1.

Introduction

Plano lenses, whether with the convex side facing the eye or the concave side toward it, are notably useful for spectacle testing through neutralization, being adaptable to various forms.¹ Lenses with zero diopter, commonly referred to as plano lenses, find widespread use in specialized eyewear, including sunglasses, sports glasses, snow goggles, safety glasses, goggles, and masks. These serve the dual purpose of eye protection, facial covering, and aesthetic appeal, holding a significant position in the global market each year. In 2020, the global market size for safety glasses amounted to US$ 1686.98 million, projected to reach US$ 1692.16 million by 2027, indicating a growth rate of 1.80% between 2021 and 2027.² Additionally, the global sports sunglasses market achieved a value of US$ 2.9 billion in 2021. The IMARC Group anticipates this market to escalate to US$ 3.7 billion by 2027, demonstrating a compound annual growth rate (CAGR) of 4.07% from 2022 to 2027.³ The testing of plano lenses necessitates the assessment of diopter and astigmatism, requiring adherence to a specified tolerance range.

The design of such measuring instruments must align with the standards outlined in section 9.4 of ANSI Z87.1-2020,⁴ which details the test requirements for diopter, astigmatism, clarity, is an American National Standard used for safety purposes such as safety glasses, safety goggles, side shields, and other eye and face protection devices. The industry benchmark for eye and face protectors has undergone six revisions and has been incorporated into regulatory guidelines by the U.S. Occupational Safety and Health Administration. These regulations mandate employers to provide appropriate safety glasses and face protectors. However, existing measuring instruments with optical components meeting these specifications often feature elongated and narrow path lengths, resulting in inefficient utilization of space within the instrument configuration. This setup restricts the intervention of personnel or objects during the measurement process. Practical laboratory planning frequently encounters challenges where available space is insufficient, leading to wasted or inefficiently used areas. Over recent years, advancements in semiconductor manufacturing processes and photonic technology have led to the transformation of cumbersome laboratory instruments into compact, field-deployable devices.^5,6 With the rapid evolution of technology and the increasing demand for high-quality, low-defect-rate products, the importance of automatic optical inspection (AOI) technology has amplified. AOI technology is intelligent, flexible, and yields more consistent results compared to manual visual inspection.⁷ It is considered robust and capable of replacing human inspectors who may experience fatigue and monotony during inspection tasks.⁸

Recognizing the efficiency and increased accuracy of automated techniques compared to manual estimations,^9,10 this study embarked on refining an advanced optical inspection (AOI) design and developing a sample fixture-based measurement mechanism. The primary goal was to substantially reduce manpower needs and operational measurement times. Moreover, considerable efforts were made to downsize the machine, reducing installation space requirements while ensuring maintained inspection quality. We anticipate that these advancements will not only contribute to the field of advanced optical inspection (AOI) but also serve as a valuable reference for informing future revisions of ANSI Z87.1.

2.

Existing Techniques

2.1.

Optical Inspection Techniques

Optical inspection techniques can be broadly categorized into two primary types: manual optical inspection, conducted by human inspectors, and AOI, which utilizes an image sensor and processor.⁹ As per section 9.4 of ANSI Z87.1-2020, the calibration layout for diopter and astigmatism in optometry instruments should include an eyepiece, manual focusing mechanism, telescope, lens fixtures, and a light box for pattern testing. The apparatus should feature an eight-power calibrated telescope (preferably equipped with a reticle), with a minimum aperture of 19 mm (0.75 in.) for plano-spherical lenses and 7 mm (0.28 in.) for plano-toric or plano-aspheric lenses. It should also include a fixture to hold the protector in the test position and a combined sunburst and resolution test pattern.

