JPS63201589A - Automatic foreign matter checking device for food - Google Patents

Abstract

PURPOSE:To check foreign matters in online base by recording an X-ray transmitted optical image on an electrified surface insulating layer as a potential difference and sticking a fine particle toner after opening the trap electric charge and picking up a formed image by a TV camera. CONSTITUTION:A high voltage is applied to a photoconductor layer 52 of a photosensitive part 24 for a prescribed time by an electrifying part 26 to uniformly electrify a surface 54A of a surface transparent insulating layer 54 with plus electric charge. When a roast ham 7 reaches the position just under a soft X-ray tube 2 by a belt conveyor 10, a tube voltage is applied to the soft X-ray tube 2 and minus electric charge is corona-discharged. The surface 54A of the photosensitive part 24 on which soft X drays are projected from the soft X-ray tube 2 and the potential difference corresponding to the transmitted optical image is generated is through out exposed to a single beam from an exposure part 28 to release the trap electric charge. A formed electrostatic latent image is developed to a visible image by electrostatic sticking of a fine particle toner 30 in a developing part 32, and this image is picked up by an image pickup part 34 while being reflectively illuminated by an illuminating part 33 and is inputted to a signal processing part 38.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to an automatic food foreign matter inspection device for inspecting foreign matter in foods such as meat and processed meat, and particularly relates to an automatic food foreign matter inspection device that irradiates the food with X-rays The present invention relates to an automatic food foreign matter inspection device that inspects foreign matter in real time based on transmitted light.

[Conventional technology]

Conventionally, in the manufacturing stage of foods such as raw meat and processed meat, foreign substances (e.g. metal, bone, cartilage, wood,
(vinyl, air bubbles, etc.) are being inspected. For example, as a foreign substance inspection method for inspecting raw meat and processed meat for foreign substances, there is an electromagnetic inspection method that uses an electromagnetic field. However, although the electromagnetic inspection method can easily detect metal foreign objects, the detection accuracy of non-magnetic metals is poor, and the detection sensitivity of non-magnetic metals is almost the same as that of raw meat and processed meat itself. N is not good. For this reason, the electromagnetic inspection method is not widely used for inspecting foreign substances in raw meat and processed meat.

Therefore, the electromagnetic inspection method has now been replaced, and soft X
Direct visual inspection of the X-ray transmitted image formed based on the X-ray transmitted light image after irradiation with rays, or visual inspection by photographing the X-ray transmitted image with a high-sensitivity TV camera such as an image orthicon TV camera. Soft X-ray inspection methods are widely used. The soft X-ray inspection method is capable of recognizing various types of foreign objects, although the size of the foreign objects that can be identified varies depending on the physical properties of the foreign objects.

The inspection method of soft X-ray inspection equipment (hereinafter referred to as soft A solid-state imaging panel inspection method in which an XIK transmission image on a solid-state imaging panel is captured by a high-sensitivity TV camera for visual judgment. An image in which an X-ray transmission image on a phosphor plate is directly captured by an image intensity fire TV camera for visual judgment. There are four types: the Intensity-Fi 7-TV left camera inspection method, and the X-ray film inspection method in which an X-ray transmitted image is directly printed on an X-ray film and visually judged after development.

The foreign object recognition limits of soft X-ray inspection equipment using these four inspection methods are based on example test results of simulated foreign objects (such as steel balls, wood chips, blisters, voids, bone souls, and muscles) mixed into roast ham. I will explain.

In the fluorescent plate inspection method, the X-ray transmission image of the roast ham formed on the fluorescent plate is taken through FL, 6, FL, and 5 mm lenses with a minimum object illuminance of 0.5 Lu and a high-sensitivity TV camera that can take more pictures. When the image is displayed on a monitor screen and judged visually, the diameter of the steel ball is approximately 2.5 mm, the soul of bone is approximately 5 mm square, the void is approximately 1011 Iffl square, the piece of wood is approximately 10 ml 11 square, and the muscle is approximately 10 III wide. x 5 m
Items with a thickness of m or more can be recognized. However, I cannot recognize the blisters.

In the solid-state imaging panel inspection method, an X-ray transmission image of the roast ham formed on the solid-state imaging panel is used as Fl.

