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April 2015 Number 102



Light we cannot see

“The Plover and the Clover can be told apart with ease,
  By paying close attention to the habits of the Bees...”

From How to Tell the Birds from the Flowers (1907) by Robert Williams Wood, pioneer of infrared photography.

 Lost Fun Zone, North Kivu, eastern Congo, 2012, digital C print captured with Kodak Aerochrome Infrared false color film. Image courtesy of the artist and Jack Shainman Gallery © Richard Mosse.

Ask a five-year-old: “Do you think there is light we cannot see?” From a logically thinking child you might receive the reply: “I cannot see the light in my sister’s bedroom, but I know it’s on.” Or perhaps you get a “What do you think?” response from your precocious granddaughter, who suspects that you are leading somewhere with your question. But the most likely response is a “No.” To a five-year-old, light is something we see by and that we can see; if we cannot see it, it should not be called light.

 The IR output of a TV controller as seen by a smartphone camera. The red indicator lamp (lower left) is visible to the naked eye. However, the two lights at the top are visible only to the camera. Question: Why do they have a whitish-purple color

The door is open for an educational discussion with this child in the Year of Light. Because, of course, there is light we cannot see.

Take that child into a darkened room, point a television controller in his direction, push a but- ton, and ask him if he sees a light coming out of the end. “No,” is his response. Then have him point a smartphone camera at the controller and push the button again. “I see something!,” he exclaims. Or rather, as you point out to him, the camera sees something – and he sees the image formed by the camera (see image opposite).

As adults, we know something about infrared and ultraviolet light. We probably know that bees can see in the ultraviolet portion of the spec- trum. And we may be aware that some cameras can take photos of wild animals at night with invisible infrared light. But do we know how? Do we know why digital cameras recording infrared light take better pictures on hazy days than do ordinary digital cameras? Do we know why the IR lights in the TV remote controller appear to the camera with a whitish purple color instead of a deep red? Or – here is a good one – do we know just how many different kinds of photoreceptors are found in the eyes of the mantis shrimp that are sensitive only to the UV? (The answer is six! [1]). Some of us do; many of us do not.

 The Silver Ragwort flower as viewed in visible light (left) and ultraviolet light (right). Note the emphasis in the UV of the pollen-hosting portion of the flower. Photos by Dave Kennard via Wikimedia Commons ( licenses/by-sa/3.0).

Here are some interesting facts concerning IR and UV we may use to entice that youngster into learning more about light: • Clothing washed in some detergents comes out “whiter than white”. The reason: the detergent contains compounds that fluoresce when illuminated with UV light. The invisible UV light is converted to visible blue light, making the whites both brighter and less yellow. (Interestingly, some of the new LED lamps produce little to no UV, and the whites from the laundry are, well, just white [2].) • Dayglow paint is another example. Did you ever wonder about the origin of that really hot pink or orange color? UV light. • The sensor array elements in digital cameras are sensitive to radiation into the near- IR. Normal digital cameras have an IR-blocking filter in front of the detectors to keep the color balance of the displayed images closer to what we would see with our eyes. • Remove the filter and replace it with one that passes IR and blocks blue light and the camera will now take pictures through haze with much higher than normal contrast. Why? Because IR radiation is scattered less by haze-producing particulate matter in the air. • Shorter wavelength UV light kills bacteria. Some swimming pools use UV light to make the water safer to swim in. • Seeing something hid- den with near visible light can be an accidental outcome. In 1997, Sony released a model of their Handycam camera that included an IR sensitive “Nightshot” effect that allowed one to see in low light. A rash of (perhaps poorly-researched) news reports in 1998 to the effect that the camera, when used in daylight with a red filter, could see through clothing caused Sony to halt shipments and adjust the Handycam to only perform Night- shot with slower and blurrier shutter speeds [3].

Back to the TV controller and the ability of a smartphone camera to “see” the IR light. In the early 1970s, when room-temperature semicon- ductor diode lasers were just becoming available, an occasional “I can see the output!” was heard. Why the astonishment? Because light at the wave- length of those devices, in the near-IR, was not supposed to be visible. Obviously it was – indeed, research has shown that some people can see light as far into the IR as 1064 nm [4] – but to be seen the source must have the spectral brightness of a laser – typically more than 100,000 times that of the Sun [5]. These early experimenters with the psychophysics of IR lasers reported seeing a deep red color. It was only the long-wave receptors of the retina that were being stimulated, and the per- ceived color was thus the deepest possible red. So why does the mobile phone camera see the infra- red LED as producing whitish purple light? The sensor elements in a digital camera are equipped with red, green, and blue filters that allow images in each of the three primary colors to be formed, subsequently to be merged by internal circuitry for display. Ideally, these filters pass no IR, but clearly they do pass some. And it is not simply the red-filtered detector elements that respond to the IR. Some light must pass the blue and green fil- ters as well. The result, when rendered as a three- color combination by the camera display, is the color shown in the figure – at least for the cam- era used for this article. (For the courageous, [6] provides detailed instructions on how to remove the IR-blocking filter from the DSLR Nikon D70 camera, along with some photographic results.)

