Endoscope system and method for inspecting electronic endoscope

An electronic endoscope has a light guide for leading a light and a solid-state image sensor for capturing an image of a human body cavity during illumination. In inspecting the number of broken optical fibers in the light guide, a cap is attached to a distal portion of the electronic endoscope. The cap has a test chart. The solid-state image sensor captures an image of the test chart which is illuminated with the light transmitted through the light guide. A photometric circuit calculates an average luminance value “Y” of the test chart from a chart image signal. A broken fiber number calculator calculates the number “N” of broken optical fibers that satisfies N=M×(1−Y/I). “Y” represents an average luminance value of the test chart. “I” represents an ideal average luminance value when all optical fibers are conducting, and “M” represents the total number of the optical fibers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an endoscope system that is constituted of an electronic endoscope, a processor device and a light source device, and a method for inspecting the electronic endoscope.

2. Description Related to the Prior Art

An endoscope system is constituted of an electronic endoscope, a processor device and a light source device. The electronic endoscope has a solid-state image sensor at its distal portion for capturing an image of a human body cavity to output an image signal. The processor device receives the image signal from the solid-state image sensor and produces an image to display it on a monitor. The light source device supplies the electronic endoscope with light. Inside the electronic endoscope, a light guide extends. The light from the light source device is led into the distal portion of the electronic endoscope through the light guide, and is incident on the human body cavity from lighting windows provided in the distal portion.

The light guide consists of a plurality of optical fibers tied in a bundle and a binder such as a tape for wrapping the bundle. Since the optical fibers become rigid with a lapse of time, the optical fibers gradually deteriorate and finally snap due to stress applied in using the electronic endoscope. Accordingly, JPA No. 2006-55664 discloses to cover the optical fibers with a flexible tube which has holes formed at regular intervals, for the purpose of preventing a break of the optical fibers. Such a flexible tube, however, cannot always prevent the break because the optical fibers necessarily deteriorate with time.

The break of the optical fibers in the light guide causes reduction in the amount of light exiting from the light guide in accordance with the number of broken optical fibers. Thus, JPA No. 2002-58640 discloses to provide an aperture stop between an incident end of the light guide and a light source in order to keep the amount of exit light constant. The amount of light exiting from the light guide is measured based on an image signal from the solid-state image sensor. The aperture stop is actuated on the basis of a measurement result to control the amount of exit light. Adjustment by the aperture stop, however, is insufficient in a case where a predetermined number or more of optical fibers have already snapped. In this case, a shortage of light exiting from the light guide darkens the image, so that the light guide needs repairing.

A conventional endoscope system cannot detect the break of the optical fibers in the light guide. Therefore, a user cannot grasp appropriate timing of repairing the light guide, and hence it may happen that a doctor has found a shortage of exit light after inserting the electronic endoscope into a patient's body, and the electronic endoscope has to be pulled out and replaced. To prevent such an event, it is conceivable to repair the light guide early on before the electronic endoscope is short of the exit light. However, repair at an early stage causes increase in costs.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an electronic endoscope inspection method that detects the number of broken optical fibers in a light guide, and to provide an endoscope system that correctly notifies a user of repair timing of the light guide on the basis of an inspection result.

An endoscope system is constituted of a cap having a test chart, a photometric circuit and a broken fiber number calculator. The cap is attached to a distal portion of an electronic endoscope so that a solid-state image sensor captures an image of the test chart. The photometric circuit measures an average luminance value of the test chart from an image signal of the test chart. The broken fiber number calculator calculates the number “N” of broken optical fibers that satisfies the following expression:
N=M×(1−Y/I)
wherein, “Y” represents the average luminance value detected by the photometric circuit. “I” represents an ideal average luminance value when all optical fibers are conducting, and “M” represents the total number of the optical fibers.

In the endoscope system, the processor device may contain the photometric circuit and the broken fiber number calculator.

In that case, the processor device may further comprise a first notification section for notifying a user of the number of the broken optical fibers.

The processor device may further comprise a second notification section for issuing a warning message, when the number of the broken optical fibers reaches or exceeds a predetermined number.

The processor device may further comprise an operation unit, a count detector and a third notification section. The operation unit causes the endoscope system to set a light guide inspection mode. The number of the broken optical fibers is calculated in the light guide inspection mode. The count detector detects the number of times the electronic endoscope has been used. The third notification section issues a message that reminds a user of inspection of the light guide, when the number detected by the count detector coincides with a predetermined number.

