JPH0420943A - Image blur suppressing device for camera - Google Patents

Abstract

PURPOSE:To eliminate the discontinuity of a finder image to smoothly suppress the image blur by executing the starting operation and the terminating operation of image blur suppression by an operation control means at the time of entrance of the signal output from a blur detecting means to a prescribed set range. CONSTITUTION:A CPU 100 starts the initial operation to set a port output inverting ISCONT to the high level. Since the output of an inverter 82 is in the low level in this state, a switch 80 is turned off and a switch 81 is turned on, and the operation of a variable vertical angle prism 41 is disconnected from an angular displacement detecting means and it is stopped in the center position. When the output of the angular displacement detecting means is higher than a reference voltage Vc or is lower than the voltage -Vc, an input the inverse of HDIN signal to the CPU 100 goes to the high level; but when it is between voltages Vc and -Vc, this signal goes to the low level. When the signal is in the low level, the output of the angular displacement detecting means is connected to a coil 98, and the vertical angle of the prism 41 is changed by the coil 98 to start suppression of the image blur. When a timer circuit 213 reaches a prescribed time, the blur suppressing operation is terminated.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is directed to a camera that detects camera shake due to hand shake, etc., and suppresses the camera shake by performing optical correction based on this information. The present invention relates to an image blur suppression device.

[Prior Art] Modern photographic equipment, such as cameras, has automated almost all of the functions necessary for photographing, such as determining exposure and focusing, and the number of photographic failures caused by the functions of the camera itself has become extremely low. Recently, efforts have been made to develop cameras that automatically suppress photographing failures caused by causes other than the camera, such as blurring of photographed images due to vibrations such as camera shake.

Normally, in order to prevent camera shake from occurring due to camera shutter release, etc., camera shake (vibration) is detected and image stabilization is performed based on that information. It is necessary to equip the camera with a means that can properly perform this, that is, an image blur suppression device.

FIG. 7 is a schematic block diagram showing an example of a conventional image blur suppressing device. First, the blur detecting means will be explained. This is configured as an optical angular displacement detecting means. That is, a floating body 302 that can freely rotate around a rotation axis 303 is supported in a case 301 that is filled with a liquid whose viscosity is appropriately selected and is arranged to move together with the camera (or lens barrel). For example, the current camera (lens barrel)
Assuming that there is a shake in the camera and the camera is rotated by θIn with respect to the coordinate system of absolute space, then case 3 above occurs.
01 moves in unison with the camera. In contrast, the liquid within the case remains stationary relative to absolute space due to inertial force. In other words, the floating body 302 and the case 301 are
This means that the portion has been relatively rotated by θIrl corresponding to the above portion.

Therefore, light from a light emitting element 306 fixed to the camera (lens barrel) is reflected by the floating body 302, and the reflected light is received by the light receiving element 305, which is connected to the light receiving element 305. The position detection circuit 304 can detect the angular displacement due to the shake.

On the other hand, to explain the mechanism for keeping the image on the image plane apparently still, the conventional example shown in Fig. 7 has a certain refraction inside an accordion-shaped container formed by connecting two transparent plates with a bellows. Variable apex angle prism 4 by enclosing a liquid with a certain ratio
1 is constructed and used as an optical axis decentering means.

Then, the actuator 31, which is a solenoid, connects the transparent plate on the subject side to the fixed transparent plate on the shooting lens 308 side.
3, the camera can be tilted as appropriate to change the photographing optical axis. In other words, the transparent plate on the subject side of the variable apex angle prism 41 is θ. , the photographic optical path that passes through the photographic lens 308 and enters the film surface 309 is
. .

and will rotate relative to the optical axis according to a proportionality constant determined by the refractive index of the enclosed liquid.

As described above, in the camera shown in FIG. 7, the optical angle detection means using the rotatable floating body 302 is used to detect the shake (vibration) that occurs in the camera, and to respond to the detected shake. By varying the apex angle of the variable apex angle prism by the corresponding angle, even if the camera is blurred, it is possible to always guide the incident light from the subject to the same position on the film surface 309, thereby suppressing blur in the photographed image. There is.

