What Is an IR Bandpass Filter?

Paul Reed

Infrared (IR) light has a wavelength or frequency that is longer than red, and is not visible to the human eye. People can see light in wavelengths from reds to violets, a range called the visible light spectrum. IR light is useful for a wide range of chemical testing, security motion detection, and astronomy. An IR bandpass filter is a glass or crystal flat filter with a coating that blocks all light frequencies except for infrared light.

Hidden details found with IR light can be seen in Leonardo Da Vinci's painting of Mona Lisa.
Hidden details found with IR light can be seen in Leonardo Da Vinci's painting of Mona Lisa.

Many chemical tests use infrared light for determining both the chemical make-up or composition of products, and their reactions with other chemicals. Laboratory devices look at chemicals in various light frequencies, including infrared, both as solids and liquids. An IR bandpass filter is used when the device needs to block other frequencies that might confuse or interfere with the machine's analysis.

Ancient cave paintings have shown enhaced details when exposed to infrared light.
Ancient cave paintings have shown enhaced details when exposed to infrared light.

Some chemical analysis can be done by burning a sample in a flame and looking at the light that results from it. Different atoms will change the light frequencies from the standard flame colors, which can be seen in the infrared range with an IR bandpass filter. This analytical device is called a flame photometer or spectrophotometer.

When certain materials are exposed to different light frequencies, they can fluoresce, which is a glowing effect caused by light reacting with molecules. Some natural minerals will release infrared light when exposed to blue-green light frequencies. Photographs can be taken using a camera fitted with an IR bandpass filter to block out the visible light, but allow the IR to be viewed.

Infrared light has proven valuable for investigation of artwork and in archaeology, the study of ancient cultures and buildings. Artists over many centuries produced paintings that were damaged or hidden by later attempts to repair or restore the paintings. Paintings exposed to infrared light will often show hidden images, even older paintings covered by a different work when the artist reused their canvases. Ancient cave paintings have also shown enhanced details when exposed to infrared light.

An example of hidden details found with infrared light is found in Leonardo Da Vinci's painting of Mona Lisa. Painted in the early 16th century, the painting was very complex, with an estimated 30 layers of various paints applied by the artist. Later attempts to preserve or repair the painting had hidden many details, as well as the effects of time and exposure to air. In the early 21st century, an infrared camera was fitted with an IR bandpass filter, and scientists exposed the painting to infrared light. Fine details of the Mona Lisa's hair, clothing and other details were clearly visible when the images were examined.

Cameras can be used to view objects at night in the dark, either by seeing the infrared energy a person or animal gives off, or by using infrared light sent out from the camera. Many infrared security cameras contain small light-emitting diodes (LEDs) that give primarily infrared light. These LEDs can be fitted with an IR bandpass filter to ensure only specific infrared frequencies are used to light an area covered by the camera lens, which improves the quality of the image.

Infrared light is widely used in astronomy, the study of stars and planets in the visible universe. Dust clouds that cover large areas of space can hide stars at great distances from Earth. IR filters attached to camera equipment can view and record images from telescopes on Earth or in Earth-orbiting satellites. Many details not visible in normal light can be clearly seen in infrared photographs, and these details help astronomers understand the nature of how our universe formed and changes.

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