Laser differential reflection-confocal focal-length measurement

Jiamiao Yang; Lirong Qiu; Weiqian Zhao; Hualing Wu

doi:10.1364/OE.20.026027

1. Introduction

The focal length is a key parameter that reflects performance of a lens. The focal length is related to the curvature radii of the lens surfaces, refractive index of the lens material, and lens thickness, among others. The actual focal length always deviates from the theoretical value as a result of manufacturing and assembly errors. Therefore, the precise measurement of the lens focal length is a core problem in the field of optical measurement.

Various methods for focal-length measurement have been proposed. Classical methods such as nodal slide and image magnification are based on the principles of geometry optics. Although these methods are easy to setup and manipulate, high accuracy is difficult to achieve because of subjectivity-related measurement errors. To improve the measurement accuracy, some novel methods based on the principle of physical optics have been presented. For example, moiré deflectometry [1] and Talbot interferometry [2–4] use a moiré fringe to determine the lens focal length. Their measurement accuracies (lower than 0.36%) are limited by the manual measurement of the inclination of the moiré fringes [3]. Grating shearing interferometry measures the lens focal length by using the relative lateral shift between the undiffracted zero order and diffracted first order caused by the grating. The measurement accuracy (lower than 0.4%) is limited by the accuracy of charge-coupled device (CCD) measurements [5]. Lau phase interferometry uses the slope of a phase map to determine the lens focal length [6,7]. The measurement accuracy (lower than 0.2%) is influenced by the aberrations of an unwrapped phase map [7]. The circular Dammann grating method uses the diffraction characteristics of a circular Dammann grating at the focal plane of a lens to locate the focus of the test lens. The identification accuracy of the focus (lower than 0.1%) is limited by the definition of a diffraction pattern [8]. Phase-shift interferometry identifies the image point of the test lens using interference fringes, and then calculates the lens focal length by Gaussian or Newtonian equations [9,10]. The measurement accuracy reaches 0.01% [10], which is the highest accuracy among those of existing measurement methods. Other proposed methods include the fiber-optic autocollimation method [11], reference plant method [12] and Hartmann-Shack method [13]. However, their measurement accuracies need to be improved for engineering measurements.

The accuracies of existing focal-length measurement methods are lower than 0.01%, which cannot meet the demands of some sophisticated technologies [14,15]. In the previous work, we have presented a laser differential confocal ultra-long focal length measurement with the relative error of about 0.01% [16], but it is not suitable for the short focal-length measurement. Therefore, a laser differential reflection-confocal focal-length measurement (DRCFM) method is proposed in this paper. DRCFM uses differential confocal intensity detection to identify the lens focus and vertex of the lens last surface with high accuracy. The method also has a strong anti-interference capability [17]. Theoretical analyses and preliminary experiments show that the DRCFM relative measurement error is less than 10 ppm. Thus, DRCFM can enable the high-accuracy measurement of the lens focal length.

2. DRCFM principle

Our group has previously established that the null point of the axial intensity response signal corresponds to the focus of the objective in a differential confocal system [17]. On this basis, DRCFM uses null points Q_A and Q_B of differential confocal response signals I_A and I_B to identify precisely the lens focus and vertex of the lens last surface, as shown in Fig. 1 . The distance between these two null positions is then determined to facilitate the high-accuracy measurement of the lens back focal length l_F'.

l_{F}^{'} =2 | z_{A} - z_{B} |,

where z_A and z_B are the position coordinates of reflector R corresponding to null points Q_A and Q_B, respectively.

Fig. 1 DRCFM principle. PBS is the polarized beam splitter, P is the one-fourth wave plate, L_c is the collimating lens, L_t is the test lens, R is the reflector, BS is the beam splitter, CCD1 and CCD2 are detectors, MO1 and MO2 are microscope objectives, M is the offset of the VPHs from the focus of L_c, and DMI is distance measurement interferometer.