In addition, in the conventional device shown in FIG. 7, the angular displacement θ performed by the variable apex angle prism. , is detected by a position detection circuit 312 placed near the variable apex angle prism,
Position detection circuit 304 detected as camera shake
Angle 0 plate which is the output of . From the signal indicating the above angular displacement θ
. , and the subtracted output is amplified by an amplifier circuit 314 and then input to a driver circuit 316 via a phase compensation circuit 315. Therefore, as a result, the drive of the actuator 313 is feedback-controlled, and highly accurate control is achieved.

[Problems to be Solved by the Invention] Incidentally, in the image blur suppressing device described in FIG. This is done by turning on/off a manual switch, which serves as operation control means for the image blur suppressing device.

That is, input from the angular displacement detection means to the actuator 313 is performed by turning the manual switch 320 on and off.

In such a configuration, the detection output from the optical angular displacement detection means described above is transmitted to the driver 316 of the variable apex angle prism.
(in other words, to start image blur suppression), there are no conditions related to the state of the angular displacement detection means (particularly the relative rotational state of the floating body and the case).
When turned on, a signal indicating the shake state detected by the angular displacement detecting means at that time is directly input to the driver 316.

However, although this type of control is not enough to be considered a failure as a device that satisfies the function of suppressing image blur, it does result in a system that is difficult to use when using a camera equipped with this device. Problems are pointed out.

For example, if the position detection signal from the angular displacement detection means deviates significantly from the normal center position, the switch 3
20 is turned on and image blur control is started, the transparent plate of the variable apex angle prism 41 on the subject side will suddenly be displaced from an inclined state to a parallel state.

This causes a sudden change in the optical path. For example, in cameras such as TTL, which view the subject through the viewfinder through the photographic optical system, discontinuity (so-called skipping) of the viewfinder image occurs, causing a great discomfort to the photographer. It has the disadvantage of giving

The present invention has been made to solve the problems of such conventional devices.

That is, an object of the present invention is to solve the problems of the conventional image blur suppressing device as described above, thereby eliminating discontinuities that appear in the viewfinder image, and suppressing camera image blur without causing discomfort to the photographer. The goal is to provide equipment.

[Means and effects for solving the problem] The image blur suppressing device of the present invention that achieves the above object is characterized by a blur detecting means for detecting blur with respect to the camera space, and an image blur suppressing device based on a signal from the blur detecting means. An image blur suppressing device for a camera comprising: an optical axis changing means for changing the photographing optical axis by using a camera; and an operation control means for receiving an external signal and determining when to start and end the operation of the optical axis changing means. The control means has a comparison means for determining whether the signal output from the shake detection means is within a predetermined setting range, and uses the output of the comparison means as one of the conditions for determining the operation start point. The structure is such that it is designed to do so.

Another feature of the apparatus of the present invention is that the operation control means is provided so as to use the output of the comparison means as one of the conditions for determining the end point of the operation.

According to this configuration, the image blur suppressing device starts or ends its operation only when the signal output from the shake detection means is within a predetermined setting range, so that sudden tilting of the optical axis is prevented. I'm never invited.

In the image blur suppressing device of the present invention, the signal input from the outside to the operation control means is not particularly limited. Output from an operable manual switch, output from a zoom lens position detection means provided as a means to start or end image blur suppression, and output from a zoom lens position detection means provided as a means to start or end image blur suppression, and output from an image blur suppression device when the power supply voltage is not within a predetermined range. An example of the output from the power supply voltage monitoring means that is set to be inactive can be given.

[Embodiments] The present invention will be described in detail below based on embodiments shown in the drawings.

Note that in FIGS. 1 to 6, the same reference numerals are used for common elements for convenience, and detailed explanations are partially omitted. Further, in the explanation of the flowcharts in FIGS. 2, 4, and 6, the numbers indicate the numbers of processing procedures (steps).

Embodiment 1 FIG. 1 is a block diagram illustrating an outline of an image blur suppressing device according to Embodiment 1, and FIG. 2 is a flowchart illustrating an outline of a processing procedure.

This example device shows an example in which the start and end of the image blur suppression operation is operated from the outside using a manual switch.

In these figures, the angular displacement detection means for detecting vibrations such as camera shake is constructed by filling a cylindrical case 2 with a liquid 3 having a predetermined refractive index. The structure includes a floating body 4 that is rotatable around an axis.

This floating body 4 is held in a predetermined position by a closed magnetic circuit made up of permanent magnets 1 provided so as to surround the case 2 when there is no wobbling.

