MTF environmental temperature change measuring device

Abstract

To provide a measuring method and a device capable of measuring MTF at high spatial frequencies of 250 lines / mm or more for a temperature in a photographing camera's operating environment.SOLUTION: A lens 1 under test is surrounded by a thermostatic chamber 5, an entrance window 6a is disposed at the top and an exit window 6b is disposed at the bottom. A chart image at an image forming point 3 of a first chart 10 is photographed from the bottom by an objective lens 12. The objective lens used here has a long working distance WD (Working Distance) of 10 mm or more. This optical system configuration makes it possible to perform highly accurate MTF measurement without an imaging optical system being affected by severe test temperature changes in the thermostatic chamber. Further, the configuration of an orthographic projection optical system can be adopted, so that a slit line width of the chart 10 is reduced by an optical system magnification of a collimator and the lens under test, and a high-definition chart image can be formed on the image forming point, thereby making it possible to measure the MTF characteristics of high spatial frequencies.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a measuring device and an inspection device for assuring the MTF characteristics of photographic cameras such as vehicle-mounted cameras, mobile phone cameras, and CCTV cameras with respect to environmental temperatures. [Background technology]

[0002] As an example of a camera, the structure of a typical vehicle-mounted camera is shown in Figure 1. It is composed of a photographing lens unit 100 that captures light beams from a target object and forms an image, and a camera section 14 that converts the image captured on the image sensor 15 into an electrical signal on a circuit board 16, performs image processing (exposure correction, aberration correction, color correction, etc.) as necessary, and outputs the signal to an external device.

[0003] Here, if the environmental temperature fluctuates, the refractive index, radius of curvature, lens thickness, and lens spacing of each lens in the lens unit will change, which may result in changes in focus and other optical aberrations (distortion, field curvature, etc.), leading to a deterioration in the MTF characteristics, which indicate resolution performance.

[0004] From the perspective of national security, in-vehicle cameras in particular require safety assurance based on extremely strict standards, including the international standard ISO26262, the automotive industry QMS standard TS16949, and cybersecurity (CS) regulations, and they are required to guarantee the MTF (resolution characteristic) over a temperature range from the extreme cold of -40°C to the maximum temperature inside the vehicle of approximately 125°C. Moreover, in recent years, the pixel size of image sensors has become increasingly fine, which has led to demands for higher resolution in photographic lens units, and the evaluation of MTF characteristics at high frequencies is now required.

[0005] In recent years, the pixel size of image sensors has increased to 2 μm, and it has become necessary to evaluate the MTF at a Nyquist frequency Fn equivalent to 250 lines / mm (Fn = 1 / (2 × 0.002)).

[0006] MTF measurement devices at room temperature generally use an orthogonal projection method, which makes it possible to evaluate MTF at frequencies up to about 300 lines / mm. However, for reasons described below, existing MTF temperature characteristic measurement devices often use an inverse projection optical system to evaluate the temperature characteristics of MTF, and as a result, MTF evaluation is limited to a frequency of about 100 to 150 lines / mm. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 10-68674 [Patent Document 2] JP 2003-50183 A [Patent Document 3] JP 2004-294076 A [Patent Document 4] JP 2019-49524 A [Patent Document 5] JP 2022-191392 A Summary of the Invention [Problem to be solved by the invention]

[0008] The resolution performance of a camera is generally tested by measuring the Modulation Transfer Function (MTF). Many methods and devices for testing MTF have been proposed and are on the market, and they can be broadly divided into orthogonal projection and inverse projection methods.

[0009] The orthogonal projection method is a method in which a chart is placed on the object side of the lens to be tested, and the MTF value is output from the chart image formed on the lens to be tested, and has been proposed in JP-A-10-68674, JP-A-2003-50183, JP-A-2004-294076, etc. As described below, the orthogonal projection method has the advantage of being able to test high-frequency resolution and making high use of the light quantity of the measuring instrument's light source (halogen, etc.). On the other hand, when measuring MTF with a wide angle of view, it is necessary to move the image sensor or use a large sensor. Therefore, when prioritizing applications that shorten the number of inspections and the inspection time, the back projection method is considered to be slightly more advantageous.

