JPS6021792Y2 - Defect detection device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in an apparatus for detecting defects in a transparent sheet-like body using coherent laser light.

When detecting defects in transparent objects such as glass plates using a defect detection device that uses a laser as a light source, laser light has good monochromaticity and is a coherent light source, so it is difficult to detect defects due to multiple reflections on the surface of the transparent object. Interference is inevitable.

The signal level due to this interference is almost non-negligible compared to the defective signal level due to reflected light from the object to be inspected.
Detection sensitivity for defect detection cannot be secured.

One possible way to avoid this interference is to change the light source from a laser beam to another type of incoherent light, but it is easy to use a fine, parallel, and stable output of a laser beam as a light source for a defect detection device. The benefits of being able to receive a loan are something that is hard to give up.

In addition, to distinguish between noise signals and defect signals, for example, as in Japanese Patent Application Laid-Open No. 50-144488, reflected light and transmitted light from an object to be inspected are photoelectrically converted using different photoelectric converters, and then these signals are converted into A detection device that multiplies each output of a photoelectric converter has been proposed, but in a detection device using a multiplier in this way, the waveform of the defective signal is expanded and the waveform of the noise is also expanded. , it cannot be canceled unless the noise level of at least one of them is zero, and especially when the signal/noise ratio of the defective signal is small, the difference from the noise does not become that large.

Therefore, it is not suitable for devices where the noise level due to interference is high, such as when using a coherent light source.

In a defect detection device that uses laser light as a light source, this invention cancels out complementary noise to lower only the interference signal level, expands the defect signal, and secures the detection level by combining the reflected signal and the transmitted signal. By adjusting the amplitude of one alternating current component of the signal according to the amplitude of the other and adding the outputs of both signals, the high level of noise that is a problem when using coherent laser light can be effectively eliminated. This is to ensure that defective signals can be detected.

FIG. 1 is an explanatory diagram of a defect detector that detects defects by irradiating light onto a transparent or semi-transparent object. If the object is transparent, most of it will pass through the glass plate 4, the transmitted light 3t will enter the transmitted light receiver (transmitted signal detector) 5, and some of it will be transmitted through the glass plate 4. It is reflected and enters a reflected light receiver (reflected signal detector) 6 as reflected light 3r.

However, the transmitted light 3t and the reflected light 3r interfere with each other due to multiple reflections within the glass plate 4, resulting in primary, secondary, etc.
Since it can be divided into components due to the interference of..., R=Rm+'! 'Ri (i=Ot 1.2"")R=
Tm+"Ti (i=0.1.2"") However, R...The total output of the reflected light 3r Rm・
...Average output Ri of reflected light 3r...
- Fluctuation of output due to interference of reflected light 3r Molecule...Total output Tm of transmitted light 3t...Average output Ti of transmitted light 3t...
...This is the variation in output due to interference of the transmitted light 3t.

By the way, according to the 5tokes relational expression, when light is reflected at the interface between an optically sparse substance and a dense substance, a phase difference of half a wavelength is generated.

Therefore, if one of the reflection variation Ri and the transmission variation numerator i increases, the other decreases.

Here, if we add the total reflected power R and the total transmitted power T, we get A11R+T=A-Rm+Tm Σ+, (A-Ri+Ti)...
(Formula 1) Since the reflection fluctuation amount Ri and the transmission fluctuation numerator i have opposite signs, the constant A can be determined so that A −Ri+Ti=0.

This constant A can be determined by measuring the interference amplitude of the reflected total output R and the transmitted total output T in advance, and then calculating the ratio of the interference amplitudes.

However, the interference amplitude differs depending on the angle of incidence θ on the glass plate 4, the thickness of the glass plate 4, and the material of the glass plate.

When the output signals of the total reflected power R and the transmitted total power T from the object to be inspected are measured with separate light receivers (signal detectors), these signals include signals due to interference and signals due to defects.

Signals due to interference are output in opposite phases to reflected signals and transmitted signals, but signals due to defects are output at the same place, at the same time, and with the same sign.

At the defective part, light is scattered or absorbed and the output decreases, but the reflected light Rf and transmitted light Tf at the defective part are as follows: Rf= (1-Q) Rm-1'Ri Tf= (1-a) Tm+'FTi Also, α is the decrease in output of the defective part relative to the normal part.

Therefore, the output of only the defective part where interference is canceled by the defective part is A-Rf+Tf= (1-af (A 11Rm+T
m) Jugi (A-Ri+Ti) = (1-α)(A-
Rm+Tm) m1ll16 (Equation 2)%Equation%Here, the light output of the normal area and defective area can be expressed by (Equation 1) and (Equation 2).

FIGS. 2 and 3 show a block diagram and waveform diagrams of each part of an embodiment of the present invention.

5 is a transmitted light receiver (transmitted signal detector), 6 is a reflected light receiver (reflected signal detector), 7 is an amplifier (amplification degree can be changed), 8 is an adder, and 9 is an automatic gain adjuster (AGC). ), 1
0 is the output end.