6, fl, minimum subject illuminance 0. through a 6mm lens.
When an image is taken with a high-sensitivity TV camera that can take pictures from 5L-, and the image is displayed on a monitor screen and visually judged, the steel ball has a diameter of about 1.5 mm, the bone soul has a diameter of about 2.5 mm, and a void. Objects with a size of about 7.511 IIm square or more can be recognized. Pieces of wood and muscle can be recognized to the same size as with the fluorescent plate examination method, and blisters are similarly not recognized.

In the image intensifier-TV left camera scanning method, an X-ray transmitted image of the roast ham formed on a fluorescent plate is captured with high brightness by an image intensifier-TV camera, and the image is displayed on a monitor screen for visual judgment. In this case, visual recognition of the raw image without image processing shows that the steel ball has a diameter of about 0.1 square, the bone soul has a diameter of about 2.5 square, and the void has a diameter of about 5 square.
IIIff1 square, and pieces of wood larger than approximately 7.5 mm square can be recognized. Furthermore, blisters with a diameter of about 2.0 mm or more can be recognized, although they could not be recognized using the two inspection methods described above. Furthermore, when the X-ray transmission image was integrated and displayed several dozen times on a monitor screen and visually judged, the diameter of the steel ball was approximately 0°5i+m, the bone soul was approximately 1.0 mm square, and the void was Approximately 511 II 11 squares, wood piece approximately 5.0 mm square, muscle approximately 10 Illff1 width
Items with a thickness of 5 mm or more can be recognized. Further, in this inspection method, for example, even if 20 screens of X-ray transmission images are integrated, it only takes about 0.6 seconds.

In the X-ray film inspection method, an X-ray transmission image is recorded on an X-ray film, and when the X-ray transmission image formed on the developed X-ray film is visually inspected, a steel ball with a diameter of approximately 0.
5 crucian carp, about 1.0 kaku of bone soul, about 2.5 ml of void
1 square, approximately 2.5 ma + square for a piece of wood, approximately 5 IQm for a muscle width x 2 ++un thickness or more is S2.
I can understand.

As described above, the X-ray film inspection method is the best among the inspection methods in recognizing various foreign substances in roast ham.

[Problem that the invention seeks to solve]

However, the X-ray film inspection method can only be used offline because it involves a time-consuming process of developing the X-ray transmitted image exposed on the X-ray film with a developer and making it into a film. There is a problem in which skill is required to accurately recognize foreign objects from an X-ray image of a film.

In view of the problems of the conventional technology described above, the present invention has developed an online system that has foreign object recognition performance equivalent to or better than that of a soft The purpose of this research is to provide an automatic food foreign substance inspection device that can be used in various ways.

[Means for solving problems]

In order to achieve the above-mentioned object, the food foreign substance inspection device of the present invention irradiates X-rays into food such as meat transported by a conveyance means such as a belt conveyor, and
An automatic food foreign matter inspection device that inspects food for foreign matter based on transmitted radiation light includes a photosensitive means on which a photosensitive member sensitive to X-rays is formed, and a surface of the photosensitive member of the photosensitive means that is uniformly charged. A charge charging means for charging, an exposure means for exposing the entire surface of the photosensitive member uniformly charged by the charge charging means, and electrostatically attaching toner to the photosensitive member that has been entirely exposed by the exposure means. a developing means for illuminating the photosensitive member of the photosensitive means;
an imaging means for taking an image of the photosensitive member illuminated by the illumination means; a signal processing means for processing an output signal from the imaging means into a predetermined image; and an image processing means for processing an image based on the signal from the signal processing means. The apparatus is characterized in that it comprises an image display means for displaying images, and a drive means for synchronously driving the photosensitive member and a conveyance means such as a belt conveyor.

[Effect]

In the automatic foreign matter inspection device for food according to the present invention, the surface insulating layer of the photosensitive part is uniformly charged by corona discharge, and then the object to be inspected is irradiated with soft X-rays to coat the surface insulating layer of the photosensitive part. The X-ray transmitted light image is recorded as a potential difference, and the surface insulating layer of the photosensitive area is exposed to light over the entire surface to release the trapped charges, after which fine particle toner is electrostatically deposited, and the X-ray transmitted image formed by this fine particle toner is recorded. The images are taken directly with a TV camera while shining reflected lighting, allowing foreign object inspection to be carried out online.

