JPH0145791B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an overcurrent detection and protection method for a photomultiplier tube, and more specifically, the present invention relates to an overcurrent detection and protection method for a photomultiplier tube. This invention relates to an overcurrent detection and protection method for a photomultiplier tube used in an image scanning reading method in which image information is read by detecting light containing image information obtained by a photomultiplier tube. It is.

Conventionally, a beam light such as a laser beam is scanned two-dimensionally on a sheet on which image information is recorded, and then the sheet is irradiated with the beam light, resulting in light containing image information ( For example, reflected light,
An image scanning reading method is widely used in which image information recorded on a sheet is read by detecting transmitted light using a photomultiplier tube.

Examples of devices using such an image scanning reading method using a photomultiplier include image reading devices such as computer input devices and facsimiles. In this device, image reading is specifically performed as follows.

A beam light is scanned and irradiated two-dimensionally onto a sheet having image information consisting of a density pattern, that is, an original, and the resulting reflected light (in the case of a paper original) or transmitted light (in the case of a film original) is obtained. The image is read by receiving light with a photomultiplier tube and outputting the image information recorded on the document as a serial electrical signal.The obtained electrical signal is transmitted to a distant place by a transmission system and then processed for image processing. ,
Various processes such as storage on a magnetic recording medium are performed.

In addition, a radiation imaging system using a stimulable phosphor sheet proposed by the applicant (Japanese Patent Laid-Open No.
No. 55-12429, No. 56-11395, etc.) also use the above-mentioned image scanning reading method using a photomultiplier tube. In this method, a stimulable phosphor sheet is irradiated with excitation light, and the stimulated luminescent light generated by the excitation is read photoelectrically.

Here, the storage fluorophore refers to radiation (X-rays, α
When the phosphor is irradiated with radiation (beta rays, gamma rays, ultraviolet rays, etc.), a portion of this radiation energy is accumulated in the phosphor, and when this phosphor is irradiated with excitation light such as visible light, the energy is accumulated. A phosphor that exhibits stimulated luminescence depending on energy.

In a radiation imaging system using such a stimulable phosphor, image reading is specifically performed as follows.

By irradiating a stimulable phosphor sheet with radiation such as X-rays through an object such as a human body, excitation light such as a laser beam is applied two-dimensionally onto the stimulable phosphor sheet on which a radiation transmitted image has been accumulated and recorded. Image reading is performed by scanning the image to generate stimulated luminescence light, photoelectrically reading out the resulting stimulated luminescence light using a photomultiplier tube to obtain an image signal, and based on the obtained image signal. A visible image is output to recording materials such as photographic light-sensitive materials, CRTs, etc.

A radiographic imaging system using this stimulable phosphor sheet has the extremely practical advantage of being able to record images over an extremely wide radiation exposure range compared to radiographic systems using conventional silver halide photography. are doing. That is, in a storage fluorophore,
It is recognized that the amount of emitted light that is stimulated to emit light due to excitation after accumulation is proportional to the amount of radiation exposure over an extremely wide range, and therefore the amount of radiation exposure can vary considerably depending on various imaging conditions. Even if the light intensity fluctuates, the light intensity of the emitted light is read by the photoelectric conversion means by setting the reading gain to an appropriate value and converted into an electrical signal, and this electrical signal is used to convert recording materials such as photosensitive materials, CRTs, etc. By outputting the image as a visible image on a display device, a radiation image that is not affected by fluctuations in radiation exposure amount can be obtained.

In addition, after converting the radiation image information accumulated and recorded in the stimulable phosphor into an electrical signal, appropriate signal processing is performed, and this electrical signal can be used for recording materials such as photosensitive materials and display devices such as CRT. Observation interpretation aptitude (diagnosis aptitude) by outputting as a visual image
It is also possible to obtain an extremely large effect of being able to obtain excellent radiographic images.

By the way, the apparatus according to the above-described image scanning reading method using a photomultiplier tube as a photodetector has the following serious problem.

Photomultiplier tubes are suitable for detecting extremely weak light, but if a photomultiplier tube is exposed to strong light (so bright that it appears bright to the naked eye) while voltage is applied to it, an excessive photocurrent will flow. However, if this condition continues, the photomultiplier tube will be damaged. Therefore, in the image scanning reading device based on the image scanning reading method using the photomultiplier tube described above, the photomultiplier tube is not damaged due to light entering the device from the outside. The device has a structure that completely blocks out light from the outside.

However, due to poor assembly, the light shielding of the device may be incomplete, or the device may not be properly adjusted.
During maintenance, etc., after a part of the equipment is opened, the opened part of the equipment is closed again due to carelessness, etc., and the equipment is not completely shielded from external light. There was a problem in that when a voltage was applied to the photomultiplier tube to start it, the photomultiplier tube was easily damaged. Such problems actually occur quite frequently, and since photomultiplier tubes are quite expensive, it would be economically disadvantageous to damage the photomultiplier tube for the reasons mentioned above.

