# Measurement System for Finger Skin Displacement on a Textured Surface Using Index Matching
## 5 Discussion  
In this paper, a measurement system for direct skin displacement on a textured surface was proposed. To verify the validity of this method, the degree of optical misalignment was verified via simulation, and the measurement accuracy was verified using the implemented measurement system. A simulation to estimate the refractive index change and the optical misalignment caused by the texture angle revealed that this system can measure skin displacement directly related to the SAI and RAI activities on the textured surface. Previous measurements of skin displacement on textured surfaces have mainly been made on PCs having wide receptive fields. It is noteworthy that the firing thresholds of the SAI and RAI can now be measured under appropriate conditions. However, it is difficult to measure high-frequency (>200 Hz) vibrations with an amplitude of less than 1 \(\upmu\)m, which leads to the activity of the PCs in this system. One way to solve this problem is to attach an acceleration sensor to the nail [2] or a device that can measure the vibration propagated from the fingertip to the middle of the finger in a high-frequency band [26].  
The experimental results suggest that skin displacement on a textured surface can be measured in oil with a noise level of approximately 30 \(\upmu\)m. The results are less than the ideal values obtained by the simulation. Although this value is better than the firing thresholds of the SAI, the accuracy is below the firing thresholds of the RAI (18 \(\upmu\)m (4 Hz) at maximum and 0.7 \(\upmu\)m (50 Hz) at minimum [27]). The present results are inferior in accuracy to those of existing systems used for tactile measurements (e.g., accelerometers, laser Doppler sensors). However, with an accuracy below the firing threshold of the tactile receptors, our system can measure skin deformation on a texture, and also observe it spatially and temporally. These results satisfy the requirements for reproduction on a tactile display.  
There are three possible reasons for the low measurement accuracy in this study: (i) the wide angle of view, (ii) the marker-tracking system, and (iii) hardware implementation. For (i), the current measurement system requires a wide angle of view, because the finger traces the texture, which results in poor per-pixel resolution. To solve this problem, the resolution of the camera should be improved or the texture should be moved by fixing the fingertips. In the latter case, the fingertips can be measured up to the very edge of the angle of view when the finger is not moved, resulting in a lower millimeter-per-pixel value and a higher resolution. However, this may have some effect on roughness perception, because it would be a passive touch instead of an active touch. For (ii), the software can be improved to reduce noise, including a more robust calculation of finger location with adaptive filtering, because the marker location is subtracted from the finger location, and the error of the finger location effects all markers. Additionally, the accuracy of the marker displacement estimation can be improved by improving the resolution of the finger fixation as mentioned in the solution of (i) because, in the current setup, one pixel corresponds to 30 \(\upmu\)m. For (iii), because we used Lego in this hardware implementation, distortions were generated while moving the artificial fingers. To solve this problem, it will be necessary to use more rigid materials such as aluminum frames in the future.  
A possible application of this research is the measurement of the spatiotemporal displacement of skin displacement on a variety of natural textures. The proposed measurement system enables us to analyze texture clustering using spatiotemporal information, whereas previous studies used vibration measurements from accelerometers, etc. Furthermore, the displacement data obtained here will enable us to study the vibration presentation algorithm on a tactile display. Another possible application is the measurement of skin displacements in areas other than the fingertips.