GB1601642A - Apparatus for measuring optical transfer function - Google Patents

Description

(54) APPARATUS FOR MEASURING OPTICAL TRANSFER FUNCTION (71) We, CANON KABUSHIKI KAISHA, a Japanese Company of 30-2, 3-chome, Shimomaruko, Ohta-ku, Tokyo, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to apparatus for measuring optical transfer function (hereinafter referred to as "OTF" in its abridged form).

To measure OTF it known in the art to use a measuring method in which a line image or an object slit is formed by the lens to be tested, the distribution of intensity of the image is measured and then it is Fourier-transformed. This known method can be classified further into two methods, that is, a normal projection method and an inverted projection method, taking into consideration the difference in positional relation between the object slit and the test lens. In the normal projection method, a measurement of OTF is carried out under the same positional relation as that for the actual use of the test lens.On the contrary, in the inverted projection method. the object slit is positioned at the position whereat an image is to be formed when the test lens is actually used and the line image is formed at the position which is normally to be a position for the object, and the intensity distribution of the line image is measured. For a lens which constitutes a reducing image forming system, for example, a lens of the type generally used as a photographic lens the inverted projection method is more often employed rather than the normal projection method. This is because if the normal projection method be used for such a lens, the line image formed would be very small, and this in turn would require a high mechanical accuracy for the mechanism used in scanning the image.

The spatial frequency of OTF is defined by the spatial freqency VN at the image side of the lens in the position of normal use (normal projection). Therefore, when the inverted projection method is used, the spatial frequency VR of OTF determined by this method must be scaled in terms of spatial frequency for normal projection using the image forming magnification p and according to the formula:: VN = ssR. Furthermore, for both the normal projection and inverted projection methods, the determination of OFT involves making a correction in respect of the slit width by scaling the known width of the object slit into the width at the image side based on the image forming magnification ss. The correction of slit width is necessary to correct the error of OFT measurement caused by the fact that the OTF measurement is in theory made using an infinitely narrow slit whereas in practice the object slit possessess a finite width.

Figure 1 illustrates the manner in which the above mentioned image forming magnification ss can be obtained in the conventional OTF measurement technique.

Referring to Figure 1,, the reference numeral 1 designates an object plane, 2 is an image forming lens the OTF of which is to be measured, 3 is an image plane, 4 is optical axis and 5 is principal ray. The coordinate on the object plane is designated by t; and the coordinate on the image plane is designated by x. According to the conventional measuring method of image forming magnification, measurement is made to obtain ss = XO/t;O at any suitable position on the object plane, for example at the position 1/2 of the maximum object height: t ;0 wherein X0 is the height of the image of O on the plane 3.Alternatively, the distance between the lens 2 and the object and the distance between the lens 2 and the image are measured to obtain the paraxial lateral magnification po Based upon the found value of p = XOIW" (hereinafter referred to as mean lateral magnification) or that of (3,, the necessary scaling of spatial frequency mentioned above is carried out.

However, there exists usually a distortional aberration in the test lens and therefore the magnification in the formation of the image of the slit for the desired OTF measurement varies depending upon the position and orientation of the slit. This means that an accurate OTF can not be determined using the above mentioned single value of paraxial lateral magnification or mean lateral magnification which is, in the conventional measuring methods, used as a representative of various image forming magnifications in all the positions of an object.

According to the invention, therefore, there is provided apparatus for measuring an optical transfer function of an optical element comprising means for forming an optical test object from which said optical element is to produce a detectable image; means for detecting for each of a plurality of different positions of said test object relative to the optical element, the image produced by said optical element at an image position conjugate with that object position relative to the optical element and for producing an electrical representation of the said image, and means for determining from said electrical representation for each said test object position the image forming magnification with which the said image is formed, and for determining the corrected optical transfer function by performing a calculation in which compensation is made for varying values of said image forming magnification for said different test object positions.

