JP2685198B2 - Wall thickness measuring device for bottomed cylindrical containers - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to various beverage products (hereinafter referred to as juices).
The present invention relates to a device for measuring the wall thickness of a plastic container (hereinafter, referred to as a container) having a bottomed cylindrical shape for filling with.

[Conventional technology]

Recently, juices are sold in large containers. The problem in this container is the heat of the juice at the time of filling the juice and the deformation of the container due to the decrease in the pressure inside the container after cooling.

That is, in general, this type of container is a PET (polyethylene
terephthalate) resin material, but this PET
The resin itself is poor in heat resistance. Therefore, the juices have a temperature of about 85 ° C during filling, so the internal pressure drops when the lid is cooled during filling or after filling, and the body collapses inward. There is a risk. This is more likely to occur especially in long large containers. In order to prevent this deformation, there is a means for strengthening by forming a column portion having an uneven cross section on the side wall of the container in the axial direction and devising it formally, but this is not always sufficient.

Therefore, it is considered that the heat resistance and the strength are improved by mixing the PET resin with the heat resistant resin. As a method of inspecting this heat resistance, it is possible to measure the wall thickness of the container. This is because a certain amount of wall thickness is required to ensure heat resistance.

Examples of the method for measuring the wall thickness include a direct destruction method in which the body of the container is cut and measured, and a non-contact or non-destructive method using various light rays or ultrasonic waves. Since the container is made by a continuous automatic manufacturing line here, non-contact and non-destructive inspection methods are used, but in the past, as a device that measures the thickness of the container with high precision by non-contact and non-destructive method. There was no suitable one. Therefore, only an unannounced inspection can be performed by an appropriate sampling method, and thus it is difficult to perform 100% inspection. Moreover, the number of measurement points is limited to one and at most several points, and it is difficult to obtain detailed data over the entire circumference of the container or the entire axial direction.

[Problems to be solved by the invention]

Therefore, the present inventor has provided a measuring device capable of accurately measuring the thickness of a container over the entire region of the container by a non-contact, non-destructive method (Japanese Patent Application No. 62-316081).

However, as the efficiency and speed of the continuous automatic production line for containers increase, the container thickness measuring device is also required to have higher speed and higher accuracy.

An object of the present invention is to provide a wall thickness measuring device that can achieve the above object.

[Means for solving the problem]

In order to achieve the above-mentioned object of the present invention, in a device for measuring the wall thickness of a transparent or semi-transparent bottomed cylindrical container having one end opened, (1) a plurality of light emitting parts are provided in the axial direction of the container. A light projector for emitting a plurality of measurement lights on the side wall surface of the container is integrated with the light projector (2) while keeping a constant relative position at all times, and the light emitter faces each light emitting portion of the light projector. A light receiver having a plurality of light receiving portions in the axial direction of the container for receiving the measurement light transmitted through the container and converting it into an electric signal; (3) an electric signal output from the light receiver and a wall thickness of the container A calculator for calculating the wall thickness of the container at the measurement position from the correlation; and (4) moving the container and the light receiver in the axial direction of the container while maintaining their relative positions. A measuring position changing means for changing the thickness measuring position, and (5) before By the measurement position changing means, one of the light projector and the light receiver is inserted into the container through the opening of the container, and the other one is arranged outside the container, and (6) the light is simultaneously emitted from the light projector. The plurality of measurement lights transmitted through the container are received by the light receiver, so that the wall thicknesses at a plurality of points of the container can be measured simultaneously and in a non-contact manner.

[Action]

According to the above configuration of the present invention, the light projector has a plurality of light emitting units in the axial direction of the container, and the light receiver faces the light emitting units of the light projecting unit and has a plurality of light emitting units in the axial direction of the container. And the positional relationship between the light projector and the light receiver is always constant, one of the light projector and the light receiver is inserted into the container from the opening of the container, and the other one is the container. It is placed outside of and the measurement is performed. The plurality of measurement lights emitted from the respective light emitting portions of the light projector pass through the side wall of the container and reach the corresponding light receiving portions of the light receiver. During this period, the measurement light from the projector is attenuated when it is transmitted, and the amount of attenuation, that is, the amount of transmission, has a certain correlation with the wall thickness. The light receiver converts the received light into an electric signal. Since the converted electrical signal corresponds to the amount of transmitted measurement light,
The wall thickness measurement value is output from the calculator based on the correlation between the electric signal and the wall thickness.

