While not normally a part of the Reciprocal Optical Test, it is somewhat important to determine the path that various sun rays travel before hitting the pot. There are three types of paths. They are: Direct rays that hit the pot with no reflection, Primary reflection where the rays are reflected once before hitting the pot, and Secondary reflection where the rays are reflected two or occasionally more times before hitting the pot. The intensity of the rays hitting the pot will vary with the path. For example, if the solar intensity is 1000 W/m2, the diffuse component is 10% (which does not contribute to useful reflection), and the reflectivity of the reflective surface is 80%, the corresponding intensities will be: 1000 W/m2 for direct rays, 1000(0.9)(0.8)=720 W/m2 for primary reflection, and 1000(0.9)(0.8)2=576 W/m2 for double secondary reflection.
In the tests reported earlier in this paper, all of the effective area was treated as primary reflection. This underweighted the direct rays from the pot, but it was thought that this might not be too important when comparing cookers using the same pot. However, the preliminary test of the CooKit yielded some unexpected results. For example, Figure 9 shows 213 watt peak power for a sun elevation angle of 60 degrees with the front reflector at about 35 degrees. This peak was not present in a previous computer simulation of the CooKit performance. It was believed that this unexpected peak was primarily due to secondary reflection that had not been included in the computer simulation. Therefore, an investigation was conducted to determine the contribution from each of the three paths.
A laser level was attached to a tripod and was used to check the path of the incoming sun’s rays. For example, Figure 17 shows the laser level with the beam reflecting from the front flap, then from the side reflector and then hitting the pot. This verified that the illuminated areas on the left and right ends of the front flap were from secondary reflections. Another secondary reflection was found where the beam hit the lower front portion of the side reflectors, then the bottom of the cooker and then the pot. Black construction paper was then carefully placed on the cooker where it would eliminate the secondary reflections without affecting the primary reflections. Most of the bottom of the cooker was covered as well as the left and right ends of the front flap. Figure 18 shows the CooKit tilted at 60 degrees with the front flap in the upper position. The upper portion of the figure has the black paper in place to eliminate the secondary reflections. The lower portion has the black paper removed.
The CooKit was tested for sun angles from 15 to 90 degrees with the black paper in place and with the black paper removed. Also, at each sun angle, the direct radiation component was determined by using GIMP to determine the number of number of pixels in the pail in each photo. Figure 19 shows the results of these tests.