JP2003215255A - X-ray detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an X-ray detector for improving resolution characteristics and brightness characteristics. <P>SOLUTION: The X-ray detector comprises a scintillator layer 38 for converting incident X rays to light, a photodiode 13 for converting light that enters through an upper electrode 13b from the scintillator layer to signal charges, a capacitor 15 for accumulating signal charges, and a thin-film transistor 14 for controlling the reading of signal charges that are accumulated in the capacitor 15 for charging. The X-ray detector has a reflection prevention member 39 at the gap between the scintillator layer 38 and the upper electrode 13b. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an X-ray detector for detecting an X-ray image or the like.

[0002]

2. Description of the Related Art In a medical diagnostic apparatus or the like, for example, X-rays are used for photographing an object, and an X-ray detector is used for detecting an X-ray image of the object. An image tube is used as the X-ray detector, for example. In recent years, as a new-generation X-ray detector, a planar X-ray detector in which a plurality of pixel units for X-ray detection are two-dimensionally arranged on a plane has attracted attention.

The flat X-ray detector is constructed so as to output an X-ray image taken by X-ray or a real-time X-ray image as a digital signal. A solid-state element is used for the flat panel X-ray detector, and it is expected to improve image quality performance and stability.

As the flat panel X-ray detector, those for general radiography for capturing a still image with a relatively large dose and those for chest radiography have already been developed and commercialized. In the near future, even higher performance will enable real-time detection of X-ray movies with 30 or more screens per second under fluoroscopy dose, and commercialization of products applied to fields such as cardiovascular system and digestive system. Is expected. In order to detect such X-ray moving images, improvement of S / N and development of real-time processing technology for minute signals are required.

The flat type X-ray detector can be roughly classified into a direct type and an indirect type. The direct method is a-S
In this method, a photoconductive film such as e is used to directly convert X-rays into signal charges, and the converted signal charges are stored in a charge storage capacitor. The indirect method is a method in which X-rays are converted into visible light by a scintillator layer, the converted visible light is further converted into signal charges by an a-Si photodiode, CCD or the like, and stored in a charge storage capacitor.

The direct system has a structure in which signal charges generated by the incidence of X-rays are guided to a charge storage capacitor by utilizing a high electric field. In the case of this configuration, the resolution characteristic is almost defined by the pixel pitch. On the other hand, in the indirect method, the visible light converted by the scintillator layer diffuses or scatters before reaching the photodiode, and the resolution deteriorates. Therefore, the indirect method requires improvement in resolution characteristics.

In the direct method, in order to improve the absorption rate of X-rays, a photoconductive film of a-Se, for example, is formed with a thickness of about 1 mm. Further, in order to increase the generation rate of signal charges per X-ray photon and to guide the generated signal charges to the storage capacitor without being trapped by the defect level in the film, the direction perpendicular to the bias electric field is further increased. In order to suppress the diffusion of signal charges to the
A strong bias electric field of 10 V / μm is applied to both ends of the photoconductive film of e. Therefore, the film thickness of the a-Se photoconductive film is 1
In the case of about mm, a high voltage of about 10 kV is applied.

Comparing the direct method and the indirect method, the direct method is superior in resolution characteristics. However, it is necessary to protect the TFT having a low operating voltage from a high voltage, which makes it difficult to secure reliability. In addition, there is a problem that a photoconductive material having low dark current characteristics, high sensitivity characteristics, thermal stability, etc. cannot be easily obtained.

The indirect method uses a photo diode or CCD. Therefore, application of high voltage is not required, and dielectric breakdown due to high voltage does not occur. In addition, scintillator materials and photo diode materials are known techniques, and can be easily commercialized as compared with the direct method.

However, there is a problem that the resolution characteristic is inferior to that of the direct method. This is partly due to the spread of light from the scintillator layer to the photo diode. In addition, the gap formed between the scintillator layer and the photo diode will also cause deterioration of resolution.

For example, the indirect method has a characteristic that the light reflectance between the scintillator layer and the photo diode is changed depending on the distance between the two. Therefore, if there is a gap between the scintillator layer and the photo diode, the reflectance varies depending on the size of the gap, the brightness changes, and the resolution decreases.

