Infrared radiation imaging

Imaging > Imaging Techniques > Infrared radiation imaging

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Infrared Imaging

Infrared Imaging Terminology
Wavelength Range	Recording Medium	Techniques	Image Produced
700 - 1000 nm -NIR (Near Infrared) -IR-A (CIE1 700-1400 nm)	Film	-Reflected infrared photography -Transmitted infrared photography -False-color infrared photography	-Reflected infrared photograph -Transmitted infrared photograph -False-color infrared photograph
	Digital capture/Electronic systems	-Reflected infrared digital photography -Transmitted infrared digital photography -False-color infrared digital photography	-Reflected infrared digital photograph -Transmitted infrared digital photograph -False-color infrared digital photograph
1000 - 3000 nm -SWIR (Short Wave Infrared) -IR-B (CIE 1400-3000 nm)	Electronic systems	-Infrared Reflectography (IRR)2 -Infrared Transmittography3	-Infrared reflectogram -Infrared transmittogram
3000 - 20000 nm -MWIR (Mid Wave Infrared, 3000-5000 nm) -LWIR (Long Wave Infrared, 7000-14,000 nm) -VLWIR (Very Long Wave Infrared, 12,000-30,000 nm) -IR-C (CIE 3000-1,000,000 nm)	Electronic systems	-Thermal imaging -Thermography	-Thermal image -Thermogram

This table is adapted from Dan Kushel’s Infrared Imaging Terminology Table (2015 version, SUNY Buffalo State course materials).

Introduction

What is IR imaging?

Infrared (IR) imaging is a technique used to capture images based on infrared radiation, which is invisible to the human eye. This type of imaging relies on the heat emitted by objects, allowing for the visualization of details not visible in regular light. In the context of conservation, IR imaging is especially valuable for revealing underlying layers of artworks, such as preparatory sketches, underdrawings, or previous restoration interventions, by penetrating through surface layers like paint or varnish. It is used in art conservation to analyze and understand the condition of artworks, identify historical techniques, and support restoration efforts without causing any physical damage.

What is IR radiation?

Infrared (IR) radiation is a type of electromagnetic radiation that lies between the visible light spectrum and microwaves. It has longer wavelengths than visible light, typically ranging from 700 nanometers (nm) to 1 millimeter (mm). While it is invisible to the naked eye, it is perceived as heat. All objects emit infrared radiation based on their temperature, with warmer objects emitting more IR radiation. In imaging applications, IR radiation is captured to produce visual representations, making it possible to see features and patterns not detectable by visible light alone.

History of IR imaging

Since the discovery of infrared radiation by Frederick William Herschel in 1800, numerous forms of infrared detection have been developed. These optical infrared detectors fall into two categories: (1) photon detectors, and (2) thermal detectors.  Over the years, advancements in infrared detector technology have been primarily driven by the military, with the seeds of modern infrared detection being developed during World War II.  

The conservation field began using infrared detection technology to image artistic works in the 1930s, when infrared film first became commercially available.  Conservators have used a series of different IR detection devices throughout the 20th and 21th centuries, including infrared film, infrared vidicon television monitor systems, and modified DSLR cameras.  Infrared light and visible light interact with objects differently, absorbing and scattering wavelengths in different ways, which allow for certain aspects of artistic works to be imaged during infrared imaging that are not available to the naked eye. Conservators capitalize on this, and use IR imaging for many purposes, including: to expose underdrawings, to detect changes made during the construction or after the construction of an artistic work, to clarify obscured inscriptions, and to aid in the identification of materials.

Sir William Herschel

Frederick William Herschel discovered infrared radiation on February 11th,1800, while conducting an experiment designed to measure the temperature of different colors of light.  In this experiment, Herschel used a prism to divide sunlight into its constituent colors, and then measured the distribution of energy in each color using a thermometer. The highest temperature that he measured in this experiment was just beyond the range of red visible light, which led him to infer that there must be a form of radiation beyond the red visible spectrum, which is now known as infrared radiation.

