A small volume direct projection system

Abstract

The present invention relates to the field of projection technology, and discloses a small-volume direct projection system. It includes: a light valve, an illumination prism, an image biasing mirror, and a projection lens arranged in sequence from the incident direction of the light to the exit direction of the light; the aperture is located in the middle of the projection lens; the third lens, the fourth lens, and the fifth lens are combined into a triple-cemented lens, and the chromatic aberration of the projection lens is corrected by using a combination of high, low, and high refractive index; the second lens is adjustably arranged in the lens barrel, and is used to compensate for the image quality degradation caused by the tilt of the triple-cemented lens and the front group lens group; the first lens, the second lens, the sixth lens, and the seventh lens are symmetrically arranged on both sides of the aperture, and are used to balance the aberration of the projection lens. The present invention can compensate for the degradation of the imaging quality in the projection lens by adjusting the eccentric position of the second lens, and the overall structure is simple, the volume is small, the assembly is simple, and the production cost is low.

Description

Technical Field

[0001] The present invention relates to the field of projection technology, and in particular to a small-volume direct projection system. Background Art

[0002] Direct projection system, also known as front projection system, is a common installation method in projection technology. Its main feature is that the audience and the projector are on the same side relative to the projection screen, that is, the projector directly projects the image or video onto the screen for the audience to watch, and is widely used in various industries.

[0003] However, the direct projection systems on the market have the following problems:

[0004] 1. During the production and assembly of the direct projection system, each lens is fixed by a pre-processed retaining ring to meet the design distance requirements between each lens. However, some small deviations are inevitable during the assembly process, resulting in problems such as tilted or eccentric installation of the lens, which causes a certain degree of decline in the imaging quality of the entire lens after assembly.

[0005] 2. In order to pursue high brightness, laser light source will be used. High brightness will cause uneven color and halo of the entire imaging picture, which will also reduce the imaging quality.

[0006] Therefore, there is an urgent need for a small-volume direct projection system that can compensate for assembly errors during the assembly process, thereby reducing aberrations, improving the image quality of the lens, and making the color of the image uniform and low-cost. Summary of the invention

[0007] In order to solve the above technical problems, the present invention provides a small-volume direct projection system for generating high-quality images, comprising: a light valve, an illumination prism, an image biasing mirror and a projection lens arranged in sequence from the light incident direction to the light exit direction;

[0008] The projection lens is provided with a rear lens group and a front lens group in sequence on the same optical axis from the incident direction of the light to the exit direction of the light, the rear lens group includes a first lens, a second lens, a third lens, a fourth lens, and a fifth lens, and the front lens group includes a sixth lens, a seventh lens, an eighth lens, and a ninth lens;

[0009] An aperture is provided between the front lens group and the rear lens group, and the aperture is located in the middle of the projection lens;

[0010] The third lens, the fourth lens and the fifth lens are combined into a triplet lens, and the chromatic aberration of the projection lens is corrected by using a combination of high, low and high refractive index;

[0011] The second lens is disposed in the lens barrel, and the position of the second lens is adjustable, so as to compensate for the degradation of image quality caused by the tilt of the triplet lens and the front lens group;

[0012] The first lens, the second lens, the sixth lens and the seventh lens are symmetrically arranged on both sides of the aperture, and are used to balance the aberration of the projection lens.

[0013] Furthermore, the second lens is mounted on the barrel wall of the lens barrel, is symmetrical along the circumferential direction and is threadedly connected to the barrel wall with a plurality of adjustment screws, the adjustment screws penetrate the barrel wall of the lens barrel and abut against the circumference of the second lens, and are used to adjust the eccentric position of the second lens.

[0014] Furthermore, the second lens is a glass spherical lens, which is used to correct the chromatic aberration of red, green and blue colors entering the second lens.

[0015] Furthermore, the first lens is a glass spherical lens, which is used to correct the angle and telecentricity of light entering the first lens.

