That is, the maximum response is obtained when the radiation reaches the sensor perpendicularly (the sun at its zenith), while no response is obtained when the sun is on the horizon (an angle of incidence of 90��) and the response is half of the maximum when the incident radiation is at 60��. Therefore, it can be deduced from the definition that a pyranometer must have a ��directional�� response or, as it is usually termed, a cosine response to emphasise the fact that its response must ideally be analogous to the cosine function. The difference between the pyranometer’s real response and the ideal cosine response is termed cosine error [2,3].Pyranometers are widely used in meteorology, climatology, agriculture [4], solar energy studies [5] and building physics.

In photovoltaic solar installations they are normally mounted with the sensor surface in the plane of the panel. In spite of the interest in measuring solar radiation, the use of Brefeldin_A pyranometers is still not very widespread outside the field of research, probably due to their high cost.The element that characterises a pyranometer is the sensor it uses, which may be thermal (thermopile) or photovoltaic. Photovoltaic sensors are a cheap alternative, whose only advantage in principle over thermopiles in measuring radiation, aside from their price, is their response speed. Thus, while photodiode-based pyranometers have a response time of around 10 ��s [6], in those based on thermopiles, response time ranges between 1 and 10 s, thus making them less suitable for measuring very rapid changes in radiation.

The influence of temperature on pyranometer’s measurement is also well known. Although this influence exists, it is lower in thermopile pyranometers [1,7-10] than in photodiode devices [11-14].With regard to integrating a pyranometer into an instrumentation system (generally into any measuring device), there is a series of very important factors to take into consideration, namely: ease of connection, signal degradation due to the transmission process [15].In order to achieve the objective proposed in this work, designing and building a photodiode-based pyranometer [16] with similar characteristics to those of a thermopile-based device, also incorporating significant connection, measuring and programming utilities [17], the authors have analysed and corrected both the defects mentioned in literature and those observed during the testing of various commercial units. That is, the pyranometer developed has the following original features:Excellent cosine response guaranteed by both the level gauge (to guarantee horizontality), which is incorporated, and by the specifically designed solar radiation diffuser.