Projection optical system and image projection device

Abstract

The present disclosure is a projection optical system having therein an intermediate imaging position that is conjugate to a reduction conjugate point on a reduction side and an enlargement conjugate point on an enlargement side, the projection optical system comprising a first sub-optical system and a second sub-optical system disposed on the enlargement side from the first sub-optical system. The first sub-optical system includes a plurality of lenses L1-L11 and a stop ST. The second sub-optical system includes a prism PM. The prism PM has a first transmission surface T1 positioned closest to the first sub-optical system on an optical path between the first sub-optical system and the enlargement conjugate point, a second transmission surface T2 positioned closest to the enlargement conjugate point, and a reflection surface group including a second reflection surface R2 positioned closest to the second transmission surface T2 on an optical path between the first transmission surface T1 and the second transmission surface T2. The intermediate imaging position is inside the prism. A dielectric multilayer film not including a metal layer is formed on the second reflection surface R2.

Description

Projection optical system and image projection device

[0001] The present disclosure relates to a projection optical system using a prism, and also to an image projection device using such a projection optical system.

[0002] Patent documents 1 and 2 disclose optical systems that use a prism to enable short-focus and large-screen projection, and mention providing a metal reflective film, a dielectric multilayer film, or a composite film of a metal and a dielectric multilayer film on the reflective surface of the prism.

[0003] Patent No. 6605635 Patent No. 4331290

[0004] The present disclosure provides a projection optical system that enables short-focus, large-screen projection and reduces color unevenness and drift in images, and also provides an image projection device that uses such a projection optical system.

[0005] One aspect of the present disclosure is a projection optical system having a reduction conjugate point on the reduction side and an enlargement conjugate point on the enlargement side, and having an intermediate imaging position therein that is conjugate to the reduction conjugate point and the enlargement conjugate point, respectively, the projection optical system comprising: a first sub-optical system; and a second sub-optical system arranged on the enlargement side of the first sub-optical system, the first sub-optical system including a plurality of lenses, the second sub-optical system including a prism formed of a transparent medium, the prism having a group of reflecting surfaces including a first transmitting surface located closest to the first sub-optical system on an optical path between the first sub-optical system and the enlargement conjugate point, a second transmitting surface located closest to the enlargement conjugate point, and a second reflecting surface located closest to the second transmitting surface on the optical path between the first transmitting surface and the second transmitting surface, all or a part of the intermediate imaging position is located inside the prism, and a dielectric multilayer film that does not include a metal layer is formed on the second reflecting surface.

[0006] Another aspect of the present disclosure is a projection optical system having a reduction conjugate point on the reduction side and an enlargement conjugate point on the enlargement side, and having an intermediate imaging position therein that is conjugate to the reduction conjugate point and the enlargement conjugate point, respectively, the projection optical system comprising: a first sub-optical system; and a second sub-optical system arranged on the enlargement side of the first sub-optical system, wherein the first sub-optical system includes a plurality of lenses, and the second sub-optical system includes a prism formed of a transparent medium, the prism having a group of reflecting surfaces including a first transmitting surface located closest to the first sub-optical system on an optical path between the first sub-optical system and the enlargement conjugate point, a second transmitting surface located closest to the enlargement conjugate point, and a second reflecting surface located closest to the second transmitting surface on the optical path between the first transmitting surface and the second transmitting surface, all or a part of the intermediate imaging position is located inside the prism, and the second reflecting surface is formed with a coating layer that reflects both a first light ray having an incident angle that results in total reflection and a second light ray having an incident angle that does not result in total reflection.

[0007] In addition, an image projection device according to another aspect of the present disclosure includes the above-described projection optical system, an image forming element that generates an image to be projected onto a screen via the projection optical system, and a light source that supplies light to the image forming element.

[0008] The projection optical system according to the present disclosure enables short-focus and large-screen projection, and in particular reduces color unevenness in images projected onto a screen, and further suppresses APL (Average Picture Level: average brightness) drift.

[0009] Layout diagram showing the optical system 1 according to Example 1; Layout diagram showing the optical system 1 according to Example 2; Layout diagram showing the optical system 1 according to Example 3; Illustrative diagram showing the reflection of light inside a prism PM having two reflecting surfaces R1 and R2; Reflection coating structure (comparative example) having a reflection-enhancing coating FB and a metal reflecting film FA formed thereon, and its reflection spectral characteristics; Reflection coating structure having a dielectric multilayer film FM without a metal layer formed thereon, and its reflection spectral characteristics; Graphs showing an example of the reflection spectral characteristics of a typical dielectric multilayer film; Fig. 8(A) is a graph showing an example of the reflection spectral characteristics of another typical dielectric multilayer film; Fig. 8(B) is a graph showing an example of the emission spectral characteristics of a light source used in a projection device; Fig. 8(C) is a definition of the valley of a ripple that appears in the reflection spectrum; Illustrative diagram showing the shape of the footprint on the first reflecting surface R1 and the second reflecting surface R2 according to Examples 1 to 3; Fig. 10(A) is the reflection spectral characteristics (incident angle: 0°) of a dielectric multilayer film (Table 4, 64 layers) formed on glass KVC80; Figure 10(B) shows the reflection spectrum characteristics of the same dielectric multilayer film (incident angle: (critical angle -5°) to (critical angle -1°)). Figure 10(C) shows the reflection spectrum characteristics of the same dielectric multilayer film (incident angle: critical angle 36.2°). Figure 11(A) shows the reflection spectrum characteristics of a dielectric multilayer film (Table 5, 54 layers) formed on glass KVC80 (incident angle: 0°). Figure 11(B) shows the reflection spectrum characteristics of the same dielectric multilayer film (incident angle: (critical angle -5°) to (critical angle -1°)). Figure 11(C) shows the reflection spectrum characteristics of the same dielectric multilayer film (incident angle: critical angle 36.2°). Figure 12(A) shows the reflection spectrum characteristics of a dielectric multilayer film (Table 6, 84 layers) formed on glass KVC80 (incident angle: 0°). Figure 12(B) shows the reflection spectrum characteristics of the same dielectric multilayer film (incident angle: (critical angle -5°) to (critical angle -1°)). Figure 12(C) shows the reflection spectrum characteristics of the same dielectric multilayer film (incident angle: critical angle 36.2°). Figure 13(A) shows the reflection spectrum characteristics of a dielectric multilayer film (Table 10, 64 layers) formed on glass KSKLD5 (incident angle: 0°). Figure 13(B) shows the reflection spectrum characteristics of the same dielectric multilayer film (incident angle: (critical angle -5°) to (critical angle -1°)). Figure 13(C) shows the reflection spectrum characteristics of the same dielectric multilayer film (incident angle: critical angle 39.0°).FIG. 14(A) shows the reflection spectrum characteristics of a dielectric multilayer film (Table 11, 54 layers) formed on glass KSKLD5 (incident angle: 0°). FIG. 14(B) shows the reflection spectrum characteristics of the same dielectric multilayer film (incident angle: (critical angle -5°) to (critical angle -1°)). FIG. 14(C) shows the reflection spectrum characteristics of the same dielectric multilayer film (incident angle: critical angle 39.0°). FIG. 15(A) shows the reflection spectrum characteristics of a dielectric multilayer film (Table 12, 84 layers) formed on glass KSKLD5 (incident angle: 0°). FIG. 15(B) shows the reflection spectrum characteristics of the same dielectric multilayer film (incident angle: (critical angle -5°) to (critical angle -1°)). FIG. 15(C) shows the reflection spectrum characteristics of the same dielectric multilayer film (incident angle: critical angle 39.0°). Block diagram showing an example of an image projection device according to the present disclosure.