The processor device may further comprise an amplifier for amplifying the image signal and a gain adjuster for adjusting gain of the amplifier to correct decrease in the average luminance value with respect to the ideal average luminance value.

In the endoscope system, the test chart is preferably provided with a distance-measuring area. The processing unit further comprises a distance-measuring circuit for detecting distance from the distal portion to the test chart on the basis of the size of the distance-measuring area obtained from the image signal. The ideal average luminance value is corrected based on the distance detected by the distance-measuring circuit.

The distance-measuring area may be a circular black area.

The test chart may have a photometric area of 18% gray, and the photometric circuit calculates the average luminance value of the test chart from luminance of the photometric area obtained from the image signal.

In the endoscope system, the light source device may comprise a light source, an aperture stop mechanism, a timer, a memory and an aperture stop controller. The light source emits the light. The aperture stop mechanism is disposed between the light source and the light guide, and leads the light into the light guide. The timer counts used time of the light source. The memory stores light source property data which indicates the relation between a light emission amount and the used time of the light source. The aperture stop controller retrieves a light damping rate in the light emission amount from the used time and the light source property data, and controls opening of the aperture stop mechanism so that the amount of light led into the light guide is made constant irrespective of the light damping rate.

When the light damping rate is larger than a predetermined value and the amount of light led into the light guide is less than a predetermined value though the aperture stop mechanism is open to its maximum, the ideal average luminance value is corrected based on the light damping rate.

A method for inspecting an electronic endoscope comprises the steps of attaching a cap with a test chart to a distal portion of an electronic endoscope, illuminating the test chart with light which is emitted from a light source and transmitted through a light guide, capturing an image of the test chart by a solid-state image sensor, detecting an average luminance value of light obtained from an image signal outputted from the solid-state image sensor, and calculating the number “N” of broken optical fibers that satisfies the following expression:
N=M×(1−Y/I)
Wherein, “Y” represents the detected average luminance value. “I” represents an ideal average luminance value when all optical fibers are conducting, and “M” represents the total number of the optical fibers.

The endoscope system according to the present invention can correctly detect the number of broken optical fibers in the light guide. Therefore, the user can grasp appropriate timing of replacing the electronic endoscope and hence effectively use the electronic endoscope.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

InFIG. 1, an endoscope system2consists of an electronic endoscope10, a processor device11, a light source device12and a cap40(seeFIG. 3). The electronic endoscope10is provided with a flexible insert section13that is introduced into a human body cavity, an operation section14that is joined to a base end of the insert section13, and a universal cord15that is connected to the processor11and the light source device12.

At an end of the insert section13is provided a distal portion16that contains a solid-state image sensor (CCD)50for capturing an optical image of a target body part to inspect. Behind the distal portion16, a bending portion17consisting of a number of linked ring-like segments is provided. By operating an angle knob18on the operation section14, a number of wires extending in the insert section13are pulled and pushed to bend the bending portion17from side to side and up and down. Thus, the distal portion16is directed to the target body part inside the human body cavity.

To an end of the universal cord15, a multi-connector19is attached. The connector19is detachably connected not only to the processor device11but also to the light source device12. The processor device11is electrically connected to the light source device12via the connector19, and has control over the endoscope system2.

The processor device11feeds power to the electronic endoscope10through a transmission cable extending in the universal cord15, and controls the actuation of the solid-state image sensor50. The processor device11receives an image signal outputted from the solid-state image sensor50, and subjects the image signal to various kinds of signal processing to produce image data. An image produced from the image data is displayed on a monitor20, which is connected to the processor device11with a cable, as an endoscope image.

As shown inFIG. 2, a front face16aof the distal portion16is provided with an image capturing window30, lighting windows31, a medical instrument outlet32and an air/water nozzle33. The image capturing window30is disposed in the upper middle of the front face16a. Behind the image capturing window, the solid-state image sensor50is disposed through an objective lens system53and a prism54(refer toFIG. 6).

The two lighting windows31symmetric with respect to the image capturing window30projects light that is guided from the light source device12through a light guide80and a lens83(refer toFIG. 6) to the target body part. The medical instrument outlet32is connected to a medical instrument insertion port21(refer toFIG. 1) on the operation section14through a channel extending in the insert section13. A medical instrument with a forceps, a needle, a diathermy knife or the like at its tip is inserted into the medical instrument insertion port21in order to protrude the tip of the instrument from the medical instrument outlet32to the target body part.