[0010] The back projection method is a method in which a chart is placed on the image side of the test lens and the MTF value is output from the chart image formed on the object side by the test lens, and has been proposed in JP 2019-49524 A and JP 2022-191392 A. Compared to the orthogonal projection method, the back projection method has an advantage in terms of the number of inspections and inspection time because it can measure the MTF values ​​of multiple angles of view almost simultaneously by forming a chart corresponding to each angle of view on the image plane. On the other hand, since the line width of the chart determines the measurement resolution, the current slit line width of about 5 μm is the limit, making it difficult to measure MTF at frequencies of 100 lines / mm or more.

[0011] Although some devices capable of measuring the temperature characteristics of MTF have been proposed, there are almost no relevant documents published in patent documents, papers, etc. Therefore, an example using the orthogonal projection method is shown in Figure 2. The lens 1 to be inspected is surrounded by a thermostatic chamber, the outer walls of which are covered with insulating material to maintain as much thermal insulation as possible from the outside air, and the temperature inside the thermostatic chamber is varied by introducing temperature-controlled gas or air ranging from low to high temperatures.

[0012] The resolution chart 10 is irradiated with light from the light guide 7, converted into a substantially parallel light beam by the collimator lens 11, and then focused by the test lens 1 to form a chart image (aerial image) at the imaging point 3, thus constituting a so-called orthogonal projection optical system. This chart image is re-formed on the image sensor 15 via the microscope objective lens 12. The MTF value can be calculated from the contrast of this image or by Fourier transform calculation, etc.

[0013] Furthermore, by measuring the MTF while defocusing the photographing optical system unit 31, which incorporates the objective lens 12 and the image sensor 15, in the optical axis direction, the so-called MTF through-focus characteristics (defocus vs. MTF characteristics) can be measured, and the tolerance of the MTF to focus deviation can be evaluated. In addition, the projection optical system unit 30 incorporating the collimator 11 and chart 10 is changed in the direction of the angle of view α 25, and the photographing optical system unit 31 is moved in the image height direction y to track the chart image near the center of the image sensor, thereby making it possible to measure the MTF characteristics with respect to the angle of view α (image height y). Furthermore, by rotating the θ stage 26 around the optical axis of the lens 1 under test, the MTF characteristics in all directions can also be measured.

[0014] As described above, by using the method shown in FIG. 2, if the same focus position is measured at each temperature, it is in principle possible to measure the variation in focus position, that is, the variation in flange back deviation. However, because the tip of the objective lens is designed to fit inside the bottom of the thermostatic bath, the focal length of the microscope objective lens becomes unstable due to the effect of the temperature of the thermostatic bath. Furthermore, because the incident light side (incident light beam from the collimator) and the exit side of the thermostatic bath are not completely isolated from the outside temperature, temperature distribution occurs inside the thermostatic bath and air fluctuations occur due to the temperature difference with the air outside the thermostatic bath, distorting the chart image, resulting in a significant deterioration in MTF measurement accuracy.

[0015] As described above, it was difficult to accurately measure the temperature variation characteristics of MTF using the conventional MTF temperature characteristic measuring device shown in Fig. 2. Therefore, MTF temperature characteristic evaluation devices using the back projection method have been proposed and sold. This method is also not clearly disclosed in documents such as patent documents, so an overview of the method is shown in Fig. 3.

[0016] The lens 1 to be tested is surrounded by a thermostatic chamber 5, the outer walls of which are covered with insulating material to maintain as much insulation from the outside air as possible, and a glass entrance window is installed on the incident light beam side. Temperature-controlled gas or air, ranging from low to high, can be inserted into the thermostatic chamber to maintain a constant temperature inside the chamber. This is a so-called back projection optical system in which a resolution chart 10 is placed near the image position of the test lens 1. Here, the chart 10 usually uses a simple parallel flat glass member with a cross slit or the like formed on the glass plate, so even if large temperature fluctuations occur in the thermostatic chamber from extremely low to high temperatures, they do not affect the deterioration of the measurement optical system performance. Therefore, it is possible to place the chart inside the thermostatic chamber.

[0017] The chart 10 is illuminated with a light beam from a light guide 7, passes through the test lens, and a chart image is formed on an image sensor 15 by a collimator lens 11. The temperature characteristics of the MTF value of this chart image can be evaluated by the contrast method or Fourier transform calculation.

