The Human Retina

Learn Photo Editing

Learn Photo Editing

Get Instant Access

The human eye combines an image-forming lens and a versatile detector into one compact unit. The detector in the eye is called the retina; it is an array of cells that detects light by the breakdown of the chemical rhodopsin. The breakdown products of rhodopsin trigger nerve responses that send an encoded signal to the brain through the optic nerve, where the signals are interpreted as a visual scene.

The lens in the eye has a focal length of about 16 millimeters, and a pupil that varies in aperture from 7 to 1.5 millimeters (yielding focal ratios from//2.3 to fl 11) to adjust the amount of light admitted to the eye. The retina is located on the curved interior of the eye, opposite the lens and if flattened, would measure about 40 millimeters in diameter. Because the retina is fully integrated into the structure of the eye, it cannot be attached directly to a telescope, but must be used in conjunction with an afocal lens system consisting of an eyepiece and the objective lens of the eye itself.

Nevertheless, for comparison with photographic and CCD detectors, we shall examine the properties of the retina as a detector. It contains some 100,000,000 light-sensing cells of two types: rods and cones. Rod cells cover the

Figure 1.5 The eye is a high-performance imaging system containing a lens, detector array, and an on-board image processing system in the neural network overlying the rods and cones. The eye is so sensitive that under optimal conditions, an observer can see a flash of light consisting of ten photons.

Figure 1.5 The eye is a high-performance imaging system containing a lens, detector array, and an on-board image processing system in the neural network overlying the rods and cones. The eye is so sensitive that under optimal conditions, an observer can see a flash of light consisting of ten photons.

entire retina, and work well at low light levels. Cone cells are clustered near the optical axis of the lens, and operate best at high light levels. Individual cone cells are optimized to detect light at different wavelengths to provide color vision.

Near the optical axis, the retina consists of cone cells—each about 2 microns in diameter—packed to a density of about 1,000 per square millimeter. The cones match the diffraction limit for the lens of the eye, providing an angular resolution of about 80 seconds of arc. Away from the optical axis, resolution is much lower. The light-sensitive parts of the rods and cones are nearly 100% efficient at absorbing photons, but because they are located on the back side of the retinal tissue, approximately half the incident light is lost in passing through the overlying neural network. This layer of nerve cells mediates against noise by sensing light only when several adjacent cones or rods are simultaneously triggered. Because of these losses, the overall quantum efficiency of the retina is about 15% at the peak rod-cell sensitivity at 505 nm wavelength. The effective integration time of the retina is 100 to 200 milliseconds (consistent with its function in a moving animal), and the generation of the signal is continuous.

In addition to filtering noise, the network of nerve cells preprocesses the signals generated by the light-sensing cells so as to detect edges, lines, and small differences in color. Thus, the signals that travel to the brain are not raw brightness data, but partially processed information on the shape, size, and color of objects in the visual field.

Was this article helpful?

0 0
Learn Photoshop Now

Learn Photoshop Now

This first volume will guide you through the basics of Photoshop. Well start at the beginning and slowly be working our way through to the more advanced stuff but dont worry its all aimed at the total newbie.

Get My Free Ebook


Post a comment