GB2103356A - Image analysis - Google Patents

Abstract

In apparatus for detecting blemishes and defects in a surface, light is shone on the surface and its image analysed by a camera 10. The output of the camera is digitised by an analogue to digital converter 16. To provide contrast enhancement, the input to the converter 16 has a variable DC offset 20 applied to it to remove the effect of the mean background illumination so as to improve the sensitivity of the output of the converter 16 to signal variations resulting from surface defects, the offset being dependent upon the intensity of the light sensed by the camera 10 during preceding scan cycles. <IMAGE>

Description

SPECIFICATION Image analysis The present invention relates to image analysis and is particularly concerned with apparatus for detecting blemishes and defects in a surface.

It has been proposed in order to detect defects in a surface to illuminate the surface and to analyse the reflection by the use of a CCD (charge couple device) array. When the output of such an array is digitised, in orderto limit digital processing a limitation is placed on the number of digital bits by the rate at which the analogue to digital converter is required to operate and also the escalating cost.

It is normally desired that the output of the analogue to digital converter should be a linear representation of the video signal so that the ability of the apparatus to detect defects should be the same regardless of the intensity of the light source and the reflectivity of the surface. As a consequence, the minimum difference between intensities which can be resolved, it is determined by the maximum intensity divided by the maximum digital output of the AID converter.

The present invention seeks to permit contrast enhancement without increasing the number of the bits at the output of the A/D converter.

In accordance with the present invention, there is provided apparatus for detecting defects in a surface which comprise an optoelectrical transducer for scanning the surface, an analogue to digital convertes for digitising the output of the transducer and a processor for analysing the digital values representative of the intensity of the radiation emanating from the surface to detect surface defects, wherein a DC offset is applied to the signal received by the analogue to digital converter from the transducer during each scan cycle, the offset being dependent upon the intensity sensed by the transducer during preceding scan cycles.The negative DC feedback enables the dynamic range of the analogue to digital converter to be substantially less than the dynamic range of the output signal of the transducer thereby improving the resolution of the system by providing constrast enhancement.

The invention will now be described further, by way of example, with reference to the accompanying drawing which shows a block circuit diagram of the apparatus in accordance with the invention.

When digitising any transducer output, the dynamic range of the analogue to digital converter is normally matched to the minimum and maximum output signal of the transducer. However, on occasion, the transducer may require digitising at a very fast rate and due to cost or technological reasons it may be found impossible to match the transducer signal to noise ratio with the result that an analogue to digital converter of reduced resolution would normally have to be selected, giving rise to a reduced system performance.

The transducer in the present case is a CCD (charge couple device) photosensitive linear array which typically has a signal to noise ratio of 2500;1.

To preserve this in the following analogue to digital converter would require digitising to an accuracy better than 11 bits. The apparatus shown in the Figure enables an 8 bit analogue to digital converter to provide a resolution approaching the limits possible with the signal to noise ratio of the charge couple device array.

The CCD array, designated 10, is connected by way of an input resistor 12 and a voltage follower 14 to the input of the analogue to digital converter 16.

The input of the voltage follower 14 is further connected to ground by way of two current sinks 18 and 20 of which the latter is variable.

The output of the analogue to digitial converter 16 is connected to a digital processor (not shown) and to the input of a photosite averager 22. The output of the averager 22 is connected to a digital comparator 24 having a second input receiving a constant count equal to 128. The output of the comparator 24 is fed to an up down counter 26 the count of which is converted by means of the digital analogue converter 28 to a signal for controlling the variable current sink 20.

The CCD array typically has a peak signal of 1 volt and a noise of 400 microvolts. The analogue digital converter 16 has a more limited input range of only 0 to 100 millivolts, the 8 bits giving a maximum count of 256.

When scanning an essentially flat surface with only minor blemishes, the video signal from the CCD array 10 will consist of a pedestal voltage which remains constant across the scan with superimposed signal corresponding to surface defects. The feed back from the output of the A/D converter 16 to the voltage follower 14 is intended to offset the input signal in such a manner that the pedestal voltage corresponds substantially to a count of 128 equal to half the maximum count of the converter 16. Because the offset voltage follows the pedestal voltage, the full range of the analogue to digital converter can be used to digitise signals corresponding only to surface defects rather than the overall emitted radiation thereby enabling significant contrast enhancement, the enhancement being tenfold in the case of the given example.

The two current sinks 18 and 20 carry a current which when summed gives rise to a voltage drop across the resistor 12 equal to the offset voltage.

Thus the voltage at the input of the voltage follower is less than the output of the CCD array by the offset voltage. The voltage difference is converted and the digital count over a whole cycle is averaged by the averager 22. If the average exceeds 128 the digital comparator 24 will produce a signal to alter the state of the updown counter 26 by 1 unit in a first direction whereas if the mean value is less than 128 then the digital comparator 24 will alter the count in the counter 26 by 1 unit in the opposite direction. The remaining count in the counter 26 is converted into an analogue signal by the D/A converter 28 to fine tune the variable current sink 20 such that the photosite average over a scan cycle should be 128.

Since in each scan the updown counter can only be varied by one count, the feed back loop is very slow and does not respond quickly to changes in intensity affecting only one or two scans but instead sets a mean which follows long term trends in the video signal dictated by intensity of the illuminating light source and the reflectivity of the light source.

It should be understood that though the term light has been used in the specification, the term is intended to be understood broadly to cover radiation outside the part of the spectrum visible by the human eye.

Claims (2)

1. Apparatus for detecting defects in a surface which comprises an optoelectrical transducer for scanning the surface, an analogue to digital conver tes for digitising the output of the transducer and a processor for analysing the digital values representative of the intensity of the radiation emanating from the surface to detect surface defects, wherein a DC offset is applied to the signal received by the analogue to digital converter from the transducer during each scan cycle, the offset being dependent upon the intensity sensed by the transducer during preceding scan cycles.

2. Apparatus for detecting defects in a surface constructed substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.