Historically, fluorescent and quartz halogen lighting sources have been used most commonly. In recent years, LED technology has improved in stability, intensity, and cost-effectiveness; however, it is still not as cost-effective for large area lighting, particularly compared with fluorescent sources. However, if application flexibility, output stability, and longevity are important parameters, then LED lighting might be more appropriate. Depending on the exact lighting requirements, oftentimes you can use more than one source type for a specific implementation, and most vision experts agree that one source type cannot adequately solve all lighting issues.

Again, start in the 'Develop' section of Lightroom. Then, open the “Lens Correction” window as you did for the auto-correction, but instead of remaining on the “Profile” box, switch to the “Manual” menu. You should have the same settings as in the photo above.

IR light is considerably more effective at penetrating polymer materials than the short wavelengths, such as UV or blue, and even red in some cases (see Figure 13). Conversely, it is this lack of penetration depth that makes blue light more useful for imaging shallow surface features of black rubber compounds or laser etchings, for instance.

Chromaticaberration example

By shooting in RAW you’ll have more data recorded which you can experiment with using the settings of post-production softwares such as Adobe Lightroom or Adobe Camera Raw. With a little bit of practice, you’ll manage to remove chromatic aberration almost completely from your pictures.

Fully understanding the immediate inspection area’s physical requirements and limitations, in a 3D space, is critical. In particular, depending on the specific inspection requirements, the use of robotic pick-and-place machines or pre-existing, but necessary, support structures, may severely limit the choice of effective lighting solutions by forcing a compromise in not only the type of lighting but also its geometry, working distance, intensity, and pattern. For example, you may determine that a diffuse light source is required but cannot be applied because of limited close-up, top-down access. Inspection on high-speed lines may require intense continuous light or a strobe light to freeze motion, and of course large objects present an altogether different challenge for lighting. Additionally, consistent part placement and presentation are also important, particularly depending on which features are being inspected; however, even lighting for inconsistencies in part placement and presentation can be developed, as a last resort, if fully understood.

Figure 22 illustrates potential application fields for the different lighting techniques based on the two most prevalent gross surface characteristics: (1) surface flatness and texture and (2) surface reflectivity.

Let’s start with the easiest option. In order to find the automatic CA correction in Lightroom, make sure you are in the 'Develop' section of the software.

Figure 6. On the left, the bottom of a soda can is illuminated with a bright field ring light but shows poor contrast, uneven lighting, and specular reflections. On the right, the soda can is imaged with diffuse light, creating an even background so the code can be read.

This diagram plots surface reflectivity, divided into three categories—matte, mirror, and mixed—versus surface flatness and texture or topography. As you move right and downward on the diagram, more specialized lighting geometries and structured lighting types are necessary.

In simpler words, and as you can see from the photo below here (which is a roughly 100% crop of the original), chromatic aberration consists of some annoying colour halos that appear where they shouldn’t be. In this case, you can clearly see them along the edge of the mountain.

Theoretically speaking, the perfect lens should be able to focus all the wavelengths of colours into one point, called “circle of least confusion”, where chromatic aberration is minimised. In reality though, different colours of light hit the lens at different speeds (and so, at different times), causing different types of chromatic aberrations to occur.

Understanding how to manipulate and enhance sample contrast using the four cornerstones is crucial in meeting the three acceptance criteria for assessing the quality and robustness of lighting. Effecting contrast changes through geometry involves moving the sample, light, and/or camera positions until you find a suitable configuration. For example, a coaxial ring light (one mounted around the camera) may generate hotspot glare on a semireflective barcode surface, but by simply moving the light off-axis, the hotspot glare is also moved out of the camera’s view. Contrast changes through structure or the shape of the light projected on the sample is generally light head– or lighting technique–specific (see the Lighting Techniques section in Part 2 of this series). Contrast changes through color lighting are related to differential color absorbance versus reflectance (See Sample/Light Interaction).

Figure 22. Lighting Technique Application Fields: Surface Shape Versus Surface Reflectivity Detail (Although not shown, any light technique is generally effective in the Geometry Independent Area of the diagram.)

Polarizing filters, when applied in pairs, one between the light and sample and the other between the sample and camera, and typically affixed to the lens through screw threads, are useful for detecting structural lattice damage in otherwise transparent samples (Figure 14).