Now, a conventional X-ray detector will be described with reference to FIG. FIG. 8 is a cross-sectional view in which one pixel unit portion is extracted, and a thin film transistor (hereinafter referred to as TFT) 82 and a storage capacitor 83 are formed on an insulating substrate 81 such as glass. The TFT 82 is an insulating substrate 81
A gate electrode G formed thereon, an insulating film 84 covering the gate electrode G, a semiconductor film 85 formed on the insulating film 84,
It is composed of a source electrode S, a drain electrode D, and the like provided on the semiconductor film 85.

The storage capacitor 83 comprises a lower electrode 86 formed on the insulating substrate 81, an insulating film 84 extending from the gate electrode G to the lower electrode 84, an upper electrode 87 provided on the insulating film 84, and the like. Has been done. Upper electrode 8
7 is electrically connected to the drain electrode D.

A flattening layer 88 made of resin is provided above the TFT 82 and the storage capacitor 83, and the flattening layer 8 is formed.
A photodiode 89 is formed on the surface 8. The photodiode 89 is provided for each pixel unit. Through holes 90 are formed in the flattening layer 88, and the lower surface electrode 89a below the photodiode 89 in the drawing is the drain electrode D.
It is also electrically connected to the storage capacitor 83.

The upper surface electrode 89b above the photodiode 89 is formed of, for example, an ITO transparent electrode film, and the scintillator layer 91 is formed on the upper surface electrode 89b. Further, a light reflecting portion 92 is formed on the scintillator layer 91. Reference numeral 93 indicates a gap generated between the upper electrode 89b and the scintillator layer 91 when they are brought into close contact with each other.

The pixel unit having the above-described structure is formed two-dimensionally in the vertical direction and the horizontal direction to form a planar X-ray detector. Then, X-rays incident from the outside are scintillator layer 9
It is converted into light at 1. This light is converted into a signal charge by the photodiode 89, and is stored in the storage capacitor 83 via the drain electrode D. The signal charge stored in the storage capacitor 83 is then transferred to the gate electrode G of the TFT 82.
When an ON signal is applied to the source electrode S, the source electrode S outputs the electrical signal.

[0017]

In the case of a conventional X-ray detector, the scintillator layer is composed of a scintillator material such as CsI: Na or CsI: Tl and an X-ray emission fluorescent material containing Gd2O2S: Tb powder as a main component. It is made of a body resin or the like and is formed by, for example, a vacuum vapor deposition method.

The TFT which constitutes the switch portion of the X-ray detector has a characteristic that it is vulnerable to high temperatures (140 ° C. or higher). Therefore, when forming the scintillator layer, the TFT
Instead of vapor deposition on a photoelectric conversion substrate having a photodiode or the like formed thereon, for example, a method of vapor depositing a scintillator layer on another substrate to be a light reflecting member and then bringing the two substrates into close contact is used. At this time, since the substrate has irregularities, the scintillator layer and the photodiode are not completely adhered to each other, and a gap (reference numeral 93 in FIG. 8) is generated between them,
The reflectance of the upper surface electrode of the photodiode, for example, the ITO transparent electrode changes.

Here, the reflection characteristics of the X-ray detector described above will be described with reference to FIG. The horizontal axis represents the gap (eg, air layer thickness (μm)), and the vertical axis represents the reflectance (%), which is the characteristic when light (wavelength 550 nm) is incident perpendicularly to the boundary.

The reflection characteristic P depends on the scintillator material (Cs
The refractive index of I: Na) is 1.7 to 1.8, and the phosphor resin (G
The refractive index of d2 O2 S) is 1.7, the refractive index of the ITO transparent electrode is 2.0, and the scintillator layer (refractive index nS = 1.
7) and the ITO transparent electrode (refractive index nd = 2.0), an air layer (refractive index n1 = 1.0) is generated.
The calculation result of the light reflectance (wavelength 550 nm) at the TO transparent electrode is shown.

As can be seen from the reflection characteristic P, the reflectance changes depending on the thickness of the air layer. Therefore, when the scintillator layer and the ITO transparent electrode are not in close contact, the brightness changes by about 30% at the maximum for each pixel unit. As a result, the resolution characteristic is deteriorated and a good X-ray image cannot be obtained.

An object of the present invention is to solve the above-mentioned drawbacks and to provide an X-ray detector having improved resolution characteristics and brightness characteristics.

