Patent Description:
In an endoscope apparatus including a medical endoscope that is used to observe and treat the inside of a living body, an illumination window and an observation window are provided in a distal end of an endoscope insertion part, and illumination light is emitted from the illumination window and an observation image is acquired through the observation window. Light from, for example, a light source device such as a xenon lamp, a laser light source, and a light emitting diode (LED) is guided to the illumination window by a light guide such as an optical fiber, and is emitted from the illumination window.

An illumination optical system is disposed at a distal end portion of the light guide of the endoscope. In a case where the light guided by the light guide is emitted as it is, a range of a light emission angle is narrow and a light irradiation range is narrowed. Therefore, it has been performed that the illumination optical system is disposed at the distal end portion of the light guide to diffuse the light and expand the light irradiation range.

As such an illumination optical system, a system in which a plurality of lenses are combined, a system in which a member that diffuses light is disposed, and the like have been used.

For example, <CIT> describes an endoscope apparatus provided with a function of detecting a light transmission loss, the endoscope apparatus comprising a first light source, a first light guide member that introduces output light of the first light source and guides the light to a distal end of an insertion part which is inserted into a subject, a wavelength conversion member that is disposed in a light emission end of the first light guide member, a second light source, a second light guide member that introduces output light of the second light source and guides the light to the distal end of the insertion part, a light diffusion member that is disposed in a light emission end of the second light guide member, a light source driving unit that generates a driving signal according to a target light amount set for each of the first and second light sources and drives the first and second light sources, a temperature sensor that detects a temperature of heat generated from the wavelength conversion member and the light diffusion member, a storage unit that stores a first temperature change rate caused by the heat generated from the wavelength conversion member corresponding to a driving signal intensity of the first light source and a second temperature change rate caused by the heat generated from the light diffusion member corresponding to a driving signal intensity of the second light source, and a light transmission loss detection unit that compares a third temperature change rate of a temperature detected by the temperature sensor, and the first temperature change rate and the second temperature change rate, that determines that a light transmission loss occurs in the second light guide member in a case where the third temperature change rate matches the first temperature change rate, and that determines that a light transmission loss occurs in the first light guide member in a case where the third temperature change rate matches the second temperature change rate.

Further, <CIT> describes a light projecting unit for endoscope which is provided at a distal end portion of an endoscope and which irradiates a subject with illumination light, the light projecting unit for endoscope comprising a wavelength conversion member that absorbs a part of light having a predetermined wavelength, converts the wavelength to generate fluorescence, and transmits remaining light to emit illumination light including the light having the predetermined wavelength and the fluorescence, and a spread angle enlarging unit that scatters the illumination light emitted from the wavelength conversion member and enlarges a spread angle of the illumination light.

<CIT> describes that the spread angle enlarging unit is a member in which a filler is mixed into a resin. <CIT> discloses an illumination optical system for an endoscope utilizing a light-guiding member to direct and emit light from a source, a wavelength conversion member to modify the emitted light's wavelength, an outer sheath that encloses the wavelength conversion member with one end sealed and the other featuring an opening, a transmittance member sealing the open end, and a metallic reflective film surrounding the wavelength conversion member to reflect the converted light while being enclosed within the outer sheath. <CIT> discloses an endoscopic illumination device, which irradiates a subject with illumination light through illumination windows, and comprises a blue semiconductor light source, a red semiconductor light source, a light guide member configured to guide blue and red light to the illumination windows, and light-emitting members disposed at the light-emission end of the light guide member within the illumination windows.

<CIT> describes a lighting system including: at least one laser light source exiting laser beams; a coupling optical system for coupling the laser beams exited from the laser source, with optical fibers; a collimating optical system shaping the laser beams exited from the optical fibers, into a parallel light flux; a diffusion member disposed near a rear-side focal position of the collimating optical system and diffusing the parallel light flux to generate a secondary light source; and a condenser optical system forming an image of the secondary light source at an incidence end of a light guide at a prescribed paraxial lateral magnification.

