[Updated 2019-07-18] Godox sells a medium-power flash called AD200 with interchangeable heads and several accessories like light modifiers and remote wireless triggers with TTL exposure metering and high speed synchronization capabilities. This gives a lot of flexibility in its use. After a few separate purchases I now own the AD200 and the H200, H200J and H200R heads, an Xpro-O TTL Wireless Flash Trigger, and several light modifiers, all of them branded Godox. (The same flash and accessories are also available under other brand names.)
I will start with the heads, as their radiation emission characteristics are most important when needing an UV-A or NIR radiation source for imaging. The standard “speedlight” or “Fresnel” head, type “H200“, has a plastic Fresnel lens with a yellowish tint, and so this head emits almost no UV radiation with a sharp cut-off of the emission spectrum at around 415 nm. At the NIR end it emits past 900 nm, the limit of the calibration of the spectrometer I used. The newest head is a compact round head called “H200R” which comes with a glass diffuser disk that seems to transmit UV-A radiation rather well. The diffuser can be rather easily removed and replaced with a 72 mm step ring so as to be able to screw in normal 72 mm-thread camera filters. The emission spectrum has a gradual tail towards short wavelengths reaching as far as 320 nm or so. At the NIR end it emits past 900 nm. The bare-bulb head “H200J” provides a socket for use of the lamp “AD-FT200” which seems to vary depending on the production batch on how well filtered it is to block UV radiation. The one I received with my AD200 emits slightly more UV-A than the round head, relative to the amount of radiation in the range 400 nm to 700 nm. It does also emit NIR radiation strongly. It can be used together with a reflector, AD-S2, which is accompanied by a removable plastic diffuser which transmits UV-A radiation quite well, at least when new. The AD-FT200 flash tube is enclosed in a thick glass envelope. Some other lamps, just bare flash tubes can be adapted to the H200J head. It is not easy to find a flash tube of high enough power rating and small enough size to fit in this head. I have used flash tubes with a lower power rating, the FT-ST-S40w from www.xenonflashtubes.com, with some success. Because of its power rating it is necessary to let the tube cool-down between flashes. It is likely that not using it at full power may extend its otherwise short life as a result of using it outside manufacturer-provided maximum power-rating specifications. As the total emission with the FT-ST-S40w is significantly less than with the AD-FT200 the gain in relative UV-A output does not translate into increased total UV-A output. I have recently bought two slightly larger flash tubes (FT-118 and an FT-218) flash tube from www.xenonflashtubes.com. The FT-118 is rated at 60 J and the FT-218 at 100 J. The FT-118 is a tight fit but still usable in the bare-bulb head “H200J” with the reflector. I haven’t tried yet the FT-218 but the fit is extremely tight, and maybe not usable.
With very good filters on the objective the AD-FT200 may be the best option, but the less favourable ratio between UV-A and NIR radiation could lead to IR leaks in UV-A photographs. The speedlight head and the round head include a white LED array as modelling light. The round head allows full power and two levels of dimming for the modelling light, in the speedlight head only full-power and off settings are available. The modelling light does not emit UV-A or NIR so it is useful only for framing and focussing in visible light. The heads described above are all flash heads. There is also a LED-only continuous-light head available, the “AD-L” with 3.6W—rather weak— output power, which I have not tested. Its output can be dimmed remotely and it’s colour temperature is rated at 5600K±200K.
The spectral irradiance output from the different heads at 1/128 power setting are shown below. These measurements were done with an Ocean Optics Maya 2000Pro array spectrometer using only roughly similar distances between the heads and the input optics, a Bentham diffuser. Data was acquired from within R with packages from the R for Photobiology suite (see https://www.r4photobiology.info/) including ‘rOmniDriver’ and ‘ooacquire’.
These measurements are summarized by expressing the UV-A radiation output relative to the visible light output, both expressed in photons.
Next, I describe the light output geometry of the different heads. Many modern automatic camera-mounted flash/speedlight units have a built-in zoom that can adjust the angle of the emitted light beam to match the focal length of the objective in use, even when zooming. As is the case with studio flash units, the heads available for the AD200 lack a zoom and the light beam geometry can be controlled only by means of accessories. However, there are intrinsic differences among the heads available for the AD200 in the range of light beam geometries that can be achieved. The H200 speedlight head focuses its light output into a constrained rectangular area, which can be expanded with diffusers and limited with barn-doors or honey-comb grids. The H200J bare-lamp head provides maximum control of the light beam with the multiple accessories available, and used as is, without any modifier of reflector, it provides the broadest beam. Accessories for the H200J include different reflectors that can be used to concentrate the beam, diffusers to be used with the reflectors to even out the spatial distribution of the light, different honeycomb grids to restrict the width of the beam to different angles, barn-doors allowing asymmetric beams and snoots with a honeycomb grid to obtain very narrow light beams. It is important to remember that honeycomb grids, barn-doors and snoots do not concentrate the light beam energy into a smaller area but instead limit the spread by absorption. In contrast lenses and reflectors do focus the beam concentrating the emitted radiation into a smaller area. When interested in the UV-A and NIR regions, spectral transmittance of Fresnel and other lenses, and diffusers becomes of interest as these optical devises have the potential to absorb radiation in these regions specially if designed for visible radiation. The round head, H200R, has a fixed reflector and a diffuser screwed down in place. Additional accessories are available: honeycomb grid, barn-doors and diffusion dome. The same is true for the H200 speedlight head. Filters or “gels” can be used with the different heads.
