Photo sensor, display panel having the same and display device having the display panel

A display device that includes a first substrate having pixel electrodes; a second substrate having a color filter corresponding to the pixel electrodes to display images; a photo switching element disposed at the first substrate; a red or green color filter corresponding to the photo switching element formed at the second substrate to sense an amount of external light; a driving controller configured to output a driving control signal responsive to the amount of external light sensed by the light sensing unit; and a light generation unit configured to provide the display unit with an internal light controlled by the driving control signal. This photo sensor is well suited to human-eye luminosity and uses external light to determine how much backlight is needed.

This application claims priority to Korean Patent Application No. 2007-0045339, filed on May, 10th, 2007, and all the benefits accruing therefrom under 35 U.S.C §119, and the contents of which in its entirety are herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photo sensor, a display panel having the same and a display device having a display panel.

2. Description of the Related Art

Generally, a liquid crystal display device is classified as either a transmissive type liquid crystal display device, which displays images using an internal light source such as a backlight assembly, or a transmissive and reflective type liquid crystal display device which displays images using an internal light source or by reflecting an external incident light.

The transmissive and reflective type display device controls power supplied to a backlight assembly in response to an intensity of the external incident light. Specifically, when the external incident light has a lower intensity, the transmissive and reflective type display device operates in a transmission mode such that the backlight assembly is turned on and internal light transmitted by the backlight assembly is used to display images. When the external incident light has a higher intensity, the transmissive and reflective type display device operates in a reflective mode such that the backlight assembly is turned off and the external incident light is reflected to display images. Additionally, a gamma level is automatically adjusted corresponding to either the transmission mode or the reflective mode so that an image displaying quality is improved.

Thus, power consumption of the transmissive and reflective type display device is reduced by controlling the power supplied to the backlight assembly in response to the intensity of the external incident light. Additionally, when the gamma level is adjusted according to a respective operational mode of the liquid crystal display device, the image displaying quality is improved. Accordingly, a photo sensor disposed on a display panel of the liquid crystal display device to sense the intensity of the external incident light is required to reduce the power consumption of the liquid crystal display device.

SUMMARY OF THE INVENTION

Accordingly, the present invention is provided to substantially obviate one or more problems due to limitations and disadvantages of the related art. Exemplary embodiments of the present invention provide a photo sensor suited to human eye luminosity.

In an exemplary embodiment of the present invention, a photo sensor having a photo switching element is disposed at a first substrate, and a red color filter or a green color filter corresponding to a channel of the photo switching element is disposed at a second substrate facing the first substrate. In the present invention it is recognized that having photo switching elements corresponding to a red or green color filter is well suited to human eye luminosity.

The channel of the photo switching element includes an amorphous silicon layer and the thickness of the amorphous silicon layer is 500-2000 Å. The photo sensor further includes a liquid crystal layer disposed between the first substrate and the second substrate. The liquid crystal layer is used for a normally black mode display.

Exemplary embodiments of the present invention also provide a display panel having the above photo sensor. In some exemplary embodiments of the present invention, the display panel includes an array substrate, a liquid crystal layer and an opposing substrate. The array substrate has a first switching element in the active area and a second switching element in the sensing area. The opposing substrate is combined with the array substrate to receive the liquid crystal layer and the opposing substrate has a red or green color filter corresponding to the second switching element. The array substrate further has a storage capacitor applying a voltage to the second switching element. The red, green and blue color filters corresponding to the active area are formed on the opposing substrate. The opposing substrate further includes a transparent pattern.

Exemplary embodiments of the present invention also provide a display device having the above display panel. In some exemplary embodiments of the present invention, the display device includes a display unit, a photo sensing unit, a driving controller and a light generation unit. The display unit has pixel electrodes formed at a first substrate and a color filter disposed at a second substrate corresponding to the pixel electrodes. The photo sensing unit includes a photo switching element disposed at the first substrate and a red or a green color filter corresponding to the photo switching element formed at the second substrate to sense an amount of external light. The driving controller outputs a driving control signal responsive to the amount of external light sensed by the light sensing unit. The light generation unit provides the display unit with an internal light controlled by the driving control signal.