[0023]

According to the present invention, there is provided an X-ray light conversion section for converting incident X-rays into light, and a photoelectric conversion section for converting the light input from the X-ray light conversion section through electrodes into signal charges. An X-ray detector comprising: a storage unit that stores the signal charge; and a switching unit that controls reading of the signal charge stored in the storage unit. At least one layer made of an antireflection member made of a material different from that of the X-ray light conversion unit and the electrode is provided between them.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described with reference to the schematic circuit configuration diagram of FIG. Reference numeral 11 denotes a photoelectric conversion unit, and the photoelectric conversion unit 11 two-dimensionally arranges a plurality of pixel units 12 on an insulating substrate such as glass in a row direction (for example, a horizontal direction in the drawing) and a column direction (for example, a vertical direction in the drawing). It is arranged and configured. The plurality of pixel units 12 are formed in the same structure, and nine pixel units 12a to 12i are shown in FIG.

For example, in the case of the pixel unit 12i, a thin film transistor (hereinafter referred to as a TFT) which constitutes a photodiode 13 for converting light into a signal charge and a switching portion.
14, a charge storage unit for storing signal charges, for example, a storage capacitor 15 and the like. The TFT 14 has a gate electrode G, a source electrode S, and a drain electrode D, and the drain electrode D is electrically connected to the photodiode 13 and the storage capacitor 15.

A control circuit 16 for controlling the operating state of the TFT 14, for example, ON / OFF, is provided outside the photoelectric conversion section 11. The control circuit 16 includes a plurality of control lines 17
Is provided. In the figure, for example, the first to fourth four
Individual control lines 171-174 are shown. Each control line 17 has a pixel unit 12 located in the same row.
Is connected to the gate electrode G of the TFT 14. For example, the first control line 171 is connected to the gate electrodes G of the pixel units 12a to 12c.

A plurality of data lines 18 are provided in the column direction. In the figure, for example, four first to fourth data lines 181 to 184 are shown. Each data line 18 has a pixel unit 12 located in the same column.
Is connected to the source electrode S of the TFT 14. For example, the first data line 181 has pixel units 12a, 12
It is connected to the source electrodes S of d and 12 g. Each data line 17 is connected to a corresponding charge amplifier 19.

The charge amplifier 19 is composed of, for example, an operational amplifier, one input terminal a1 of which is connected to the data line 18, and the other input terminal a2 of which is grounded. A capacitor C is connected between the one input terminal a1 and the output terminal b to have an integration function. A switch SW is connected in parallel with the capacitor C, and the switch SW is closed to discharge the electric charge remaining in the capacitor C, for example.

Each charge amplifier 19 is connected to a parallel / serial converter 20 which converts a plurality of electric signals input in parallel into serial signals and outputs the serial signals. The parallel / serial converter 20 is connected to an analog-digital converter 21 that converts an analog signal into a digital signal.

Next, the light conversion section will be described with reference to FIG. 2 is a cross-sectional view in which one pixel unit portion is extracted, and portions corresponding to those in FIG. TF on the insulating substrate 31 such as glass
A T14 and a storage capacitor 15 are formed.
The TFT 14 includes a gate electrode G formed on the insulating substrate 31, an insulating film 32 covering the gate electrode G, a semiconductor film 33 formed on the insulating film 32, a source electrode S provided on the semiconductor film 33, and a drain electrode. D and so on.

The storage capacitor 15 is composed of a lower electrode 34 formed on the insulating substrate 31, an insulating film 32 extending from the gate electrode G to the lower electrode 34, an upper electrode 35 provided on the insulating film 32, and the like. Has been done. Upper electrode 3
5 is electrically connected to the drain electrode D.

A resin flattening layer 36 is provided above the TFT 14 and the storage capacitor 15, and the photodiode 13 is formed on the flattening layer 36. The photodiode 13 is provided for each pixel by an a-Si pn diode, a PIN diode, or the like. Flattening layer 3
Through hole 37 is formed in 6 and photodiode 1
The lower surface electrode 13a on the lower side of FIG. 3 is electrically connected to the drain electrode D and the storage capacitor 15.

