Ultra High Performance Quartz lamp quality inspection.

The Ultra High Performance mercury arc lamp was developed by Philips in 1995 for use in commercial projection systems, home theater projectors and video walls. Unlike other common mercury vapor lamps used in projection systems, it is not a metal halide lamp, but uses only mercury. UHP burner
These lamps are highly efficient compared to other projection lamps and do have a long lifetime (over 10,000 hours).
To guarantee this high quality each burner (quality level is a few ppm!) will be inspected using vision systems.
The inspection of the geometrical dimensions of the burner is not described on this page, because when using good illumination it's straightforward.
For a reliable defect detection the most important issue is to create the best feasible illumination. In most cases special illumination is created to achieve the best result.

The most important inspections or defects are described here and classified in:
    In-glass defects, e.g. bubbles, cracks
    Inside balloon defects
    Outside balloon defects

In-glass defects:

Bubbles

Basically bubbles can be detected using a back light. If the resolution of the optics and camera is sufficient, the bubble is visible as a ring so the inner side is bright.
Bubble in Glass Flat Backlight Bubble in Glass V Backlight To create a good contrast the illumination angle has to be as small as possible which results in a broad and dark balloon contour.
The disadvantage of this method is the decrease of the scan area.
To solve this problem an other type of illumination with a large illumination angle including a small dark line in the back of the illumination can be used.
This thin line shows a thin small contour of the inner chamber of the balloon.
When the illumination angle is getting too large the contrast will decrease as shown in the sub images.

Bubble in Glass, subimage Flat Backlight Bubble in Glass, subimage V Backlight Bubble in Glass, subimage Flat Backlight int profile Bubble in Glass, subimage V Backlight int profile Compare the intensity profiles. Left the image created with a flat back light, second left the image created using a V-shaped illumination.
Followed by the corresponding intensity profiles.
To inspect the whole lamp, the lamp has to be rotated.

Cracks

Cracks can occur in the balloon as well in the seal on both sides of the balloon.

Cracks in the balloon:

Using a normal back light this defect is almost not visible and hardly detectable. So a different type of illumination is used. The illumination is fit for the glass curve so an optimal contrast is realized.
Crack in Glass Crack in Glass analyse Crack in Glass, other types In the image (top side, red arrow) the crack is visible. The subimage and the intensity profile (over the green line) shows the crack and the realized contrast.
The background noise is very low so the intensity can be increased if necessary without disturbing the detection quality significantly. The length of the crack is about 200 micrometer (scale is 10 micron per pixel).
Detection of this crack is straightforward using a suitable blob detection algorithm, with or without dynamic thresholding. The length, width and area can be measured. To inspect the whole lamp, the lamp has to be rotated. Two different cracks are shown in the sub images.

Cracks in the seal:

Using a normal back light this defect is almost not visible and hardly detectable. In the image (red arrow) the damage is hardly visible.
Crack in seal Crack in seal, analyse Crack in seal, analyse Crack in seal, analyse The intensity profile shows very low contrast. The illumination used for cracks in the balloon even doesn't give a sufficient result.
So a different and complex type of illumination, especially created for cylindrical shaped glass, is used.
Using this illumination a dark field or a bright field images can be created. Sub images for bright field as well for dark field including the intensity profile are shown.

Compare the contrast values between a normal back light and the special illumination.
The contrast is very good so a reliable detection can be performed using a suitable blob detection. The length, width and area can be measured. To inspect the whole lamp, the lamp has to be rotated.

Inside Balloon defects

Clean burner Inside the balloon two electrodes are visible. Between these two electrodes a light bow will be created.
This image is created with a back light and a small thin line on the back to create a perfect contour of the balloon chamber.
The image is not good enough to detect de quality of the electrodes. Therefore the thin black line has to be removed.
At that particular part at the inside of the balloon it's not allowed to have any dark precipitation or small crystal parts.
This will disturb the light which results in a not perfect beam.



Black precipitation
Basically small precipitation (dark parts) can be detected using a back light. Light will be absorbed by these dark parts.
Burner precipitation Burner precipitation
To get a large as possible scan area and a well defined contour of the balloon chamber, a V-shape illumination and a thin center line is used.
Now the inner contour can easily be detected. Using a smart algorithm the amount of precipitation within the balloon chamber can be measured.
Compare the two sub images and the difference in precipitation.


Crystallization
When using a back light crystallization shows a very weak dark area, almost comparable with black precipitation.
Burner crystallization Burner crystallization
Important is also to know if this defect is crystallization or black precipitation. This information is important for a better tuning of the production proces.
To make the difference and to create the best contrast a complete other illumination is used to visualize this defect. Here the white particles on the inside of the balloon chamber are visible when the illumination is under a certain angle.
Sub images show these particles. Because of the inhomogeneity of the background the software must be well designed.
The realized contrast is very good so the detection does not cause any problem using this type of illumination. On the other hand

Outside balloon defects

The most complex detectable defects on the outside of the balloon is a very thin layer of condensated quartz vapour.
Burner vapour Burner vapour Burner vapour This thin layer is not visible and therefore not detectable by a dark field or a bright field setup.
To visualize this defect a special illumination is developed. Because of the round shape of the balloon this illumination is special created for this type of product.
Using complex structural lighting a very good contrast can be realized.

Burner vapour rotating.
Compare the images using a back light and the structured light. The measurement is getting more complex so complex software is realized to detect the defect.
The images show only the upper part of the balloon and of course for a complete detection the product has to rotated.
Here a rotating scan is shown.