Quite a few sites on UV photography seem to link to this page. They are welcome to do so, but for several years I did not update the contents of this page. Other pages of my site contain much more relevant information. Therefore, I decided to partly update the contents of this page, and to provide links to other pages on this site. However, if you want to make sure that you did not miss any subsequent additions on UV photography, always check the main index of the photography section.
What is UV
In addition to visible light (i.e., wavelengths ranging from 400 to 750 nm) and infrared, sunlight at ground level contains UV-A ( 400-320 nm), UV-B (320-280 nm) and small amounts of UV-C (280-200 nm) light. Above the atmosphere, UV-D, or vacuum UV, is an important part of the solar spectrum. Virtually no UV-D passes through the atmosphere. At even shorter wavelengths, the UV range grades into the soft-X-ray range.
UV-B is absorbed by most materials normally used to manufacture lenses, and digital camera sensors also possess low sensitivities in these ranges. UV-C is rapidly absorbed by atmospheric oxygen, and at ground level solar UV-C is irrelevant to photography. UV-D is very rapidly absorbed by atmospheric gases, including nitrogen.
How I arrived to UV photography
My early trials with near-UV photography, almost a decade ago, were not much successful, because of very large technical problems that stack up to make the odds almost impossible. Few photographers dealt with -at the time - this very exotic branch of photography, and reliable information was much more difficult to come by.
NIR photography, for initial experimentation, may require only a cheap, easy-to-find filter and a long exposure time (albeit modern digital cameras are much better than older ones at reducing the amount of NIR passing through their built-in filters to uselessly low levels). Getting even entry-level equipment for UV photography, instead, is both expensive and time-consuming. Essentially, cameras, digital sensors, lenses and ordinary filters are all designed to cut out UV light, and you have to swim upstream in order to obtain odd bits and pieces that, by accident or design, work in the opposite way of what is normally desired.
It is now much easier to start out in UV photography than a few years ago, thanks to current bulletin boards like http://www.ultravioletphotography.com and www.fotozones.com (and here) that freely distribute and discuss technical information. Nonetheless, UV photography requires a considerable determination, time and money to obtain the necessary equipment, and additional determination and time to learn how to properly use this equipment. At the risk of repeating myself:
The reward is that we can document and study a world otherwise invisible to us. In addition to its visual appeal, UV photography has numerous uses in science, technology and forensics. However, not too many scientists have used UV photography to document macroscopic objects (aside for flowers and birds plumage, which have been the subject of hundreds, or possibly thousands of papers). As an example, when I decided to look at the shells of land snails in UV (and IR) and did not quite understand what I was looking at, I found out that apparently no other scientist had thought of doing the same thing, and I was able to put to good use what I learned about UV photography in the following paper:
A realistic time- and action plan goes as follows:
Cameras and camera sensors
Camera sensors are moderately sensitive to UV-A, albeit to a lesser degree than to the visible range (400-750 nm). At shorter wavelengths than UV-A, the sensitivity of ordinary camera sensors quickly decreases with decreasing wavelength. This is due to a combination of factors, of which the most important is probably absorption by the Bayer color filters and microlenses built on top of the sensor chip. Depending on its thickness and composition, the glass window covering the chip and sealing it within its ceramic package can be an obstacle to transmission or UV-B and UV-C, but usually it is fairly well to very transparent to UV-A. Anti-reflection coatings of this window are typically designed to be most effective in the visible range, and some of them may reflect UV to varying degrees.
Videocamera sensors without microlenses and Bayer filters can be used in the UV-A and UV-B ranges. The window protecting the sensor is sometimes devoid of coatings, or treated with coatings optimized for UV transmission. The same type of videocamera sensor is sometimes completely devoid of a protective window, and completely exposed to whatever radiation passes through the lens. In this case, the sensor is extremely vulnerable to damage (e.g., atmospheric humidity may corrode the vacuum-deposited aluminium electrical pathways, thus requiring the use of gold for this purpose, and any dust particles are impossible to remove without damaging the sensor), and the lens should never be removed from the camera. This equipment is expensive and has specific technical uses, but it is simply the wrong type of equipment to start out in UV photography.
Fujifilm used to make DSLR models for UV (officially, only down to 380 nm) and multispectral photography. They are no longer available, and it is much more practical to modify a general-purpose DSLR or mirrorless camera that to try and get one of these special cameras.