[0010] Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. However, more detailed explanation than necessary may be omitted. For example, detailed explanation of well-known matters or redundant explanation of substantially the same configuration may be omitted. This is to avoid unnecessary redundancy in the following explanation and to facilitate understanding by those skilled in the art.

[0011] The applicant provides the accompanying drawings and the following description to enable those skilled in the art to fully understand the present disclosure, and does not intend for them to limit the subject matter described in the claims.

[0012] Below, various embodiments of the optical system according to the present disclosure will be described. In each embodiment, a case will be described in which the optical system is used in a projector (an example of an image projection device) that projects image light of an original image SA, which is obtained by spatially modulating incident light using an image forming element such as a liquid crystal display or a DMD (digital micromirror device) based on an image signal, onto a screen. That is, the optical system according to the present disclosure can be used to enlarge and project onto the screen an original image SA on an image forming element arranged on the reduction side by placing a screen (not shown) on an extension of the enlargement side. However, the projection surface is not limited to a screen. Projection surfaces also include walls, ceilings, floors, and windows inside homes, stores, and vehicles and aircraft used for transportation.

[0013] First Embodiment An optical system according to a first embodiment of the present disclosure will be described below with reference to FIGS.

[0014] (Example 1) FIG. 1 is a layout diagram showing an optical system 1 according to Example 1, where FIG. 1(A) is a side view showing the YZ plane and FIG. 1(B) is a top view showing the XZ plane. The optical system 1 includes a first sub-optical system including a plurality of lens elements and an aperture stop ST, and a second sub-optical system including a prism PM. In FIG. 1(A), a reduction conjugate point, which is an image formation position on the reduction side, is located to the left of the optical axis OA, and a magnification conjugate point, which is an image formation position on the magnification side, is located to the upper left of the optical axis OA. The second sub-optical system is located on the magnification side of the first sub-optical system on the optical path. When the optical system 1 is used in an image projection device such as a projector, an image forming element is installed at the reduction conjugate point, and a screen is installed at the magnification conjugate point.

[0015] Furthermore, an intermediate imaging position that is conjugate to each of the reduction conjugate point and the enlargement conjugate point is located inside the optical system 1. As shown by the dashed lines in Figure 1, both the Y-direction intermediate image IMy and the X-direction intermediate image IMx exist at this intermediate imaging position inside the prism PM. Furthermore, the entire intermediate imaging position exists inside the prism PM.

[0016] The first sub-optical system includes, in order from the reduction side to the magnification side, an optical element PA and lens elements L1 to L11. The optical element PA represents an optical element such as a TIR (total internal reflection) prism, a prism for color separation or color synthesis, an optical filter, a parallel plate glass, a quartz low-pass filter, or an infrared cut filter. A reduction conjugate point is set at a position a predetermined distance from the reduction-side end face of the optical element PA, and the original image SA is placed here. In Example 1, this predetermined distance is zero, and the original image SA is located directly at the reduction-side end face of the optical element PA.

[0017] The optical element PA has two parallel, flat transmitting surfaces (S1, S2). For the surface numbers, refer to the numerical examples described later. Lens element L1 has a biconvex shape (S3, S4). Lens element L2 has a negative meniscus shape with a convex surface facing the reduction side (S5, S6). Lens element L3 has a biconvex shape (S7, S8). Lens element L4 has a negative meniscus shape with a convex surface facing the reduction side (S9, S10). Lens element L5 has a biconvex shape (S10, S11). Lens elements L4 and L5 are cemented together to form a compound lens. Lens element L6 has a negative meniscus shape with a convex surface facing the reduction side (S12, S13). Lens element L7 has a biconvex shape (S15, S16). Lens element L8 has a positive meniscus shape with a convex surface facing the reduction side (S17, S18). Lens element L9 has a negative meniscus shape with a convex surface facing the magnification side (S19, S20). Lens element L10 has a biconcave shape (S21, S22). Lens element L11 has a negative meniscus shape with a convex surface facing the magnification side (S23, S24). These lens elements L1 to L110 are rotationally symmetric lenses having surface shapes that are rotationally symmetric about the optical axis OA of the first sub-optical system, and portions through which light rays do not pass may be removed as necessary.

[0018] The aperture stop ST defines the range through which the light beam passes through the optical system 1, and is positioned between the reduction conjugate point and the intermediate image position described above. As an example, the aperture stop ST is located between the lens element L6 and the lens element L7 (S14).

[0019] The second sub-optical system includes a prism PM made of a transparent medium, such as glass or synthetic resin. The prism PM has a plurality of optical surfaces: a first transmitting surface T1 located closest to the first sub-optical system on the optical path between the first sub-optical system and the magnification conjugate point; a second transmitting surface T2 located closest to the magnification conjugate point; and a first reflecting surface R1 located closest to the first transmitting surface T1 and a second reflecting surface R2 located closest to the second transmitting surface T2 on the optical path between the first transmitting surface T1 and the second transmitting surface T2. The first reflecting surface R1 and the second reflecting surface R2 form a reflecting surface group. The first transmitting surface T1 has a free-form surface shape with a convex surface facing the magnification side (S25). The first reflecting surface R1 has a free-form surface shape with a concave surface facing in the direction in which light rays incident on the first reflecting surface R1 are reflected (S26). The second reflecting surface R2 has a free-form shape with a convex surface facing in the direction in which the light ray incident on the second reflecting surface R2 is reflected (S27). The second transmitting surface T2 has a free-form shape with a convex surface facing the magnification side (S28).

[0020] Regarding the three-dimensional shape of the prism PM, for example, the first transmitting surface T1 is curved so that the concave surface faces in the -Z direction, the second transmitting surface T2 has a shape like a partial dome that covers the other optical surfaces from above, the first reflecting surface R1 faces the first transmitting surface T1, and the second reflecting surface R2 faces the second transmitting surface T2.

