The term absorption is not only used for absorption processes, but also often for related quantities, e.g. instead of absorption coefficient.

I find that compositions with front light or strong side light are a good starting place to learn about visible or infrared light. Front light means you are standing between the subject and the light source, facing your subject with the light source to your back – just watch out for your shadow in the shot!

A big surprise for me was how the different minerals in the rocks in Death Valley render in infrared. Another shocker was that not all foliage will be white. For example, the short needle evergreens in Yellowstone National Park render black, much like visible light, whereas long-needle pine trees glow infrared. You won’t know until you make an image.

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Non-transparent objects can be attributed an absorptance, which is the fraction of incident light which is absorbed rather than transmitted, reflected or scattered.

Try experimenting with subjects other than green foliage, such as architecture, monolith rocks on land or at the edge of the sea, cacti or other plants, or morning fog in the trees.

A monochrome image on the camera display will instantly reveal your composition. Since my end goal is processing for black-and-white, the monochrome JPG is very close to the final processed IR image.

Side light is usually a light source to the left or right of the camera. The lower the sun is in the sky, the stronger the light will be. Watch for deep shadows on the “shade” side of your subject. Make sure you always have details in the shadows.

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Most beginners only shoot infrared during the spring and summer (peak green foliage) and only when the sun is very bright and high in the sky. You can tell what time of day a photograph was taken by looking at the shadows. If the shadows are tight and close to the subject, the sun is high, and it is probably summer. If the shadows are long and away from the light source, the image was taken around sunrise or sunset.

In some special cases, nearly all of the absorbed light causes fluorescence rather than heat, and there can be even a net cooling effect (→ laser cooling). It may even happen that at some (typically longer) wavelengths one obtains laser amplification for strong enough excitation of the medium, usually involving a population inversion. The medium may then generate laser radiation which may remove a substantial fraction of the deposited energy.

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Absorption in a semi-transparent medium is usually quantified with an absorption coefficient, telling which fraction of the optical power is lost per unit length. The inverse of an absorption coefficient is called an absorption length. The absorption of a given length of material (e.g. of a plate with a certain thickness) can be quantified with an absorbance.

Now that the camera is set up for black-and-white capture, how do I see the infrared light? I cheat by looking at the monochrome JPG! In most cases, I lean on my years of success and failure in different conditions, but to start, I suggest looking for where visible light (sunlight or moonlight) reflects off the subject in the scene. Where is the light source? Where are you standing in relation to the scene and the light source? What areas are awash in visible light? Where are the shadows? Your “best” shot might be behind you!

Over seven years ago, I stopped making images in color and now only shoot with infrared-converted cameras. I have found that 720nm gives me the broadest range of tonalities for black-and-white, compared to 590nm or 850nm. For me, 590nm is too close to the visible light spectrum, which means my mid-tones would suffer, and 850nm has too much contrast and would require bracketing and blending to capture the full dynamic range.

Infrared light still surprises me constantly. I am never certain what I will get until I press the shutter. Test the light by making the shot. Review it to see the possibility. If the image works for you, keep it. If it doesn’t, ditch it. That’s the workflow. Clients in the field learn quickly not to ask me, “Do you think this shot will look good in infrared?” I always respond, “I don’t know. Make a shot, and let’s look at it.” Very quickly, they understand I am trying to encourage experimentation. When I see it, I will know if the shot worked and has potential through processing. I learn more from my failures than from my successes.

A general distinction is between intrinsic and extrinsic absorption. Extrinsic absorption (also sometimes called parasitic absorption) results from things which could in principle be avoided – for example, from impurities and structural defects which could be absent in pure high quality material. Intrinsic absorption results from basic properties of the pure material.

Linear absorption means that the absorption coefficient is independent of the optical intensity. There are also nonlinear absorption processes, where the absorption coefficient is a linear or higher-order function of the intensity. For example, two-photon absorption is a process where two photons are absorbed simultaneously, and the absorption coefficient rises linearly with the intensity. Multiphoton absorption processes of higher order are often involved in laser-induced damage caused by intense laser pulses.

