JPS58203405A - Temperature compensating mechanism of lens system - Google Patents

Abstract

PURPOSE:To make the temperature compensation of a lens system, where the lens back has a negative coefficient of temperature, possible, by forming a collimator lens and the lens of a read optical system with plastic and setting properly the relation between lens materials and coefficients of thermal expansion of a collimator holder and a barrel. CONSTITUTION:The barrel is formed with a triple cylinder consisting of three cylinders 19, 20, and 21, and the innermost inside cylinder 19 holding a lens 8 of an astigmatism generating optical system and the outermost outside cylinder 21 holding a light receiving face 10 are formed with materials having a relatively low coefficient of expansion such as metals or plastics packed with glass fibers, and the middle cylinder 20 connecting the end of the inside cylinder 19 on the opposite side of the lens 8 and the end of the outside cylinder nearer to the lens 8 is formed with materials having a relatively high coefficient of thermal expansion such as plastics. In this constitution, thermal expansion of the inside cylinder and the outside cylinder can be ignored in comparison with that of the middle cylinder because coefficients of thermal expansion of inside and outside cylinders are sufficiently lower than that of the middle cylinder; and as a result, when the temperature rises, the middle cylinder has the right end stopped and has the left end expanded left, and the light receiving face is drawn left, namely, toward the lens 8 to allow the light receiving face to follow up the reduction of the lens back.

Description

[Detailed description of the invention]