[0021] Because the prism PM integrates the first transmitting surface T1, the second transmitting surface T2, the first reflecting surface R1, and the second reflecting surface R2, assembly adjustments between optical components can be reduced, resulting in lower costs. Furthermore, the optical surfaces of the prism PM that have power, such as the first transmitting surface T1, the second transmitting surface T2, and the first reflecting surface R1, are formed as free-form surfaces that do not have an axis of rotational symmetry, i.e., have different curvatures along the X and Y axes. Using free-form surfaces that can define different curvatures along the X and Y axes for the optical surfaces of the prism increases the degree of freedom in correcting distortion, which can also be expected to shorten the overall length of the first sub-optical system.

[0022] A dielectric multilayer film that does not include a metal layer is formed on both the first and second reflecting surfaces R1 and R2 of the prism PM, or on only the second reflecting surface R2. Specific examples of this dielectric multilayer film will be described later.

[0023] Example 2 FIG. 2 is a layout diagram showing an optical system 1 according to Example 2, where FIG. 2(A) is a side view showing the YZ plane and FIG. 2(B) is a top view showing the XZ plane. This optical system 1 has a configuration similar to that of Example 1, and therefore a description that overlaps with Example 1 will be omitted. The optical system 1 includes a first sub-optical system including a plurality of lens elements and an aperture stop ST, and a second sub-optical system including a prism PM. In FIG. 2(A), a reduction conjugate point, which is an image formation position on the reduction side, is located to the left of the optical axis OA, and a magnification conjugate point, which is an image formation position on the magnification side, is located to the upper left of the optical axis OA. The second sub-optical system is located on the magnification side of the first sub-optical system on the optical path.

[0024] Furthermore, an intermediate imaging position that is conjugate to each of the reduction conjugate point and the enlargement conjugate point is located inside the optical system 1. As in Figure 1, both the Y-direction intermediate image IMy and the X-direction intermediate image IMx exist at this intermediate imaging position inside the prism PM. Furthermore, the entire intermediate imaging position exists inside the prism PM.

[0025] The first sub-optical system includes, in order from the reduction side to the enlargement side, an optical element PA and lens elements L1 to L7. A reduction conjugate point is set at a position a predetermined distance from the reduction-side end face of the optical element PA, and the original image SA is placed at this point.

[0026] The optical element PA has two parallel, flat transmitting surfaces (S1, S2). For the surface numbers, see the numerical examples described later. The lens element L1 has a positive meniscus shape with a convex surface facing the reduction side (S3, S4). The lens element L2 has a biconvex shape (S5, S6). The lens element L3 has a biconcave shape (S7, S8). The lens element L4 has a biconvex shape (S9, S10). The lens element L5 has a positive meniscus shape with a convex surface facing the reduction side (S13, S14). The lens element L6 has a positive meniscus shape with a convex surface facing the reduction side (S15, S16). The lens element L7 has a biconcave shape (S17, S18). These lens elements L1 to L7 are rotationally symmetric lenses having surface shapes that are rotationally symmetric about the optical axis OA of the first sub-optical system, and portions through which light rays do not pass may be removed as necessary.

[0027] The aperture stop ST defines the range through which the light beam passes through the optical system 1, and is positioned between the reduction conjugate point and the intermediate image position described above. As an example, the aperture stop ST is located between the lens element L4 and the lens element L5 (S11).

[0028] The prism PM has a plurality of optical surfaces, including a first transmitting surface T1 located closest to the first sub-optical system on the optical path between the first sub-optical system and the magnification conjugate point, a second transmitting surface T2 located closest to the magnification conjugate point, and a first reflecting surface R1 and a second reflecting surface R2 located closest to the first transmitting surface T1 and the second transmitting surface T2 on the optical path between the first transmitting surface T1 and the second transmitting surface T2. The first reflecting surface R1 and the second reflecting surface R2 form a group of reflecting surfaces. The first transmitting surface T1 has a free-form surface shape with a convex surface facing the magnification side (S19). The first reflecting surface R1 has a free-form surface shape with a concave surface facing in the direction in which light rays incident on the first reflecting surface R1 are reflected (S20). The second reflecting surface R2 has a free-form surface shape with a convex surface facing in the direction in which light rays incident on the second reflecting surface R2 are reflected (S21). The second transmitting surface T2 has a free-form curved shape with a convex surface facing the magnification side (S22).

[0029] A dielectric multilayer film that does not include a metal layer is formed on both the first and second reflecting surfaces R1 and R2 of the prism PM, or on only the second reflecting surface R2. Specific examples of this dielectric multilayer film will be described later.

[0030] Example 3 FIG. 3 is a layout diagram showing an optical system 1 according to Example 3, where FIG. 3A is a side view showing the YZ plane and FIG. 3B is a top view showing the XZ plane. This optical system 1 has a configuration similar to that of Example 1, and therefore, a description that overlaps with Example 1 will be omitted. The optical system 1 includes a first sub-optical system including a plurality of lens elements and an aperture stop ST, and a second sub-optical system including a prism PM. In FIG. 3A, a reduction conjugate point, which is an image formation position on the reduction side, is located to the left of the optical axis OA, and a magnification conjugate point, which is an image formation position on the magnification side, is located to the upper left of the optical axis OA. The second sub-optical system is located on the magnification side of the first sub-optical system on the optical path.

[0031] Furthermore, an intermediate imaging position that is conjugate to the reduction conjugate point and the enlargement conjugate point is located inside the optical system 1. As in Figure 1, at this intermediate imaging position, both a Y-direction intermediate image IMy and an X-direction intermediate image IMx exist inside the prism PM. Furthermore, a part of the intermediate imaging position exists inside the prism PM.

[0032] The first sub-optical system includes, in order from the reduction side to the enlargement side, an optical element PA and lens elements L1 to L7. A reduction conjugate point is set at a position a predetermined distance from the reduction-side end face of the optical element PA, and the original image SA is placed at this point.

[0033] The optical element PA has two parallel, flat transmitting surfaces (S1, S2). For the surface numbers, refer to the numerical examples described later. The lens element L1 has a positive meniscus shape with a convex surface facing the reduction side (S3, S4). The lens element L2 has a biconvex shape (S5, S6). The lens element L3 has a biconcave shape (S7, S8). The lens element L4 has a biconvex shape (S9, S10). The lens element L5 has a positive meniscus shape with a convex surface facing the reduction side (S13, S14). The lens element L6 has a positive meniscus shape with a convex surface facing the reduction side (S15, S16). The lens element L7 has a biconcave shape with a negative meniscus shape with a convex surface facing the reduction side (S17, S18). These lens elements L1 to L7 are rotationally symmetric lenses having a surface shape that is rotationally symmetric about the optical axis OA of the first sub-optical system, and portions through which light rays do not pass may be removed as necessary.