The photodiode 13 is formed in a region that does not overlap the storage capacitor 15 and the TFT 14, for example. However, when the light receiving area of the photodiode 13 is increased, an insulating layer may be provided on the TFT 14 and the storage capacitor 15, and the photodiode 13 may be provided in the entire area of the pixel unit.

The upper surface electrode 13b above the photodiode 13 is formed by depositing an ITO transparent electrode, for example, and the scintillator layer 38 is formed on the upper surface electrode 13b. At this time, the upper surface electrode 13b and the scintillator layer 3
An antireflection member 39 is filled in the gap between the eight. Further, the light reflecting portion 4 is formed on the surface of the scintillator layer 38 on the upper side in the drawing.
0 is formed.

The antireflection member 39 is formed of a dielectric material such as a transparent adhesive or paraxylene. The scintillator layer 38 is made of, for example, CsI: Na or CsI: Tl.
Etc. and a material such as an X-ray emitting phosphor resin whose main component is a powder of Gd2 O2 S: Tb. The light reflector 40 is composed of, for example, an Al substrate and a reflective film provided on the surface of the Al substrate on the side of the scintillator layer 38, and reflects the fluorescence generated in the scintillator layer 38 and directs it toward the photodiode 13. Act on.

The above-mentioned pixel unit is two-dimensionally formed in the vertical and horizontal directions on the insulating substrate 31 to form a planar X-ray detector.

In the above structure, when a bias voltage is applied between the upper surface electrode 13b and the lower surface electrode 13a of the photodiode 13 and an X-ray is incident from the outside, the scintillator layer 38 converts it into fluorescence. This fluorescence is converted into a signal charge by the photodiode 13 and stored in the storage capacitor 15 via the drain electrode D. The signal charge stored in the storage capacitor 15 is output from the source electrode S as an electric signal when an ON signal is applied to the gate electrode G of the TFT 14.

Here, the reflection characteristics when the antireflection member 39 is provided between the upper surface electrode 13b and the scintillator layer 38 will be described with reference to FIG. In FIG. 3, the horizontal axis represents the thickness (μm) of the light transmitting film, and the vertical axis represents the reflectance (%).

The reflection characteristic Q is that the wavelength of the light converted by the scintillator layer 38 is 550 nm, and the scintillator layer (Cs
I: Na, refractive index ns = 1.7, and upper surface electrode of the photodiode (hereinafter referred to as ITO transparent electrode. Refractive index nd
= 2.0), a calculation result in the case where an antireflection member (refractive index n1 = 1.7) having substantially the same refractive index as the scintillator layer is satisfied is shown. As can be seen from the reflection characteristic Q,
The reflectance of the ITO transparent electrode is almost constant regardless of the thickness of the antireflection member.

Therefore, for example, when a scintillator layer formed on a substrate and a photodiode formed on another substrate are brought into close contact with each other, when a gap occurs between the scintillator layer and the photodiode, the reflection occurs in the gap. If the prevention member is filled, the reflectance becomes uniform.
As a result, the change in brightness for each pixel unit is reduced, and the resolution characteristics are improved.

The characteristic shown in FIG. 3 is the refractive index n1 of the antireflection member.
And the scintillator layer has the same refractive index ns.
However, when the refractive index n1 of the antireflection member and the refractive index nd of the ITO transparent electrode have the same value, the same effect can be obtained.

The refractive index n1 of the antireflection member is, for example, larger than the refractive index ns of the scintillator layer adjacent to one side and the refractive index n of the ITO electrode adjacent to the other side.
An intermediate value smaller than d, for example, the square root of the product of the refractive index ns and the refractive index nd n1 = 1.84 (n1 ² = ns × n
Also in the case of setting d), the change in reflectance due to the thickness of the antireflection member is small.

Next, another embodiment of the present invention will be described with reference to FIG.
Will be described with reference to. In FIG. 4, parts corresponding to those in FIG.

In this embodiment, an intermediate value between the scintillator layer 38 and the refractive index of air (refractive index n1 = 1) is provided on the surface of the scintillator layer (refractive index ns = 1.7) 38 on the ITO transparent electrode side. Antireflection member (refractive index n2 = 1.31:
This is a case where n2 ² = ns × n1) 39 is formed with a thickness of 0.1 μm and the gap between the antireflection member 39 and the ITO transparent electrode is an air layer 41.

