Ophthalmic testing devices

Optical measuring and testing apparatus incorporates a holographically recorded, single-frequency, optically thin phase grating. When this phase grating is illuminated by a quasi-monochromatic, spatially coherent light source, it acts as a basic common path interferometer and constitutes a highly efficient source for a high contrast, stable, interference fringe pattern. Various apparatus embodiments incorporating the phase grating are described which permit (1) measurement of central and peripheral retinal acuity, (2) variable contrast testing to measure the ability of the eye to detect low contrast stimuli, (3) measurement of visually evoked responses to help diagnose retinal-neurological dysfunction, and (4) the testing of optical lenses.

BACKGROUND OF THE INVENTION 
This invention relates generally to the field of optical measuring and 
testing, and more specifically to apparatus incorporating interference 
fringe pattern generators for retinal acuity and related testing. 
Ophthalmologists use a variety of techniques to measure ophthalmic and 
related functions and characteristics. Some of these measurements indicate 
retinal acuity at both the central and peripheral retinal regions. Others 
measure neurological response to a range of visual stimuli. 
For example, ophthalmologists use apparatus of the type that implements 
either Moire or interference techniques to test and measure retinal 
acuity. This measurement is obtained by varying the "fineness" of the 
fringes projected onto the retina and monitoring the patient's ability to 
resolve them. The patient's ability to resolve a fringe pattern of a 
certain "fineness" converts directly into a measurement of retinal acuity. 
Tests of peripheral vision can lead to an early diagnosis of glaucoma. 
Prior instruments of this general type used to measure the acuity of the 
central field of retina have not been employed successfully to measure the 
acuity of the eccentric region of the retina which is the area associated 
with peripheral vision. This is mainly because of their inability to 
project interference fringe patterns onto those eccentric regions of the 
retina, i.e. they do not have a sufficiently wide field. Resultantly, 
today, testing of peripheral vision is accomplished by flashing light at a 
variety of locations oblique to the patient's line of sight. The patient's 
ability or inability to detect those flashes at different points within a 
peripheral field of view is directly related to the size of the patient's 
visual field, but not necessarily to the acuity of the peripheral or 
eccentric regions of the retina. Therefore, such testing does not really 
provide an accurate indication of peripheral acuity. 
Measurements of neurological response to spatially and temporally varying 
visual stimuli are useful in diagnosing other problems including 
retinal-neurological dysfunction. During testing, evoked potentials from 
the brain are produced in response to a visual stimulus. The most common 
visual stimulus today is a phase-reversing checkerboard or bar pattern 
displayed on a television screen. 
All the foregoing tests and measurements using many current techniques 
require clear ocular media with reasonably normal refractive properties. 
If the media are not clear, as in the case of a patient afflicted with 
cataracts, the tests are not always valid. However, if a procedure were 
available for performing these tests independently of the opacity and 
refractive properties of the eye, better diagnosis could be made. 
Generally, laser produced interference fringe patterns provide a basis for 
instruments that measure retinal acuity because they can be projected onto 
the retina independently of ocular refractive errors and minor ocular 
media opacities. 
There are two basic methods for producing fringe patterns: (1) an 
interferometric technique that utilizes interference phenomena, and (2) a 
Moire technique that utilizes shadow casting and/or pattern 
multiplication. 
There are a wide variety of measuring and testing procedures that utilize 
interference fringe patterns and there are many ways to produce and 
control interference fringes. Generally, an interference fringe pattern is 
produced when at least two coherent beams of light are brought together 
and interact. When two coherent beams interact, they destructively 
interfere to produce dark spots or bands and constructively interfere to 
produce bright spots or bands. 
Moire fringes are produced when two similar, geometrically regular patterns 
consisting of well defined clear and opaque areas are juxtaposed and 
transilluminated. Some examples of geometrically regular patterns used to 
generate Moire fringes include (1) Ronchi rulings, (2) sets of concentric 
circles, and (3) radial grids. The generation of Moire fringes can be 
considered as shadow casting; that is, the shadow of the first pattern 
falling onto the second pattern produces the Moire fringes. The 
mathematical function describing Moire fringes is obtained by multiplying 
the intensity transmissions or irradiances of the overlapped geometrically 
regular patterns. 