UV and multispectral conversion
Virtually all sensors in general-purpose digital cameras are covered with a separate UV- and IR-blocking filter. In the large majority of cases, and especially in modern cameras, this filter is very effective for its intended purpose, and must be removed to allow imaging in the UV (and IR) range. Often, this filter also carries two thin layers of birefringent material, which provide anti-aliasing. Increasingly often, this anti-aliasing filter is absent, and aliasing and the related color moiré artifacts are corrected by the camera software.
In DSLRs, this filter is usually replaced with a window of optical glass transparent to the wavelengths of interest. This is necessary because simply removing the filter changes the optical distance between lens mount and sensor, while the optical distance between lens mount and the ground glass of the optical viewfinder is unchanged. Removing the filter without replacing it results in two separate problems:
In so-called multispectral conversions, a window transparent to most or all wavelengths that can be recorded by the sensor replaces the built-in filter. In UV conversions, a UV-pass filter is instead used as replacement. A variety of IR-pass filters are also used as replacements to allow IR photography. This modification is essentially permanent. Therefore, the most versatile conversion is the multispectral type, which requires a filter to be mounted in front (or at the rear) of the camera lens to isolate a subset of the light spectrum.
Most DSLRs allow the distance between ground glass and lens mount to be adjusted, and in some camera models the adjustment range may be large enough to allow viewfinder focus to be recalibrated after removal of the UV and IR filter, without needing a replacement window. The thickness of the original UV- and IR-cut filter is very variable among camera makes and models, and this also plays a role in the availability of this solution. This type of recalibration does not correct the problem of lenses no longer focusing at infinity. For this reason, DSLR conversions usually replace the built-in filter with a transparent window or filter. On the other hand, when using a mirrorless camera and lenses mounted on an adapter, it is possible to simply remove the built-in filter without replacing it, and to use a slightly shorter adapter to restore infinity focus with lenses that can be mounted on an adapter (native lenses designed for direct mounting on the specific mirrorless cameras, without an adapter, remain unable of focusing at infinity).
The infinity focusing position of many lenses can be recalibrated, and in some cases the adjustment range is large enough to allow focusing at infinity on cameras where the built-in filter has been removed but not replaced. This is the case, for instance, of the Novoflex Noflexar 35 mm.
Among Nikon DSLRs, the old D70 and D70s are frequently reported to be particularly suitable for UV photography. Many other models are also suitable, albeit not all. Several Micro 4/3 cameras also work well. I discuss here a Panasonic G3 that I modified for multispectral (including UV) photography with very good results. In the past, it was often stated that Canon DSLRs were unsuitable. It was also stated in the past that CCD sensors were more suitable that CMOS sensors, but this is not supported by the current knowledge. Very capable UV cameras like the Panasonic G3 and GH series have CMOS sensors. It is stated on some sites that some of these cameras are suitable for UV photography even without modification. I prefer to qualify this statement by saying that some of these unmodified cameras allow imaging in the (very) near UV range at the price of extremely long exposure times, even in the presence of abundant UV radiation, but certainly cannot be called optimal for this application.
A further factor to consider is that live view is very convenient for framing and focusing with a UV-pass filter mounted on the lens (provided that a sufficiently strong continuous UV illumination is available). Live view also allows a precise focusing (by electronically magnifying a portion of the live view image), although this requires a stronger UV source than necessary for viewing the whole frame. Older cameras like the Nikon D70 lack live view, and focusing and framing must be carried out without a UV-pass filter. UV focus shift (see below) and, at high magnification, the thickness of the filter can slightly alter the plane of focus, and with these older cameras this problem becomes evident only after examining the pictures.
Links to cameras for UV photography on this site:
Special lenses can be designed for UV-A and UV-B photography. This includes, for instance, the Nikon UV Nikkor 105 mm (and the apparently identical Tochigi Nikon UV Nikkor 105 mm), CoastalOpt Apo Macro 60 mm and UV Rodagon 60 mm. None of these lenses are cheap, and most of them are no longer manufactured. Several of these lenses also have relatively long focal lengths that limit their usefulness for landscape photography.
At least some of the general-purpose photographic lenses also transmit rather low amounts of these wavelengths (see links below).Therefore, it is possible to use modified general-purpose digital cameras and general-purpose lenses to take pictures in the near UV range, at least for static subjects that allow relatively long exposures.