1.3.2  Film

In 1873, H. W. Vogel discovered that adding certain dyes to silver halide emulsions extended the spectral imaging capacity of photography, which opened the door to infrared photography. Professor Robert Williams Wood published the first infrared photographs in the Photographic Journal of the Royal Photographic Society in October of 1910.  However, the process of sensitizing photographic plates to red light was difficult, so widespread use of infrared photography did not take off until the 1930s, when infrared film became commercially available.  Conservators began using infrared film to image artistic works in this time period, but faced many challenges, including the need for specialized equipment and techniques.  Furthermore, there was no fast or easy way to tell whether a given artistic work was worth imaging using IR, which made this a laborious, time-intensive, and sometimes fruitless process.  

1.3.3 Vidicon

In the mid-1960s, vidicon video tubes were developed, which allowed infrared images to be viewed on TV monitors.  This technology was introduced to the conservation field by J.R.J. Van Asperen De Boer, who proposed using infrared vidicon television systems to perform infrared reflectography of paintings.  This technology made the reflectogram immediately visible on a television monitor, which could then be photographed for posterity.  The system’s video recorder could also store the imaged content.  Vidicon systems allowed most paintings to be examined in situ. Vidicon systems allowed imaging areas that ranged in size, were effective at revealing underdrawings that were obscured by certain pigments, and had improved IR sensitivity.

1.3.4 Digital Imaging

Solid state sensors were introduced to the conservation field in the 1990s.  These capture devices often have improved IR sensitivity, but have poor spatial resolution, which require mosaicking multiple images in order to get a comprehensive result.  Following the development of digital photography, physical IR film that was used in conservation contexts was eventually discontinued. Now, many conservators use modified DSLR cameras to perform IR imaging.  These DSLR cameras have inherent sensitivity limited to the near infrared spectrum, so are a useful and high resolution option for IR imaging.

Infrared Imaging Techniques

Reflected infrared digital photography

Reflected infrared digital photography (IR) is an imaging technique that can be used in the investigation of cultural heritage items and utilizes reflected infrared radiation between the wavelengths of ~ 700 - 1100 nm. Historically, reflected infrared photography included film formats, but today it is commonly carried out with a modified commercial camera (e.g. DSLR) with a silicon-based sensor (CMOS or CCD). The item is exposed to infrared radiation and the response in the infrared region is captured in the resulting image, which is called a reflected infrared photograph.

Reflected infrared photography can be used to reveal features hidden to the naked eye, as materials respond differently to IR radiation than they do to visible light. Reflected infrared photography can be used to visualize subsurface layers when pigments closer to the surface are transparent to IR radiation, such as underdrawings in paintings. Pigments that behave similarly in the visible region (i.e. appear the same color) can be distinguished from each other with IR photography if they exhibit unique behaviors in the IR region.

Equipment

Cameras

Modified cameras

Because most DSLR sensors (CMOS and CCD) are sensitive to a spectral range of ~ 350 - 1100 nm, these cameras have the capacity to record information in the near infrared (~ 700 - 1100 nm). However, internal camera filtration and photo-electric processing during image capture are built into most commercially available cameras, and require modification to acquire reflected infrared digital photographs. Pre-modified cameras are available for purchase.

Camera conversions

The process of converting a camera for near infrared photography involves removing an internal filter (also known as a hot mirror) that blocks infrared. The removal of this filter allows the detection of a wider range of wavelengths. The infrared blocking filter is built into most commercial cameras for more accurate color rendering. Thus, removing this filter allows the camera to see beyond what the human eye can see.

As part of the modification, the user must determine if they wish to replace the internal filtration with a specific bandpass or edge filter (permanent) or replace the filtration with a full spectrum filter (UV-Vis-IR). A full spectrum modification gives the user the option to change the filtration and thus sensitivity of the camera in the UV, IR, or visible regions. Many conservators find the full spectrum modification the most versatile for a variety of imaging techniques.

It is important to note that some hot-mirrors are coupled with anti-aliasing, or low-pass filtration. Anti-aliasing filters reduce moire in conjunction with the color filter array.

Professional service or pre-converted cameras

Converted cameras for near infrared photography can be purchased from multiple suppliers, including https://www.lifepixel.com/, https://kolarivision.com/, and https://maxmax.com/. Professional mail-in conversion services for commercially available cameras are also available from these suppliers. It is possible to convert a commercially available camera with skill and acceptance of risk.