Exemplary embodiments of the present invention also provide a method of manufacturing a display device having the above photo sensor. In some exemplary embodiments of the invention, the method comprises forming pixel electrodes on a first substrate, forming a color filter corresponding to the pixel electrodes on a second substrate, positioning a photo switching element on the first substrate and positioning a red or green color filter on the second substrate corresponding to the photo switching element. The photo switching element comprises an amorphous silicon layer of which thickness is 500-2000 Å. Forming a blue color filter and a light blocking layer on the second substrate, disposing a liquid crystal for a normally black mode between the first substrate and the second substrate and disposing a light blocking layer opposite to the blue color filter may be further comprised.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1is a schematic figure illustrating a photo sensor according to an exemplary embodiment of the present invention.

Referring toFIG. 1, a photo sensor is disposed on a first substrate2and Red, Green and Blue color filters5,6and7are formed on a second substrate1facing the first substrate2. A liquid crystal layer4is disposed between the first and second substrates2and1. A back Light Unit10is disposed below the first substrate2.

The photo sensors are disposed opposite to the red or green color filters5,6. But in this embodiment of the invention, no photo sensor is disposed opposite to the blue color filter since the human eye is not as sensitive to light in the blue range as discussed in relation toFIGS. 2 and 3below.

A light blocking material or opaque material9is formed opposite to the blue color filters to block light from the back light unit10. The photo sensor may have an amorphous silicon layer of which resistance can be changed according to the intensity of light.

External light (EL) through the red or green color filters is introduced to the photo sensors. The amount of current flowing through the photo sensor is varied in response to the intensity of the external light introduced to the photo sensor. So, The photo sensors make a signal according to the intensity of the external light.

A light blocking layer8is disposed at the second substrate1. The light blocking layer8is used to block unnecessary light from the back light unit10below the first substrate2.

FIG. 2is a graph of relative luminosity efficiency of a human eye. The X axis is the wave length of light, and the y axis is relative luminosity efficiency. As the wave length of the light is increased, the relative luminosity efficiency of a human eye is increased and goes to a peak and then decreases.FIG. 2shows that a human eye can easily recognize visible light from about 500 nm to about 630 nm which has the relative luminosity efficiency larger than 0.2.

Moreover, the light whose wave length is smaller than 420 nm and larger than 670 nm cannot be recognized by a human eye. The external light whose wave length is smaller than 420 nm and larger than 670 nm of the external light cannot be recognized by the human eye, so this external light does not need to be sensed by the photo sensor. Although that range of light is sensed by the photo sensor, it is not suitable for the human eye and therefore the user cannot detect a difference in display quality at those wavelengths.

For example, if an ultraviolet rich light (<500 nm) is introduced to the photo sensor, it is not recognized by the human eye (the relative luminosity efficiency<0.2). When light at these wavelengths is introduced to the photo sensor, and if the photo sensor reacts to the all the introduced light, the photo induced current will flow and the backlight of the device will decrease. The quality of the display will therefore decrease giving degraded display output.

Similarly, an infrared rich light (>635 nm) will have the same effects as outlined above.

FIG. 3is a spectrum of a color filter substrate. The spectrum of the color filter substrate shows a transmittance of the color filter substrate according to the wave length of the light. InFIG. 3, when light having a wavelength of 400˜500 nm is introduced to the color filter, the transmittance of the blue color filter is higher than that of the red or green color filter. When light having a wavelength of 500˜600 nm light is introduced to the color filter, the transmittance of the green color filter is higher than that of the red or blue color filter. When light having a wave length of 600˜700 nm light is introduced to the color filter, the transmittance of the red color filter is higher than that of the green or blue color filter.

ComparingFIG. 2withFIG. 3, light having a wavelength of 500˜630 nm, which is well recognized by human eye, has a high transmittance, over 80%, when it is introduced through the red color filter or green color filter. Light having a high transmittance introduced through the blue color filter is closer to the ultra-violet ray wavelength (˜500 nm), which is rarely recognized by human eyes.

This suggests that the light introduced to the photo sensor through the blue color filter may include a light not recognized by human eyes. So, the photo sensor should preclude the blue color filter. In other words, when ultra-violet rich external light is introduced to the device, the photo sensor should not react to this kind of light. FromFIG. 2andFIG. 3, it is recommended in one embodiment of the invention to eliminate photo sensors corresponding to the blue color filter in order for the device to have luminosity suitable to the human eye. In other embodiments of the invention it may be important to limit the effect of the light from the blue color filters.