Fringes generated by both interference and Moire techniques are used by 
ophthalmologists for testing retinal acuity. In one such apparatus, light 
from a laser is divided into two coherent beams by an optical element 
consisting of two adjoined dove prisms. These two beams are converged and 
directed into the eye where they interact to produce an interference 
fringe pattern on the retina. 
In another apparatus used in the field of ophthalmology, a laser source and 
an ordinary Ronchi ruling form an interference fringe pattern. The laser 
source produces a laser beam that is directed to the Ronchi ruling. The 
Ronchi ruling splits the incident beam into multiple coherent beams of 
widely varying strengths. It is necessary to use complicated motions of 
numerous optical and mechanical components to select only two coherent 
beams and to control the spacing of interference fringes eventually 
projected onto the retina. In yet another ophthalmic apparatus, two Ronchi 
rulings are used that produce Moire fringes which are eventually imaged 
onto the retina. 
Certain disadvantages exist in apparatus that utilize the interferometric 
techniques to form fringe patterns in ophthalmic applications. For 
example, in such apparatus the two light beams generally travel through 
different light paths that contain distinct optical elements. If the 
elements in each path are not matched optically, aberrations distort the 
fringe pattern. Matched optical elements can eliminate the aberration 
problem; however, they significantly increase the overall expense of the 
apparatus. Moreover, this apparatus is subject to various outside 
influences, such as vibration and thermal change. These influences can 
cause fringe pattern motion or noise and lead to improper measurements. 
Moire techniques also have many limitations. When small spacings and high 
accuracies are required, the geometrically regular patterns used to 
generate Moire fringes are quite difficult and expensive to produce. In 
applications where one ruling moves next to a fixed ruling, the spacing 
between the rulings must be held constant or errors result. Also, Moire 
fringes are localized, i.e., they exist in a very small region of space, 
and additional optical components are often required to image the Moire 
fringes into desired regions. 
Recently, an amplitude grating and a spatially coherent, 
quasi-monochromatic light source have been used to generate interference 
fringes. An amplitude grating is a generally transparent to 
semi-transparent media whose opacity is altered in accordance with some 
spatially periodic pattern. An amplitude grating "breaks up" or diffracts 
an incoming beam of light into a series of diffracted cones or orders. The 
strength, or amount, of light in each order depends upon the exact shape 
of the periodic opacity of the amplitude grating. Although various 
diffracted orders could be approximately the same strength, scalar 
diffraction theory for a thin amplitude grating predicts that the dominant 
strength will lie in the zero order undiffracted light and that the 
strength of other diffracted orders will vary. Indeed, practical 
applications bear out this prediction. 
In U.S. Pat. No. 3,738,753, issued June 12, 1973, Huntley proposes to pass 
light from a source through an amplitude grating to produce different 
order cones of diffracted light: for example, zero order and first order 
cones. To compensate for the different intensities, the diffracted light 
cones are reflected back through the grating. After the second passage 
through the grating, the zero order cone of the reflected first order cone 
and the first order cone of the reflected zero order cone have equal 
strengths and are combined to form a high contrast interference fringe 
field. This double pass system is quite stable because it closely 
approximates a common path interferometer. In a common path 
interferometer, the interfering beams traverse the same optical path. 
Therefore, perturbations affect both beams simultaneously and do not 
distort the output fringe pattern which is sensitive only to differences 
between the two optical paths. However, problems in such a double pass 
system do occur because it is difficult to control grating substrate 
aberrations and mirror-grating separation. 
Further improvements have been made with the advent of holographically 
produced amplitude gratings. Holographic amplitude gratings are produced 
by exposing a high resolution photographic emulsion to the precise 
interference pattern of a laser two-beam interferometer. During ordinary 
photographic processing, the photosensitive silver halide in the emulsion 
converts into opaque metallic silver to form the amplitude grating. 
In an application of one such holographic grating, a double frequency 
holographic grating produces a so called "shearing" pattern. See U.S. Pat. 
No. 3,829,219, issued 1974 to Wyant, and U.S. Pat. No. 4,118,124 issued 
Oct. 3, 1978 to Matsuda. This grating is produced by sequentially exposing 
a single photographic emulsion to a first laser interference pattern of a 
first spatial frequency, f.sub.1, and then to a second laser interference 
pattern of a second spatial frequency, f.sub.2. Equal amplitude 
transmission modulations at both frequencies f.sub.1 and f.sub.2 are 
achieved by adjusting the exposure to the first and second laser patterns. 