DIY

• Liveview/soldering
• Internal replacement filter
• External filtration
• Auto-focus offset/recalibration

Full spectrum vs IR dedicated

Full spectrum cameras and IR dedicated cameras serve distinct purposes in infrared photography and imaging. Full spectrum cameras are modified cameras that capture a wide range of wavelengths, including ultraviolet (UV), visible, and infrared (IR) light. By removing the internal UV/IR cut filter, full spectrum cameras can be used for various types of photography depending on the external filters placed on the lens. This versatility allows photographers to capture UV, visible, or IR images by simply swapping filters, making them highly adaptable for a range of applications. However, this also means that a full spectrum camera must be precisely filtered for the desired imaging wavelength, and improper filtering can lead to mixed spectral data.

In contrast, IR dedicated cameras are specifically modified to capture only infrared light, with built-in filters that block visible and UV wavelengths, allowing only infrared wavelengths to reach the sensor. These cameras are optimized for infrared photography, delivering sharper, clearer IR images without the need to switch filters. IR dedicated cameras are ideal for professionals focused exclusively on infrared work, such as in art conservation or scientific research, where precise and consistent IR imaging is essential. While less versatile than full spectrum cameras, IR dedicated cameras offer superior performance and image quality in the infrared range.

Apochromatic Lenses

Due to infrared wavelengths being longer than visible in the electromagnetic spectrum, apochromatic lenses are often used. An apochromatic lens allows the user to focus the lens using visible light to retain image sharpness while capturing in the infrared region. If an apochromatic lens is not used, the lens will have to be refocused when switching between capturing visible and infrared images.

These two apochromatic lenses are commonly referenced as high quality for infrared imaging: the JenOptik UV-VIS-IR 60mm 1:4 and the Nikon UV-Nikkor 105mm f/4.5s

This list, released by Kolari Vision, lists lenses with bad hot spots for infrared photography:

https://kolarivishttps://kolarivision.com/articles/lens-hotspot-list/ion.com/articles/lens-hotspot-list/

Filters for Reflected Infrared Digital Photography (IR)

Filters play a crucial role in reflected infrared digital photography, allowing for the selective capture of infrared light while blocking visible light. The use of specialized IR filters ensures that the camera sensor only detects the IR radiation reflected off the object being photographed, providing a clear view of details not visible in standard photography.

IR-Pass Filters (Camera-Side Filters)

IR-pass filters, also known as infrared filters, are mounted on the camera lens to block visible light (typically below 700 nm) while allowing infrared light (wavelengths above 700 nm) to pass through. These filters are essential for isolating the reflected IR radiation, enabling the camera to capture images based solely on the infrared reflectance of the subject.

There are different types of IR-pass filters based on the specific wavelength they allow to pass. Common filter types include:

• 720 nm Filter: This filter blocks visible light and starts transmitting light from around 720 nm onwards. It is one of the most commonly used filters for general IR photography, as it provides a good balance between near-IR and visible light sensitivity, offering a more classic IR look with dark foliage and bright skies.
• 850 nm Filter: This filter allows only infrared light above 850 nm to reach the sensor, providing a more pronounced infrared effect. This filter is ideal for capturing more dramatic, high-contrast infrared images, with greater emphasis on the IR reflection and minimal visible light interference.
• 950 nm Filter: This filter transmits infrared light from 950 nm and above, resulting in very deep infrared images. This filter is less commonly used for general IR photography but can be useful in highly specialized applications that require minimal visible light interference.

IR-pass filters are crucial for ensuring that the captured images consist predominantly of infrared light, making it possible to study the reflective properties of various materials. This technique is especially useful in art conservation, archaeology, and material science, where details hidden under the surface can be revealed through reflected IR photography.

UV/IR Cut Filters (Light Source-Side Filters)

UV/IR cut filters are used on the light source in reflected infrared photography to ensure that only visible light is emitted if needed in combination with reflected IR techniques. In some applications, these filters prevent unwanted ultraviolet (UV) and infrared light from reaching the object, allowing the focus to remain on the interaction of visible light with the subject. This combination of visible light and reflected IR imaging can provide complementary data.

