GB2307752A

GB2307752A - Optical defect scanning

Info

Publication number: GB2307752A
Application number: GB9624701A
Authority: GB
Inventors: Wolfgang Haubold; Thomas Zimmermann
Original assignee: LASER SORTER GmbH
Current assignee: LASER SORTER GmbH
Priority date: 1995-11-29
Filing date: 1996-11-27
Publication date: 1997-06-04
Also published as: FR2741698A1; DE19544481A1; GB9624701D0

Description

I:) Defect Scanning Arrangement 2307752 The invention relates to the

identification of 5 defects in or on materials.

When using cameras in the detection of defects on or in materials, such as glass sheets, transparent foils, paper webs, etc, high demands are placed on the illumination employed at the time. With illumination times which are usually short, the light intensity has to be very high. Long service life, high stability and freedom from noise are further demands on an illumination system.

In the past, very good results have been obtained with halogen filament lamps. The service lives of these lamps amount to about 2000 hours, the efficiency, however, is very low and the heat build- up is very high.

According to the present invention there is provided the combination of one or more halogen-metal vapour lamps and a camera in association with delivery means for passing a material to be scanned for defects in front of the camera.

Such an arrangement enables light output to be increased over when using conventional halogen lamps and the illumination arrangement is extremely longlived and shock-proof.

Halogen-metal vapour lamps are high-quality light sources, which are marked by high light output. Metal admixtures in the gas discharge increase the light efficiency of the order of four-fold in comparison with filament lamps. Their shock resistance corresponds approximately to that of a shock-proof filament lamp.

In order to focus the light beam which is produced, the lamps are advantageously arranged at the focal point of an elliptical mirror or even a parabolic 1 0--- mirror.

If the light intensity of a lamp is not sufficient, then at least two lamps can be arranged next to each other, with their light beams overlapping in the plane which is to be tested.

By arranging a lightscattering screen with as high a transparency as possible, or an optic, in front of the lamps, the illumination can be improved further depending on the instance of use.

For a better understanding of the invention and to show how the same can be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, wherein; Figure 1 shows schematically one basic form of illumination arrangement comprised by the combination of the invention; and Figures 2 to 9 show variants of such illumination arrangement.

In Figure 1, the basic construction of an illumination arrangement is shown. Because, when using line cameras, only one line is examined, or when using TDI-cameras only a few lines are examined, the light can be concentrated on a relatively narrow line. An elliptical mirror or parabolic mirror 5 is used for focusing. The high and even illumination of the test line is achieved by arranging in rows a variable number of light sources or lamps 3 shown installed in a housing 9 and which overlap in the test plane. The residual ripple illumination depends on the scattering effect of the material to be tested and the distance of the individual light sources 3. It can be improved by the additional use of a onedimensional lightdiffusion screen 6 or by cylindrical lenses arranged in rows.

Figure 2 shows an arrangement for scatter testing.

The material 1 to be tested acts as a diffusor. In all figures, a camera is denoted by the reference symbol 2.

The lamps 3 are located in an elliptical mirror 5. The light beam is indicated by the reference symbol 4.

AS shown in Figure 3, if there is a bright surface and a very narrow reflection beam, it can be necessary to arrange a diffusing screen 6 between light source 3 and test line 1.

Figure 4 shows an arrangement for detecting deflecting defects in mirror reflection. Arranged between lamp 3 and the material 1 to be investigated is a diffusing screen 6 having a dark field 8.

A transmission examination of paper 1, for example, is shown in Figure 5. Dirt and thin areas can be distinguished by the signal direction; the pinhole defect type, however, is problematic, because, depending on the position of a pinhole, the camera 2 shines directly into a lamp 3 (signal positive) or to the side of a lamp 3 (signal negative).

Figures 6 and 7 show variants which in the case of the pinhole defect type produce positive signals. In Figure 6, a weakly dimming diffusing screen 6 is located directly beneath the web 1; in Figure 7 a weakly dimming lightdiffusing strip 6 is located on the open end pane of the illumination unit or between the open end pane and the material. A bright field examination for absorbing defects in transparent materials 1 is shown in Figure 8. Should deflecting defects also be found in addition to the absorbing defects, the bright field has to be reduced. 30 Figure 9 shows a test arrangement which permits a dark field examination. It is used to detect deflecting defects. Especially when testing in diffuse transmission in the case of fast-moving webs 1, it can happen that the luminous intensity is insufficient. Here there is the possibility (see Figure 10) of focusing the light 4

1 onto a relatively narrow strip by means of cylindrical lenses or an optic 7, and consequently greatly increasing the intensity. In a preferred embodiment, the individual illumination sources 3 are installed in a housing 9 as in Figure 1. The length of the housing 9 depends in each case on the width to be checked. Advantageously, the illumination arrangements can also be produced from single spotlights. These are displaceably fastened on a rail, the length of which is dependent on the test width. Such an illumination system would be marked by great flexibility and would make further construction superfluous. Advantages of such a system are:

is 1) matching to test width merely by lengthening the fastening rail, 2) altering the luminous intensity by altering the number of lamps or by exchanging the types of lamp (150/400 W), 3) optimal accessibility to the lights and lamps for maintenance, repair, and eliminating the need for recurrent construction work for the illumination unit.

a 1

Claims

1. The combination of one or more halogen-metal vapour lamps and a camera in association with delivery means for passing a material to be scanned for defects in front of the camera.

2. The combination of claim 1, which has said lamp(s) arranged at the focal point of an elliptical mirror or parabolic mirror in order to focus the light beam which is produced.

3. The combination of claim 1 or 2, wherein two said lamps are arranged next to each other so that in use, the light beams of the lamps overlap in the plane to be checked.

4. The combination according to any one of claims 1 to 3, wherein a light-diffusing screen or an optic is arranged in front of the lamp(s).

5. The combination of claim 1, having one or more halogen-metal vapour lamps.

6. The combination of claim 1, substantially as hereinbefore described with reference to any one of the accompanying drawings.