Optical Sensors

Optical sensors detect electromagnetic radiation with
wavelength between 0.2 μm (ultraviolet) and several 10 μm (far
infrared). Two different classes of optical sensor applications
have to be distinguished.
In the first group of applications the light source is given
by external conditions of the application, e.g. the sun as a
source of visible light or the human body as a source of far
infrared radiation in the range of 5 μm to 10 μm. These are
applications with an external light source. In most of
these cases the purpose of the optical sensors is to measure
the intensity of this electromagnetic radiation like a photometer
in a photo camera. The radiation is converted into an electrical
signal which is a measure of the intensity of the electromagnetic
radiation. Since the different types of optical sensors only cover
a certain bandwidth, the most important task for the engineer
is to select a sensor with a sensitivity range that matches the
spectral distribution of the given light source.
The second group are applications with an internal light
source. The electromagnetic radiation is only a means to an
end to detect e.g. the position, size or shape of an object. In
this case the sensing system not only contains an optical sensor,
but also a light source and the engineer is free in choosing the
light source and the optical sensor. Very often it is convenient
to choose infrared light between 800 nm and 1000 nm which
is not visible for the human eye. For these wavelengths powerful
LEDs as a light source are available (see table 1) and the
spectral sensitivity of phototransistors and photodiodes made
of silicon match perfectly to these infrared LEDs. Well-known
examples are optical interrupters where the light, generated by
an infrared LED, is detected by a silicon phototransistor.