The present invention relates to a temperature compensation mechanism when lens back changes due to temperature changes. Making all or part of the lens system a plastic lens is very advantageous for mass production of lens machining surfaces, especially aspherical lenses, and using aspherical lenses is extremely effective in removing aberrations. Plastic lenses are increasingly being used for these reasons. However, plastics generally have a higher coefficient of thermal expansion than glass or metals, and their refractive index decreases as the temperature rises, so plastic lenses increase in size due to temperature increases, which increases the focal length and decreases the refractive index of the upper layer. Since the focal length also becomes longer, the effects of both are added together, resulting in a large change in focal length. As a result, the lens back of an optical device containing a plastic lens often becomes longer as the temperature rises, but depending on the configuration of the lens system, the lens bank becomes shorter with a temperature rise of 1, that is, the lens back increases. It may have a negative temperature coefficient. Conventionally, mechanisms have been proposed for performing temperature compensation when the lens bank has a positive temperature coefficient, but no proposal has yet been made regarding temperature compensation when the lens back has a negative temperature coefficient. SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned situation and has an object to provide temperature compensation for a lens system in which a lens bank has a negative temperature coefficient. In order to clarify the purpose of the present invention more specifically, an optical system in which the lens back has a negative temperature coefficient will be explained. 1st
The figure shows an optical system of a conventional optical disc reader. 2 is a semiconductor laser, 2 is a collimating lens, 3 is a collimating holder, 4 is a polarizing beam splitter, 5 is a 1/41 wavelength plate, 6 is a focus lens, and 7 is an optical disk. The above configuration is an illumination optical system. The diameter of the illumination light spot on the optical disk is on the order of . . Since a single spherical lens cannot produce a lens with such slight aberrations, the collimating lens is usually a combination of at least two lenses. Since the surface of the optical disk is a mirror surface, the light incident on the optical disk travels backward along the incident optical path, is reflected by the splitter surface 4.' of the beam splitter 4., passes through the lens 8.8', and becomes an astigmatic beam. The light is incident on the light receiving surface 10. 9 is a lens barrel. The section from the focus lens 6 to the light receiving surface 10 is a reading optical system. The reading optical system is responsible for three functions: autofocus of the focus lens 6, tracking servo (operation for causing the reading device to follow the track of the optical disc), and output of a reading signal. Lens 8 is a spherical plano-convex lens, and 8' is a flat cylindrical lens, which converts the light beam transmitted through 8°8' into an astigmatic light beam. The light-receiving surface is positioned at the circle of least confusion of this astigmatic light beam. The light-receiving surface 10 is divided into four cross-shaped quadrants, and light-receiving elements 101 to 104 are arranged in each quadrant. When the light-receiving surface 10 deviates from the position of the circle of least confusion of the incident light beam, the irradiation spot for improving light reception becomes as indicated by the dotted line a or b in the figure, and the outputs of adjacent light-receiving elements are no longer equal. Therefore, from the output power of the light receiving element 101 and FO3, 102 and 104.
By subtracting the sum of the outputs of the subtraction circuit A, it is possible to detect whether the circle of least confusion of the light beam incident on the light receiving surface 1 (H) is before or after the light receiving surface. Since the focus lens 6 corresponds to the front focus or the rear focus with respect to the optical disc, autofocus of the focus lens is performed using the output of A. In such an optical system, the collimating lens 6 and the lenses of the reading optical system are made of plastic. Then, aspherical lenses with high precision can be mass-produced, and by using aspherical lenses, the number of lenses can be reduced and weight reduction can be achieved.However, other problems arise when the lenses are made of plastic. Since the coefficient of thermal expansion is larger and the change in refractive index due to temperature is also large, in addition to the increase in size due to temperature rise, the increase in focal length due to temperature increase in the refractive index causes a large shift in the focal position. The problem can be solved in principle by the following way of thinking. Figure 2A shows a simplified illumination optical system. fc is the focal length of the collimating lens 2 at room temperature (example: 20°C); Similarly, F' c is the focal length at a high temperature (440° C.). A semiconductor laser 1 serving as a light source is located at the focal point of a collimating lens 2 at room temperature. At high temperatures, the light source is located at position 1' due to thermal expansion of the collimator holder 3, and is located closer to the lens 2 than the focal position at high temperatures. This is because, as mentioned above, the increase in focal length is greater than the amount of expansion of the collimator holder 3 because the increase in the dimensions of the lens 2 and the change in refractive index overlap. At high temperatures, the light source 1 is located inside the focal point of the lens 2, so the light transmitted through the collimating lens 2 is not a parallel beam of light but a slightly diverging beam of light. Assuming that autofocus is functioning correctly, the light that has passed through the focus lens 6 will be converged on the optical disk 7 even at high temperatures. Therefore, the reflected light from the optical disk travels the same path as the incident light. That is, the light enters the lens 8t8' of the reading optical system as a slightly convergent light beam. FIG. 2B shows a simplified reading optical system. Bfc is the combined focal length of lenses 8 and 8' at room temperature (
Since it is an astigmatic beam, the focal point is the position of the circle of least confusion with respect to the parallel incident beam). Similarly, εFc is the composite focal length at high temperatures, and ε is the magnification that indicates how many times the focal length of the collimating lens 2 is designed to be the composite focal length of the lens 8° 8'. At room temperature, the light receiving surface 10 is the lens 8, 8'
at the focal point of the composite lens. When the temperature is high, the lens barrel 9 also extends, but the focal length increases further, so the light-receiving surface during the high temperature is located inside lO' (closer to the lens 8.8') than the focal position at the high temperature. If the circle of least confusion at high temperature is at this 10' position, there will be no problem at all, but if the two positions are misaligned, the autofocus will move the focus lens to reduce this misalignment to 0, so the focus lens 6 On the contrary, it ends up being out of focus. By appropriately setting the relationship between the coefficient of thermal expansion of the lens material and the coefficient of thermal expansion of the collimator holder 3 and lens barrel 9, it is possible to eliminate the deviation caused by temperature changes between the light receiving surface and the position of the circle of least confusion in the reading optical system. Think about getting rid of it. In general, the change in the focal length of a lens due to temperature is expressed as follows in the thin lens approximation: f≦(1+αT)×22+×f Old... Old... [1
1n'-1 Here, f: Focal length at reference temperature f': Original focal length at temperature change T α: Coefficient of linear expansion of lens material n: Refractive index at reference temperature n': Original focal length at temperature change T The original refractive index is nZ ex n + βT β: the temperature coefficient of the refractive index, and equation (1) is obtained. At T=40°C, β is 110XIO'/'c
If rl is −1,5 in terms of degree, the denominator of the above equation can be seen as 1, so △f, = (α−”)Tf...
It can be approximated by (3). In the above formula, [α
-β/(n-1)'1% is called the coefficient of change in focal length due to temperature and is expressed by fγ. For example, in the case of acrylic resin, it is α-9X 10757 ·c β--10XIO7°C n-1,48366 (wavelength 800 nm), so it becomes fγ-2,968X10 /°c. Returning to Fig. 4, if we assume that the reading optical system has exactly the same configuration as the illumination optical system, including focal length dimensions and materials, the light receiving surface 10 will be
and light source 1 optically correspond to each other, and the light reflected by the optical disk travels the same optical path as the incident light and returns to the light source.