According to the study by the inventors, it has been found that light utilization efficiency is not sufficient in a case where light guided by a light guide and emitted is diffused by using a diffusion member in which a filler is mixed into a resin, as described in <CIT>. Low light utilization efficiency causes a problem that an amount of heat generated becomes large.

Specifically, since the filler is an isotropic scattering material, light incident on the filler is diffused in all directions. Therefore, a large amount of light is scattered in a direction other than an emission direction, and the efficiency is decreased. It has also been performed that a reflection plate is provided on a surface except an emission surface and the light scattered in the direction other than the emission direction is made to be reflected, but the light strikes the filler again and is isotropically scattered. Although the light is emitted to an outside by repeating the reflection by the reflection plate and the isotropic scattering by the filler, the efficiency is deteriorated because the reflection and the scattering are repeated many times.

An object of the present invention is to solve the above-described problem based on the prior art and to provide an illumination optical system for endoscope having high light utilization efficiency.

In order to achieve the object, the present invention provides an illumination optical system for endoscope comprising the features of independent claim <NUM>. Further embodiments are provided by the dependent claims.

According to the present invention, it is possible to provide an illumination optical system for endoscope having high light utilization efficiency.

Hereinafter, an illumination optical system for endoscope of an embodiment of the present invention will be described in detail on the basis of a preferred embodiment shown in the accompanying drawings.

It should be noted that the drawings described below are examples illustrating the present invention, and the present invention is not limited to the drawings shown below.

In the following, "to" indicating the numerical range includes the numerical values described on both sides thereof. For example, in a case where ε is a numerical value α to a numerical value β, the range of ε is a range including the numerical value α and the numerical value β, and is represented by α ≤ ε ≤ β in mathematical symbols.

In addition, "whole surface" and the like include an error range generally allowed in the relevant technical field.

First, an endoscope system having an endoscope having the illumination optical system for endoscope of the embodiment of the present invention will be described.

<FIG> is a schematic view showing an example of the endoscope system having the endoscope having the illumination optical system for endoscope of the embodiment of the present invention.

An endoscope system <NUM> irradiates an observation site in a living body (inside a subject), which is an observation target, with illumination light, picks up an image of the observation site, generates a display image of the observation site on the basis of an image signal obtained by the image pick-up, and displays the display image.

The endoscope system <NUM> has the same configuration as a conventionally known endoscope system except that the endoscope system <NUM> has the illumination optical system for endoscope of the embodiment of the present invention.

As shown in <FIG>, the endoscope system <NUM> comprises an endoscope <NUM> that picks up an image of an observation site in a living body (inside a subject), which is an observation target, a processor device <NUM> that generates a display image of the observation site on the basis of an image signal obtained by the image pick-up, a light source device for endoscope (hereinafter, simply referred to as a light source device) <NUM> that supplies illumination light with which the observation site is irradiated, to the endoscope <NUM>, and a monitor <NUM> that displays the display image. An operational input unit <NUM>, such as a keyboard and a mouse, is connected to the processor device <NUM>.

The endoscope <NUM> comprises an insertion part 12a that is inserted into the subject, such as the inside of a body of a patient, and an operation part 12b provided at a proximal end portion of the insertion part 12a. In the endoscope <NUM>, a side of the insertion part 12a is a side of the distal end. The operation part 12b of the endoscope <NUM> is connected to the processor device <NUM> via a signal line <NUM>. The endoscope <NUM> is, for example, a forward-viewing type rigid endoscope, such as a laparoscope.

The processor device <NUM> receives the image signal output from an image pick-up unit of the endoscope <NUM> via the signal line <NUM>, generates a video signal, and outputs the video signal to the monitor <NUM>. By doing so, the display image of the observation site, such as the inside of the body, is displayed on the monitor <NUM>.

The operation part 12b of the endoscope <NUM> is connected to the light source device <NUM> via a light guide <NUM>. Light from the light source device <NUM> is supplied to the light guide <NUM>, and the light is emitted from a distal end of the endoscope <NUM>.