In accordance with the invention it is therefore possible to improve the accuracy of measurement with the OTF measuring apparatus of inverted projection type described above by using an image forming magnification that is different from those hitherto used for the purpose such as paraxial lateral magnification and mean lateral magnification.

In measuring the local image forming magnification with the OTF measuring apparatus disclosed herein in accordance with the invention, a pattern having an extremely small spatial expansion is used as the object and an image of the pattern is formed by the optical test element so as to measure the expansion thereof in the image space. The fundamental principle for the measurement of local image forming magnification according to the invention is embodied in the apparatus described herein by the use, as the object pattern of small spatial expansion of a slit. However, the shape of the object used as such a pattern is not limitative and may be, for example, a double slit, a pattern in the form of plural slits with an increased number of slits and a pattern in a form of rectangular opening.

Accordingly, in the apparatus according to the invention as defined above, the means for forming an optical test object prefereably comprises means defining a first slit and means for illuminating the first slit so that in use light travels through said first slit to enable said optical element to form the image of said first slit at said image position.

Further. the means for detecting may include means defining a second slit and photoelectric detection means arranged to receive from the second slit light from the image of the first slit formed by the optical element at the second slit, and to produce said electrical representation of said image. Therefore, the OTF of the test element and the local image forming magnification at that time are calculated at the same time using the signal coming from the photoelectric transforming element.

Preferred embodiments of the invention will now be described by way of example with reference to Figures 2 to 8 of the accompanying drawings, in which: Figure 1 is a schematic view for explaining the image forming magnification used in measuring OTF according to the prior art; Figure 2 is a schematic view for explaining the local image forming magnification used in measuring OTF with an apparatus according to the invention; Figures 3 through 5 are explanatory views for showing the manner how to measure the local image forming magnification used in the measurement of OTF with an apparatus according to the invention Figure 6 is a schematic view of OTF measuring apparatus showing an embodiment of the invention; and Figures 7 and 8 are block diagrams showing two different embodiments of OTF measuring apparatus according to the invention.

The local image forming magnification used in the OTF measuring apparatus disclosed herein is explained with reference to Figure 2 in which the same reference numerals as used in Figure 1 designate the same members as those in Figure 1. In Figure 2, a pattern of a small width d5 on an object plane 1 is projected on an image plane 3 through a test optical element comprising an image forming lens 2. When the width of the pattern image formed on the image plane is dx, then the local image forming magnification ss of the lens 2 is expressed by: dx = .

Figure 3 is shown to explain the manner in which the local image forming magnification is measured with apparatus to be described herein.

Referring to Figure 3, there is disposed at the height of an object to be measured a double slit 6 composed of two openings spaced from each other by a very small distance 6. The double slit 6 is illuminated from the behind thereof (not shown) to form a image 7 of the slit on the image plane 3 through the test lens 2.

Figure 4(A) diagrammatically shows the distribution of intensity of light (8, 9) passed through the double slit 6 on the object plane. In a similar manner, Figure 4(B) shows the intensity distribution of the image 7 of double slit formed on the image plane in which the two line images 10 and 11 have a distance of 6' therebetween. Since the distance 6 is an extremely small value, the local image forming magnification can be obtained with a sufficiently good approximation using the following formula: dx o' ss .

6 More particularly, a known distance is preliminarily given to the double slit of object and the distance o' between the line images formed in the image space is measured.

A method for measuring the distance 6' will be described referring to Figure 4(C). The pattern (12. 13) illustrated in Figure 4(C) is that obtained by differentiating the scanning output of the intensity distribution of the linear image (10, 11) shown in Figure 4(B). At the peak point of each the line image there exists a zero-cross. Therefore, 6' can be measured with a high accuracy by reading the distance between two zero-crosses corresponding to the peak positions of the two line images.

While the distribution curve of intensity of the linear image illustrated in Figure 4(B) is of a symmetrical shape. the measuring method of local image forming magnification allows to determine precisely the magnification also for such a case where exists asymmetric aberration such as coma aberration. This measuring method can not be adversely affected thereby. The distance 6 between two openings in the double slit is extremely small and therefore the variation caused by the aberration therebetween becomes negligibly small.

