Monday, October 21, 2013

Medical Mondays: Eyeshine and Red-Eye on Photos

Hello all and happy middle-of-October! Can you believe it's gotten this far into fall? I swear a few days ago it was August. What the--?

Anyway.

Today I'm blogging about tapetum lucidum. What is that?

It's this:

Hello cutie lemur! Source: Wikipedia
Tapetum lucidum is the reflective layer behind the retina (the inner surface of the eye, which is the light sensitive tissue that picks up images seen) of an animal eye that reflects light. It's the reason why when you shine light at your dog or cat at night, you see their eyes glow.

The color of the eyeshine you see depends on the angle of the light hitting the animal's eye, and the kind of animal. Each species has its own set of different crystals that make up the tapetum lucidum, and that is why:
  • Some fish have white eyeshine
  • Horses have blue eyeshine
  • Cats, dogs and raccoons have green eyeshine
  • Rodents and birds have red eyeshine
What about humans? Well, humans and most other primates don't have a tapetum lucidum.

What about red-eye in photographs? This occurs when the camera flash is very close to the lens that is taking the photograph. The light hits the choroid (a blood vessel layer behind the retina) and reflect back into the camera, which records the photo. The red color comes from the redness of the blood 
vessels.

Source: Wikipedia
Why do some people always have red-eye in photos? Or the reverse question, why do people like me never get red-eye? People who have lighter skin tone and blue eyes often have less melanin (dark pigment) on the inner surface of their eye (the fundus, which contains the retina, optic disc, fovea and macula) so more light is able to pass through to the choroid. In people like me, the melanin in my fundus (geez, that sound SO PERSONAL) absorbs that light and very little gets reflected back by my choroid. 

How did they do this in Blade Runner?

Rachel and Deckard have a moment. (Source: Warner Bros.)
Man, wasn't that cool??? They showed the eyeshine of an artificial owl too, and the eyeshine of both Rachel (a replicant) and the owl highlighted the fact that they were both artificial. Beautiful and perfect, but fake.

What the director did was shine a light straight at Rachel that was extremely close to the camera lens. If you look carefully, you'll see that Han Solo (whoops, I mean Indiana. *slaps face* I mean Harrison Ford!) has a faint eyeshine, but you don't notice it in the movie because Rachel closer and in focus.

What's the red reflex?
This is a reflex that doctors look for when we sometimes shine light into your eye during an exam. In kids, the reflex is very bright. When a more white reflex is observed instead (called leucocoria), that is usually a sign of pathology, like an eye cancer (retinoblastoma in kids), cataracts in older individuals, or other problems. 

Well! Everything you needed to know about eyeshine! Thanks for stopping by and leave any questions in the comments. :)

22 comments:

Shelly said...

Finally, an explanation I can understand about eyeshine! And that video of the raccoon- fascinatingly horrifying!

Nicole Zoltack said...

Wow, that's really interesting!

Old Kitty said...
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prerna pickett said...

oooh, I never noticed the eye shine on Harrison Ford. Fascinating stuff!

Alex J. Cavanaugh said...

Yes, fascinating stuff! Never knew we couldn't reflect light the way animals can.

Casey L. Clark said...

Very interesting! I love Medical Mondays! :)

Southpaw said...

Very cool stuff. I remember reading a book where hunters were specific about the color of eyeshine.

mooderino said...

So it doesn't mean they're possessed by the devil? Are you sure?

Rock said...
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Natalie Aguirre said...

Thanks for sharing. I had no idea what causes red eye in pictures.

Deb Salisbury said...

Very cool! I'd wondered about Rachel's eyes.

My sighted cats have green reflections, but my blind cat's eyes reflected red.

Kelly Polark said...

This is so cool!!

J E Oneil said...

Good info! I always wondered how that worked.

Shelley Munro said...

Fascinating stuff. It's interesting how the different animals have different colored eyeshine.

BECKY said...

Hi Lydia! Gosh, it's been way too long since I stopped by. Your blog has a whole new look! (probably not that new, at all, though, huh?!)
I've always enjoyed all your posts, and especially Medical Mondays. I hope to be able to stop and comment more often. Have a wonderful rest of the week!

Barbara Watson said...

Eyeshine bugs me most when photographing my dog with a flash. When I try to reduce the 'red-eye' with a photo editor, it tell me, "No red-eye detected. Now I know: It's because dogs have green eyeshine. :-)

Rachna Chhabria said...

I would always wonder about the red eyes in photographs.Thanks for clearing that doubt.

LD Masterson said...

I always wondered why pictures of my dogs had the "green eye" effect, but I figured it was the same as our "red eye" - just a different color. Thanks for setting me straight.

Dianne K. Salerni said...

I have fair skin and blue eyes. I always have red-eye in photos unless my husband uses the "anti-red-eye" setting. That sets off a double flash, which for some reason combats the red-eye effect. *shrugs*

Angela Brown said...

I've often wondered how the eye shine happened but hadn't searched it out. This is why I love coming to your blog because I can find out some of the coolest things!!

Ron Smith said...

Now I can finally rest easy that I know this.

What a fun and informative post!

Great blog you have.

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