In anti-glare spectacles comprising a frame and a pair of lenses, each lens comprises a substantially circular central zone of substantially complete transparency and an adjoining light-absorbing annular zone over an angular range of substantially 45.degree. in which the light absorption increases sharply from the inside of the annulus and drops towards the outside of the annulus by the square of the angle. An outer zone of each lens has a low absorption.

The invention relates to anti-glare spectacles or glasses comprising a 
conventional frame and lenses held therein to be centralised with the 
pupils, the lenses exhibiting a different transparency of absorption over 
their area. 
Anti-glare spectacles of this kind are known and numerous embodiments are 
offered primarily to motorists. In the case of the known anti-glare 
spectacles for motorists, the lenses are tinted at the centre and top and 
completely transparent in their lower regions. There can be a gradual 
transition from complete transparency in the lower region to complete 
absorption at the top. When wearing these spectacles, the motorist moves 
his head downwardly when experiencing glare by the headlights of an 
oncoming vehicle and hence cannot see objects in front of him. The beams 
from the headlights thereby strike zones of ever increasing absorption so 
that the glare light intensity at the eye E.sub.B1 is reduced. When 
wearing such spectacles, the motorist must constantly raise and lower his 
head. In another known type of anti-glare spectacles for motorists, the 
lenses are strongly tinted at one side and towards the other side they 
have a decreasing tint. On the whole, the tinting in one lens extends over 
about half the total width of the lens and in the other lens about one 
third. The arrangement of these different zones of complete transparency 
on the left-hand or right-hand lens and the arrangement of the tinted 
regions on the one side or the other side of the lenses depends on whether 
the anti-glare spectacles are to be worn by motorists in countries having 
left-hand or right-hand traffic (U.S. Pat. No. 3,512,880). When wearing 
these spectacles, the motorist must constantly shake his head to the left 
and right. On one side, these spectacles give the motorist an unhindered 
peripheral view. For this reason street lanterns, which also can cause 
glare, are not covered. Generally speaking, the biggest disadvantage of 
the known anti-glare spectacles resides in the fact that objects which 
ought to be seen also fall into the absorption zone but this should be 
avoided on all accounts. 
Physiological glare reduces the perceptability of the eye and occurs if one 
or more glaring lights are disposed in the field of view or if the field 
of view itself has a high luminance. The detrimental influence is caused 
by stray light which is created when light beams pass through the eye 
media, primarily in the cornea, the non-homogeneously structured lens of 
the eye and in the retinal layers themselves. The lens of the eye changes 
with an increase in age, which leads to a reduction in the width of 
accomodation and to more formation of stray light. Older persons are 
therefore more sensitive to glare than are younger people. 
The reduction in the perceptability of the eye under glare is caused by the 
fact that the image of the object to be viewed and that is always formed 
at the centre of the retina, the fovea centralis, has a haze of stray 
light superimposed on it whereby the contrast of this image is reduced. 
The casual relationships of physiological glare have often been 
investigated and discussed in publications. Glare is stated to be 
expressed by a uniform field surrounding the viewed object having a 
luminance that requires the same threshold value of the viewed object as 
under glare. This surrounding field luminance is equivalent to the effect 
of stray-light created in the eye and is termed equivalent stray-light 
luminance L.sub.saq. The relationship is given numerically by the formula: 
EQU L.sub.saq =K.multidot.E.sub.B1 /.theta..sup.2 (Equation 1) 
wherein 
L.sub.saq is in cd/m.sup.2, 
K is an age-dependent constant amount to about 10 for the age group between 
20 and 30, 
E.sub.B1 is the illuminance in Lx produced by the source of glare in the 
plane of the eye normal to the direction of viewing, and 
.theta. is the angle between the viewed object and the centre of the source 
of glare in degrees. 
With several sources of glare in the field of view, the individual 
proportions of stray light are superimposed according to the formula: 
##EQU1## 
The exponent of .theta. applies only for .theta..gtoreq.2.degree.. 
Equation 2 experimentally proves that the stray light calculation is also 
applicable to any desired surrounding fields, and thus the surrounding can 
be considered as being made up from small light sources and disposed 
around a central field of about 2.degree. diameter. It has been found that 
a bright surrounding field reduces the contrast of a foveally viewed 
object, which leads to an increase of its threshold. This detrimental 
influence in viewing can be eliminated if one reduces the luminance of the 
source of glare in the field of view and thus the amount of the stray 
light created in the eye media. 
The present invention is based on these considerations and leads to 
anti-glare spectacles in which the absorptions of the lenses are designed 
according to physiological considerations. Accordingly, it is an object of 
the present invention to provide anti-glare spectacles constructed so that 
the equivalent stray-light luminance L.sub.saq over the entire field of 
view becomes constant. The wearer of these spectacles will then be 
confronted with adsorbing zones only at places where sources of glare 
appear in the field of view and experience not the hazardous effect on the 
ability to view the street in front of him. 
In spectacles of the aforementioned kind, this object is achieved in 
accordance with the invention with the aid of the features in the appended 
claims. The spectacles are suitable for wearing at a place of work where 
glare might occur because of reflections or bright surfaces around the 
viewed object on the work surface, as well as for motorists. In the latter 
case, a desirable feature is that each lens includes a completely 
transparent lower zone bounded by a horizontal line and a line subtending 
an angle of about 50.degree. with the vertical. In general, no glare 
sources occur in this region of the field of view from oncoming vehicles. 