The two line images 10 and 11 shown in Figure 4(B) are entirely the same in shape as each other although they may not be of a svmmetric shape and therefore even when there exists any asymmetric aberration. the distances 6' remains unchanged and there occurs only an equal shift of the two zero-cross positions.

On the contrary, according to the prior art method in which the image forming magnification is obtained by using ss = xt ,. a measurement of image height has to be made using the optical axis as its base line. Therefore, even if the distortional aberration is completely removed, there may be caused an error of measurement for the reason that when the peak positions of the line images are shifted due to any asymmetric aberration such as coma aberration, the measured value of the image forming magnification is varied depending upon the amount of the shift of the image at the measured position. Such a problem of measuring error involved in the prior at method is automatically solved according to the apparatus disclosed herein.

Figure 5 shows another form of measurement in which a pattern in the form of rectangular opening is used in place of the above described double slit. The rectangular opening (slit) possesses a width of 6 and is disposed in the object plane 1 in the optical arrangement shown in Figure 3. Figure 5(A) shows the intensity distribution 14 of the light passed through the rectangular opening and Figure 5(B) shows the intensity distribution 15 of light of the line image formed on the image plane 5 through the lens 2. One method for obtaining the value of width 6' of the line image 15 is that after shaping the wave form of the image and then differentiating it. the section where the differentiated signal (Figure 5(C) ) remains zero is measured. In this manner, the length of 6' becomes known.Therefore, the local image forming magnification can be obtained by calculating the formula: 6' p 6 Figure 6 schematically shows an embodiment of the OTF measuring machine according to the invention.

A beam of light emitted from a light source 18 illuminates an object slit 20 through a condenser lens 19. The opening 21 of the slit constitutes a pattern the spacial expansion of which is extremely small. In case of the object slit 20 shown in Figure 6, the opening pattern is a double slit 21. The beam of light passed through the double slit 21 enters a test lens 22 and then forms an image of the double slit on a scanning slit 24. The reference numeral 23 designates the distribution of intensity of the formed image. The intensity distribution of the line images is detected by scanning the scanning slit 24 in the direction of arrow A.The electrical signal coming from a photoelectric transducer 25 is processed by a signal processing circuit 26 later described in detail so as to compute the value of OTF in real time which is corrected by using the above described local image forming magnification. The found value is displayed by a display unit 27 or a printer unit 28.

The object slit 20 and the scanning slit 24 are disposed at conjugate positions relative to the test lens 22. Thus, the object slit 20 is provided in the position whereat the lens 22, when normally used, will form an image and the scanning slit 24 is provided in the position whereat an object will be positioned when the lens is normally used. The value of OTF of the test lens 22 is measured every time when the angle n is gradually changed which the lens forms with the optical axis 29 of the measuring optical system.

Now. a detailed description will be made of two circuit arrangements which may constitute the signal processing circuit 26 for obtaining the value of OTF in real time corrected using the local image forming magnification, referring to Figures 7 and 8.

The signal processing circuit shown in Figure 7 is one example of circuits adoptable in the case where a double slit is used in an opening pattern of the object slit 20. The reference numeral 31 designates a cylinder slit in which a spiral opening 32 is provided. An image of the above described double slit is formed on the cylinder slit 31 through a test lens not shown. Therefore, when the cylinder slit 31 rotates about its rotational axis 34, the image 33 of the opening pattern is scanned and the intensity distribution thereof is detected by a photoelectric transducer 35.

