Micscape Lite: Micscape contributors sharing their other interests.
Near infrared studies of autumn leaves.
by David Walker, UK
My interest in exploring microscopy outside the visible spectrum using both near IR and UV also extends to conventional photography. I use a home modified older Sony S75 3.3 Mp camera with excellent Zeiss 3X optical lens for monochrome near IR studies. A selection of galleries have been shared in previous Micscape Lite articles.
Recently I have been exploring autumn landscapes where was finding that leaves with strong autumn colours reflected NIR almost as brightly as green leaves, the latter noted for this property where they give the near white other worldly look to NIR imagery. To gain an insight into how autumn leaf colours are seen in reflected near IR I brought a selection of leaves indoors to do side by side comparisons of visible and NIR. (Bright diffuse sunlight from a large window, Hoya R72 filter, custom white balance from grass brightly lit. Tonal balances in Photoshop adjusted to match shadow and highlights, mid-tones not adjusted manually. Visible images, iPhone 7.)
Series 1: two pairs, visible cf 720 nm shots.
This series of graded autumn colouring for silver birch shows that other leaf pigments reflected NIR well. A further selection below. Note the poorer reflectivity of the dark brown spots where leaf structure is being lost.
Four sycamore leaves.
Oak, rowan and a pine.
The deep red of a dwarf maple reflect NIR well. The single horse chestnut leaf cluster with a range of colours from green through yellow, light browns to dark brown usefully captures in one image the relative reflectivity. The very dark brown and curling far right is the leaf structure breaking down where NIR reflectivity drops considerably.
Series 2: visible cf NIR at 720 (Hoya), 760, 850 nm (latter two cheap China 'dHD' brand). Custom white balance redone for each filter.
I invariably use the 720 nm filter which retains some deep red light in the visible above 700 nm. This gives usable handheld shots at ISO 100 as the old Sony S75 does not have a good low noise, long exposure performance. For interest, filters further into the near infrared were used above.
Comments: The set of images clearly show that leaves lacking chlorophyll still reflect near infrared strongly. Online resources discuss that chlorophyll does not predominantly reflect green nor NIR and illustrated with reflectance spectra. It is the underlying plant structure which reflects. The dark brown curled parts of the horse chestnut above loses its NIR reflectivity because underlying leaf structure is being lost.
Useful references: Many detailed studies have been made on how vegetation reflects near infrared as satellite cameras use this part of the spectrum to monitor vegetation types and changes caused by both Nature and man.
Chlorophyll does not reflect green light – how to correct a misconception, Olli Virtanen, Emanuella Constantinidou & Esa Tyystjärvi. Journal of Biological Education, Volume 56, 2022 - Issue 52, pp. 552-559.
Physical and physiological basis for the reflectance of visible and near-infrared radiation from vegetation, Edward B. Knipling. Remote Sensing of Environment, Volume 1, Issue 3, Summer 1970, pp. 155-159. This paper was in the public domain on the Internet Archive when downloaded this month but curiously can no longer find on the archive. The original linked to lies behind a paywall.
Comments to the author David Walker are welcomed.
Published in the November 2023 edition of Micscape.
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