Nikon EL-Nikkor enlarger lenses are often used for UV photography (see links below). Once regarded as the best general-purpose lenses for UV photography, they transmit less deeply into the UV than the lenses mentioned above. Nonetheless, they provide good alternatives for wavelengths down to approximately 365 -380 nm. The EL-Nikkor 63 mm f/3.5 was once regarded as singularly good for this purpose, but several other models (especially the older series in metal barrels) are just as suitable.
A problem with many general-purpose lenses used in UV photography is a so-called focus shift between the visible and UV ranges (strictly speaking, this is not focus shift, which is a different phenomenon, but a type of chromatic aberration). This problem is much less serious when focusing with live view in UV-only imaging, but must still be taken into account in multispectral photography, when superposing images recorded at widely different wavelengths.
Links to lenses for UV photography on this site:
CoastalOpt 60 mm f/4 Apo (also here)
UV photography normally requires a filter that blocks longer wavelengths than UV, so that they do not contribute to pictures. When photographers talk about UV filters, usually they mean filters that are opaque to UV and transmit longer wavelengths. These filters are more properly called UV-cut filters. This is the opposite of what we need for UV photography. What we want is UV-pass filters. A special problem with digital cameras is that their sensors are very sensitive to near infrared (NIR), and any NIR that leaks through a UV-pass filter will usually spoil the recorded image.
Traditionally, UV-pass filters were made from different types of ionic glass. Virtually all these types of glass also transmit NIR. For this reason, traditional types of UV-pass filters are essentially useless for digital UV photography, unless combined with a NIR-cut filter. Multiple filters increase the risk of internal reflections and loss of contrast.
The most frequently used modern filters for UV photography are of three types:
Dielectric coatings have variable spectral transmission properties, depending on the angle of incidence of the radiation. For this reason, they should not be used with wideangle lenses. Anti-reflection coatings are similar to dielectric coatings, but typically consist of fewer layers. Some of the traditional UV-pass ionic filters are available in anti-reflection coated versions (e.g. from Thorlabs) and perform better than their uncoated counterparts in terms of image contrast. However, anti-reflection coatings do not solve the NIR-leak problem.
Many traditional types of ionic UV-pass optical glass deteriorate on exposure to air and/or humidity. This deterioration makes the surface of the filter "matte" and strongly degrades its performance. Some of these types of glass, depending on the climate and humidity, may visibly deteriorate in as little as a few months, typically developing a visible "flowering", "icing", or moss-like matte areas, on their optical surfaces (see above example). This degradation may look like molds, but is not of biological origin, and cleaning the filter only makes it worse. In other types of UV-pass ionic glass, deterioration is not immediately visible, but cleaning one of these filters causes microscopic glass particles to detach from its optical surfaces and makes the filter unusable for UV photography.
Unless the optical surfaces of these ionic glass types are protected by sufficiently thick dielectric coatings or by sandwiching with more resistant types of glass and air- and humidity-proof optical glue, these filters are only usable for photometry (although they may require a periodic re-calibration of the equipment), but not for imaging and photography. The filter in the above images has a dielectric coating on the opposite surface, which is still pristine. Any scratch in dielectric coatings, however, opens the way to more significant, and steadily growing, damage. In extreme cases, degradation that starts at the edges of a sandwich- or combination-type filter, where the ionic glass is exposed to air, can eat its way through the glass toward more central regions of the filter. In these cases, the filter edges should be air-proof sealed with epoxy or silicone within a metal cell.
Other UV-pass filter types exist, but are typically very expensive. For instance, so-called solar blind filters use a nano-scale grid of metal wires on a fused quartz substrate to reflect higher wavelengths. A partially similar technology, which uses parallel nanowires, allows the construction of polarizing filters effective in the UV.
Strong and/or lengthy exposure to UV radiation degrades many types of UV-pass filters and results in a lower UV transmission. This is sometimes called solarization (and is unrelated to the darkroom printing process known with the same name). Typically, this type of degradation does not affect image quality, but does require a longer exposure or stronger UV source. This is especially likely to happen to filters placed on a UV source, rather than on a camera lens. For some uses, manufacturers recommend replacing the UV-pass filters after six months or one year of use, although this recommendation may be dictated by the necessity of avoiding a combination of solarization and air degradation.