Ordinarily, the two sequential exposures are identical, but if f.sub.1 and 
f.sub.2 are very different or if one laser pattern is in red light and the 
other is in green light, the sequential exposures must be compensated for 
the spectral and frequency responses of the photographic plate. These 
exposure adjustments to achieve equal amplitude transmission modulations 
in f.sub.1 and f.sub.2 are usually done by trial and error. 
Upon illumination with spatially coherent, quasimonochromatic light, this 
double frequency grating produces two first order light cones of equal 
strength, one light cone being associated with each of the f.sub.1 and 
f.sub.2 frequencies. These two first order light cones interact to form a 
very stable, high contrast fringe pattern. Such a double frequency 
holographic shearing interferometer also is a common path interferometer. 
It is simple to construct. However, in this interferometer it is necessary 
to separate the zero order cone from the interacting first order cones. 
This separation requirement limits the f/number of the input light cone 
and the amount of shear obtainable. Moreover, if the two first order cones 
have high diffraction angles an astigmatic distortion of the output fringe 
field exists. In addition, the efficiency, or ratio of output fringe field 
power to input power, is only about 2%. 
For many years people have bleached photographically recorded amplitude 
gratings to obtain "phase gratings". One basic type of such bleaching, 
known as volume bleaching, chemically converts the opaque silver in the 
photographic emulsion into a transparent, high index silver salt. A second 
type of bleaching, known as tanning, chemically removes the developed 
silver within the emulsion and leaves a void. A tanned phase grating has a 
corrugated surface. Whereas an amplitude grating selectively absorbs 
light, a bleached phase grating selectively introduces phase delays across 
the input light beam. As a result, a phase grating is much more efficient 
than an amplitude grating; that is, the ratio of first order power to 
input power is greater. 
However, bleached gratings are generally characterized by substantial 
problems. They are very noisy and also may deterioriate physically back 
into amplitude gratings upon extended exposure to light. Bleached gratings 
also have a lower spatial frequency response than amplitude gratings. 
Although volume bleached gratings are less noisy and have a higher spatial 
frequency response than their tanned counterparts, they generally are 
weaker and less efficient. 
The efficiency of a volume bleached grating can be increased by increasing 
its thickness. However, any substantial increase in thickness drastically 
changes the basic diffraction properties of the grating. Any amplitude or 
phase grating can be considered optically thick when the optical thickness 
of the emulsion is more than five times the grating spacing. A grating can 
be considered optically thin if the optical thickness of the emulsion is 
less than half the grating spacing. Properties of thick gratings are 
accurately predicted by electromagnetic theory, while properties of thin 
gratings are described by scalar diffraction theory. For example, a thick 
phase grating output consists of only the zero order and one first order 
diffracted cones. In addition, diffraction takes place only for a plane 
wave input at a certain specified angle with respect to the grating. On 
the other hand, a thin grating of the same spacing produces multiple 
orders (i.e. the 0, .+-.1, .+-.2, .+-.3, etc. orders) with either a 
spherical wave or plane wave input at an arbitrary angle with respect to 
the grating. 
Distinctions between optically thin amplitude and optically thin phase 
gratings are accurately predicted by scalar diffraction theory. When a 
pure sinusoidal amplitude transmission perturbation exists in a thin 
amplitude grating, only the zero and .+-.1 diffracted orders exist. When a 
pure sinusoidal phase perturbation occurs in a thin phase grating, many 
orders (e.g., the 0, .+-.1, .+-.2, .+-.3, and other orders) are observed. 
The strengths of the phase grating orders are proportional to the 
normalized Bessel functions [J.sub.n (m/2)].sup.2, where n is the order 
number (e.g., n equals 0, .+-.1, .+-.2, . . . ) and m is the strength, or 
magnitude, of the phase perturbation in radians. When the amplitude 
grating perturbation departs from a pure sinusoidal form, additional 
diffracted orders are generated. The strengths of these additional orders 
are directly related to the strengths of the Fourier components associated 
with the grating perturbation function. 
With a phase grating, the diffracted orders associated with a 
non-sinusoidal phase perturbation are predicted by convolving the 
individual outputs from each Fourier component of the phase perturbation. 