[0016] Furthermore, the third lens, the fourth lens and the fifth lens in the triplet lens are all glass spherical lenses, and the refractive index range of the third lens is [1.5, 1.7], the refractive index range of the fourth lens is [1.45, 1.65], and the refractive index range of the fifth lens is [1.5, 1.7].

[0017] Further, the sixth lens, the seventh lens and the eighth lens are all glass spherical lenses;

[0018] The sixth lens is a meniscus lens bent toward the light source, the seventh lens is a biconvex lens, and the eighth lens is a biconcave lens;

[0019] The refractive index range of the eighth lens is [1.5, 1.7), and the Abbe number range is [20, 95], which reduces the aberration of the projection lens.

[0020] Furthermore, the ninth lens is a resin aspherical lens, and is shaped as a meniscus lens bent toward the light source, and is used to magnify the magnification of the projection lens and correct the TV distortion of the projection lens.

[0021] Furthermore, the projection lens has positive optical power;

[0022] Among them, the optical focal powers of the first lens, the second lens, the third lens, the fourth lens, the fifth lens, the sixth lens, the seventh lens, the eighth lens and the ninth lens are set to be positive focal power, positive focal power, positive focal power, positive focal power, negative focal power, positive focal power, positive focal power, negative focal power, and negative focal power respectively.

[0023] Furthermore, the illumination prism, the image biasing mirror and the projection lens form a refractive system, the total length of the refractive system is L1, the distance from the light valve to the first lens is the back working distance BFL, and BFL meets the condition: 0.015<BFL / L1<1.3.

[0024] Furthermore, the total length of the projection lens is L2, and the total focal length of the projection lens is f, which satisfies the condition: 2<L2 / f<15.

[0025] The embodiments of the present invention have the following technical effects:

[0026] 1. In the present invention, a second lens with adjustable position is arranged in the lens. The second lens plays the function of adjusting the eccentric position alone, and other lenses used in groups are not selected. When the lens has been assembled and the existing assembly is not changed, other lens groups used in groups to achieve specific functions are not affected. The second lens alone can adjust the eccentric position to compensate for the image quality degradation caused by the eccentricity and tilt of the triplet lens and the front lens group, thereby improving the imaging quality of the direct projection system.

[0027] 2. In the present application, the third lens, the fourth lens and the fifth lens are combined into a triplet lens, and a combination of high, low and high refractive index is adopted, which works together with other lenses to correct the chromatic aberration of the projection lens, reduce the chromatic aberration of the three primary colors of red, green and blue, and evenly distribute the color of the picture, thereby further improving the picture restoration degree. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0029] Figure 1 A schematic structural diagram of a small-volume direct projection system provided by an embodiment of the present invention;

[0030] Figure 2 A diagram showing the adjustment structure of a second lens in a small-volume direct projection system provided by an embodiment of the present invention;

[0031] Figure 3 A cross-sectional view of a second lens adjustment structure in a small-volume direct projection system provided by an embodiment of the present invention;

[0032] Figure 4 A vertical axis chromatic aberration diagram of a small volume direct projection system provided by an embodiment of the present invention;

[0033] Figure 5 An 80-inch light modulation transfer function (MTF) diagram of a small-volume direct projection system provided by an embodiment of the present invention;

[0034] Figure 6 A 60-inch light modulation transfer function (MTF) diagram of a small-volume direct projection system provided by an embodiment of the present invention;

[0035] Figure 7 A 120-inch light modulation transfer function (MTF) diagram of a small-volume direct projection system provided by an embodiment of the present invention;

[0036] Figure 8 A light spot array diagram under different field of view conditions of a small-volume direct projection system provided by an embodiment of the present invention;

[0037] Fig. 9 A TV distortion diagram of a small-volume direct projection system provided by an embodiment of the present invention.

[0038] Reference numerals:

[0039] 1-projection lens; 2-aperture; 3-light valve; 4-illumination prism; 5-image bias mirror; 6-lens barrel; 7-adjustment screw; 11-first lens; 12-second lens; 13-third lens; 14-fourth lens; 15-fifth lens; 16-sixth lens; 17-seventh lens; 18-eighth lens; 19-ninth lens. DETAILED DESCRIPTION

[0040] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work belong to the scope of protection of the present invention.

