Collecting visual evidence of bats at roost entrances

Aim
Recording bats and their behaviour around roost entrances can be extremely useful for a number of reasons: as evidence to present to a client, to demonstrate or test for a change in behaviour during or after mitigation, and as a back-up system to record the presence of the quieter bats like the brown long-eared. We tested two night vision systems at a lesser horseshoe maternity roost.

Yukon Stringer 5 x 50 Night Vision MonocularPulsar Quantum S Series Thermal Imaging Scope

Methods

We set up two very different night vision video recorders on tripods near the entrance of a large lesser horseshoe roost near Totnes, Devon. The first was the Yukon Stringer 5 x 50 Night Vision Monocular, a very reasonably priced Generation One night vision device with a built-in video recorder. The second was the Pulsar Quantum HD38S Thermal Imaging Scope, a thermal imaging camera with a 30 Hz refresh rate coupled with a Yukon MPR Mobile Player / Recorder. Both were used to film bats as they emerged from the roost entrance and as they flew around the garage within which the roost entrance is sited.

Results
The two videos below demonstrate close-up and distance footage from both the Yukon Stringer and the Pulsar Quantum:

Video: Surveying a bat maternity roost - Close upVideo: Surveying a bat maternity roost - At a distance

Discussion

Image Quality: The Pulsar Quantum produced some very high quality video that was clear and easy to interpret. The results from the Yukon Stringer are slightly less clear but are still of sufficiently high quality for most purposes.

Usability: The Yukon Stringer does have a much narrower depth of field and due to the fixed zoom it proved very hard to get any decent footage of the bats flying around within the garage space.

Battery Life: The only drawback was the short battery life of both the Quantum and the Yukon MPR Mobile Recorder. To get round this we used the EPS5 External Battery on the Pulsar Quantum and changed the batteries of the Yukon MPR regularly – not the ideal solution but the cheapest way we know of to get some really impressive thermal imaging video.

Night Vision Scopes at NHBS

Night vision scopes work by collecting available light, normally from the moon and stars, and amplifying this to generate a meaningful image. The main component of any scope is the Image Intensifier, a vacuum tube that differs in design depending on the generation of scope (more info on generations later!). The image intensifier collects particles of light (photons) and focuses them onto a photocathode. The photocathode absorbs these photons and converts them to electrons, which are subsequently amplified and projected onto a green phosphor screen at the rear of the tube. To do this the photocathode needs a power supply, normally from commercially available batteries.

When the electrons hit the phosphor screen, the screen emits visible green light that the user can see. Therefore, you do not look through a night vision scope, but rather look at an amplified electronic image on a phosphor screen. As the phosphor screen emits light in exactly the same pattern and intensity as the light collected by the objective lens, the screen image corresponds to the actual scene in front of the scope. The phosphor screen is coloured green because the human eye can differentiate more shades of green than any other phosphor colour.

There is a bewildering array of night vision scopes available for amateur, commercial and military purposes. NHBS has selected a range of scopes and accessories that match the needs of amateur and commercial users so we hope there is no chance you are going to get shot at whilst using them! All our scopes have been tried and tested by wildlife and land management professionals and so provide versatile and durable solutions to seeing at night. However, even amongst our range there are subtle differences that make some scopes more suitable than others for certain uses. Below is a list of criteria to consider when making your choice. If you are still unsure, then please contact us as we would be happy to provide further advice.

For more on this blog, see the NHBS Quick Guide: Night Vision Scopes