<FIG> shows an enlarged perspective view of a distal end portion 12d of the endoscope <NUM>.

As shown in <FIG>, in the distal end portion 12d (distal end surface) of the endoscope <NUM>, as an example, an observation window <NUM> of an image pick-up optical system, an illumination window <NUM> of an illumination optical system, a forcep channel <NUM>, and an air and water supply channel that communicates with an air and water supply nozzle <NUM> are disposed. In the example shown in <FIG>, two illumination windows <NUM> are provided, and the two illumination windows <NUM> are disposed on both sides of the observation window <NUM> interposed therebetween.

The illumination optical system for endoscope of the embodiment of the present invention is disposed in such an illumination window <NUM>, propagates the light guided by the light guide <NUM>, and emits the light from the illumination window.

<FIG> shows a side view schematically showing an illumination unit having the light guide and the illumination optical system for endoscope of the embodiment of the present invention.

An illumination unit <NUM> shown in <FIG> has the light guide <NUM> and an illumination optical system for endoscope <NUM> disposed on an end surface of the light guide <NUM>.

The light guide <NUM> is a light transmission member, and is formed of, for example, a bundle of a plurality of optical fiber strands. As described above, the light guide <NUM> guides the light supplied from the light source device <NUM> and emits the light from the end surface of the light guide <NUM>. Since the illumination optical system for endoscope <NUM> is disposed on the end surface of the light guide <NUM>, the light emitted from the end surface of the light guide <NUM> is incident on the illumination optical system for endoscope <NUM>.

<FIG> shows a top view of the illumination unit <NUM> of <FIG> as viewed from an a direction, and <FIG> shows a cross-sectional view taken along a line B-B of <FIG>.

As shown in <FIG>, the illumination optical system for endoscope <NUM> has a diffusion plate <NUM>.

The diffusion plate <NUM> is used to diffuse the light emitted from the end surface of the light guide <NUM> and to widen a light irradiation range.

As shown in <FIG>, the diffusion plate <NUM> has a light guide unit <NUM> and a diffusion surface <NUM>.

The diffusion surface <NUM> is formed on a surface of the diffusion plate <NUM> on a side in contact with the light guide <NUM>. The diffusion surface <NUM> consists of a holographic diffusion plate, and diffuses and transmits the light emitted from the end surface of the light guide <NUM>.

The holographic diffusion plate has a surface-uneven structure formed so that the reproduced light becomes diffused light that is diffused in any angle range. As an example, the shape is as shown in Fig. <NUM> (a) of Holographic diffuser by use of a silver halide sensitized gelatin process, <NPL>.

With the holographic diffusion plate having such a surface uneven structure, the light emitted from the light guide <NUM> is diffused in a predetermined angle range.

In the present invention, the light guide unit <NUM> is used as a substrate and a holographic diffusion plate is formed on the light guide unit <NUM>.

A method for forming the holographic diffusion plate is not particularly limited, and a conventionally known forming method can be used.

For example, the sol-gel method can be used to form the holographic diffusion plate. Specifically, a solution (sol) containing SiO<NUM>, which is a material of the holographic diffusion plate, is prepared, applied on the substrate, and then gelled, and in a state in which a master (mold) to which the surface uneven structure designed by the computer-generated hologram can be transferred is pressed against the gelled coating film, the coating film is heated and cured, so that the holographic diffusion plate can be formed.

Glass can be used as the substrate for the holographic diffusion plate. The material of the holographic diffusion plate is a composite material including glass.

The light diffused on the diffusion surface <NUM> is incident on the light guide unit <NUM>.