On the circumferential edge portion of the cylinder slit 31, there are provided a number of holes 36 at regular intervals. These holes 36 are illuminated by means of an optical system comprising a light source 37, an image forming lens 38 and a light transmissive medium 39 such as optical fiber. As the cylinder slit 31 rotates at a uniform speed, the beam of light coming from the illumination system (37, 38 and 39) passes through the holes 36 at regular time intervals and then it is detected by a photoelectric transducer 40. The signal generated from the above mentioned photoelectric transducer 35 is amplified in an amplifying circuit 41 and thereafter the signal is divided into two, the one of which is used to obtain the local image forming magnification and the other becomes an indication of the intensity distribution of the line images used in measuring OTF.

The output signal a of the amplifier 41 indicative of the intensity distribtuion of the double slit line images is introducd into a slicer 42 which cuts out the upper portion of the signal to form a signal b. The signal b is differentiated in a differentiation circuit 43 to form a signal c which is further transformed into a signal d by a clipper 44 which clips of the negative portion of the signal c. The width D1 of the signal d corresponds to the width D of the image 33 of double slit formed on the cylinder slit 31. The signal d is transformed by a bistable multivibrator 45 into a rectangular signal e the width of which is D,. The signal e is then put in a gate circuit 46.

On the other hand, the signal coming from the above described photoelectric transducer 40 is introduced into a clipper circuit 50 through an amplifying circuit 47 (f). a slicer 48 (g) and a differentiation circuit 49 (h). successively in this order. The clipper circuit 50 puts out a signal i in a form of pulse having a regular interval and the signal i is put in the gate circuit 46. The gate circuit allows the pulse signal i to pass through only during the time interval D, of the signal e. A pulse counter 51 counts the number of the pulses passed through the gate circuit 46. The length per pulse can be calculated preliminarily using the known values such as the distance between the holes 36. the rotational velocity of the cylinder slit 31 and the like. Therefore, the number of pulses counted by the pulse counter 51 is directly transformed into the slit width D of the double slit image 33 and based on the latter the local image forming magnification p = D/D,, is calculated by a divider circuit 52. wherein D,, is opening width of the double slit actually used at that time as an object.

The signal e put out from the bistable multivibrator 45 is delivered also into a differentiation circuit 53 in addition to the gate circuit 46. The signal k from the differentiation circuit 53 is put into a clipper 54 which clips off the negative portion of the signal so as to form a signal f which is then put into a monostable multivibrator 55.

On the other hand, a signal coming from the amplifier 41 is introduced into a delay circuit 56. This delay circuit is provided in order that the signal n from the circuit 56 and the signal m from the monostable multivibrator 55 both of which are derived from the same amplifier 41 may be synchronized. The signal m from the monostable multivibrator 55 is put in a gate circuit 57 into which the signal n is also introduced. Among the signals n put in the gate circuit 57, only those signals are allowed to come out from the gate circuit which are derived from the beam of light passed through the one of openings in the double slit. Such a signal as come out from the gate is designated by the reference character o.The signal o is subjected to a Fourier transformation in a Fourier transformer 58 to form an OTF signal P before correction and the OTF signal thus formed is introduced into a frequency scale operating circuit 59. In the circuit 59, the local image forming magnification p obtained by the above divider 52 is added to the scale on the frequency axis indicative of the OTF signal P so as to correct only the scale on the frequency axis. In this manner, the aimed OTF is obtained.

Figure 8 shows one example of signal processing circuits adoptable for another case where a rectangular opening is used as the opening pattern of slit. In Figure 8, the same reference numerals and characters as used in Figure 7 designate the same members and elements.

Referring to Figure 8, the line image 61 of the rectangular opening formed on the cylinder slit 31 is detected by the photoelectric transducer 35 and amplified by the amplifying circuit 41. A portion of the signal a' from the amplifying circuit 41 is shaped into a rectangular signal b' by a wave form reformation circuit 62. The rectangular signal b' is introduced into the gate circuit 46. On the other hand, a pulse signal formed by the previously described circuit (47. 48, 49 and 50) is applied to the gate circuit. The pulse signal is allowed to pass through the gate circuit 46 only during the time of the signal b' from the wave form reformation circuit 62 being put into the gate circuit. The pulse counter 51 counts the number of pulses passed through the gate circuit.After converting the pulse number into the width D' of the line image 61, the local image forming magnification ss = D'ID0, is calculated by the dividing circuit 52, wherein Dot,' is width of opening slit actually used.