Although UV/IR cut filters are more commonly used in reflection imaging with visible light, they may sometimes be employed in controlled setups to prevent unwanted IR contamination from the light source, particularly in complex workflows that combine multiple imaging techniques.

Light Sources for Reflected Infrared Digital Photography (IR)

In reflected infrared digital photography, the choice of light source is critical for generating high-quality images. The light source needs to emit sufficient infrared radiation to reflect off the object being photographed, allowing the camera to capture the reflected IR light. Common light sources for IR photography include natural light, incandescent lamps, and specialized infrared lamps.

Natural Light (Sunlight)

Natural sunlight is a widely used light source for reflected infrared photography due to its abundance of infrared radiation. Sunlight emits a broad spectrum of electromagnetic radiation, including a significant amount of IR light, particularly in the near-infrared range (700 nm to 1200 nm). Outdoors, reflected IR photography often utilizes the sun as the primary light source to capture landscape or architectural features.

The advantages of using sunlight include its availability and the fact that it provides a broad range of infrared wavelengths, which allows for versatile imaging. However, the intensity of sunlight can vary depending on weather conditions and time of day, making it less predictable for controlled or indoor settings.

Incandescent Lamps

Incandescent lamps are commonly used in reflected infrared photography due to their ability to emit a broad spectrum of light, including a strong IR component. These lamps, particularly tungsten-halogen bulbs, emit significant amounts of infrared radiation in addition to visible light. Incandescent lamps are particularly useful in indoor environments where sunlight is not available, and the IR output can be controlled.

One downside of incandescent lamps is that they also emit visible light, which must be filtered out using an IR-pass filter on the camera. Despite this, incandescent lamps remain a popular choice because they are widely available and affordable, and they produce sufficient infrared radiation for most photography applications.

Infrared Lamps

For more specialized applications, infrared lamps are often used as a dedicated light source for IR photography. These lamps are designed to emit primarily infrared radiation, minimizing the amount of visible light produced. Infrared lamps, such as IR LEDs and tungsten-halogen IR bulbs, provide consistent, controlled IR illumination, making them ideal for indoor and laboratory settings where precise IR reflectance measurements are required.

• Tungsten-Halogen IR Bulbs: These bulbs emit a strong infrared spectrum and are commonly used in art conservation and scientific studies. They provide a stable IR output and can be used to highlight specific areas of interest in an object.
• IR LEDs: These light-emitting diodes are designed to emit in the infrared spectrum, typically around 850 nm or 950 nm. IR LEDs offer focused IR illumination and are often used in scientific and industrial IR photography setups where precision and control are required.

Fluorescent Lamps

Fluorescent lamps can also emit some infrared radiation, though their IR output is generally weaker compared to incandescent and halogen lamps. Fluorescent lamps are not the most common choice for infrared photography, but they may be used in certain situations where only moderate IR output is needed. Since they emit more visible light than infrared, additional filtering is usually required.

Infrared Lasers

In very specialized applications, such as scientific experiments or material studies, infrared lasers may be used as a light source. These lasers emit highly focused, monochromatic infrared light at specific wavelengths, providing unparalleled control over the illumination. Infrared lasers are not commonly used in standard IR photography but are invaluable in precise experiments where exact wavelength control is necessary.

Calibration in Reflected Infrared Digital Photography (IR)

Calibration is a critical step in reflected infrared digital photography, ensuring the accuracy, consistency, and reliability of the images captured. Proper calibration helps adjust the camera, filters, light sources, and processing workflow to account for the specific properties of the materials being photographed and the environmental conditions of the shoot. By setting a standardized baseline for capturing IR images, calibration ensures that the infrared reflectance data is accurate, comparable, and useful for analysis in applications such as art conservation, archaeology, material science, and scientific research.