Figure 15. A change in light/sample, camera geometry, or type may be more effective than applying polarizers to stop glare. (a) Coaxial ring light without polarizers. (b) Coaxial ring light with polarizers (note some residual glare). (c) Off-axis (light axis parallel to the sample long axis) ring light without polarizers. (d) Coaxial ring light without polarizers. (e) Coaxial ring light with polarizers (note: 2 ½ f-stop opening).

Consider all the information from these evaluations together with the available optics, lighting types, techniques, and the four cornerstones to develop a sample-appropriate lighting solution that meets the three acceptance criteria.

Longitudinal aberration (LCA), also called “bokeh fringing”, occurs when the lens is not able to properly focus all the different wavelengths of colour on a determined focal plane. You can easily recognise this kind of chromatic aberration since you’ll see the unwanted colour fringing all around the frame, even in the centre, especially in the out-of-focus parts of the image.

Each inspection is different, thus it is possible, for example, for lighting solutions that meet acceptance criteria one and two to be effective only provided there are no inconsistencies in part size, shape, orientation, placement, or environmental variables such as ambient light contribution (see Figure 1).

Chromaticaberrationeffect

When you have accumulated and analyzed the information from these areas, with respect to the specific sample and inspection requirements, you can achieve the primary goal of machine vision lighting analysis—to provide sample-appropriate lighting that meets three acceptance criteria consistently:

In this article, you will learn all about chromatic aberration, why it occurs and the different types of chromatic aberration that you can observe. You'll also discover some of the best techniques to avoid it both on the field and in post-production, so that your shots won’t be affected by chromatic aberration anymore!

High contrast scenes are the most sensitive areas to chromatic aberration, where it is more visible. Of course, if the shot you have in mind requires you to work in high contrast situations, then you won't be able to do much about it and you should still take the shot. However, if you have the chance to choose between shooting with some high contrasts in the frame or not, I’d rather go for the latter one since you won’t have to deal with the evidence of chromatic aberration later on.

Figure 7. The 2D dot peen matrix code on the left is illuminated by bright field ring light. The right is imaged with a low angle linear dark field light. A simple change in light pattern creates a more effective and robust inspection.

The second option is to play with these parameters by yourself, which is what I did in this case as the eye-drop wasn’t working perfectly.

You’ll be able to easily see how much that lens is affected by chromatic aberration when you open the shot you took on Lightroom and zoom in on the dots a little bit.

There are three active methods for dealing with ambient light: (1) high-power strobing with short duration pulses, (2) physical enclosures, and (3) pass filters. Which method is applied is a function of many factors, most of which are discussed in some detail in later sections. High-power strobing simply overwhelms and washes out the ambient contribution, but has disadvantages in ergonomics, cost, and implementation effort, plus not all sources, such as fluorescent, can be strobed. If you cannot employ strobing, and if the application calls for using a color camera, multispectral white light is necessary for accurate color reproduction and balance. In this circumstance, a narrow wavelength pass filter is ineffective, as it will block a major portion of the white light contribution, and thus an enclosure is the best choice.

The application of some techniques requires a specific light and geometry, or relative placement of the camera, sample, and light—others do not. For example, a standard bright field bar light may also be used in dark-field mode; whereas a diffuse light is used exclusively as such.

Figure 13. In the populated PCB the penetration of red is 660 nm (left image) and IR 880 nm light. Notice the better penetration of IR despite the red blooming out from the hole in the top center of the board.

The lighting sources now commonly used in machine vision are fluorescent, quartz halogen, LED, metal halide (mercury), and xenon.

Spherical aberration

Diffuse, or full bright field lighting, is most commonly used on shiny specular or mixed reflectivity samples where even but multidirectional light is needed. Several implementations of diffuse lighting are generally available, but there are three primary types (Figures 17a–c), with hemispherical dome/cylinder or on-axis being the most common.

No matter the level of analysis, and understanding, there is quite often no substitute for actually testing the two or three light types and techniques first on the bench, then in actual floor implementation whenever possible. And when designing the vision inspection and parts handling/presentation from scratch, it is best to get the lighting solution in place first, then build the remainder of the inspection around the lighting requirements.

What about if you want to place your subject in the bottom or top thirds of the frame, or close to the edges? Well, you can place it in the centre while you are shooting and then crop the image to achieve the composition you were looking for!

The objective of this detailed analysis and application of what might be termed a “tool box” of lighting types, techniques, tips, and tricks is to help you arrive at an optimal lighting solution that takes into account and balances issues of ergonomics, cost, efficiency, and consistent application. This helps you to better direct your time, effort, and resources—items better used in other critical aspects of vision system design, testing, and implementation.