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When reflector R moves near position A along the optical axis of test lens L_t, the measurement beam reflected from R is again reflected by the polarized beam splitter (PBS) onto the beam splitter (BS). The two measurement beams split by the BS are received by the virtual pinholes (VPH1 and VPH2). The normalized intensity signals I_A₁(ν₂,u, + u_M) and I_A₂(ν₂,u,-u_M) received from CCD1 and CCD2 can be obtained by the Huygens–Fresnel diffraction integral formula.

\begin{array}{l} I_{A 1} (ν_{2}, u, + u_{M}) = | \int_{0}^{\infty} {\int_{0}^{1} P_{t} (ρ) P_{c} (ρ) \exp (\frac{i u ρ^{2}}{2}) J_{0} (ν_{1} ρ) ρ d ρ} \times \\ {{\int_{0}^{1} P_{t} (ρ) P_{c} (ρ) \exp [i ρ^{2} (\frac{u}{2} - \frac{u_{M}}{2})] \int_{0}^{2 π} J_{0} (ρ \sqrt{ν_{1}^{2} + ν_{2}^{2} + 2 ν_{1} ν_{2} \cos θ}) d θ ρ d ρ} ν_{1} d ν_{1} |}^{2}, \end{array}

\begin{array}{l} I_{A 2} (ν_{2}, u, - u_{M}) = | \int_{0}^{\infty} {\int_{0}^{1} P_{t} (ρ) P_{c} (ρ) \exp (\frac{i u ρ^{2}}{2}) J_{0} (ν_{1} ρ) ρ d ρ} \times \\ {{\int_{0}^{1} P_{t} (ρ) P_{c} (ρ) \exp [i ρ^{2} (\frac{u}{2} + \frac{u_{M}}{2})] \int_{0}^{2 π} J_{0} (ρ \sqrt{ν_{1}^{2} + ν_{2}^{2} + 2 ν_{1} ν_{2} \cos θ}) d θ ρ d ρ} ν_{1} d ν_{1} |}^{2}, \end{array}

where

{\begin{cases} ν_{1} = \frac{π}{λ} \frac{D}{{f^{'}}_{t}} r_{1} \\ u = \frac{π}{2 λ} z \frac{D^{2}}{{f^{'}}_{t}^{2}} \end{cases} and {\begin{cases} ν_{2} = \frac{π}{λ} \frac{D}{f_{c}^{'}} r_{2} \\ u_{M} = \frac{π}{2 λ} M \frac{D^{2}}{{f^{'}}_{c}^{2}} \end{cases} .

J₀ is a zero-order Bessel function, ρ is the radial normalized radius of a pupil, θ is the angle of variable ρ in the polar coordinate, r₁ is the radial coordinate on reflector R, v₁ is the normalized coordinate of variable r₁, z is the axial displacement between reflector R and position A, u is the normalized coordinate of variable z, r₂ is the radial coordinate on the object plane of VPH1 or VPH2, v₂ is the normalized coordinate of variable r₂, M is the offset of the VPHs from the focus of collimating lens L_c, u_M is the normalized offset of variable M, P_t(ρ) is the pupil function of test lens L_t, P_c(ρ) is the pupil function of collimating lens L_c, D is the effective aperture of L_c and L_t which is equal to the smaller aperture between L_c and L_t, D/f_c' is the effective relative aperture of L_c, and D/f_t' is the effective relative aperture of L_t. To ensure the full aperture measurement of L_t, the clear aperture of L_c should be no smaller than that of L_t.

When P_c(ρ) = 1 and P_t(ρ) = 1, the differential confocal response signal I_A(u,u_M) is obtained through the differential subtraction of I_A₁(0,u, + u_M) and I_A₂(0,u,-u_M).

\begin{array}{l} I_{A} (u, u_{M}) = I_{A 1} (0, u, + u_{M}) - I_{A 2} (0, u, - u_{M}) \\ = {[\frac{\sin (u / 2 - u_{M} / 4)}{u / 2 - u_{M} / 4}]}^{2} - {[\frac{\sin (u / 2 + u_{M} / 4)}{u / 2 + u_{M} / 4}]}^{2} . \end{array}

As shown in Fig. 1, null point Q_A of differential confocal response signal I_A(u,u_M) precisely corresponds to position A such that reflector R is exactly at the focus of L_t.

When reflector R moves near position B along the optical axis of L_t, a part of the measurement beam is reflected by the last surface of L_t. The overlap area of the measurement beam and the last surface of L_t is so small that the effect of the surface curvature on the differential confocal response signal is negligible. When the distance between reflector R and position B is z, the distance between the focusing point of the measurement beam and the vertex of the L_t last surface is 2z. Therefore, the differential confocal response signal I_B(u,u_M) obtained through the differential subtraction of I_B₁(0,u, + u_M) and I_B₂(0,u,-u_M) can be derived as follows.