Referring to theFIG. 2andFIG. 3, light having a wavelength over 630 nm which is closer to the infrared ray, is not recognizable to the human eye. This suggests that precluding the light over 630 nm introduced through red color filter to the photo sensor is recommended for making a device suitable to luminosity suitable to the human eye. If light over 630 nm is introduced to the photo sensor, and the photo sensor reacts to this light, then the photo induced current will flow in the device. Some of the signal will go to the driving circuit and the backlight current will be reduced creating a darker display panel. It may be difficult for the user to view the display under this condition. To solve this problem, the thickness of the amorphous silicon layer may be controlled.

FIG. 4is a graph of a light absorptance according to the thickness of the amorphous silicon layer.

Referring toFIG. 4, the solid line is the absorptance when the thickness of the amorphous silicon layer is 2000 Å and the dotted line is the absorptance when the thickness of the amorphous silicon layer is 3000 Å. As the wave length is increased, the light absorption is apt to decrease. The absorptance when the thickness of amorphous silicon layer is 2000 Å is lower than the absorptance when the thickness of a-Si is 3000 Å. This means that the absorptance is lower as the amorphous silicon layer is thinner. The longer wave length is less absorbed into the a-Si layer and increases the penetration of the amorphous silicon layer.

To lower the absorptance of the light which is unrecognized to the human eyes, (i.e. light over 630 nm which is introduced through the red color filter), the thickness of the amorphous silicon layer should be thin. When the thickness is smaller, the current that flows into the photo sensor decreases. By controlling the thickness of the amorphous silicon layer, absorptance and transmittance of light are also controlled.

FIG. 5is a spectrum of a photo sensor according to an exemplary embodiment of the present invention.

The thickness of the amorphous silicon layer of the photo switching device is about 500˜2000 Å. A Red or Green color filter is disposed in the upper substrate corresponding to the photo switching device. In this condition, the spectrum ofFIG. 5is taken. This spectrum is very similar to the relative luminosity efficiency drawn inFIG. 2.

The photo sensor reacts to the light which is recognizable to the human eye.

FIG. 6is a plan view illustrating a photo sensor according to an exemplary embodiment of the present invention.

FIG. 7is a cross sectional view taken along line I-I′ inFIG. 6.

Referring toFIGS. 6 and 7, the photo sensor includes a photo switching element110formed on a first substrate100and a color filter220formed on a second substrate200facing the first substrate100. A liquid crystal layer300is disposed between the first and second substrates100and200. The liquid crystal layer may be in a normally black mode in which the display device displays a black image when a voltage is not supplied.

Particularly, the first substrate100includes a photo switching element (e.g., amorphous silicon thin film transistor (a-Si TFT))110formed on a first base substrate101. The photo switching element110includes an amorphous silicon layer for forming a channel.

The photo switching element110has a gate electrode111formed from a first metal layer, and source electrode113and a drain electrode114formed from a second metal layer. A semiconductor layer112is disposed between the gate electrode111and the source and drain electrodes113and114. The semiconductor layer112includes an activation layer112aand a resistive contact layer112b.

A portion of the resistive contact layer112bcorresponding to the source and drain electrodes113and114is removed to form a channel (CH) in the semiconductor layer112, and the activation layer112ais exposed through the channel (CH). A resistance of the channel (CH) is varied in response to the amount of external light introduced to channel (CH), and accordingly the amount of current flowing through the channel (CH) is varied in response to the amount of the external light introduced to the channel (CH).

The semiconductor layer112is formed from an amorphous silicon layer, and the thickness of the semiconductor layer may be smaller than 2000 Å. Desirably it is 500˜2000 Å.

A gate insulation layer102is disposed over the gate electrode111and a passivation layer103is disposed over the source and the drain electrodes113and114, and exposed portions of the gate insulation layer102.

The second substrate200includes a second base substrate201, color filter layer220and a light blocking layer210disposed at the second base substrate201. A dummy red color filter or a dummy green color filter is disposed at the second base substrate201facing the above photo switching element. The dummy color filter is larger than the semiconductor layer. The light blocking layer210is used to blocking the unnecessary light from the back light (not drawn) below the first substrate100.