The test pattern, featuring the sunburst lines with resolution patterns (Fig. 1), should be positioned at a distance of 10.67 m (35 ft) from the telescope’s objective lens and should be adequately illuminated, either backlit or by other means, to conduct the test effectively (Fig. 2). The telescope comprises the eyepiece F1, the objective lens F2, and the focal length adjustment mechanism. In the absence of a sample, when the target is clearly visible, the distance between the eyepiece F1 and the objective lens F2 is “D,” and the distance between the objective lens and the target is “d.” Upon placing the sample T3, the image of the target becomes unclear. At this point, adjusting the distance “D” between F1 and F2 makes the target visible again, resulting in a variation denoted as “$\delta$.”

Fig. 1

The test resolution pattern with a radial sunburst power target superimposed according to ANSI Z87.1-9.4.

Fig. 2

Schematic of the calibration instrument designed for optometry applications in accordance with the ANSI/ISEA Z87.1-2020 standard.

The “YT-306 Telescope Optical Tester (US Standard) - Plano Eyewear Dioptometer” used in this study is manufactured by the Yin-Tsung Company and meets the specified specifications.¹¹ Figures 3(a) & 3(b) showcase the telescope, which is equipped with an adjustment mechanism, and the target, featuring a light box.

Fig. 3

(a) Part of the diopter testing system and (b) target with backlight.

2.2.

Manual Optical Inspection

Within the diopter test system, the operator modifies the focal length of the telescope to assess the clarity of a distant target. When a lens sample is positioned in front of the telescope, a spherical lens will reveal only one distinct target, as depicted in Fig. 4(a). Conversely, astigmatism will display two clear targets—D1 and D2, showcased in Figs. 4(b) & 4(c), calculated in accordance with Z87.1-9.4.3¹² as follows:

Eq. (1)

$$\text{Diopters:}\frac{(\mathrm{D}1+\mathrm{D}2)}{2},(\text{standard range}\pm 0.06\mathrm{D}),$$

Eq. (2)

$$\text{Astigmatism Power}:|\mathrm{D}1-\mathrm{D}2|,(\text{standard range}0.06\mathrm{D}).$$

Fig. 4

Graphics at different focal lengths for Z87.1-2020 (a) without astigmatism, (b) clear D1 image at 135 deg, and (c) clear D2 image at 45 deg.

2.3.

Original Automatic Optical Inspection

The initial AOI method involved visual observation and manual focus to compute the diopter and astigmatism, illustrated in Fig. 5.

Fig. 5

System architecture of optometry automatic detection instrument.

Computer vision technology is extensively applied across numerous practical domains today, encompassing optical character recognition, machine inspection, retail, 3D model construction (photogrammetry), medical imaging, automotive safety, match move, motion capture (mocap), surveillance, fingerprint recognition, and biometrics, among others.¹³

In this study, we adhere to the principles of AOI techniques as established in “optometry automatic detection technology.”¹⁴ We employ computer vision through the use of cameras and auto-focus modules, substituting human visual observation and manual focus with machine vision and computer-controlled focus mechanisms. This transition enhances detection speed and reduces measurement discrepancies.

3.

Methods

3.2.

Improvement of Original AOI

3.2.1.

Telescope group and target group

The AOI telescope’s design incorporates an automatic focusing feature. This mechanism involves securing the front lens and utilizing a stepping motor to move the slide, thereby adjusting the position of the rear lens barrel (camera) to change the focal length. The camera captures a contrast image, systematically scanning at fixed intervals to determine the optimal focus position’s potential range. Subsequently, the computer calculates the position with the highest contrast, facilitating automatic focus adjustment.

For proper alignment with the optical axis of the target group, the telescope assembly requires a structure capable of tilting up and down and rotating left and right on the mounting base, as depicted in Fig. 6(a), allowing adjustment of the telescope’s orientation. To prevent potential errors in computer judgment, the sunburst pattern for diopter and astigmatism measurement and the fringe pattern for sharpness measurement have been divided into separate left and right targets. Consequently, the target device necessitates lateral movement capability. To address this, a stepping motor controls the sliding table, which acts as a switching platform securely holding the high-precision reduced optical target. This setup involves placing a lens in front of the target to establish a parallel optical path with the telescope lens, significantly reducing the distance. Additionally, the light source is positioned behind the target, as illustrated in Fig. 6(b).