I_{B} (u, u_{M}) = {[\frac{\sin (u - u_{M} / 4)}{u - u_{M} / 4}]}^{2} - {[\frac{\sin (u + u_{M} / 4)}{u + u_{M} / 4}]}^{2} .

As shown in Fig. 1, null point Q_B of differential confocal response signal I_B(u,u_M) precisely corresponds to position B such that the measurement beam is focused on the vertex of the L_t last surface.

When the other parameters of L_t are given, the distance between the L_t second principle point and the vertex of the L_t last surface can be calculated by using ray tracing formulas. Then, the effective focal length of L_t can be indirectly measured. Generally, the distance between the L_t second principle point and the vertex of the L_t last surface is so small that the resulting measurement error is negligible. With a single lens as an example, the effective focal length can be calculated as follows.

f_{t}' = l_{F}^{'} + \frac{r {}_{2}b}{n (r {}_{2}- r_{1}) + (n - 1) b},

where r₁ is the curvature radius of the L_t front surface, r₂ is the curvature radius of the L_t back surface, n is the L_t refractive index, and b is the L_t thickness.

3. Effect analyses of key parameters

3.1 Focusing sensitivities

Sensitivities S_A(0,u_M) at null point Q_A and S_B(0,u_M) at null point Q_B can be obtained by differentiating Eqs. (5) and (6) on u, respectively.

S_{A} (0, u_{M}) = | {\frac{\partial I_{A} (u, u_{M})}{\partial u} |}_{u = 0} | = | {2 \sin \frac{u_{M}}{4} (\sin \frac{u_{M}}{4} - \frac{u_{M}}{4} \cos \frac{u_{M}}{4}) / (\frac{u_{M}}{4})}^{3} |,

S_{B} (0, u_{M}) = | {\frac{\partial I_{B} (u, u_{M})}{\partial u} |}_{u = 0} | = | {4 \sin \frac{u_{M}}{4} (\sin \frac{u_{M}}{4} - \frac{u_{M}}{4} \cos \frac{u_{M}}{4}) / (\frac{u_{M}}{4})}^{3} | .

Figure 2(a) shows that differential confocal response signals I_A(u,u_M) and I_B(u,u_M) have the largest sensitivities S_A_max and S_B_max at null points Q_A and Q_B when u_M = 5.21.

Fig. 2 Determination of the best offset of the VPHs: (a) focusing sensitivity curves, (b) differential confocal response signals.

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S_{A \max} = S_{A} (0, 5.21) = 0.54 and S_{B \max} = S_{B} (0, 5.21) = 1.08.

Figure 2(b) shows simulation curves I_A(u,u_M) and I_B(u,u_M) with u_M = 5.21. The identification at the vertex of the L_t last surface is clearly much more sensitive than that at the L_t focus.

When u_M = 5.21, DRCFM focusing errors σz_A and σz_B at the L_t focus and the vertex of the L_t last surface can be described as follows.

σ z_{A} = \frac{δ I_{A} (u, u_{M})}{S_{A max}} \frac{2 λ}{π {(D / f_{t}^{'})}^{2}} = \frac{2 λ}{0.54 π \cdot S N R \cdot {(D / f_{t}^{'})}^{2}},

σ z_{B} = \frac{δ I_{B} (u, u_{M})}{S_{B max}} \frac{2 λ}{π {(D / f_{t}^{'})}^{2}} = \frac{2 λ}{1.08 π \cdot S N R \cdot {(D / f_{t}^{'})}^{2}},

where δI_A(u,u_M) and δI_B(u,u_M) are the random noises of the differential confocal response signals caused by the CCDs, and SNR is the signal-to-noise ratio of the VPHs. Equations (11) and (12) show that the identification precision increases as relative aperture D/f_t' increases.

3.2 Effect of axial alignment errors

As shown in Fig. 3 , axis m of the measurement beam, optical axis s of L_t, measurement axis p of the distance measurement interferometer (DMI), and optical axis q of reflector R should be coincident, whereas the deviation angles between them always exist in practice. Using the autocollimation method without L_t in the measurement path, the angle between axes m and q can be aligned such that the effect of this angle on the measurement can be negligible. If the angle between axes m and s is defined as α, and the angle between axes m and p is defined as β, the back focal length measurement error can be calculated using the following geometric relationship.

Fig. 3 Angles between DRCFM axes.

