Summary of December 4th – Experiment to get pixel width of rear lights in a picture

Logbook noting:

The experiment was divided into three parts mainly. The aim of this experiment was to find the relationship between real distance and pixel with of red lights in a picture. Because the consideration of safety, this experiment couldn’t be finished outdoors. So in the lab, I just use two red papers to represent the rear lights of front car. In the first part, I set the light width to be 2 meters and take photos in the distance of 5, 10, 15, 20, 25, 30 meters. Then recorded the pixel width of red lights at different distance. The second and third part were almost same except changing lights width to be 1.8m and 1.5. The pictures below shows the procedure of that experiment. 

Set lights width to be 2 meters

     

Take pictures at different distances. 

5 meters:

10 meters:

15 meters:

20 meters:

25 meters:

30 meters:

Then set lights width to be 1.8m and 1.5m and did the same measurement. 

Comments:

The similar triangle model is shown in this figure:

Figure 1: Similar triangle illustration 

According to rules of similar triangles, the relationship between real distance and image view is:

Next problem was how to get the pixel width of red lights in a picture. I didn’t have photo tools to calculate the pixel width between two red papers. I chose a very strait way to measure. I opened the album in Mac and use screen shot. I used the scale plate of the system to the total picture length and to find the length between two red papers in that picture. The technical data of iPhone 6 shows that there are 1334 pixels a line and 750 pixels a row. I multiplied the ratio of red papers in that picture to the total length of that picture and total pixels a line. In this way, I got the approximate pixel width of that red papers.

However, in the following days, I found that the normal lights width of a car is around 1.5m to 1.7m. The picture taken in the picture mode is not a whole screen so that I can’t use 1334 as the total pixels. Fortunately, I changed the resolution later on and avoid this huge error successfully. The chart below shows the relationship between real distance to the front car and the pixel width of front car’s rear lights in a picture. 

Figure 2: Relationship between real distance and pixel width of lights in a picture