[0041] Figure 1 A structural diagram of a small volume direct projection system provided by an embodiment of the present invention is shown in FIG. Figure 1 As shown, a small-volume direct projection system in the present application comprises a light valve 3, an illumination prism 4, an image biasing mirror 5 and a projection lens 1 which are arranged in sequence from the light incident direction to the light exit direction;

[0042] Furthermore, the illumination prism 4, the image biasing mirror 5 and the projection lens 1 form a refractive system, the total length of the refractive system is L1, the distance from the light valve 3 to the first lens 11 is the back working distance BFL, and BFL meets the condition: 0.015<BFL / L1<1.3.

[0043] The projection lens 1 is provided with a rear lens group and a front lens group in sequence on the same optical axis from the incident direction of the light to the exit direction of the light, the rear lens group includes a first lens 11, a second lens 12, a third lens 13, a fourth lens 14, and a fifth lens 15, and the front lens group includes a sixth lens 16, a seventh lens 17, an eighth lens 18, and a ninth lens 19;

[0044] Furthermore, the total length of the projection lens 1 is L2, and the total focal length of the projection lens 1 is f, which satisfies the condition: 2<L2 / f<15. The optical-mechanical back focus, lens length, optical-mechanical total length, field angle, and image height of the direct projection system have a mutually restrictive relationship.

[0045] Furthermore, the first lens 11 is a glass spherical lens, which is used to correct the angle and telecentricity of the light entering the first lens 11 .

[0046] For example, since the higher the refractive index, the stronger the light refraction ability is, the first lens 11 is a glass spherical lens, and is made of a material with a relatively high refractive index, with a refractive index range of [1.7, 2.0], which corrects the angle and telecentricity of the light entering the first lens 11 .

[0047] Furthermore, the third lens 13, the fourth lens 14 and the fifth lens 15 in the triplet lens are all glass spherical lenses, and the refractive index of the third lens 13 is in the range of [1.5, 1.7], the refractive index of the fourth lens 14 is in the range of [1.45, 1.65], and the refractive index of the fifth lens 15 is in the range of [1.5, 1.7]. Different refractive indices have different refractive powers for the three colors of red, green and blue, resulting in chromatic aberration. Through this combination, after a beam of white light passes through the triplet lens, the decomposition power of the three colors of red, green and blue becomes weaker, the color distribution of the picture is uniform, and the picture restoration degree is further improved.

[0048] Further, the sixth lens 16, the seventh lens 17 and the eighth lens 18 are all glass spherical lenses;

[0049] The sixth lens 16 is a meniscus lens bent toward the light source, the seventh lens 17 is a biconvex lens, and the eighth lens 18 is a biconcave lens;

[0050] The refractive index range of the eighth lens 18 is [1.5, 1.7), and the Abbe number range is [20, 95], which reduces the aberration of the projection lens 1.

[0051] Furthermore, the ninth lens 19 is a resin aspherical lens, and is in the shape of a meniscus lens bent toward the light source, and is used to magnify the magnification of the projection lens 1 and correct the TV distortion of the projection lens 1 .

[0052] Since the ninth lens 19 is far away from the optical machine and has relatively weak temperature sensitivity, a resin aspherical lens is used, which has a meniscus shape that bends toward the light source. At the same time, since the lens is also far away from the aperture 2, the field of view is large. The use of an aspherical surface can reduce system distortion aberration and improve imaging quality.

[0053] The nine lenses of the projection lens 1, namely the first lens 11, the second lens 12, the third lens 13, the fourth lens 14, the fifth lens 15, the sixth lens 16, the seventh lens 17 and the eighth lens 18 are all glass spherical lenses, and the ninth lens 19 is a resin aspherical lens, which are all made of commonly used materials to reduce manufacturing costs.