The reflection characteristic in this case is indicated by reference symbol R in FIG. The vertical axis of FIG. 5 is the reflectance (%), the horizontal axis is the gap, for example, the thickness (μm) of the air layer 41. As can be seen from the reflection characteristic R, the reflectance is around 10%, but the scintillator layer 3
Even if a gap (air layer) is generated between 8 and the ITO electrode,
The change in reflectance at the ITO transparent electrode due to the thickness of the air layer is small.

Next, another embodiment of the present invention will be described with reference to FIG.
Will be described with reference to. In FIG. 6, parts corresponding to those in FIG. 3 are designated by the same reference numerals, and overlapping description will be partially omitted.

In this embodiment, on the surface of the scintillator layer (refractive index ns = 1.7) 38 on the ITO electrode side, a refractive index intermediate between the scintillator layer 38 and the ITO transparent electrode (nd = 2.0) is used. Antireflection layer of a material having (refractive index n1
= 1.84: n1 ² = ns × nd) 39a 0.1 μm
And the gap between the first antireflection layer 39a and the ITO transparent electrode is filled with the second antireflection layer 39b made of a material having substantially the same refractive index as the ITO transparent electrode.

The reflection characteristics in this case are shown in FIG. The vertical axis of FIG. 7 is the reflectance (%), and the horizontal axis is the thickness of the second antireflection layer (μ
In m), as can be seen from the reflection characteristic S, the reflectance at the ITO transparent electrode is constant regardless of the thickness of the antireflection layer on the ITO transparent electrode, and the reflectance becomes lower than that of the structure of FIG. ing.

In FIG. 6, the gap between the first antireflection layer and the ITO transparent electrode is filled with the second antireflection layer having the same refractive index as the ITO transparent electrode. However, the scintillator layer and the first
The refractive index of the antireflection layer is set to the same value, and the second antireflection layer is made of a material having a refractive index of an intermediate value between the scintillator layer and the ITO transparent electrode, and is 0.1 μm on the ITO transparent electrode.
The same effect can be obtained even if it is formed to a thickness of m.

For the antireflection member used in the above embodiment, for example, a solid, liquid or gas dielectric material different from the scintillator layer or the ITO transparent electrode is used.

[0051]

According to the present invention, an X-ray detector having good reflection characteristics and resolution characteristics can be realized.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram for explaining an embodiment of the present invention.

FIG. 2 is a structural diagram for explaining a pixel unit used in an embodiment of the present invention.

FIG. 3 is a characteristic diagram for explaining a reflection characteristic of the embodiment of the present invention.

FIG. 4 is a structural diagram illustrating a pixel unit used in another embodiment of the present invention.

FIG. 5 is a characteristic diagram for explaining reflection characteristics of another embodiment of the present invention.

FIG. 6 is a structural diagram illustrating a pixel unit used in another embodiment of the present invention.

FIG. 7 is a characteristic diagram for explaining a reflection characteristic of another embodiment of the present invention.

FIG. 8 is a structural diagram for explaining a pixel unit used in a conventional example.

FIG. 9 is a characteristic diagram for explaining a reflection characteristic of a conventional example.

[Explanation of symbols]

11 ... Photoelectric conversion unit 12 ... Pixel unit 13 ... Photodiode 14 ... Thin film transistor (TFT) 15 ... Storage capacitor 16 ... Control circuit 17 ... Control line 18 ... Data line 19 ... Charge amplifier 20 ... Parallel / serial converter 21 ... Analog-to-digital converter 31 ... Insulating substrate 32 ... Insulating film 33 ... Semiconductor film 34 ... Lower electrode 35 ... Upper electrode 36 ... Planarization layer 37 ... Through hole 38 ... Scintillator layer 39 ... Antireflection member 40 ... Light reflection part

   ─────────────────────────────────────────────────── ─── Continued front page    F term (reference) 2G088 EE01 FF02 GG19 JJ05 JJ09                 4M118 AA10 AB01 BA05 CA03 CA19                       CA40 CB06 CB11 FB03 FB09                       FB13 FB16 FB24 FB25

Claims (8)

