2020 so far; LED vs. actinic part 3

When I started mothing almost exactly two years ago, I decided not to use MV lights: I didn't want to buy equipment for which bulbs were no longer made or imported, and I also wanted something that I could run off a battery. I initially bought a 15W actinic strip light (with both mains and 12V DC drivers), but I was interested in LEDs. A year later, having read a lot about LED lights for moth traps, I concluded that those on the market at the time were more-or-less divided into two groups: those that were effective and very expensive, and those that were cheap but not terribly effective. I thought that there was a lot of potential in this area and I wanted to experiment, so starting in late June 2019 I started to design and construct my own LED light. You can read my original post on this here, describing what I built.

At the end of 2019 I made another post (here) with a progress report. As we are now half-way through 2020, I thought I would give a further update. In 2019 I was running a single trap, alternating between LED and actinic lights on different nights. Variations in weather and moon between "actinic" nights and "LED" nights made comparison rather difficult. At the end of the year, I worked out how (i.e. where!) I could run two traps in the garden simultaneously, so I bought another identical trap, spent the first three weeks of January on maintenance and for all of 2020 I have run them in parallel. I have tried to operate the traps twice per week in order to get enough data for comparison, and as there is a difference between the two trapping sites, I alternate the lights between the two locations. During 2020 I have made some other changes, which I'll mention shortly.

The results for these last six months as a whole are below:

2020 H1 Results	Actinic	LED	Overall
Nights run	40	40	40
Species caught	154	166	210
Macro species	101	109	133
Micro species	53	57	77
Best night for species	23-Jun	23-Jun	23-Jun
Species on best night	47	46	73
Best night for moths	23-Jun	15-Jun	23-Jun
Individuals on best night	233	211	392
Total number of moths caught	1136	1449	2585

This makes the lights look similarly effective, with the LED slightly ahead of the actinic. However, this masks the effect of changes I made in April and May, prior to which the LED was slightly behind the actinic. With lockdown, I decided to increase the power of the LEDs, exploiting the availability of mains power in the garden. The LEDs had originally been a total of 14 one-watt components, but in April I replaced them with eighteen 3W LEDs - adding more ultraviolet components. However, the LEDs were not driven at their full 3W rating: I limited the total power to about 18W, compared to the 13W of the original version. The reason for limiting the power is that it is difficult to dissipate the heat; even though I have mounted them on an aluminium heatsink and used thermal paste, they are not tightly bonded and so there is a risk of overheating.

I then found that the UV LEDs were extremely inefficient: this is a common problem at UV wavelengths, but hard to identify because most datasheets do not show the radiation flux (ϕe) that they emit. Only about 2% of the electrical energy was being converted to light and the rest was coming out as heat. After more research, I identified a source of UV LEDs that are about fourteen times as efficient, so in May I replaced six of the LEDs with these better versions. Heat dissipation is still a problem and I am limiting the current so as to keep chip temperatures below a conservative 70°C, meaning that the total power to the LEDs is just under 20W (the drivers consume another 2W).

These changes have made quite a striking impact on the relative performance, best illustrated by the cumulative number of moths caught. Since making the changes, the LED light catches about 35% more moths and 13% more species than the actinic.

7 May 2020 onwards	Actinic	LED	Overall
Nights run	16	16	16
Total species caught…	127	144	182
… of which macros	81	93	112
… of which micros	46	51	70
Total moths caught…	934	1275	2209
… of which macros	605	909	1514
… of which micros	329	366	695

I will continue this experiment for the rest of the year. I may make more changes to the LEDs, but when I think I have finalised the configuration, I will then look to improve the bonding so as to be able to run at a higher power: better bonding generally means it's harder to swap LEDs. I will probably also produce a new set of drivers that will power a subset of the LEDs at a lower current, in order to restore the option for compatibility with running off a battery. Finally, when I am no longer taking the light to bits to change components, I will use a less Heath-Robinson (pun acknowledged) way of mounting the light on the trap, and add baffles - but that might be a job for 2021.

Tim Arnold

Newton Longville, Bucks