The light guide unit <NUM> is a plate-shaped member that guides the light diffused on the diffusion surface <NUM> and that emits the light from an emission surface 26a on a side opposite to a surface on which the diffusion surface <NUM> is formed. The light guide unit <NUM> consists of sapphire glass having a high refractive index, and when light is emitted from the emission surface 26a, refracts the light at the interface due to a difference in refractive index from the outside to further widen the light irradiation range. Specifically, the refractive index of sapphire glass is about <NUM>. For example, the emission surface 26a is in contact with an air layer, and when light is emitted from the emission surface 26a, the light is refracted at the interface with the air layer due to a relationship with the refractive index of air (n = <NUM>).

Further, the sapphire glass has a Vickers hardness of about <NUM> GPa, which is very hard. Therefore, the thickness of the light guide unit <NUM> can be reduced.

The thickness of the diffusion plate <NUM> (the light guide unit <NUM> + the diffusion surface <NUM>) is preferably <NUM> to <NUM>, more preferably <NUM> to <NUM>, and even more preferably <NUM> to <NUM>. Since the thickness of the diffusion surface <NUM> (holographic diffusion plate) is very thin, the thickness of the light guide unit <NUM> can be approximated to the thickness of the diffusion plate <NUM>.

In the example shown in <FIG>, the end surface of the light guide <NUM> has a circular shape when viewed from a direction perpendicular to the emission surface 26a of the light guide unit <NUM> of the diffusion plate <NUM> on a side opposite to the diffusion surface <NUM>, that is, when viewed from an a direction in <FIG>. Further, the emission surface 26a when viewed from the direction perpendicular to the emission surface 26a also has a circular shape. Further, the center of the end surface of the light guide <NUM> and the center of the emission surface 26a, when viewed from the direction perpendicular to the emission surface 26a are disposed so as to match each other.

Further, the diameter φ<NUM> of the emission surface 26a is larger than the diameter φ<NUM> of the end surface of the light guide <NUM>.

Therefore, in the example shown in <FIG>, the emission surface 26a includes the end surface of the light guide <NUM> when viewed from the direction perpendicular to the emission surface 26a.

Here, in the present invention, in a case where a shortest distance between the edge side of the emission surface 26a and the edge side of the end surface of the light guide <NUM>, in the in-plane direction of the emission surface 26a, is denoted by L, and the thickness of the diffusion plate <NUM> in the direction perpendicular to the emission surface 26a is denoted by t, it is preferable that t/L ≤ <NUM> is satisfied. In the example shown in <FIG>, the shortest distance L is (φ<NUM> - φ<NUM>)/<NUM>.

The action of the illumination optical system for endoscope having such a configuration will be described with reference to <FIG>.

First, a case where light is incident on the diffusion plate <NUM> perpendicularly to the surface of the diffusion surface <NUM> from a side of the diffusion surface <NUM> is considered. As shown in <FIG>, the incident light is diffused by the diffusion surface <NUM> and is guided while spreading in the light guide unit <NUM>. <FIG> shows the diffused light and the angular distribution in the light guide unit <NUM>. In the distribution shown in <FIG>, the light intensity (relative intensity) in a direction of an angle of <NUM>° (vertical direction) is highest, and the intensity is decreased as the angle is increased. In <FIG>, as an example, a full angle at half maximum of the intensity of the light diffused on the diffusion surface <NUM> is <NUM>°.

In a case where the light diffused on the diffusion surface <NUM> is emitted from the light guide unit <NUM>, the light is refracted in response to the difference in refractive index from the outside (air), and is emitted. Therefore, as shown in <FIG>, the light is emitted with a further spread. <FIG> shows the angular distribution of the light emitted from the light guide unit <NUM>. In the examples shown in <FIG> and <FIG>, the full angle at half maximum of the intensity of the light emitted from the light guide unit <NUM> is <NUM>°.

In this way, the diffusion plate <NUM> can emit light having an angular distribution due to the light diffusion on the diffusion surface <NUM> and the refraction when light is emitted from the light guide unit <NUM>.

Next, the light emitted from the light guide will be described. As shown in <FIG>, the light emitted from the light guide <NUM> is emitted at a predetermined light distribution angle. For example, in a case where light is emitted into the air (n = <NUM>), the light distribution angle of the light emitted from the light guide is <NUM>°.