In this case, the wall thickness measurement in the axial direction of the container enables simultaneous multi-point measurement by arbitrarily determining the number of light emitting parts of the light projector and the number of light receiving parts of the light receiver, and the measurement position changing means. As a result, the relative positional relationship between the light projector and the light receiver and the container can be arbitrarily changed, so that high-speed and highly accurate non-contact and non-destructive measurement can be performed over the entire region of the container.

〔Example〕

An embodiment of the wall thickness measuring apparatus of the present invention will be described with reference to the drawings.

Outline Configuration FIG. 1 shows an outline configuration of one embodiment of the wall thickness measuring apparatus of the present invention. The wall thickness measuring device of the present invention comprises a plurality of light emitting parts (4a, 4a in the illustrated example) arranged in the axial direction of the container 1 arranged outside the container 1.
b, 4c and 4d) and a light projector 4 which emits measurement light 3a, 3b, 3c and 3d from each light emitting part to the side wall of the container 1, and a light projector 4 which is inserted into the inner portion of the container 1 Each light emitting unit 4a, 4b,
Container 1 so as to face 4c and 4d with a certain space.
A light receiver 5 having a plurality of light receiving portions 5a, 5b, 5c and 5d in the axial direction of the light source, a calculator 6 for calculating the wall thickness of the measurement portion of the container 1 based on the output signal of the light receiver 5, and the light projector 4 And an elevating means 8 for integrally elevating the light receiver 5, and a rotating means 9 for rotating the container 1 in the circumferential direction of the container 1 at the measurement position.
And is provided. In this structure, the elevating means 8 and the rotating means 9 constitute a measuring position changing means. That is, the elevating means 8 is the axial measuring position changing means of the container, and the rotating means is the circumferential measuring position changing means of the container.

Container 1 The container 1 to be measured is made of a synthetic resin material in which a heat resistant resin such as a polyarydrate resin is mixed in a PET resin, and is transparent or translucent. This container 1
Is formed by a co-injection molding method in which both resins are simultaneously injected. The structure of the side wall in the thickness direction is PET on both sides.
The resin has a three-layer structure in which the heat resistant resin is interposed in the intermediate layer.

Projector 4 As shown in FIG. 1, the projector 4 comprises four light emitting parts 4a, 4b, 4c arranged at predetermined positions in the axial direction of a cylindrical casing 12 having an axis in the same direction as the axial direction of the container 1. And 4d.

FIG. 2 shows the lowermost light emitting portion 4a among the four light emitting portions, and the light emitting portion 4a and other light emitting portions have the same structure.

The light emitting part 4a is housed in a cylindrical casing 12, and a light source 13 and a concave mirror located laterally (left in the illustrated example) of the light source 13 so as to face the wall surface of the container 1 through the light source 13.
14, a chopper 15 located on the side of the light source 13 near the container 1 (to the right in the illustrated example), and a cylindrical light guide portion that horizontally projects from the side wall of the casing 12 in the container 1 direction.
11 and a window portion provided on the end surface of the light guide portion 11 on the container 1 side.
16 and a lens system 19 installed in the light guide unit 11. As the light source 13, a light source that emits infrared light as measurement light, such as a nichrome wire, is used. It is preferable to use infrared rays having a wavelength of 2 to 5 μm. The reason is that if infrared light with a long wavelength is used, the optical path formed by the lens system becomes long, and the light guide section described later becomes long,
Practically hindering the device configuration, and
This is because infrared rays of 15 to 18 μm require a black body furnace and are expensive.

The concave mirror 14 collects infrared rays from the light source 13 and makes them enter the lens system 19.

The chopper 15 is for chopping the light reflected by the concave mirror 14 and advancing to the lens system 19 to have an intermittent shape (that is, a cross-seeding waveform). The reason for this chopping is that drift and offset occur due to the characteristics of the photoelectric conversion element provided in the light receiver 5 which will be described later, and it is necessary to remove these fluctuation factors and perform highly accurate measurement. For this purpose, the waveform is temporarily converted to a crossing waveform to cancel the drift and offset, and then converted to DC again. As the chopper type, an electric type that chops an electric signal to be supplied to the light source 13 and a constant light emission from the light source 13 as in this embodiment, and the light is emitted from the chopper plate 17 at a predetermined rotation speed by the motor 18. A mechanical type that rotates and intermittently shields light is considered.