In this very important part of the field of view for observing traffic, 
the spectacles therefore retain their full transparency. 
The invention therefore utilizes the relationships given in Equations 1 and 
2 in order to reduce glare from light sources such as is encountered in 
traffic at night. A lens placed in front of the eye is optically 
constructed so that it has very little light absorption in a central zone 
of the visual field, i.e. the place where fixed objects are perceived. The 
user of the spectacles according to the invention is therefore always left 
with a central zone in which he has full visibility. Following this 
central transparent zone, the absorption of the lens increases sharply 
until it reaches a maximum of 0.8 to 0.85 approximately 7.5.degree. from 
the centre in order to reduce the intensity of glare occurring 
particularly in this zone through the headlights of oncoming vehicles, 
which leads to a reduction in the formation of stray-light. The angles 
stated relate to a spacing of the lens from the nodal point of the eyes of 
26 mm. The absorption is then reduced again with a gradient following the 
law E.sub.B1 .multidot..theta..sup.-2, to approach zero at approximately 
20.degree. from the centre. This creates a zone of lower absorption at the 
periphery of the field of view. This feature serves to maintain unhindered 
peripheral view at night time because the visual information available 
from the periphery of the field of view is important for traffic safety. 
The transparent peripheral zone becomes possible because the effect of a 
source of glare is reduced with the square of the angle .theta.. With the 
selected construction of the absorption gradient for the lens following 
E.sub.B1 .multidot..theta..sup.-2 glare remains practically constant when 
passing an oncoming vehicle. In one form of the invention, the course of 
the gradient for the light transmission is circular so as similarly to 
reduce the sources of glare occurring in the upper half of the field of 
view, for example street lanterns. On the other hand, no sources of glare 
need to be expected in the lower part of the field of view in which there 
is an optically transparent zone. 
It is a further feature of the invention that the transitions from 
optically dense to optically thin zones are gradual. It has been found 
experimentally that the perceptability of objects in surrounding fields of 
small sizes increases again, and this cannot be explained by stray-light. 
Other tests have shown that voluntary eye movements cause a defect in 
adaptation of the retinal area used for perception through the marginal 
contrast between the surrounding field and the internal field which is 
temporarily formed on the fovea. Even if the edge of the clear central 
zone is sharp adjacent the zone of high absorption, this will not be 
sharply imaged on the retina because the optical medium is disposed far 
within the near point of the eye. A detrimental effect would nevertheless 
become noticeable, especially because of the added effect of involuntary 
eye movements, so that the line of sight can pass through the lens of the 
spectacles even near the rim of the central zone. For this reason, the 
transitions of the absorptions should be gradual as indicated in the 
accompanying drawings. 
The basic construction of the anti-glare spectacles according to the 
invention is intended for use at work places where the requirements for 
visibility are high and the generally bright surfaces surrounding the 
viewed object could reduce its contrast and thus perceptability, for 
example in surgery. Similarly, glare could be produced from strong light 
sources or reflections.

In the FIGS. 1 and 2 embodiments, a central zone 12 of the lens 10 is 
completely transparent. This central zone is adjoined radially by an 
annular zone 14 of increasing absorption. In the FIG. 1 embodiment, 
absorption reaches a maximum value of about 0.85 and in the FIG. 2 
embodiment a maximum value of about 0.55 to about 0.6. This is followed by 
a peripheral zone 16 giving complete transmission of light. In the FIG. 1 
embodiment which is intended for a motorist, a sector 18 at the bottom 
right-hand portion of the lens is completely transparent. The FIG. 2 
embodiment intended for workplaces shows a strictly circular construction 
without any transparent lower zone. In this case the maximum absorption is 
0.5 to 0.55 so that the objects occurring around the absorption zone are 
not excessively weakened and the field of view will not appear excessively 
non-homogeneous. Calculations show that in a homogeneous surrounding field 
a reduction in stray-light by about 15% is achieved with the spectacles 
according to FIG. 2. 
The course of transmission or absorption over the width or radius of a lens 
is shown in FIGS. 3 and 4. In FIG. 4, the transmission (upwardly along the 
ordinate) or absorption (downwardly along the ordinate) is plotted against 
the angle .theta. in degrees or against the radius in mm, curve 1 
representing spectacles for use at night-time and curve 2 spectacles for 
use in daylight. Without considering the absorption occurring through the 
glass itself or the substrate, both FIGS. 3 and 4 show that there is full 
transmission of light in the central zone over an angle of about 
11.degree. or 5 mm. This is followed by a zone of increasing absorption 
from about 5 to about 7.5-8 mm. The rise is practically linear. There is 
then a zone of constant absorption. E.sub.B1, which is the intensity of 
illumination produced at the eye by a passing automobile, is in this case 
substantially constant. After about 15 mm, absorption then drops off 
proportionally to .theta..sup.2. Here, E.sub.B1 remains practically 
constant. The light absorbing zone thus extends over an annular range of 
substantially 45.degree.. At the periphery, there is again complete 
transmission of light.