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Shoot year-round. Shoot long exposures of clouds or water. One would think brown sea or marsh grasses in the winter would not pop white in infrared, but they do! Chlorophyll gives plants their green color, which turns white in 720nm and 850nm black-and-white infrared. Even though the brown shade of the dried grasses might imply the plant is dead, the chlorophyll still reflects the infrared light all year.

As absorption coefficients are wavelength-dependent, one often produces absorption spectra, showing an absorption coefficient as a function of wavelength or optical frequency.

Remember to use your lens hood! And always take the time to look at your preview images thoroughly. I find it very hard to remove flares in post-processing, and it is best to try another shot while you are at the location. I use a Hoodman loupe in bright sun to cover the camera screen. It can be incredibly frustrating when you get a terrific shot, but it is unusable due to faint flare issues.

There are also many cases where a material contains some absorbing dopant while the host material itself exhibits only negligible absorption. This is the case for solid-state (doped-insulator) gain media.

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Shooting into the sun or full moon usually results in flares when using infrared filters, either on the lens or sensor. Flare can be a creative concept, but it is a deal-breaker for me. Flares should not be confused with a lens hotspot. Lenses have special coatings to cut down on flare, but those lens coatings are ineffective when using an infrared-converted camera. Many times, when I review an image on the back of the camera and see a flare, all that is needed is to slightly change the camera’s position relative to the angle of the light source or change the focal length (longer or shorter).

More specific terms: infrared absorption, excited-state absorption, pump absorption, light-induced absorption, multiphonon absorption, multiphoton absorption, two-photon absorption, pump absorption

Absorption of light can also have electrical effects. For example, there are photoresistors, where the electrical resistance is reduced by absorbed light. In photodiodes and phototransistors, one exploits the internal photoelectric effect, related to the excitation of electric carriers by light absorption.

Although our eyes only see visible light, here are some observations and an approach to learning how to “see” infrared light. With a few tips, you will be well on your way to understanding infrared light.

Further, the modified population in electronic states can substantially modify the absorption at the wavelength of the absorbed light and also at other wavelengths. It has already been mentioned above that absorption may be saturated. In other cases, light absorption is strongly increased by the light-induced changes of the state of matter. That is often exploited in laser material processing, where the initial absorption e.g. by a metal is weak, but strongly increases once the material is strongly excited (anomalous absorption). In various materials, one may obtain excited-state absorption at wavelengths where the material would normally not be absorbing. In semiconductors, at high intensities one obtains free carrier absorption.

Various types of processes, which would in principle be avoidable, lead to extrinsic absorption for example in optical glasses, in nonlinear crystal materials and in laser crystals:

If absorption is caused by some absorbing dopant, the contribution to the absorption per dopant atom or ion is often quantified with an absorption cross-section.

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If the incident light is in a coherent state, exhibiting the standard shot noise level, the extra noise added through linear absorption is just enough to keep the residual light at the shot noise level (which is relatively stronger for weaker light).

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Consider shooting during the blue and golden hours as well. Your images will have a magical feeling at those times of the day. I prefer the early morning, about an hour and a half to two hours before sunrise. For me, by the time the sun comes up, the light becomes harsh, and the shadows can be very dark.

Even simple linear absorption processes introduce some amount of quantum noise. This can be intuitively understood by considering that some of the incident photon are randomly removed, while other photons remain in the light beam. An initially perfectly regular stream of photons (→ amplitude-squeezed light) would thus be converted into a random stream of photons, exhibiting some intensity noise.

Penelope Taylor is a New Jersey-based photographic landscape artist. Her father introduced her to film cameras in her early teens. Her passion is traveling throughout the United States and Canada to capture her favorite subject — landscapes. In 2015, she began to exclusively produce work using Nikon digital cameras converted for infrared photography. In 2017, Penelope began to offer infrared-specific landscape photo workshops where she teaches capture and black-and-white processing techniques. She is available for group lectures and one-on-one instruction in person or online.

