The experiment

The PV modules have been replaced by a wooden target that has identical dimensions. The target was painted white for better contrast in the pictures. On that target a set of parallel lines in different colours are drawn accurately with a 2 cm difference between each of them. These lines are used as reference edges in the target from which the irregularities of the reflected pattern (Figure 2) can be observed. A set of pictures were taken with a digital camera placed at a distance perpendicular to the concentrator’s axis which is oriented towards it. A movable platform was used to insure the correct location of the camera along the normal to the concentrator. The principle of the two-point perspective is used to detect, with the aid of numerical software, the edges of the target.

A geometrical algorithm is then used to calculate the normal vector to reference line. An error map

of the normal vectors for each mirror is further constructed and the RMSE calculated. The above procedures were made for a vertical and tilted concentrator positions.


Fig. 2. Reflection of the reference coloured lines of the wooden target 4. The geometric algorithm

Figure 3 shows a camera placed at a distance S normal to the plane of the concentrator. The target (absorber) is located at the focal plane at a distance f. A mirror X meters from the centre of the concentrator and inclined an angle ad degrees, shows an image i of the edge or the reference lines of the target. The normal vector n of any point on the mirror’s surface is theoretically inclined with respect to the concentrator plane with an angle nth-

nth = 90 — ad (1)

From simple geometrical relations it can be proved that the values of angles a and a are related to real tilt angle qexp which is computed as per equation (4). Irregularities in the mirror’s surface affect the tilt angle of the normal vector n. Errors are computed from the discrepancies between the theoretical and experimental values of q.

a’ = arctan(Xm)


. X + Ax — A d. a = arctan( ) f — Ay


nexp = 90 — (a+2a)










Fig. 3. The Absorber Reflection Method algorithm



The algorithm requires the knowledge of the image coordinates of the reference line i or more precisely the length Xm in space dimensions. The image coordinates of points along the reference lines are first located in pixels and then transformed into actual distances as shown in the following section. The straight reference lines will appear deformed if the surface underneath is not totally flat.

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