When the lens barrel is shaken and the floating body 4 rotates relative to the case 2 as described above, the amount of rotation is detected by the optical detection means.

That is, a light emitting element (for example, an infrared light emitting diode IRED)
The signal light emitted from the floating body 4 is reflected by the surface of the floating body 4 and sent to a light receiving element for position detection (for example, a semiconductor device detection element P).
SD) 5, the position of incidence of the signal light on the light receiving element 5 changes due to the relative rotation, and as a result, the output currents (1) and I of the light receiving element 5 change.

Then, the output current I is amplified by a current-voltage conversion circuit composed of an operational amplifier 13, a resistor 14, and a capacitor 15.
The signal is input to a subtraction circuit composed of 9 and 20.

-4, the output current I is amplified by a current-voltage conversion circuit composed of an operational amplifier 10, a resistor 11, and a capacitor 12, and is input to an adder circuit composed of an operational amplifier 21, resistors 22, 23, 24, and 25. Ru.

Here, the output of the adder circuit is the operational amplifier 26 and the resistor 27.
．． 28.31, capacitor 29, transistor 30
The voltage is input to an IRED driver circuit composed of , and feedback control is performed so that the output of the adder circuit as a whole becomes equal to the reference voltage vC.

On the other hand, the amount of displacement of the variable apex angle prism 41 is determined by the detection device 43.
and 44, and is input to a current-voltage conversion circuit, a subtraction circuit, and an addition circuit composed of circuit elements 50 to 71 similar to the above-mentioned angular displacement detection means, and as a whole, an operational amplifier 56 and resistors 57, 58, and 59. , and 60 from the subtraction circuit.

As described above, each output of the operational amplifiers 16 and 56 has a value corresponding to the amount of angular displacement with respect to absolute space and the amount of vertical angle displacement of the variable apex angle prism 41, and the output of the operational amplifier 16 is connected to the inverter 82. The operational amplifier 5 is connected to the resistor 83 via the analog switch 80 which is
The output of 6 is connected to a resistor 84 and to an inverting input terminal of an operational amplifier 85 to which a feedback resistor 86 is connected.

Further, an analog switch 81 is connected to the resistor 83 in parallel with the analog switch 80, and one input of the analog switch 81 is connected to GND.
The angular displacement detecting means and the variable apex angle prism 41 are connected to each other, and constitute an operation control means of the present invention for turning on and off the circuit including the angular displacement detecting means and the variable apex angle prism 41.

The circuit composed of the operational amplifier 85, resistors 83, 84, and 86 is originally known as an adder circuit, but
For example, in this example, the polarity of the output representing the movement of the variable apex angle prism 41 is inverted with respect to the output of the angular displacement detection means, and as a result, it is a subtraction circuit, and the optical axis change of the present invention is performed. This constitutes means for determining the correction amount (in this example, the driving amount of the displacement of the apex angle of the variable apex angle prism 41).

Next, the output of the operational amplifier 85 is input to a phase lead compensation circuit composed of a capacitor 91 and resistors 92 and 93, and after phase compensation of the entire feedback system is performed, it is input to a buffer amplifier 90. . Then, this one output is input to a power amplification circuit composed of an operational amplifier 97. Other outputs are operational amplifier 94, resistor 95.
It is input to an inverting type power amplifier circuit consisting of 96.

Next, the outputs from these two power amplifier circuits are input to a coil 98 which is an actuator of the variable apex angle prism, and the operation of this coil 98 controls the apex angle of the variable apex angle prism 41 to be changed. These buffer amplifiers 9
In this example, the two power amplifier circuits and the coil constitute the optical axis changing means of the present invention that changes (rotates) the optical axis.

In addition, the outputs of the comparators 87 and 88 are input to the CPU 100 via the ND gate 89 to N.
Connected to IN.

Here, the positive reference voltage ■c is applied to the non-inverting input terminal of the comparator 87, and the negative reference voltage !c is applied to the inverting input terminal of the comparator 88. ! ! The voltage -Vc is connected and the operational amplifier 16
The output of the operational amplifier 16 is compared with the reference voltage, and the output of the operational amplifier 16 is V
It constitutes a so-called window comparator in which the input HDIN becomes L level only when it falls within the range of c and -Vc, and this corresponds to the comparison means of the present invention.

