IAM―Efficiency adjusting factor

IAM is an acronym for Incidence Angle Modifier and is simply a numeric value with refers to the amount of available solar radiation striking the absorber of the collector A value of 1 is achieved when the collector is perpendicular to the suns rays, and therefore receiving maximum radiation.
Any collector that is installed at a fixed angle (generally equal to the location's latitude), will experience decreased radiation levels (IAM value < 1) in the morning and afternoon when the sun is not perpendicular to the absorber surface.
When comparing performance between flat plate collectors, evacuated tube collectors with a flat absorber, or those that using reflective panels,IAM is not very important because they usually have a fairly similar set of transversal and longitudinal IAM values. The value of most concern for fixed angle collectors is transversal IAM, as this reflects the solar radiation 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 Century Sun solar collector are as follows:

As you can see FROM the above graph, Century Sun 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 40-50o there is no light lost between the tubes, and no tube overlap, hence a peak in relative performance. This is ideal, as during this period (mid morning through mid afternoon) solar isolation levels are quite highest. The peak at 70o 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。

Because of the round shape of the solar tubes, the absorber passively tracks the sun FROM 40o either side of midday (9:20am to 2:40pm). The cosine adjusted IAM values confirm this, as the collector maintains a value of close to 1 up until 40o, beyond which the tubes start to overlap and the relative surface area decreases. Flat plate collectors, and other collectors with flat absorbers display a fairly standard bell curve, only peaking at midday.

As the angle increases, the tubes start to overlap, shading each other. They are still facing the sun, but the actual surface area of absorber exposed to the sunlight is reduced. Only a small amount of sunlight falls beyond 40o (early morning and late afternoon), and so this decrease surface area has minimal influence on the total daily energy output of the collector.