[0034] The aperture stop ST defines the range through which the light beam passes through the optical system 1, and is positioned between the reduction conjugate point and the intermediate image position described above. As an example, the aperture stop ST is located between the lens element L4 and the lens element L5 (S11).

[0035] The prism PM has a plurality of optical surfaces, including a first transmitting surface T1 located closest to the first sub-optical system on the optical path between the first sub-optical system and the magnification conjugate point, a second transmitting surface T2 located closest to the magnification conjugate point, and a first reflecting surface R1 and a second reflecting surface R2 located closest to the first transmitting surface T1 and the second transmitting surface T2 on the optical path between the first transmitting surface T1 and the second transmitting surface T2. The first reflecting surface R1 and the second reflecting surface R2 form a group of reflecting surfaces. The first transmitting surface T1 has a free-form surface shape with a convex surface facing the magnification side (S19). The first reflecting surface R1 has a free-form surface shape with a concave surface facing in the direction in which light rays incident on the first reflecting surface R1 are reflected (S20). The second reflecting surface R2 has a free-form surface shape with a convex surface facing in the direction in which light rays incident on the second reflecting surface R2 are reflected (S21). The second transmitting surface T2 has a free-form curved shape with a convex surface facing the magnification side (S22).

[0036] A dielectric multilayer film that does not include a metal layer is formed on both the first and second reflecting surfaces R1 and R2 of the prism PM, or on only the second reflecting surface R2. Specific examples of this dielectric multilayer film will be described later.

[0037] Next, conditions that can be satisfied by the optical system according to this embodiment will be described. Note that, although multiple conditions are specified for the optical system according to each example, it is possible to satisfy all of these multiple conditions, or to satisfy individual conditions to obtain the corresponding effects.

[0038] This embodiment is a projection optical system having a reduction conjugate point on the reduction side and an enlargement conjugate point on the enlargement side, and having an intermediate imaging position therein that is conjugate to the reduction conjugate point and the enlargement conjugate point, respectively, and comprising: a first sub-optical system; and a second sub-optical system arranged on the enlargement side of the first sub-optical system, wherein the first sub-optical system includes a plurality of lenses L1 to L11, and the second sub-optical system includes a prism PM formed of a transparent medium, and the prism PM has a group of reflecting surfaces including a first transmitting surface T1 located closest to the first sub-optical system on the optical path between the first sub-optical system and the enlargement conjugate point, a second transmitting surface T2 located closest to the enlargement conjugate point, and a second reflecting surface R2 located closest to the second transmitting surface T2 on the optical path between the first transmitting surface T1 and the second transmitting surface T2, all or part of the intermediate imaging position is located inside the prism, and a dielectric multilayer film that does not include a metal layer is formed on the second reflecting surface R2.

[0039] 4 is an explanatory diagram showing the reflection of light inside a prism PM having two reflecting surfaces R1 and R2. In this specification, a prism PM having a reflecting surface group including two reflecting surfaces R1 and R2 is illustrated as an example, but the same applies to prisms having a reflecting surface group including one or three or more reflecting surfaces.

[0040] 5A shows a comparative example of a reflective coating structure in which a reflective coating FB and a metallic reflective film FA are formed on the surface of a substrate (prism material) SUB. The reflective coating FB is generally formed of a dielectric multilayer film, and the metallic reflective film FA is formed of, for example, Al, Au, Ag, Ni, or Cr.

[0041] Figures 5(B) and (C) are graphs showing the reflection spectrum characteristics of the reflective coating of Figure 5(A). The vertical axis represents reflectance, and the horizontal axis represents light wavelength. As shown in Figure 5(B), when the incident angle θ is 0° (normal incidence on the reflective surface), the reflectance remains almost constant within the visible light range. On the other hand, as shown in Figure 5(C), as the incident angle θ increases (e.g., θ = 15° to 30°) (oblique incidence on the reflective surface), the reflection spectrum shifts overall to the shorter wavelength side, with a decrease in reflection of red light in particular. As a result, a white image projected onto a screen appears white near the center, but bluish near the periphery.

[0042] In particular, free-form prisms, due to their ultra-short focal length and ultra-shifting, have a large range of change in the angle of incidence on the prism's reflecting surface, and even light rays exceeding the critical angle are incident. In particular, the metal reflective film FA does not cause total reflection even when the angle of incidence exceeds the critical angle. Therefore, the structure of the metal reflective film FA and the reflection-enhancing coating FB shown in Figure 5(A) will cause color unevenness in the peripheral area of ​​the image.

[0043] Furthermore, the metal reflective film FA generates heat as it absorbs light, which causes the shape and curvature of the reflective surface to fluctuate due to thermal expansion of the prism. Particularly during long-term high-brightness projection, APL (Average Picture Level) drift can cause out-of-focus images, coma aberration, and field curvature.

[0044] 6A shows a reflective coating structure in which a dielectric multilayer film FM that does not include a metal layer is formed on the surface of a substrate SUB. The dielectric multilayer film FM is generally formed of a dielectric multilayer film in which high-refractive index materials and low-refractive index materials are alternately stacked.

[0045] 6(B) and (C) are graphs showing the reflection spectrum characteristics of the reflective coating of FIG. 6(A). The vertical axis represents reflectance, and the horizontal axis represents the wavelength of light. As shown in FIG. 6(B), when the incident angle θ is 0°, the reflectance exhibits a nearly constant value within the visible light range, and the flat range is extended compared to the reflective coating of FIG. 23(B). Furthermore, as shown in FIG. 6(C), as the incident angle θ increases (e.g., θ = 15° to 30°), the reflection spectrum shifts overall to the shorter wavelength side, but the reflectance of red light does not change significantly.

[0046] By adopting a dielectric multilayer film that does not contain a metal layer, it is possible to improve the reflection characteristics up to total reflection and maintain good reflection characteristics even for light with large angular changes. In addition, since there is no light absorption by the metal reflection film, heat generation on the reflection surface can be suppressed and APL drift can be reduced.

[0047] In the projection optical system according to this embodiment, the average reflectivity of the dielectric multilayer film for S-polarized light and P-polarized light may be greater than 95% for incident light whose incident angle is between the critical angle of 5 degrees and the critical angle and whose wavelength is in the range of 440 nm or more and 480 nm or less.