Additionally, in this example, the externally actuated switch 102
8~Switch 102k is switch interface 10
1 to CPljlClO through switch 10
2a-102 is communicated to the CPU 100.

With the above configuration, even if the lens barrel moves due to camera shake, etc., the variable apex angle prism 41 is driven to an angular displacement corresponding to the relative angular displacement of the floating body 4, and the subject image on the camera image plane remains unchanged. Can remain stationary.

Next, the operation of starting and ending the image blur suppression operation in this example will be explained based on the flowchart of FIG. 2.

First, the CPt 1100 starts an initial operation by a power-on reset circuit (not shown) and sets the boat output l5CONT to H level.

In this state, the output of the inverter 82 is at the L level, so the analog switch 80 is turned off and the analog switch 81 is turned on, and the operation of the variable apex angle prism 41 is separated from the signal from the angular displacement detection means, and the center position The camera remains stationary at , and the image blur suppression operation remains off. In addition, in the initial operation, the latch 15ON, which stores the state of each switch inside the CPU 100, is activated.
The value of L is set to L level (step 200,
201),.

Next, the state of this latch l5ONL is determined (step 202), and if this value is at L level, the process proceeds to step 203, and if it is at H level, the process proceeds to step 210.

Steps 203 to 209 are a startup routine for the image blur suppressing device, and in step 203, the state of I5SW, which corresponds to the state of switch 102a shown in FIG. 1, is read via switch interface 101. If l5SW is off, it is determined that the state of switch 102a remains off and the process returns to step 202, but if rssw is on, it is assumed that the state of switch 102a has changed from off to on. Judgment step 204
Then, control for starting the suppression operation is started.

In step 204, the timer circuit 103 built in CPIJloo is started, and then in step 2
At step 05, the state of the input HDIN is determined.

While the output of the angular displacement detection means is greater than the reference voltage Vc or less than the reference voltage -Vc, the output of the window comparator, that is, the input to the CPU 100) IDIN
The signal at the input HDIN is at the H level, and the process proceeds to step 206. When the output falls between the reference voltages Vc and -Vc, the signal at the input HDIN becomes the L level, and the process then proceeds to step 207. move on.

In step 206, it is determined whether the timer (circuit 103) has reached a predetermined time Tε. If T4 has not been reached, the process returns to step 205; if T has been reached,
Assuming that the output has never been within the range of the reference voltages Vc and -Vc within the set time, the determination operation is ended and the process proceeds to step 207.

By setting the port output l5CONT to L level in step 207, the analog switch 80 is turned on, and the output of the angular displacement detection means is connected to the coil 98.
Image blur suppression operation is started.

Next, in step 208, the latch l5ONL is set to H.
level, the switch state is memorized, the timer is stopped in step 209, and the timer is stopped again in step 202.
Return to

Steps 210 to 216 are the end routine of the image blur suppressing device, and when the latch l5ONL is at the H level in step 202, the process proceeds to step 210, where the state of l5SW is determined.

If rssw is on, the process returns to step 202 as described above, but if it is off, it is determined that the switch 102g has changed from on to off, and the process proceeds to step 211, where control is performed to end the image blur suppression operation. Start.

In step 211, the timer circuit 103 is started, and then in step 212, the state of the input (IDIN) is determined.

Here, as in the case of steps 205 to 206 described above, while the value of the operational amplifier 16, which is the output of the angular displacement detection means, is greater than the reference voltage Vc or less than the reference voltage -Vc, the input HDIN signal is At this time, the process proceeds to step 213, where the output is set to the reference voltage Vc.
When the voltage falls within the range of -Vc and -Vc, the input HDIN signal becomes L level, and in this case, the process advances to step 214.

In step 213, it is determined whether the timer value has reached the predetermined time TE, and if it has not reached TE, the process returns to step 212, and if it has reached Tt, the same process as in steps 205 and 206 described above is performed. The determination operation is completed and the process proceeds to step 214.

In step 214, the boat output l5CON is set to H level, the analog switch 80 is turned off, and the analog switch 81 is turned on, thereby disconnecting the output of the angular displacement detection means from the coil 98, and the image blur suppressing operation is completed.