FIG. 8is a plan view illustrating a display area and a photo-sensing area of the second substrate.

The second substrate has an AA (active area) and a SA (sensing area). The AA is an active area, which is a real display area, and the SA is a dummy area, which does not display images. At the SA, the external light is introduced to the photo sensor, so the device senses the amount of the external light.

The photo sensor is disposed under the SA area, especially under a dummy Red or Green color filter located near the SA area. The photo sensor is not disposed under the dummy blue filter. Under the blue filter, a metal opaque organic layer or light blocking layer is disposed on the first substrate in order to block light from the back light assembly and the external light.

In theFIG. 8, the photo sensor is disposed in the upper, lower, left and right side of the active area. The photo sensor may be disposed at least one part of the upper, lower, left and right side of the active area. For example, the photo sensor may be disposed in the left and right side of the active area, and the photo sensor may be at the upper and lower side of the active area.

FIG. 9is a plan view partially illustrating a display panel according to the exemplary embodiment of the present invention andFIG. 10is a cross sectional view taken along line II-II′ inFIG. 9.

Referring toFIGS. 9 and 10, the display panel includes an array substrate400and a color filter substrate500.

The array substrate400includes an active area AA and a photo sensing area SA for sensing light. Data lines DL, gate lines GL, switching elements410coupled to the data lines DL and gate lines GL, and pixel electrodes430coupled to the respective switching elements410are disposed in the active area AA. A reflection portion having a reflection electrode can be further comprised in the active area AA.

Photo switching elements450coupled to the readout lines ROL and the sensing gate lines SGL are disposed in the sensing area SA. The sensing area can be disposed at the upper, lower, left or right side of the active area AA.

The color filter substrate500has a color filter520disposed in a first region corresponding to the active region AA and a dummy color filter521disposed in a second region corresponding to the sensing area SA.

A space for each unit pixel, which is filled with the color filter520and the dummy color filter521, is defined by a light blocking layer510. The color filter520includes respective color filters for red (R), green (G) and blue (B) colors to represent corresponding colors of incident light and the cooler filter520may further include cyan (C), magenta (M), yellow (Y), black (B) or transparent pattern. The dummy color filter layer521includes a color filter that cause a light having a wave length of 500 nm˜630 nm, which is well recognized to human eyes. For example, Red or Green color filters are formed but Blue color filters, which are rarely recognizable to human eyes, are not formed in dummy color filter area.

Referring still toFIGS. 9 and 10, the display panel includes an array substrate400, a color filter substrate500facing the array substrate400and a liquid crystal layer600disposed between the array substrate400and the color filter substrate500.

The array substrate400includes a transparent substrate401, the switching element410, a pixel electrode430for a liquid crystal capacitor (CLC), a photo switching element450and a storage capacitor (CST1) for a photo switching element. A storage capacitor (not shown in the drawings) connected to the pixel electrode430may be further included.

The switching element410includes a first gate electrode411, a first semiconductor layer412, a first source electrode413and a first drain electrode414. The first gate electrode411is disposed at the transparent substrate401and a gate insulation layer403is disposed over the first gate electrode411. The first semiconductor layer412is disposed over the gate insulation layer403corresponding to the first gate electrode411.

The first semiconductor layer412includes a first activation layer412aand a first resistive contact layer412bdisposed on the first activation layer412a. A portion of the first resistive contact layer412bis removed between the source and drain electrodes413and414so that a channel (CH1) through which the activation layer412ais exposed is formed in the first semiconductor layer412. A passivation layer405is disposed at the first source and drain electrodes413and414.

A portion of the passivation layer405disposed at the first drain electrode414is removed to form a contact hole416. The pixel electrode430for a liquid crystal capacitor (CLC) is electrically coupled to the drain electrode414through the contact hole416.

The photo switching element450includes a second gate electrode451(electrically connected to a sensing gate line SGL), a second semiconductor layer452, a second source electrode453(electrically connected to a voltage line VL) and a second drain electrode454. The second gate electrode451is disposed at the transparent substrate401and the gate insulation layer403is disposed over the second gate electrode451. The second semiconductor layer452is disposed over the gate insulation layer403corresponding to the second gate electrode451.

