Infrared Radiation Theory
Unlike light, infrared waves are not visible to the human eye, but humans can detect it as heat. In
the electromagnetic spectrum, infrared radiation is located in the middle of visible and
microwave regions. This waves usually have a wavelength of 0.75 and 1000µm [1]. The infrared
spectrum is divided into three segments; the near IR, mid IR, and far IR. "Near infrared" light is
closest in wavelength to visible light and "far infrared" is closer to the microwave region of the
electromagnetic spectrum. The longer, far infrared wavelengths are about the size of a pin head
and the shorter, near infrared ones are the size of cells, or are microscopic, far infrared waves are
thermal. In other words, we experience this type of infrared radiation every day in the form of
heat! The heat that we feel from sunlight, a fire, a radiator or a warm sidewalk is infrared
[2]. Infrared was first studied by William Herschel when he conducted an experiment measuring
the difference in temperature between the colors in visible spectrum, he placed thermometers in
each colors and the result showed an increase temperature from blue to red, When he noticed an
even warmer temperature measurement just beyond the red end of the visible spectrum, Herschel
had discovered infrared light [3]. When an object is hot, they emit a visible light such as fire.
Unlike fire, humans are also producing heat but only emit infrared waves [4]. Human eye cannot
see these waves but tools can sense infrared energy such as infrared cameras.
Wavelengths and Electromagnetic Waves (Photo: Apiste Global)
References
[1] Alexander Chilton, "The working principle and working of infrared sensors," Azo Sensors, 2014.
[2] Univie AC, "The infrared," 2010.
[3] Troy Benesch, "Infrared Radiation History".
[4] National Aeronautics and Space Administration, Science Mission Directorate. , "Nasa Share the
Science," 2010.
IR Sensors
IR Sensors or Infrared radiation sensors are electronic devices that detects thermal radiation from
an object. When an object heats up, it emits infrared radiation that is invisible to our eyes, but it
can be detected by this sensor. All objects which have a temperature greater than absolute zero (0
Kelvin) contains thermal energy and are sources of infrared radiation as a result [3]. IR sensors
are type of photodiode that is sensitive to IR light emitted by an object [3]. When an IR light
touches the photodiode, the resistance and output voltage will vary in proportion to the
magnitude of the IR light received. An infrared sensor circuit is one of the basic and popular
sensor module in an electronic device. This sensor is analogous to human’s visionary senses,
which can be used to detect obstacles and it is one of the common applications in real-time.
IR Sensor Circuit (Photo by: elprocus.com)
There are five basic elements used in a typical infrared detection system: infrared source,
transmission medium, optics, infrared detector or receiver, and signal processing. You can use
infrared lasers and infrared LEDs with specific wavelengths as infrared light sources. The three
main types of media used for infrared transmission are vacuum, atmosphere, and fiber optics.
Optical components are used to focus infrared radiation or to limit the spectral response. Optical
lenses made of quartz, germanium and silicon are used to focus infrared radiation. Infrared
receivers include photodiodes and phototransistors. Important specifications for infrared
receivers include photosensitivity, detectability, and noise equivalent power. The output of the
infrared detector is so small that the signal processing is done by an amplifier.
IR Sensor (Photo by: electronicscomp.com)