The light guide portion 11 has a cylindrical shape that horizontally protrudes from the side wall of the casing 12 in the direction of the container 1 at a position corresponding to the light source 13 and the concave mirror 14, and the window portion 16 is provided on the end surface on the container 1 side. Is provided. The measuring light 3 reflected by the concave mirror 14 and crossed by the chopper 15 is guided to the lens system 19 composed of a plurality of lenses formed in the light guide portion 11 and provided in the window portion 16. The lens 20 which is the final stage lens makes a parallel light beam and projects it toward the side wall of the container 1. The measuring light 3 projected on the side wall of the container 1 is spot light.

The casing 12 containing the light emitting portion is attached to the casing 36 of the elevating means 8.

Light Receiver 5 As shown in FIGS. 1 to 4, the light receiver 5 faces the plurality of light emitting portions 4a, 4b, 4c and 4d of the light projector 4 and is provided with a support member 21 so as to be positioned at a predetermined interval. Multiple receivers 5
The support member is attached to the casing 36 of the elevating means 8 having a, 5b, 5c and 5d. Therefore,
The light projector 4 and the light receiver 5 are integrated while always maintaining a constant relative positional relationship. The light receiving portion (the light receiving portion 5a is shown in the illustrated example, and the other light receiving portions also have the same structure) will be described with reference to FIG. The light receiving section 5a includes a photoelectric conversion element 23 mounted on the support member 21 and an interference filter 22 provided on the front surface thereof. The interference filter 22 has a specific wavelength (2.6 μm) according to the spectral characteristics of the photoelectric conversion element 23.
, And blocks unnecessary wavelengths of other disturbance noise. The photoelectric conversion element 23 can be used at room temperature, and it is preferable that the photoelectric conversion element 23 has a peak spectral characteristic at a wavelength of 2 to 5 μm corresponding to the measurement light 3 in order to improve the SN ratio. As such a photoelectric conversion element, "PbS
(Lead / Sulfur) photoelectric conversion element can be used.

It should be noted that by changing the number or combination of installation positions of the light emitting portion of the projector 4 and the light receiving portion of the light receiver 5,
The measurement position in the axial direction of the container 1 or the number of measurement points can be arbitrarily set.

In this way, by providing a plurality of light emitting units and light receiving units in the axial direction of the container 1 and enabling simultaneous measurement at a plurality of measurement points, the time required for measuring the axial wall thickness of the container 1 can be reduced. It can be greatly shortened.

Electric Signal Processing System As shown in FIG. 1, the electric signal processing system of the present apparatus is a chopper circuit 26 that drives the chopper 15, and a DC waveform (peak) of the output signal of the photoelectric conversion element using the chopper output signal A as a timing signal. A signal processing circuit 27 for converting into a hold waveform) and an arithmetic circuit 28 for calculating the wall thickness t of the measurement portion of the container 1 based on the processed output signal.
And the rotational position of the rotating means 9 described later (that is, the container 1
Rotary encoder 31 to detect the circumferential position of the
And a potentiometer 32 for detecting the axial position of the container 1 of the light emitting part of the projector 4 and the light receiving part of the light receiver 5, and the circumferential direction of the container 1 based on the calculation output signal (thickness value t) and the encoder output signal. A display device 29 for displaying the thickness distribution and for displaying the axial thickness distribution of the container 1 based on the calculation output signal (thickness value t) corresponding to each light receiving portion.

The chopper circuit 26 rotates the DC motor 18 at a predetermined number of rotations, and rotates the chopper plate 17 having intermittent cutouts (not shown) in the circumferential direction, thereby outputting the measuring light 3 having a crosswise waveform. It is for. The chopper circuit 26 outputs a timing signal A corresponding to the chopping cycle of the measuring light 3 (Fig. 6 (a)). The timing signal A is input to the signal processing circuit 27.