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σ_{a x i a l} \approx {f^{'}}_{t} (\frac{\cos β}{\cos α} - 1) .

Angle β is diminished through the careful alignment of axes p and m, and it should be adjusted only while establishing the system. When the focal length of L_c in the DRCFM system is 1000 mm, angle β can be reduced to less than 3 second. Angle α between axes m and s can be easily controlled within 2 minute by careful alignment.

3.3 Effect of two VPHs with different offsets

As shown in Fig. 4 , when the offsets of the two VPHs from the L_c focus are different, null points Q_A and Q_B of the differential confocal response signals deviate from positions A and B, respectively. Thus, the l_F' measurements change.

Fig. 4 Light path schematics with different offsets.

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Let the offsets of two VPHs from the L_c focus be M and -M + δ_M, their corresponding normalized offsets be u_M and -u_M + u_δM, the offsets of Q_A from position A and of Q_B from position B in the measurement process be Δl₁ and Δl₂, and their normalized values be Δu₁ and Δu₂.

Then, the differential confocal response signals I_A(u,u_M) and I_B(u,u_M) are obtained according to Eqs. (5) and (6).

I_{A} (u, u_{M}) = {\frac{\sin (u / 2 - u_{M} / 4)}{u / 2 - u_{M} / 4}}^{2} - {\frac{\sin [u / 2 + (u_{M} - u_{δ M}) / 4]}{u / 2 + (u_{M} - u_{δ M}) / 4}}^{2},

I_{B} (u, u_{M}) = {\frac{\sin (u - u_{M} / 4)}{u - u_{M} / 4}}^{2} - {\frac{\sin [u + (u_{M} - u_{δ M}) / 4]}{u + (u_{M} - u_{δ M}) / 4}}^{2} .

From Eqs. (14) and (15),

Δ u_{1} = \frac{u_{δ M}}{4} and Δ u_{2} = \frac{u_{δ M}}{8} .

From Eq. (4), the respective actual offsets of the corresponding positions Q_A and Q_B are

Δ l_{1} = \frac{1}{4} \frac{{f^{'}}_{t}^{2}}{{f^{'}}_{c}^{2}} δ_{M} and Δ l_{2} = \frac{1}{8} \frac{{f^{'}}_{t}^{2}}{{f^{'}}_{c}^{2}} δ_{M} .

Therefore, when the deviation of VPHs axial offset is δ_M, back focal length measurement error σ_offset caused by δ_M can be obtained through Eq. (1).

σ_{o f f s e t} = 2 (Δ l_{1} - Δ l_{2}) = \frac{1}{4} \frac{{f^{'}}_{t}^{2}}{{f^{'}}_{c}^{2}} δ_{M} .

Using a grating scale as an aided adjustment of the VPH offsets, offset error δ_M can be easily controlled within 10 μm, which is limited by both the position accuracy of the grating scale and the focusing accuracy of the L_c focus.

3.4 Distance measurement error

As shown in Fig. 1, the distance between positions A and B is measured by a single-frequency laser interferometer, and its measurement error is

σ_{L} = 1 \times L ppm,

where L is the measurement distance.

3.5 Synthesis error

Considering the effect of the aforementioned errors on the measurement results of l_F', the DRCFM total measurement error σl_F' is

σ l_{F}^{'} = \sqrt{{(\frac{\partial l_{F}^{'}}{\partial L} σ_{L})}^{2} + {(\frac{\partial l_{F}^{'}}{\partial z_{A}} σ z_{A})}^{2} + {(\frac{\partial l_{F}^{'}}{\partial z_{B}} σ z_{B})}^{2} + σ_{o f f s e t}^{2} + σ_{a x i a l}^{2}} .

According to Eq. (1), the respective error transfer coefficients of σ_L, σz_A, and σz_B are

\frac{\partial l_{F}^{'}}{\partial L} = 2, \frac{\partial l_{F}^{'}}{\partial z_{A}} = 2, and \frac{\partial l_{F}^{'}}{\partial z_{B}} = 2.

Using Eqs. (20) and (21), the DRCFM synthesis measurement error is

σ l_{F}^{'} = \sqrt{4 σ_{L}^{2} + 4 σ z_{A}^{2} + 4 σ z_{B}^{2} + σ_{o f f s e t}^{2} + σ_{a x i a l}^{2}} .

4. Experiments

4.1 Experimental setup

The experimental setup shown in Fig. 5 is established based on Fig. 1 to verify the validity of DRCFM.

Fig. 5 Experimental setup. (1) He-Ne laser, (2) single-mode fiber, (3) VPH1, (4) VPH2, (5) beam splitter, (6) polarized beam splitter, (7) one-fourth wave plate, (8) aiming system, (9) beam splitter, (10) collimating lens, (11) test lens, (12) reflector, (13) air bearing slider, (14) XL-80 laser interferometer produced by Renishaw, and (15) air sensor of XL-80.

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The environmental conditions in the measurement laboratory are pressure = (102540 ± 60) Pa, temperature = (20 ± 0.2) °C, and relative humidity = (40 ± 4) %. In the experiment, a He-Ne laser with a wavelength of 632.8 nm is used as the light source. An achromatic lens (produced by LINOS Photonics GmbH & Co.) with a focal length of 1000 mm and a diameter of 100 mm is used as collimating lens L_c. An XL-80 laser interferometer (produced by Renishaw) is used as the distance measurement interferometer. A high-accuracy air bearing slider with a range of 1300 mm and a straightness of 0.1 μm is used as the motion rail. The CCDs used are OK-AM1100 with a pixel size of 8 μm. The SNR of the VPHs is 100:1, and the magnification of the VPH microscope objectives is 10 × . Test lens L_t is a cemented doublet lens with a diameter of 20 mm and a focal length of 200 mm, and its nominal back focal length l_F' is 197.3 mm ( ± 2%).

4.2 Experiment results

As shown in Fig. 6 , when reflector R moves across position A along the optical axis during measurement, DRCFM uses null point Q_A of differential confocal response signal I_A(z) to determine precisely the focus of L_t. The position coordinate of reflector R corresponding to point Q_A is z_A = −99.17363 mm.

Fig. 6 Back focal length measurement curves.

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Similarly, when reflector R moves across position B, DRCFM uses null point Q_B of differential confocal response signal I_B(z) to determine precisely the vertex of the L_t last surface. The position coordinate of reflector R corresponding to point Q_B is z_B = 0.17223 mm.

Therefore, the back focal length l_F' of L_t is 2|z_A - z_B| = 198.6917 mm, and the repeatability achieved from ten measurements is σ_test = 1.6 μm.

Using Eqs. (11)–(13), (18), and (19), the errors existing in DRCFM are σ_L = 0.1 μm, σz_A = 0.75 μm, σz_B = 0.37 μm, σ_axial = 0.03 μm, and σ_offset = 0.1 μm. The system error obtained using Eq. (22) is

\begin{array}{l} σ l_{F}^{'} = \sqrt{4 σ_{L}^{2} + 4 σ z_{A}^{2} + 4 σ z_{B}^{2} + σ_{a x i a l}^{2} + σ_{o f f s e t}^{2}} \\ = \sqrt{4 \times {0.1}^{2} + 4 \times {0.75}^{2} + 4 \times {0.37}^{2} + {0.03}^{2} + {0.1}^{2}} = 1.7 μ m . \end{array}

The relative error is

δ = \frac{σ l_{F}^{'}}{l_{F}^{'}} \times 100 % = \frac{1.7}{198.6918 \times 1000} \times 100 % \approx 0.00086 % = 8.6 ppm .

Considering the environmental effects and some negligible errors, the relative error of DRCFM can be less than 10 ppm.

It can be concluded from the above error analyses that the focusing errors σz_A and σz_B have more significant effect on the measurement accuracy. So, the measurement accuracy increases as the relative aperture of test lens increases.

5. Conclusions

The proposed DRCFM uses the null points of differential confocal response signals to identify precisely the lens focus and vertex of the lens last surface. Consequently, the lens focal length can be measured with high accuracy. Theoretical analyses and experimental results show that the proposed approach has a relative error of less than 10 ppm and has the following advantages.

1) It improves significantly the identification precision at the lens focus and vertex of the lens last surface because it has the best linearity and sensitivity at the null point of a differential confocal response signal.
2) It can measure the lens spherical aberration in combination with annular pupil filtering technology.
3) It has higher measurement accuracy and stronger anti-interference capability than existing approaches.

Therefore, DRCFM is a feasible method for the high-accuracy measurement of the lens focal length.

Acknowledgments

The authors gratefully acknowledge the support of the Major National Scientific Instrument and Equipment Development Project of China (No. 2011YQ040136), the National Science Foundation of China (No. 91123014, 60927012), and the Nature Science Association Foundation (No. 11076004).