Infrared Reflectography

Infrared reflectography is a technique that uses wavelengths in the infrared range of the electromagnetic spectrum to penetrate through opaque paint layers and reveal otherwise invisible elements of the composition. Infrared light is absorbed by carbon-rich materials and reflected back from light-colored elements such as a white ground. Compared to reflected infrared photography, detector systems record a wider range of wavelengths – in different infrared spectral bands from 900 nm to 2500 nm – allowing for even greater penetration of different colors. As most cameras are relatively low resolution, infrared captures are acquired in small segments while scanning the surface of an artwork. These are digitally assembled to produce a full picture called an infrared reflectogram mosaic.

Silicon based CCD and CMOS sensors common in most commercial cameras will not be able to detect wavelengths beyond ~1000 nm. As such, a specialized camera with a different type of detector or sensor (eg: vidicon tubes, InGaAs) will be required.

Example Uses

Introduced in the late 1960's, it was initially successful primarily in revealing underdrawing in Early Netherlandish painting. Increasingly sophisticated sensors using the full range of the infrared spectrum with higher sensitivity and better resolution are now able to reveal other features of a painting, such as pentimenti and underpainted sketches, even on paintings with a darker ground.

Camera/Sensor Type	Wavelength Range (nm)	Examples	Additional Notes
Vidicon	Up to 2000nm	Hamamatsu Vidicon C2400	Fell out of use due to stitching process, sensors are still good
InGaAs sensors	900nm-1700nm	FLIR SC2500-NIR, Osiris and Apollo Cameras by Opus Instruments
PtSi sensors	1200nm-2500nm	Kodak 310-21X, Mitsubishi M600
InSb sensors	1000nm-3500nm (??)	Santa Barbara Focalplane SBF187
Commonly used filters	-	Astronomy J, H, and K filters

Transmitted Infrared Imaging

Transmission Infrared (IR) Imaging is a non-destructive analytical technique used in various fields, including art conservation, material science, and biomedical research, to visualize and analyze the internal structure and composition of objects. Unlike reflection-based IR imaging, where the infrared light is directed at the surface and the reflected energy is captured, transmission IR imaging involves passing infrared radiation through an object, allowing for the detection of the material's internal features and composition based on how the radiation is absorbed or transmitted.

Transmission IR imaging is based on the interaction between infrared radiation and the material through which it passes. Different materials absorb IR radiation at specific wavelengths, depending on their chemical bonds and molecular structure. By analyzing the transmitted IR radiation, information about the object's internal layers, hidden features, and material composition can be obtained. This is especially useful for objects that are opaque or multilayered, such as paintings, textiles, or biological tissues.

Applications in Art Conservation

In art conservation, transmission IR imaging is particularly valuable for analyzing artworks and cultural heritage objects. It helps conservators and researchers detect underlying sketches, layers of paint, and varnishes that are not visible to the naked eye. For example, in paintings, this technique can reveal changes made by the artist during the creative process or prior restorations that may affect the artwork's historical significance or structural integrity.

A famous example of transmission IR imaging in art conservation is its use in the study of Leonardo da Vinci’s works, where underlying drawings and compositional changes were discovered, offering insight into the artist’s methods and creative process.

Equipment

Transmission IR imaging typically involves an infrared light source that emits radiation across a broad range of IR wavelengths (from near-infrared to mid-infrared, depending on the application). The object is positioned between the light source and a sensor, which captures the transmitted IR radiation. The sensor can be a specialized IR camera or detector, such as:

• InGaAs sensors for near-infrared detection (900 nm - 1700 nm)
• PtSi sensors for extended wavelength ranges (1200 nm - 2500 nm)
• InSb sensors for even broader ranges (up to 3500 nm)

The captured image is processed to identify patterns of absorption or transmission, which indicate different materials or internal structures.

Limitations and Considerations

While transmission IR imaging is a powerful tool, it is not without limitations. The method is less effective for highly opaque materials, as IR radiation may not penetrate deeply enough to reveal hidden features. Additionally, the technique requires careful calibration to ensure accurate interpretation of the transmitted radiation.

The effectiveness of transmission IR imaging also depends on the wavelength of the IR radiation used and the material properties of the object being examined. For instance, organic materials, like canvas or wood, may transmit IR radiation differently than metallic surfaces or pigments used in painting.