As shown in FIG. 5, the signal processing circuit 27 uses the timing signal A obtained by delaying the timing signal A by a predetermined time constant.
_A primary delay circuit (integrator circuit) 61 that outputs ₁ and a secondary delay circuit (integrator circuit) 62 that outputs a timing signal A ₂ that is obtained by further delaying the delayed timing signal A ₁ ; Edge detecting circuit (differentiating circuit) 34 for detecting the rising edge of the timing signal A ₂ delayed by a delay time, a connection circuit 35 for converting the edge detecting signal A ₃ into a TTL level signal voltage, and resetting the converted signal. A peak hold circuit 63 that peak-holds the output signal of the photoelectric conversion element 23 as the input signal R is configured. Note that the delay circuit does not necessarily need to have a two-stage configuration, and may be one stage if one circuit can secure a required delay time constant.

The first-order delay circuit 61 and the second-order delay circuit 62 are active circuits using CR integrator circuits (C ₁ , C ₂ , R) and operational amplifiers, and output the chopper circuit output signal A shown in FIG. ₁ and A ₂ (FIGS. 6 (b) and (c)) are delayed.
The time constant τ can be adjusted by making the resistance R of the CR integrating circuit variable.

The edge detection circuit 34 is a differentiating circuit with a capacitor C ₀ interposed, and detects the rising edge of the timing signal A ₂ to generate an edge detection circuit output signal A ₃ (Fig. 6 (d)). The output waveform is actually Is a differential waveform.

The connection circuit 35 outputs the reset signal R at a TTL level (logic signal level 5V) in order to match the signal level of the peak hold circuit 63, and an open collector circuit is used as the circuit element. Reset signal R
Is applied to the reset input terminal of the peak hold circuit 63.

The peak hold circuit 63 uses an external holding capacitor C _H to hold the peak level of the output signal B from the photoelectric conversion element 23 to the next peak, and a value corresponding to the wall thickness t of the measurement portion of the container 1. Is output. This is equivalent to converting the alternating signal of the chopper cycle into a DC signal. A high-pass filter 33 is a high-pass filter for removing low-frequency drift components included in the output signal B of the photoelectric conversion element 23.

The arithmetic circuit 23 outputs the output (= the amount of transmitted measurement light of the body wall of the container 1) obtained by the photoelectric conversion element 23 and the wall thickness T of the container 1.
The wall thickness t is calculated based on the correlation with. Here, FIG. 7 shows a graph of the correlation. FIG. 7 shows a case in which the output voltage V is divided by the initial output voltage V ₀ in consideration of the initial output voltage of the photoelectric conversion element 23 (including the variation) to make it dimensionless. That is, the dimensionless output voltage V ^* is given by V ^* = 83 × exp (−1.367t) in the experimental example, and this equation is modified to obtain the wall thickness t. Is required. Here, V ^* = V / V ₀ × 100, V: actual output voltage, V ₀ : initial output voltage. Therefore, if the voltage V of the output signal B of the photoelectric conversion element 23 is obtained, the wall thickness t can be calculated immediately. The arithmetic circuit 28 can be realized by using a logarithmic amplifier set to a constant satisfying the above equation (1), or by using a computer such as a personal computer.

As the display means 29, for example, an XY recorder or the like can be used. When displaying the circumferential thickness distribution of the container 1, the circumferential measurement position signal E is input from the encoder 31 to the X-axis input terminal, and the output signal D of the arithmetic circuit 28 is input to the Y-axis input terminal.
(Thickness t) can be easily obtained by inputting, and it can be checked whether or not it conforms to the standard. When displaying the axial thickness distribution of the container 1, the axial measurement position signal F from the potentiometer 32 is input to the X-axis input terminal and the output signal D of the arithmetic circuit 28 is input to the Y-axis input terminal. You can ask.

Ascending / descending means 8 As shown in FIGS. 3 and 4, the ascending / descending means 8 includes an ascending / descending motor 37 attached to a casing 36, an ascending / descending guide 39 provided on a column 38, and an ascending / descending operation. The elevation guide rod 40 and a holding member 41 for supporting the elevation guide rod 40 are provided. Casing 36
A rack (not shown) is provided at a portion of the elevating guide 39, and a pinion (not shown) meshing with the rack is
The casing 36 is vertically moved by rotating the pinion by a lifting motor 37.

Therefore, the light projector 4 and the light receiver 5 integrated with the casing 36 also move up and down at the same time. Further, by providing the lock lever 43 at an arbitrary position of the holding member 41, the movement of the elevating guide rod 40 can be restrained and locked and held at an arbitrary position.

