while a distance of 200 pixels would be expected in absence of distortion, the radial distortion, at that point, would be
Distortion = (198-200)/200 = -2/200 = 1%
Fig.3: “pincushion” type distortion “barrel” type distortion
Positive radial distortion is also called “pincushion” distortion, negative radial distortion is called “barrel” distortion: note that the distortion depends on the radial position and can also change of sign. Distortion can be also viewed as a 2D geometrical transformation of the real world into the virtual space created by the lens; as this transformation is not perfectly linear but is approaching 2nd or 3rd degree polynomials, the image becomes slightly stretched and deformed.
Common optics show distortion values ranging from some percent to some tens percent, making precise measurement really difficult; things get even worse when non-telecentric lenses are used. Since most machine vision optics have originally been developed for video-surveillance or photography applications, relevant distortion values have usually been considered acceptable, as the human eye can compensate distortion errors up to 1-2%. In some cases, like in fish-eye lenses or webcam-style lenses, distortion is intentionally introduced to make the lens work on large angles also providing an even illumination of the detector (in these cases distortion is helpful in reducing cosine-to-the-fourth law effects).
High quality telecentric lenses normally show a very low distortion degree, in the range of 0,1%; although this amount seems to be very small it would actually result into measurement errors approaching the size of one pixel of an high resolution camera. For this reason, in most applications, distortion has to be software calibrated: a precise pattern (whose geometrical accuracy must be at least ten times better than the needed measurement accuracy) is placed at the center of the field depth; distortion is then computed at several image points and, based on these data, the software algorithm transforms the native image into a distortion-free image. Few people know that the distortion also depends upon the distance of the object, not only upon the optics itself; for this reason it is very important
Few people know that the distortion also depends upon the distance of the object, not only upon the optics itself; for this reason it is very important that the nominal working distance is strictly respected.
A fine perpendicular alignment between the lens and the inspected object is recommended in order to avoid non-axially symmetric distortion effects. Trapezoidal distortion (also known as “keystone” or “thin prism” effect) is another important parameter to be minimized in an optical inspection system as it is asymmetric and very difficult to software calibrate. Lens focusing mechanism can also introduce some symmetric or non-symmetric distortion effect because of mechanical play or optical element decentering.
Fig. 4: on the left an image of a distortion pattern taken with a telecentric lens, where no radial or trapezoidal distortion is present. In the middle the image of the same pattern showing strong radial distortion. On the right an example of trapezoidal distortion.
(3) Perspective Errors limitation
When using common optics to image 3D objects (non completely flat objects) far objects will look smaller than close objects. As a consequence, when objects like a cylindrical cavity are imaged, the top and the bottom crown edges will appear to be concentric although the two circles are perfectly identical.
On the contrary, by means of a telecentric lens, the bottom crown edge will disappear because the two crown edges are perfectly overlapping.
Fig. 5: Common optics showing significant image perspective error (on the left). A telecentric lens is able to cancel any perspective effect (on the right).
This effect is due to the specific path of the rays: in the case of common optics, any geometric information that is “parallel” to the main optical axis also shows a component on the detector plane direction, while in a telecentric lens this perpendicular component is totally absent.
One could describe a common lens as a mathematical function building a correspondence between the 3-dimensional object space and the 2-dimensional detector (image) space while a telecentric would build a 2D-2D correspondance as would not display an object’s third dimension thus making it the perfect component for profile imaging and measurement.
Fig. 6: Common optics (left) project longitudinal geometrical information onto the detector, while telecentric lenses are not.
(4) Good image resolution
Image resolution is decribed by CTF (contrast transfer function) which quantifies the contrast ratio at a given spatial frequency on the camera detector plane, expressed in lp/mm (line pairs per millimeter).
Fig. 7: good and bad contrast achieved with optics of varying CTF looking at a standard USAF test pattern.
Quite often, machine vision integrators tend to combine cameras having tons of small pixels with cheap, poor resolution lenses, resulting in blurred images; the resolution provided by telecentric lenses is compatible with very small pixel sizes and high resolution cameras thus increasing the measurement resolution.
(5) No edge position uncertainty
When common back lighting an object it can often be difficult to determine the exact position of its edges.
This can happen because the bright pixels in the background tend to overlap with the dark pixels at the object edges. Moreover, if the object is highly 3D-shaped, also a border effect could furtherly limit the measurement precision; as shown in the following drawing, rays grazing the object edges at certain incidence angles could be reflected by the surface, but still be collected by the lens.
The lens would then see those rays as if they were coming from behind the object; as a result, slices of the image could disappear, thus making the measurement very much imprecise and unstable.
Fig. 8: Border effects in a common imaging lens are strongly reduced by means of a telecentric lens
This effect can be efficiently limited by means of a telecentric lens: if the pupil aperture is small enough, the only reflected rays which could enter the lens would be those nearly parallel to the optical main axis.
As these rays are affected by very small deflection, the reflection from the object surface doesn’t jeopardize the measurement accuracy.
To get rid of such issues, Parallel light Background light (also called “collimated” or “telecentric”) illuminators can be interfaced to telecentric lenses, taking care of matching the lens aperture and FOV with the collimated source divergence. With this option, all the light coming out of the illuminator is collected by the lens and delivered onto the detector, allowing extremely high signal-to-noise ratios and incredibly low exposure times. On the other hand, only “expected” rays come into the imaging lens so that no problems occur at the borders.
Fig. 9: Parallel light Background light (Collimated light, telecentric) illumination projects only the expected rays into the imaging system.
We captured some images use using 2 different combinations of lens and background (one is common lens and diffused background light as used in general contact angle measurement instrument, another is used telecentric lens and Parallel light Background light)
Fig. 10: Image of pendant drop used Parallel light Background light and telecentric lens. We can find little change range of sharpness at image edge when zooming it about 1500X.
Fig. 11: Image of pendant drop used common Background light and lens. We can find more change range of sharpness at image edge when zooming it about 1500X.
Fig. 12: Image of sessile drop with little change range of sharpness used Parallel light Background light and telecentric lens.
Fig. 13: Image of sessile drop with more change range of sharpness used common Background light and lens.
(5) Not disturbed by flare veiling glare
Flare veiling glare will not disturb image capture process.
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