Another portion of the signal a' come out from the amplifying circuit 41 is differentiated in the differentiation circuit 63. The output signal c' of the differentiation circuit 63 is put into the gate circuit 57 through the delay circuit 56. A portion of the signal b' issued from the wave form reformation circuit 62 is also put into the gate circuit 57 as a rectangular signal d' after passing through differentiation circuit 53, clipper 54 and monostable multivibrator 55. The delay circuir 56 is provided in order to synchronize the signals c' and d' transmitted to the gate circuit 57 from the amplifying circuit 41 passing two different paths each other.The function of the signal d' from the monostable multivibrator 55 is to make pass through the gate only the fore portion e' of the signal c' which is obtained by a differentiaion of the rising-up portion of the above described signal a'. The signal e' put out from the gate circuit 57 is Fourier-transformed in the Fourier transforming circuit 58 to form a signal g' and then using the local image forming magnification p obtained by the divider 52 there is made a correction of the scale on the frequency axis in the frequency scale operator circuit 59 in a manner as described above so that the desired value of OTF may be obtained.

Various advantageous effects can be obtained by carrying out scaling of spatial frequency using the local image forming magnification according to the invention. The following is one example to demonstrate one of the effects obtainable according to the invention: When OTF of a lens having a distortion of - 5% is measured in the sagital direction at the spatial frequency of 50 lines/mm according to the inverted projection method, a scaling of the spatial frequency using the paraxial lateral magnification will result in finding of the value of the spatial frequency of 52.5 lines/mm that is 2.5 lines/mm higher than the real frequency. This may cause a significant error of measurement particularly when the gain of OTF changes sharply in the vicinity of the frequency. On the contrary, the use of local image forming magnification allows one to completely correct such an error caused by distortion.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details can be made therein without departing from the scope of the invention.

Claims (9)

WHAT WE CLAIM IS:

1. Apparatus for measuring an optical transfer function of an optical element comprising means for forming an optical test object from which said optical element is to produce a detectable image means for detecting for each of a plurality of different positions of said test object relative to the optical element, the image produced by said optical element at an image position conjugate with that object position relative to the optical element and for producing a electrical representation of the said image, and means for determining from said electrical representation for each said test object position the image forming magnification with which the said image is formed, and for determining the corrected optical transfer function by performing a calculation in which compensation is made for varying values of said image forming magnification for said different test object positions.

2. Apparatus according to claim 1 wherein said means for forming an optical test object comprises means defining a first slit and means for illuminating the first slit so that in use light travels through said first slit to enable said optical element to form the image of said first slit at said image position.

3. Apparatus according to claim 2 wherein said means for detecting includes means defining a second slit and photoelectric detection means arranged to receive from the second slit light from the image of the first slit formed by the optical element at the second slit, and to produce said electrical representation of said image.

4. Apparatus according to any of claims 1 to 3, wherein said means for determining comprises a first circuit for determining an uncorrected optical transfer function, a second circuit for determining the image forming magnification and means for determining from said uncorrected optical transfer function and said image forming magnification the said corrected optical transfer function.

5. Apparatus as claimed in claim 2 or claim 3 or in claim 4 when dependent on claim 2 wherein said first slit is a double slit.

6. Apparatus as claimed in claim 2 or claim 3 or in claim 4 when dependent on claim 2 wherein said first slit is a single rectangular slit.

7. Apparatus for measuring an optical transfer function of an optical element substantially as herein described with reference to Figure 6 of the accompanying drawings.

8. Apparatus for measuring an optical transfer function of an optical element substantially as herein described with reference to Figure 7 of the accompanying drawings.

9. Apparatus for measuring an optical transfer function of an optical element substantially as herein described with reference to Figure 8 of the accompanying drawings.