The watering/airing nozzle33ejects water or air from an air/water reservoir contained in the light source device12to the image capturing window30or the target body part in response to actuation of a watering/airing button21(refer toFIG. 1) on the operation section14.

The endoscope system2has a light guide inspection mode for inspecting the light guide80. In this mode, as shown inFIG. 3, the cap40for tightly sealing the front face16ais attached to the distal portion16of the electronic endoscope10. The cap40is in the shape of a cylinder with a bottom. The cap40includes an insertion slot41that has the approximately same diameter as the distal portion16, a light-shielding hole42continuous from the insertion slot41, and a test chart43fitted into a bottom face42aof the light-shielding hole42. The diameter of the light-shielding hole42is smaller than that of the insertion slot41. When the distal portion16of the electronic endoscope10is inserted into the insertion slot41, the rim of the front face16acomes into contact with a step44between the insertion slot41and the light-shielding hole42, and the front face16afaces to the test chart43. The light illuminates the test chart43through the lighting windows31. The solid-state image sensor50captures an image of the illuminated test chart43through the image capturing window30.

The test chart43, as shown inFIG. 4, has a distance-measuring area43aand a photometric area43b. The distance-measuring area43ais a circular black area disposed in the center of the test chart43. The distance-measuring area43ais used for measuring a distance L (seeFIG. 3) from the test chart43to the front face16aof the distal portion16. The photometric area43bis an 18% gray area (gray of 18% reflectivity) disposed on the periphery of the distance-measuring area43a. The photometric area43bis used for white balance correction, in addition to the inspection of the light guide80.

As shown inFIG. 5, the light guide80consists of a bundle of many optical fibers81whose periphery is covered with a binder82such as a tape. Although all the optical fibers81are tied into a single bundle on a light incident side of the light guide80, the bundle is branched off in two and led to the two lighting windows31on a light exit side.

Referring toFIG. 6, the electronic endoscope10has the solid-state image sensor50disposed in the distal portion16, and an analog front end processor (AFE)51and a memory52disposed in the operation section14. The solid-state image sensor50such as a CCD image sensor is so disposed that object light passing through the objective lens system53and the prism54is incident upon its light receiving surface. The light receiving surface is equipped with a color filter having a plurality of color segments (for example, primary-colors filter of Bayer arrangement).

The AFE51is constituted of a correlated double sampling circuit (CDS)55, an automatic gain controller (AGC)56being an amplifier and an analog-to-digital converter (A/D)57. The CDS55applies correlated double sampling processing to the image signal from the solid-state image sensor50in order to remove reset noise and amplifier noise. The AGC56amplifies the image signal without noise by gain that the processor device11has designated. The A/D57converts the amplified image signal into a digital signal of a predetermined bit number, and inputs it to the processor device11through the connector19.

The memory52is a nonvolatile memory such as a flash memory. The memory52stores identification data for identifying the model of the electronic endoscope10and the number of times the electronic endoscope10has been used. No sooner is the electronic endoscope10connected to the processor device11, than the processor device11reads out the identification data and the number of times the electronic endoscope10has been used.

The processor device11includes a CPU58, a timing generator (TG)59, an isolation device (ID)60, another CPU61, a digital signal processor (DSP)62, a digital-to-analog converter (D/A)63, a photometric circuit64, a distance-measuring circuit65, a memory66and an operation unit67. The CPU48controls actuation of the electronic endoscope10. The TG49generates various timing pulses. The ID60electrically separates the electronic endoscope10from the processor device11. The CPU61controls actuation of the processor device11. The DSP62applies image processing to the digital signal to produce image data. The D/A63converts the image data produced by the DSP62into an analog signal, and outputs it to the monitor20. The photometric circuit64calculates luminance of light supplied through the light guide80in the light guide inspection mode. The distance-measuring circuit65calculates the distance “L” from the test chart43to the front face16aof the distal portion16in that mode. The memory66stores a table that lists the total number of the optical fibers81contained in the light guide80on a model of electronic endoscope10basis. The operation unit67inputs a control signal to the CPU61in response to actuation of a user. The CPU61is provided with a broken fiber number calculator68that calculates the number of broken optical fibers81in the light guide80.

Upon connecting the electronic endoscope10to the processor device11, the CPU58reads the identification data and the number of times the electronic endoscope10has been used out of the memory52, and inputs them to the CPU61via the ID60. The CPU58also increments the number which is read out of the memory52by “1”, and rewrites a new number into the memory52. The identification data is used in the light guide inspection mode. The number of times the electronic endoscope10has been used is available for the purpose of reminding a user of execution of a light guide inspection. In this embodiment, the CPU58functions as a count detector.