[0018] By measuring the MTF while defocusing the chart 10 in the optical axis direction, it is possible to measure the so-called MTF through-focus characteristics (defocus vs. MTF characteristics). In addition, by moving the projection optical unit 30 incorporating the chart 10 and the light guide illumination 7 in the image height y direction and changing the photographing optical system unit 31 incorporating the collimator 11 and the image sensor 15 in the field angle α direction 25 so that the chart image is photographed near the center of the image sensor, it is possible to measure the MTF characteristics with respect to the field angle α (image height y). Furthermore, by rotating the θ stage 26 around the optical axis of the test lens 1, it is possible to measure the MTF characteristics in all directions.

[0019] As described above, the existing method shown in Fig. 3 has high thermostatic properties and a high thermal insulation effect with the outside air, so there is little air fluctuation caused by temperature differences with the outside air and no distortion of the chart image. In addition, by adopting a back projection optical system, only the chart 10 is placed inside the thermostatic chamber, so even severe temperature fluctuations in the thermostatic chamber have almost no effect on deterioration of the measurement optical system performance. Therefore, it is currently considered to be the best method for evaluating the temperature characteristics of MTF and flange back.

[0020] However, in the case of a back projection optical system, the measurable frequency characteristics on the image plane are limited by the resolution of the chart 10, that is, the slit line width. Ordinary charts are produced in the same way as semiconductor manufacturing processes, by forming a film of chrome or the like on a glass substrate, then exposing and etching it to form a cross chart or edge chart, but the guaranteed accuracy of line width and edges is generally about 5 μm. Therefore, the maximum frequency Fm for MTF measurement is limited to about 100 lines / mm (Fm = 1 / [line width x 2] = 1 / (0.005 mm x 2)).

[0021] Due to the demand for miniaturization and high density in recent imaging elements, pixel sizes of around 2 μm are becoming commonplace, and the evaluation frequency for image resolution, i.e., the Nyquist frequency Fn, needs to be evaluated at 250 lines / mm (Fn = 1 / [line width × 2] = 1 / (0.002 mm × 2)), and the inability to evaluate this has become a problem.

[0022] On the other hand, in the orthogonal projection method, as shown in the conventional example in Fig. 2, the chart image is formed on the test lens 1 after passing through the collimator 11, but since the optical magnification of the collimator and the test lens is usually used at 10 times or more, the chart image on the image plane can be formed at a 10 times higher frequency, and theoretically, frequencies up to about 1000 lines / mm can be measured. In reality, the orthogonal projection method can measure MTF values ​​up to about 300 lines / mm due to restrictions such as the number of bits and noise of the shooting signal. However, the conventional orthogonal projection method MTF temperature characteristic evaluation measuring device Fig. 2 has a problem of low MTF measurement accuracy because the microscope objective lens is affected by the temperature of the thermostatic bath as described in

[0014] . [Means for solving the problem]

[0023] The object of the present invention is to provide a method and apparatus that can ensure highly accurate measurement of the temperature characteristics of MTF in a stable constant temperature environment even in a wide range of temperature environments from low to high, and that can measure high-frequency MTF of 250 lines / mm or more.

[0024] "Explanation of claim 1" The overall configuration of the present invention is shown in Figure 4. The basic structure of the present invention is an orthogonal projection type MTF measuring device, in which the light beam from a first chart 10 on the illumination light source side is collimated by a collimator 11 and then enters the test lens 1, where a chart image (aerial image) is formed at an imaging point 3. Here, when evaluating MTF performance at a finite distance, a finite system lens (a lens whose object point is not at infinity but at a finite distance) may be used instead of the collimator 11.

[0025] The test lens 1 is surrounded by a thermostatic chamber 5, and has an entrance window 6a at the top and an exit window 6b at the bottom. A chart image at the image-forming point 3 of the first chart 10 is captured from the bottom by an objective lens 12 having a long working distance. The objective lens used here has a working distance (WD), which is the distance from the tip of the objective lens to the object surface being photographed by the objective lens, that is longer than that of the objective lens used in the conventional measuring machine shown in Figure 2.