[0048] FIG. 7 is a graph showing an example of the reflection spectrum characteristics of a typical dielectric multilayer film. The vertical axis represents the average reflectance of S-polarized light and P-polarized light, and the horizontal axis represents the wavelength of light. When the incident angle θ is 0°, the reflectance is flat at approximately 1.0 in the wavelength range from approximately 450 nm to approximately 890 nm. However, as the wavelength increases from approximately 890 nm, the reflectance decreases with ripples including multiple peaks and valleys, and at approximately 970 nm, the reflectance becomes approximately 0.1 or less. As the incident angle θ increases, the entire reflection spectrum tends to shift toward shorter wavelengths. When the incident angle θ is 15°, the reflectance decreases with ripples at wavelengths longer than approximately 860 nm. When the incident angle θ is 30°, the reflectance decreases with ripples at wavelengths longer than approximately 740 nm. When the incident angle θ is 45° or 60°, the reflectance is greater than the critical angle of the dielectric multilayer film, and therefore the reflectance remains flat at approximately 1.0 up to a wavelength of 1000 nm.

[0049] FIG. 8A is a graph showing an example of the reflectance spectrum characteristics of another typical dielectric multilayer film, with the vertical axis representing the average reflectance of S-polarized and P-polarized light, and the horizontal axis representing the wavelength of light. FIG. 8B is a graph showing an example of the emission spectrum characteristics of a light source used in a projection device, with the vertical axis representing the relative light intensity, and the horizontal axis representing the wavelength of light. When a high-brightness color LED is used as the light source, blue light exhibits a Gaussian emission spectrum ranging from 440 to 480 nm with a peak wavelength of approximately 460 nm. Green light exhibits a Gaussian emission spectrum ranging from 500 to 600 nm with a peak wavelength of approximately 545 nm. Red light exhibits a Gaussian emission spectrum ranging from 575 to 700 nm with a peak wavelength of approximately 610 nm. Blue light tends to have a narrower emission spectrum width than green and red light, and it can be seen that the light source's light intensity is particularly low in the range of 480 to 510 nm.

[0050] On the other hand, as shown in the upper graph, the dielectric multilayer film exhibits reflection spectrum characteristics with multiple ripples, and the position of the valley of the ripple changes depending on the incident angle θ = 35°, 36°, 37°, 38°, and 39°, which are less than the critical angle (= 40°).

[0051] Therefore, by setting the average reflectance of the dielectric multilayer film to be greater than 95% for S-polarized light and P-polarized light with an incident angle between 5 degrees and the critical angle and a wavelength in the range of 440 nm to 480 nm, the reflection spectrum characteristics for blue light become nearly constant, thereby suppressing color unevenness of blue light. Note that the reflectance of the dielectric multilayer film can be calculated using Fresnel's formula, and since it differs between P-polarized light, which has an electric field parallel to the plane of incidence, and S-polarized light, which has an electric field perpendicular to the plane of incidence, the average of the reflectance of P-polarized light and the reflectance of S-polarized light is used.

[0052] In the projection optical system according to this embodiment, the average reflectivity of the S-polarized light and the P-polarized light of the dielectric multilayer film may have a ripple that is 95% or less for incident light whose incident angle is between the critical angle and 5 degrees or less, and whose wavelength is in the range of greater than 480 nm and less than 510 nm.

[0053] As shown in Figure 8A, the presence of a ripple valley in the reflectance spectrum of the dielectric multilayer film in the range of 480 to 510 nm, where the light intensity of the light source is low, can suppress color unevenness in blue and green light. The ripple valley can be defined as the region where the reflectance is 95% or less, as shown in Figure 8C.

[0054] In the projection optical system according to this embodiment, the group of reflecting surfaces includes, in order from the reduction side on the optical path, a first reflecting surface R1 and a second reflecting surface R2, the absolute value of the optical power of the first reflecting surface R1 is greater than the absolute value of the optical power of the second reflecting surface R2, and the dielectric multilayer film may be formed on both the first reflecting surface R1 and the second reflecting surface R2, or only on the second reflecting surface R2.

[0055] With this configuration, light from the light source is more concentrated on the second reflecting surface R2 than on the first reflecting surface R1, and heat generation due to light absorption is also greater. Therefore, the dielectric multilayer film may be formed on both the first reflecting surface R1 and the second reflecting surface R2, or, from a cost perspective, it may be formed only on the second reflecting surface R2. This makes it possible to suppress thermal expansion of the second reflecting surface.

[0056] In the projection optical system according to this embodiment, the intermediate image position may be between the first transmitting surface T1 and the group of reflecting surfaces.

[0057] With this configuration, the prism PM can be made smaller, and the first reflecting surface R1 can be made less susceptible to the effects of heat.

[0058] In the projection optical system of this embodiment, the long diameter A of the footprint on the second reflecting surface R2 of the first principal ray closest to the optical axis OA and the long diameter B of the footprint on the second reflecting surface R2 of the second principal ray farthest from the optical axis OA may satisfy B≧3×A.

[0059] FIG. 9 is an explanatory diagram showing the shapes of the footprints on the first reflecting surface R1 and the second reflecting surface R2 in Examples 1 to 3. In Examples 1 to 3, the first principal ray passes through a position near the bottom of the first reflecting surface R1 and then passes through a position near the top of the second reflecting surface R2. The second principal ray passes through a position near the top of the first reflecting surface R1 and then passes through a position near the center of the second reflecting surface R2. The footprint of the first principal ray tends to be larger than the footprint of the second principal ray, and this tendency is particularly pronounced on the second reflecting surface R2. If the footprint shape is large in the peripheral area of ​​the image where the second principal ray is involved, increased heat generation due to light absorption at the second reflecting surface R2 can easily cause image quality degradation such as coma aberration and drift only in the peripheral area of ​​the image. As a countermeasure, image quality degradation due to heat generation can be suppressed by providing a dielectric multilayer film on the second reflecting surface R2.

[0060] In the projection optical system according to this embodiment, the second reflecting surface R2 may reflect both the first light ray having an incident angle that causes total reflection and the second light ray having an incident angle that does not cause total reflection.

[0061] With this configuration, the first light that is totally reflected by the second reflecting surface R2 has a reflectance of 100%, which is the most efficient. In addition, the design of the dielectric multilayer film only needs to be optimized for the range of incident angles from 0° to the total reflection angle, so the reflection characteristics up to total reflection can be improved.

[0062] In the projection optical system according to this embodiment, the second reflecting surface R2 may be incident with light rays whose incident angle with respect to the normal of the incident surface of each light ray traveling on the second reflecting surface R2 is 25° or less and 60° or more.