The CPU58drives the TG59on the basis of an operation start command from the CPU61. The TG49generates drive pulses (a vertical/horizontal scanning pulse, a reset pulse and the like) for the solid-state image sensor50and a synchronization pulse for the AFE51, and inputs them into the electronic endoscope10through the connector19. The solid-state image sensor50captures an image in response to the drive pulses from the TG59, and outputs an image signal. The CPU58also adjusts gain of the AGC56. In this embodiment, the CPU58functions as a gain adjuster.

The TG59also supplies the DSP62, the CPU61and the like through the ID60with a synchronization pulse for signal processing. The ID60is an isolator made of a photocoupler and the like. The image signal is inputted from the AFE51to the DSP62through the ID60.

The DSP62subjects the inputted image signal to color separation, color interpolation, gain correction, white balance correction, gamma correction and the like to produce image data.

The photometric circuit64and the distance-measuring circuit65operate in the light guide inspection mode. When the operation unit67is operated with the cap40attached to the distal portion16of the electronic endoscope10, the electronic endoscope10is set into the light guide inspection mode. The light transmitted through the light guide80illuminates the test chart43. During illumination, the solid-state image sensor50captures an image of the test chart43to output an image signal. The photometric circuit64receives the image signal of a single frame from the AFE51, and adds up a luminance value of every pixel in an effective area. Then, the AFE51divides a total luminance value by a total pixel number to obtain an average luminance value “Y”. The distance-measuring circuit65detects an area corresponding to the distance-measuring area43aof the test chart43from the image data produced by the DSP62, and calculates the distance “L” from the test chart43to the front face16aof the distal portion16on the basis of the size of the detected area. The distance-measuring circuit65has a function expressing the relation between the size of the distance-measuring area43aand the distance “L”, and hence obtains the distance “L” by proportional calculation.

The broken fiber number calculator68, which operates in the light guide test mode, calculates the number “N” of broken optical fibers out of the optical fibers81in accordance with the following expression (1):
N=M×(1−Y/I(R,L))  (1)
Wherein, “M” represents the total number of the optical fibers81. “Y” represents the average luminance value, and “I(R,L)” represents an ideal average luminance value.

The total number “M” of the optical fibers81is read out of the memory66in accordance with the identification data the CPU58has read. The average luminance value “Y” is detected by the photometric circuit64, as described above. The ideal average luminance value “I(R,L)” is an estimated average luminance value of the test chart43on the assumption that there are no broken optical fibers81in the light guide80(non defect state). The ideal average luminance value “I(R,L)” is a function of a damping rate “R” in the amount of light emitted from a light source70and the distance “L”. The distance “L” is an adjustment parameter for adjusting a detection error due to attachment misalignment between the cap40and the distal portion16.

When the endoscope system2is set into the light guide inspection mode, the CPU61displays the detected number “N” on the monitor20. At the same time, the CPU61issues a command to the CPU58, and adjusts the gain of the AGC56so as to eliminate a decrease “D (=I(R,L)−Y)” in the average luminance value that is caused by a shortage of light due to the broken optical fibers81, in other words, to be D=0. Also, the CPU61displays a warning message for urging the user to replace the electronic endoscope10on the monitor20, when the number “N” is a predetermined number “N0” (for example, a half of the total number “M”) or more.

In addition, the CPU61displays a message for reminding the user of the inspection of the light guide80whenever the number of times the electronic endoscope10was used, which the CPU58has read out of the memory52, coincides with predetermined numbers (for example, 100, 200, 300 . . . ). In this embodiment, the CPU61and the monitor20function as first to third notification sections.

The light source device12is constituted of a light source70such as a xenon lamp and a halogen lamp, a light source driver71for driving the light source70, an aperture stop mechanism72and a condenser lens73disposed between the light source70and the light guide80, a CPU74for controlling the light source driver71and the aperture stop mechanism72by communicating with the CPU61of the processor device11, a timer75for counting the used time of the light source70and a memory76for storing light source property data. The aperture stop mechanism72increases or decreases the amount of light incident upon the light guide80. The condense lens73condenses light passing through the aperture stop mechanism72, and leads it into an entry of the light guide80.

The memory76stores the light source property data as shown inFIG. 7. The light source property data shows the relation between a light emission amount and the used time in driving the light source70under a fixed condition. As is apparent inFIG. 7, the light emission amount of the light source70is damped with increase in the used time. The CPU74obtains the light emission amount “Q(T)” of the light source70at used time “T” counted by the timer75, and calculates a light damping rate “R (=1−Q(T)/Q(0))”.

