I originally posted this blog in 2012. You’re asking about IR light and the North Nest, so I thought I would repost it here.
We are starting to get questions about the night-time illumination of our nests, so I wanted to write about visible light, invisible ‘light’, and our IR illuminators. What we see – the world of visible light – is just a tiny fraction of all the types of light that exist. Infrared light, gamma rays, X-rays, ultraviolet light, microwaves and radio waves are types of invisible light that we group into spectra based on wavelength.
The illustration below shows the entire electromagnetic spectrum. The IR portion, aka our night lights, hangs just off the red end of the visible spectrum. It spans the frequency range from 780 nm to 300,000 nm (.78 to 300 um) and has a longer wavelength and lower frequency than visible light.
Birds see a very different world. They have four color cones instead of three, more cones and rods to see color and light, a higher proportion of cones to rods (at least in diurnal birds), and different peak sensitivities to light. As the graph below illustrates, we have narrower spectral sensitivity than birds and are much less sensitive to certain wavelengths of light. However, like humans, birds do not see above about 700 nanometers. While some birds range into UV, IR light is as invisible to them as it is to us.
Absorption peaks for visible light: humans and European Starlings
How can we use an IR illuminator if we can’t see IR light?
The IR spectrum starts at about 780 nm. The LED IR illuminators used on our camera emit ‘invisible’ light at a wavelength of 850 nm: well above the 700 nm visibility limit we share with birds, but well below the high-energy ‘hot’ end of the spectrum.
Here is how it works! The illuminators shine IR light into the darkness. The camera’s IR sensor detects IR and focuses it onto a CCD (Charge Coupled Device) imager chip, which has a spectral response of up to roughly 1,000nm. The chip maps IR wavelengths of light into the visible spectrum via tiny electric charges generated by IR light striking an array of tiny sensor cells. Since IR doesn’t contain color information, at least in spectra that we can see, the camera generates a monochrome image.
Mammals and birds can’t see IR light, but our camera can. We use low power IR illumination devices that are eight feet or more from the animals we are watching. They are not hot and do not emit dangerous radiation. A chip inside the camera does the work of converting the IR image into visible light via tiny electric charges. IR does not carry color information in spectra that we can see, so it generates a monochrome image.
Why do the animals we watch have ‘flashlight eyes’? Since they can’t see IR light, their pupils remain fully dilated. The light reflects off the tapetum lucidem, a layer of tissue behind the retina, which dramatically increases the light available to the animal’s photoreceptors. This gives it superior night vision and results in the flashlight effect. Their eyes wouldn’t be quite so big and bold if their pupils contracted to block out the light.
Experimenting with IR light
To see IR light for yourself, take an IR-based television remote. Press a button and look at the LED on the end. You won’t be able to see anything. Now, take a digital camera or phone camera, point it at the remote to take a photo, and press a button on the TV remote. You should now be able to see the infrared light. Like our camera, your camera uses a CCD chip that maps IR down into visible light for humans. But neither we nor the eagles can see IR without technological assistance. Again, our locations are dark at night.
The following sources helped me understand all this: