MTE UV 301 torch
UV LED torches, or flashlights, are commonly available on eBay and other online sources, but are not yet commonplace in hardware stores. A good UV torch is a useful tool in scientific photography, for example:
Other uses for UV torches include detecting counterfeit banknotes, altered documents and paintings, coolant leaks in air-conditioning and automotive systems, repaired damage to the paint in cars, and certain types of molds in food and produce. UV torches and larger UV sources have also applications in a variety of forensic examinations, and in industrial processes for curing UV-sensitive glues and paints.
This page discusses a small, battery-operated LED torch with a single LED mounted in its head. UV LEDs are also used, single or in arrays, in larger and heavier torches, as well as mains-powered UV lamps. Other types of UV sources are used when more power and/or a broader UV spectrum are desired. Deuterium lamps are typically used in spectrometers. Hydrogen lamps are cheaper than deuterium lamps but have some drawbacks. Xenon arc lamps are used for high-power sources with relatively continuous spectra, and since they emit also in the visible range they require filtering if only UV is desired. Halogen and mercury lamps are also common UV sources. Even some HID lamps for automotive applications, which are halogen arc lamps, may produce sufficient amounts of UV to be useful as UV sources. Fluorescent tubes with quartz glass and devoid of phosphor linings are used to produce mainly UV-C radiation for disinfection.
UV LEDs are identified mainly by their nominal power, i.e., the amount of electricity they can be fed with, without overheating, when mounted on an adequate heat sink. The peak wavelength of their emission is also a necessary specification. A third key parameter is the actual power of the emitted UV radiation, which is only a small portion of the nominal power and may vary wildly among LED models and production batches. This is also where the specifications provided by the LED manufacturers often become fuzzy or remain unavailable.
For power UV LEDs, nominal powers are often specified as 1 W, 3 W or sometimes 5 W. All these LEDs need a relatively large heat sink, proportional to the nominal power. Multi-chip arrays mounted in a single package are also available, up to 100 W. Above 5 W, the required heat sinks become very large, and in most cases fans are also necessary to prevent overheating of the LEDs, which quickly destroys them. Nominal power specifications provided by sellers or manufacturers are sometimes not credible. I have seen UV LEDs sold as 5 W but accompanied by maximum voltage and current figures that clearly indicate a maximum nominal power of only 3 W (power = voltage * current). Unless specified otherwise, nominal power is continuous.
UV torches that use large numbers of low-power LEDs are generally not as powerful as a torch that uses a single 3 W die. A characteristic of low-cost torches that use a dozen or more low-power UV LEDs is that these LEDs are often poorly sorted and emit at slightly different wavelengths. In practice, the UV emitted by these torches is distributed across a slightly broader spectrum, which is not necessarily a bad things when used to stimulate VIS fluorescence. Certain fluorescing substances are sensitive to specific UV wavelengths, and using a broader emission spectrum has a better chance of making them fluoresce.
It is sometimes possible to temporarily overload a LED for a very short time (a fraction of a second). This is sometimes called "flash mode". Unless the LED is designed to be used in this way, overloading a LED does not produce a proportionally stronger emission (i.e., doubling the power with respect to the nominal specifications, in most cases, may produce no more than a 20% increase in emission).
The peak wavelength specifications of UV LEDs are often unreliable. This is especially true for LEDs sold on eBay and/or produced in China. LEDs specified as 365 nm often emit at 380 or 390 nm, which severely limits their usefulness. LEDs that emit at 405 nm (i.e., in the visible range) are sometimes advertised as "UV LEDs". The peak wavelength of LEDs and their efficiency (in particular, the maximum amount of emitted radiation) may also vary with each production batch. One of the reasons for the unpredictable quality of no-brand UV LEDs purchased on eBay is probably that many of them are seconds or rejects left over after the best ones in each batch were selected for sale through more profitable channels.
UV LED torches available for online order often use low-cost LEDs, and their performance is therefore limited by these components. If you are looking for a reliable and honestly rated 365 nm LED torch, you should be suspicious of prices lower than about 150 US$, and before purchasing you should look for reviews and counsel by knowledgeable users (i.e., experienced UV photographers, since LED torch collectors and enthusiasts may not have the equipment or experience necessary for an objective evaluation of UV torches).
LEDs that emit shorter wavelengths than 365 nm are available, but they tend to be very expensive and to emit only minuscule amounts of radiation.
