RU2006809C1 - Method of measuring lens transmission gain - Google Patents

Abstract

FIELD: test equipment. SUBSTANCE: three lenses are taken for testing 1, 2 and 3. Combinations of two lenses mounted in series onto the same optical axis are chosen in turn. Spherical mirror 7 is installed by its center of curvature in the focus of lens in front of lens 1. The mirror is irradiated by convergent beam which has aperture angle being close to aperture angle of lenses under test. Photodetector 8 is installed in front of tested lenses in the path of radiation flux, reflected from mirror 7, and signal a is registered. Mirror 7 is installed behind lens 2 - center of curvature is in the case in its focus. Lenses 1 and 2 are irradiated by convergent beam with aperture angle being close to aperture angle of lenses under test. Parallel pass of the beams are formed between the first and the second controlled lenses, and signal b is registered. Then signals c and d are detected in the output of photodetector after the first and the third and the second lenses and the third lenses are installed in radiation flux. Transmission gains of the lenses are calculated according to taken results. EFFECT: improved precision. 3 dwg

Description

 The invention relates to the control of measuring equipment, and in particular to methods for measuring the transmittance of lenses and lenses.

 A known method for measuring the transmittance of lenses is that they illuminate the lens with a diverging beam of rays, divide this beam into two light beams by installing a beam-splitting translucent plate in front of the lens, form a parallel beam of rays at the output of the lens from the first beam-splitting plate of the first beam, reflect this beam in the opposite direction with a flat autocollimation mirror and focus the beam of rays reflected from the plane mirror and the beam splitter onto the photodetector.

Основным недостатком указанного способа является невысокая точность измерений, обусловленная влиянием погрешностей изготовления отражающих покрытий плоского автоколлимационного и сферического эталонного зеркал, погрешностей аттестации коэффициентов отражения этих зеркал и нестабильность их коэффициентов отражения. Кроме того, наблюдаются потери (75-90% и более) потока излучения на светоделительной пластине.The second light beam reflected from the beam splitter is reflected with a reference spherical mirror and is also focused on a photodetector. Consistently block the opaque curtains reflected from the beam splitter and transmitted by its light beams, the beam of rays and register signals U ₁ and U ₂ at the output of the photodetector, proportional to the radiation flux before and after passing through the measured lens. According to the results obtained, the transmittance is calculated by the formula
r =

The main disadvantage of this method is the low measurement accuracy due to the influence of errors in the manufacture of reflective coatings of flat autocollimation and spherical reference mirrors, certification errors for the reflection coefficients of these mirrors and the instability of their reflection coefficients. In addition, there are losses (75-90% or more) of the radiation flux on the beam splitter plate.

; r₂=

;

=

Однако и данный способ измерения коэффициентов пропускания объективов характеризуется низкой точностью измерений, обусловленной его малой чувствительностью (при измерении излучение проходит через объектив только один раз). Другим существенным недостатком известного способа является необходимость перемещения фотоприемника при измерении сигнала на входе в пары объективов и на выходе из объективов. При проведении высокоточных измерений это обстоятельство может служить источником дополнительных погрешностей, так как применяемые при таких измерениях (обычно многократно повторяемых) фотоприемники весьма чувствительны к неизбежному при их перемещениях механическому воздействию, что может приводить к нестабильности сигнала на выходе фотоприемника.Closest to the invention in technical essence is a method for measuring the transmittance of the lenses, which consists in taking at least three lenses as controlled, introducing combinations of two lenses in series into the radiation flux, forming a parallel path of the rays between them. In this case, the lenses are illuminated with a diverging beam of rays with an aperture angle close to the aperture angle of the lenses being checked, signals a and b are recorded at the output of the photodetector when it is installed before and after radiation passes through the first and second lenses, signals c and d are recorded at the output of the photodetector when it is installed after the radiation passes through the first and third, and second and third lenses, and according to the results obtained, the transmittance of the lenses is determined by the formulas
r ₁ =

; r ₂ =

;

=

However, this method of measuring the transmittance of the lenses is characterized by low measurement accuracy due to its low sensitivity (when measuring radiation passes through the lens only once). Another significant disadvantage of this method is the need to move the photodetector when measuring the signal at the input to the pairs of lenses and at the exit from the lenses. When conducting high-precision measurements, this circumstance can serve as a source of additional errors, since the photodetectors used in such measurements (usually repeatedly repeated) are very sensitive to the mechanical action that is inevitable during their movements, which can lead to instability of the signal at the output of the photodetector.

 The aim of the invention is to improve the accuracy of measurements of the transmittance of the lenses.

