Pupil size and light intensity

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Alun Harford - 27 Jan 2006 15:34 GMT

I'm wondering if anybody can tell me (approximately) what the relationship
is between the size of the pupil and light intensity. I know that changing
the size of the pupil only allows for fine tuning the amount of light we
"see", and also may change due to emotional state etc.

I would expect that increasing the amount of light enterring the eye would
quickly cause the pupil to constrict. Then, as the photoreceptors adjust
chemically, the pupil will slowly dilate back to its original size.

I'm trying to work out (approximately) a function of pupil size over time
when a light is shined into the eye. Can anybody point me to any useful
information on the topic?

Thanks,

Alun Harford

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Don W - 27 Jan 2006 22:13 GMT

Maybe...

http://retina.anatomy.upenn.edu/~lance/eye/pupil.html

> I'm wondering if anybody can tell me (approximately) what the relationship
> is between the size of the pupil and light intensity. I know that changing
[quoted text clipped - 12 lines]
>
> Alun Harford

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Alun Harford - 30 Jan 2006 00:35 GMT

> Maybe...
>
> http://retina.anatomy.upenn.edu/~lance/eye/pupil.html

That's certainly interesting, thanks.

It does, however, seem to suggest that iris size is determined solely by
light intensity, yet a quick experiment with a torch and a mirror suggests
that this is not the case.
When I shine a torch into my eye (through the mirror) and look the the size
of my pupil, it clearly starts big, goes small, and then "settles" on
something in the middle.

(Interestingly, it's far from a smooth transition - the iris seems to
twitch)

Does anybody know of anything else?

Alun Harford

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Don W - 31 Jan 2006 01:44 GMT

> (Interestingly, it's far from a smooth transition - the iris seems to
> twitch)
>
> Does anybody know of anything else?
>
> Alun Harford

In remembering .... some years ago, published in IEEE Transactions (I
think), there was a study on focusing a small beam so that it would be split
(like approx in half) by an arc of the iris. By the correct size and
positioning, the pupil size was driven into a state of oscillation. (Cannot
give accurate reference to this).

Don W.

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Mike Tyner - 31 Jan 2006 03:28 GMT

> In remembering .... some years ago, published in IEEE Transactions (I
> think), there was a study on focusing a small beam so that it would be
> split (like approx in half) by an arc of the iris. By the correct size
> and positioning, the pupil size was driven into a state of oscillation.
> (Cannot give accurate reference to this).

No mystery. It's pretty easy to set up these oscillations with a slit lamp
microscope.

It's also easy to see the iris moving spontaneously without such a stimulus.

Normal "pupillary unrest" is sometimes called "hippus" but that term is more
conventionally reserved for more vigorous abnormal movements.

-MT

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