Here, a case where the end surface of the light guide <NUM> has layers having different refractive indexes is considered. For example, as shown in <FIG>, in a case where a first layer <NUM> (corresponding to a diffusion surface <NUM> having no diffusivity) having a refractive index of <NUM> is disposed on the end surface of the light guide <NUM>, and a second layer <NUM> (corresponding to the light guide unit <NUM>) having a refractive index of <NUM> is disposed on the surface of the first layer <NUM>, light emitted from the end surface of the light guide becomes light having a light distribution angle of <NUM>° due to the difference in refractive index between the light guide <NUM> and the first layer <NUM>, and is guided into the first layer <NUM> and reaches the second layer <NUM>. In a case where the light is incident on the second layer <NUM> from the first layer <NUM>, the light becomes light having a light distribution angle of <NUM>° due to the difference in refractive index between the first layer <NUM> and the second layer <NUM>, and is guided into the second layer <NUM> and is emitted from the surface of the second layer <NUM>. In a case where the light is emitted from the surface of the second layer <NUM>, the light becomes light having a light distribution angle of <NUM>° due to the difference in refractive index between the second layer <NUM> and the air, and is emitted. That is, the light distribution angle of the light that is emitted into the air is the same as the light distribution angle of the light that is directly emitted from the light guide even in a case where another layer is provided between the end surface of the light guide <NUM> and the air layer.

Since the diffusion surface <NUM> (holographic diffusion plate) consists of a composite material containing glass, the refractive index is about <NUM>.

As in the present invention, in a case where the diffusion plate <NUM> having a function of diffusing light is disposed on the end surface of the light guide <NUM>, as shown in <FIG>, the above-described alignment characteristics of the light guide <NUM> and the scattering characteristics of the diffusion plate <NUM> are integrated to determine the light distribution characteristics of the emitted light.

As described above, with the illumination optical system for endoscope of the embodiment of the present invention using the diffusion plate <NUM> having the diffusion surface <NUM> and the light guide unit <NUM>, the irradiation range of the emitted light can be made wider than the light distribution angle of the light that is directly emitted from the light guide.

In the example shown in <FIG> and the like, the spread angle of the light caused by diffusion is <NUM>°, but as shown in <FIG>, the angle is a full angle at half maximum, and light irradiation can actually be performed in a wider angle range. For example, in the example shown in <FIG>, the angle at which the light intensity is about <NUM>% is about ± <NUM>°. Therefore, in a case where the light distribution characteristics of the light guide <NUM> are combined, irradiation with light having an intensity of <NUM>% or more can be performed in a range of about <NUM>°.

Here, as shown in <FIG>, the maximum emission angle θ<NUM> of the light emitted from the distal end of the endoscope need only be about <NUM>°.

The incident angle θ<NUM> in a case where the emission angle θ<NUM> of the light emitted from the illumination optical system for endoscope of the embodiment of the present invention is <NUM>°, is calculated as θ<NUM> = <NUM>° from the refractive index n<NUM> = <NUM> of the sapphire glass and the refractive index n<NUM> = <NUM> of the air layer.

As shown by L<NUM> in <FIG>, in a case where the light L<NUM> emitted from a position of the edge side of the end surface of the light guide <NUM> and incident on the diffusion plate <NUM> is emitted at a position of the edge side of the emission surface 26a, t/L = <NUM>/tan θ<NUM> is satisfied. As described above, the incident angle θ<NUM> at which the emission angle θ<NUM> of the light emitted from the emission surface 26a is <NUM>° is <NUM>°, so that t/L is <NUM>.

In a case where t/L is larger than <NUM>, the light that is guided into the light guide unit <NUM> reaches the side surface of the light guide unit <NUM>. Since the side surface of the light guide unit <NUM> is bonded to the distal end portion of the insertion part of the endoscope by, for example, an adhesive <NUM> or brazing, the light that has reached the side surface of the light guide unit <NUM> is not totally reflected and is absorbed by the adhesive <NUM> or the like. Therefore, the amount of light emitted from the emission surface 26a is decreased, and the light utilization efficiency is decreased.