For maximum durability in UV photography use, one should choose only filters with optical surfaces coated with dielectric layers, if possible use them on the camera lens rather than light source, and protect them from excessive humidity and strong light when not in use. The use of lens shades left permanently attached to UV-pass filters, in addition to improving image contrast and reducing the risk of internal flare (see here), also reduces the risk of accidentally touching the filter surfaces.
Clean the filters only when really necessary. Normally, dust accumulating on a filter has no effect whatsoever on image quality, especially when using a sufficiently deep and narrow lens shade. Blow dust away with a rubber blower (not your breath, which is far from clean). Molds, on the other hand, should be cleaned out before they have a chance of etching themselves into filter coatings or optical surfaces. Fingerprints and grease marks are more likely to affect image quality, and should be cleaned out with a lens cleaning fluid. I use denaturated ethyl alcohol for this purpose, which works for me and does not damage optical surfaces and most plastics. The chemicals added to denaturated alcohol may vary in different countries, and some types may be unsuitable because they leave haloes after evaporating. Use lab grade ethyl or propyl alcohol if you have access to laboratory supplies. Avoid rubbing alcohol, strong organic solvents, or any cleaning fluids that contain added oils. De-ionized or distilled water can also be used and is safe on almost all optical surfaces, but is less effective than alcohol on grease and stuck grime.
Wipe optical surfaces only with a new (never before used) optical-grade paper towel, and only with the towel moist (not dripping) with alcohol, lens cleaner or water. Throw away a lens-cleaning paper towel immediately after using it, to make sure you will not re-use it by mistake. The portion of surface of the lens towel that will wipe an optical surface must not touch or wipe anything else (including your fingers) before being used. Never wipe an optical surface with a dry lens cleaning towel, or with anything else than a lens towel (I do not trust reusable micro-pore towels on optical surfaces, although they are fine for other surfaces). Practice cleaning a piece of glass or a cheap UV-cut filter with a new cleaning fluid or lens cleaning towel type, before using them on expensive equipment.
Sun lotion, cosmetics and all sorts of spray cans should be kept well away from photographic equipment. They can be very difficult to remove from optical surfaces and some of them block UV radiation extremely well.
Links to filters for UV photography on this site:
Sources of UV radiation
Warning. UV damages the eyes and skin (especially UV-B and UV-C, but UV-A is by no means safe), so you should not stare into UV light sources and avoid skin exposure. Even if the sources are not powerful, exposure to UV is compounded by the fact that the eye's iris does not contract in the presence of UV, thus increasing the effective irradiation of the eye. It is also a good idea to switch off the UV sources when not in use and to wear protecting goggles that cut UV radiation (e.g., made by UVEX and specifically classified for UV protection, not just ski or fashion goggles), to prevent long-term exposure to indirect UV during photography sessions. Some prescription glasses do not cut UV, and many cheap sunglasses are transparent to UV.
As UV-only sources, initially I tried both "black light" fluorescent tubes (of the type that can be screwed into an ordinary socket for a light bulb) and UV LED banks (which can be mounted in 12 V sockets for halogen lights). The latter seem to produce more blue and indigo light and less UV than fluorescent tubes (at least in the models I tried). LED banks can be battery-powered or fed with a 12V DC power supply, but they draw so little current that the 12 V solid-state transformers used for halogen lights sometimes are fooled into thinking that no lamp is present, and refuse to provide a current. Ordinary incandescent light bulbs (especially those of higher power and higher Kelvin temperatures) do emit potentially usable amounts of UV, but they also emit a lot more IR. I tested additional UV sources here and here.
Specialist continuous light sources are capable of producing relatively large amounts of UV (e.g., for adhesive curing and fluorescence microscopy). The tube of electronic flashes does produce a lot of UV, which normally is filtered out by a yellowish coating on the outer surface of the tube (sometimes the filter is colored glass used to make the tube, and therefore is not removable). The plastic exit window of flashes also cuts UV, and sometimes has a distinct yellowish color that indicates the presence of UV-absorbing materials.
There are a few specialist electronic flashes designed to provide unfiltered UV, but they are scarce, and probably mostly discontinued (this includes the Nikon SB-140). In principle, one might attempt to convert a general-purpose electronic flash with uncoated tube by removing the plastic exit window. Among old (and usually cheap) battery-operated flash models known to have been modified with success are the Vivitar 285 (and its more recent and more expensive revival, the 285HV) and the Metz Mecablitz 45 CT-1.