The second semiconductor layer (452) includes an amorphous silicon layer, and the thickness of the amorphous silicon layer is 500-2000 Å. The second semiconductor layer (452) is formed using the same process as used with the first semiconductor layer, and the thicknesses of the first semiconductor layer and the second semiconductor layer are different from each other. The first and second semiconductor layers are formed by a chemical vapor deposition method. The first semiconductor layer (412) in the active area (AA) has a thickness of 2000-4000 Å but the second semiconductor (452) in the photo sensing area (SA) has a thickness of 500-2000 Å.

The second semiconductor layer452includes a second activation layer452aand a second resistive contact layer452bdisposed on the second activation layer452a. A portion of the second resistive contact layer452bis removed between the second source and drain electrodes453and454so that a second channel (CH2) is disposed at the first semiconductor layer452. A passivation layer405is disposed over the second source and drain electrodes453and454.

The sensing gate line (SGL) is extended in the horizontal direction (X), and supplies a sensing gate signal to the photo switching element (450) from the external circuit. The voltage line (VL) is extended in the vertical direction, and supplies a Von voltage from the external circuit to the photo switching element (450) through the storage capacitor (CS1). When the external light is introduced to the second channel (CH2) of the photo switching element (450), the photo induced current flows to the readout line (ROL) via the second source electrode (453). The photo induced current is a kind of photo sensing signal and includes information corresponding to the external light.

The readout line (ROL) is extended in the vertical direction (Y), and transfers the photo induced current of the photo switching element (450) to the external driver integrated chip (IC) (not drawn).

The storage capacitor (CST1) for the photo switching element is defined by a lower storage electrode (CST1a), which is formed when the voltage line (VL) is formed, and an upper storage electrode (CST1b) is formed when the second drain electrode (454) is formed.

The first and second gate electrodes411and451, the first and second source and drain electrodes413,414,453and454, the sensing gate line (SGL), voltage line (VL) and the readout line (ROL) may be formed as a single metal layer or a multi metal layer. The single or multi metal layer may include, for example, aluminum (Al), silver (Ag), copper (Cu), molybdenum (Mo), an alloy of aluminum, silver, copper or molybdenum, chromium (Cr), tantalum (Ta) or titanium (Ti), etc.

The color filter substrate500includes a transparent substrate501, the light blocking layer510, the color filter520, the dummy color filter521, a protective layer530and a common electrode layer540.

Particularly, the light blocking layer510defines a space for each unit pixel corresponding to the pixel electrode430.

The color filter520includes color filters for red, green and blue. A space for each unit pixel defined by the light blocking layer510is filled with the color filter520. The color filter520may further include transparent pattern, cyan (C), magenta (M), yellow (Y) or black (B).

The protective layer530is disposed at the light blocking layer510and the color filter520, and functions as a planarization film and a protective film. The common electrode layer540is a transparent conductive layer to which an electrode of the liquid crystal capacitor (CLC) is coupled. A common voltage is applied to the common electrode layer540.

The dummy color filter521is disposed above the photo switching element450. The size of the dummy color filter521may be larger than the size of the second channel452of the photo switching element450in the array substrate400.

FIG. 11is a schematic plan view illustrating a display device according to an exemplary embodiment of the present invention.

Referring toFIG. 11, a liquid crystal display panel700includes a display area DA for displaying images and first and second peripheral areas PA1and PA2adjacent to the display area DA. The display area DA includes an active area AA for displaying images and a light sensing area SA for sensing an intensity of external light EL.

In the active area AA, switching elements TR1are coupled to gate lines GL1through GLn and data lines DL1through DLm. In the light sensing area SA, a light sensor730including a photo switching element TR2for outputting a first voltage V1(seeFIG. 11) responsive to the intensity of the external light EL and a reset unit740for resetting the light sensor730are disposed.

A gate driver circuit710for outputting gate signals to the gate lines GL1through GLn is disposed in the first peripheral area PA1. The gate driver circuit710may be implemented as a shift register including stages SRC1through SRCn+1 that are sequentially connected to one another. The stages SRC through SRCn+1 of the shift register output gate signals to corresponding gate lines GL1through GLn. The last stage SRCn+1 is a first dummy stage for driving an n-th stage SRCn.