Infrared Luminescence

2.4.1 Visible Induced Infrared Luminescence (VIL)

Visible Induced Infrared Luminescence (VIL), is an imaging technique used in the mapping of certain pigments by illuminating an objects with a excitation wavelength in the 400-540nm region, which causes an emission (luminescence) in the near infrared region (NIR) 700-1100nm. A change occurs between the excitation and the emission, when the wavelength gets longer and there is a decrease in intensity. This change is called Stokes Shift. This luminescence is most visible with cadmium and Egyptian blue pigments. Although Visible Induced Infrared Luminescence has its roots in analogue film photography, advances in both digital sensors and LED sources have made this imaging method more accessible in recent years.

Common uses:

Cadmium pigments are commonly imaged using Visible Induced Infrared Luminescence. It is useful to identify and map cadmium pigments, applied by the artist or as inpainting, in artwork and artifacts that were made after 1817 (reference: https://www.winsornewton.com/na/articles/colours/spotlight-on-cadmium-yellow/). Cadmium pigments induce intense luminescence.

Egyptian Blue is a pigment that was first used as early as 2500 BCE and was seemingly forgotten by 800 CE (reference: http://www.artinsociety.com/egyptian-blue-the-colour-of-technology.html). Egyptian blue causes intense emission with the Visible Induced Infrared Luminescence imaging method. This technique is extremely useful for identifying and mapping the location of this pigment and is useful even when no pigment is apparent in visible light.

Madder Lake pigments show some luminescence with this imaging method, but are not as intense as Cadmium and Egyptian Blue pigments.

Equipment

Cameras

Modified DSLR

Just like in UV-induced IR luminescence, visible-induced IR luminescence can be captured using modified DSLR cameras that have been adapted to be sensitive to IR radiation. By removing the internal IR cut filter, these cameras can detect the infrared emissions from the material excited by visible light. While affordable and accessible, modified DSLRs may lack the sensitivity of scientific-grade IR cameras but are still useful for many conservation and research purposes.

Sensors

CMOS and CCD sensors are used to detect the emitted IR radiation. These sensors are capable of capturing the faint infrared light emitted after visible light excitation. However, managing sensor noise is essential for producing clear and accurate images. Similar to other luminescence imaging methods, techniques like dark frame subtraction (capturing an image in complete darkness and subtracting the noise) are used to minimize the effects of sensor noise and enhance the visibility of the luminescent features.

Light Sources

Visible Light

The light source used for visible-induced IR luminescence imaging emits light in the visible spectrum (400 nm to 700 nm). Common sources include LED lights, tungsten-halogen lamps, and xenon lamps, which provide the necessary excitation energy to stimulate the materials and produce IR luminescence. The choice of light source depends on the type of material being examined and its luminescence response. For example, specific pigments may respond more efficiently to certain wavelengths of visible light.

Filters

Camera-Side Filters

An IR-pass filter is placed on the camera lens to block out visible light and allow only the infrared radiation emitted by the material to be captured. This ensures that the recorded image represents the luminescence in the IR range rather than any visible light reflections from the source.

Light Source-Side Filters

In cases where a narrow range of visible light is required for excitation, visible-pass filters can be applied to the light source to filter out unwanted wavelengths, allowing only the desired portion of the visible spectrum to reach the object. This helps enhance the luminescence effect by exciting the material with the optimal wavelength of light.

Calibration

Targets & References

As with other imaging techniques, calibration is critical to ensure accurate results. Calibration targets with known luminescence properties under visible light excitation are used to verify that the equipment (camera, filters, and light source) is properly set up and functioning. This helps in accurately interpreting the emitted infrared luminescence.

Behavioral Calibration

Behavioral calibration involves adjusting the system based on the specific luminescence response of the material being studied. Different materials exhibit varying degrees of luminescence under visible light, and calibration is necessary to account for these variations. Calibration ensures consistent and reliable imaging results across different materials and conditions.

Ultraviolet Induced Infrared Luminescence (UVL)

Ultraviolet induced Infrared Luminescence (UVIL) is a relatively new method in imaging. The technique is used for mapping materials by excitation in the Ultra violet region(max 356nm) in order to capture luminescence in the near  infrared region (700-1200nm).  The technique and theory behind UVIL has  derived from Visible Induced Infrared Luminiscence (VIL) as a complimentary technique in multiband imaging.