〔Example〕

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the automatic food product inspection device according to the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 shows a schematic overall configuration of an automatic foreign matter inspection device for roast ham to which the automatic foreign matter inspection device for food according to the present invention is applied. In the same figure, the automatic foreign matter inspection device for roast ham mainly includes a first sealed shield room 4 which houses a soft X-ray tube 2 and is electromagnetically sealed, and an automatic foreign matter inspection device 6 for foods according to the present invention which is housed in an electromagnetic sealed room 4. A belt conveyor 1 with fins that continuously conveys a second sealed shield room 8, first and second sealed shield rooms 4.8, and roast ham 7.
0 and prevents soft X-rays from dispersing to the outside; a driving device 23 that rotates the belt conveyor 10; and a photosensitive section 24 (described later); and a power source for each electric circuit section of the device. The device belt conveyor 10, which is composed of a power supply device that supplies power, a control box (not shown) that controls the main body of the device, and a photosensitive section 24 are indicated by a dashed line 2.
The rotary drive mechanisms of the drive group Wt23 conceptually shown at 5 and 27 synchronously rotate in a fixed direction.

Overall shielding box 12 into which the belt conveyor 10 enters and exits
The openings 13 and 14 are respectively provided with diffusion prevention tunnels 16 and 18 that prevent soft X-rays from diffusing to the outside.
Furthermore, multiple XII shielding screens (not shown) made of lead rubber are installed. The roast ham 7 is continuously conveyed by the belt conveyor 10 in the direction shown by the arrow 20, and the roast ham 7 that has been inspected and found to be contaminated with foreign matter is moved to the side of the belt conveyor 10 by the air cylinder type pusher 22. It will be automatically transported to the destination.

The external dimensions of the automatic foreign matter inspection device for roast ham are 3.7 m wide x 1.8 m high x 1 m deep, and the total weight is approximately -
It is 1t.

The food foreign substance automatic inspection device 6 mainly uses a photoconductive semiconductor selenium (Se+5eT) which rotates in the direction shown by the arrow 21.
A photosensitive section 24 consisting of a photosensitive drum or the like sputtered with an X-ray photosensitive material such as e), a charge charging section 26 that charges the surface of the photosensitive section 24 with a uniform charge prior to X-ray exposure, and a photosensitive section 24 An exposure section 28 consisting of a xenon lamp or the like that exposes the entire surface of
an illumination section 33 that applies reflected illumination to an X-ray transmission image formed of fine particle toner on the surface of the photosensitive section 24, and a TV camera (-dimensional image An imaging unit 34 consisting of a sensor (TV camera), etc., and a signal processing unit that processes the electric signal from the imaging unit 34 into a video signal and records the frame, as well as detecting the level of a luminance signal corresponding to one horizontal scan of the image. A display unit 4 consisting of a circuit 38 and a monitor TV etc. that displays an image based on the output signal from the signal processing circuit 38
It is composed of 0. Further, a drive signal (not shown) is output from the signal processing circuit 38 to the bushing 22, and the bushing 22 operates based on this drive signal.

Incidentally, the basic structure of the photosensitive section 24 of this embodiment is as shown in FIG. In the same figure, photosensitive [24
A metal film electrode 48, an insulating layer 50, a photoconductor layer (Se + Se Te) 52, and a surface transparent insulating layer (polyester) 54 are sequentially formed on the lower side of the substrate 46. An X-ray transmission image is formed by the fine particle toner.

Next, with reference to FIG. 3, an outline of the operation of the automatic foreign matter inspection apparatus for roast ham constructed as described above will be described.

