# Laser Lens Size Measurement Using Swept-Source Optical Coherence Tomography
## 3 Results  
The imaging experiment was performed on a laser focusing lens used in the laser projector to measure the ROC of the focusing lens. A focusing lens with a ROC of 1.25 mm was used for the measuring sample. The charge coupled device (CCD) image and the surface profile of the Laser focusing lens are shown in Figure 3. The measurement area is \(2.4\times 3.0\) mm as shown in Figure 3a. The laser confocal microscope (LEXT OLS4000, Olympus) with objective lens MPLAPON50X (NA = 0.95, Olympus), 1X optical zoom and field of view is \(256\times 256\)\(\upmu\)m was used to measure ROC of the sample. Figure 4 shows the profile image of the laser focusing lens measured by the laser confocal microscope. The experiment was performed on the laser focusing lens, which measured position was marked by the horizontal line at the central of image as shown in Figure 4a. The profile curve is shown in Figure 4b. From Figure 4c, we can get the ROC of 1.2408 mm. The imaging experiments were then performed by the proposed SS-OCT system. Figure 5 shows the 2D OCT images at different location of the focusing lens. From Figure 5, we can see the profile curve and the inner structure of the focusing lens. The light lines in Figure 5 represent the upper surface and the lower surface of the focusing lens, respectively. The light lines occurred at the boundary when the refractive index changed suddenly. A series of straight lines in Figure 5 were noises caused by strong light reflection from the two edge of the focusing  
Figure 4: Profile image was measured by the laser confocal microscope. Figure 3: The images of the focusing lens. (**a**) CCD image; (**b**) surface profile. lens. From Figure 5a-c, the length of the upper curve gradually increased with the position changes. Figure 5c is the 2D OCT image of the center of the focusing lens. We can get the ROC value of the lens from Figure 5c. The OCT images were first grayed by using the function of \"rgb2 gray\" in MatLab R2015b (MathWorks, United States). Second, the function of \"medfilt2\", \"imopen\" and \"bwareapen\" were used for filter morphologic openings to remove all small, connected objects from the binary image. Finally, the fitting curve was obtained using the function of \"polyfit\" and \"polyval\", as shown in Figure 6.  
In Figure 6, the blue points were retrieved from the curve of the OCT image and the red curve is the fitting curve. We can get the ROC of the focusing lens from the curve. The discontinuity part of the curve was removed because of the noise impact during the image processing. A circle fitting was adopted and the fitting function \(F\) can be expressed as  
\[F=\left(x-x_{1}\right)^{2}+\left(y-y_{1}\right)^{2}-r^{2} \tag{11}\]  
Figure 5: Two-dimensional optical coherence tomography (OCT) images of the focusing lens with different location. Figure 5 shows the two-dimensional optical coherence tomography images at different location of the focusing lens were performed by the proposed SS-OCT system. The profile curve and the inner structure of the focusing lens were obtained and shown in red rectangle. The light lines at the top and down in red rectangle represent the upper surface and the lower surface of the focusing lens, respectively. The light lines occur at the boundary when the refractive index changes suddenly. A series of straight lines on the left and right sides in the red rectangle are noises caused by strong light reflection from the two edge of the focusing lens. From Figure 5a–c, the length of the upper curve gradually increases with the position changes. Figure 5c is the two-dimensional optical coherence tomography image of the center location of the focusing lens. We can get the ROC value of the lens from Figure 5c. Figure 6: Graph of fitting curve. where \(x,y\), is the circle center coordinates, respectively; \(x_{1},y_{1}\), represents the offset of the circle center coordinates relative to the zero coordinate, respectively; \(r\) is the radius of circle. The function of \"nlnfit\" was employed to get the optimal solution when F = 0.