With mercury-based (Hg) lamps now prohibited from manufacture and the availability of new data on moth behaviour concerning their attraction to light (see previous post), it seems appropriate to re-think the design of an LED moth-lamp. I have decided to explore the use of LED tapes. These are available in many different colours (both broad and narrow wavelength) and importantly, the use of UV light for curing the resins used in 3D printing and other manufacturing processes, means that there are UV tapes available with outputs at 395 and 365nm. Many of the LED tapes can be powered from a 12V or lower voltage source and thus can readily be run from a battery. Further, they are available with a silicon coating that renders them waterproof. Their mechanical flexibility and adhesive backing allows them to be wound around a former – this can either be a plastic moulding such as a pipe or something that has been 3D printed. Normally, power LEDs require a heat-sink – usually a sheet of metal or a purpose-built finned aluminium heat-sink. However, because LED tapes have a significant area of copper backing that acts as a heat-sink and the individual LEDs in what is called 5050 tape have a power rating of only 0.25W, no additional heat-sink is required. 5050 LED tapes are very efficient. Per metre they have 60 LEDs, which produce up to a 1000 Lux, and consume only about 5W. The distributed nature of their output means that though precautions should still be taken to protect one’s eyes, they are likely to be a great deal less dangerous than a light source that packs the same output into a much smaller area. There is good reason to believe that LED tape could rival or perhaps even outperform much higher power Hg bulbs. A 200W high-pressure Hg bulb produces about 10,000 Lux which would imply that 10 metres of tape could rival it for total light output. However, Hg bulbs produce some of their light at wavelengths other than those around the peak spectral sensitivities of the receptors in the moth eye that are thought to contribute to the behaviours that attract them to light at night. Although it is hard to calculate how bright an LED light source should be to rival its Hg counterpart, in theory (?) it could be as low as 10W of tape. The prototype below is based of 2 x 3 metre lengths of LED tape that consumes 30W at 12V.
Given that many apparently transparent materials have poor UV transmittance it seemed pertinent to check the transmittance of the silicon coating used on the waterproof version of UV tapes.
Date from the manufacturers of the silicon coating would suggest that it is very transparent to all visible wavelengths and also to UV, having a transmittance approaching 90% at 365nm. Thus, there is no reason not use the IP65 version of the LED tapes designed for outdoor use.
With these properties in mind, I have produced a crude prototype intended for testing on a Skinner-type moth trap. The most appropriate UV tape would appear to be 365nm. However, this tape is relatively expensive (8 to 12 Euros a metre). That said, were it available the cost of a mercury bulb and associated components would be significantly greater. In the prototype illustrated below, two 3 metre lengths of tape have been employed; 365nm and cool white. The cool white tape produces a lot of light at the blue end of the spectrum and is very cheap. The twp tapes are spiral wound onto a 45cm long former cut from a length of 50mm PVC piping. This diameter of pipe is commonly used for the evacuation of water from sinks etc. and it is thick and very strong. Because some of the 5050 LEDs will face downwards, a reflector should perhaps be used to throw as much light as possible up into the sky – a simple flat sheet of mirror finish aluminium or aluminium foil should suffice. However, in the event I bent a thin piece of brushed aluminium sheet to make a small reflector. The device illustrated should be easy to DIY manufacture and with a little ingenuity the 3D-printed brackets could be replaced with PVC ‘pipe end blanks’ and supports made from wood or other more readily-available materials. It will be of interest to compare the performance of the ‘hybrid’ UV/cool white lamp with that of one based solely on cool white which is much cheaper than its UV counterpart, and as stated above, has the larger part of its output concentrated at the blue end of the visible spectrum but is eye-safe. The UV + cool white design shown here cost about 50 Euros to manufacture. While the prototype is perhaps too long to be used upright as might be required in some other moth trap designs, it would be relatively easy to 3D print an elliptical former with a much greater surface area – the former shown below is for a lamp equivalent to the power of the prototype but that would have a height of only 18cm. 3D printing allows for a base that could screw into an ES27 holder, to be screwed to a flat surface or hung in front of a white sheet.
The brackets to hold the pipe were 3D printed and are not intended as the final product. Many plastics degrade in UV light, becoming brittle and fragile. The PVC pipe used as a former in the light shown is perhaps unlikely to suffer such problems because it is designed for outdoor use. The brackets supporting the tube were printed in ‘leftover’ PLA – the final design will need to use ASA or another similar plastic that does not degrade as easily as PLA. Hopefully, PLA should last long enough to test the lamp…we will see!?
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