Therefore, we are considering a method to protect the photomultiplier tube by stopping the voltage applied to the photomultiplier tube when the output current from the photomultiplier tube exceeds a certain value. However, in this case, even if no external light is incident inside the device, the intensity of reflected light, transmitted light, or stimulated luminescence light obtained by irradiating the beam light onto the sheet on which image information is recorded is calculated. Even if the voltage is large according to the actual image information, the voltage applied to the photomultiplier tube is stopped, which is not preferred in practice.

Therefore, an object of the present invention is to two-dimensionally scan a sheet on which image information is recorded with a beam of light such as a laser beam, and to transmit the light containing the image information obtained thereby to a photomultiplier tube. To provide a method for protecting a photomultiplier tube used in an image scanning reading method for reading image information by detecting the photomultiplier tube only from light incident on the inside of the device from the outside. There is a particular thing.

The overcurrent detection and protection method for a photomultiplier tube according to the present invention uses a light beam to perform main scanning and sub-scanning to two-dimensionally scan a sheet on which image information is recorded. voltage applied to the photomultiplier tube when the main scanning position of the beam light is outside the sheet and the output current value of the photomultiplier tube is greater than a predetermined reference level. It is characterized by lowering the value of.

The reference level is a value that does not exceed the rated current value of the photomultiplier tube, and the photomultiplier tube detects light containing image information obtained by scanning a sheet on which normal image information is recorded. It is sufficient if the value is equal to or less than the maximum output current value.

On the other hand, in the embodiment of the present invention, if the inside of the device is completely shielded from light, when the beam light reaches the outside of the sheet, the output from the photomultiplier tube becomes approximately 0; If light is incident inside the device, the output from the photomultiplier tube should exhibit a certain finite value, so the lower limit of the reference level is the noise level that mainly depends on the dark current of the photomultiplier tube. It's okay.

Therefore, in practice, it is preferable to set the reference level to a low value, but since noise levels that depend on dark current may cause malfunctions, it is preferable to use a low value, for example, one to two orders of magnitude lower than the rated current value of the photomultiplier tube. It is preferable to use the level as a reference level.

Note that in the present invention, reducing the value of the voltage applied to the photomultiplier tube includes completely reducing the value of the voltage to 0.

According to the present invention, no matter what kind of image information is recorded on the sheet, the output current value from the photomultiplier tube when the main scanning position of the beam light is outside the sheet is higher than the reference level. If so, it can be determined that external light is entering the inside of the device, and therefore the photomultiplier tube can be protected only against light that enters the inside of the device from the outside.

Hereinafter, the present invention will be explained in detail with reference to the drawings.

FIG. 1 is a schematic perspective view showing a preferred embodiment of the present invention, which is based on a radiographic imaging system using a stimulable phosphor sheet.

In this embodiment, reading is performed as follows. A laser beam 2 from a laser light source 1 is scanned in a main scanning direction A onto a stimulable phosphor sheet 4 by a galvanometer mirror 3. This main scanning is performed with an amplitude larger than the width of the stimulable phosphor sheet 4. The stimulable phosphor sheet 4 is moved in the sub-scanning direction B and is two-dimensionally scanned by the laser beam 2. The light emitted from the scanned stimulable phosphor sheet 4 by the scanning of the laser beam 2, the stimulated luminescent light, enters the light transmitting member 5 from the incident end 5a of the light transmitting member 5 made of a transparent sheet, and is transmitted into the light transmitting member 5. is transmitted and focused on the light receiving surface of the photomultiplier tube 6. The emission end of the light transmission member 5 is annular to match the shape of the circular light-receiving surface of the photomultiplier tube 6, and light is transmitted from one linear end to one end of the annular shape by total reflection.

A cut filter is inserted between the photomultiplier tube 6 and the annular exit end of the light transmission member 5, which does not allow the wavelength range of the laser light 2 to pass through, but allows only the wavelength range of the stimulated luminescence light to pass through. The wavelength range of the laser beam 2 is set to 600 to 700 nm so as not to overlap with the wavelength range of 300 to 500 nm of stimulated luminescence light, and the cut filter is a long wavelength cut filter that cuts wavelengths longer than 500 nm. Therefore, stimulated luminescence from the stimulable phosphor sheet can be detected with a high S/N ratio.

A high voltage obtained by boosting the voltage output from a power source 7 by a booster 8 is applied to the photomultiplier tube 6 .