Common uses:

• UVIL is being used to verify traces of Egyptian blue, madder lakes, and cadmium pigments (see VIL). More recently it has been discovered that UVIL can be used for distinguishing between anatase and rutile titanium oxide. Further research into the use of UVIL is being conducted.

Equipment

Cameras

Modified DSLR

Digital Single-Lens Reflex (DSLR) cameras, commonly used for regular photography, can be modified to capture UV-induced IR luminescence. This modification typically involves the removal of the internal IR cut filter, allowing the camera’s sensor to become sensitive to both UV and IR light. By adding external filters (discussed below), the camera can be used to image only the IR light emitted by the material after UV exposure.

Modified DSLRs are popular because they are more affordable than scientific-grade cameras and can still provide high-resolution images. However, their performance in terms of sensitivity and noise reduction is generally lower than that of professional IR cameras.

Sensors

CMOS/CCD Sensors

Modern cameras, including modified DSLRs, use CMOS (complementary metal-oxide-semiconductor) or CCD (charge-coupled device) sensors to detect light. For UV-induced IR luminescence, these sensors need to be sensitive to IR wavelengths. However, such sensors can be prone to noise, especially in low-light situations. This noise can obscure the faint luminescence emitted by certain materials.

To reduce noise, techniques like dark frame subtraction (capturing a black image with the same exposure settings and subtracting it from the luminescent image) are often used. This helps improve image clarity by eliminating sensor noise unrelated to the luminescent signal.

Light Sources

UV Light

The UV light source is a critical component in UV-induced IR luminescence imaging. UV light sources emit radiation in the UV-A (320-400 nm) or UV-B (280-320 nm) range, which is absorbed by the material and induces the emission of IR radiation. Common UV light sources include UV LEDs and mercury vapor lamps.

To ensure that the UV light source emits only the desired wavelength, UV-pass filters are sometimes applied to the light source, preventing visible or infrared light from interfering with the excitation process.

Filters

Camera-Side Filters

In UV-induced IR luminescence imaging, IR-pass filters are placed on the camera lens. These filters block visible light and UV light, allowing only the infrared radiation emitted by the object to pass through. This ensures that the captured image represents the material's luminescence in the IR range rather than any reflections or stray light from the UV source.

Light Source-Side Filters

UV-pass filters are applied to the light source to ensure that only UV light is used for excitation, blocking out any visible or infrared light from the light source itself. This ensures that the IR radiation detected by the camera originates solely from the luminescence of the material.

Capture Considerations

Exposure

There are some general guidelines provided in (Dyer, Verri, and Cupitt 2013, 61-63, 68). Exposure depends on three camera settings: aperture or f-stop, shutter speed, and ISO (Dyer, Verri, and Cupitt 2013, 61). Dyer et al. recommends starting with an aperture of f/8. Smaller apertures (higher f-stop numbers) can generally be used. The shutter speed can also range from ⅛ sec for continuous light source to 1/200 sec for flashes (Dyer, Verri, and Cupitt 2013, 68). A low ISO is beneficial to reduce the amount of noise captured. A spectrally neutral target that will reflect equal amounts of all wavelengths (Spectralon) is helpful to ensure consistent exposure.

Focus differential for IR vs Visible light

As mentioned in the “Apochromatic Lenses” section, infrared wavelengths are longer than visible wavelengths. Therefore, infrared and visible wavelengths will behave differently (specifically, they will bend at different angles) when they travel through the same photographic lens; this is called chromatic aberration. This results in the different wavelengths having different focal points; this means that the same focus setting will not produce a sharp image for a visible and infrared image. An apochromatic lens can be used to correct this difference. However, if an apochromatic lens is not used, the camera lens will need to be refocused before capturing each photo if switching between infrared and visible reflected light photos.

The live view mode on the camera can be used to focus once the appropriate filters are in place. Tethering the camera to a computer monitor can help when focusing in the live view mode.