; τ₂=

; τ₃=

На фиг. 1 показаны оптическая измерительная схема и ход лучей в ней при проведении измерений фотоприемник установлен непосредственно в фокальной плоскости контролируемого объектива; на фиг. 2 - то же, фотоприемник установлен в плоскости изображения дополнительной проекционной системы; на фиг. 3 - то же, фотоприемники установлены в зеркале по разные стороны от оптической оси измеряемых объективов.This goal is achieved by the fact that with the method of measuring the transmittance of the lenses, which consists in taking at least three lenses as controlled, they are introduced into the radiation flux by combinations of two lenses in series, form a parallel path of the rays between them, while the lenses illuminate with a diverging beam of rays with an aperture angle close to the aperture angle of the monitored lenses, register signals a and b at the output of the photodetector when it is installed before and after passing through radiation of the first and second lenses, signals c and d are recorded at the output of the photodetector when it is installed after passing the radiation of the first and third and second and third lenses, respectively, signal a is recorded at the output of the photodetector when radiation is received from it reflected from the spherical mirror installed in front of the first lenses with the center of curvature at the focus of this lens, and the signals b, c, d are recorded respectively at the output of the photodetector, when it receives radiation reflected from the set and the second beam along the ray lens spherical mirror with its center of curvature at the focus of the lens and again passed through the test lens, the lens transmittance τ _1, τ _2, τ ₃ is determined by the formulas:
τ ₁ =

; τ ₂ =

; τ ₃ =

In FIG. 1 shows the optical measuring circuit and the path of the rays in it during measurements, the photodetector is installed directly in the focal plane of the controlled lens; in FIG. 2 - the same, the photodetector is installed in the image plane of the additional projection system; in FIG. 3 - the same, photodetectors are installed in the mirror on different sides from the optical axis of the measured lenses.

Two 1, 2 out of three 1-3 controlled lenses are placed on one optical axis. The lens 3 is placed out of the light beam, while all the lenses (devices for mounting them) are made interchangeable in the seats. In front of the controlled lenses 1 and 2, a focusing system 4 with a focus in the focal plane of the controlled lens 1 and an aperture diaphragm 5 installed in front of it are located at an angle α to their optical axis 5. A collimated radiation source 6 (for example, a collimator with a monochromatic illuminator) is placed in front of the aperture diaphragm 5, on the same optical axis with the focusing system 4. Directly behind the focusing system 4 with a focal length f _f ¹ there is a spherical mirror 7 with the center of curvature in the focal plane f the oculating system 4 at a distance Δ = R˙tgα from its focus, R is the radius of curvature of the spherical mirror 7, made from the sphere with the possibility of outputting the lenses 1 and 2 from the optical axis. The photodetector 8 is located near the center of curvature of the spherical mirror 7 the photodetector area is shifted by a distance Δ = R˙tgα from the optical axis of the monitored lenses 1 and 2, from the opposite side from the focus of the system 4. The output of the photodetector 8 is combined with the input of the recording device 9.

 The device can be additionally equipped with a small mirror 10 installed near the focus of the focusing system 4 in the middle also at a distance Δ = R˙tgα from the optical axis of the controlled lenses and the projection system 11 (see Fig. 2). The photodetector 8 is placed behind the projection system 11 in the image plane of this system, while the restrictions on the design and dimensions of the photodetector 8 are practically removed.

 To increase labor productivity, the device can be additionally equipped, in addition to the mirror 10 and the projection system 11, a switching mirror 12, while the spherical mirror 7 and the controlled lenses 1, 2 are placed on opposite sides from the optical axis of the focusing system 4 (see Fig. 3) . The switching mirror 12 is located near the image of the radiation source 6 and is rotatable around an axis passing through the focus of the focusing system 4. The mirror 10 is also rotatable, while the axis of rotation passes through the middle of its mirror surface parallel to the axis of rotation of the mirror 12. Projection system 11 is located on the same axis with the focusing system 4, perpendicular to the optical axis of the controlled lenses 1 and 2.

 The method of measuring the transmittance of the lens is as follows.

 Three positive lenses 1, 2, and 3 are taken as controlled. Lenses 1 and 2 are mounted in the course of the beam of radiation sequentially on one optical axis behind the focusing system 4 with an aperture 5 installed in front of it. The focus of the lens 1 is aligned with the plane of the actual image of the collimated radiation source 6.

 After the focusing system 4, a spherical mirror 7 is installed with the center of curvature in the plane of the actual image of the radiation source 6 constructed by the focusing system 4.

The main beam of the diverging beam of rays after the focusing system 4, passing through the center of the image of the collimated radiation source 6, is tilted by an angle α≈0.5-2.0 ^about the optical axis of the lenses 1 and 2, while the converging beam of rays reflected from the spherical mirror 7 is collected at a distance Δ '= 2R˙tgα from the image of the radiation source 6 constructed by the focusing system 4. The photodetector 8 is installed near the image of the radiation source 6 so that the elements of its body do not overlap the beam at the output of the focusing system 4 (Fig. 1).