The MTE UV 301
Nichia is one of few manufacturers with experience in producing reliable 365 nm power LEDs. Its 1 W and 3 W 365 nm LEDs are known to be among the best. Therefore, it is a safe bet to choose a torch that uses one of these 365 nm Nichia UV LEDs, like the MTE UV 301.
MTE is apparently a Singapore-based company, but its web site does not explain what the MTE acronym means. It markets a number of white LED torch models and two UV LED torch models, including the MTE UV 301 discussed on this page.
The first impression upon handling this torch is that it is expensive but solid and well-built. The weight including battery is around 180 g. It uses a Nichia NCSU033B diode, which according to MTE emits 658 mW of radiation when fed with 3 W of power. Nichia specifies an emission between roughly 380 and 640 mw for different grades of the NCSU033B, with an average of 450 mW, so the MTE figure is overoptimistic. Nonetheless, this is a state-of-the-art efficiency for UV LEDs. In fact, this amount of emitted radiation at 356 nm is quite a lot, and you must avoid radiation exposure to the eyes. You should always wear UV-protective polycarbonate goggles or a polycarbonate full-face mask when using this torch. It is also a good idea to avoid skin exposure to direct radiation or strong reflections, and to cover all body parts, including face and hands, with appropriate clothing during extensive use of this torch.
Kits including this torch vary slightly. Some include a holster for the torch and spare O-rings, while others include battery and battery charger. A wrist strap is also included.
The torch is switched on and off with a rubber button on its "butt". There is only one power setting. The front protective glass is flat, transparent and smooth (i.e., not a lens or a diffuser) and has a diameter of about 40 mm. The head cannot be focused.
It is not completely true that this torch cannot be focused. The head and body of the torch are separate pieces, joined by a thread. The joint is sealed by an o-ring.
The MTE UV 301 uses one 18650 rechargeable battery. I don't know exactly how long one battery charge lasts, but I would guess a few hours. The manufacturer specifies that a battery charge lasts 4 hours, which seems reasonable. The torch should not be covered or wrapped in cloth or plastic during prolonged operation, because a free flow of air around the torch head is necessary for a proper cooling. In normal operation, and with the flash head exposed to air, the head becomes warm after a few minutes, but never hot.
A potential problem is that larger dust particles that settle onto the window of the head produce visible shadows on the beam projected onto an object a few tens of cm away. The window should also not be touched with one's fingers (skin oils and fingerprints absorb UV), and should be kept clean with a mild solvent (e.g., ethyl or propyl alcohol without oil additives). It is not enough to try and wipe away fingerprints with a cloth.
The LED emits a relatively faint but easily detectable VIS radiation. This is not a problem in typical UV photography, where a UV-pass, VIS-cut filter is almost invariably mounted on the camera lens. In this case, one of the purposes of the filter is blocking any VIS radiation emitted by fluorescence of the subject. The amount of VIS emitted by the LED is sufficiently small not to be a problem in the photography of VIS fluorescence stimulated by UV, except in rare cases where a very faint fluorescence must be recorded. Therefore, for most applications it is not necessary to mount a UV-pass, VIS-cut fiter on the head of this torch.
The "goose-bump" reflector provides a relatively homogeneous central region of the beam without obvious "donuts" or irregularities. However, the illuminated area has a brighter central spot that gradually fades away laterally. The diameter of this center spot is very approximately 50 mm at a distance of 38 cm from the front window. The beam fades away gradually from the central spot, and the total illuminated circle has a diameter of 40 cm at the same distance, with a relatively well-defined outermost border. The beam shape and width are easily observed by illuminating a white typing paper sheet (see above figure). Typing paper contains whiteners that fluoresce blue in the presence of UV.
It is difficult to decide whether it could be possible to design a better reflector for this torch, in order to concentrate the "spillage" now projected outside the central hotspot. This might increase the intensity of the hotspot by another 20 to 50%. On the other hand, if the spillage comes directly from the LED die and is not bounced by the torch reflector, it is difficult to concentrate this radiation. Possibly, this could be done by carefully designing a combined reflector and UV-transparent convergent front lens that match each other, or by mounting a small aspherical convergent lens a short distance in front of the LED die to catch and collimate the spillage but allow sideways radiation to reach the reflector unimpeded.
The central hotspot emitted by this torch is often too narrow for using the torch at a short distance to a relatively large subject. In these cases, I found a thin (1-2 mm) sheet of closed-cell white synthetic foam to work as a satisfactory diffuser, placed directly in front of the torch. A disk of this foam is often found at the bottom of the plastic boxes containing photographic filters. Contrary to my expectations, this material does not substantially block UV. Enlarging the irradiated spot by diffusing the beam (with any type of diffuser) results of course in a lower intensity of the radiation per unit of irradiated surface, and requires a longer exposure time.