[0026] By configuring the measurement optical system of the present invention, there are no measurement optical systems other than the lens to be tested inside the thermostatic chamber, and the entrance window 6a and exit window 6b separate the thermostatic chamber from the outside air, so the temperature stability inside the thermostatic chamber is extremely high. Furthermore, since the objective lens observes the chart image of the lens to be tested from outside the thermostatic chamber through the exit window 6a, it is not affected by the severe temperature of the thermostatic chamber, and there is no focal length fluctuation or deterioration of optical performance. When an objective lens with a short working distance is used as in Figure 2 of the conventional method, as described in

[0014] , there are problems such as the objective lens being affected by the temperature of the thermostatic chamber, causing the focal length of the microscope to become unstable, the temperature stability inside the thermostatic chamber being low, and air fluctuations distorting the chart image, resulting in a significant deterioration in MTF measurement accuracy. However, by configuring a measurement optical system using an objective lens with a long working distance according to the present invention, it is possible to ensure high temperature stability of the thermostatic chamber and measure the MTF temperature characteristics with high accuracy.

[0027] The long working distance objective lens of the present invention will be described in detail with reference to Fig. 5, which is an enlarged view of the periphery of the test lens 1 in Fig. 4. Here, WD is the working distance of the objective lens 12, FB is the flange back distance of the test lens, and the glass thickness of the exit window 6a is represented by T1 in the case of a single-pane structure and by T2 in the case of a double-pane structure. D1 is the distance from the image point of the test lens 1 to the exit window 6b, and D2 is the distance from the exit window 6b to the tip of the objective lens. ΔH is the movement amount of the objective lens 12 when measuring the MTF through-focus characteristic, including the movement amount and the margin.

[0028] Here, although the exit window 6b is thermally insulated from the inside of the thermostatic chamber and the air outside the thermostatic chamber, the area about 3 mm from the surface is thermally unstable, and in the worst case, air fluctuations may adversely affect the MTF measurement results. For this reason, it is desirable to set D1>3mm, D2>3mm. Also, it is desirable to ensure that the objective lens movement amount is 3mm or more, including the MTF through-focus characteristics and margins. For this reason, it is desirable to set ΔH>3mm. Next, the glass thickness of the exit window 6b is desirably T>1mm to ensure surface accuracy. Therefore, it is desirable to set WD > 10 mm (WD = D1 + D2 + T1 + ΔH = 3 + 3 + 1 + 3 = 10).

[0029] In order to improve the heat insulation effect at the exit window 6b and to prevent condensation during low-temperature measurements, it is effective to use a double-pane glass structure. In this case, if two pieces of glass with a thickness of 1 mm are used and the space between them is 1 mm, it is desirable to set the thickness T2 of the double-pane structure to > 3 mm. Therefore, when exit window 6b has a double window glass structure, the working distance is desirably set to WD>12 mm (WD=D1+D2+T1+ΔH=3+3+3+3=12).

[0030] The objective lens used in MTF measurement is required to have high optical performance. In particular, since the numerical aperture of the photographic lens used in vehicle-mounted lenses is about NA0.2 to 0.5, the numerical aperture of the objective lens needs to be NA>0.5. The lenses used in the conventional example in Figure 2 are often objective lenses for microscopes. However, for microscope objective lenses with NA exceeding 0.5, the working distance WD is generally 5 mm or less, and objective lenses with a WD exceeding 10 mm are rarely commercially available. In addition, if a glass member such as the exit window 6b is inserted in the path of the imaging light beam from the objective lens, spherical aberration will occur. Therefore, it is necessary to design the exit window so that the spherical aberration caused by the thickness of the glass material can be corrected.

[0031] Therefore, the objective lens used in the present invention is characterized in that the working distance WD is 10 mm or more, the numerical aperture NA is 0.5 or more, and the spherical aberration due to the exit window is corrected.