How do bees see light we cannot, and what in fact do they see? Humans have three color receptors in the retina, peaking at 560, 530, and 420 nm. Visible light of a particular color produces different ratios of stimulation of these receptors with the perception of color result- ing. Bees, on the other hand, have receptors that peak at 540, 430, and 340 nm, the last of which is in the near-UV [7]. A honey bee can thus see things that we cannot. This ability proves useful to the bee, since – possibly through co- evolution? – many flowers have UV patterns on their petals that direct the bee to the site of the nectar and pollen. See the figure opposite (top). Interestingly, the Bee’s long wavelength 540 nm band receptor is not sensitive to red light, and what we see as red f lowers appear black to the bee [8]. For some particularly beautiful pseudoc- olored near-UV photos of flowers by professional nature photographer Bjørn Rørslett, see [9].

What “color” do bees see when they see ultraviolet light? That is like asking someone, “Do you see the color red the same way that I do? – only more difficult. We can reasonably specu- late that the psychophysical response to view- ing a particular wavelength is the same among humans, i.e., you see red the same as I do. But bees? We have no idea.

 William Herschel, astronomer and discoverer of infrared light.

The stories of the discovery of IR and UV light can teach our youngsters something about the relation bet ween curiosit y and science, and also about the scientific method. In 1800, astronomer William Herschel noted that he could sense heat from the Sun’s rays even when he used darkened glasses to strongly attenuate the light reaching his eyes. Motivated by curios- ity, he separated sunlight into its colors with a prism and, as suggested in the figure opposite, measured the temperature rise in thermom- eters at different places along the spectrum. Red light provided more heat than did green or blue. But something beyond red caused an even greater rise in temperature! Herschel showed that these invisible “calorific rays” behaved like light in that they could be ref lected, refracted, absorbed, and transmitted in similar fashion. For an excellent discussion of this topic well- suited for young people, see [11].

A year after Herschel’s discovery of IR light, Johann Ritter, guided both by curiosity and by an appreciation for the symmetry so frequently observed in nature, looked for evidence of invis- ible light at the violet end of the sunlight spec- trum [12]. He found that evidence through the increased rate at which silver chloride turned black in the space beyond the violet part of the spectrum, where no sunlight was visible – a rate that exceeded that in the blue and violet portions. He referred to this invisible light as “oxidizing rays” or “chemical rays,” the name ultraviolet being introduced only later.

 Infrared imagery of Pablo Picasso’s bowtied man underpainting revealed beneath Picasso’s 1901 painting, The Blue Room 1901. From [10].

Crime-show watchers know that UV light plays a role in forensic investigations. Semen f luo- resces under UV illumination, even after years of drying. Kerosene, sometimes used in arson, f luoresces a bright blue. And UV-f luorescing powders are often used to enhance fingerprints, which can then be photographed without the necessity to “lift” them from the surface. In June of 2014, the Associated Press reported on the image of an underpainting hidden beneath the surface of The Phillips Collection’s The Blue Room (1901) by Pablo Picasso. The underpainting was made visible by means of IR illumination. Indeed, IR illumination has been used success- fully in tracing the “prehistory” of many paint- ings, not only by Picasso but by Da Vinci and many others. An interesting article on how to become an art investigator is to be found at [13].

The most esthetically pleasing exploitation of IR light is doubtless in photography. Artists have always been captivated by the near-visible in the broadest sense – imagery produced that is near, but not actually accessible, to human vision, like the blurring of time with long shutter speeds to create a feeling of romance, or freezing time for dramatic effect with short exposures, macro enlargement to abstract a form, or multiple expo- sures for an ethereal effect. The capture with special films of near-visible light performs a spe- cial role: it is still relatable to our common per- ception but is at the same time affected, exalted. A simple image search for “high dynamic range [HDR] photography” or “infrared photography” brings up a host of images that are not scientific, but that see beyond the spectrum of human per- ception for aesthetic effect.

Some art photographers are seeking in earlier film-based methods characteristics that are unre- alistic but expressive. For example, Kodak Aero- chrome, a “false color” infrared film stock made for aerial photography, has been popularized by the Irish artist Richard Mosse, who represented Ireland at the prestigious 55th Venice Biennale art fair [14].

 “Washington Monument, Washington, D.C.,” by Carol M Highsmith (2007).

Aerochrome, which is sensitive out to approxi- mately 900 nm in the IR, was designed to capture the near-visible by suppressing some visible light. Blue light is filtered out with a No. 12 Yellow fil- ter, and the entire spectrum is then shifted: in prints, IR appears red, red appears green, and green appears blue. The film was created for aerial photography of vegetation; the strong near- infrared ref lectivity of leaf cell walls makes for purple–pink blooms in the photos, easily distin- guished from other surfaces. This high IR reflec- tivity of plants is the effect (known as the “Wood Effect” after R W Wood) that results in bright white vegetation in black and white IR photogra- phy. Mosse has used this effect in photographing rebel militia fighters in the Congo: an invisible war shot with invisible light. As demonstrated in his 2011 photo, “Man-Size, North Kivu, eastern Congo”, the camouf lage outfits, which would normally be hard to see against the vegetation background, stand out against the bright fuch- sia foliage as viewed by the Aerochrome film. Indeed, Kodak color infrared-sensitive films were originally designed for reconnaissance and cam- ouflage detection and were referred to as CD, or camouflage detection films.

