RU2276778C1 - Method for determining distortion of long-focus objectives - Google Patents

Abstract

FIELD: optical engineering, possible use for measuring distortion of long-focus large-dimensional objectives.

SUBSTANCE: method includes measuring linear values of images in focal plane of optical bench of glass scale participants, positioned along field of view of tested objective by means of two ocular micrometers.

EFFECT: higher precision of distortion detection due to excluded usage of angle-measuring devices.

5 dwg

Description

The invention relates to the field of optical instrumentation and can be used, in particular, in the certification of long-focus large-sized lenses to control radial distortion.

A known method of measuring distortion using a nodal point. It consists in measuring with a microscope the displacement of the autocollimation image of the test object when the test lens is rotated relative to the zero position by symmetrical angles. But this method has insufficient accuracy and complexity of alignment of the test lens when installing the rear nodal point on the axis of rotation of the lens holder (N.P. Gvozdeva, K.I. Korkina "Theory of optical systems and optical measurements", Moscow "Engineering" 1981, p. .364-365).

The closest is the method for determining distortion described in OST 3-5396-83 "Aerial lenses. Method for measuring radial photogrammetric distortion". It lies in the fact that the test lens is mounted on a rotary device of the optical bench, combining the plane of the entrance pupil of the lens with the vertical axis of the rotary device. The test lens is fixed so that its first lens is located on the side of the mirror lens of the optical bench, and behind the last lens of the test lens is placed a glass scale perpendicular to its optical axis. The measurement of radial photogrammetric distortion comes down to measuring the angles at which the images of the strokes of the glass scale are visible, first in the center and then in the field of view. This method does not provide high quality when measuring distortion in large telephoto lenses.

The objective of the invention is to provide a method for determining the distortion of telephoto lenses with improved characteristics.

The technical result is to increase the accuracy of determining the distortion of long-focus large-sized lenses by eliminating the need to use goniometric measuring devices.

This is achieved by the fact that in the method for determining the distortion of telephoto lenses, in which the test lens is mounted on the rotary device of the optical bench, combining the plane of the entrance pupil of the lens with the vertical axis of the rotary device, the first lens of the test lens is placed on the side of the mirror lens of the optical bench, and behind the last lens place a glass scale perpendicular to the optical axis of the test lens, then combine the crosshair of the eyepiece of the optical bench with the image the focal plane of the optical bench of the glass scale, while the zero stroke is on the optical axis of the test lens, and the image of this bar is on the optical axis of the optical bench, in contrast to the well-known crosshairs of two eyepiece-micrometers combine with the image in the focal plane of the optical bench of the strokes located symmetrically with respect to the zero stroke of the glass scale at a small distance from it, and take readings of two micro-screws of each eyepiece-micrometer, then rotate the rotary device the lens holder of the optical bench and combine the crosshairs of the two aforementioned eyepiece micrometers with the images of the glass bar strokes located along the field of view of the test lens, starting from the zero stroke, the distance between these strokes on the glass scale should be the same as between the above symmetrically located relative to the zero stroke by strokes, and take readings on two microscrews of two eyepiece-micrometers.

The invention is illustrated by drawings.

Figure 1 presents the layout of the test lens when measuring the magnitude of the images of the sections of the glass scale located along the field of view of the test lens.

Figure 2 shows a diagram for measuring the magnitude of a portion of a glass scale located symmetrically relative to the optical axis of the lens.

Figure 3 presents graphs of the distortion values of a large telephoto lens, measured by the method according to the invention.

Figure 4 presents the averaged values for the same distortion values.

Figure 5 presents graphs of the distortion values of a large telephoto lens, measured according to OST 3-5396-83 "Aerial lenses. Method for measuring radial photogrammetric distortion".

The long-focus lens under test 1 (Fig. 1) is mounted on the lens holder of a rotary device (not shown) of the optical bench so that the plane of the entrance pupil of the test lens 1 coincides with the vertical axis of the rotary device, the first lens of the test lens 1 is directed towards the mirror lens 2 of the optical bench and behind the last lens a glass scale 3 is placed perpendicular to the optical axis of lens 1. Next, the crosshairs of the eyepiece micrometers 4 and 5 are combined (the eyepiece micrometers are located on the optical tables 6 and 7 with microscrews of longitudinal and transverse movement with a graduation price of 0.01 mm) with images located symmetrically relative to the optical axis of the lens 1 (Fig. 2) in the focal plane of the optical bench of the strokes of the glass scale 3 illuminated by the backlight 9 -LB40- 2. Read the micrometer screws x ' ₁ , x' ₂ . The difference (x ' ₁ -x' ₂ ) is taken as the reference point, the rotator is rotated by an angle corresponding to the "i-th" interval of the glass scale. The crosshairs of the eyepiece-micrometers are combined with the images of the strokes corresponding to the "i-th" interval of the glass scale. Take samples x ₁ , x ₂ .

Calculate Δl _{± i} value corresponding to the difference in the size of the images of the section of the glass scale, taken as the reference point and the measured section of the glass scale 3:

Δl _{± i} = (x ' ₁ -x' ₂ ) - (x ₁ -x ₂ ) _{± i} .

