Phosphorescent spherical marker for precise image measurements

In this paper, we propose a phosphorescent spherical marker to improve precision of detecting the marker position in captured images, which leads to precise image measurements. The phosphorescent spherical marker is excited by a UV light source to emit phosphorescence from the marker. Only phosphorescence emitted from the marker is captured by a camera without any other light source. In the captured images, the intensity contrast between the marker and background is high because the images contain only the intensity information of phosphorescence emitted from the marker instead of the reflected UV light. This high contrast leads to success of edge detection of the marker contour with high precision. The phosphorescent spherical marker of 25.4 mm in diameter has high sphericity less than 100 µm. The marker position in the captured images can be detected with high precision by fitting a circle to detected edge points on the marker contour because of the precise edge detection and the high sphericity. Standard deviation of detected position of the phosphorescent spherical marker was evaluated as an index of precision of marker detection. The marker placed 500 mm in front of a camera was captured 30 times. The camera was Nikon D800E which resolution was 7360 x 4912 pixels. The lens installed in the camera was AF S NIKKOR 50mm f/1.8G. In the captured images, the diameter of the marker was about 690 pixels. The precision of detecting the phosphorescent spherical marker was 0.0084 pixels in standard deviation, which corresponded to 0.31 µm. The relative precision were 1.1 x 10 ⁶ and 1.7 x 10 ⁶ in horizontal and vertical directions respectively. These results show a possibility of applying image measurements with the phosphorescent spherical marker to precise motion of machines.