Fig. 6

(a) Telescope group and (b) target group.

3.2.2.

Jig platform

To meet the diverse requirements for measuring different samples, the use of various fixture types is necessary. For instance, when evaluating glasses as a single unit, a fixture for placing and adjusting the glasses, depicted in Fig. 7(a), is utilized. Alternatively, a separate fixture, shown in Fig. 7(c) and designed to position samples on a model resembling the human head, is employed.

Fig. 7

Jig platform (a) Glasses jig, (b) mobile sliding platform, and (c) human head jig.

Because the device operates with a single optical path, scenarios that require separate measurements for left and right eyes necessitate an automatic adjustment of the sample position. To achieve comprehensive automation in measurement procedures, a sliding table controlled by a stepping motor, functioning as a switching platform, as illustrated in Fig. 7(b), is employed. Once the desired measurement position is set, this mechanism automatically shifts between the left and right eye positions, ensuring more precise measurement data and expediting the process.

3.3.

Target Making

3.3.1.

Estimation of target scale

Applying the thin lens imaging formula: $1/|p|+1/|q|=1/f$, and calculating the imaging magnification: $m=h^{\prime }/h=q/p$, as represented in Fig. 8(a), provides the basis for further analysis.

Fig. 8

(a) Imaging principle of thin lens, (b) imaging map of the original structure, and (c) imaging map of the target in this study.

By utilizing the target size from the initial specification,⁶ the magnification is determined: $m=h^{\prime }/h=q/p=0.24/10.67=0.02249$. Therefore, the camera’s target imaging size is reduced to 0.02249 times, as illustrated in Fig. 8(b).

To maintain the target imaging size ratio of the new structure to that of the original structure (both at a ratio of 0.02249), an additional lens with the same focal length as the original structure is needed in front of the target, as shown in Fig. 8(c), denoting $h^{\prime }=h_1$.

3.3.2.

Reduced production of new target

In this research, the necessity to significantly reduce the measurement distance also requires a proportional reduction in the target size. Specifically, the measurement distance is diminished from the original standard of 10.67 to 0.24 m, and consequently, the target size must be reduced by the ratio of 0.02249. This reduced target size poses a challenge as the minimum line width of the target pattern shrinks from 0.25 mm (h) to 0.0056225 mm $(\mathrm{h}1)=5.6225\mu \mathrm{m}$ after applying the reduction ratio of 0.02249. Conventional printing technologies are incapable of producing such minute line widths. As a result, the only viable solution is to utilize mask manufacturing technology adhering to the semiconductor standard process to create a precision optical target.

A photo-mask substrate typically consists of a $6{\mathrm{in.}}^2$ sheet ($\sim 152{\mathrm{mm}}^2$) sheet of fused silica. During photolithography, this substrate is adorned with a pattern of opaque and transparent areas that are projected onto the substrate. In the subsequent patterning step, numerous continuous pattern designs guide the material deposition or removal on the substrate. Finally, the completed pattern is cut out for use, as depicted in Fig. 9.

Fig. 9

(a) Complete finished photomask and (b) photomask design used in the radial sunburst image and clear line image.

3.3.3.

Optical path adjustment

To begin, secure the target holder and position its objective lens at a distance of 0.24 m from the target pattern [p1 in Fig. 8(c)]. Subsequently, fix the telescope assembly at an appropriate distance ensuring the transmission of parallel light within this range, a distance dictated by the fixture platform to accommodate the placement of test samples. Following this, adjust the distance between the telescope’s objective lens and the camera to 0.24 m [q in Fig. 8(c)]. Using the image relayed by the camera, make further adjustments involving tilting up and down as well as rotating left and right to finely calibrate the orientation of the telescope assembly. This adjustment is crucial to ensure complete image capture by the camera.

4.