[0054] Furthermore, the projection lens 1 has positive optical power;

[0055] Among them, the optical focal powers of the first lens 11, the second lens 12, the third lens 13, the fourth lens 14, the fifth lens 15, the sixth lens 16, the seventh lens 17, the eighth lens 18 and the ninth lens 19 are set to be positive focal power, positive focal power, positive focal power, positive focal power, negative focal power, positive focal power, positive focal power, negative focal power, and negative focal power respectively.

[0056] An aperture 2 is provided between the front lens group and the rear lens group, and the aperture 2 is located in the middle of the projection lens 1;

[0057] The aperture 2 is disposed in the middle of the projection lens 1, which is beneficial for keeping the apertures of the lenses in the front lens group and the rear lens group basically consistent, and at the same time shortening the volume of the projection lens 1 by coordinating the lens materials and shapes.

[0058] Exemplarily, the stop 2 is an aperture stop.

[0059] A small-volume direct projection system in the present application has a compact structure and a small volume. The volume of the direct projection system is shortened by setting the aperture in the middle of the lens and coordinating the lens materials and shapes on both sides of the aperture, so that the total length of the direct projection system can be less than 59 mm.

[0060] The total length of the direct projection system is too long, and its convenience and portability are poor. It is relatively bulky and has a high processing cost. If its total length is too short, it lacks processability, increases the difficulty of processing, and is not mass-producible. Therefore, in order to meet both processability and portability, the aperture in this application is set in the middle of the lens and the lens materials and shapes on both sides of the aperture are matched to shorten the total length of the projection system to less than 59mm.

[0061] The third lens 13, the fourth lens 14 and the fifth lens 15 are combined into a triplet lens, and the chromatic aberration of the projection lens 1 is corrected by using a combination of high, low and high refractive index.

[0062] The second lens 12 is disposed in the lens barrel 6, and the position of the second lens 12 is adjustable, so as to compensate for the degradation of image quality caused by the tilt of the triplet lens and the front lens group;

[0063] The second lens 12 is mounted on the wall of the lens barrel 6 , symmetrically along the circumferential direction and threadedly connected to the wall with a plurality of adjustment screws 7 . The adjustment screws 7 penetrate the wall of the lens barrel 6 and abut against the circumference of the second lens 12 , so as to adjust the eccentric position of the second lens 12 .

[0064] Figure 2 A diagram of the adjustment structure of the second lens in a small-volume direct projection system provided by an embodiment of the present invention, Figure 3 A cross-sectional view of a second lens adjustment structure in a small volume direct projection system provided by an embodiment of the present invention; Figure 2 and Figure 3 As shown, the second lens 12 is mounted on the wall of the lens barrel 6, and is symmetrical along the circumferential direction and threadedly connected with four adjustment screws 7. The screws penetrate the wall of the lens barrel 6 and abut against the circumference of the second lens 12, and are used to adjust the position of the second lens 12 in four directions: up, down, left, and right.

[0065] By rotating the adjusting screw 7, the position of the second lens 12 in the lens barrel 6 can be moved up, down, left, or right, so as to adjust the eccentricity of the second lens 12, thereby compensating for the eccentricity of the triplet lens assembly and the image quality degradation caused by the tilted installation of the front group of lenses.

[0066] In the present invention, a second lens with adjustable position is arranged in the lens, and the second lens alone performs the function of adjusting the eccentric position, and other lenses used in groups, such as the third lens, the fourth lens or the fifth lens, are not selected. In this way, when the lens has been assembled and the existing assembly is not changed, other lens groups used in groups to achieve specific functions are not affected. The second lens alone can perform eccentric position adjustment to compensate for the image quality degradation caused by the tilt of the three-cemented lens and the front lens group, thereby improving the imaging quality of the direct projection system.

[0067] Furthermore, the second lens 12 is a glass spherical lens, which is used to correct the chromatic aberration of the three colors of red, green and blue entering the second lens 12 .

[0068] The first lens 11 , the second lens 12 , the sixth lens 16 , and the seventh lens 17 are symmetrically arranged on both sides of the aperture 2 to balance the aberration of the projection lens 1 .