Further, a limit switch or the like may be provided on the elevating guide 39 or the holding member 41 to elevate the casing 36 in stages. In this case, after the casing 36 descends to a predetermined position and the measurement is completed, the limit switch is operated to operate each light emitting portion of the light projector 4 and the light receiver 5.
The casing 12 is moved up or down corresponding to a distance of about half the installation interval of each of the light-receiving parts, and measurement is performed at the position after the movement, thereby measuring the number of times the number of the light-emitting parts or the light-receiving parts. Measurement at points can be done easily.

Alternatively, only the light receiver 5 may be integrated with the casing 36 so that only the light receiver 5 can be moved up and down, and the light projector 4 can be fixed at a predetermined position.

Rotating Means 9 Various means can be used as the rotating means 9, but the first means
The inventors of the present invention proposed in FIGS. 3, 3 and 4 (Japanese Patent Application No. 62-7).
No. 743) shows the rotating means based on the rotating mechanism.
In FIG. 1, FIG. 3, and FIG. 4, the rotating means 9 holds the collar 53 at the mouth of the container 1 at three places by rollers, holds the container 1 in an upright state, and rotates it. That is, the rotating means 9 is a shaft 45 equipped with a drive roller 44 for rotating the container 1.
Roller support rod having one set of rollers and a rotatable roller 46
The rotary shaft 48 of the roller support rod 47 has two sets 47, and the two sets of roller support rods 47, 47 are fixed on a base 49 so that the shaft 45 is sandwiched. Therefore, the three rollers 44, 46, 46 are arranged in a triangular shape, and the upper part of the drive roller 44 is provided with a rotating gear.
50 is fixed and engages with a drive gear connected to an output shaft of a motor 52 on a base 51 provided on the base 49 of a support rod 53. The roller support rod 47 has a three-pronged shape centered on the rotation shaft 48.

And two roller support rods 4 centering on the roller rotation shaft 48
7,47 spreads to the left and right, and when the collar 53 at the mouth of the container 1 enters between the two roller support rods 47,47, the roller support rods 47,47 close and the three rollers 44,46,46 constricted. The portion holds the brim 53 at the mouth of the container 1. Here, when the motor 52 is activated, the roller 44 rotates, and the frictional force causes the container 1 and the two rotatable rollers 46, 46 to rotate.

The rotation cycle of the container 1 due to the rotation of the roller 44 is not synchronized with the chopper output signal A, and the container 1 rotates independently. Therefore, in order to clarify the circumferential position of the container 1, an encoder 31 is provided, which enables circumferential measurement of the container 1. Note that the rotation of the container 1 by the roller 44 can be independently rotated in consideration of a toilet bowl or the like that arbitrarily changes the rotation of the container 1.

 Next, a series of measurement operations will be described.

First, the container 1 to be measured is gripped by the rollers 44, 46, 46. Then, the elevating means 8 lowers the projector 4 and the light receiver 5 together with the casing 36, and inserts the light receiver 5 into the container 1 through the opening 2 of the container 1 (see FIGS. 3 and 4).
See figure). After stopping at a predetermined measurement position, the measurement light 3 is sent out from each light emitting unit. At this time, the measuring light 3 is chopped by the chopper 15. The measurement light 3 is emitted from the outer circumference of the container 1 via the lens system 19 in the light guide section 11. The plurality of measurement lights that have passed through the side wall of the container 1 reach the corresponding photoelectric conversion elements 23 of the light receiver 5 and output an output signal B corresponding to the amount of transmission thereof (FIG. 5, FIG. 6 (f). )). The output signal B of the optical conversion element 23 is a signal processing circuit.
Entered in 27. On the other hand, the chopper output signal A from the chopper circuit 26 is also input to the signal processing circuit 27 (FIGS. 5 and 6 (a)).

In the signal processing circuit 27, the phase of the chopper circuit output signal A is delayed by the two-stage delay circuits 61 and 62 (FIG. 6 (c)). The purpose of this delay is that it is necessary to synchronize in order to sample the peak value in the peak hold circuit 63 (FIG. 6 (e)). The lag phase τ is detected by the edge detection circuit 34, and at this timing, the reset signal R (that is, the edge detection circuit output signal A ₃ , FIG. 6C) is given to the peak hold circuit 63. Therefore, when the reset signal R is applied, the peak hold circuit 63 resets the peak value held so far and newly samples the peak value at that time (FIG. 6 (e)). This sampling value C corresponds to the wall thickness value t of the container 1 at that time. The sampling value C is provided to the arithmetic circuit 28. The arithmetic circuit 28 outputs a signal D of the corresponding thickness value t according to the equation (1).