Subsequently, in step 215, the latch 15ONL is set to L level to store the switch state, and in step 218, the timer is stopped, and then the process returns to step 202.

In the device described above, the reference voltages VC and -Vc
Since the value of is set to a value extremely close to the GND level, the amount of shift when the image blur suppression operation changes from off to on (steps 203 to 2o9) is small, and the detected camera shake is usually Since the signal is a periodic signal, the output of the angular displacement detection means always crosses the GND level at intervals of several seconds, so that continuous operation is possible even if there is some time delay.

The same applies when the image blur suppression operation changes from on to off, and as soon as the output of the angular displacement detection means falls within the predetermined range,
Since the image blur suppression operation is ended, discontinuity in the finder image does not occur.

Example 2 Figure 3 and '! Figure J4 explains the image blur suppressing device according to the second embodiment of the present invention. In this example, external signals for starting and ending the image blur suppressing operation are applied to the zoom position of the photographic lens. It is planned to be carried out on the basis of

Reference numerals 1 to 103 in FIG. 3 indicate the same elements as in FIG. 1 of the first embodiment, and part numbers 110 to 118 indicate structures related to the zoom mechanism, which is a characteristic structure of this example.

That is, in FIG. 3, 111 is a variable resistor,
The resistance value is set to change according to the zoom position of the photographing lens, and the voltage set by the variable resistor 111 of the reference voltage KVC is applied to the A/D converter 11.
0, and is input to the CPU 100.

Also, transistor 112.113.115.117
The bridge circuit constitutes a motor driver circuit for the motor 118 that actually controls zooming of the photographic lens, and determines the direction of zooming based on a control signal from the CPU 100.

Next, operations for starting and ending the image blur suppression operation in this example will be explained based on the flowchart of FIG. 4.

In FIG. 4, steps 200 to 202 are the same as the 'tS2 diagram of the first embodiment, and when the latch l5ONL is at the L level, the process proceeds to step 250, and the latch l5ONL is at the H level.
In the case of level, the process moves to step 251.

In step 250, in order to activate the image blur suppression device,
The zoom position of the photographic lens is determined.

In other words, the preset zoom control allows the shooting lens to change from the wide (WIDE) side to the telephoto (position E) side.
Assuming that it is moving to the side, the zoom position f
is a predetermined value D (the value of D increases toward the telephoto side)
If it is smaller, the process returns to step 202, but if the zoom position moves to the telephoto side and the value of f becomes thicker than D, the process proceeds to step 204.

Steps 204 to 209 are the startup routine of the image blur suppression device, and as explained in the first embodiment,
When the output of the angular displacement detection means falls within a predetermined level range, the image blur suppression operation is started.

In step 251, the zoom position of the photographic lens is determined in order to terminate the image blur suppression device.

That is, assuming that the photographic lens is moving from the telephoto side to the wide side by preset zooming control, if the zoom position f is larger than a predetermined value, the process returns to step 202, but if the zoom position is When the value of ``D'' becomes smaller than D after moving to the wide side, the process advances to step 211.

Steps 211 to 216 are the end routine of the image blur suppressing device 1, and as explained in Embodiment 1, the image blur suppressing operation ends when the output of the angular displacement detection means falls within a predetermined level range. do.

Therefore, in this example as well as in the first embodiment, discontinuity in the finder image does not occur either when the device is started up or when it is shut down.

Embodiment 3 FIGS. 5 and 6 illustrate an image blur suppressing device according to Embodiment 3 of the present invention.

Reference numerals 1 to 103 in FIG. 5 indicate the same elements as in FIG. 1 of the first embodiment, and part numbers 120 to 126 relate to the configuration of the power supply voltage monitoring means, which is a feature of this embodiment.

In FIG. 5, a value obtained by dividing the power supply voltage ■ec by a resistor 125.126 is input to the non-inverting input terminal of the comparator 120, a reference voltage Vc is connected to the non-inverting terminal, and the output of the comparator 120 BCNG is CPU
100 is input.

Further, a constant current circuit composed of an operational amplifier 121 and a resistor 123 supplies a constant current to a coil 122, and the on/off control of this driver is controlled by the pressure CLO of the CPU 100.
N via transistor 124.

Next, operations for starting and ending the image blur suppression operation in this example will be explained based on the flowchart of FIG. 6.

