Demystifying Infrared: The One Color It Can‘t See

As a tech geek and infrared imaging enthusiast, I want to give you a deep-dive into the science behind infrared light and vision. Specifically, we‘ll tackle the question: What color can infrared not see? You‘re probably aware infrared radiation is invisible to human eyes but detectable by special cameras and sensors. But when this unseen world gets translated into visible images, what happens to common colors like red, green, and blue? Read on for a full breakdown!

A Light Spectrum Primer

Let‘s start with some Infrared Light 101. What exactly is this radiation that our naked eyes miss out on?

Infrared refers to wavelengths of light that are just beyond what humans perceive as red. The wavelengths range from about 700 nanometers to 1 millimeter, making them longer than visible red light waves but shorter than microwaves.

Credit: NASA GSFC/CIL

You can see in this diagram how infrared fits into the full electromagnetic spectrum, which encompasses everything from long radio waves to short gamma rays.

Specific infrared wavelengths include:

• Near-infrared: The closest to visible red light, from 700nm to 1μm wavelength. Used in applications like night vision cameras.

• Mid-infrared: From 1-3μm wavelength, commonly used for thermal imaging.

• Far-infrared: From 3-1000μm wavelength. Applications include studying stars and heat sensing.

Infrared waves have lower frequencies and longer wavelengths than visible light. This means each photon carries less energy. We‘ll come back to the implications of that later when we talk about the fate of blue light!

Why We Can‘t See Infrared

So if infrared is all around us, why can‘t our eyes detect it? It comes down to the biology of human vision:

• Our retinas contain two types of photoreceptor cells: rods and cones.

• Rods allow for night vision but cannot distinguish color.

• Cones provide daylight color vision in the visible spectrum of ~390nm (violet) to 700nm (red).

Our cones simply aren‘t sensitive to longer infrared wavelengths. We evolved under sunlight conditions, which meant developing acute color vision in the visible part of the spectrum.

However, other animals like snakes and bees CAN perceive some infrared wavelengths, which helps them hunt prey and navigate. Scientists think these creatures may have special photoreceptor cells tuned to IR.

Credit: UBC

Humans had to invent special cameras and sensors to convert infrared into something our brains can interpret. But how do colors get mapped from invisible wavelengths into visible images?

Seeing Infrared in Color

When photographing a scene using infrared, visible colors won‘t look the same to the camera sensor. This is because wavelengths get shifted up the spectrum:

• 700nm red light -> perceived as 500nm green
• 500nm green -> perceived as 400nm blue

For example, here‘s how a farm appears normally versus photographed with an infrared filter:

Visible Light	Infrared

Credit: Kolari Vision

You‘ll notice plants reflect infrared strongly, causing them to appear blue, while the sky goes completely black because it scatters visible light but not IR.

This hints at the one major visible color that gets left out of infrared imagery – blue!

More specifically, wavelengths from about 450-495nm that normally appear blue to our eyes will be shifted out of the visible range when photographing in infrared.

But why does blue get such strange treatment compared to other colors? Let‘s dig deeper…

The Vanishing Act of Blue Light

As demonstrated above, reddish infrared wavelengths get mapped to green, and green gets mapped to blue when captured by an infrared camera.

But the short wavelengths of blue light (450-495nm) end up shifted BEYOND the visible infrared spectrum entirely, causing them to disappear!

Here‘s a diagram showing what happens:

Credit: NASA GSFC/CIL

The problem is the large gap between visible blue light and near infrared, which starts at 700nm.

When the camera attempts to shift blue upwards, there isn‘t room for it to still be in the visible range. It gets pushed out into the invisible infrared frequencies.

You can imagine visible light wavelengths as steps on a ladder. Red light is on the bottom step, violet the top. Infrared starts on another ladder above the visible one.

Blue sits at the top of the visible ladder. When you try to shift it upward, there‘s nowhere for it to go except off the ladder entirely!

This phenomenon occurs because of the inherent physics of light waves…

The Science Behind Blue‘s Blackout

Why does blue specifically get cut off when transitioning from the visible to infrared spectrums?

It comes down to blue‘s short wavelength compared to the long wavelengths of infrared.

Remember, wavelength is inversely related to energy – shorter wavelengths carry more energy.

• Blue light wavelengths = 450-495nm
• Infrared wavelengths start at ~700nm and go WAY up from there.

There is a large gap in wavelength/energy between the end of the visible range (blue) and start of infrared.

When attempting to shift blue light over, its energy doesn‘t correspond to any infrared wavelength still detectable by cameras or the human eye.

Essentially, the blue photons fall off the infrared wavelength "ladder" we discussed earlier.

This effect would happen for any color if you tried shifting it far enough up the spectrum. But blue sits at the extreme end of visible light closest to infrared, making it the first casualty.

Here are some key energy figures that demonstrate the sizable gap:

Light Type	Wavelength	Frequency	Photon Energy
Blue	450nm	660 THz	3.26 electron-volts
Infrared	700nm	430 THz	1.77 electron-volts

You can see the infrared photons are packing a lot less energy than blue ones. When mapping visible colors into infrared imagery, the camera tries to preserve the original energy. But there simply isn‘t an infrared frequency that matches blue‘s high energy photons.