References and links

1. E. Keren, K. M. Kreske, and O. Kafri, “Universal method for determining the focal length of optical systems by moire deflectometry,” Appl. Opt. 27(8), 1383–1385 (1988). [CrossRef] [PubMed]

2. C.-W. Chang and D.-C. Su, “An improved technique of measuring the focal length of a lens,” Opt. Commun. 73(4), 257–262 (1989). [CrossRef]

3. P. Singh, M. S. Faridi, C. Shakher, and R. S. Sirohi, “Measurement of focal length with phase-shifting Talbot interferometry,” Appl. Opt. 44(9), 1572–1576 (2005). [CrossRef] [PubMed]

4. K. V. Sriram, M. P. Kothiyal, and R. S. Sirohi, “Direct determination of focal length by using Talbot interferometry,” Appl. Opt. 31(28), 5984–5987 (1992). [CrossRef] [PubMed]

5. F. Lei and L. K. Dang, “Measuring the focal length of optical systems by grating shearing interferometry,” Appl. Opt. 33(28), 6603–6608 (1994). [CrossRef] [PubMed]

6. M. Thakur and C. Shakher, “Evaluation of the focal distance of lenses by white-light Lau phase interferometry,” Appl. Opt. 41(10), 1841–1845 (2002). [CrossRef] [PubMed]

7. C. J. Tay, M. Thakur, L. Chen, and C. Shakher, “Measurement of focal length of lens using phase shifting Lau phase interferometry,” Opt. Commun. 248(4-6), 339–345 (2005). [CrossRef]

8. S. Zhao, J. F. Wen, and P. S. Chung, “Simple focal-length measurement technique with a circular Dammann grating,” Appl. Opt. 46(1), 44–49 (2007). [CrossRef] [PubMed]

9. Y. P. Kumar and S. Chatterjee, “Technique for the focal-length measurement of positive lenses using Fizeau interferometry,” Appl. Opt. 48(4), 730–736 (2009). [CrossRef] [PubMed]

10. Y. Xiang, “Focus retrocollimated interferometry for focal-length measurements,” Appl. Opt. 41(19), 3886–3889 (2002). [CrossRef] [PubMed]

11. I. K. Ilev, “Simple fiber-optic autocollimation method for determining the focal lengths of optical elements,” Opt. Lett. 20(6), 527–529 (1995). [CrossRef] [PubMed]

12. D.-H. Kim, D. Shi, and I. K. Ilev, “Alternative method for measuring effective focal length of lenses using the front and back surface reflections from a reference plate,” Appl. Opt. 50(26), 5163–5168 (2011). [CrossRef] [PubMed]

13. J.- Wu, J.- Chen, A.- Xu, X.-y. Gao, and S. Zhuang, “Focal length measurement based on Hartmann-Shack principle,” Optik (Stuttg.) 123(6), 485–488 (2012). [CrossRef]

14. J. Wu, J. Chen, A. Xu, and X. Gao, “Uncollimated light beam illumination during the ocular aberration detection and its impact on the measurement accuracy by using Hartmann-Shack wavefront sensor,” Proc. SPIE 7508, 75080V, 75080V-12 (2009). [CrossRef]

15. T. G. Parham, T. J. McCarville, and M. A. Johnson, “Focal length measurements for the National Ignition Facility large lenses,” Optical Fabrication and Testing (OFT 2002), paper: OWD8.

16. W. Zhao, R. Sun, L. Qiu, and D. Sha, “Laser differential confocal ultra-long focal length measurement,” Opt. Express 17(22), 20051–20062 (2009). [CrossRef] [PubMed]

17. W. Zhao, J. Tan, and L. Qiu, “Bipolar absolute differential confocal approach to higher spatial resolution,” Opt. Express 12(21), 5013–5021 (2004). [CrossRef] [PubMed]

Laser differential reflection-confocal focal-length measurement

Abstract

1. Introduction

2. DRCFM principle

3. Effect analyses of key parameters

3.1 Focusing sensitivities

3.2 Effect of axial alignment errors

3.3 Effect of two VPHs with different offsets

3.4 Distance measurement error

3.5 Synthesis error

4. Experiments

4.1 Experimental setup

4.2 Experiment results

5. Conclusions

Acknowledgments

References and links

Cited By

Figures (6)

Equations (24)

Optics Express