Verification and Results

Two experiments are conducted in this study to validate the accuracy and adherence to the specifications of the newly designed system. In the first experiment, the study employed the new architectural design and utilized six standard diopter test lenses calibrated by the National Physical Laboratory (NPL), as indicated in their certificate of calibration for six test lenses and two prisms dated 2022 (Table 1). The intention was to utilize these test lenses in the measurement process.

Table 1

Lens specifications.

The measurement outcomes were assessed against the certificate of calibration reference: 2022010181/1 (stating a measurement uncertainty within ±0.002D).¹⁵ Each lens underwent 33 random measurements, with all resulting values falling within the allowable error range, as demonstrated in Tables 2 and 3.

Table 2

New architecture design measurement results (tester 1).

Table 3

New architecture design measurement results (tester 2).

The second experiment involved selecting three pairs of goggles that had satisfactorily passed the Z87.1 test. These goggles conformed to the Z87.1 acceptance standard, where the tolerance range fell within $D\pm 0.06$, as illustrated in Fig. 10.

Fig. 10

Three pairs of goggles that had satisfactorily passed the Z87.1 test.

Subsequently, two testers conducted individual tests using both the standard instrument (at a distance of 10.67 m) and the newly designed instrument. To facilitate a comparison between measurements, each sample underwent random measurements 33 times, and the average measurements were recorded. The resulting data from these tests is presented in Tables 4Table 5Table 6–7.

Table 4

Z87.1 standard instruments (tester 1).

Table 5

New instruments (tester 1).

Table 6

Z87.1 Standard instruments (tester 2).

Table 7

New instruments (tester 2).

The measurement results vary between the two instruments and among different personnel as depicted in Fig. 11, within the range of $D\pm 0.01$.

Fig. 11

The measurement results vary between the two instruments and among different personnel.

5.

Discussions

Utilizing parallel light, as illustrated in Fig. 12, the new instrument is unaffected by variations in optical path length. By integrating the optometry automatic detection instrument system depicted in Fig. 5, along with the additional space required for the measurement fixture and the correction length of the target size, the overall dimensions can be drastically reduced to $<1\mathrm{m}$.

Fig. 12

Optical path diagram of the architecture.

Reducing the target size and capturing images can be affected by dust on the precision optical target, impacting the automatic interpretation of images. Hence, it is essential to assemble precision optical targets within a clean room environment to ensure the acquisition of dust-free images. Moreover, to prevent interpretation errors caused by excessive lines, the radial sunburst image and the clear line image should be measured separately. Upon downsizing the machine, each component can be enclosed within a sealed casing to mitigate external light influence and fluctuations in airflow, facilitating easier installation and transportation.

The second set of experimental results in this article reveals that the measurements obtained with the scaled-down new automated equipment in this study closely align with those recorded by the Z87.1 standard equipment. As depicted in Fig. 11, the automated inspection with the new equipment proves to be more accurate, attributed to the reduction of human operating errors. Furthermore, it is noteworthy that the measurement range of Z87.1 diopter is only $\pm 0.12\mathrm{D}$, whereas the new device in this study has an expanded measurement range, reaching up to $\pm 0.30\mathrm{D}$.

Despite the mentioned advantages of the new instrument, its measurement results may still be susceptible to operator errors in lens placement, such as tilt, and the quality of the sample itself, which includes factors like bubbles, distortion, aberrations, and material characteristics. It is crucial to acknowledge that the accuracy of the test results in this study relies on the specifications of six standard wafers. Consequently, the measurement accuracy for samples outside these standard wafer specifications remains uncertain.

6.

Conclusion

After the actual test in this study, it has been verified that the measurement results of this new architecture design are reliable. Because the measurement laboratory needs to control the temperature and humidity of the environment, the costs will depend on the size of the space. The smaller the lab, the lower the cost. This new architectural design reduces the measurement distance from 10.67M to 0.24M and the target pattern scale to the ratio of 0.02249. When making precision optical targets, it is necessary to use the mask manufacturing technology of the semiconductor standard process, which can reduce the size of the overall equipment, not limited by the laboratory space, and save time and installation costs during installation.