[0032] "Explanation of claim 2" In the conventional example shown in Fig. 3, a back projection optical system is used that allows only the chart to be placed near the thermostatic chamber in order to deal with the severe thermal changes of the thermostatic chamber. However, as described in

[0020] and

[0021] , in the case of a back projection optical system, the slit line width of the chart is the limit of the MTF measurement frequency, and with the current slit line width of 5 μm, the limit is 100 lines / mm, and the pixel size of 250 lines / mm, which is equivalent to 2 μm, which is currently required, cannot be evaluated. The measurement optical system of the present invention can realize an orthogonal projection optical system without placing the measurement optical system inside the thermostatic chamber by employing an objective lens with a long working distance. Effect of the Invention

[0033] The conventional method of measuring the temperature characteristics of MTF is an orthogonal projection method (Fig. 2), but the objective lens is subject to the harsh temperature change control of the thermostatic chamber, making it difficult to perform stable MTF evaluation. The conventional method (Fig. 3) is a back projection method, so MTF evaluation was limited to about 100 lines / mm. This invention develops a new long working distance objective lens that enables evaluation of the MTF performance of the test lens from outside the thermostatic chamber, enabling stable MTF measurement without the measurement optical system itself being affected by the temperature of the thermostatic chamber. Furthermore, because an orthogonal projection optical system can be used, MTF evaluation at high frequencies of 250 lines / mm or more is possible. The present invention provides a method and an apparatus that can measure and guarantee MTF characteristics with high accuracy even in a wide range of temperature environments, from low to high temperatures, and can measure high-frequency MTF. [Brief description of the drawings]

[0034] [Figure 1] FIG. 1 is a structural diagram of a typical vehicle-mounted camera. [Diagram 2] This is an example of a conventional MTF temperature characteristic measuring device that uses the orthogonal projection method. [Diagram 3] This is an example of a conventional MTF temperature characteristic measuring device that uses the back projection method. [Figure 4] FIG. 1 is a configuration diagram of an MT temperature characteristic measuring device according to the present invention. [Diagram 5] 5 is an enlarged configuration diagram of the periphery of the test lens 1 in FIG. 4. [Figure 6] This shows an example of measurement of MTF frequency characteristics. [Figure 7] 13 shows an example of measurement of the MTF through-focus characteristics at each angle of view. [Figure 8] This shows an example of measurement of MTF characteristics (MTF map) in all directions. [Figure 9] FIG. 2 is a structural diagram showing an image of the rotation of the bottom of the thermostatic chamber of the present invention. [Explanation of symbols]

[0035] 1 Test lens, 2 Flange surface, 3 Imaging point, 4 Lens holder, 5 Thermostatic chamber, 5a thermostatic gas inlet, 5b thermostatic gas outlet 5c Constant temperature chamber bottom, 5d Constant temperature chamber bulkhead 6 thermostatic chamber window, 6a entrance window, 6b exit window 7 light guide, 8 filter, 9 diffuser, 10 chart, 11 collimator, 12 objective lens, 13 imaging lens, 14 camera unit, 15 image sensor, 16 Circuit board, 17 Connector, 25 α (angle of view) stage, 26 θ (around the optical axis) stage, 27 Y,Z (image height, optical axis direction) stage, 30 projection optical system unit, 31 photographing optical system unit, 40 Test lens optical axis adjustment jig 50 Device base 100 Photographing lens unit DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLES

[0036] According to the present invention, it is possible to measure the MTF temperature characteristics of high spatial frequencies using an orthogonal projection optical system as shown in Figure 4. One embodiment of this is shown below. In FIG. 4, MTF measurement is performed with the following specifications: ◇Measurement optical system conditions Chart minimum line width: ΔS=10μm=0.01mm (general slit line width) Test lens focal length: fs=5mm Collimator focal length: fc=50mm Objective lens magnification: Mi=50x ◇Chart image at imaging point 3 of the test lens Orthogonal projection optical system: The optical magnification Ms from the chart to the image point is Ms=fc / fs=50mm / 5mm=10x The chart image line width at the focal point: ΔSz is as follows: ΔSz=ΔS / Ms=10μm / 10 times=1μm The maximum frequency Fm on the image point is Fm = 1 / (2 × ΔSz) = 500 pieces / mm Therefore, a chart image with a maximum MTF performance of 500 lines / mm is formed at the image point. ◇Chart image on the image sensor of the measuring device The chart image line width on the image sensor: ΔSi is as follows: ΔSi=ΔSz×Mi=1μm×50 times=50μm The maximum frequency Fi on the image sensor is Fi = 1 / (2 × ΔSi) = 10 pieces / mm

