Friday, November 11, 2016

Science: Reflecting on Plants




The primary subject of near infrared photography are plants; leaves particularly.  They appear ghostly, bright; often angelically white, sometimes in pastels of pink, mint, lavender or sky blue, depending on the filter used. Even when taking on a green tone with certain optical filters, near-infrared reflectance off of leaves appear between twice to quince times brighter than the darker green tone we see with the naked eye.  A basic principle is that color of any object depends on the slice of light from the sun's full spectrum that is reflected off the object and reaches our eyes.
A healthy leaf, or granny-smith apple reflects green light, which is what we see.  However, it also reflects light we can't see, in the near infrared.  If you want to capture better infrared photos of foliage, knowledge about what causes the dramatic increase in near-infrared reflection, and when it happens, could aid you in this pursuit.


 
To understand why this happens, let’s dissect a leaf.  There are four main layers in a cross-section of leaf, going from the top sun-side through the middle and to the underside: top/underside epidermis, palisade mesophyll, spongy mesophyll, intraspace/ion-exchange/veins.


Blue and red light from the sun are absorbed by chloroplasts (chlorophyll containing cells), popping off electrons from CO2, and transformed through a bevy of chemical reactions into O2, C and eventually storage energies such as sugars. Green light is not as heavily absorbed because the metal complexes of chlorophyll do not have transitions in the green energy.  Roughly 15-20% of green light is reflected from the leaf surface (at and just under the top epidermis), a fraction transmitted through the leaf, and the rest absorbed by deeper layers which are heated by the sun’s green light energy. 

The plot below is a common way of describing the color response of  various elements (i.e., filters, of scene reflectance from a leaf, the sensing response of the human eye or of a modified camera, etc.). The vertical axis shows the relative response level, in this case, the amount of light absorbed by chlorophyll (and carotenoids from fruit).  The horizontal axis shows the wavelength or color. Most people are not familiar with wavelength numbers. The human eye can see blue (400nm), to green (500nm) to red (600nm), and begins to fade at longer wavelengths beyond 700nm. The plot shows that Chlorophyll A absorbs light (meaning it keeps the light in the molecule and converts it to chemical energy) in the deep blue (430nm) and in the far red (680nm).
Have you ever lain in the cool grass on a hot summer day? If you touch a leaf on a warm sunny day, it won’t feel as hot to the touch as you would expect, and much less than even the light gray paint of a car.  That’s mostly because the bulk of the sun’s energy, residing in the infrared, is only very slightly absorbed by the leaf. 


Again I offer a plot, where the vertical axis shows the relative response level, in this case, the reflection of light off of leaves, from 0 to 100% (1.0).  The horizontal axis shows the wavelength or color. The plot shows that near infrared light is reflected at about 50-60%, and most of the remaining amount is transmitted through the leaf (not shown), with a small percent absorbed as it reflects multiple times inside the leaf (not shown). 
Try photographing a leaf on the top and on the bottom and you may find it is about as bright on both sides.  You may even find photos of trees and leaves are as interesting looking up through them at the sun and sky as photographing them from beyond their canopy.








You may also notice that leaves and grass look softer in the near infrared than in the green. When NIR light enters the leaf, it reflects in the interior many times, creating a diffuse broad-angle reflection or scattered transmission on the underside.  This effect is even pronounced on bright cloudless days, while shadows form crisp edges, but the leaves remain softened.

Why are leaves red in fall? When the leaf begins dying, the chlorophlasts (and their chlorophyll pigment/metal complexes) decrease in absorbing the red light, such that instead of absorbing red it begins reflecting in increasing amounts of red, causing the green to mix with red, transitioning from yellow to orange to red tones as autumn (and leaf death) progresses. As the blue light also increases reflectance, the leaf appears brown and even gray upon complete death.

With most infrared photography filters, obtaining fall colors is difficult because far more light reflects in the infrared than in the green or redder autumn.  However, in future blog I will describe custom filters that I use to pull out various colors even in NIR photography.


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