Saturday, April 29, 2006

Wonders of the human eye

Current read:
Pirenne MH, ‘Vision and the Eye’, C. Tinling and Co., 1967, London.

The eye: an optical sensor so utterly magnificent that it is consistently used in arguments in favour of religious doctrine.

Disregarding this irrelevant hoo-ha, let’s talk a little about the human eye itself.

The eye can work with a huge variation of illumination, from direct sunlight to illumination by mere starlight (I have experienced this on a clear moonless night far removed from civilisation- the environment remains slightly visible).

The most obvious mechanism is the iris, which regulates the size of the pupil (the orifice that allows passage of light towards the retina). In dimly lit conditions, the pupil dilates to allow more light to strike the retina (the photo-detector), thus improving sight.

However, there is an obvious limit to how much the pupils can dilate. To further extend the operating range of the eye, the retina is equipped with 2 different photo-detectors: rod cells and cone cells. Cones work well with bright light and also respond to colouration of light; rod cells respond to dim light and do not give colour information.

Thus in bright conditions, the rods may be saturated (that is, their output is at maximum) but the cones will be able to provide information regarding the quantity of light falling on each cell. Conversely, the cones will return a zero signal in dim conditions while the rods give good data.

This effect can be observed when one looks at the stars. Bear in mind that cone cells are more densely clustered in the middle of the retina and rod cells are distributed around the periphery. It is often noted that the star that one is explicitly trying to stare at persistently fades from sight- cone cells are not very good at sensing dim light sources. The way to observe a star is to stare away from it and look at it using your periphery vision where the rod cells dominate (and give good low-light seeing).

Compared to a photographic camera, the situation is analogous to having a mixed sensor of say, ISO 50 and ISO 400.

Apart from the variable aperture found in the pupil and the mixed sensor in the retina, the eye has one more trick up its sleeve that further extends its operating range. The light detecting cells have the ability of changing their sensitivity, although this effect is not instant but requires durations on to the order of minutes.

Returning to the photographic camera analogy, one might find that the sensors automatically change from ISO 50 and 400 to ISO 200 and 1600 respectively.

In experiments that attempt to deduce the minimum light required for detection, the test subject is usually dark-adapted (enclosed in a completely dark room) for up to 45 minutes to allow this sensitivity to increase to its maximum. Initially, the sensitivity increases rapidly and this increase slows down until it reaches its maximum.

Experiments conducted in the middle 20th century showed that the eye can detect incredibly dim light flashes. A suitable dark-adapted test subject can detect light flashes of blue-green in colour (wavelength of 507μm) of merely 50 to 150 quanta, corresponding to a range of 0.02 to 0.06 femto-Joules (10^-15 J).

Efficiency of the light sensing element in rod cells

Rod cells contain a pinkish pigment rhodopsin, also referred to as ‘visual purple’. When exposed to light, this pigment changes from pink to clear- absorption of energy provided by light bleaches the pigment. The reaction stimulates nervous endings in the rod cells, thus giving the resulting information on amount of light falling on each rod cell.

The previous experiment showed that about 50 to 150 photons need to enter the eye for the flash to be detected. However, not all this light is absorbed by the rhodposin. Absorption occurs at every stage the light passes through- the cornea (the outermost layer of the eye), the aqueous humour (a fluid between the lens and the cornea), the lens, the vitreous humour (the gelatinous mass in the eyeball), the finally rhodopsin.

However, the rhodopsin does not absorb all light falling on it, and some is transmitted to other parts of the slightly reflective retina. Light reflected by the retina then reverses in direction and exits the eye in the same manner it entered, again with a proportion being absorbed at each stage it passes through.

As already stated, it is known that 50 to 150 photons need to enter the eye for detection to occur, but how much must be absorbed by the rhodopsin itself?

To find out, very dim light of known intensity is shone into the dark-adapted eye of a test subject. The quantity of light that is reflected back out of the pupil is measured. By comparing the two values, the quantity of light absorbed by the entire eye is easy to deduce.

Next, a bright burst of light is shone into the eye. This serves to bleach the rhodopsin so that it no longer absorbs more light.

Immediately after, before the rhodopsin returns to its pink hue, another dim light of known intensity is shone into the eye, and the reflected intensity measured. Again, the quantity absorbed by the entire eye is easily deduced.

By comparing the absorption of light with and without rhodopsin, the quantity of light absorbed by rhodopsin can be inferred.

It was found that a mere 5 to 15 photons are required for optical stimulation, which is remarkably close to the theoretical limit of 1 photon.