Removing the flash window of the Vivitar 285 is difficult and messy (it is actually better to just cut a rectangular hole in the window), while doing it with the Metz 45 is easy. The Metz 45 CT-1 has been suggested as a UV source for film UV photography (even without removing the front plastic window, which means it would only be usable down to 360-370 nm) as early as the 1990s. It is frequent and cheap on the second-hand market, and recommended by a few UV photographers. Once the window is removed, I found it usable at least down to 325 nm, and possibly 300 nm. It produces a significantly higher amount of UV than the Vivitar 285.
The original rechargeable Ni-Cd battery packs of the Metz 45 CT-1 are almost always unusable. A replacement battery holder for 6 AA alkaline batteries, made in China, is cheap and commonly available. It is often mentioned on web sites and bulletin boards that the rechargeable and alkaline battery packs are different, and use different electrical contacts (which is true). However, I and other photographers have been successful in loading the unmodified alkaline battery holder with six Eneloop nickel hydride batteries, and using the flash in this way without any obvious problems. With these batteries, voltage is about 1.5 V lower than with alkalines, and the electronics seem to be able to cope with this difference.
Studio strobes can be modified by replacing a UV-coated tube with an uncoated one. Uncoated replacement tubes for studio strobes are cheaper than the ones coated with a UV-cut layer. Cheap, amateur-level studio strobes made in China sometimes use uncoated tubes. These cheap strobes are neither durable, nor reliable, nor especially powerful, but they may be an acceptable compromise for studio UV photography. See here and here for an updated discussion of studio flash units for UV photography.
A word of caution when modifying
electronic flashes: their capacitors store a potentially deadly high-voltage
charge even when the power is turned off and power cords and batteries are removed. The
capacitor may remain charged for hours or days, especially in battery-operated units and cheap studio units that do not actively discharge the capacitor when power is switched off.
In my personal experience, flash units with uncoated tubes are by far the most practical sources of radiation for UV photography (above examples). With powerful ones, I can record radiation down to about 300 nm (imaged as blue in the picture at the right; see details here). However, a continuous UV source is also very handy, because it allows focusing and framing the subject in live view, without removing the UV-pass filter from the lens. UV LED torches are useful for this purpose down to about 365 nm, but LEDS that emit shorter wavelengths are weak and expensive. LEDs nominally specified as emitting 365 nm radiation often emit 370, 375 or even 380 nm instead. This is especially common in cheap Chinese UV flashlights sold on eBay (some of them are even known to emit only at 405 nm, which is in the visible range).
Certain types of specialized fluorescent tubes emit a more broadband UV radiation in the UV-A, UV-B and UV-C ranges. Different types of tubes must be used for different wavelength bands. As a rule, their emission is too week for photography, and their usefulness mainly in framing and focusing.
Links to electronic flash for UV photography on this site:
Links to continuous UV sources for UV photography on this site:
More questions than answers
The effects of fluorescent tubes and electronic flash on certain subjects can be subtly different. In the above example, electronic flash was used at the left, and a "UV-B" fluorescent tube at the right. The petals near the center of the flower and the filaments of the stamina reacted very differently with the two sources. This is possibly due to a different spectral emission of the two sources, but probably also to the UV behavior of certain biological materials being strongly dependent on the angle of illumination.
Sometimes, I record UV images that I don't quite understand, like the above. This filter transmits between 320 and 330 nm, which I know from previous tests to be recorded as green or a slightly yellowish green. Then where does the red in the above image come from, and why is it clearly restricted to the flower and not the background? UV around 370 nm is typically imaged as orange, rusty or ruddy, but this particular flower reflects almost no UV at these wavelengths, and in addition, this filter has very sharp cutoffs at the borders of its range. NIR at wavelengths just above the visible range is imaged as violet or pink, but this filter shows no evidence of NIR leaks with other subjects. Perhaps the flower is selectively reflecting at 330 nm (which is imaged as a slightly yellowish green, and therefore contains some red) but not 220 nm (imaged as a purer green)? So far, I have no good explanation. This is the first time I get such a combination of "UV colors".
Just for fun, the same flower with two other UV-pass filters. A second flower (extreme right) was not invited but suddenly decided to pop into the picture unannounced.