Such a multiple convolution reveals complicated phase relationships 
between multiple orders associated with just one particular Fourier 
component. In addition, diffracted orders corresponding to sum and 
difference frequencies are generated when the phase perturbation consists 
of more than one fundamental spatial frequency. For example, one might 
consider bleaching the previously discussed double-frequency holographic 
grating to improve its poor efficiency. Although bleaching will increase 
the overall efficiency of such a grating, the bleached grating, in 
accordance with the convolutional operation, produces sum and difference 
frequency diffraction cones that are in addition to and that interact with 
the desired fundamental frequency diffraction cones. It is then possible 
for the sum and difference frequency diffraction cones to destroy the 
fringe field. 
SUMMARY 
Therefore, it is the object of this invention to provide an improved 
holographic phase grating for producing a high contrast interference 
pattern that is useful in ophthalmic applications. 
Another object of this invention is to provide an improved holographic 
grating that is useful in the testing of retinal acuity. 
Still another object of this invention is to provide an improved 
holographic phase grating that is useful in the testing of peripheral 
vision. 
Still yet another object of this invention is to provide an improved 
holographic phase grating that is useful in the testing of visual evoked 
responses. 
Yet another object of this invention is to provide apparatus for testing 
retinal acuity. 
Yet still another object of this invention is to provide apparatus for 
testing peripheral retinal acuity. 
Yet another object of this invention is to provide apparatus for testing 
visually evoked responses. 
A further object is to provide apparatus in the nature of a focimeter for 
measuring the focal length of an optical element such as a lens and 
testing it for aberration. 
In accordance with my invention, I use a single frequency holographic phase 
grating in ophthalmic testing equipment. A spatially coherent light source 
illuminates the grating to produce diverging diffractions, in conical or 
rectangular form, of different order. By "different order", I mean 
diffractions whose order numbers have different absolute values. In two 
diffractions of different order, the diffractions have equal strength and 
overlap thereby to produce a bright, high constrast, low noise 
interference pattern. I place a focusing element between the light source 
and grating to produce a spatially coherent source of light at a focal 
point that is slightly displaced from the grating. Other optical elements 
positioned in the resulting interference fringe field project the 
interference pattern through the eye and onto the retina. 
Various controls in the optical path enable many ophthalmic measurements, 
including visual evoked response measurements and visual acuity 
measurements in the central and eccentric regions of the retina even in 
the presence of corneal or eye lens opacities known as cataracts, or other 
refractive effects. 
The above and further objects and advantages of this invention may be 
better understood by referring to the following description taken in 
conjunction with the accompanying drawings.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
A. Holographic Grating 
FIG. 1 depicts, in diagrammatic form, the arrangement of apparatus 
necessary for exposing a photographic plate during the production of a 
holographic phase grating. The holographic phase grating produced in 
accordance with the arrangement shown in FIG. 1 and the procedures 
outlined in FIG. 2 are essential to the operation of the diverse 
embodiments of the invention that are shown in the other figures. 
Specifically, this apparatus includes a laser source 10 which directs 
light along an axis 11. The other apparatus in FIG. 1 splits the light 
into parts that travel over two separate paths and are then brought back 
together to expose a photographic plate 12. 
A conventional beamsplitter 13 separates the light into two parts. A first 
part travels along a first path that includes mirrors 14 and 15 for 
reflecting the light into an objective lens and pinhole 16, thereby to 
produce a spherical wave that emanates from a point source at the pinhole. 
The wave appears in a cone 17 and is directed toward the photographic 
plate along an axis 18. The second path established by the beamsplitter 13 
includes a mirror 20 and an objective lens and pinhole 21 that produce a 
spherical wave cone 22 that emanates from a point source at that pinhole 
along an axis 23. The light waves from these two point sources combine; 
they destructively interfere to produce dark bands and constructively 
interfere to produce bright bands at the photographic plate 12. 
The photographic plate 12 mounts on a rotary table which positions the 
photographic plate 12 and accurately establishes an angle .theta. between 
the axes 18 and 23. The spatial frequency, .delta., of the interference 
pattern at plate 12 is closely approximated by the equation 
##EQU1## 
where .lambda. is the laser wavelength. Although the fringes produced at 
the plate 12 are slightly hyperbolic, they are excellent approximations to 
rectilinear bands and therefore are shown as such in various figures. 