[Actually, it cannot return because there is a beam splitter), so no deviation occurs between the light-receiving surface 10 and the circle of least confusion of the incident light beam, regardless of temperature changes. However, in general, the focal length of the reading optical system is designed to be ε times longer than the focal length of the illumination optical system, so the thermal expansion coefficient of the lens barrel 9 is
The coefficient of thermal expansion must be smaller than that of Therefore, this general case will be explained below. In FIG. 2A, when the temperature increases by T, the distance between the collimating lens and the virtual image 1'' of the light source 1' formed by the collimating lens 2 is b.If the expansion coefficient of the collimating holder 3 is γ, then the collimating lens The distance from 2 to light source 1' is fc (1 + γT), according to the lens formula.
□ In addition, in Figure 2B, where X is the distance to the convergence point of the light that converges at a distance bl behind the lens 8.8', (5) When lenses 2 and 8.8' are in symmetrical positions with respect to the beam splitter surface of beam splitter 4, b = b',
Furthermore, since the purpose is to match the light receiving surface 10' with the convergence point (circle of least confusion) of the light flux, X-εfc (l+γI
T) where γ' is the expansion coefficient of the lens barrel 9. Using these relationships to rewrite equation (5), if metal is used for the lens barrel 9,
Since the expansion coefficient of metal can be selected to be about one order of magnitude smaller than that of plastic, γ' in the first term on the left side of the above equation (6) can be regarded as ○. In addition, if we rearrange equation (6) using fγ-α-Qiao introduced earlier, we can find γ from this: ” = 1-1 k-ri m-(1- k) ■+γT ε 1+fγ・T ε From this, 2≧L・・・・・・・・・(7) γ″″fγ・
T1ε T'! In the range of 4.0°C, fγ・T(ε), so the above equation (7) is γ−1,000*f 7−′; “4 items”−−T...
...(8). When acrylic resin is used as the lens material, the above-mentioned weight + rfr = 2.968xl Omm/'C, and ε
is usually in the range of 2 to 3, so γ-1,
5~2X10 mm/'C. However, plastic materials with a coefficient of thermal expansion of 2 x lOmm at 7°C tend to have low molding accuracy, and currently plastic materials with good molding accuracy have a coefficient of thermal expansion of 1 to 1. ．． 5X
Since it is about 10 mm/°C, the expansion coefficient is slightly smaller than that required by the above equation (8). Therefore, if the collimator holder 3 is made of such a plastic material with good molding precision, the focal point of the reading optical system will be smaller than the lens 8, It will be close to 8. That is, in the reading optical system, the lens back is shortened due to the temperature rise. The present invention is intended to provide a correction means when the lens bank changes in the opposite direction to the normal one due to the above-mentioned temperature change. The present invention will be explained below with reference to Examples. FIG. 8 shows an example in which the present invention is applied to the configuration of the reading optical system of the above-mentioned optical disc reading device. According to the above theory, the collimating lens 2 and the lens 8 of the astigmatism generating optical system are made of acrylic resin, the lens barrel 9 is made of a material with a small thermal expansion coefficient such as metal, and the collimating holder 3 is made of a material with a small thermal expansion coefficient. Squid should be made of plastic material with a temperature of 1.5 to 41 2 x 10 7°C.
If only a material with a value of about 1XlO/° C. can be selected for this purpose, the lens barrel 9 must have a negative coefficient of thermal expansion. Third
In the illustrated embodiment, the lens barrel 9 in FIG.
The innermost inner tube 19 that holds the lens 8 of the astigmatic optical system and the outermost outer tube 21 that holds the light receiving surface 10 are made of metal or glass fiber. It is made of a material with a relatively small coefficient of expansion, such as filled plastic, and is connected to the end of the inner cylinder 19 opposite to the lens 8 and the outer cylinder 2.
The intermediate tube 20 connecting the end of the lens 1 closer to the lens 8 is made of a material having a relatively large coefficient of thermal expansion, such as plastic. With this configuration, the coefficient of thermal expansion of the cylinder and outer cylinder is sufficiently smaller than that of the intermediate cylinder, so the thermal expansion can be ignored compared to the intermediate cylinder.Eventually, when the temperature rises, the right end of the intermediate cylinder remains stationary and the left end moves to the left. Stretch and pull the light-receiving surface to the left, that is, toward the lens 8,
Follows the shortening of the lens bank. The above example is a special optical system in which the lens bank has a negative temperature coefficient, but even in the case of photographic lenses, there are cases where the lens bank has a negative temperature coefficient depending on the combination of lens materials. . The embodiment shown in FIG. 4 is suitable for such a case. The configuration of this lens system is shown in the table below. In the same table, the number in the second row from the left is the surface number of the lens, and the object side force is counted as 1, 2, 3, etc. The object-side seventh surface of the fourth lens 104 is an aspherical surface. Lens surface number / curvature size 5 mm) axis vertical spacing (ITII
TI) Refractive index Dispersion r C1nd Vd 1 1], 386 101 2.602 1.772
5 4.9.602 4.8.4.86 ], 075 1 3-59.590 102 0.929 1.805
1825.4.04, 19.258 3°7651 5 25.4.4.0 103 2.323 1.581
4.4. 4.0.706 -26.84.2 7.116 1 "7 -7.4.4.7 ro+ o,929''"
1.4.91.4. (j57.4, 08 -13.4.