These two apochromatic lenses are commonly referenced as high quality for infrared imaging: the JenOptik UV-VIS-IR 60mm 1:4 and the Nikon UV-Nikkor 105mm f/4.5s

This list, released by Kolari Vision, lists lenses with bad hot spots for infrared photography:

https://kolarivishttps://kolarivision.com/articles/lens-hotspot-list/ion.com/articles/lens-hotspot-list/

Filters

UV/Vis blocking filter

To capture infrared photographs, you must remove (or cut) the UV and visible light. To capture visible light photographs, you must cut the UV and infrared. These can be accomplished by placing a filter between the recorded scene and the sensor.

With a “full spectrum” conversion, there is no filter to block UV or visible light from the sensor so a filter would be added to the lens to allow only infrared to reach the sensor. This allows the user to change filters to choose desired wavelengths to capture. Alternatively, during the conversion process, such a filter can be added to the sensor, some may find this simpler as you do not need to deal with external filters but this restricts the ability to change filters to capture different ranges of wavelengths.

The choice of filter is key in being able to capture the information that you want. There are multiple types of filters including long pass filters that allow all wavelengths longer than a given point or a band-pass filter that allows through (passes) a range of wavelengths and blocks wavelengths above and below the band.

These filters are available from many of the companies that offer camera conversions. Peca also offers kits with multiple filters at http://www.ir-uv.com/

Tunable filters

There are liquid crystal tunable filters (LCTF). They are usually expensive, complicated, and often only work with smaller sensor cameras. They do allow for extraordinary flexibility.

Filter adaptor rings

Because filters can be expensive, it is advisable to purchase filters with the diameter which matches your largest lens. You can then use step-up rings to adapt the smaller diameter lenses you have to the larger filter size. Brass adapter rings are less susceptible to cross-threading than aluminum ones.

IR Calibration

There is no real standard calibration in capturing infrared images, but it is important to understand the use and needs of your images. If you wish to be able to compare one image to another image, having a set of agreed standards is critical.

In order to compare two images, it is critical to know that they were captured in a consistent manner. For example, an object that is naturally dark in the IR region could be subjectively exposed lighter to make it more visible compared to an object that is lighter in the IR region and imaged with a normal exposure; this could lead to an incorrect interpretation that the two objects reflect similar amounts of IR. To that end, it is critical that we calibrate our imaging systems for some level of consistency. If a calibrated image is too dark to see or illustrate a point, it is often possible and advisable to create a derivative image that is manipulated to show the desired aspect. However it is much more difficult and often impossible to bring an image that was made subjectively into a known standard.

There are many variables at play when trying to capture consistent images. For example, different sensors have varying sensitivities to different wavelengths and different lights have varying power at different wavelengths which lead to variation even with exact same camera settings. Additionally, different power settings on lights and the distance and angle of the lights to the object will vary the exposure. The first step is to try to be as consistent as possible in as many of these variables, but that is not always possible and even with everything kept consistent, the power out of a light might vary over time, so some form of calibration is required.

One way to ensure consistency is using a white target. A white target is valuable to ensure consistent exposure and set a white point. It is important to note that while a white target made for photography like the white patch X-Rite/MacBeth Color Checker or a Kodak White/Gray card may be better than nothing, they are designed for visible wavelengths and it is not ensured that they will reflect the same percentage of energy at a given IR wavelength as they do at a visible wavelength. They are cheap and available and for SWIR they are better than nothing but are not entirely reliable particularly when trying to compare images captured at different wavelengths. A Spectralon target [purchase link: https://www.edmundoptics.com/p/white-reflectance-standard-includes-99-standard/11120/ ] is designed to reflect energy evenly from 250 nm to 2500 nm and will be extremely consistent; they also often come with specific reference measurement data for the target. PTFE (teflon) targets are also generally good and may be found cheaper [need to reference paper to compare effectiveness: B. K. Tsai, D. W. Allen, L. M. Hanssen, B. Wilthan, and J. Zeng, “A comparison of optical properties between high density and low density sintered PTFE,” Proc. SPIE 7065, 70650Y (2008).]

With a white target captured, one can at least adjust exposure/brightness to ensure that is consistent. When setting the exposure, one should reference the CIELAB L* values as the RGB values will vary depending on the gamma of the color profile.

References and resources

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