Lens quality inspection.

In almost every cell phone a camera is included. Typical resolution of these cameras is 5 Mega pixel and are still increasing. These cameras require also a high resolution and high quality lens.
The lens itself does have a diameter of 2.5 mm and contains 2 separated optical parts. (so 4 surfaces)
On the other hand these lenses must be relative cheap so they are produces in large quantity. At this moment the quality of each lens is checked manually using a microscope. This is time consuming and very expensive.
Inspection using a vision system would decrease the inspection cost but detection the defects is not obvious. Lens background incl line profile JAIMS

Because these camera lenses are produced on wafers (diameter about 200mm) with about 3000 lenses per wafer, a circuit board inspection system would be a obvious inspection system to check the lens quality.
A lot of these high speed printed circuit board inspection scanners use a front light to illuminate the scenery.
Because of the front lens surface is not a flat surface but does have a special curved shape the reflected front light will diverge.
The created image does have a not homogeneous intensity profile which results in a low contrast at the edge of the lens and a high intensity in the center of the lens
When using a back light (not used for circuit board inspection of course), the lens does his job and focuses the light in the focus point in front of the camera. Result, again a divergence of the light.
And again a bright lens center and a rapidly decrease of light to the outside of the lens, results in a low defect contrast at the edge of the lens. The intensity profile of this image is given in the line profile graph. Here an image of the lens surface including a defect is shown. Line profile standard illumination JAIMS
Using a dynamic threshold algorithm a detection is possible but the reliability is far from optimal. So a normal front light or a normal back light is not suited for this type of inspection because the inhomogeneity of the by camera received light which result in a decrease of contrast.
The images with the yellow rectangles show also less contrast for defects within that area. Also the defect just outside the centre under the green line does have a very low contrast.
The very dark defect (red line) shows a good contrast, this defect absorbs light heavily.

Conclusion: for lens quality inspection special illumination, to create a good defect contrast, is necessary.
So first of all a good image with a good contrast must be created. Next step is a reliable detection algorithm.

Therefore J.A.I.M.S. developed a few illumination techniques and shows here several lens defects including contrast results using the more sophisticated J.A.I.M.S. illumination technique.

J.A.I.M.S. illumination

As mentioned the lens does have a diameter of about 2.5 mm. Defects on the lens surface (e.g. stain, particles, scratch width) larger than 20 micron has to be detected.
Since the surface of the lens does have a certain curvature, focus depth will also be one of the major problems to create a overall sharp image.
To create sufficient resolution (pixel size of about 0.33 micron) a 5 Mega pixel black and white camera is used.

JAIMS Front Light illumination lens M01 Lens 1: Front light. (Abbreviate as FLBF)
The image created using the front light is shown in the left image. The intensity profile three defects are also shown.
The defects are visible but the local contrast for all three is relative low, anyway not optimal.

JAIMS Back Light Ill Special M01 Lens 1: (J.A.I.M.S.) Special back light illumination. (Abbreviate as BLDF)
The image shows also a dark field image, the defect are visible as a bright particle.
Compare the intensity profiles (Line profile) and a typical kind of defect is very good visible (top and center) but an other type, in this case a light absorbing defect, is hardly visible and not detectable (bottom).
This type of illumination gives a nice result for a special type of defects but it's not the best solution for this lens quality inspection.

JAIMS Back Light illumination lens M01 Lens 1: (J.A.I.M.S.) Sophisticated back light illumination. (Abbreviate as BLBF)
The images created using the J.A.I.M.S. Sophisticated back light illumination results in an image with a very good contrast.
The diameter of the dark defect (bottom lens) is about 12micron and because of the extreme good contrast very good and reliable detectable.
The other two defects needs slightly more software because of the thin defect border, but very good and therefore reliable detectable.

Overall intensity lens1 Front Ill JAIMS Overall intensity lens1 Back Ill JAIMS Compare the overall intensity between the front light (FLBF) and the sophisticated back light (BLBF).


No image is shown of lens 2 using the FLBF illumination, the contrast is not sufficient enough for any detection.

JAIMS Front Light ILL M02 Lens 2: Front light. (FLBF)
The image created using the front light is shown in the left image.
The intensity profile of two defects are also shown.
The intensity profile (left and right) on top and bottom are out of focus, the intensity profile in the middle is in focus.
The defect is still visible but the contrast for detection is relative low.

JAIMS Back light Ill Special M02 Lens 2: (J.A.I.M.S.) Special back light illumination. (BLBF)
The images created using the Special back light illumination results in an image with a very good contrast.
The intensity profile (left and right) on top and bottom are out of focus, the intensity profile in the middle is in focus.
The defect is very good visible and the contrast for detection is very good.
The scratch in the yellow marked area is less well detectable and need some extra software effort. In the FLBF illumination this scratch is hardly visible.
The width of this scratch is 2 micron.

JAIMS Back Light Ill Special Fixed Threshold M02 The 5 sequential images show the result using a simple fixed threshold algorithm. The most left and right image are out of focus, the image in the middle is in focus.
The small spot marked with a yellow arrow is a defect of 8x9 micron.
In this situation using this illumination and camera very small defects are detectable.

JAIMS 3 types illumination M03 Here an extra set of images, left a FLBF, center a BLDF and right a BLBF created image.
The scratch left bottom does have a width of 4 micron.
Compare the moon shaped defect on the right top. The BLBF illumination gives the best contrast.


And again:

Lighting is the key.


For more detailed information about Lens Inspection download the pdf (only in Dutch available, sorry): Lens kwaliteit inspectie.pdf