Prior to X-ray exposure, a high voltage of about 2° IKV is applied for a predetermined time to the photoconductor layer (Se + Se Te layer) 52 of the photosensitive section 24 by the charging section 26, thereby uniformly generating ten charges due to corona discharge. is charged on the surface 54Δ of the surface transparent insulating layer 54 (see FIG. 3 (Δ)). Next, when the roast ham 7, which is being continuously conveyed by the belt conveyor 10 that is driven to rotate in synchronization with the photosensitive section 24, reaches just below the soft X-ray tube 2, a tube voltage of approximately 70 KV is applied to the soft X-ray tube 2 ( Tube current 4Qm
The high voltage of A) is applied for about 0.4 seconds and one charge is corona discharged. This allows the soft X-ray tube 2 to
The roast ham 7 is irradiated with the beam and is placed on the surface 54A of the photosensitive section 240.
A potential difference is generated corresponding to the X-ray transmitted light image (Fig. 3 (B)).
), this surface 54A is entirely exposed to single light from the exposure unit 28 to release the trapped charges (see Figure 3 (
(See C). The electrostatic latent image formed by releasing the trapped charges is visualized by the electrostatic adhesion of particulate toner 30 in the developing section 32 (see FIG. 3(D)). The X-ray transmission image appearing on the surface 54A of the photosensitive section 24 is captured by the imaging section 34 while receiving reflected illumination of constant illuminance from the illumination section 33, and is converted into a predetermined electrical signal. (See Figure 3 (E)).

The signal processing circuit 38 processes the electric signal into a video signal and records the frame, and also detects the level of a luminance signal corresponding to one horizontal scanning line.

The display section 40 displays, for example, an X-ray transmission image shown in FIG. 4(A) based on the output signal from the signal processing circuit 38.

In the X-ray transmission image shown in FIG. 4(A), 46 indicates a steel ball, 48 indicates a bone, 50 indicates a piece of wood, and 52 indicates a fat layer.

Furthermore, the signal processing circuit 38 is configured as shown in FIG.
- The negative polarity luminance signal (see FIG. 4(B)) of the video signal for one horizontal scanning line corresponding to the display image indicated by line A is processed into a binarized signal for each predetermined threshold value. For example, by setting the first threshold value to Vl (V) so that signals with a level higher than this are extracted, the binarized signals corresponding to the steel ball 46 and the wood piece 50 are changed to 46A and 50A. (See FIG. 4(C)). Furthermore, by setting the second threshold value to V2 (V) and extracting signals at a level lower than this value, a binarized signal 48A corresponding to the wood piece 48 can be obtained (Fig. 4 (D)). ). In addition, in FIG. 4(D), in addition to the binary signal 48A indicating the wood piece 48, the binary signal 54.5 corresponds to both ends of the roast ham 7.
4 is generated, but this signal 54.54 may be multiplied by a predetermined window function to eliminate it. As a result, Figure 4 (C
) and the two binarized signal waveforms shown in FIG. 4(D) are synthesized into the binarized signal waveform shown in FIG. 4(E).

The signal waveforms shown in FIGS. 4(B), (C), (D), and (E) are displayed on the display unit 44 together with the xi transmission image shown in FIG. 4(A). Also, in this way, roast ham 7
If a foreign object is detected inside, the busher 22 operates based on a drive signal from the signal processing circuit, and automatically carries out the roast ham 7 to the side of the belt conveyor lOO.

According to the device of this embodiment, the simulated foreign matter in roast ham is measured by using a steel ball with a diameter of about 0.2 mm, a bone chunk with a diameter of about 0.5 u square, a void with a diameter of about 1 mm square, and a piece of wood with a diameter of about 1.5 u square or more. Large objects can be recognized well. Also, the diameter of the blister is about 0.5
If it's bigger than your torso, then t! ! The fat layer can also be recognized.

Furthermore, since the imaging time for one roast ham is approximately 0.8 seconds, online inspection is possible.

As described above, in the automatic foreign matter inspection device for roast ham of this embodiment, fine particle toner electrostatically adhered to a drum-shaped photosensitive area is transferred onto a transparent film, and then this transferred fine particle toner is fixed. Without going through the processing process,
An X-ray transmission image formed by fine particle toner electrostatically deposited directly on a photosensitive drum is captured by a TV camera under constant illuminance, and the X-ray transmission image is displayed on a monitor TV. In addition to inspecting the presence of foreign matter by directly observing the displayed image, it is also possible to detect the level of the luminance signal of the video signal that represents the image for one horizontal scan, and then check the level of the luminance signal corresponding to the signal level.
It is configured so that it can be processed into a digitized signal.

In addition, in this embodiment, an X-ray transmitted image formed by fine particle toner on the surface insulating layer on the photosensitive drum is directly captured and imaged with a TV camera while applying reflected illumination, thereby obtaining an X-ray transmitted image. As I explained about the method, the X-ray transmission image on the photosensitive drum is not directly captured, but is transferred onto an inexpensive film that does not require fixing processing, and this transferred image is directly captured with a TV camera while being illuminated with transmission illumination. A method of obtaining an X-ray transmission image by imaging may be adopted.