[0063] This configuration facilitates ultra-wide-angle illumination by widening the angle of incidence at the second reflecting surface R2. On the other hand, using a metal reflective layer makes it difficult to compensate for wide angles. Therefore, the reflective film is constructed using a dielectric multilayer film, and the film design ensures reflectivity across the wavelength range in use. Light rays with angles of incidence greater than the critical angle are totally reflected, thereby increasing reflectivity across the wavelength range and angle of incidence in use and reducing color unevenness. Furthermore, even if ripples occur in the reflectance spectrum characteristics of the dielectric multilayer film, they are difficult to pinpoint and reduce. This is because ripples occur at a specific angle, and allowing light to be incident at a wide angle averages out the characteristics, reducing their impact.

[0064] In the projection optical system according to this embodiment, an air layer may exist on the back surface of the effective area of ​​the second reflecting surface R2, and the prism may come into contact with an external member in an area other than the back surface of the effective area of ​​the second reflecting surface R2.

[0065] With this configuration, most of the light at the second reflecting surface R2 is reflected by the dielectric multilayer film, but some of the light passes through the dielectric multilayer film and irradiates the external member, generating heat due to light absorption. The presence of an air layer in the effective area of ​​the second reflecting surface R2 prevents the heat from reaching the effective area, thereby suppressing drift.

[0066] In the projection optical system according to this embodiment, an air layer having a thickness of 5 mm or more may be present on the back side of the effective area of ​​the second reflecting surface R2.

[0067] This configuration reliably prevents heat generated in the external member from reaching the effective area, thereby suppressing drift.

[0068] In the projection optical system according to this embodiment, the dielectric multilayer film may be made up of 54 or more layers in which layers having different refractive indices are alternately stacked.

[0069] With this configuration, high reflectance can be ensured over a wide range of incident angles in the wavelength range of 450 to 680 nm, and color unevenness can be suppressed.

[0070] In the projection optical system according to this embodiment, the dielectric multilayer film may have an extinction coefficient of 0.1 or less at room temperature (20° C. to 30° C.) for incident light with a wavelength of 632.8 nm.

[0071] With this configuration, heat generation due to light absorption by the dielectric multilayer film is reduced, and drift can be suppressed. ２ O ５ exhibits a refractive index of 2.316 and an extinction coefficient of 0.000 at a wavelength of 632.8 nm. ２ exhibits a refractive index of 1.965 and an extinction coefficient of 0.011 at a wavelength of 632.8 nm.

[0072] In the projection optical system according to this embodiment, the dielectric multilayer film may be configured by alternately stacking high refractive index layers having a refractive index of 2.0 or more and low refractive index layers having a refractive index of 1.6 or less.

[0073] With this configuration, the reflectance of the dielectric multilayer film can be increased. The high refractive index layer having a refractive index of 2.0 or more is, for example, CeO ２ (cerium oxide, refractive index n=2.2 @ wavelength 550 nm), Nb ２ O ５ (niobium pentoxide, n=2.33@500 nm), SnO ２ (tin oxide, n=2@550 nm), Ta ２ O ５ (tantalum pentoxide, n=2.16@550nm), Ti ３ O ５ (titanium pentoxide, n = 2.3 to 2.55 @ 550 nm), TiO (titanium monoxide, n = 2.3 to 2.55 @ 550 nm), TiO ２ (titanium dioxide, n=2.3-2.55@550 nm), WO ３ (tungsten oxide, n=2.2 @ 550 nm), ZnO (zinc oxide, n=2.1 @ 550 nm), ZrO ２ (zirconium oxide, n = 2.05 @ 550 nm), ZRT2 (ZrO ２ + TiO ２ , n=2.1@550 nm), ZnS (zinc sulfide, n=2.35@550 nm), etc. can be used.

[0074] The low refractive index layer having a refractive index of 1.6 or less is, for example, SiO２ (Silicon oxide, n=1.46@500nm), AlF ３ (aluminum fluoride, n=1.38@550 nm), BaF ２ (barium fluoride, n=1.48@550 nm), CaF ２ (calcium fluoride, n = 1.23-1.45 @ 550 nm), LiF (lithium fluoride, n = 1.3 @ 550 nm), MgF ２ (magnesium fluoride, n=1.38 to 1.4 @ 550 nm), NaF (sodium fluoride, n=1.34 @ 550 nm), etc. can be used.

[0075] In the projection optical system according to this embodiment, the second reflecting surface R2 may have a reflectance of 95% or more over the wavelength range of 450 to 850 nm at normal incidence due to the dielectric multilayer film.

[0076] With this configuration, high reflectance can be ensured over the wavelength range of 450 to 850 nm, thereby suppressing color unevenness.

[0077] In the projection optical system according to this embodiment, the prism PM may be made of glass.

[0078] With this configuration, the linear expansion coefficient of glass is small, so that the change in shape due to temperature change is small, and drift can be suppressed.

[0079] The projection optical system according to this embodiment may project light of 3000 lumens or more.

[0080] With this configuration, a bright projected image can be obtained even with a projection range of 150 inches or more.

[0081] In the projection optical system according to this embodiment, a protective layer may be formed on the second reflecting surface R2 on the side of the dielectric multilayer film opposite to the prism PM.

[0082] According to this configuration, the presence of the protective layer can prevent the dielectric multilayer film from deteriorating over time. ２ ) and magnesium fluoride (MgF ２ ) etc.

[0083] Furthermore, this embodiment relates to a projection optical system having a reduction conjugate point on the reduction side and an enlargement conjugate point on the enlargement side, and having an intermediate imaging position therein that is conjugate to the reduction conjugate point and the enlargement conjugate point, the projection optical system comprising: a first sub-optical system; and a second sub-optical system arranged on the enlargement side of the first sub-optical system, the first sub-optical system including a plurality of lenses L1 to L11, the second sub-optical system including a prism PM formed of a transparent medium, the prism PM having a group of reflecting surfaces including a first transmitting surface T1 located closest to the first sub-optical system on an optical path between the first sub-optical system and the enlargement conjugate point, a second transmitting surface T2 located closest to the enlargement conjugate point, and a second reflecting surface R2 located closest to the second transmitting surface on the optical path between the first transmitting surface T1 and the second transmitting surface T2, all or a part of the intermediate imaging position exists inside the prism, The second reflecting surface R2 is provided with a coating layer (for example, a dielectric multilayer film) that reflects both the first light ray having an incident angle that causes total reflection and the second light ray having an incident angle that does not cause total reflection.

[0084] With this configuration, the first light that is totally reflected by the second reflecting surface R2 has a reflectance of 100%, which is the most efficient. In addition, the design of the dielectric multilayer film only needs to be optimized for the range of incident angles from 0° to the total reflection angle, so the reflection characteristics up to total reflection can be improved.

[0085] In the projection optical system according to this embodiment, the coating layer may be formed on all of the reflecting surfaces of the reflecting surface group.

[0086] This configuration can improve the reflection characteristics of the group of reflecting surfaces.