The heads can be used remotely. The body of the AD200 is heavier than that of camera-mounted flashes. It is not possible to install the AD200 body in the flash socket, while it does have two 1/4 screw holes for mounting it on a tripod or other support. One very useful accessory, is a 1.8 m extension cable, EC200, that allows separating the relatively light-weight flash heads from the rather heavy body of the flash, containing the power electronics and battery. This allows mechanically attaching the flash head to the flash socket in a camera using the built-in male flash socket in the extension cable’s head end. A magic arm can be used to hold the head or the head can be used hand-held without strain. The use of the extension cable, decreases slightly the radiation output of the flash head due to its electrical resistance. The cable is thick because of the need to keep electrical resistance low.
The AD200 can be controlled manually using its built-in buttons and LCD screen. It can be triggered remotely by wire or through wireless triggers. The AD200 is most useful when intelligent wireless (radio) triggers/controllers of Godox’s 2.4G X System are used, as they allow full automatic control by the camera and/or remote manual control and triggering. The AD200 supports TTL-metered flash power control and high speed-syncing at all shutter speeds with many cameras. As the AD200 is camera agnostic, by means of camera-specific X-System wireless triggers it can be used with cameras of different brands (currently Canon, Nikon, Sony, Fujifilm X and GFX, Olympus/Panasonic, Pentax). I have used my flash only with Olympus E-M1 (“Mk I”) micro-four-thirds cameras.
The AD200 as a light source for photography in the UV-A region has the advantage of stopping movement. It works well for close-up and macro photography in the UV-A, allowing high quality images with a hand-held camera set at moderate ISO and under breeze conditions under which flowers and plants move. Outdoors, sunlight provides enough UV-A for image composition through the EVF and autofocus in a full-spectrum converted Olympus E-M1. However, when the sun elevation is low or indoors with normal artificial illumination, there is too little UV-A for the EVF to be useful or autofocus to work, making both manual and autofocus impossible. Under these conditions using a tripod or other camera support is a must. I use a Convoy 2+ UV-A flashlight with a 365 nm Nichia LED as an aid for image framing and manual focusing and the AD200 for image exposure.
In the case of UV-A photography the choice of a filter good at blocking NIR is crucial when using a flash, as Xenon tubes strongly emit NIR radiation. Most UV-A-pass filters do “leak” in the NIR region, to a small extent even the Baader U filer. The StraightedgeU filter seems to be the best at rejecting NIR and, with its maximum transmittance near 390 nm, also the best filter at transmitting the UV-A emission from flashes with xenon tubes with a glass envelope. On the other hand, for NIR photography most long-pass filters will block the UV radiation from the flash letting through enough NIR radiation to overwhelm UV “leaks” if they exist. Some visible blocking filters fluoresce when exposed to UV radiation, and will need to be stacked with UV rejecting filters such Tiffen Haze 2A to avoid fluorescence.
Bare xenon flash tubes, even if made of glass, emit enough UV radiation to make it a must to avoid direct exposure or the use of eye protection when used in a darkened or dimly lighted room. It should be also possible to adapt xenon flash tubes with a quartz envelope to the H200J head. As these tubes emit a lot of UV all the way down to 250 nm and shorter wavelengths, extreme care should be used to avoid hurting/damaging the subjects photographed. With quartz xenon tubes the photographer and anybody else exposed to the flash should use suitable eye and skin protection. As modified digital cameras are not sensitive to UV-B it is most effective and safest to filter out the shorter wavelengths at the flash. Good eye protection is needed (e.g. with EN170 certification and UV400 rating with no gaps on any sides that could allow either direct or reflected UV ingress), specially if working under conditions where pupils dilate, such as dim visible light or darkness. Skin exposure at close range should be also avoided.
Modifying a flash should be done only by someone knowledgeable of the risks involved in electronics circuits with lethally high voltage and in the exposure to damaging radiation. If not professionally trained, play it safe and get someone else to modify the flash and advice on how to protect yourself from ultraviolet radiation!