The image value of the i-th interval of the glass scale l _{± i is} calculated by the formula

l _{± i} = l ₀ -Δl _{± i} .

Where l ₀ = f ' _CK · 2 · tg (α' / 2),

where f ' _ck is the focal length of the optical bench,

α 'is the angle corresponding to the scale interval of the ruler h located symmetrically with respect to the optical axis of the lens (the zero stroke coincides with the optical axis of the lens) when the lens is not deployed relative to the optical axis of the bench (figure 2):

where f ' _r is the focal length of the lens, previously measured by the goniometric method in accordance with GOST 13095-85 "Lenses. Methods of measuring the focal length".

The angle α _{± i} corresponding to ± i of the scale interval is calculated by the formula

The angle A ± i corresponding to the distance from the zero stroke to the corresponding ± i stroke is calculated by the formula

The photogrammetric focal length is determined by the formula:

where d _{± i} is the scale interval from the zero stroke to the corresponding ± i stroke of the ruler. Photogrammetric distortion is calculated by the formula:

_{Yd ± i = L ± i -f} 'φ · tgA ± i ± Δ · tg ² A _{± i,}

± Δ - correction for the distance between the autocollimation point and the point of symmetry in accordance with OST3-5396-83 (the minus sign corresponds to negative values of the intervals of the ruler scale):

Optical distortion is found by the formula:

y _{0 ± i} = d _{± i} -f ' _about · tgA _{± i} ± Δ · tg ² A _{± i} .

Thus, this method for determining the distortion of telephoto lenses is based on measuring the difference between the linear magnitude of the image in the focal plane of the bench of the glass scale section located symmetrically with respect to the optical axis of the test lens and images of the same glass scale sections located along the field of view of the test lens. In this case, there is no need to use goniometer devices that must be used according to the method of determining distortion described in OST 3-5396-83 "Aerial lenses. Method for measuring radial photogrammetric distortion", and linear measurements are made with greater accuracy, since a portion of the glass scale is observed in focal plane of an optical bench at high visible magnification

where f ' _ck is the focal length of the optical bench,

f ' _about - the focal length of the test lens,

G _ok - a visible increase in the eyepiece.

For comparative analysis, multiple measurements of the distortion of a large-sized telephoto lens according to OST 3-5396-83 "Aerial lenses. A method of measuring radial photogrammetric distortion" (graphs of the results are shown in Fig. 5) and a method for determining distortion in telephoto large-sized lenses (test results graphs are shown in figure 3, figure 4), the results of which were compared with the calculated values of the distortion of the test lens.

Statistical processing of the results was performed with a confidence probability of P = 0.95. Confidence limits (random errors of the measurement result were determined by the formula

ε = ± t · S,

where t is the Student's coefficient (t-3,182 for 3 measurements according to OST 3-5396-83 "Aerial lenses. The method of measuring radial photogrammetric distortion"; t = 2,776 for 5 measurements by the method of determining distortion in long-focus large-sized lenses), S - standard deviation of the arithmetic mean of the measurement results.

The confidence limits ε of the random error of the optical distortion measurement result at the points of the field of view of the test lens for the method described in OST 3-5396-83 were ± 0.0742 mm, for the method for determining distortion for long-focus large-sized lenses ± 0.0082 mm. The confidence limits ε of the random error of the result of measuring the photogrammetric distortion at the points of the field of view of the test lens for the first method was ± 0.0768 mm, for the second method ± 0.0032 mm.

The maximum deviation of the arithmetic mean of the optical distortion from the calculated value was ± 0.0312 mm for the first method, ± 0.0020 mm for the second method. The maximum deviation of the arithmetic mean of the photogrammetric distortion from the calculated value was ± 0.0328 mm for the first method, ± 0.0022 mm for the second method.

As you can see, the method for determining distortion in long-focus large-sized lenses significantly exceeds the control method according to OST 3-5396-83 "Aerial lenses. The method of measuring radial photogrammetric distortion" in terms of measurement accuracy.

Claims (1)

A method for determining the distortion of telephoto lenses, namely, that they fix the test lens on the rotary device of the optical bench, combining the plane of the entrance pupil of the lens with the vertical axis of the rotary device, the first lens of the test lens is placed on the mirror side of the optical bench, and the last lens of the test lens is placed a glass scale perpendicular to its optical axis, then combine the crosshair of the eyepiece of the optical bench with the image in the focal plane of the optical bench of the glass scale, while the zero stroke should be on the optical axis of the test lens, and the image of this stroke on the optical axis of the optical bench, characterized in that the crosshairs of the two eyepiece-micrometers are combined with the image in the focal plane of the optical bench of the strokes located symmetrically relative to the zero stroke of the glass scale at a small distance from it, and take readings of two microscrews of each eyepiece-micrometer, then rotate the rotary device about the lens holder of the optical bench and combine the crosshairs of the two eyepiece-micrometers with the images of the glass scale strokes located along the field of view of the test lens, starting from the zero stroke, the distance between these strokes on the glass scale should be equal to the distance between the above strokes symmetrically located relative to the zero stroke, and take readings on two microscrews of two eyepiece-micrometers.

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