[0037] From the above, in the case of an MTF temperature characteristic measuring instrument with an orthogonal projection optical system having the measurement optical system configuration of the present invention, a typical resolution chart line width ΔS (10 μm) is reduced by the optical system magnification Ms (10x) of the collimator and the lens to be tested, resulting in a chart image line width ΔSz (1 μm) at the imaging point, and an image equivalent to a high frequency frequency Fm (500 lines / mm) is formed. This is magnified to a magnification Mi (50x) by the objective lens optical system, re-imaged on the image sensor and enlarged to a chart line width ΔSi (50μm), and the MTF can be analyzed at a low frequency Fi (10 lines / mm), making it possible to measure MTF with high precision. In other words, if an orthogonal projection optical system is used, it becomes possible to measure the MTF characteristics of high spatial frequencies without using a high-definition resolution chart line width.

[0038] This embodiment is merely an example, and MTF can be measured by changing the resolution chart line width, collimator lens focal length, etc. according to the maximum measurement frequency Fm and the focal length of the lens to be tested. In recent years, the resolution of image sensors in photographic cameras has improved, and sensors with pixel sizes of 2 μm or less are now being used. In this case, the Nyquist frequency Fn = (1 / ([pixel size] x 2) = (1 / ([2 μm] x 2) = 250 lines / mm, and MTF measuring devices are also required to be able to perform evaluations at 250 lines / mm. By changing the chart line width and collimator lens focal length of the orthogonal projection optical system of the present invention, it is possible to easily measure MTF with spatial frequencies of 250 lines / mm or more, and in principle even 1000 lines / mm or more. However, due to the influence of electrical noise of the chart image captured on the image and air fluctuation noise of the optical image, the actual limit for Fm is thought to be around 600 lines / mm. Even so, even if the pixel size of image sensors continues to increase in the future, it will still be possible to fully accommodate pixel sizes of 1 μm or less.

[0039] "Explanation of claim 3" By optimizing the slit line width of the resolution chart and the focal length of the collimator using an MTF temperature characteristic measuring device for the orthogonal projection optical system based on the present invention, MTF evaluation is possible at spatial frequencies of 250 lines / mm or more.

[0040] Generally, an MTF measuring device must be able to perform the following evaluations: a.MTF frequency response: It is an index for evaluating the frequency characteristics of the resolution of the test lens, and ideally, it can also determine the MTF resolution performance at the Nyquist frequency of the image sensor of the photographing camera. As mentioned above, the present invention makes it possible to evaluate the MTF at frequencies exceeding 250 lines / mm. An example of the measured MTF frequency characteristics is shown in Figure 6. b.MTF through focus characteristics (defocus vs MTF characteristics): This is an index for evaluating the tolerance of the resolution with respect to the focus position shift of the test lens, and can be measured by measuring the MTF while moving the Z stage 27 in the optical axis direction z in FIG. c.MTF characteristics for angle of view α (image height y): This is an index for evaluating the resolution relative to the angle of view of the test lens. In FIG. 4, the angle of incidence is changed on the α (angle of view) stage 25, and the Z stage 27 is caused to follow so that the chart image is formed approximately in the center of the image sensor 15, thereby enabling the MTF at each image height to be measured. FIG. 7 shows a measurement example of the MTF through-focus characteristic b for each angle of view. d. Omnidirectional MTF characteristics: This is an index for evaluating the resolution in all directions of the test lens, i.e., in the entire area of ​​the image sensor of the shooting camera. In FIG. 4, by rotating the θ stage 26 around the optical axis of the test lens 1, the MTF characteristics in all directions can also be measured. Fig. 8 shows an example of measurement of MTF characteristics (MTF map) in all directions (all over the image sensor).

[0041] Here, each of the above MTF characteristics can be measured and evaluated using the conventional method shown in Figures 2 and 3. However, in order to particularly evaluate the MTF characteristic c for the angle of view α (image height y) and the omnidirectional MTF characteristic d, it is necessary to install an optical axis adjustment jig 40 for the test lens to align the optical axis of the test lens with the optical axis of the measuring device optical system, and a mechanism 26 that can rotate around the optical axis of the test lens, near the test lens 1. In conventional measuring devices capable of evaluating the MTF temperature characteristics, the optical axis adjustment jig for the test lens and a mechanism that can rotate around the optical axis of the test lens are installed inside or near the thermostatic chamber, which requires a large heat capacity and a large thermostatic chamber, and it takes a very long time for the thermostatic chamber to reach a constant temperature (stable temperature).