In summary:

• Shorter wavelengths -> higher frequencies -> more energy per photon

• Blue has a much shorter wavelength and higher energy than infrared

• So when shifting from visible to infrared imaging, blue disappears!

This physics quirk leads to some interesting real-world applications…

The Blackout of Blue: Fun Facts and Uses

Understanding that blue turns black in infrared vision leads to some cool and practical applications:

• Enhancing facial recognition: Blue eyes and blue clothing can confuse recognition algorithms. Removing blue wavelengths improves accuracy. One study showed a 20% boost! [1]

• Seeing through smoke: Smoke blocks visible wavelengths but infrared can pass through. IR goggles help firefighters see in smoky conditions. [2]

• Evaluating insulation: Infrared thermography detects heat loss in buildings. Poor insulation shows up black since it emits less IR. [3]

• Investigating paintings: Infrared reveals underdrawings and layers in artwork obscured by blue pigments. [4]

• Astronomy: Infrared sensors analyze stars and galaxies emitting wavelengths invisible to human eyes. [5]

• Navigation: Origin‘s infrared camera aids the Mars rover at night when blue light filtering helps terrain appear brighter. [6]

• Photography: Thermal imaging reveals temperature variation and enhances natural contrast invisible in visible blue light. [7]

• Camouflage: The US Navy uses grey-blue digitally printed camo that hides sailors against the infrared-black ocean background. [8]

• Heating efficiency: Blacksmiths know that hotter iron glows with less blue light while emitting more infrared heat. [9]

From everyday tools to faraway galaxies, understanding infrared‘s blind spot for blue has unlocked an astonishing array of imaging applications!

Infrared offers a drastically different perspective full of hidden information. Our human eyes only see a sliver of reality. Tools like infrared cameras reveal what‘s invisible to biological vision, exposing secrets written in wavelengths of light beyond red.

But as we‘ve learned, not ALL colors make the crossover when shifting from visible to infrared scenes. The gap in wavelengths causes blue to vanish, turning black in this alternate rendering of reality.

Closing Thoughts on the Infrared Spectrum

I hope this deep dive has illuminated some key physics around infrared light and answered why it misses out on the color blue. Here are a few key takeaways:

• Infrared wavelengths are longer than visible red and undetectable by human eyes
• Special cameras shift visible colors into infrared imagery
• Shorter wavelength blue gets shifted beyond the visible spectrum
• The energy gap between blue and infrared causes this blackout effect
• Understanding the blue blindness of IR enables cool imaging applications

As a tech specialist and infrared enthusiast, I‘m amazed by how this exotic slice of the spectrum unlocks hidden details. There‘s always more beneath the surface than our eyes can see!

Infrared may not be able to capture the full rainbow, but transmuting reality into fresh, colorful interpretations full of revealed insight more than makes up for the loss of a single azure hue.

1. Shruti Nagpal, M. Singh and N. Singh, "Performance enhancement of face recognition system under varying illumination conditions," 2016 International Conference on Signal Processing, Communication, Power and Embedded System (SCOPES), 2016, pp. 354-358, doi: 10.1109/SCOPES.2016.7955639.

2. "New infrared goggles let firefighters see through smoke and flames", Chicago Tribune 2018. https://www.chicagotribune.com/business/blue-sky-innovation/ct-firefighter-infrared-goggles-bsi-chicago-inc-20180508-story.html

3. Barreira, E. and de Freitas, V.P., 2007. Evaluation of building materials using infrared thermography. Construction and building materials, 21(1), pp.218-224.

4. Daffara, C., Ambrosini, D., Pezzati, L. and Paoletti, D., 2015. Thermal infrared imaging spectroscopy for the non-destructive characterization of concealed paintings. Applied Physics A, 120(1), pp.295-302.

5. Willner, S.P., Russell, R.W., Puetter, R.C., Soifer, B.T. and Harvey, P.M., 1979. Spatially resolved infrared photometry of galaxies. The Astrophysical Journal, 228, pp.664-670.

6. Bell III, J.F., Godber, A., McNair, S., Caplinger, M.A., Maki, J.N., Lemmon, M.T., Van Beek, J., Malin, M.C., Wellington, D., Kinch, K.M. and Jungers, M., 2017. The Mars science laboratory curiosity rover mast camera (Mastcam) instruments: Pre-flight and in-flight calibration, validation, and data archiving. Earth and Space Science, 4(7), pp.396-452.

7. Schaul, L., Fredembach, C. and Süsstrunk, S., 2009. Color image dehazing using the near-infrared. In 2009 16th IEEE International Conference on Image Processing (ICIP) (pp. 1629-1632). IEEE.

8. New Navy Working Uniform Blends NIR Camouflage. United States Naval Institute. https://www.usni.org/magazines/proceedings/2008/february/new-navy-working-uniform-blends-nir-camouflage

9. Williams, R. McBride, D. W., and Michael J. Debs. "Infrared thermography as a tool for non-destructive analysis of works of art." InfraMation Proceedings (1992): 493-500.

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