# Experimental Validation of Non-Marker Simple Image Displacement Measurements for Railway Bridges
## 4 Verification Results by Model Bridge
### Discussion for Practical Application II: Effect of Weather (Illuminance)  
In the practical application of non-marker image measurement, environmental conditions are a major concern and influence measurement accuracy. Here, the authors focus on the brightness of the shooting environment and consider this environment from the perspective of illuminance based on the results of experiments on model bridges.  
Figure 16 shows the vibration test setting of a model bridge used to examine the effect of illuminance. The illuminance was varied in this test by turning off the LED light located on the right-hand side of the figure, which was one of two LED lights used as lighting. Figure 17 shows the illuminance when the LED light was on and off at each measurement point. The illuminance exhibited certain errors because it was converted from the brightness on the captured image through the illuminance performance of the LED light. In fact, although Ch0 and Ch2 at LED OFF have higher illuminance than at LED ON, this is an error due to the effect of conversion from brightness to illuminance. However, it is similar to the measured value with the illuminometer, and no problem was encountered in making a rough evaluation of the brightness of the surrounding environment. When the LED light was on, an illuminance of ~1000-3000 k was obtained at all measurement points. By turning off the LED light, the measurement environment was set to 800 k for Ch3 and under 500 lx for Ch4-Ch6.  
Figure 16: Experimental situation of illuminance changes due to LED light on/off.  
Figure 17: Illuminance change due to LED light on/off.  
Figure 15: Calculation result of the feasibility rate according to the camera distance and span length.  
Figure 18 shows the influence of turning the LED light on and off on the accuracy of the non-marker image measurement. The result of turning the LED light on was the same as that shown in Figure 11, and the standard deviation was \(\sim\)0.004-0.007 pixels, with the exception of Ch6. However, when the LED light was off, a large error occurred in Ch4-Ch6, where the illuminance was <500 lx. Because the highest displacement was between 0.035 and 0.017 pixels, it is safe to assume that, on average, an error of the same magnitude as the original waveform amplitude occurred on Ch4-Ch6, thus making measurement difficult. Because the precision of Ch3 was nearly identical to that when the LED light was on, 500 lx was used as a guideline for the non-target image measurement.  
Table 8 shows a guideline for general illuminance under each climatic and environmental condition in Japan (Ref. [29]). The table shows the applicability of the non-marker image measurement for illuminance. If 500 lx is used as the applicability guideline, the illuminance may be insufficient immediately after sunset or just before sunrise; however, it can be measured even if it is sunny or cloudy during the day. In addition to illuminance, the application in rainy weather conditions is challenging because of the effects of raindrops moving into the image and the waterproofness of the equipment; these concerns will be addressed in the future.