Frogs and leaf growth

Back in January I noted research by into infrared reflectance of insects carried out by Michael Mielewczik and others. Michael has contacted me again about two more papers on similar subjects.

The first is Non-Invasive Measurement of Frog Skin Reflectivity in High Spatial Resolution Using a Dual Hyperspectral Approach [1] (on PLOS ONE here with a PDF here).



As before, the team used a camera with filtering that split near-infrared (specifically the red-edge between 675-775 nm) and blue to explore the 'colour' of frog skin. They also used a two further hyperspectral cameras sensitive to visible and near-infrared between 400 and 1000 nm and to SWIR (short wave infrared) between 1000 and 2500 nm. This image is of agalychnis callidryas using the red-edge camera.

I've come across hyperspectral cameras before and they're quite fascinating devices. They produce a multi-dimensional image where each of the pixels in the x and y plane have a complete spectrum recorded in the z axis ... so z records intensity at a range of wavelengths. This means that you can choose which wavelength (or wavelengths) to view the scene after the fact. This multiplies the amount of data dramatically of course.

The second paper uses infrared imaging to help a study of leaf growth. The paper is Diel leaf growth of soybean: a novel method to analyze two-dimensional leaf expansion in high temporal resolution based on a marker tracking approach (Martrack Leaf) [2], available on the Plant Methods web site. This study used dark beads attached to the margins of a leaf and a camera fitted with a 940nm narrow bandpass filter. At this wavelength the leaf is brighter than the beads which makes image analysis easier.

[1] Pinto F, Mielewczik M, Liebisch F, Walter A, Greven H, et al. (2013) PLoS ONE 8(9): e73234. doi:10.1371/journal.pone.0073234
[2] Mielewczik M, Friedli M, Kirchgessner N, Walter A. Plant Methods 2013, 9:30 doi:10.1186/1746-4811-9-30

[Note: corrected information about the hyperspectral camera added 31 October]

The physics of near-infrared photography

Klaus Mangold (a photographer), Joseph A Shaw and Michael Vollmer (who are physicists) have just published a paper, The physics of near-infrared photography in the European Journal of Physics. This is the best technical paper on the subject that I've seen since Clark's book Photography by Infrared (which went out of print in 1984).

The European Journal of Physics has a policy of making papers freely available for 30 days from publication, although you will need to set up an online account to access it.

Amongst other things the paper tells us that red wine, Diet Coke and even espresso coffee are transparent to near-infrared wavelengths.

This is the URL: stacks.iop.org/EJP/34/S51

The citation is Eur. J. Phys. 34 (2013) S51–S71

The Pi NoIR: inexpensive nocturnal wildlife video

A further quick camera-related post. Andrew Back, an enthusiast for open hardware, has been using a Raspberry Pi (the tiny cheap computer) with an unfiltered CMOS imaging chip to shoot night-time wildlife. In this case it's, so far, mostly slugs and bugs caught using time-lapse photography.

See all the gory detail on the Design Spark blog.

30 October: Andrew continues posting his results on the Design Spark blog. Here's a time-lapse movie of the landscape near Hebden Bridge.

Thermal imaging reveals wildlife secrets

The BBC's new natural history series The Great British Year has shown some fascinating, and very artistic, uses of thermal imagery. (Best if you expand the videos ... but sorry they probably won't work outside the UK.)



And there's a movie showing something of the technology.



Shows you don't have to wander the plains of Africa to get exciting thermal footage, and I have to say it's nice to see something more subtle than the usual blue/red heat pattern. Also worth a read (and with the 'usual' colouring in many cases) is a web page on 10 wildlife secrets revealed by thermal cameras ... even a hot plant!

New IR camera is user-configurable

New near-infrared cameras are few and far between and there is an increasing choice of thermal imagers, even if they tend (with a notable exception) to be very expensive. However, cameras working in the gap between the two are much rarer beasts.

Episensors of Bolingbrook, Illinois have announced a new camera working in the short wave infrared (which lies just beyond photographic/near infrared) and which is intriguing, not just because they describe it as 'low cost' (not sure just how low) but also because of its versatility. Here's a paragraph from their press-release.

The infrared camera company Episensors, Inc. recently launched a new type of portable infrared camera called the Night SWEEP-1 (“NS-1”). Infrared cameras can see light that is invisible to the human eye and provide imaging at night and through obscurants like smoke and fog. What sets NS-1 apart is its portability and customizability, which allows scientists, researchers, and others to utilize the camera in the field, without sacrificing the capability of swapping between short-wave, mid-wave and long-wave infrared focal plane arrays, lenses and other components. Based on a patent-pending design, this infrared camera system is fully customizable. The camera can be configured with a pour fill Dewar or a closed cycle Integrated Dewar Cooler Assembly (IDCA) depending on the customer’s preference.

An excellent technical note on their web site explains the wavelength domain this camera covers. It's notable not just because the user can change the imaged band but that the extended SWIR (short-wave-ir) band, between 1 and 3 µm (1000 and 3000 nm) not only has some haze and smoke penetration ability but also contains a sweet spot where there is some smoke penetration but also the radiation goes through glass. Output resolution is 320 by 256 with plans for 640 by 512. The digital resolution is 14 bit and I assume having a supercooled sensor (that Dewar referred to in the note is a thermos flask of something like liquid nitrogen) will give a low noise floor.

So this camera is a kind of infrared SLR and operates between photographic infrared (which ends around 1500 nm) and the thermal bands and operates using reflected radiation (from the sun for example) while thermal imagers show radiation from the objects themselves. I believe this mid-infrared imaging is sometimes referred to as reflectography and has applications in art restoration amongst other things.

Check out the videos on the web site. They look like photographic infrared rather than thermal but you will notice some smoke penetration. It'll come down to particle size and by configuring the camera the user will be able to balance haze and smoke penetration against things like glass transparency. The nearest I've seen to this in other devices is where a single unit combines two different cameras.

Whether we will see the NS1 on this side of the Atlantic is currently debatable as some of the technology is export-restricted.