 At the output of the photodetector 8 using a recording device 9 register a signal proportional to the radiation flux at the entrance to the monitored lenses 1 and 2.

 A spherical mirror 7 is mounted behind the lenses 1 and 2 with the center of curvature at the focus of the lens 2. A parallel beam of rays at the output of the lens 1 is focused by the lens 2 in its focal plane. Next, the beam of rays is reflected from the spherical mirror 7, again passes through the lenses 1 and 2, and is focused in the focal plane of the lens 2 on the photodetector 8 (Fig. 1), the signal b is proportional to the radiation flux twice passed through the lenses 1 and 2.

 The considered operations of measuring the signal at the output of the recording device 9 when installing a spherical mirror 7 before and after the measured lenses are repeated for pairs of lenses 1 and 3 and 2 and 3, introducing alternately into the beam of rays and recording the values of c and d for each pair of lenses.

(2) где D_а - диаметр апертурной диафрагмы 5;
f_ф - фокусное расстояние фокусирующего объектива 4;
U_к = arctg

(3) где D_к - световой диаметр измеряемых объективов;
f_к - фокусное расстояние контролируемых объективов.The measured lenses 1-3 and the spherical mirror 7 are illuminated when measuring with a diverging beam of rays with an aperture angle U _f close to the aperture angle U _{to the} measured lenses, while
U _f <U _k (1),
U _f = arctg

(2) where D _a is the diameter of the aperture diaphragm 5;
f _f - the focal length of the focusing lens 4;
U _to = arctg

(3) where D _to - the light diameter of the measured lenses;
f _to - the focal length of the controlled lenses.

; τ $\genfrac{}{}{0ex}{}{\text{2}}{\text{1}}$ ×τ $\genfrac{}{}{0ex}{}{\text{2}}{\text{3}}$ = c/a; τ $\genfrac{}{}{0ex}{}{\text{2}}{\text{2}}$ ×τ $\genfrac{}{}{0ex}{}{\text{2}}{\text{3}}$ = d/a
Решают систему уравнений относительно коэффициентов τ₁, τ₂, τ₃пропускания объективов 1, 2 и 3 и получают

₍₄₎

₍₅₎

₍₆₎
Коэффициент отражения сферического зеркала 7 при измерениях предложенным способом не влияет на точность измерений, т. к. зеркало 7 участвует в измерениях сигналов на выходе фотоприемника 8 до и после контролируемых объективов.Based on the measurement results, the proposed method can be composed of a system of equations:
τ $\genfrac{}{}{0ex}{}{\text{2}}{\text{1}}$ × τ $\genfrac{}{}{0ex}{}{\text{2}}{\text{2}}$ = b / a

; τ $\genfrac{}{}{0ex}{}{\text{2}}{\text{1}}$ × τ $\genfrac{}{}{0ex}{}{\text{2}}{\text{3}}$ = c / a; τ $\genfrac{}{}{0ex}{}{\text{2}}{\text{2}}$ × τ $\genfrac{}{}{0ex}{}{\text{2}}{\text{3}}$ = d / a
Solve the system of equations for the coefficients τ ₁ , τ ₂ , τ ₃ transmission lenses 1, 2 and 3 and get

₍₄₎

₍₅₎

₍₆₎
The reflection coefficient of the spherical mirror 7 during measurements by the proposed method does not affect the measurement accuracy, since the mirror 7 is involved in measuring the signals at the output of the photodetector 8 before and after the controlled lenses.

 When installing during the beam of rays after the focusing system 4 of the mirror 10, the photodetector 8 is placed in the image plane of the projection system 11.

 There are lenses of such a design that it is rather difficult to place a concave spherical mirror 6 between the focal plane and the last surface of the lens. In this case, it is easiest to change the path of the rays so that the lenses 1 and 2 and the spherical mirror 7 are placed on the measurements on opposite sides of the optical axis of the focusing system 4 (Fig. 3). For these purposes, you can use the switching mirror 12, the rotation of which together with the mirror 10 provides a consistent measurement of signals a and b. Compared to the measurement circuitry of FIG. 1, 2, such a measurement scheme allows the mirror 7 to be reinstalled without having to remove the lenses 1 and 2 from the optical axis of the focusing system, which somewhat simplifies measurements (especially when they are repeated many times). Moreover, with this version of the measuring circuit, two identical spherical mirrors 7 can be used, the reinstallation of which to eliminate the influence of differences in the reflection coefficients of the mirrors can be carried out less often than when using one mirror 7. One of the mirrors is fixed by the center of curvature at the focus of the focusing system 4 , the other is mounted behind the controlled lenses 1 and 2.