In FIG. 6, steps 200 to 202 are the same as in FIG. 2 of the first embodiment, and constitute a startup routine for the image blur suppressing device, and when the latch l5ONL is at the L level, the process proceeds to step 260, and the latch l5ONL is at the H level. In the case of level, the process moves to step 261.

In step 260, the power supply voltage vcc is set to a predetermined level VC.
Determining whether it is greater than

If the output CLON of CPLIloo is at the L level in advance and the coil 122 is energized, and the value of the power supply voltage VCC is smaller than the predetermined level vc, the process returns to step 202, but if it is larger, the process returns to step 202. 20
Proceed to step 4. From step 204 to step 209, the image blur suppressing operation is started when the output of the angular displacement detecting means falls within a predetermined level range, similarly to FIG. 2 of the first embodiment.

Further, if the latch l5ONL is at the H level in step 202, the process proceeds to step 261.

Steps 261.211 to 216 are the end routine of the image blur suppression device, and in step 216 it is determined whether the value of the power supply voltage vcc is below a predetermined level, and if the power supply voltage vcc is greater than the predetermined level ■c The process returns to step 202 to continue the image blur suppression operation, but if vce becomes smaller than level vc, the process proceeds to step 211.

Steps 211 to 216 are the same as in the case of FIG. 2 of the first embodiment, and the image blur suppression operation is ended when the output of the angular displacement detection sensor falls within a predetermined level range.

Therefore, in this example as well as in the first embodiment, discontinuity in the finder image does not occur either when the device is started up or when it is shut down.

[Effects of the Invention] As described above, according to the present invention, when a signal prompting the image blur suppressing device to start and end the image blur suppressing operation is input from the outside, the control is not immediately switched;
By adopting a configuration that starts and ends image blur suppression for the first time when the output of the angular displacement detection means, which becomes a periodic signal under certain conditions, falls within a predetermined level range, the photographic lens The viewfinder image that passes through the lens does not become discontinuous, and the image blur suppression operation can be turned on and off smoothly, which has the effect of preventing the photographer from feeling uncomfortable due to the discontinuity of the image.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the image blur suppressing device of the present invention, and FIG. 2 is a flowchart showing an outline of its processing procedure. FIG. 3 is a block diagram of a second embodiment of the image blur suppressing device of the present invention, and FIG. 4 is a flowchart showing an outline of its processing procedure. FIG. 5 is a block diagram of a third embodiment of the image blur suppressing device of the present invention, and FIG. 6 is a flowchart showing an outline of its processing procedure. FIG. 7 is a block diagram of an embodiment of a conventional image blur suppressing device. 1: Permanent magnet 2: Case 3: Liquid 4: Floating body 40: Photographic lens 41: Variable apex angle prism 100:
CPU 101: Switch interface 102a: Switch 103: Timer circuit 110
:＾/D converter 111: Variable resistor 112.113, 115.117 : Resistor 118: Zoom motor 120: Comparator 121: Obeambu 122: Coil 123: Resistor 124:) Run sister 1 flood 4 Ru Figure 2 Figure 4 Figure 6

Claims (1)

[Scope of Claims] 1. A shake detection means for detecting shake in the camera space; an optical axis changing means for changing the photographing optical axis based on a signal from the shake detection means; An image blur suppression device for a camera comprising an operation control means for determining the start and end points of operation of the optical axis changing means, wherein the operation control means is configured such that the signal output from the shake detection means is within a predetermined setting range. 1. An image blur suppressing device for a camera, comprising a comparing means for determining whether or not the above-mentioned operation starts. 2. An image blur suppressing device for a camera according to claim 1, wherein the operation control means is provided so as to use the output of the comparison means as one of the conditions for determining the time point at which the operation ends. 3. The image blur suppressing device for a camera according to claim 1 or 2, wherein the signal input from the outside to the operation control means is an output from a manual switch for starting or terminating the image blur suppressing device. . 4. An image blur suppressing device for a camera according to claim 1 or 2, wherein the signal input from the outside to the operation control means is an output from a photographing optical system. 5. According to claim 1 or 2, the power supply voltage monitoring means disables the image blur suppressing device when the signal inputted from the outside to the operation control means causes the power supply voltage to be not within a predetermined range. An image blur suppression device for a camera, characterized in that the output is: