[neilep (OP)]

Hello,

I'm just wondering, when I turn my light off and my eyes have to adjust to the dark.....is it a physical thing that is happenning ?...y'know like a lens focusing...or is it a biochemical reaction thing ?...like photosensitive paper etc.

I also notice that when the light is turned on(and you've had your eyes closed for a while) it doesn't bother me very much but others HATE it with relish.....what's that all about ?

Thank you for your kind consideration in this matter

Neil

'Men are the same as women...just inside out !'  

[Rokitansky]

This is a classical physiology question. Generaly, there are three ways of the adaptation on darkness.

1st  Neural adaptation is the fastest, It includes neurons in the optical pathway and in the brain. It`s not very great, it allows the eye to adapt to 2-3X lower degree of lightness.

2nd Changing the diametar ot the eye pupil accounts for up to 30X change in the degree of adaptaton. It`s also fast.

3rd In the dark, quantity of photosensitve pigment in rods and cons is increasing in the absance of light. This can take several minuts for full adaptation, but the degree of it is very high, up to 25 000 X. Cons are being adapted faster then rods, but they adapt to a far less extent then cons.

All three types of adaptation are performed simultaniously.

[neilep (OP)]

Thanks Darko (or now that my eyes have adjusted perhaps I can call you Notsodarko !!)

I appreciate you clearing that up for me.....and helping me understand the wonder of my eye adjustment.

Cheers

Neil


'Men are the same as women...just inside out !'  

[Rokitansky]

Sure Neil   [][][]

Nice game of words !

[tweener]

Very good answer Darko!  Thanks!

----
John - The Eternal Pessimist.

[neilep (OP)]

Gonna put my eye adjustment capability to the test now !!..night night !!!

'Men are the same as women...just inside out !'  

[bezoar]

Yep, and guys, FYI, the reason your female significant other can walk all the way across the house before she turns on a light is because women see better in the dark than men.  Anatomical fact, and I think it's because we have more rods, or is it cones.  Anyway, our eyes are slightly different from yours.

[neilep (OP)]

....my eyes are blue and generally bloodshot most of the time !!...Nancy !!..do you think it's worth me trying eyedrops ?...............and it's good that you girlies can see better in the dark cos it means you can do the cleaning without having to put the lights on and disturb us blokes

'Men are the same as women...just inside out !'  

[Donnah]

Feel pretty safe and smug making a comment like that with an ocean between us, eh Neil.  Just remember, not only can women see in the dark better, but we can remember imagined slights and petty details forever!!!  (And it's just a matter of time before I'm on your side of the ocean.)

[neilep (OP)]

Uh Oh !!...*gulp*

'Men are the same as women...just inside out !'  

[bezoar]

And furthermore, when we combine our energies, that ocean between us won't help.  You might need more than eyedrops soon!

By the way, my eyes are green, and they get bloodshot, more now that I'm older.  I use the Visine drops from time to time, but I think the bloodshot eyes might be a companion to light colored eyes cause I quit herbal entertainment a LONG time ago.

[neilep (OP)]

I'm going to have to become an authority on camouflage methinks !!!...hell hath no fury eh ?.....still... on the way over don't forget to bring me some dinner, and I'll leave my shoes out cos they need a polish........and...my eyes are bloodshot cos I'm tired most of the time !!, not for any other reason.... *tee hee*

'Men are the same as women...just inside out !'  

[chris]

The changes that take place in the retina, in order to optimise it for different light conditions, are very complicated. I'll explain it when I have a few hours to spare.

But for now, to add one other piece to the jigsaw :

There are 2 populations of photoreceptors (light-sensitive cells) in the retina. These are rods and cones.

The rods are much more numerous (there are about 125 million per eye) and are smaller than the cones (of which there are about 6 million per eye).

Rods are more sensitive to light than cones, but are not colour-selective which is why you see only black and white in the dark. Increased rod sensitivity comes at the cost of reduced resolution (acuity), which is where the cones come in.

But during the daytime it is too bright for the sensitive rods so they are masked by a layer of melanin (the substance responsible for a suntan) at the back of the eyeball.

When it gets dark suddenly (if you go into a dark place from somewhere well lit) the reason that your eyes take a while to adjust is partly down to the delay in unmasking the rods from their protective pigment layer.

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

[neilep (OP)]

Thanks Chris.....fascinating stuff.. what about strobe lighting ? what is happenning there ? do the rods and cones.
 just reach a happy medium ?...also, are you able to give a 'brief' reason why photosensitve seizures occur ?

'Men are the same as women...just inside out !'  

[Rokitansky]

quote:Originally posted by chris






But during the daytime it is too bright for the sensitive rods so they are masked by a layer of melanin (the substance responsible for a suntan) at the back of the eyeball.

When it gets dark suddenly (if you go into a dark place from somewhere well lit) the reason that your eyes take a while to adjust is partly down to the delay in unmasking the rods from their protective pigment layer.

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

Yes, it`s true. However, I think that masking rods with melanin is more concerned with preventing the reflection of fotons from the back of the eyeball during a daytime. It doesn`t affect rods (cons) sensitivity. It prevents flashes (sparkling) in our visual field.

[chris]

Both statements are in fact correct.

The melanised layer at the back of the eyeball serves to absorp stray photons so that they don't hit any other photoreceptors and affect visual acuity.

The pigment epithelium also enshrouds the rods during the day (or times of high light intensity) to prevent them interfering with the more precise (higher resolution) vision subserved by cones. At daytime light intensities rod photoreceptors would be saturated and unable to provide any meaningful contribution to vision other than telling you that there is a lot of bright light about ! You experience this when you walk out of a dim building into a brightly lit street and temporarily cannot see...

And Neil, in answer to your question about strobes, that's essentially where the expression "a blinding flash of light" comes from. When your eyes have adapted to low-light conditions and you suddenly unleash a flash of bright light into them, the photoreceptors are saturated and all they record is a bright light. They also do the biochemical equivalent of 'flinching' so you see poorly for a few seconds afterwards.

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

[cdurk5]

hi, i was wondering, i recently found out that the tapetum lucidum in some animals is what helps them to see, and is also what causes the glow when a light is in their eyes.  Now i wonder, in this is where you experts from earlier come in, has there been any research in helping the human eye see in the dark better? im talking about some type of surgery to a normal eye would help it see better in the dark, possibly creating a "tapetum lucidum" like thing that could be placed behind a human's retina? this may all be a stupid question but im interested in it, and if this was even posssible to do, how this might distort our acuity, not only being more sensitive to light, or effects to our colors we already see, but the actual contrast we see in. any and all information on this would be appreciated.
thanks
Chris

[qazibasit]

Rokitansky is right it is all due to the rods and cones and they helps the eye to adjust in light and darkness and i think your rods do not totally disintegrate thats why you didnt feel bad when you comes in light after a long time in darkness.

[chris]

Hi Chris

animals that have a tapetum lucidum, like cats and dogs, are usually nocturnal hunters so, for them, the benefit is in being able to see better at night. The downside is that in the day time, when there is far more light around, acuity will suffer because the light is being bounced around inside the eye and stimulating additional photoreceptors.

I think it would be very difficult to insert a tapetum lucidum into a human because it is a structural component of the back of the eyeball.

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

[qazibasit]

well cdurk if you are sure about this tapetum lucidum then i will appreciate you to give me such a knowledge coz i was unaware of this fact.