Cellule solari

IREDs are LEDs which emit infrared light with a wavelength
in the range of 780 nm to 1550 nm. Although there is a
variety of different wavelengths possible in this range, only few
of them are used in standard optoelectronic sensors. The most
important ones are 950nm, 880 nm and 850 nm. In some special
applications like optical spectroscopy or blood oximetry
special wavelengths are used. Similar to the LEDs in the visible
range the material of the chip determines the wavelength.
The emission angle of the IRED is defined as the direction
where the radiant intensity has decreased to a value of 50% of
the radiant intensity on the optical axis (forward direction). The
distance of the chip to the lens determines the emission angle
of the LED (see Fig. 1). Since the size of the IRED-chip (typically 300 μm) is usually not negligible compared to the diameter of the lens of the IRED, there is a minimum emission angle of about 10° for standard IREDs. If there is no lens or other optical elements the theoretical emission angle is 120° (Lambert-radiator).
The radiant intensity of the IREDs is defined as the optical
power which is emitted into a cone of 1 sr (sterad, spherical
angle) along the optical axis of the system. The unit is mW/sr. In
most sensor applications the radiant intensity of an IRED is much
more important than the total optical power in mW. The radiant
intensity is dependent on the emission angle while the total
optical power is independent from the emission angle. The radiant
intensity is always measured at direction 0° (i.e. on the optical
axis) and most IRED are classified into classes (ranks) of
different radiant intensity.