Solar panels and 'reflective' grass

I was interested to read in today's Guardian that BP are researching the most reflective kinds of grass to plant underneath solar panels; panels that pick up on top and underneath. The story points out that "bifacial panels can increase electricity output by almost 15% – but this can be much higher if the ground beneath the panel is particularly reflective".

Although the story doesn't mention it, regular readers and fans of infrared photography will know that grass (as well as other foliage) strongly reflects near-infrared light due to the retro-reflective effect of those wavelengths travelling through plant cells. Chlorophyl is transparent to near-infrared radiation. The effect is the same one that makes snow appear white, which is why infrared photos and snowy scenes can be confused.

The Solarquotes blog in 2017 looked at the proportion of solar radiation that a solar panel can exploit. Their context was about UV but if you scroll down the page you'll see a diagram that shows that a silicon solar panel will make use of radiation between 400 and 1100 nanometres. Since visible light extends from about 400 nm to about 650 nm you can see that including near-infrared more than doubles the available energy bandwidth.

All this makes planting grass underneath bi-directional solar cells a logical thing to do. That reflective grass is not just fun for infrared photography then ... or grazing!

Animals as thermal 'detectors'

Thermal imaging cameras being as expensive as they are, it's interesting to see how observation of animal behaviour can provide clues to some aspects of temperature distribution in the environment. The classic is how to detect that a car has only recently arrived at a property ... if you can't touch the bonnet (hood). A thermal camera will show a warm spot on the bonnet, or (in classic detective mode) you might see that a cat has decided to lie there. Pigeons can also be useful detectors. You will often see birds roosting on top of one building in a group and this is often simply because that roof is warmer; either because of a local heat source such as an air outlet or because that roof is less insulated than the others. Snow will fulfil the same function.

A similar, but inverse, phenomenon has been observed by researchers in Australia. They were using thermal imaging to study how koalas regulate their body temperature, given the hot climate. Hugging trees was one mechanism, the tree trunk being cooler than the surroundings. The animals were observed moving from the top leaves where they feed in winter down to cooler parts of the tree in summer. Conveniently this would provide a perch where the koala could either lie spread on top of a shady branch (as some big cats are seen to do) or wedge themselves in a junction between a large branch and the trunk. Thermal imaging revealed how the koala uses the tree trunk and/or branch as a heat sink.

The study is published (freely accessible) in the Royal Society journal Biology Letters [Biol. Lett. June 2014 vol. 10 no. 6 20140235] and you can read (and see) more on the BBC web site which also outlines other research into how animals can exploit microclimates in trees and other means to combat high temperatures.

Similar themes can be explored from an earlier blog post on toucans and trees.

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 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!

Michael Nichols lions

I know from the work of Colin Jackson at the BBC Natural History Unit that you get some fascinating shots using infrared cameras with lions at night. You would imagine that National Geographic would also come up with something impressive when covering the same subject.

Michael Nichols spent months photographing lions on the Serengeti and I'll do no more than direct you to a page of his images (some infrared) on the NatGeo web site ... especially a mesmerising shot of a lion called C-Boy. The accompanying piece appeared in the August 2013 edition of the magazine.

I'll be returning to the annals of National Geographic in future posts as they played a significant role in several historic infrared imaging adventures.

BBC Natural History infrared shooting tips

I've known Colin Jackson from the BBC Natural History Unit in Bristol for a few years. He has presented his work, filming wildlife at night using near-infrared and thermal cameras, at both the Infrared 100 symposium and at IBC.

I just came across a video from June 2012, tips on infrared filming, on the BBC Academy of Production web site. In it, Colin is shown using what he calls the 'kebab shop camera', a standard definition surveillance camera made by Ikegami. These days he is using a Canon DSLR with its infrared-blocking filter removed which will produce HD resolution images. You should also know that, if memory serves, his infrared lighting is at 920 mn.

What he doesn't talk about in this video is using thermal imaging. There, as you probably know, you don't need lighting as a thermal imager shows the photons generated by the objects themselves.

Insect infrared reflectance

In the previous post I wrote about the work of Tristán and Michaud in Costa Rica and their papers from the early 1910s on infrared. They were somewhat prolific with their infrared research, including a paper on the appearance of soils, skin and insects over the next few years.

I had been searching for a copy of the article they published in Scientific American on how insects, especially butterflies, look in infrared. Very kindly Michael Mielewczik from the Institut für Agrarwissenschaften (agricultural science) in Zurich sent me not only a scan of the SciAm pages but also a paper he co-wrote on a very similar subject.

Tristan and Michaud had black and white infrared film to work with but Michael and his colleagues have made good use of a digital camera with no infrared blocking and bandwidth-limited to 675-800 nm (deep red/near infrared). The resulting colour palette, presumably due to the optical properties of the beyer filtration on the sensor, is mostly red/magenta. A photograph of a green tree python is particularly striking but the paper really concentrates on insects. Here's an example image showing visible and infrared reflectance characteristics of a group of preserved rose chafer Caelorrhina superba beetles.

The different infrared tones can be clearly seen (B).

The full citation of the paper is:

Mielewczik M, Liebisch F, Walter A, Greven H (2012) Near-Infrared (NIR)-Reflectance in Insects - Phenetic Studies of 181 Species. Entomologie heute 24:183-215

... and you can access the paper as a PDF on the ETH Zurich web site.