The presence of ambient light input can have a tremendous impact on the quality and consistency of inspections, particularly when using a multispectral source such as white light. The most common ambient contributors are overhead factory lights and sunlight, but occasionally errant vision-specific task lighting from other inspection stations or even other stations in the same workcell can have an impact.

Chromaticaberration in eyes

In those applications requiring high light intensity, such as high-speed inspections, it may be useful to match the source’s spectral output with the spectral sensitivity of your particular vision camera (Figure 4). For example, CMOS sensor-based cameras are more IR sensitive than their charge-coupled device (CCD) counterparts, imparting a significant sensitivity advantage in light-starved inspection settings when using IR LED or IR-rich Tungsten sources.

Chromaticaberration in glasses

As you can see in the photo below, I played a bit with the Defringe Green slider and the Green Hue, to find the right point where all the chromatic aberration is removed.

As you may have understood by reading the definition, chromatic aberration is an optical problem, so it all depends on the lens you are using. In general, high quality, expensive lenses will logically minimise chromatic aberration better than basic lenses. However, it's impossible to find a lens without chromatic aberration, since it’s a physiological problem of all lenses when exposed to light.

Figure 3. Light Source Relative Intensity Versus Spectral Content (The bar at the bottom denotes the approximate human visible wavelength range.)

A useful property of axial diffuse lighting is that in this case, rather than rejecting or avoiding specular glare, you may actually take advantage of the glare if it can be isolated specifically to uniquely define the feature(s) of interest required for a consistent and robust inspection.

Chromatic aberration is an effect that occurs when a lens is not able to properly refract all the wavelengths of colour in the same point. It’s quite a common problem in photography that affects almost all lenses, though high-quality lenses will present with less chromatic aberration compared to lower-quality ones.

We've talked about what chromatic aberration is, why it exists and what it looks like. At this point, you may be quite discouraged, thinking that you've just discovered a new “problem” to deal with when taking pictures.

To deal with this, you have two options: the first one is to use a prime lens, which will handle chromatic aberration much better, while the second one is not to use the minimum/maximum focal lengths of your zoom lens and just to use the ones in between. You can crop a little bit or do a panorama to replicate the minimum/maximum focal lengths of the lens if you wish.

Chromaticaberration in games

Figure 21. The peanut brittle bag on the left is under a bright field ring light. On the right, it is under a dark field ring light—note the seam is very visible.

Unfortunately, as if one wasn’t enough, there are two different types of chromatic aberration; the longitudinal (axial) chromatic aberration and the lateral (transverse) chromatic aberration. We’ll talk about them separately now to understand what the differences actually are.

Figure 1. The cellophane wrappers on packs of note cards shows the left meets all three acceptance criteria whereas the right meets only criteria one and two. In this circumstance, the wrinkles are not precluding a good barcode reading. But what if the wrinkles were in a different place in the next pack on the line?

Perhaps no other aspect of vision system design and implementation has consistently caused more delays, cost overruns, and general consternation than lighting. Historically, lighting often was the last aspect specified, developed, or funded, if at all. This approach was not entirely unwarranted, as until recently there was no real vision-specific lighting on the market, meaning lighting solutions typically consisted of standard incandescent or fluorescent consumer products, with various amounts of ambient contribution.

Longitudinal aberration is typical of long focal lengths and wide prime lenses. If you spot some kind of longitudinal aberration, you can easily minimise it by stopping down your aperture.

Figure 11. The motor oil bottle on the left is illuminated with a red 660 nm ring light. On the right, the bottle is illuminated with a 360 nm UV fluorescent light.

How to fixchromaticaberration

Effective application of dark field lighting relies on the fact that much of the light incident on a mirrored surface that would otherwise flood the scene as a hotspot glare, is reflected away from rather than toward the camera. The relatively small amount of light that is reflected back into the camera is what happened to catch an edge of a small feature on the surface, satisfying the “angle of reflection equals the angle of incidence” equation (see Figure 21 for another example).

At this point, all you have to do is tick the “Remove Chromatic Aberration” box, and let the software do everything for you.

Dark field lighting can be subdivided into circular and linear, or directional types, the former requiring a specific light head geometry design. This type of lighting is characterized by low or medium angle of light incidence, typically requiring close proximity, particularly for the circular light head varieties (Figure 19).