[0042] In the conventional example of FIG. 3, in order to measure the MTF in all directions, the thermostatic chamber has a double structure, and below the lens receiving base 4, an optical axis adjustment jig 40 for the test lens 1 to align the optical axis of the test lens 1 with the optical axis of the measuring instrument optical system, and a θ stage 26 that can rotate around the optical axis of the test lens are installed. Although the thermostatic chamber has a double structure, the thermal insulation between the optical axis adjustment jig 40 of the lens to be inspected and the θ stage 26 is not sufficient due to the influence of heat conduction with the lens receiving base 4. Therefore, it is necessary to control the temperature of these jigs and stages to a certain degree. Therefore, before a constant temperature of an arbitrary temperature is reached, it is necessary to achieve a constant temperature to a certain degree for the large heat capacity of the jigs and stages, and it takes more than 30 minutes for the vicinity of the lens to be inspected to reach a stable constant temperature. The MTF temperature characteristics of the test lens are measured and evaluated at least at room temperature, a high temperature (125°C), and a low temperature (-40°C), which poses a problem of taking a long time for measurement.

[0043] "Explanation of claim 5" In the present invention, as shown in FIG. 4, only the test lens 1, the lens holder 4, and the test lens optical axis adjustment jig 40 are installed inside the thermostatic chamber 5, and the heat capacity of these components and the volume inside the thermostatic chamber are much smaller than in the conventional example. Therefore, when evaluating the MTF temperature characteristics, the time required to reach the desired temperature is within about 10 minutes, and since the heat insulating effect is also high, the temperature distribution inside the thermostatic chamber can be kept small.

[0044] "Explanation of claim 4" As described above, in order to keep the number of parts in the thermostatic chamber, i.e., the heat capacity, as small as possible, the θ (around the optical axis) stage 26 is a mechanism that is completely separated from the thermostatic chamber 5 in Fig. 4. Therefore, the thermostatic chamber bottom 5c of the thermostatic chamber 5 is a rotatable mechanism, and the θ stage 26 is installed outside the thermostatic chamber and is a mechanism that rotates the thermostatic chamber bottom.

[0045] An image of the rotation mechanism at the bottom of the thermostatic chamber is shown in Fig. 9. The thermostatic chamber bottom 5c of the thermostatic chamber 5 has a structure separated from the thermostatic chamber main body so as to be rotatable, thereby enabling measurement in all directions.

Claims

1. A measuring device for measuring the modulation transfer function (MTF value) of a test lens with respect to the environmental temperature, comprising: a thermostatic chamber as a means for varying the environmental temperature; a test lens placed inside the thermostatic chamber; an entrance window for admitting a light beam to the test lens and an exit window for observing the imaging position of the test lens; and an orthogonal projection optical system for observing a chart image at the imaging position from outside the exit window of the thermostatic chamber with an objective lens having a long working distance, wherein the MTF measurement method or the measuring device thereof is characterized in that the MTF is calculated from the observed chart image.

2. 2. The MTF measuring method or measuring device according to claim 1, wherein the objective lens has a working distance WD of 10 mm or more, a numerical aperture NA of 0.5 or more, and spherical aberration occurring due to the thickness of the glass material of the exit window is corrected.

3. 2. The MTF measuring method or device according to claim 1, wherein in the orthogonal projection optics, a slit line width of the resolution chart and a focal length of the collimator are set so that MTF evaluation is possible at a spatial frequency of 250 lines / mm or more.

4. 2. The MTF measuring method or device according to claim 1, wherein the bottom of said thermostatic chamber has a function of being rotatable about the optical axis of the lens to be tested, thereby enabling MTF measurement in all directions.

5. 2. The MTF measuring method or device according to claim 1, wherein a stand for placing a lens to be tested and a jig for adjusting an optical axis of the lens are stored inside the thermostatic chamber.

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