My current kit for UV photography
In the end, my current, satisfactorily working solution for UV photography involves a few unusual pieces of equipment (not counting UV light sources, and various adapters, bits and pieces needed to make this equipment work together):
Things that I attempted but did not work well
I attempted several of the commonly discussed "fixes" for near-ultraviolet photography, (i.e., attempts at using and/or modifying lenses not designed for this purpose), with scarce success. At the time not being able to afford a true UV lens, which would have removed most problems, I tried, for instance:
Fluorescence is not UV photography
UV photography is very different from the photography of UV-excited fluorescence in the visible range. Fluorescence is the emission of longer wavelengths than those of incident light. Thus, illumination with UV radiation can cause the subject to fluoresce by emitting visible light. This is, for all practical purposes, photography in the visible range, and in fact it does not require special lenses and may even be carried out with a UV-cut filter on the lens. The above pictures are an example of UV-excited visible fluorescence. Some of the markings on a banknote, which are almost invisible in ordinary light (left), fluoresce strongly when illuminated with near-UV light (right). Visible light illumination was used in both pictures (in addition to UV illumination in the second picture).
Some materials fluoresce in the near IR when illuminated with visible light. This fluorescence is typically weak, but has scientific applications. In rare cases, materials fluoresce at shorter wavelengths than the incident radiation (by absorbing two photons in quick succession before releasing their energy as a single photon). Also this type of fluorescence is typically very weak.
Fluorescent materials release photons a very short time after capturing incident photons. For practical purposes, absorption and fluorescent emission are simultaneous. Phosphorescence is partly similar to fluorescence, but phosphorescent materials capture photons and store their energy for a longer time (even minutes or hours) before releasing it. This characteristic allows the imaging of phosphorescence by starting the exposure after the stimulating radiation source has been switched off, and in this way requires no filter to cut out the incident wavelengths.
Phosphorescent materials also emit photons when exposed to other types of radiation than photons. A small amount of radioactive material is sometimes mixed with phosphorescent materials to make them weakly glow even in the absence of any ambient light. This use is now less common than in the past, because of health concerns about low-dosage radiation.
Quite a few web sites talk about "underwater UV photography". In all cases I am aware of, they do not deal with UV photography, but with UV-excited fluorescence in the visible range (or sometimes, blue LED-excited fluorescence in the visible range). Many of their images are very interesting, but I have yet to see true examples of underwater UV photography.
The above picture shows visible fluorescence of a white cardboard sheet illuminated by the UV fluorescent tube visible in the picture. The fluorescence in the picture is much brighter that the surface of the UV bulb. This picture was taken without additional light sources. This example shows that, even with a UV-only light source, often it is necessary to use a UV-pass, visible-cut filter on the camera, in order to eliminate visible fluorescence of the subject that may overwhelm its actual UV reflectance. Some UV fluorescent tubes do emit also large amounts of IR.
Some people report "seeing" UV light. Most of these people do not possess unusual capabilities. There are documented genetic traits in humans that produce minor differences in spectral response of the eye photo-pigments, and some people are indeed slightly tetra-chromatic, but they do not include UV vision. Instead, those who report seeing UV at improbably short wavelengths may be experiencing UV-induced fluorescence of parts of their eyes, which causes a stimulation of the retina in the visible range. The most common result of fluorescence of the cornea, crystalline of vitreous humor is a perception of "haziness" in the presence of UV. If the retina itself is fluorescing, instead, the images of bright UV sources can be sharp. This is especially likely in cataract patients that have had the crystalline surgically removed and are using prosthetic lenses transparent to UV. In other cases, reports of people "seeing" the UV output of a monochromator are simply seeing the equipment leaking radiation in the visible range.
The eye crystalline acts as a strong UV-cut filter, and usually prevents UV damage to the retina. The crystalline itself, however, can be damaged by long-term UV exposure, often leading to cataracts. In people who have had the crystalline surgically removed (usually to restore vision in cataract patients), UV from sunlight reaches the retina almost unimpeded. As a result, the retina is easily damaged by UV (and perhaps even by blue and violet light), and these persons must protect their eyes from sunlight with UV-absorbing glasses or contact lenses. They may also need to wear UV protecting lenses when indoors in buildings with large windows. In my house, double-pane windows transmit about 50% of solar UV at 340 nm, and still a relatively high amount at 325 nm.
More links to UV photography on this site:
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