Partial bright field lighting is the most commonly used vision lighting technique, and is the most familiar lighting used every day, including sunlight. This type of lighting is distinguished from full bright field in that it is directional, typically from a point source and, because of its directional nature, it is a good choice for generating contrast and enhancing topographic detail. It is much less effective, however when used on-axis with specular surfaces, generating the familiar “hotspot” reflection.

Chromaticaberration

Dispersion also has something to do with chromatic aberration: the refracting index (a number that tells you how fast the light is able to pass through a certain material) of the lens elements is different for various colours. For example, blue wavelengths of light are able to pass faster through a lens compared to red wavelengths of light. This difference in terms of refracting index affects the focus in your photos, and that’s when you get some of those annoying colourful halos along the edges.

The following sequence of lighting analysis assumes a working knowledge of lighting types, camera sensitivities, and optics and familiarity with illumination techniques and the four cornerstones of vision illumination. You can use it as a checklist, but it is by no means comprehensive. It does, however, provide a good working foundation for a standardized method that you can modify and/or expand for the inspection’s requirements.

Most manufacturers of vision lighting products also offer lights with various combinations of techniques available in the same light, and at least in the case of LED-based varieties, each of the techniques may be individually addressable. This circumstance allows for greater flexibility and also reduces potential costs when many different inspections can be accomplished in a single station rather than two. If the application conditions and limitations of each of these lighting techniques, as well as the intricacies of the inspection environment and sample/light interactions are well understood, it is possible to develop an effective lighting solution that meets the three acceptance criteria.

TCA won’t be minimised if you stop down the aperture of your lens, since it doesn’t depend on it. You can recognise this kind of chromatic aberration as it appears along high contrast edges, where perhaps there is a sharp transition between bright and dark areas. It is typical of short focal lengths.

Fluorescent, quartz halogen, and LED are the most widely used lighting types in machine vision, particularly for small- to medium-scale inspection stations. Metal halide, xenon, and high-pressure sodium are more typically used in large-scale applications or in areas requiring a very bright source. Metal halide, also known as mercury, is often used in microscopy because it has many discrete wavelength peaks, which complements the use of filters for fluorescence studies. A xenon source is useful for applications requiring a very bright strobe light. Figure 2 shows the advantages and disadvantages of fluorescent, quartz halogen, and LED lighting types and relevant selection criteria, as applied to machine vision. For example, whereas LED lighting has a longer life expectancy, quartz halogen lighting may be the choice for a particular inspection because it offers greater intensity.

The more common colours of chromatic aberration that occur are magenta (hence purple fringing) or green, but they can be also blue, red, yellow, etc.

At this point you should know a lot more about chromatic aberration, from what it is to how to avoid it, both on the field and then in post-production. Chromatic aberration can be an annoying problem, but if you see that some of your pictures are affected by it, don’t panic! It’s quite easy to control them on the field and even easier to remove them with Lightroom.

As might be expected, the Geometry Independent Area implies that relatively flat and diffuse surfaces do not require specific lighting, but rather any light technique may be effective, provided it meets all the other criteria necessary, such as working distance, access, brightness, and projected pattern.

Figure 10. Candy pieces are imaged under (a) white light and a color CCD camera, (b) white light and a black and white camera, (c) red light, lightening both the red and yellow and darkening the blue, (d) red and green light, yielding yellow, lightening the yellow more than the red, (e) green light, lightening the green and blue and darkening the red, (f) blue light, lightening the blue and darkening the others.

Figure 5. The left image shows nyloc nuts with a UV ring light, but flooded with red 660 nm “ambient” light. The goal is to determine nylon presence/absence. Given the large ambient contribution, it is difficult to get sufficient contrast from the relatively low-yield blue fluoresced light from the sample. The right image has the same lighting, except a 510 nm short pass filter was installed on the camera lens, effectively blocking the red “ambient” light and allowing the blue 450 nm light to pass.

How a sample’s surface interacts with task-specific and ambient light is related to many factors, including the gross surface shape, geometry, and reflectivity as well as its composition, topography, and color. Some combination of these factors determines how much light, and in what manner, is reflected to the camera, and subsequently available for acquisition, processing, and measurement. For example, a curved, specular surface, such as the bottom of a soda can (Figure 6), reflects a directional light source differently from a flat, diffuse surface such as copy paper. Similarly, a topographic surface, such as a populated PCB, reflects differently from a flat but finely textured or dimpled (Figure 7) surface, depending on the light type and geometry.