On the other hand, in the present invention, with t/L ≤ <NUM> satisfied, it is possible to prevent the light incident on the diffusion plate <NUM> from the light guide <NUM> from reaching the side surface of the light guide unit <NUM>, and from being absorbed on the side surface by the adhesive <NUM> or the like, so that it is possible to prevent the decrease in light utilization efficiency. In addition, since the light utilization efficiency can be increased, it is possible to restrain the increase in the amount of heat generated.

Here, in a case where the diameter φ2 of the light guide <NUM> is fixed, it is necessary that the thickness t of the diffusion plate <NUM> is reduced and/or the diameter φ<NUM> of the emission surface 26a of the diffusion plate <NUM> is increased.

Since it is desired that the distal end portion of the endoscope is made thin, it is difficult to increase the diameter φ<NUM> of the emission surface 26a of the diffusion plate <NUM>. Meanwhile, in a case where the thickness t of the diffusion plate <NUM> is reduced, durability such as cracking of the diffusion plate <NUM> becomes a problem.

In response to this, in the present invention, sapphire glass having high hardness is used for the light guide unit <NUM> of the diffusion plate <NUM>, so that cracking can be restrained even in a case where the thickness t is reduced. Therefore, the diffusion plate <NUM> can be made smaller and thinner. Further, with the light guide unit <NUM> made thinner, it is possible to restrain the decrease in utilization efficiency caused by the occurrence of light loss in the light guide unit <NUM>.

In a case where the end surface of the light guide <NUM> has a circular shape, the diameter φ<NUM> of the end surface is preferably <NUM> to <NUM> from the viewpoint of ensuring the flexibility of the endoscope insertion part by which light from the light source device can be guided with high efficiency.

The light distribution angle of the light emitted from the end surface of the light guide <NUM> is preferably <NUM>° or more.

Here, the light distribution angle is the spread angle of the light when the light is emitted into the air from the end surface of the light guide <NUM>.

The diffusion angle at half maximum (full angle at half maximum of the diffused light) of the diffusion plate <NUM> is preferably <NUM>° or more.

Here, the diffusion angle at half maximum is the full angle at half maximum of the diffused light described with reference to <FIG>.

Here, in the examples shown in <FIG>, the end surface of the light guide <NUM> has a circular shape, but the present invention is not limited thereto. The shape of the end surface of the light guide <NUM> can be any shape such as an elliptical shape, a polygonal shape, and an amorphous shape.

Similarly, the emission surface 26a of the diffusion plate <NUM> has a circular shape, but the present invention is not limited thereto. The shape of the emission surface 26a of the diffusion plate <NUM> can be any shape such as an elliptical shape, a polygonal shape, and an amorphous shape.

In the examples shown in <FIG>, the shape of the end surface of the light guide <NUM> and the shape of the emission surface 26a of the diffusion plate <NUM> are similar to each other, but the present invention is not limited thereto, and the shapes may be different from each other.

Further, in the examples shown in <FIG>, the center of the end surface of the light guide <NUM> and the center of the emission surface 26a of the diffusion plate <NUM> are disposed so as to match each other in the plane direction, but the present invention is not limited thereto, and the center of the end surface of the light guide <NUM> and the center of the emission surface 26a of the diffusion plate <NUM> may deviate from each other.

For example, <FIG> show an example of an illumination unit having another example of the illumination optical system for endoscope of the embodiment of the present invention.

<FIG> is a top view of the illumination unit, and <FIG> is a cross-sectional view taken along a line C-C of <FIG>.