52 *The seventh surface is an aspherical surface, and in this aspherical surface, the distance IWnX to the tangent plane at each incident height Y with respect to the tangent plane at the seventh vertex is given by Co = l /r7 . However, here, ε-1, C1=, -0C2=0.
19995X10-4 C3=-0,11264X10. C4=0.4.7989X10''-7 C6=0.194.15X10-10 Focal length f...35mm F number 1:2.8
Angle of view 21.1=60” Astrabank L
, B, = 15.606 mm In this example, lens 1
The material of lenses 01, 102, and 103 is glass, and the lens 104.
It is Kablastic (acrylic). To explain the effects of temperature on this acrylic lens, since this lens is a concave lens, the lens becomes larger as the temperature rises, and the refractive index also decreases, so the negative power decreases, the focal length becomes shorter, and the lens back becomes shorter. It becomes like this. Considering that only the shape and refractive index of this plastic lens changes, and that the distance between it and other optical elements is almost constant, the amount of temperature fluctuation in the lens back caused by temperature changes in the plastic lens is approximately -0,002 mm/°C. . Therefore, for a change of ±4.0°C, there is a change of +o, osmm. Next, in FIG. 44, these glass lenses 101.
A temperature compensation mechanism using a holder for 102 and 103 and a plastic lens 104 will be described. A glass lens 102 is fitted into the lens holder 105 from the front in the optical axis direction, and the glass lens 102 is inserted through a spacer plate 106.
is inserted, and is held down by the holding screw 1071. On the other hand, a glass lens 103 is fitted from the opposite direction and held down by a holding screw 108, and a plastic lens 104 is further fitted and held down by a holding screw 109. The material of this lens holder 105 is made of a material with a relatively small coefficient of thermal expansion (for example, metal or plastic filled with glass fiber). A temperature compensation holder 110 is attached to the focal point side end 1'05a of the holder. Methods for this attachment include screwing, heat tightening, and insert molding during molding. This temperature compensation holter 110 is formed to extend in a direction away from the focal plane side and is provided with a certain length, and is made of a material having a relatively large coefficient of thermal expansion (for example, plastic, etc.).
made of things. The other end 110 of the holder 110
A second lens holder 111 that is integral with the camera body and maintains a distance from the focal direction is attached to this lens. The second lens holder 111 is made of a material with a relatively small coefficient of thermal expansion (for example, metal or plastic filled with glass fiber). As mentioned above, if the temperature fluctuation of this lens system is -0,002 mm/°C, then the thermal expansion coefficient of the material of the temperature compensation holder 110 is 1.
When the temperature is 5xlO7℃, the length of the boulder 1]0 is 0
．． 002/1.5X10 = 13.3, the lens back will be almost constant regardless of temperature. In other words, the amount of temperature change in the lens back of the lens system is
When the coefficient of thermal expansion of the holder 110 is Y, the length L of the holder 110 is given by L=lx/Yj. (The second lens holder 105 and the second lens holder 11
The coefficient of thermal expansion of 1 is more than an order of magnitude smaller, so it is ignored). Next, another example of temperature compensation in the same lens system as above in FIG. 5 will be described. Focusing on the fluctuation of lens back when the lens spacing changes in this lens system, we can see that the glass lens 103 and the plastic lens 10
When the distance 11 from 4 increases by 0.1 mm, the lens bank becomes 0.
．． 217 becomes shorter. Therefore, with respect to the original focal plane, it will be closer to the 0.11111Il lens side. As mentioned above, the temperature fluctuation of the plastic lens itself is -0
,002mm/°C, so the variation with respect to the temperature of the interval 11 is 0.1 0.002X --0,0017mm/°c0.11
7. Just make it as short as possible. Temperature compensation holder 1 is attached to the focal plane side end 112-a of the second lens holder 112 that holds lenses 101, 102, 103.
13 are fixedly installed. A second lens holder 114 is fixed to the other end 113a of the temperature compensation holder 113. A plastic lens 104 is fitted into the second lens holder 114. The material of the lens holder 112, the second lens holder 114, and the frame 15 is a material with a relatively small coefficient of thermal expansion, such as metal or glass fiber-containing plastic. The material of the temperature compensation holder 113 is plastic or the like having a relatively large coefficient of thermal expansion.In addition, in FIG. 6, when the distance between the glass lens 101 and the glass lens 102 increases by 0.1 mm, the lens bank becomes 0. It is 34.8 mm shorter. Therefore, it is closer to the dish lens side by 0.248 mm with respect to the original focal plane. As in the previous example, the variation of the interval 12 with respect to temperature is 0.002 x 0.110 mm. 24.8=0.00070
4. Make it shorter by mm/'C. The length of the temperature compensation radar should be 41.67 mm. To explain the structure, the glass lens 101 is the second lens holder 1
16 by caulking or the like, and a temperature compensation holder 117 is fixed to the end portion 116a by screws or the like. A second lens holder 118 is fixed to the other end 117a of this temperature compensation holder 117. This second lens holder 118 includes a glass lens 10.
2, 103 and a plastic lens 104 are fixedly attached by caulking or the like. This second lens HONDA-118 is attached to a 9-coid mechanism or a fixed camera body. In the present invention, with the above-described configuration, temperature compensation is performed for a lens system in which the lens bank has a negative temperature coefficient, so even when plastic lenses are used for all or part of the lens group, the It becomes possible to obtain good imaging conditions over a temperature range.