In this case as well, since the transcription is completed instantaneously, online processing is possible in almost real time.

Next, FIG. 5 shows a schematic overall configuration of an automatic foreign matter inspection device for roast ham according to another embodiment to which the automatic foreign matter inspection device for foods according to the present invention is applied. In FIG. 5, the same elements as those shown in FIG. 1 are denoted by the same reference numerals, and descriptions of the elements will be omitted. In the same figure, 4
2A, 42B142C, 42D, 42E, 42F, 42
G and 42H are photosensitive plates sputtered with the same photoconductive semiconductor selenium or other X-ray photosensitive material as the photosensitive drum of the photosensitive section 24 shown in FIG. 1; 44 is each photosensitive plate (42A, 42B,
42C, 42D, 42E, 42F, 42G, 42H) in the direction indicated by arrow 21. The belt 44 is rotationally driven in synchronization with the belt conveyor 10 by a rotational drive mechanism of the drive device 23 schematically indicated by a dashed line 27 .

Referring to FIG. 3, an outline of the operation of the automatic foreign matter inspection apparatus for roast ham constructed as described above will be described.

Prior to X-ray exposure, for example, the photosensitive plate 42 is
Approximately 2.
A high voltage of IKV is applied for a predetermined period of time, and as a result, a uniform charge due to corona discharge is applied to the surface 5 of the surface transparent insulating layer 54.
It is charged to 4A (see FIG. 3(A)). Next, the roast ham 7, which is being continuously conveyed by the belt conveyor 10 that is driven to rotate in synchronization with the photosensitive plate 42A, reaches the position directly below the soft X-ray tube 2, and the photosensitive plate 24Δ is located at the position where the photosensitive plate 42C is currently located. The tube voltage 7 is applied to the soft X-ray tube 2 when it is transferred to the
A high voltage of 0 KV (tube current 40 mA) is applied for about 0.4 seconds, and -charge is corona discharged. As a result, soft X-rays are irradiated from the soft X-ray tube 2, and a potential difference corresponding to the X-ray transmitted light image of the roast ham 7 is generated on the surface 54A of the photosensitive plate 42A. The entire surface is exposed with a single light beam from the exposure section 28 at the location where the trapped charges are released (see FIG. 3(C)).
. The electrostatic latent image formed by the release of the trapped charges is visualized by electrostatically depositing the particulate toner 30 in the developing section 32 at the location where the photosensitive plate 42E is currently located (see FIG. 3). (See D). Photosensitive plate 4
The X-ray transmission image appearing on the surface 54A of 2A is
The photosensitive plate 4 is currently receiving reflected illumination with a constant illuminance from
After the image is captured by the imaging unit 34 at the location where the 2G is located and converted into a predetermined electrical signal, it is manually input to the signal processing circuit 38 (see FIG. 3(E)).

The signal processing circuit 38 processes the electric signal into a video signal and records the frame, and also detects the level of the luminance signal corresponding to the horizontal scan.

The display unit 40 similarly displays the X-ray transmission surface image and the binarized signal waveform (see FIG. 4) shown in the above-described embodiment.Furthermore, in this way, foreign matter in the roast ham 7 is detected. When this happens, the busher 22 operates based on the drive signal from the signal processing circuit, and automatically carries out the roast ham 7 to the side of the belt conveyor 10.

The apparatus of this embodiment can recognize foreign objects (steel balls, bone lumps, voids, wood chips, blisters, fat layers) of the same size as those of the previously described embodiments.

As described above, in the automatic foreign matter inspection device for roast ham of this embodiment, fine particle toner electrostatically adhered to a flat photosensitive area is transferred onto a transparent film, and then this transferred fine particle toner is subjected to a fixing process. The X-ray transmission image formed by the particulate toner electrostatically adhered directly to the photosensitive plate is captured by a TV camera under constant illuminance, and the X-ray transmission image is displayed on a monitor TV. In addition to inspecting the presence of foreign matter by directly observing the displayed image, it is also possible to detect the level of the luminance signal of the video signal representing the image for one horizontal scan and convert it into a binary signal in accordance with the signal level. It is configured so that it can be processed.