[0069] Embodiment 1

[0070] In this embodiment, the relevant parameters of each lens, including the refractive index nd, Abbe number Vd, radius of curvature R, and coefficient of thermal expansion CT, are shown in Table 1. The data in the column of the radius of curvature represent the right radius / left radius respectively. When measuring the coefficient of thermal expansion, linear thermal expansion is simulated to simulate the linear thermal expansion effect of the material under temperature change, which is the ratio of the length change per Kelvin.

[0071] Table 1 Lens Parameter Table

[0072] In this embodiment, the system structure parameters of the projection lens 1 satisfy the following conditions: effective focal length (EFL) = 12.7 mm, offset is 96% < OFFSET < 104%, resolution is 93 lp / mm, the projected image size is 20 - 190 inches, and the projection ratio ≤ 1.2.

[0073] Coefficient of expansion: Compensate for the distance change caused by the thermal expansion and contraction of each device, and affect the imaging effect of the image.

[0074] Radius of curvature: Shorten the distance between each lens. The smaller the number of the radius, the more curved it is, and the better the distance reduction effect. If the vertex of the arc where the mirror surface is located is on the left side of the center of curvature, the radius of curvature of the mirror surface is positive. If the vertex is on the right side of the center of curvature, the radius of curvature of the mirror surface is negative. For example, when looking at a biconvex lens from the side, the radius of curvature of its left surface is positive, and the radius of curvature of its right surface is negative, while the biconcave lens is just the opposite.

[0075] Exemplarily, for the first lens 11 in Table 1, the vertex of the arc of the left surface is on the left side of its center of curvature, so the radius of curvature of this mirror surface is positive. The vertex of the arc of the right surface is on the right side of its center of curvature, so the radius of curvature of this mirror surface is negative. The radius of curvature of the left surface 38.576 is smaller than the radius of curvature of the right surface 123.5, so the left surface of the first lens 11 is more curved than the right surface.

[0076] For the ninth lens 19, the vertex of the arc of the left surface is on the left side of its center of curvature, so the radius of curvature of this mirror surface is positive. The vertex of the arc of the right surface is also on the left side of its center of curvature, so the radius of curvature of this mirror surface is also positive. The radius of curvature of the left surface 25.36 is larger than the radius of curvature of the right surface 8.7, so the left surface of the ninth lens 19 is less curved than the right surface.

[0077] Figure 4A vertical axis chromatic aberration diagram of a small volume direct projection system provided by an embodiment of the present invention, the ordinate is the actual image height of the field of view, the abscissa is the error between each wavelength and the main light, lines 1, 2, and 3 in the figure represent three different wavelengths of light, where line 1 is blue light, line 2 is green light (main wavelength), and line 3 is red light. The color difference value of the system is determined by the offset of the red light and the blue light relative to the central green light under different field conditions. Generally, the color difference value is less than 0.5 pixels. In this embodiment, 1 pixel = 5.4μm, from Figure 4 It can be seen that the color difference values ​​are all less than 0.2 pixels, and the clarity of the image quality is beyond the range that the human eye can distinguish. Therefore, the color difference of the entire system is small and the color is uniform.

[0078] Figure 5 An 80-inch light modulation transfer function (MTF) diagram of a small-volume direct projection system provided by an embodiment of the present invention; Figure 6 A 60-inch light modulation transfer function (MTF) diagram of a small-volume direct projection system provided by an embodiment of the present invention; Figure 7 A 120-inch light modulation transfer function (mtf) diagram of a small-volume direct projection system provided by an embodiment of the present invention; wherein the horizontal axis is the spatial frequency, and the vertical axis OTF module value is the value of the modulation transfer function, which represents the resolution capability. The higher the vertical axis value, the stronger the resolution capability and the higher the degree of image quality restoration.