A waveform 21 is a signal waveform of the transmitted light receiver 5, which is amplified by an amplifier 7 according to the level of the interference signal to form a waveform 22, which is added to a signal waveform 23 of the reflected light receiver 6 by an adder 8.
The AGC 9 outputs the waveform 24 from the terminal 10.

In reality, there is a difference in the DC level between the reflected light and transmitted light, so compare the levels of only the AC part of the reflected light and transmitted light.
Determine the constant A so that the interference signal level is the same,
This is to cancel out the interfering bones and extract only the defective signal.

In an embodiment in which the object 4 to be inspected is transparent glass, the transmitted light 3t
The amount of light 3r is sufficiently large compared to the reflected light 3r.

Therefore, the signal output of the transmitted light receiver 5 is large and its waveform 21 has a high DC level, but the AC component due to interference is relatively small, and in contrast, the signal output of the reflected light receiver 6 is small and its waveform 22 is a low DC level, but the AC component due to interference becomes large.

Therefore, the constant A is determined by comparing the levels of only the alternating current parts of the reflected light and the transmitted light, and the output signal of the transmitted light is amplified by the amplifier 7 so that the interference signal level is the same, and the signal waveform 23 is obtained. The signal waveform 22 of the reflected light receiver is added to the adder 8.
are added together, and the AGC 9 outputs the waveform 24 from the terminal 10.

As shown in FIG. 4, the output signal waveform 22 of the reflected light receiver 6 and the output signal waveform 23 of the amplified transmitted light receiver 5 are such that the noise signals ri and ti are adjusted to approximately equal amplitudes and have opposite phases. However, since the signal if -tf due to the defect is in the same phase, the added waveform 24 has a noise signal r
Although i-ti is canceled, the defect signal if*tf is added and can be easily determined.

5 and 6 show a block diagram and a waveform diagram of another embodiment of the present invention.

A signal waveform 201 received by the transmitted light receiver 5 is passed through the filter 11 to form a waveform 202, amplified to form a waveform 203, and then added to a signal waveform 204 received by the reflected light receiver 6 to obtain an output signal 205. sell.

In other words, only the alternating current portion of the transmitted light receiver is taken out in advance, and the alternating current is amplified and added to cancel the interference.

The output level Rf' of the defect signal at this time is Rf'= 0aRm+! Ri +”+ A @Rf'+Tf = -aeA-Rm+(1-a)Tm = (1a) (A-Rm+Tm)-A-Rm, so both when only AC amplification is used and when DC component is also added. However, the magnitude of change in the defect signal remains the same.

In the interference of a glass plate, the interference between reflected light and transmitted light is originally
The phases are completely opposite, but in general there may be a slight shift during the signal processing stage, etc. In such cases, a phase shifter etc. may be inserted in the path of the reflected light or transmitted light to cancel the interference. .

The present invention can be applied not only to glass plates but also to transparent or translucent sheet-like materials such as acrylic plates and various films.

Note that an amplitude attenuator may be provided instead of the amplifier.

In this way, the present invention utilizes the fact that the phases of the interference signals contained in transmitted light and reflected light are complementary, and adjusts the amplitude of one AC component according to the amplitude of the other AC component. However, since these signals are added together, the defect signal is increased and only the interference signal is canceled out, which has the effect of eliminating the influence of interference in single-color laser defect detection and improving the sensitivity of defect detection.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram of an optical defect detection device, Fig. 2 is a block diagram of an embodiment of the present invention, Figs.
The figure is a waveform diagram thereof, and FIGS. 5 and 6 are block diagrams and waveform diagrams showing other embodiments. 1 is a light emitter, 2 is a rotating polygonal mirror, 3 is a laser beam, 4 is an object to be inspected, 5 is a transmitted light receiver, 6 is a reflected light receiver, 7 is an amplitude adjuster, 8 is an adder, 9 is a Automatic gain adjuster, 10 is the output end, 11 is the filter, ri, ti are the interference signals, if
, tfj is a defective signal.

Claims (1)

[Scope of utility model registration request] An optical scanning device that optically scans a transparent or translucent sheet-like object to be inspected with a coherent laser beam, and a surface reflection that occurs when the scanning light from this optical scanning device is reflected from the object to be inspected. A reflected signal detector that detects light and a transmitted signal detector that detects transmitted light generated when the scanning passes through the inspected object, the signal from the reflected signal detector and the transmitted signal detector In the defect detection device for the sheet-like inspected object, the amplitude of the alternating current component is converted into the amplitude of the other alternating current component by combining the signals from the output from the reflected signal detector and the output from the transmitted signal detector. 1. A defect detection device comprising: an amplitude adjuster that adjusts accordingly; and an adder that adds the output of the amplitude adjuster and the output of the other signal detector.