The CPU74, as shown inFIG. 8, sets opening of the aperture stop mechanism72in accordance with the light damping rate “R”. In other words, the opening is proportional to the light damping rate “R” within the confines of 0≦R≦0.5, in such a manner that the opening is 50% at R=0 (t=0) and becomes 100% at R=0.5. On the other hand, the opening is fixed at 100% within the confines of 0.5≦R≦1. By setting the opening like this, the amount of light incident on the light guide80is kept constant at an initial value (a half of an initial light emission amount “Q(0)”) within the confines of 0≦R≦0.5, and decreases from the initial value within the confines of 0.5<R≦1.

Accordingly, the amount of light exiting from the lighting windows31is constant within the confines of 0≦R≦0.5 irrespective of the light damping rate “R”, and decreases within the confines of 0.5<R≦1 with increase in the light damping rate “R”. Thus, the following expressions (2a) and (2b) represent the ideal average luminance value “I(R,L)” with the use of the light damping rate “R” and the distance “L” as adjustment parameters.

In the case of 0≦R≦0.5:
I(R,L)=I0×(L0/L)2:  (2a)
In the case of 0.5<R≦1:
I(R,L)=2×(1−R)×I0×(L0/L)2(2b)
Wherein, “I0” represents an ideal average luminance value of the test chart43at R=0 and L=L0. The ideal average luminance value “I0” and an ideal distance “L0” have been stored on the memory66in advance. The broken fiber number calculator68reads the values from the memory66, and calculates the number “N” of broken optical fibers81by using the above expressions (1), (2a) and (2b).

Next, the operation of the endoscope system2having aforementioned structure will be described along a flowchart ofFIG. 9. The CPU58of the processor device11first detects whether or not the electronic endoscope10is connected to the processor device11. If YES, the CPU reads the identification data and the number of times the electronic endoscope10has been used out of the memory52of the electronic endoscope10, and rewrites a new number on the memory52by incrementing the read number by “1”.

Then, the CPU61judges whether or not the number of times the electronic endoscope10has been used, which is read by the CPU58, coincides with any of the predetermined numbers (for example, 100, 200, 300 . . . ). When the number coincides with the predetermined number, the message reminding the user of the light guide inspection is displayed on the monitor20(which says, for example, “This endoscope has been used a hundred times. Please execute light guide inspection.”) When the number does not coincide with the predetermined number, on the other hand, the endoscope system2is shifted into a normal mode for capturing the image of the inside body site.

During the normal mode, when the user attaches the cap40to the distal portion of the electronic endoscope10and orders the light guide inspection from the operation unit67, the CPU61starts to inspect the light guide80. In the light guide inspection mode, firstly, the CPU58drives the solid-state image sensor50, and the CPU74drives the light source70. Thus, the solid-state image sensor50captures the image of the test chart43, while the light exiting from the light guide80is applied on the test chart43.

Since the image signal outputted from the solid-state image sensor50is inputted to the processor device11through the AFE51, the photometric circuit64detects the average luminance value “Y” of the test chart43. Also, the DSP62converts the image signal into the image data, and the distance-measuring circuit65detects the distance “L” from the test chart43to the front face16aof the distal portion16.

Then, the CPU61retrieves the total number “M” of the optical fibers81contained in the light guide80on the basis of the model of the electronic endoscope10read by the CPU58. Based on the used time of the light source70read out of the light source device12, the CPU61obtains the light damping rate “R” in the light emission amount of the light source70at that point in time. The CPU74successively sets the opening of the aperture stop mechanism72based on the obtained light damping rate “R”.

The broken fiber number calculator68calculates the number “N” of broken optical fibers81on the basis of the aforementioned expressions (1), (2a) and (2b) with the use of the detected average luminance value “Y” and distance “L” of the test chart43, the total number “M” of the optical fibers81and the light damping rate “R” in the light emission amount.

The CPU61displays the number “N” of broken optical fibers81calculated by the broken fiber number calculator68on the monitor20. When the number “N” reaches or exceeds the predetermined number “N0” (for example, a half of the total number “M”), the CPU61displays the warning message on the monitor20(which says, for example, “warning: light guide has deteriorated. Please replace endoscope.”).

The CPU58, on the other hand, adjusts the gain of the AGC56so as to eliminate the decrease “D (=I(R,L)−Y)” in the average luminance value due to a broke of the optical fibers81, and returns to the normal mode.

