Spectral transmittance of visible light by the living human eye

Specifying and controlling the optical image on the human retina

A review covering the trends that led to the current state of knowledge in the areas of: (a) schematic models of the eye, and the definition of the retinal image in terms of first-order optics; (b) the description of the actual image on the retina and methods for accessing and characterizing it; (c) available procedures for controlling the quality of the retinal image in defined situations; and (d) intra-receptoral optical effects that cause differences between the light distribution on the retinal surface and at the level of interaction with photopigment molecules.

Fundus reflectance and the measurement of crystalline lens density

We have developed an optical technique for measuring the in vivo absorption of the human crystalline lens based on using the retina as a reflector for a double-pass measurement of lens density. We compare results obtained by the reflectometric technique with results obtained in the same subjects by a psychophysical method. We find that the estimates obtained with the two techniques are highly correlated, and both measure similar effects of aging on the optical density of the lens. In addition, we show that the significant variations in retinal reflectance attributable to differences in ocular pigmentation and drusen have only a minor effect on the estimate of lens optical density. However, correcting retinal reflectances for the effect of lens absorption shows that the effect of lens absorption may be overestimated with both methods. The overestimation may arise from inaccuracies in the shape of the assumed lens spectrum.

Dark-light: model for nightblindness from the human rhodopsin Gly-90-->Asp mutation

A human rhodopsin mutation, Gly-90-->Asp (Gly90Asp), cosegregated with an unusual trait of congenital nightblindness in 22 at-risk members of a large autosomal dominant kindred. Although rhodopsin mutations typically are associated with retinal degeneration, Gly90Asp-affected subjects up to age 33 did not show clinical retinal changes. Absolute threshold for visual perception was elevated nearly 3 logarithmic units in 7 individuals tested (ages 11-64), indicating greatly compromised rod threshold signaling. However, in vivo rhodopsin density was normal. Although the 38-year-old proband could not perceive dim lights, his rod increment threshold function was normal on brighter backgrounds. The impaired rod vision for dim but not bright backgrounds is consistent with a mechanism of increased basal "dark-light" from thermal isomerization equivalent to an increase of > 10(4) over that of wild-type rhodopsin. The Gly90Asp mutation on the second transmembrane helix places an extra negative charge in the opsin pocket; this could contribute to partial deprotonation of the retinal Schiff base and thereby increase photoreceptor noise. In vitro evidence had suggested that transducin is activated by the Gly90Asp mutation in the absence of both the retinal chromophore and light, termed "constitutive activity." The apparent preservation of functioning rods despite extensive and lifelong night-blindness in this kindred is inconsistent with one current hypothesis that chronic rod activation from constitutively active mutant rhodopsin necessarily contributes significantly to photoreceptor demise in human retinal dystrophies.

Spectral transmission of the optical media of the human eye with respect to keratitis and cataract formation

The spectral transmissions of cornea, aqueous humour, lens and vitreous humour of human eyes were measured in the range from 250 nm to 800 nm in 1 nm steps using a high resolution double monochromator. Cornea and lens are good cut off filters; 1% transmission was measured in various cornea samples between 291 and 298 nm, and in different lenses between 389 and 414 nm. Aqueous humour and vitreous humour showed high transparency (80%) at lambda > 320 nm. The wavelength range of keratitis effective irradiance was found to be completely within the wavelength range of absorption of the cornea, that of cataract effective irradiance was found to be at the short wavelength end partly outside the wavelength range of absorption of the lens. It may therefore be supposed that the action spectrum of cataract obtained by animal experiments is applicable to the human eye only with certain reservations.

Spectral reflectance of the human ocular fundus

Reflectance spectra from discrete sites in the human ocular fundus were measured with an experimental reflectometer in the visible and near-infrared parts of the spectrum. The principal study population consisted of ten subjects 22 to 38 years of age with a wide range of degree of fundus melanin pigmentation. Reflectance spectra were obtained from the nasal fundus, the fovea, and an area 2.5 degrees from the fovea. Spectra were also recorded from several older subjects and from one aphakic patient with a coloboma. The reflectance spectra were found to be influenced by the degree of individual and local melanin pigmentation of the fundus, the amount of blood in the choroid, the transmission properties of the ocular media, and the discrete reflections in the stratified fundus layers. Mathematical models of the optical properties of the stratified layers are proposed and are fitted to the experimental fundus reflectance spectra. The models account for the absorption by blood, melanin, macular pigment, and ocular media, and incorporate tissue scattering and discrete reflectors corresponding to anatomical layers.