In addition, in this embodiment, an X-ray transmitted image formed by fine particle toner on a surface insulating layer on a photosensitive plate is directly captured and imaged with a TV camera while applying reflected illumination. We have explained the method of obtaining the X-ray latent image on the photosensitive plate, but instead of directly imaging it, we transfer it onto an inexpensive film that does not require fixing, and then use a TV camera to transmit this transferred image while applying transmitted illumination. A method of obtaining an X-ray transmission image by directly capturing and imaging may be adopted.

In this case as well, since the transcription is completed instantaneously, online processing is possible in almost real time.

〔Effect of the invention〕

As described above, the automatic foreign substance inspection device for food according to the present invention includes a photosensitive means formed with a photosensitive member that is sensitive to X-rays, and an electric charge that uniformly charges the surface of the photosensitive member of the photosensitive means. an exposure means for exposing the entire surface of the photosensitive member uniformly charged by the charge charging means; a developing means for electrostatically attaching toner to the photosensitive member exposed on the entire surface by the exposure means; An illumination means for illuminating the photosensitive member, an imaging means for taking an image of the photosensitive member illuminated by the illumination means, a signal processing means for processing into a predetermined image based on an output signal from the imaging means, and a signal from the signal processing means. The present invention is comprised of an image display means for displaying an image based on the image, a drive means for driving the photosensitive member, and a belt conveyor to rotate in synchronization, so that foreign substances in food can be inspected with higher accuracy than conventional inspection methods. Detection of foreign objects can be performed online and at high speed.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the general structure of an automatic foreign matter inspection device for roast ham to which the automatic food foreign matter inspection device according to the present invention is applied, and FIG. 2 is a sectional view showing the basic structure of a photosensitive drum and a photosensitive plate. , FIG. 3 is an explanatory diagram showing the operation of the photosensitive drum and photosensitive plate shown in FIG. 2, FIG. 4 is an explanatory diagram showing the display screen of an X-ray transmitted image, and FIG. FIG. 1 is a block diagram showing a schematic overall configuration of an automatic foreign matter inspection device for roast ham according to an embodiment of the present invention. 2...XWiF, ? ...Roast ham, 10.
...Belt conveyor, 22...Butsusha, 23
...Drive device, 24...Photosensitive drum, 26...
- Charge charging section, 28... Exposure section, 32... Development section, 33... Illumination section, 34... Imaging section, 38.
...Signal processing circuit, 40...Display section.

Claims (5)

[Claims] (1) In an automatic food foreign matter inspection device that irradiates X-rays into food such as meat transported by a conveyance means such as a belt conveyor and inspects foreign matter in the food based on the transmitted X-ray light from the irradiation, the above-mentioned a photosensitive means formed with a photosensitive member sensitive to X-rays; a charge charging means for uniformly charging the surface of the photosensitive member of the photosensitive means; and a charge charging means uniformly charged by the charge charging means. an exposing means for exposing the entire surface of a photosensitive member; a developing means for electrostatically attaching toner to the photosensitive member exposed entirely by the exposing means; an illuminating means for illuminating the photosensitive member of the photosensitive means; and the illuminating means an image capturing means for capturing an image of the photosensitive member illuminated by the image capturing means; a signal processing means for processing an output signal from the image capturing means into a predetermined image; and an image displaying an image based on the signal from the signal processing means. 1. An automatic food foreign matter inspection device comprising: a display means; and a drive means for synchronously driving the photosensitive member and a conveyance means such as a belt conveyor. (2) The automatic foreign matter inspection device for foods according to claim (1), wherein the photosensitive member of the photosensitive means is made of a photoconductive semiconductor selenium (Se+SeTe) or the like. (3) The photosensitive member of the photosensitive means is formed into a drum shape or a flat plate shape.
) Automatic food foreign substance detection device described in item ). (4) The food foreign substance inspection apparatus according to claim (1), wherein the signal processing means generates a video signal based on an output signal corresponding to the transmitted X-ray light from the imaging means. . (5) The automatic food foreign substance inspection apparatus according to claim (4), wherein the signal processing means processes the video signal into a binary signal according to the level of the luminance signal.