Tolerance analysis using Zemax, the ca for the small optics.
OPTI521, Sooyong Nam Abstract
A systematic procedure to perform tolerance analysis of the small optics has prented. To implement this
of this procedure, a typical cell phone camera lens has been chon to evaluate performance degradation of
the optical system. This outline could provide a starting point to understand small optics tolerancing and the instructions to u Zemax as a design and tolerancing tool.
1. Introduction
A few obvious trends in current consumer electronic devices are small and compact size but performing
high with very reasonable price. Optics inside tho products are not the exception. It is getting thinner but
has to be performing much better than before with higher pixel resolution. As a result of tho tight requirements, the tolerances are very tight and challenging. In this article, a systematic procedure for the tolerance analysis by Zemax has prented. The details about the design of three plastic lens elements are
not covered in this article.
我是你的太阳2. Procedure to perform tolerance analysis
The basic outline of the optical tolerancing procedure has prented in Fig. 1). In this analysis, as small cell
双提升phone camera lens don’t have complicated mechanical focusing features, only tho blue-boxed tasks are completed. Pink boxed tasks are done but will not be discusd in this article.
(1)
Table 1) is specification of required lens. Its F# is 2.8, pixel size is 5.6μm, and required resolution at center
is 125lp/mm. This is typical specification of fixed focus VGA cell phone camera lens. As a merit function
of the nsitivity analysis, 20% MTF drop at 45 lp/mm frequency is ud.
Table 1) Design specification
Specification item Required performance
Field of view 60 degrees
F# 2.8 Number of lens elements 3 plastic lens
Pixel size 5.6 μm
Image nsor size 3.58mm x 2.69mm
@center Resolution 125lp/mm
3. Optical layout for cell phone camera
Fig. 2) is optical layout of cell phone camera. It has three aspheric plastic lens, both sides are aspheres and there are two aperture stops between surface 3 and 6. As shown in Fig. 3), the materials are Zeonex E48R and Polycarbonate. Their indices are around 1.45~1.5. It is very clear and stable compare to conventional optical plastic. The total length from first surface to image plane is about 5.3mm and the lens diameters are from 2.6mm to 4.6mm.
Fig. 2) Optical layout with 3 plastic aspheric lens
Fig. 3) Lens data in Zemax
4. Optomechanical layout
Fig. 4) is optomechanical layout. Due to dimensional restriction and cost reduction requirement, the are manufactured from plastic injection molding process. All lens have their own individual mount so that the flange surfaces are stacked together with respect to the bore surface of the barrel. A plastic injection molded aperture stop is located between lens1 and lens2. Lens3 will be located after lens2. Retainer will be glued to the barrel to maintain lens asmbly and survived from the shock and keep all required tolerance remained.
Fig. 4) Optomechanical layout
2 3
6
9
7
8
Fig. 5) is the part of the drawing that has all the necessary tolerances are specified. At the top left corner, surface power and irregularity tolerances are specified and the note below is the index of refraction. Aspheric equation and their coefficients have described in the box at the top right corner. On the lens drawing, center thickness, element tilt, element decenter, and element spacing tolerances are specified. All the specific numbers are come from tolerance analysis.
Fig. 5) Error analysis from the lens element
5. Sensitivity table and error budget
Fig. 6 and 7 are input data to execute tolerance analysis from Zemax. Perturbed amounts are 1 fringe for radius of each surface(power), 0.01mm for thickness, 0.05 degree for surface tilt, 0.001 for
牛肉拌饭index change, 0.05 degree for element tilt, and 0.010mm for element decenter. Zemax allows us to tolerate surface irregularity by assigning tolerance operand TEZI which is simulating surface by adjusting Zernike coefficient. 0.00005mm is ud as RMS surface irregularity.
Fig. 6) Tolerance input to the Zemax Fig.7) Tolerance analysis routine in Zemax
After completed to define tolerances, the Zemax will generate tolerance table in Fig. 8). Then we can run tolerance analysis routine. From the command box shown in Fig. 7), we can calculate nsitivity to the given merit function.
As a merit function to evaluate performance degradation, percentile MTF drop at Ny/2 has lected.
Becau resolution and contrast is one of the main category to evaluate image quality of the camera. Since this system us 5.6μm pixel for the image nsor, the Nyquist frequency is 89lp/mm and Ny/2 is 45lp/mm.
科学小知识
大灰狼的故事大全Fig. 8) Tolerance data editor in Zemax
Table 2 is the result nsitivity table and error budget. Prented percentile MTF drop is from the 0 degree field at 0.632μm wave. Optical schematic has been attached on top the table to identify effects easily. From the analysis result, we can have following results:
- Lens 1 and following space : The dominant variables are index change and power tolerance for the surface 2 and surface 3 are cond dominant. Thickness and decenter tolerance is not as nsitive as previous three parameters. The effect of surface irregularity, lens1 and 2 spacing, and element tilt are almost negligible. Surface wedge is negligible through the entire system. So it doesn’t need to be
taken into consideration for this system.
坐井观天
- Lens 2 and following space : Dominant tolerances are power of two surface and spacing between lens 2 and lens 3. Thickness is next dominant one and after then other tolerances are all innsitive. Lens 2 is the least nsitive element.
不懂爱情- Lens 3 and space between image plane : Center thickness and space tolerance between lens and image plane are most dominant variables. Power tolerance of the surface 8 is cond most nsitive tolerance than others. The rest of the other tolerances are quite forgiving compare to other lens.
The RSS results have been calculated after each lens error and total RSS has calculated bad on the error of each lens. The total RSS of system degradation due to tolerated error is 17.7% of MTF drop. Therefore the design specification less then 20% at Ny/2 is achieved.
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3. Nominal
value
沉的成语4.
Perturbation
5. MTF
6.
Sensitivity
7.
Tolerances
refraction 1.5312 0.0010 0.0550 55.030.00100 2-3 Thickness(mm) 0.8822 0.0100 0.0283 2.830.01000 2.830.0000 0.0500 0.0001 0.000.10000
Decenter(mm) 0.0000 0.0100 0.0131 1.310.02000 1.3941 0.0011 0.0333 30.25
0.00220 0.0001 0.0001 2.800.00005 0.0500 0.0002 0.000.10000 2.3987 0.0066 0.0410 6.20
0.01320 0.0001 0.0035 69.360.00005 0.0500 0.0000 0.000.10000 Spacing
0.9750 0.0100 0.0006 0.06
0.01000 2
3
69
78
6. Conclusion
After completing tolerance analysis for the cell phone camera lens using Zemax, we have following conclusions,
a.From the given optical design, optomechanical layout has been prented.
b.Actual drawing with tolerance for the lens fabrication has been prented.
c.Dominant errors and negligible errors have been clarified after tolerance analysis. Lens thickness,
space between lens and power of each surfaces are dominant factors to affect performance.
d.Over all tolerance guide line has been met with 17.7% MTF drop at 45lp/mm
7. References
(1) Robert H. Ginsberg, “Outline of tolerancing (from performance specification to tolerance drawing)”, Optical Engineering, March/April 1981, Vol. 20, No. 2, pp. 175-180
(2) Zemax manual and Zemax knowledge ba website.
