Performance meansurements are normally taken with the solar insolation level measured perpendicular to the collector plane (ie. facing the same direction as the collector). When the light shines on the collector from an angle the performance changes, and this is what the IAM (Incidence Angle Modifier) values provide us, an angular performance factor. A value of 1 is achieved when the collector is perpendicular to the suns rays, and therefore receiving maximum radiation. For flat plate collectors, 1 is the maximum value, dropping off in both morning and afternoon. Evacuated tube collectors, however, often provide values in excess of 1 during these periods, as factors such as reflective panels and reflection off neighbouring tubes can influence the relative performance.

A solar collector that is mounted on a device to track the sun from sunrise to sunset (as sometimes used in PV applications) will maintain a IAM value of 1 throughout the day, as the collector is always facing the sun, and therefore providing performance output in line with the standard performance curve.

For most solar collectors currently on the market, IAM is not an important consideration when comparing performance. This is because flat plate collectors, evacuated tube collectors with a flat absorber, or those that using reflective panels usually have a fairly similar set of transversal and longitudinal IAM values. The value of most concern for fixed position collectors is transversal IAM, as this reflects the change in performance throughout the day. Longitudinal IAM is useful when looking at installation angle, and the changes in heat output throughout the year as angle of the sun above the horizon changes between winter and summer.

The longitudinal and transversal IAM values for the Apricus solar collector are as follows:

For longitudinal insolation, a value for 50^o is generally only provided, however the curve is standard for most collectors, very similar to the transversal curve for flat plate collectors (see graph below).

The following graph displays the transversal IAM values for the Apricus collector, a leading flat-plate and leading evacuated tube reflective panel solar collector.

As can be seen, the Apricus solar collector has a curve which is quite different to the other two collectors . This is due to the cylindrical absorber area, which passively tracks the sun throughout the day. At 40-50^o there is no light lost between the tubes, and no tube overlap, and reflection of neighbouring tube, hence a peak in relative performance. This is ideal, as during this period (mid morning through mid afternoon) solar isolation levels are quite high. The peak at 70^o provided by the ET-reflect is of little benefit as this angle corresponds to early morning or late afternoon when solar insolation levels are very low. The flat plate collector's IAM values drop away from 1 as the angle increases, and as such solar conversion efficiency is only at peak levels at midday.

To understand how the tubes passively track the sun throughout the day, refer to the diagram to the left. When looking at the tubes from above (0o) each tube's surface is clearly visible, and therefore exposed to the maximum amount of sunlight. At this angle however some light is lost between the tubes, and therefore because this is used as a reference point for the IAM value of 1, when the gaps close up, the IAM value with actually increase (a greater % of light shining on the collector is actually being absorbed). When the sun reaches an angle of 40o which correlates to 2h 40min before or after midday, the solar tubes are still fully visible with no gaps between, and no overlap. It is at this point that the pure IAM values reach their peak. The tubes are exposed to all the sunlight shining towards them, and all the tubes are still perpendicular to the sun.