In addition, a first driving voltage interconnection VONL to which a first driving voltage VON is applied and a second driving voltage interconnection VOFFL to which a second driving voltage VOFF is applied are formed near the gate driver circuit710in the first peripheral area PA1. Further, a scan start interconnection STL for providing a start signal ST to the first stage SRC1is formed near the first driving voltage interconnection VONL in the first peripheral area PA1.

A data driver circuit720for outputting data signals to the data lines DL1through DLm is disposed in the second peripheral area PA2. Additionally, a read out unit750is disposed in the second peripheral area PA2to convert the first voltage V1from the light sensor730into a second voltage V2.

FIG. 12is a circuit diagram illustrating an operation of the photo sensor illustrated inFIG. 11.

Referring toFIGS. 11 and 12, the liquid crystal display device700includes light sensor730, a reset unit740, a read out unit750, a driving controller760and a light generation unit800.

The light sensor730includes the photo switching element TR2and a first storage capacitor CS1. The photo switching element TR2has a drain electrode DE2electrically coupled to the first driving voltage interconnection VONL to receive the first driving voltage VON, a source electrode SE2electrically coupled to the first storage capacitor CS1and a gate electrode GE2electrically coupled to the second driving voltage interconnection VOFFL to receive the second driving voltage VOFF.

The first storage capacitor CS1includes a first electrode LE1electrically coupled to the second driving voltage interconnection VOFFL and a second electrode UE1coupled to a first read out interconnection RL1, wherein the first and second electrodes LE1and UE1are opposite to each other and include an interposing gate insulation layer therebetween. The first storage capacitor CS1is charged with the first voltage V1corresponding to a light current IPH outputted from the photo switching element TR2. The light sensor730further includes a drain capacitor electrically coupled between the source electrode SE2of the photo switching element TR2and the second driving voltage interconnection VOFFL.

The first read out interconnection RL1is coupled to the first storage capacitor CS1and the first voltage V1charged in the first storage capacitor CS1is read out through the first read out interconnection RL1.

The read out unit750includes a read out switching element TR3and a second storage capacitor CS2. The read out switching element TR3has a gate electrode GE3, a drain electrode DE3electrically coupled to the first read out interconnection RL1and a source electrode SE3electrically coupled to the second storage capacitor CS2. When the read out switching element TR3is turned on in response to the read out signal, the first voltage V1provided from the first read out interconnection RL1is transmitted to the read out switching element TR3and converted into the second voltage V2.

The second storage capacitor CS2includes a first electrode LE2coupled to the second driving voltage interconnection VOFFL and a second electrode UE2coupled to a second read out interconnection RL2, wherein the first and second electrodes LE2and UE2are opposite to each other and include an interposing gate insulation layer therebetween. The second storage capacitor CS2is charged with the second voltage V2that is provided through the read out switching element TR3.

The reset unit740initiates the light sensor unit730at predetermined periods of time. The reset unit740includes a reset switching element TR4having a gate electrode GE4for receiving the reset signal ST, a drain electrode DE4electrically coupled to the first read out interconnection RL1and a source electrode SE4coupled to the second driving voltage interconnection VOFFL to receive the second driving voltage VOFF.

The reset switching element TR4discharges the first storage capacitor CS1to the second driving voltage VOFF through the second driving voltage interconnection VOFFL in response to the reset signal ST. Therefore, the reset switching element TR4may periodically initiate or discharge the first storage capacitor CS1.

The driving controller760includes an operational amplifier (hereinafter, referred to as a comparator)761that is electrically coupled to the read out unit750. The comparator761compares a predefined reference voltage VREF with the second voltage V2outputted from the second read out interconnection RL2. The comparator761outputs a first control voltage V+ or a second control voltage V− in response to a comparison between the reference voltage VREF and the second voltage V2.

The light generation unit800is responsive to an output voltage VOUT of the driving controller760. For example, in response to the output voltage VOUT being the first control voltage V+, the light generation unit800prevents emission of an internal light IL. Additionally, in response to the output voltage VOUT being the second control voltage V−, the light generation unit800emits the internal light IL. Therefore, the internal light IL exiting from the liquid crystal display device700or a level of brightness is responsive to the intensity of the external light EL to reduce power consumption.

As described above, according to exemplary embodiments of the present invention, a photo sensor suitable to a luminosity detectable by a human eye may be made by forming a photo sensor under a Green or Red color filter and by controlling the thickness of the amorphous silicon layer.