About the author:  Leonardo Papèra is a landscape photographer based in Italy. You can find more of his work on his website or by following him on Instagram.

The properties of IR light can be useful in vision inspection for a variety of reasons. First, IR light is effective at neutralizing contrast differences based on color, primarily because reflection of IR light is based more on sample composition rather than color differences. You can use this property when less contrast, normally based on color reflectance from white light, is the effect you want (see Figure 12).

Chromatic aberration is known also as “purple fringing” or more generally as “colour fringing”. As these names suggest, it’s because you can easily recognise it as thin, colourful lines along the high-contrast edges of the frame, or in the out-of-focus parts of it.

You’ll have to do some practice if you want to remove chromatic aberration manually, but the results can be quite amazing! I always recommend that you try auto-correction first, as it can be a real time-saver!

Figure 4. Camera Sensor Absolute Quantum Efficiency Versus Wavelength (The bar at the bottom denotes approximate human visible wavelength range.)

Chromatic aberration is one of those subjects that most photographers have heard of but only a small fraction truly understand. It can be quite annoying sometimes, and if you don’t know how to remove or minimise the impact of chromatic aberration in your photos, the loss in terms of quality could be huge. So what is this phenomenon and how can you actually control it?

Do you have any tips or tricks for avoiding or removing chromatic aberration? Have you tested your own lenses for chromatic aberration? How did it turn out? Leave a comment below!

Materials reflect and/or absorb various wavelengths of light differentially, an effect that is valid for both black and white and color imaging space. Like colors reflect and surfaces are brightened; conversely, opposing colors absorb and surfaces are darkened. Using a simple color wheel of warm versus cool colors (Figure 8), you can generate differential contrast between a part and its background (Figure 9), and even differentiate color parts, given a limited, known palette of colors, with a black and white camera (Figure 10).

Well, allow me to cheer you up a little bit: it is possible to learn to control and possibly avoid chromatic aberration altogether! There are a few techniques you can use which will at least minimise the impact of chromatic aberration on your images, so let's take a look at what they are.

Using small apertures can also mean that you'll need to slow your shutter speed or raise the ISO, but it will help you reduce chromatic aberration in the long run.

With respect to the lighting environment, there are two aspects to evaluate when determining the optimal lighting solution: (1) immediate inspection environment and (2) sample/light interaction

Now, you have two options to move forward: the first one is to use the “Fringe Colour Selector” – that eye-drop that you see right on the upper left side of the 'Defringe' window. You can select the affected areas and Lightroom will try to remove the CA from those specific areas by changing some of the parameters.

This guide aims to present a standard method for developing sample-appropriate lighting rather than dwell on theoretical treatments. It details relevant aspects in a practical framework, with examples, where applicable, from the following areas:

So, let’s repeat the process as in the photo above, but instead of taking advantage of Lightroom’s magic, we’ll try to remove chromatic aberrations on our own. Why? Well, because sometimes the software auto-correction is not able to completely solve the problem, so it’s good to know how to deal with it on your own in case you are stuck with CA.

I’ll start by saying that you should shoot in RAW, not just for the chromatic aberration, but for a million other reasons. If you are passing your pictures through post processing software after you have snapped them, then you shouldn’t even consider shooting in JPEG. The only way to have a usable base shot for editing is by shooting in RAW.

So we've talked about how you can minimise longitudinal chromatic aberration by closing down the aperture of your lens. What about the transverse chromatic aberration? Well, there’s an easy trick to avoid that too! If you paid attention before, I wrote that TCA won’t appear in the middle of the frame but it will be visible just around the edges of the picture. As such, if you place your subject in the centre of the frame, it won’t be affected by chromatic aberration.

The following figures illustrate the differences in implementation and result of circular directional (partial bright field) and circular dark field lights on a mirrored surface.

Diffuse dome lights are effective at lighting curved, specular surfaces, commonly found in the automotive industry, for example. On-axis lights work in a similar fashion for flat samples and are particularly effective at enhancing differentially angled, textured, or topographic features on relatively flat objects. To be effective, diffuse lights, particularly dome varieties, require close proximity to the sample.

Figure 14. On the left, a transparent plastic six-pack can holder is shown with a red back light. The right shows the same, except for the addition of a polarizer pair, showing stress fields in the polymer.

To test how well (or bad) your lenses are doing against chromatic aberration, just Google “Chromatic Aberration Test Chart” and download one of the free models. Next, open it in full screen mode (and at max brightness) on your PC. You can also print it and take a few shots with the lens you want to test.