[Brief explanation of the drawing]

FIG. 1 is a longitudinal side view of the optical system of an optical disk reader;
FIG. 2 is a diagram illustrating changes in lens back due to temperature changes in the above device, FIG. 3 is a cross-sectional side view of an embodiment of the device of the present invention, and FIGS. FIG. 3 is a vertical sectional side view of different embodiments of the invention. ■... Light source, 2... Collimating lens, 3... Collimating holder, 8... Astigmatism generating optical system, 9...
- Lens barrel, 10...light receiving direction.・

Claims (1)

[Claims] In a lens system whose lens back has a negative temperature coefficient, the above-mentioned lens system holding frame made of a material that almost ignores the coefficient of thermal expansion fixed with respect to the focal plane of the lens system is used. A lens holding lens barrel made of a material that extends toward the focal plane and has a coefficient of thermal expansion larger than that of the holding frame is fixed, and the lens system is attached to the end of the lens holding mirror on the focal plane side. a lens holder made of a material that holds at least a portion of the lens and whose coefficient of thermal expansion is almost negligible;
A temperature compensation mechanism for a lens system characterized in that the length L of the lens holding barrel in the optical axis direction substantially satisfies the following formula: where X: amount of temperature change Y of the lens bank of the lens system: This is the coefficient of thermal expansion of the lens holding barrel. (2) The lens barrel is made up of three layers, the outermost outer tube is integrated with the part that holds the focal plane of the lens system, and -i
All the lenses of the lens system are held in the inner cylinder, the end of the intermediate cylinder on the focal plane side is connected to the inner cylinder, and the opposite end is connected to the outer cylinder. The temperature compensation mechanism for a lens system according to claim 1, wherein the material has a larger coefficient of expansion than both the inner and outer cylinders. (3) The lens back is extended by shortening the lens distance.The lens system is divided into two lens groups, front and rear, by an appropriate distance between the lenses, and the lens group near the focal plane is made into a frame that is integrated with the focal direction. Claim 1, wherein the lens group on the side farther from the focal plane is held at the end on the focal plane side of a holder whose end on the side farther from the focal plane is fixed to the frame.
Temperature compensation mechanism for the lens system described in Section 2.