The output current from the photomultiplier tube 6 is amplified by the amplifier 9 and outputted to the outside, and a part of the current from the amplifier 9 is inputted to one terminal of the comparator 10. Here, in comparator 10, comparator 1
Reference level input from the other terminal of 0
SL, and when the output current S from the photomultiplier tube 6 is greater than the reference level SL, the comparator 1
A signal S0 is generated from 0, and this signal is input to one terminal of the AND circuit 11. AND circuit 11
The other terminal of the galvanometer mirror drive speed obtained by differentiating the response signal S2 that the galvanometer mirror 3 actually responded to by the signal S1 for driving the galvanometer mirror 3, as shown in FIG. Set signal S3 to an appropriate threshold TL
A galvanometer mirror gate signal S5 for emitting a binary signal S4 and a drive signal S1 for
The timing pulse S6 obtained by the following logical product 4×5 is input. Here, this timing pulse S6 indicates that the main scanning position of the laser beam 2 is located outside the sheet 4. The output from the AND circuit 11 operates a switch 12 installed between the power supply 7 and the booster 8, and turns off the switch 12. That is, when the main scanning position of the beam light is located outside the sheet 4, it is determined whether the output S from the photomultiplier tube 6 is greater than the reference level SL, and if it is large, the photomultiplier is The voltage value applied to the tube 6 is set to zero.

The method of detecting that the main scanning position of the laser beam 2 is located outside the sheet, the circuit therefor, etc. are not limited to the above description.

Also, when scanning sheets with different widths, the time to scan the width of the sheet is different, but since light leakage etc. is generally longer than the time required to draw one scanning line (including return time), the scanning light It is sufficient to detect it when it exists somewhere other than the sheet.

In this embodiment, when the laser beam 2 scans the outside of the stimulable phosphor sheet 4, the outside of the stimulable phosphor sheet 4 is scanned so that the laser beam 2 is not detected by the photomultiplier tube 6 due to reflection. Since reflection of the laser beam is prevented in the scanning part by a black light absorber, etc., the reference level SL is set to It is only necessary to select a level that is larger than the output current value output from the photomultiplier tube, that is, the noise level depending on the dark current, and that does not cause malfunction. When reading image information using reflected light from a sheet on which image information is recorded in a density pattern on a paper document, image reading can be performed in a manner similar to this embodiment, but in this case,
There is no particular limitation on the wavelength range of the laser light, and a cut filter for cutting out stimulated luminescence light is not required.

Another embodiment of the invention is shown in FIG.

When reading out image information from a sheet on which image information is recorded in a density pattern on a film original, it is necessary to read out the image information using transmitted light. In this case, as shown in FIG.
The incident end surface 5a' of the laser beam 2 may be placed on the back surface of the sheet 13 along the scanning line scanned by the laser beam 2.

In addition, in the above embodiment, in order to protect the photomultiplier tube, the voltage applied to the photomultiplier tube is
Although the ON-OFF switch is used to completely set the voltage to 0, the same effect as in the above embodiment can also be obtained by lowering the voltage to an appropriately low value.

Furthermore, when the voltage applied to the photomultiplier tube is lowered, it is preferable to activate a means for notifying a person using a lamp, a buzzer, etc., a means for stopping image scanning, and the like.

As described above in detail, according to the overcurrent detection and protection method for a photomultiplier tube according to the present invention, the beam light is irradiated onto a sheet on which image information is recorded for image reading. When light other than reflected light, transmitted light, or stimulated luminescence light is present in the image scanning reader, the voltage applied to the photomultiplier tube is automatically reduced, making sure that the photomultiplier tube is protected.

Further, according to the present invention, even when light of an intensity that does not damage the photomultiplier tube is incident from outside the device, it can be used as a means for detecting such light leakage.

[Brief explanation of drawings]

FIG. 1 is a schematic perspective view showing a preferred embodiment of the present invention, FIG. 2 is a timing chart explaining the embodiment of the invention, and FIG. 3 is a schematic side view showing another embodiment of the invention. DESCRIPTION OF SYMBOLS 1... Laser light source, 2... Laser light, 3... Galvanometer mirror, 4... Stimulative phosphor sheet, 5, 5'... Light transmission member, 6, 6'... Photomultiplier tube, 7 ...Power supply, 8...Booster, 9...Amplifier, 10...AND circuit, 11...Timer, 12
...Switch, 13...Seat.

Claims (1)

[Scope of Claims] 1. A sheet on which image information is recorded is scanned with a beam of light with an amplitude larger than the width of the sheet, and the light containing the image information obtained from the sheet by this scanning is converted into photoelectrons. When performing image scanning reading by detecting with a multiplier tube, when the scanning position of the beam light is outside the sheet and the output current value of the photomultiplier tube is larger than a predetermined reference level, A method for overcurrent detection and protection of a photomultiplier tube, comprising reducing the value of a voltage applied to the photomultiplier tube. 2. A patent claim characterized in that the sheet is a stimulable phosphor sheet having on its surface a stimulable phosphor that stores and records the image information, and the light containing the image information is stimulated luminescence light. The overcurrent detection and protection method for a photomultiplier tube according to item 1. 3. The photomultiplier tube according to claim 1, wherein the sheet is a sheet having image information consisting of a density pattern, and the light containing the image information is transmitted light or reflected light. Overcurrent detection protection method.