As an aside to this topic -
Does shining a bright light into an animal's eyes do any damage to the eyes?
John
Wooroloo WA
-----Original Message-----
From:
On Behalf Of Andrew Hobbs
Sent: Monday, 18 November 2013 1:55 PM
To:
Subject: Eye Shine and Owlet Nightjars
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.)
Andrew
On 18/11/2013 12:18 PM, Chris Corben wrote:
> Roger
>
> 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.
>>
>> Cheers,
>>
>> Roger
>> Roger McNeill
>> Samford Valley, SEQ
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>
>
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