 Man-Size, North Kivu, eastern Congo, 2011, digital C print, 72 x 90 inches. Image courtesy of the artist and Jack Shainman Gallery, © Richard Mosse.

And what is it like to photograph with light you cannot see? Mosse, in an interview with Aaron Schuman for Aperture magazine, had the following to say: “Since infrared light is invisible to the human eye, you could say that I was literally pho- tographing blind ... Yet I was trying to represent something that is tragically real – an entrenched and endless conflict fought in a jungle by nomadic rebels of constantly shifting allegiances” [15]. What is most striking in Mosse’s photographs is how Aerochrome sees the conflict in the Congo, a combination of elements familiar and real with colors made through light that we cannot see. In Richard Mosse’s image that begins this arti- cle, look at how the surreal background blends with the natural gray-white bark of the trees and the normal flesh tones made slightly smooth and luminous by infrared’s subtle penetration of skin. The visible and the nearly visible combine like in a dream.

 “Tree Example IR” and “Tree example VIS” by Schwen. Licensed under CC BY-SA 2.5 via Wikimedia Commons.

Which returns us to our first question, why do we speak of UV and IR as light, and to what extent should we do so? The historical reasons speak for themselves: light from the sun was discovered to contain, directly adjacent to the red and violet ends of the visible spectrum, “rays” that could, respectively, heat thermometers even better than could red light or expose silver nitrate even better than could violet light. The nearly visible light, though not seen, was easily experienced, and therefore that an invisible form of light was being observed was a reasonable conclusion. Adding to the “light” aspect of this invisible radiation, it was determined that it could be diffracted, refracted, reflected, emitted and absorbed, just like visible light, as Herschel found with his prism. More recently, the imaging of near-IR and near-UV radiation with cameras add to their identity as light. With a little lift and modulation in spec- trum, as in Aerochrome film, nearly visible light can be given “false color” and then recorded in our visible spectrum. So again, why do we speak of IR and UV as “light”? Because in many regards it behaves like the light we can see. It is something just barely not ours and all around us. Our mobile phone in our pocket can see it, our remote con- trols shoot spotlights of it, and just outside our windows bugs are using it to navigate their spring foraging. And with a little experimentation, we and our five-year-old can see like the bees.

William Rhodes
Professor of Electrical & Computer Engineering and Computer Science at Florida Atlantic University
Emeritus Professor (retired) at Georgia Institute of Technology.

Geoffrey Alan Rhodes
Assistant Professor in the Department of Visual Communication Design at the School of the Art Institute of Chicago.

They are father and son.


[1] E Yong, “Nature’s Most Amazing Eyes Just Got A Bit Weirder”, National Geographic July 3, 2014.

[2] /2014/04/18/research/under-some- led-bulbs-whites-aren’t-whiter-white.

[3] “Sony’s naked cam scam?” CNNMoney, 8/14/1998.

[4] D Sliney et al., “Visual sensitivity of the eye to infrared laser radiation, J. Opt. Soc. Am. 66 (4) 339–341.

[5] D C O’Shea, W R Callen and W T Rhodes, Introduction to Lasers and Their Applications, Section 2.4 (Addison-Wesley 2007).

[6] htm.

[7] and_color.html.

[8] https://fieldguidetohummingbirds. do-we-see-what-bees-see./

[9] ANS.html.

[10] default/files/press_material/press- release-blue-room-collaboration.pdf.

[11] practical-physics/william-herschel- and-discovery-infra-red-radiation.

[12] optics/timeline/people/ritter.html.

[13] intro/ir.html.

[14] richard-mosse-at-the-venice-biennale/.

[15] Aperture, #203, Summer 2011, p56.

 Next articles:

International Commission for Optics

Bureau members (2014-2017):

President: Y. Arakawa;

Past-President: D. T. MooreTreasurer: J A Harrington;

Secretary: A M Guzmán, CREOL, The College of Optics and Photonics, University of Central Florida, PO Box 162700, 4000 Central Florida Blvd,Orlando, FL 32816-2700, USA; e-mail

Associate Secretary: G von Bally

Vice-Presidents, elected: J. Harvey, F. Höller, H. Michinel, J. Niemela, R. Ramponi, S-H Park, J. Zakrzewski, M. Zghal

Vice-Presidents, appointed: Y. J. Ding, J. C. Howell, S. Morgan, E. Rosas, P. Urbach, A Wagué, M. J. Yzuel

IUPAP Council Representative: C Cisneros

Editor in chief: A M Guzmán

Editorial committee:
K Baldwin, Australian National University, Australia;
J Dudley, Université de Franche-Comté, France;
William T Rhodes, Florida Atlantic University, USA.