[0087] Numerical examples of the optical systems according to Examples 1 to 3 will be described below. In each of the numerical examples, all lengths in the tables are in "mm" and all angles of view are in "°". Each numerical example also shows the surface type (XY polynomial surface, spherical surface, aspherical surface), radius of curvature, surface spacing, d-line refractive index, d-line Abbe number, material, refraction / reflection, decentering type, Y decentering amount, and Z decentering amount α rotation. The various quantities in each numerical example are calculated based on a wavelength of 550 nm. In each numerical example, the shape of the aspherical surface is defined by the following equation. Note that only non-zero aspherical coefficients are listed, except for the Conic coefficient k.

[0088]

[0089] Here, z: Sag amount of the surface parallel to the z axis r: Radial distance (=√(x ２ +y ２ )) c: curvature at the vertex of the surface k: Conic coefficient A to H: 4th to 18th order coefficients of r.

[0090] The shape of the free-form surface is defined by the following equation using a local orthogonal coordinate system (x, y, z) with the vertex of the surface as the origin.

[0091]

[0092]

[0093] Here, z: Sag amount of the surface parallel to the z axis r: Radial distance (=√(x ２ +y ２ )) c: curvature at the surface vertex k: conic coefficient C ｊ :monomial x ｍ y ｎ is the coefficient of .

[0094] In the following data, the i-th order x term and j-th order y term, which are free-form surface coefficients in a polynomial, are written as x**i*y**j. For example, "X**2*Y" indicates the free-form surface coefficients of the quadratic x and linear y terms in the polynomial.

[0095] (Numerical Example 1) For the optical system of Numerical Example 1 (corresponding to Example 1), lens data is shown in Table 1, aspheric shape data of the lens and data on the object height and image height in the optical path are shown in Table 2, and free-form surface shape data of the prism is shown in Table 3. Specific configurations of the dielectric multilayer film formed on the first reflecting surface R1 and / or the second reflecting surface R2 of the prism are shown in Tables 4 to 6. Note that "DAR (Decenter and Return)" in Table 1 refers to coordinate conversion between global coordinates and local coordinates during numerical calculation. Also, "f1 to f10" in Table 2 refer to the evaluated image heights. The same applies to the other numerical examples.

[0096]

[0097]

[0098]

[0099]

[0100] 10(A) to 10(C) show the reflection spectrum characteristics of a dielectric multilayer film (Table 4, 64 layers) formed on glass KVC80, where FIG. 10(A) shows the characteristics at an incident angle of 0° (vertical incidence), FIG. 10(B) shows the characteristics at incident angles of (critical angle -5°) to (critical angle -1°), and FIG. 10(C) shows the characteristics at an incident angle of 36.2°.

[0101]

[0102] 11(A) to 11(C) show the reflection spectrum characteristics of a dielectric multilayer film (Table 5, 54 layers) formed on glass KVC80, where FIG. 11(A) shows the characteristics at an incident angle of 0° (vertical incidence), FIG. 11(B) shows the characteristics at incident angles of (critical angle -5°) to (critical angle -1°), and FIG. 11(C) shows the characteristics at an incident angle of 36.2°.

[0103]

[0104] 12(A) to 12(C) show the reflection spectrum characteristics of a dielectric multilayer film (Table 6, 84 layers) formed on glass KVC80, where FIG. 12(A) shows the characteristics at an incident angle of 0° (vertical incidence), FIG. 12(B) shows the characteristics at incident angles of (critical angle -5°) to (critical angle -1°), and FIG. 12(C) shows the characteristics at an incident angle of 36.2°.

[0105] Numerical Example 2 For the optical system of Numerical Example 2 (corresponding to Example 2), lens data is shown in Table 7, aspheric shape data of the lens and data of the object height and image height in the optical path are shown in Table 8, and free-form surface shape data of the prism is shown in Table 9. Furthermore, specific configurations of the dielectric multilayer film formed on the first reflecting surface R1 and / or the second reflecting surface R2 of the prism are shown in Tables 10 to 12.

[0106]

[0107]

[0108]

[0109]

[0110] 13(A) to 13(C) show the reflection spectrum characteristics of a dielectric multilayer film (Table 10, 64 layers) formed on glass KSKLD5, where FIG. 13(A) shows the characteristics at an incident angle of 0° (vertical incidence), FIG. 13(B) shows the characteristics at incident angles of (critical angle -5°) to (critical angle -1°), and FIG. 13(C) shows the characteristics at an incident angle of a critical angle of 39.0°.

[0111]

[0112] 14(A) to 14(C) show the reflection spectrum characteristics of a dielectric multilayer film (Table 11, 54 layers) formed on glass KSKLD5, where FIG. 14(A) shows the characteristics at an incident angle of 0° (vertical incidence), FIG. 14(B) shows the characteristics at incident angles of (critical angle -5°) to (critical angle -1°), and FIG. 14(C) shows the characteristics at an incident angle of a critical angle of 39.0°.

[0113]

[0114] 15(A) to 15(C) show the reflection spectrum characteristics of a dielectric multilayer film (Table 12, 84 layers) formed on glass KSKLD5, where FIG. 15(A) shows the characteristics at an incident angle of 0° (vertical incidence), FIG. 15(B) shows the characteristics at incident angles of (critical angle -5°) to (critical angle -1°), and FIG. 15(C) shows the characteristics at an incident angle of a critical angle of 39.0°.

[0115] Numerical Example 3 For the optical system of Numerical Example 3 (corresponding to Example 3), lens data is shown in Table 13, aspheric shape data of the lens and data of the object height and image height in the optical path are shown in Table 14, and free-form surface shape data of the prism is shown in Table 15. The specific configuration of the dielectric multilayer film formed on the first reflecting surface R1 and / or the second reflecting surface R2 of the prism is the same as the dielectric multilayer film (64 layers) shown in Table 10.

[0116]

[0117]

[0118]

[0119] The upper part of Table 16 below shows, for the first reflecting surface R1 and the second reflecting surface R2 of the prism according to each of Numerical Examples 1 to 3, the numerical values ​​of the major axis A of the footprint of the first principal ray closest to the optical axis OA, the major axis B of the footprint of the second principal ray farthest from the optical axis OA, and the ratio B / A between the two.

[0120] The lower part of Table 16 below shows the minimum incident angle, maximum incident angle, refractive index of the prism, and critical angle for the first reflecting surface R1 and the second reflecting surface R2 of the prism according to each of Numerical Examples 1 to 3.