[0079] from Figure 5 As shown in the figure, when the projection screen is 80 inches, the modulation transfer function (mtf) of light of different wavelengths (0.45μm, 0.55μm, 0.643μm) is shown. In the figure, the green solid line is the meridian at 0.707F, the green dotted line is the sagittal line at 0.707F, the red solid line is the meridian at 1F, the red dotted line is the sagittal line at 1F, the blue solid line is the meridian at 0.2F, and the blue dotted line is the sagittal line at 0.2F, where F is the maximum field of view. The ordinate values ​​of the three different wavelengths of light are all over or close to 0.5, and the imaging quality of the system is very clear.

[0080] from Figure 6As shown in the figure, when the projection screen is 60 inches, in the modulation transfer function (mtf) of different wavelengths of light (0.45μm, 0.55μm, 0.643μm), the green solid line is the meridian at 0.707F, the green dotted line is the sagittal line at 0.707F, the red solid line is the meridian at 1F, the red dotted line is the sagittal line at 1F, the blue solid line is the meridian at 0.2F, and the blue dotted line is the sagittal line at 0.2F. The ordinate values ​​of the three different wavelengths of light are all over or close to 0.5, and the imaging quality of the system is very clear.

[0081] from Figure 7 As shown in the figure, when the projection screen is 120 inches, in the modulation transfer function (mtf) of different wavelengths (0.45μm, 0.55μm, 0.643μm), the green solid line is the meridian at 0.707F, the green dotted line is the sagittal line at 0.707F, the red solid line is the meridian at 1F, the red dotted line is the sagittal line at 1F, the blue solid line is the meridian at 0.2F, and the blue dotted line is the sagittal line at 0.2F. The ordinate values ​​of the three different wavelengths of light are all over or close to 0.5, and the imaging quality of the system is very clear.

[0082] Figure 8 A light spot diagram under different field of view conditions of a small-volume direct projection system provided in an embodiment of the present invention is shown in the figure, which is a schematic diagram of point light spot imaging on the screen under a certain field of view condition for three different wavelengths of light (0.45μm, 0.55μm, 0.643μm) under normalized different field of view conditions, where RMS represents the spot radius, which is generally less than 1 pixel, and GEO represents the geometric radius. Generally, GEO / RMS is less than 3, which means that assembly is relatively easy.

[0083] In this embodiment, 1 pixel = 5.4 μm. Table 2 shows the RMS value and GEO value of the light spot at 10 different viewing points. Taking a 0.47-inch DMD (digital micromirror device) as an example, the RMS value and GEO value of 10 points of equal area are sampled. It can be seen from Table 2 that the RMS value is less than 1 pixel and the GEO / RMS is less than 3, so the assembly yield of the projection lens 1 is good.

[0084] Table 2 RMS value and GEO value of the spot at different field of view points

[0085] Fig. 9 A TV distortion diagram of a small-volume direct projection system provided by an embodiment of the present invention is used to evaluate whether the image of the direct projection system has shape or angle distortion. Fig. 9As shown, the blue dot is the actual position of the light projected onto the screen, and the black line is the theoretical position. The closer the dot is to the intersection of the black lines, the smaller the TV distortion. The TV distortion of the small-volume direct projection system provided in this embodiment is -0.0154%, which has a small distortion and good imaging quality.

[0086] It should be noted that the terms used in the present invention are only for describing specific embodiments, rather than limiting the scope of the present application. As shown in the present specification, unless the context clearly indicates an exception, the words "one", "a", "a kind of" and / or "the" do not specifically refer to the singular, but may also include the plural. The terms "include", "comprise" or any other variant thereof are intended to cover non-exclusive inclusion, so that the process, method or device including a series of elements includes not only those elements, but also includes other elements not explicitly listed, or also includes elements inherent to such process, method or device. In the absence of more restrictions, the elements defined by the sentence "include one..." do not exclude the presence of other identical elements in the process, method or device including the elements.

[0087] It should also be noted that the terms "center", "up", "down", "left", "right", "vertical", "horizontal", "inside", "outside", etc., indicating the orientation or positional relationship, are based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention. Unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", etc. should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be an indirect connection through an intermediate medium, or it can be a connection between the two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0088] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the technical solutions of the embodiments of the present invention.

