## 4 Discussion
### Techniques for Measuring the Changes in the Residual Lower Limb
#### 4.1.3 Non-Contact Scanner (Laser and Optical Scanners)  
Laser and optical scanners are residual limb volume measuring techniques that do not involve any contact with the residual limb since they are non-contact scanners. For the optical scanning method, a digital image is captured from different angles with multiple positions of the residual limb, and later, a 3D model image of the residual limb's surface is created with its calculated volume. There are two ways to produce optical scanning images, which are the silhouetting method [36] and fringe projection [29; 32; 38]. In the silhouetting method, the image is created from different angles of the outside contour of the residual limb, while for fringe projection, the grid pattern is projected onto the residual limb from different angles. For the laser scanner technique, a plane laser of light is projectedonto the residual limb from different angles with multiple locations, and video images are recorded. Three-dimensional images are constructed based on the recorded data to calculate the volume. Most of the time, non-contact scanners are used to capture the shape of the residual limb for digitalising the prosthetic socket design. Nevertheless, this application may also be used to determine the residual limb volume as the software can determine the volume of the scanned residual limb. Unlike the contact scanner, the optical scanning system captures radial measurements rather than circumferential measurements [4]. From this radial measurement, [PERSON] [59] found that the better way to measure the distances on the residual limb is by using a calliper compared to using a non-contact scanner. Thus, a non-contact scanner has no benefit over circumference measurements in determining the residual limb volume change. One of the advantages of using a non-contact scanner is that it can capture the residual limb without applying any tension that may distort the residual limb's shape. It is quite easy to handle this non-contact scanner as it can adapt to limb movement during scanning [4]. Besides that, since it is a handheld scanner, it is portable and easy to carry everywhere. During the scanning process, the residual limb needs to be scanned as fast as possible since there is a high possibility for an increase or decrease in its volume as the change in the residual limb volume is very sensitive to socket doffing [5]. There are three publications on the validation of this measurement technique [30; 32; 33], and another three publications are on the validation of the scanner used in the studies conducted by [PERSON] et al. [43], [PERSON] et al. [45], and [PERSON] et al. [40]. Some scanners such as VIUScan, GoSCAN, Artec Eva, Biosculptor, and CAPOD demonstrated a high consistency of accuracy to determine the residual limb volume. However, more publications are needed in the validity test of laser scanning since several laser scanning brands in the market may show different results due to their own errors that sometimes affect the scanning results. Other than that, the proper alignment of shapes from different scanners may also be an issue in this technique. Some researchers placed fiducial markers on the residual limb to ensure the right alignment, and in commercial software packages, the residual limbs that have bony prominences or other regions of sharp curvature tend to align better than those without distinct features [1; 29].