The directional sensitivities of the Stiles' colour mechanisms

Field sensitivities of the three IIj (j = 3, 4, 5) mechanisms of Stiles were measured for monochromatic backgrounds of different wave numbers (mu)-1 traversing the eye through different points (r) displaced along a horizontal chord through the centre of the entrance pupil. Each mechanism shows an insensitivity to the direction of retinal incidence of short-wave backgrounds not previously described. The spectral densities of the centre-most part of the lens and of the macular pigment were measured on this eye. With reasonable assumptions the former allowed for correction at the receptor level of the directional sensitivity; together with the latter it allowed correction for the spectral sensitivity as well. No correction for the attenuation of the high spatial frequencies of the background as it traversed the pupil at different r was needed. The anomalies of section 2 (above) disappear after correction for losses in the eye media. After these corrections, for every mu and r, the results are well described by the parabola 'tentatively' suggested by Stiles (1939) for each mechanism, allowing only a small amount of variance attributable to experimental imprecision alone. Each mechanism is most sensitive to backgrounds going through essentially the same point of the pupil, independent of background. This result is inconsistent with a qualitative explanation of the 'hue shift' suggested by Safir, Hyams & Philpot (1971). The field sensitivity spectra for backgrounds traversing the pupil at this most effective point and at the 3.5 mm margin, are the data needed to predict this observer's brightness and colour matches of monochromatic lights passing through the entrance pupil at these two points according to a unified theory of the two Stiles-Crawford effects. In the following paper these predictions are quantified and confronted with results of the matching experiments (Alpern, Kitahara & Tamaki, 1982).

Development of scotopic sensitivity and the absorption spectrum of the human ocular media

Scotopic spectral sensitivity was measured for nine observers (aged 4.5 months to 66 years) from 400 to 650 nm (10-nm steps) by using a 42 degree naturally viewed stimulus. The dependent measure was the visually evoked cortical-potential amplitude that was phase locked to an 8-Hz flickering stimulus. Sensitivity was similar for all observers at middle and long wavelengths, but at short wavelengths there was a decrease in sensitivity with increasing age. The density of the preretinal ocular media was estimated by subtracting the log scotopic spectral-sensitivity function of each observer from the human rhodopsin-absorbance spectrum when the two sets of curves were pinned at long wavelengths. The density of the infant ocular media was lower than that for adults. To quantify the sequence of ocular-media development, scotopic sensitivity was determined for an additional 42 observers (aged 1 month to 70 years) at two spectral points: 553 nm, where the optic-media density is low, and 405-430 nm, where the density is high. From these data, optic-media density at 400 nm was calculated. Despite substantial individual differences within each age, a clear aging function emerged. Preretinal optic-media density increased monotonically from birth throughout adulthood. Thus optical density at 400 nm differs by about a factor of 22 between the average 1-month-old infant and the average 70-year-old adult.

Rhodopsin flash photolysis in man

1. Human rhodopsin in vivo was flash bleached by a 600 musec xenon flash which could deliver to the retina up to 15 rod-equivalent quanta per rhodopsin molecule, and the fraction bleached measured by fundus reflexion densitometry. 2. The curve relating fraction rhodopsin bleached to intensity of flash saturates at 0.5 to 0.6. Thus, 40-50% of the rhodopsin is left photo-regenerated by the brightest flashes. 3. Three types of densitometry experiments confirm the saturation of the bleaching curve. 4. The kinetic constants required to account for the observed photo-regeneration were somewhat discrepant with in vitro and in situ estimates from infrahuman species. Specifically, (i) the quantum efficiency of the back reaction, metarhodopsin I hv leads to rhodopsin, was inferred to be nearly as high as that of the forward reaction; and (ii) the rate of the metarhodopsin I leads to metarhodopsin II dark reaction was inferred to be less than 500 sec(-1).

The density and photosensitivity of human rhodopsin in the living retina

1. The visual pigment in a 5 degrees circular patch of the living human retina 18 degrees temporal from the fovea was studied with the Rushton retinal densitometer. The measuring light (570 nm) was selected to obviate artifacts from colour photoproducts.2. The action spectrum of a 10% bleach agrees well with the action spectrum at absolute threshold for the same patch of retina. The quantized C.I.E. scotopic spectral sensitivity curve is a good description of both spectra. Therefore, the visual pigment studied must be human rhodopsin.3. Its density has been estimated in five different ways. The results are in reasonable agreement. The optical density of human rhodopsin in vivo is about 0.35 (common logarithmic units) at its gamma(max.)4. The photosensitivity of human rhodopsin in vivo was determined by studying its rate of bleaching in response to steps of monochromatic light exposed to the dark adapted eye, by measuring the amount bleached in the steady state by monochromatic lights as well as the amount bleached by 10 sec flashes of white light.5. The results obtained by the different methods are in good agreement with each other and with previous estimates made by others using white light.6. The photosensitivity of human rhodopsin in vivo [epsilongamma(max) = 62,000 to 120,000 l./cm mole] is much higher than expected from in vitro measurements.

Prereceptor colour vision distortions in protanomalous trichromacy

1. Scotopic luminosity and fundus spectral reflexion in the protanomalous fail to confirm predictions made from the hypothesis that protanomalous photopic luminosity loss is due to an inert red-absorbing filter in his ocular media.2. If it were supposed that the luminosity losses were due to a reduced number of normal red cones, the anomaloscope mismatches could result from a prereceptor distortion such as a reduced concentration of macular pigment or a tilt of the foveal cones. Experiments exclude these two possibilities.3. An anomaloscope is described which makes it possible to measure colour-matching properties of the protanomalous eye by transcleral illumination. Such measurements exclude, as a class, hypotheses which attribute protanomalous colour-matching distortions to an inert filter localized anywhere between the cone outer segment and the cornea.4. It is concluded that the absorption spectrum of at least one of the three cone visual pigments of the protanomalous eye must differ from that of the pigments of the normal fovea.