Thanks Andrew for that excellent explanation.
Along the same vein, I have walked around the bush at Howard Springs near
Darwin at night with a head torch on, and seen many many reflections coming
from the ground from spiders eyes. And they were blue! A friend with a
hand-held torch wasn't seeing them at all until he put my head torch on. And
then we realised how many spiders we were walking on...
On 18/11/2013, at 4:54 PM, Andrew Hobbs wrote:
> There are two types of 'eye shine'.
> As Chris said it is partly simple physics. By the nature of the eye and its
> focus on the back of the eye when light is shone into it there is a
> reflection from the back of the eye. As is the case with anything being
> illuminated, at least some of the light is reflected back along the same path
> and will appear to cause the eye to 'shine' (compared to the rather
> non-reflective coat of hair, feathers etc).
> But there is another factor. Some animals (including some birds such as
> owls) which are active at night, have a layer at the back of the eye called a
> 'Tapetum Lucidum'. This layer contains mineral crystals which have the
> property of reflecting almost all the incident light back along the same path
> as the incident light. (Look up 'Corner reflector' on Wikipedia) This layer
> is behind the layer of light sensitive cells, doubling the sensitivity of the
> eye. (The cells have two chances of capturing photons; coming and going).
> When you shine a light into an animal or bird with these eyes they really do
> appear to shine very brightly. However the reflection is not perfect and some
> light is reflected back somewhat 'off axis'. In this case, because the
> reflection is so strong, you can still see a reflection even if your eyes are
> well off the same axis as the torch beam.
> In contrast, in animals (and birds) without a Tapetum Lucidum, the light is
> reflected of the Fundus at the back of the eye. This is not a good reflector
> but some does get reflected back along the axis. Because the reflection is
> so much weaker your eyes need to be much closer to the axis of the torch beam
> to see the effect.
> The colour of the eye reflection from the Tapetum Lucidum depends upon the
> properties of the crystals in the layer. It can vary more or less across the
> full visible spectrum. However in animals without that layer the reflection
> is always red. The red colour is mainly due to the haemoglobin in the blood
> vessels at the back of the eye. (This is the basis for the red eye effect in
> people. We don't have a Tapetum Lucidum.)
> On 18/11/2013 12:18 PM, Chris Corben wrote:
>> It's very simple physics. Almost anything looks brightest when the light
>> source is close to your eyes. But in addition, a bird's eye is pretty much
>> retro-reflective. That is, the light reflected by it mostly goes back along
>> the path it came from. If you think about it, you are looking at the inside
>> of a sphere, so wherever the light comes from, it is reflected back in the
>> reverse direction. The same principle is used to make road markings shine
>> brightly at night. The paint on a road is filled with tiny glass spheres, so
>> that from whatever direction the light arrives, some portion of it is
>> retro-reflected. Since the headlights of a car are not too far off the line
>> of your sight, a lot of that light comes back to your eyes, and the markings
>> look bright. If the paint was just plain gloss paint, it would be much more
>> reflective, but nearly all the light from the headlights would be reflected
>> away from the driver, and the paint would look essentially black. As does
>> smooth ice,
and for the same reason.
>> Owlet Nightjars eyes are not nearly so bright as White-throated Nightjar's
>> eyes. But even a White-throated can be seen at much greater distance with a
>> light which is close to your eyes. It makes such a difference, that you can
>> wear a headlamp and see the eyes of things like owls at reasonable distance
>> even though the headlamp is not very bright. A side benefit of this is that
>> the lower light levels will scare the bird a lot less, so you can actually
>> gain by having a lower intensity light if it is close to your eyes. A
>> headlamp is perfect for that!
>> Incidentally, if you know to look for it, you can see eye-shine of animals
>> in all sorts of unexpected ways. A classic case is to get to a place where
>> there are frogs on the surface of the water. If you get the sun straight
>> behind you (eg in early morning or late afternoon), you will be able to see
>> their eye-shines surprisingly well, especially if you use binoculars.
>> Using binoculars with a headlamp is a great way to find all sorts of
>> creatures at night. Frogs, snakes, geckoes, spiders, small mammals and even
>> bats in the right situations. Just use your binoculars to look at the spot
>> of light from the headlamp.
>> Cheers, Chris.
>> On 11/17/2013 08:39 PM, Roger McNeill wrote:
>>> A few weeks ago Gus McNab was over and we were discussing spotlighting and
>>> I mentioned how I have a good population of Owlet Nightjars on our block
>>> but I never see them at night because their eyes don't eyeshine, despite
>>> wandering the woods after hours.
>>> He told me (politely) how wrong I was and the issue was that I was not
>>> holding the torch in the right place to see it. What I needed to do was
>>> walk around like a unicorn with the torch beam emenenting from between my
>>> eyes. I (polietly) said that is faseniting, thinking that this was surely
>>> some ploy to make me look like an idiot...not that help is required...and
>>> thinking how that could possibly be true?
>>> Last night, I had a visiting bird-o who wanted to see Nightjars and other
>>> things so we decided to wander the tracks and see what we could find.
>>> First try was for White-throated Nightjars...two birds pearched up for us,
>>> brilliant eyeshine. A koala started calling back at the house so we
>>> wandered back, yep bright eye shine...we then decided to walk down to
>>> "owlet-nightjar grove" and I had three birds respond and two come in, one
>>> quite close. I put the torch on the bird and as expected no-eyeshine...but
>>> then I tried Gus' recomendation and move the torch between my eyes and wow,
>>> its eyes shown back bright red like a Christmas tree. Amazing! Gus you
>>> were right, but I have no idea what the explanitaion was or why this is the
>>> case! Thanks for the tip...wanted to share publically.
>>> Roger McNeill
>>> Samford Valley, SEQ
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> Andrew Hobbs
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