As described above, the endoscope system2has the light guide inspection mode for inspecting the light guide80of the electronic endoscope10. When the endoscope system2is put into the light guide inspection mode in such a state that the cap40with the test chart43is attached to the distal portion16of the electronic endoscope10, the number of broken optical fibers81is detected. The user can correctly grasp the number of the broken optical fibers81anytime, and hence replace the electronic endoscope10at appropriate timing. Gain adjustment by the AGC56corrects a shortage of luminance value due to a break of the optical fibers81. Therefore, it is possible to alleviate decrease in luminance of the image of the human body cavity.

The endoscope system2also detects the number of times the electronic endoscope10has been used. Whenever the number reaches to any of the predetermined numbers, the endoscope system2reminds the user of the inspection of the light guide80. Thus, the user can carry out the inspection at appropriate timing without managing an inspection schedule.

In the foregoing embodiment, gain adjustment by the AGC56corrects the decrease “D” in the average luminance value due to a break of the optical fibers81. Instead of this, as shown inFIG. 10, adjustment in the opening of the aperture stop mechanism72(that is, adjustment in the amount of light incident on the light guide80) may correct the decrease “D” in the average luminance value, after calculating and displaying the number “N” of broken optical fibers81. After the aperture stop mechanism72has already been open 100% to its maximum and the amount of incident light cannot be increased anymore, the AGC56adjusts (increases) the gain.

In the foregoing embodiments, the amount of light emitted from the light source70decreases with the lapse of time. The present invention, however, may use another light source such as a LED whose light emission amount does not damp with time. In this case, the ideal average luminance value “I(R,L)” is independent of the light damping rate “R”.

In the foregoing embodiments, the distance-measuring area43aof the test chart43is the circular black area. However, the shape and color of the distance-measuring area43ais appropriately changeable. Also, using a mechanism that always makes the distance “L” constant in attaching the cap40to the top section16of the electronic endoscope10eliminates the need for the distance-measuring area43a. In this case, the ideal average luminance value “I(R,L)” is independent of the distance “L”. The distance-measuring area43a, however, may be used for a purpose except for distance measurement, so that it is preferable that the test chart43has the distance-measuring area43aeven if the distance “L” is always constant. For example, the distance-measuring area43ais available to detect whether or not the cap40is attached to the distal portion16of the electronic endoscope10. If the distance-measuring circuit65does not detect the distance-measuring area43a, the light guide test mode is suspended to prevent an error in the inspection.

In the foregoing embodiments, the photometric area43bof the test chart43is in another color such as 25% gray, instead of 18% gray.

In the foregoing embodiments, the message reminding the user of the inspection, the message saying the detected number “N” of the broken fibers and the warning message saying that the number “N” reaches or exceeds the predetermined number are displayed on the monitor20. However, the endoscope system2may notify the user of the messages by another way such as voice.

The cap40may be connected to the processor device11or the light source device12with a string or the like. Moreover, a cap may be integrally provided in the processor device11or the light source device12.

In the foregoing embodiment, the photometric circuit64calculates the average luminance value “Y” on the basis of the image signal inputted from the AFE51. In the present invention, the photometric circuit64may calculate the average luminance value “Y” on the basis of the image data generated by the DSP62. In this case, the DSP62generates the image data by applying the gamma correction on the image signal, so that the relation between a luminance value and a light amount is nonlinear. Thus, in this case, the distance-measuring area43bof the test chart43is preferably in a gray scale of approximately 18% gray which roughly maintains the relation between the luminance value and the light amount linear.

In the foregoing embodiments, the CPU74of the light source device12sets an initial opening value at 50% in the case of the light damping rate R=0, but the initial opening value is appropriately changeable.

In the foregoing embodiment, the solid-state image sensor50is disposed in the distal portion16of the electronic endoscope10. The solid-state image sensor50may be disposed in any section in the electronic endoscope10such as the operation section14. When the solid-state image sensor50is disposed in the operation section14, an optical system such as an optical fiber may extend in the insert section so that object light incident from the image capturing window30of the distal portion16forms an image on the light receiving surface of the solid-state image sensor50.

In the present invention, the processor device11and the light source device12may be integrated into a single case.

Although the present invention has been fully described by the way of the preferred embodiment thereof with reference to the accompanying drawings, various changes and modifications will be apparent to those having skill in this field. Therefore, unless otherwise these changes and modifications depart from the scope of the present invention, they should be construed as included therein.