At this time, the signal E indicating the rotational position of the container 1 by the roller 44 (that is, the circumferential position of the container 1) is input to the display device 29, and the wall thickness calculation signal D is also input to display the same-direction thickness distribution. To be done.

Further, since the simultaneous measurement is performed at a plurality of measurement points in the axial direction of the container 1, the signal F indicating the axial position is displayed by the display device.
The thickness calculation signal D that is input to 29 and corresponds to each signal F
By recording, the axial thickness distribution can be obtained.

In the above embodiment, the case where the light projector 4 is arranged outside the container 1 and the light receiver 5 is inserted inside the container 1 has been described, but the present invention arranges the light receiver 5 outside the container 1,
The projector 4 may be inserted into the container 1.

In this case, although the size of the light projector 4 may be restricted, the restriction can be easily satisfied by configuring the light projector 4 using an optical fiber unlike the embodiment.

〔The invention's effect〕

According to the present invention, a light projector having a plurality of light emitting parts is arranged outside the container, and a light receiver having a plurality of light receiving parts provided so as to face the light emitting parts is inserted into the inside of the container from the opening. A plurality of measurement lights are emitted from the light projector, the amount of the measurement light transmitted through the container is converted into an electric signal by the light receiver, and the wall thickness of the measurement portion is calculated from the correlation between the electric signal and the wall thickness. As a result, the wall thickness can be accurately measured at a large number of measurement points simultaneously in the axial direction of the container in a very short time in a non-contact and non-destructive state. According to the measurement result, a container having a constant wall thickness can be efficiently manufactured, which can contribute to the improvement of quality. Then, by the measuring position changing means, the circumferential direction of the container,
It is possible to measure the wall thickness over the entire area in the axial direction, and it is possible to obtain detailed analysis data regarding the wall thickness together with the 100% inspection of the container, and it is possible to obtain appropriate feedback information to the manufacturing process.

[Brief description of the drawings]

1 is a schematic view showing an embodiment of the present invention, FIG. 2 is a partial sectional view showing a projector and a light receiver used in the present invention, FIG. 3 is a front view showing a mechanical structure of the present invention, and FIG. Is a side view thereof, FIG. 5 is a circuit diagram showing an electrical configuration of the present invention, FIG. 6 is a time chart of output signals of respective parts, and FIG. 7 is a correlation between wall thickness of the container and output voltage of the photoelectric conversion element. FIG. DESCRIPTION OF SYMBOLS 1 ... Container, 2 ... Opening part, 4 ... Emitter, 5 ... Photoreceiver, 6 ... Computing device, 8 ... Elevating means, 9 ... Rotating means.

Claims (1)

(57) [Claims] 1. An apparatus for measuring the wall thickness of a transparent or semi-transparent bottomed cylindrical container having an opening at one end, which comprises (1) a side of the container having a plurality of light emitting portions in the axial direction of the container. A light projector that emits a plurality of measurement lights on a wall surface is integrated with (2) the light projector while always maintaining a constant relative position, and faces the respective light emitting portions of the light projector in the axial direction of the container. A light receiver having a plurality of light receiving parts for receiving the measurement light transmitted through the container and converting it into an electric signal, and (3) the measurement based on the correlation between the electric signal output from the light receiver and the wall thickness of the container. An arithmetic unit for calculating the wall thickness of the container at the position, and (4)
(5) The measurement position changing means for changing the wall thickness measurement position of the container by moving the projector and the light receiver in the axial direction of the container while maintaining their relative positions. One of the light projector and the light receiver is inserted into the container through the opening of the container by the means, and the other one is arranged outside the container, and (6) the container is simultaneously radiated from the light projector. A thickness measuring device for a cylindrical container with a bottom, characterized in that the thickness of a plurality of points of the container is measured simultaneously and in a non-contact manner by receiving a plurality of transmitted measuring lights by the light receiver. .