Suppose you want to measure the transmittance of three lenses with a focal length f _to ¹ = 250 mm and a light diameter of 100 mm for a wave λ = 0.63 μm.

 As a radiation source, a helium-neon laser equipped with an appropriate telescopic expander can be used, the radiation receiver is a silicon photodiode with a sensitive area with a diameter of 3-10 mm.

<

(4). При выбранном фокусном расстоянии фокусирующего объектива 4 f_ф = 75 мм из выражения (4) имеем
D_a <

или D_a <

D_a < 30 мм Установив диаметр апертурной диафрагмы 5 равным 25 мм, можно измерить коэффициенты пропускания объективов в пределах световой зоны диаметром
D =

=

= 80 мм
Пусть сигнал на выходе регистрирующего устройства 9 (например вольтметра) при установке сферического зеркала 7 перед измеряемыми объективами составляет 1,00 В. Сигналы b, c и d, измеренные при установке сферического зеркала за парами контролируемых объективов 1 и 2, 1 и 3, 2 и 3, равны соответственно, 0,36, 0,45 и 0,40 В.The parallel radiation beam at the output of the expander is limited to the necessary aperture, and from expressions (1), (2) and (3) we have:

<

(4). With the selected focal length of the focusing lens 4 f _f = 75 mm from expression (4) we have
D _a <

or D _a <

D _a <30 mm By setting the diameter of the aperture diaphragm 5 to 25 mm, you can measure the transmittance of the lenses with a diameter of
D =

=

= 80 mm
Let the signal at the output of the recording device 9 (for example, a voltmeter) when installing a spherical mirror 7 in front of the measured lenses be 1.00 V. The signals b, c and d measured when installing a spherical mirror behind the pairs of monitored lenses 1 and 2, 1 and 3, 2 and 3 are respectively 0.36, 0.45 and 0.40 V.

= 0.798;

= 0.775;
τ₃=

= 0.84.Substituting the measured values of a; b; c; d in the proposed formulas (4), (5) and (6) get
τ ₁ =

= 0.798;

= 0.775;
τ ₃ =

= 0.84.

=

+

+

+

= 0.011
или Δτ/τ = 1,1%
При измерении всех трех объективов способом-прототипом с учетом того, что измеренные величины b, c, d будут в 1/r₁ 1/r₂ и 1/r₃ раз больше, чем в предложенном способе (излучение проходит измеряемые объективы только один раз), относительная погрешность измерений определяется по формуле

₊

₊

_{= 0,018}
или Δτ/τ = 1,8% . (56) Приборы и техника экспериментов, N 4, 1979, с. 237-238.The relative error Δτ / τ of the measurements of the transmittance of the lenses can be obtained by differentiating the logarithms of formulas (4), (5) and (6):
Δτ / τ = Δa / 4a + Δb / 4b + Δc / 4c + Δd / 4d (7) where Δa, Δb, Δc, Δd are the measurement errors of a ₁ , b ₁ , c, d. When Δa = Δb = Δc = Δd = 0.005 V, the relative error in measuring the transmittance of the lenses by the proposed method will be:

=

+

+

+

= 0.011
or Δτ / τ = 1.1%
When measuring all three lenses using the prototype method, taking into account that the measured values of b, c, d will be 1 / r ₁ 1 / r ₂ and 1 / r ₃ times more than in the proposed method (the radiation passes through the measured lenses only once ), the relative measurement error is determined by the formula

₊

₊

_{= 0.018}
or Δτ / τ = 1.8%. (56) Instruments and experimental techniques, N 4, 1979, p. 237-238.

 USSR author's certificate N 1435980, cl. G 01 M 11/02, 1985.

Claims (1)

THE METHOD FOR MEASURING THE LENS PASSAGE FACTOR, which consists of taking at least three lenses as controlled, introducing two combinations of two lenses in series into the radiation stream, forming a parallel path of rays between them, while the lenses are illuminated by a divergent beam of rays with an aperture angle close to the aperture angle of the controlled lenses, the signals a and b are recorded at the output of the photodetector when it is installed before and after the radiation of the first and second lenses passes, reg the signals c and d are generated at the output of the photodetector when it is installed after the radiation of the first and third and second and third lenses is transmitted, respectively, and the transmission coefficients of the lenses are determined by the results obtained, characterized in that the signal a is recorded at the output of the photodetector when radiation reflected from it a spherical mirror mounted in front of the first lens with the center of curvature at the focus of this lens, and the signals b, c, d are recorded respectively at the output of the photodetector when hit it radiation reflected from the installed behind the second beam along the ray lens spherical mirror with its center of curvature at the focus of the lens and again passed through the test lens, the lens transmittance τ _1, τ _2, τ ₃ is determined by the formulas
τ ₁ =

τ ₂ =

τ ₃ =

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