erferogram from the interferometer of Figure 7.10 level. Advances are being made in the calibration field with the use of the cornea = 1.376. Light entering the eye through the cornea then of cryogenic electrical substitution radiometers and silicon photo- passes through a chamber called the anterior chamber which is filled diode light traps, (Hoyt and Zalewski, 199()). These are emerging with a clear watery fluid called aqueous humour. The refractive technologies which will undoubtedly lead to a substantial improve- index of the aqueous humour is 1.336. ment in the uncertainties of working standards and field instru- Immersed in the aqueous humour is the iris. The iris is essentially ments. a diaphragm with an opening in the centre, the pupil. The size of the pupil is controlled by means of the circular and radial muscle fibres in the iris. The iris can expand or contract the pupil over a range The human eye from 2mm in bright light to about 8mm in darkness. The variation in pupil diameter depending on the amount of light, is an involun- As stated earlier, the science of photometry is concerned only with tary property. When doing close work the iris also closes down the the measurement of light detected by the human eye. It was stated pupil in order to increase image sharpness. Immediately behind the that the human eye is sensitive to radiant energy from 380nm to iris is the lens which is a transparent elastic body with an outer 750nm in wavelength. In fact, the limits of visibility for young eyes capsule, a less dense cortex and a denser inside core. The lens is are about 313nm to 900nm but for practical purposes, the narrower normally quite pliable with the refractive index varying from 1.406 range is representative for average eyes. The human eye is a positive in the inner core to about 1.386 at the cortex. The lens of the eye lens system and casts a real image on a light sensitive surface which changes curvature in order to focus light onto the retina. is curved to fit the curved field of the lens. In additi