If light is absorbed by atoms or molecules of a gas, light forces associated with the absorption may become relevant. They can be used for Doppler cooling, for example.

Through different kinds of processes, which are explained in the following, light can be absorbed in various media. This implies that the optical energy is converted into some other form of energy (but sometimes back again to optical energy). In most cases, the energy is eventually transformed into heat (thermal energy).

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I encourage you to be fearless. Experiment when you find something interesting to photograph. Ask yourself, “What would that look like in infrared?” When working with clients in the field, most refuse to press the shutter unless they can visualize the shot. If you only shoot infrared occasionally, chances are you don’t have enough experience to pre-visualize an image through the viewfinder in a spectrum we cannot see. The only way I learned was to shoot many frames and move around the subject until I finally “found” the infrared light.

As light carries energy, the absorption of light is associated with the deposition of energy in the absorbing medium. In most cases, that energy is mostly converted into heat, although sometimes a substantial amount of the received energy is radiated away as fluorescence.

In nonlinear absorption, does the laser pulse duration also affect the absorption coefficient alongside with the intensity?

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If you process infrared for black-and-white, no matter where you live, you can make beautiful images in the city, the mountains, the seaside, or the forest. No time of year is bad for infrared, and no subject is off-limits. I feel that setting up the infrared camera back to display in black-and-white has made me a better photographer – my compositions quickly improved the more I shot, and I understand the full spectrum of light better by working in infrared.

Impurities can also modify intrinsic absorption features – for example, shift the band gap energy and the corresponding absorption edge when a semiconductor compound is formed.

Looking at the image in monochrome on the back of the camera taught me where, how, and if the infrared light appeared in the scene. It is possible you won’t find anything that is reflecting the infrared light.

When I started down the rabbit hole of infrared, there were no books or YouTube videos regarding what I wanted to do: make black-and-white landscape images at 720nm. Lots of information was available for faux color (590nm) but nothing for black-and-white. Only by shooting thousands of infrared frames in all kinds of conditions over the years did I learn what probably would happen and how it would render given my subject (i.e., rocks, foliage, water, skies).

If absorption of light causes heating of the absorbing medium, that will subsequently lead to thermal expansion. The heating is often strongly inhomogeneous; for example, it may occur within a focused laser beam. The local thermal expansion then leads to mechanical stress in the medium, which can even result in fracture when the deposited thermal power or energy is sufficiently high. Further, the temperature causes a slight local modification of the refractive index, which (together with stress-related effects) can cause thermal lensing effects.

Evolution of output power and gain when an Yb-doped fiber laser is switched on. One can see the relaxation oscillations, with convergence towards the steady state. Each red or gray segment corresponds to 0.2 μs.

Saturable absorption can also be considered as a kind of nonlinear absorption. Here, however, the absorption coefficient is reduced under the influence of intense light, e.g. because the starting electronic level for the light absorption is depleted.

Light absorption processes e.g. in solid materials generally arise from the interaction of the electromagnetic wave with electrons, exciting those to excited energy levels. Thereafter, it takes some time (the electron–lattice thermalization time) for that energy to be transferred to the atomic nuclei, i.e., to vibration energy. That typically happens within a couple of picoseconds, and thereafter it takes far longer times to distribute that heat over some volume of the medium. That means that the thermalization, let alone the heat conduction, can take far more time than the pulse duration of a femtosecond laser. That has important implications for laser material processing with ultrafast lasers, where the involved processes cannot be understood as simply heating up the material. Instead, one is dealing with highly non-equilibrium states of matter, which can lead to rapid application of material while very nearby other material, not directly hit by the laser radiation, is not even significantly heated.

I remember the first infrared exposure I made and what a shock it was to my system when the RAW file displayed on the back of my camera. What is THAT funky thing? How do I know what I am looking at? I got the idea (I am sure others did this, but I had no knowledge of it) to set the camera to capture both JPG and RAW. I set the JPG and, therefore, the camera display to monochrome. The RAW file is untouched, but the JPG is black-and-white.