Claims

1. A small volume direct projection system, characterized in that: include: A light valve (3), an illumination prism (4), an image biasing mirror (5), and a projection lens (1) are arranged in sequence from the direction of light incidence to the direction of light emission; The projection lens (1) is provided with a rear lens group and a front lens group in sequence on the same optical axis from the incident direction of the light to the exit direction of the light, the rear lens group comprising a first lens (11), a second lens (12), a third lens (13), a fourth lens (14), and a fifth lens (15), and the front lens group comprising a sixth lens (16), a seventh lens (17), an eighth lens (18), and a ninth lens (19); An aperture (2) is provided between the front lens group and the rear lens group, and the aperture (2) is located in the middle of the projection lens (1); The third lens (13), the fourth lens (14) and the fifth lens (15) are combined into a triplet lens, the fifth lens (15) is adjacent to the aperture (2), and a combination of high, low and high refractive index is used to correct the chromatic aberration of the projection lens (1); the third lens (13), the fourth lens (14) and the fifth lens (15) in the triplet lens are all glass spherical lenses, and the refractive index range of the third lens (13) is [1.5, 1.7], the refractive index range of the fourth lens (14) is [1.45, 1.65], and the refractive index range of the fifth lens (15) is [1.5, 1.7]; The second lens (12) is arranged in the lens barrel (6); the position of the second lens (12) is adjustable, and is used to compensate for the degradation of image quality caused by the tilt of the triplet lens and the front lens group; the second lens (12) is mounted on the wall of the lens barrel (6), symmetrically along the circumferential direction, and has a plurality of adjustment screws (7) threadedly connected to the wall; the adjustment screws (7) penetrate the wall of the lens barrel (6) and abut against the circumference of the second lens (12), and are used to adjust the eccentric position of the second lens (12); the refractive index of the second lens (12) is 1.5; The first lens (11), the second lens (12), the sixth lens (16), and the seventh lens (17) are symmetrically arranged on both sides of the aperture (2) and are used to balance the aberration of the projection lens (1); The projection lens (1) has a positive optical power; wherein the optical powers of the first lens (11), the second lens (12), the third lens (13), the fourth lens (14), the fifth lens (15), the sixth lens (16), the seventh lens (17), the eighth lens (18) and the ninth lens (19) are arranged in sequence: positive optical power, positive optical power, positive optical power, positive optical power, positive optical power, negative optical power, positive optical power, positive optical power, negative optical power, negative optical power; The illumination prism (4), the image biasing mirror (5) and the projection lens (1) form a refractive system, the total length of the refractive system is L1, the distance from the light valve (3) to the first lens (11) is a back working distance BFL, and BFL meets the conditional formula: 0.015<BFL / L1<1.

3.

2. A small volume direct projection system according to claim 1, characterized in that: The second lens (12) is a glass spherical lens, used to correct the chromatic aberration of the three colors of red, green and blue entering the second lens (12).

3. A small volume direct projection system according to claim 1, characterized in that: The first lens (11) is a glass spherical lens, used to correct the angle and telecentricity of light entering the first lens (11).

4. The small-volume direct projection system according to claim 1, characterized in that: The sixth lens (16), the seventh lens (17) and the eighth lens (18) are all glass spherical lenses; The sixth lens (16) is a meniscus lens bent toward the light source, the seventh lens (17) is a biconvex lens, and the eighth lens (18) is a biconcave lens; Furthermore, the refractive index range of the eighth lens (18) is [1.5, 1.7), and the Abbe number range is [20, 95], thereby reducing the aberration of the projection lens (1).

5. The small-volume direct projection system according to claim 1, characterized in that: The ninth lens (19) is a resin aspherical lens and is in the shape of a meniscus lens that bends toward the light source, and is used to magnify the magnification of the projection lens (1) and correct the TV distortion of the projection lens (1).

6. The small-volume direct projection system according to claim 1, characterized in that: The total length of the projection lens (1) is L2, and the total focal length of the projection lens (1) is f, satisfying the condition: 2<L2 / f<15.

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