Check out the before and after: the shot above here is without the “Remove CA” box ticked. As you can see, the picture clearly shows some fringing. Meanwhile, in the photo below, there is no visible chromatic aberration at all. Magical, right?

Additionally, Figures 3 and 4 illustrate several other relevant points to consider when selecting a camera and light source:

Playing with your aperture will be particularly helpful in minimising the effect of longitudinal chromatic aberration. How? Well, easy: by stopping down at least one or two stops (or more) from the widest aperture of your lens.

Generally, zoom lenses are most affected by chromatic aberration at their minimum and maximum focal lengths. Let’s say that you have a 24-70mm lens – both at 24mm and at 70mm is where your lens will show more chromatic aberration.

Honestly, Lightroom does an excellent job at dealing with chromatic aberrations. You'll see that 99% of the time, you won’t even need to know how to manually correct them.

There are exceptions, however, to this general rule. For example, a 700 nm short pass filter, otherwise known as an IR blocker, is standard in color cameras because IR content can alter the color accuracy and balance, particularly of the green channel. Figure 5 illustrates how the use of a pass filter can block ambient light very effectively, particularly when the light of interest is low-yield fluorescence.

Figure 9. A mail stamp imaged under (a) red light, (b) green light, (c) blue light, generating less contrast than green, (d) white light, generating less contrast than either blue or green. White light contrasts all colors, but it may be a contrast compromise.

This level of in-depth analysis can and often does result in seemingly contradictory directions, and a compromise is necessary. For example, detailed sample/light interaction analysis might point to the use of the dark field lighting technique, but the inspection environment analysis indicates that the light must be remote from the part. In this instance, a more intense linear bar light(s) oriented in dark field configuration may create the contrast you want, but perhaps require more image post-processing.

We've talked about how to avoid chromatic aberrations while shooting on the field, but what about if you weren't able to reduce them for some reason and you still have some annoying colour fringing? Don’t worry, Adobe Lightroom will take care of it!

Consider not only a source’s brightness but also its spectral content (Figure 3). Microscopy applications, for example, often use a full-spectrum quartz halogen, xenon, or mercury source, particularly when imaging in color; however, a monochrome LED source is also useful for a black and white CCD camera, and also now for color applications, with the advent of “all color—RGB” and white LED light heads.

Particularly when used to block specular reflections on samples, any use of polarization filters comes with inherent compromises. The images depicted in Figure 15 demonstrate moderately effective and highly effective use of polarization filters specifically for blocking glare. In samples depicted in Figure 15a–c, you see that glare reflected from a curved surface, such as this personal care product bottle, can be controlled but not entirely eliminated (see Figure 15b center area). This is true because multiple reflection directions are produced on the curved surface from a directional light source, and polarization filters cannot block all vibration directions simultaneously, thus always leaving some areas vignetted. In this case, a more effective approach to glare control, given the flexibility to do so, is to reconsider the lighting geometry. By simple moving the light from a coaxial position around the lens to a relatively high angle, but off-axis position, you can completely eliminate all specular reflection. Conversely, for the relatively flat and planar jar top surface depicted in Figure 15d–e, you can largely remove the specular glare, producing a clear image for inspection. However, a caveat for using dual polarizers is that they can reduce the allowable light considerably—up to 2 ½ f-stops in the case of the jar top example, which could be detrimental for high-speed, light-starved inspections.

Sample composition can greatly affect what happens to task lighting impinging on a part. Some plastics may transmit light only of certain wavelength ranges and are otherwise opaque; some may not transmit, but rather internally diffuse the light; and still some may absorb the light only to re-emit it at the same wavelength or at a different wavelength (fluorescence). Fluorescence labels and dyes are commonly used in inks for the printing industry as well (Figure 11).

Illumination techniques comprise back lighting, diffuse (also known as full bright field) lighting, bright field (actually partial bright field or directional) lighting, and dark field lighting.

Lateral (or transverse) chromatic aberration (TCA) occurs when the different wavelengths of light are projected in different positions along the focal plane. Unlike the longitudinal chromatic aberration, TCA won’t appear at the centre of the frame but it will increase along the borders of the frame.

There are two options that will help you to remove chromatic aberrations in Lightroom: the first one is to make the software do its magic and go for the automatic correction, while the second one is to work on it by yourself with the manual correction.

Check for colourful halos around the edges of subjects in your images. It may be chromatic aberration. Photo by: 'Leonardo Papèra'.