[Claims] 1. An X-ray light conversion unit for converting an incident X-ray into light, a photoelectric conversion unit for converting the light input from the X-ray light conversion unit through an electrode portion into a signal charge, and accumulating the signal charge. In the X-ray detector, the X-ray detector comprises: a storage unit for controlling the reading of the signal charges stored in the storage unit; and an X-ray light conversion unit between the X-ray light conversion unit and the electrode. An X-ray detector comprising at least one layer made of an antireflection member made of a material different from that of the converter and the electrode. 2. An X-ray light conversion unit for converting incident X-rays into light, a photoelectric conversion unit for converting the light input from the X-ray light conversion unit through an electrode portion into signal charges, and accumulating the signal charges. In the X-ray detector including a storage unit for controlling the readout of the signal charges stored in the storage unit, a gap between the X-ray light conversion unit and the electrode is the X-ray light. An X-ray detector characterized by being filled with an antireflection member having a refractive index intermediate between the refractive index of the converter and the refractive index of the electrode. 3. An X-ray light conversion unit for converting an incident X-ray into light, a photoelectric conversion unit for converting the light input from the X-ray light conversion unit through an electrode portion into a signal charge, and accumulating the signal charge. In the X-ray detector including a storage unit for controlling the readout of the signal charges stored in the storage unit, a gap between the X-ray light conversion unit and the electrode is the X-ray light. An X-ray detector characterized by being filled with an antireflection member having a refractive index substantially the same as that of the conversion section. 4. An X-ray light conversion unit for converting an incident X-ray into light, a photoelectric conversion unit for converting the light input from the X-ray light conversion unit through an electrode portion into a signal charge, and accumulating the signal charge. In an X-ray detector including a storage unit for controlling the readout of the signal charges stored in the storage unit, a gap between the X-ray light conversion unit and the electrode is substantially the same as the electrode. An X-ray detector characterized by being filled with an antireflection member having the same refractive index. 5. An X-ray light conversion unit for converting incident X-rays into light, a photoelectric conversion unit for converting the light input from the X-ray light conversion unit through an electrode portion into signal charges, and accumulating the signal charges. In the X-ray detector including a storage section for controlling the readout of the signal charges stored in the storage section, an antireflection layer is provided on the electrode-side surface of the X-ray light conversion section. And the refractive index of the antireflection layer has an intermediate value between the refractive index of the material filling the gap between the antireflection layer and the electrode and the refractive index of the X-ray light conversion unit. And an X-ray detector. 6. An X-ray light conversion unit for converting incident X-rays into light, a photoelectric conversion unit for converting the light input from the X-ray light conversion unit through an electrode portion into signal charges, and accumulating the signal charges. In the X-ray detector, the anti-reflection layer is provided on the surface of the electrode on the X-ray light conversion unit side, the X-ray detector including a storage unit and a switching unit that controls reading of the signal charges stored in the storage unit. And the refractive index of the antireflection layer has an intermediate value between the refractive index of the material filling the gap between the antireflection layer and the X-ray light conversion section and the refractive index of the electrode. And an X-ray detector. 7. An X-ray light conversion unit for converting incident X-rays into light, a photoelectric conversion unit for converting the light input from the X-ray light conversion unit through an electrode portion into signal charges, and accumulating the signal charges. In the X-ray detector, the first antireflection layer is provided on a surface of the X-ray light conversion unit on the electrode side, the X-ray detector including a storage unit that controls the reading of the signal charges stored in the storage unit. And a gap between the first antireflection layer and the electrode is filled with a second antireflection layer having substantially the same refractive index as the electrode, and the refractive index of the first antireflection layer is the X-ray. An X-ray detector having an intermediate value between the refractive index of the light conversion section and the refractive index of the second antireflection layer. 8. An X-ray light conversion unit for converting incident X-rays into light, a photoelectric conversion unit for converting the light input from the X-ray light conversion unit through an electrode portion into a signal charge, and accumulating the signal charge. In the X-ray detector, the first antireflection layer is provided on a surface of the electrode on the X-ray light conversion unit side, the X-ray detector including a storage unit that controls the reading of the signal charges stored in the storage unit. And a gap between the first antireflection layer and the X-ray light conversion section is filled with a second antireflection layer having substantially the same refractive index as that of the X-ray light conversion section. X has a refractive index intermediate between the refractive index of the electrode and the refractive index of the second antireflection layer.
Line detector.