[0121]

[0122] (Embodiment 2) Hereinafter, embodiment 2 of the present disclosure will be described with reference to FIG. 16 . FIG. 16 is a block diagram showing an example of an image projection device according to the present disclosure. The image projection device 100 includes the projection optical system 1 disclosed in embodiment 1, an image forming element 101, a light source 102, a control unit 110, and the like. The image forming element 101 is configured with a liquid crystal, a DMD, or the like, and generates an image to be projected onto a screen SR via the projection optical system 1. The light source 102 is configured with an LED (light-emitting diode), a laser, or the like, and supplies light to the image forming element 101. The control unit 110 is configured with a CPU, MPU, or the like, and controls the entire device and each component. The projection optical system 1 may be configured as an interchangeable lens that can be detachably attached to the image projection device 100, or as a built-in lens integrated into the image projection device 100.

[0123] The image projection device 100 described above is capable of short-focus and large-screen projection with a compact device, thanks to the projection optical system 1 according to the first embodiment.

[0124] As described above, the embodiments have been described as disclosure of the technology in the present disclosure, and the accompanying drawings and detailed description have been provided for this purpose.

[0125] Therefore, the components shown in the accompanying drawings and detailed description may include not only essential components for solving the problem, but also components that are not essential for solving the problem in order to illustrate the above technology. Therefore, the fact that these non-essential components are shown in the accompanying drawings or detailed description should not be interpreted as immediately indicating that these non-essential components are essential.

[0126] Furthermore, since the above-described embodiments are intended to illustrate the technology of the present disclosure, various modifications, substitutions, additions, omissions, etc. may be made within the scope of the claims or their equivalents.

[0127] The present disclosure is applicable to image projection devices such as projectors and head-up displays.

Claims

1. A projection optical system having a reduction conjugate point on the reduction side and an enlargement conjugate point on the enlargement side, and having an intermediate imaging position therein that is conjugate to the reduction conjugate point and the enlargement conjugate point, respectively, comprising: a first sub-optical system; and a second sub-optical system arranged on the enlargement side of the first sub-optical system, wherein the first sub-optical system includes a plurality of lenses, and the second sub-optical system includes a prism formed of a transparent medium, the prism having a reflecting surface group including a first transmitting surface located closest to the first sub-optical system on an optical path between the first sub-optical system and the enlargement conjugate point, a second transmitting surface located closest to the enlargement conjugate point, and a second reflecting surface located closest to the second transmitting surface on the optical path between the first transmitting surface and the second transmitting surface, all or a part of the intermediate imaging position exists inside the prism, and a dielectric multilayer film that does not include a metal layer is formed on the second reflecting surface.

2. The projection optical system of claim 1, wherein the average reflectance of the dielectric multilayer film for S-polarized light and P-polarized light is greater than 95% for incident light with an incident angle between 5 degrees or less of the critical angle and the critical angle and with a wavelength in the range of 440 nm or more and 480 nm or less.

3. The projection optical system described in claim 1 or 2, wherein the average reflectance of the S-polarized light and the P-polarized light of the dielectric multilayer film has a ripple that is 95% or less for incident light with an incident angle between 5 degrees or less of the critical angle and the critical angle and a wavelength in the range of more than 480 nm and 510 nm or less.

4. The projection optical system described in claim 1, wherein the group of reflective surfaces includes, in order from the reduction side on the optical path, a first reflective surface and the second reflective surface, the absolute value of the optical power of the first reflective surface is greater than the absolute value of the optical power of the second reflective surface, and the dielectric multilayer film is formed on both the first reflective surface and the second reflective surface or only on the second reflective surface.

5. The projection optical system according to claim 1, wherein the intermediate image position is between the first transmitting surface and the group of reflecting surfaces.

6. The projection optical system of claim 1, wherein the major axis A of the footprint on the second reflecting surface of the first principal ray closest to the optical axis and the major axis B of the footprint on the second reflecting surface of the second principal ray farthest from the optical axis satisfy B≧3×A.

7. The projection optical system according to claim 1, wherein the second reflecting surface reflects both a first light ray having an incident angle that results in total reflection and a second light ray having an incident angle that does not result in total reflection.

8. The projection optical system according to claim 1, wherein the second reflecting surface is incident with light rays whose angles of incidence with respect to the normal of the incident surface of each light ray traveling on the second reflecting surface are less than 25° and more than 60°.

9. The projection optical system according to claim 1, wherein an air layer exists on the rear surface of the effective area of ​​the second reflecting surface, and the prism is in contact with an external member in an area other than the rear surface of the effective area of ​​the second reflecting surface.

10. The projection optical system according to claim 1, wherein an air layer having a thickness of 5 mm or more exists on the rear side of the effective area of ​​the second reflecting surface.

11. The projection optical system according to claim 1, wherein the dielectric multilayer film is composed of 54 or more layers in which layers having different refractive indices are alternately stacked.

12. The projection optical system according to claim 1, wherein the dielectric multilayer film has an extinction coefficient of 0.1 or less at room temperature for incident light with a wavelength of 632.8 nm.

13. The projection optical system according to claim 1, wherein the dielectric multilayer film is constructed by alternately laminating high refractive index layers having a refractive index of 2.0 or more and low refractive index layers having a refractive index of 1.6 or less.

14. The projection optical system according to claim 11, wherein the second reflecting surface has a reflectance of 95% or more for incident light having a wavelength of 450 to 850 nm at normal incidence due to the dielectric multilayer film.

15. The projection optical system of claim 1, wherein the prism is made of glass.

16. The projection optical system according to claim 1, which projects light of 3000 lumens or more.

17. The projection optical system according to claim 1, wherein a protective layer is formed on the second reflecting surface on the opposite side of the dielectric multilayer film from the prism.

18. A projection optical system having a reduction conjugate point on the reduction side and an enlargement conjugate point on the enlargement side, and having an intermediate imaging position therein that is conjugate to the reduction conjugate point and the enlargement conjugate point, respectively, comprising: a first sub-optical system; and a second sub-optical system arranged on the enlargement side of the first sub-optical system, wherein the first sub-optical system includes a plurality of lenses, and the second sub-optical system includes a prism formed of a transparent medium, the prism having a reflecting surface group including a first transmitting surface located closest to the first sub-optical system on an optical path between the first sub-optical system and the enlargement conjugate point, a second transmitting surface located closest to the enlargement conjugate point, and a second reflecting surface located closest to the second transmitting surface on the optical path between the first transmitting surface and the second transmitting surface, all or a part of the intermediate imaging position is present inside the prism, and a coating layer is formed on the second reflecting surface to reflect both a first light ray having an incident angle that results in total reflection and a second light ray having an incident angle that does not result in total reflection.

19. The projection optical system according to claim 18, wherein the coating layer is formed on all of the reflecting surfaces of the group of reflecting surfaces.

20. An image projection device comprising: a projection optical system according to claim 1 or 18; an image forming element that generates an image to be projected onto a screen via said projection optical system; and a light source that supplies light to said image forming element.

Images