In the example shown in <FIG>, each of the end surface of the light guide <NUM> and the emission surface 26a of the diffusion plate <NUM> has an amorphous shape. Further, the end surface of the light guide <NUM> and the emission surface 26a of the diffusion plate <NUM> have a non-similar shape to each other. Further, the center of the end surface of the light guide <NUM> and the center of the emission surface 26a of the diffusion plate <NUM> do not match each other. Further, in the example shown in <FIG>, the emission surface 26a includes the end surface of the light guide <NUM> when viewed from the direction perpendicular to the emission surface 26a.

In the case of the configuration shown in <FIG>, the distance between the edge side of the emission surface 26a and the edge side of the end surface of the light guide <NUM>, in the in-plane direction of the emission surface 26a, differs depending on the position, but in the present invention, a distance at a position where the distance between the edge side of the emission surface 26a and the edge side of the end surface of the light guide <NUM> is shortest, and in the examples shown in <FIG>, a distance at a position where the distance is Lmin need only be set as the shortest distance L.

Although the illumination optical system for endoscope of the embodiment of the present invention has been described in detail above, the present invention is not limited to the above-described embodiment, and needless to say, various modifications or changes may be made without departing from the gist of the present invention.

Hereinafter, the present invention will be described in more detail with reference to specific examples of the present invention.

An illumination unit having an illumination optical system for endoscope as shown in <FIG> was produced.

As the light guide, a fiber bundle having a length of <NUM> and a diameter of <NUM> was used.

The light distribution angle of the light guide in a case where an end part of the light guide is in contact with the air is <NUM>°.

Sapphire glass having a thickness of <NUM> and a diameter of <NUM> was used as a substrate, a holographic diffusion plate was produced on the sapphire glass by the above-described method, and a diffusion plate having a diffusion surface and a light guide unit was produced.

The holographic diffusion plate was designed so that light incident from the light guide at the above-described light distribution angle is diffused into an angle range of <NUM>° (full angle at half maximum).

The produced diffusion plate was disposed on the end surface of the light guide such that the diffusion surface of the diffusion plate faces the end surface of the light guide. The center of the emission surface of the diffusion plate and the center of the light guide were made to match each other. As described above, an illumination optical system for endoscope was produced.

Since L is <NUM> and t is <NUM> (<NUM>), t/L is <NUM>.

The light utilization efficiency of the produced illumination unit was evaluated.

<FIG> shows a schematic view of an evaluation system, not forming part of the claimed invention. The light guide <NUM> was connected to a xenon light source <NUM>, the distal end of a diffusion plate <NUM> side was set in the light distribution measuring device, and the light utilization efficiency and the emission angular distribution were evaluated.

As a result, it was confirmed that the light utilization efficiency is high and the light irradiation angle is widened.

Claim 1:
An illumination optical system for endoscope (<NUM>) which is provided in contact with an end surface of a light guide (<NUM>) at a distal end portion of an insertion part of an endoscope (<NUM>), the illumination optical system for endoscope (<NUM>) comprising:
a diffusion plate (<NUM>) that is provided on the end surface of the light guide (<NUM>) and diffuses light from the light guide,
wherein the diffusion plate (<NUM>) has a diffusion surface (<NUM>) that is formed on a surface on a side of the light guide and a light guide unit (<NUM>) that guides light diffused on the diffusion surface (<NUM>),
the diffusion surface (<NUM>) consists of a holographic diffusion plate that has a surface -uneven structure,
the light guide unit (<NUM>) consists of sapphire glass,
wherein an emission surface (26a) of the light guide unit (<NUM>) on a side opposite to the diffusion surface (<NUM>) includes the end surface of the light guide (<NUM>) when viewed from a direction perpendicular to a surface of the light guide unit (<NUM>) of the diffusion plate on the side opposite to the diffusion surface (<NUM>), and
in a case where a shortest distance between an edge side of the emission surface (26a) and an edge side of the end surface of the light guide, in an in-plane direction of the emission surface (26a), is denoted by L, and
a thickness of the diffusion plate (<NUM>) in a direction perpendicular to the emission surface (26a) is denoted by t,
t/L ≤ <NUM> is satisfied.