Dangerously misleading information
On the pages of the MTE web site that describe its UV torches, the description for both the MTE UV 301 and the MTE UV 302 (the latter is a 382 nm torch, i.e., it emits longer-wave radiation closer to the visible spectrum) presents both torches as "Powerful bactericidal". At other points, the descriptions claims "Designed for forensic applications，prospecting，sterilization". Another often-repeated bullet point is "Force sterilization". For example, see here.
These are dangerous claims that cannot be left unchallenged. 365 nm is not effective for sterilization at the power levels emitted by this torch. For example, Gadelmoula et al., 2010, in Mendez, ed., Current research, technology and education topics in applied microbiology and microbial biotechnology, reported that complete sterilization of E. coli with continuous 365 nm radiation produced by LEDs required a 75 minutes exposure at a radiation density of 15.3 W/cm2, which is tens to hundreds of times higher (depending on irradiation distance) than this torch can provide. UV-C (usually around 250 nm) is necessary for reliable sterilization in reasonable times, and you cannot expect the MTE UV 301 to work satisfactorily for this purpose. The claims about sterilization provided by MTE are misleading and incorrect, and trusting them may lead to uses of this torch that compromise the health, safety or life of its users. It is therefore important to counter these claims and to make users aware that irradiating a surface with the MTE UV 301 will not make it sterile, nor clean it in any way, nor make it safe from biohazards and pathogens. UV-A may instantly kill movie vampires, but real microorganisms and viruses are much more resistant.
Because of its longer peak wavelength, the MTE UV 302 is even less effective for disinfection than the MTE UV 301.
MTE was apparently already made aware of this liability, and added an illustration of a UV-C spectrum with a banner saying "The different wave length sterilization is different" (sic!). Even assuming that prospective buyers can guess what this awkward sentence means, this is not sufficient, since the "Powerful bactericidal", "Sterilization" and "Force sterilization" bullets still accompany the MTE UV 301 and 302 on the MTE site. They should be removed entirely, and replaced with a statement saying that their products cannot, and must not, be used for sterilization. The language currently used on the MTE web site, and copied on the web sites of resellers around the world, in many countries could likely constitute legal grounds for hefty punitive damages.
Real products for UV sterilization
An interesting result of several recent studies is that pulsed, or time-modulated, UV is far more effective for sterilization than continuous UV at a steady intensity. For example, Xenex uses xenon flash tubes in its robots designed for sterilizing an entire hospital room within a radius of 3 m in 5 to 10 minutes of unattended use. The UV-C emitted by its xenon lamps (presumably, the peak emission during a flash) is said to be 25,000 times stronger than solar radiation at the same wavelength. This is not surprising, since the atmosphere strongly absorbs UV-C and only minimal amounts of it reach ground level. In case you wonder, the Xenex demonstration unit seen flashing in the presence of people in newscasts is emitting only in the visible range and not UV-C. For obvious reasons, this robot cannot be operated in the presence of people, and in fact it has built-in motion sensors that stop its emission if they detect movement around the robot.
Fluorescent tubes that emit UV-C are often used to kill algae suspended in pond water. These tubes are mounted within quartz sleeves that protect them against direct contact with water, and placed in light-proof containers where water is circulated by an electric pump. Operation of these tubes outside a light-proof container is highly dangerous and should be avoided.
The Hoplite 365 by Higher Orbit Products is a UV torch with the same electrical specifications as the MTE UV 301, and may use the same UV LED. It is available in two models:
These torches are equipped with UV-pass filters that block any visible emission by the LED. Prices of the Higher Orbit Products UV torches are much higher than the MTE UV 301. This is partly explained by the use of UV-pass filters. I have no direct experience with the Hoplite UV torches.
It is impossible to compare the spot size declared for the Hoplite torches to the MTE UV 301 only on the basis of the manufacturer's data. Hopefully, the central spot in the Hoplite is more homogeneous and with less spillage than in the MTE UV 301. This is another factor that could help to justify the much higher price of the Hoplite models.
The MTE UV 301 is a compact and portable, battery-operated torch that emits over 600 mW of radiation with a peak wavelength of 365 nm. It is well-made and worth its high price. The emitted beam has a narrow central hotspot, but no rings or other irregularities. Claims made by its manufacturer that this torch can be used for sterilization are false, and may lead to sickness or death if this torch is used for sterilization or cleansing in environments contaminated by pathogens.