Increasingly better approximations to rectilinear bands are achieved by 
increasing the distance along the axes 18 and 23 between the plate 12 and 
the pinholes 16 and 21, respectively. 
The apparatus diagrammed in FIG. 1 has been used to manufacture gratings 
having the desirable properties that characterize my invention. The 
equipment is simple and relatively inexpensive. For example, the laser 10 
can comprise a TEM.sub.00 mode laser; the beamsplitter 13, a conventional 
variable density beamsplitter that enables the intensity of the two beams 
to be equalized. The mirrors 14, 15 and 20 are standard planar mirrors. 
The objective lens comprises a conventional 10.times. microscope 
objective, and the pinhole matches that objective lens. The distances 18 
and 23 are approximately 2 meters. With this specific arrangement, I am 
able to obtain a 500 line-per-millimeter interference fringe pattern over 
a 3".times.3" area with maximum fringe displacement error of about 0.00254 
millimeters. 
Once the apparatus in FIG. 1 is arranged, the emulsion on the photographic 
film can be exposed to the interference pattern as shown as Step 1 in FIG. 
2. During this exposure step, certain controls must be exercised to assure 
a holographic grating of good quality. For example, the exposure should be 
made in an environment that is not subjected to vibrations. Thermal 
disturbances should be minimized as any air flow between the beamsplitter 
13 and the photographic plate 12 can distort the resulting fringes. In 
applications where very high densities and minimal distortions are 
required, the distances along axes 18 and 23 must be increased to 5 or 
even 10 meters. Precise determinations of .lambda. and .theta. must be 
made. Although this basic apparatus can be used to produce highly accurate 
holographic phase gratings, the maximum accuracy ultimately then will be 
determined by the accuracy of angular measuring equipment, the stability 
of the single frequency laser, the optical table stability, and the 
atmospheric and thermal controls that are exercised. 
In order to produce a phase grating with special properties that enable the 
construction of the various disclosed embodiments, it is first necessary 
to produce an amplitude grating. Given the various properties of 
commercially available photographic emulsions and developers, a thin 
emulsion photographic plate and a chemically compatible developer are 
selected. A process of heavily overexposing and underdeveloping the 
emulsion reduces the optical thickness of the processed emulsion to a 
fraction of its original physical thickness. Thus, by utilizing the 
controls set forth in Steps 1 and 2 of FIG. 2, one produces an amplitude 
grating characterized by having: 
1. an optically thin emulsion conforming to scalar diffraction theory; 
2. a specific form for the absorbtion function which converts to a 
correspondingly specific phase transmission function after bleaching; and 
3. a specific amplitude or strength of the absorbtion function which 
converts to a specific peak-to-peak phase modulation after bleaching. 
Specific plate types, exposures, development times and developers are 
discussed later. 
Once the development of step 2 is complete, the photographic plate is 
washed in an acid short-stop solution in Step 3. The solution contains an 
acid hardener. A two-minute treatment in a hardening bath produces 
acceptable results. 
In Step 4, the emulsion of the photographic plate is fixed and hardened. A 
standard fixing bath and acid hardener have been used successfully, the 
plate being immersed in the bath for about ten minutes. 
Next (Step 5), the emulsion is prewashed for thirty seconds and 
hypo-cleared in a hypo clearing bath for about two minutes. In Step 6, the 
emulsion is washed (e.g., twenty minutes ih filtered water) and then 
soaked in a methanol bath until all residual sensitizing dye is removed 
(Step 7). Once the methanol bath has been completed, the plate is dried in 
a light blow air drying operation. 
All the foregoing steps are conventional photographic processing steps that 
utilize commercially available chemicals. Upon completion of Step 7, an 
amplitude grating has been produced. Steps 8 and 9 then convert this 
amplitude grating into a phase grating having the desired characteristics. 
More specifically, after the photographic plate is dried thoroughly in step 
7, it is bleached during Step 8 in a bromine vapor until the plate is 
clear. Once the bleaching operation has been completed, the plate is 
rinsed in a methanol bath to remove residual Br.sub.2 and dried thoroughly 
by a light blow air drying operation in Step 9. 
It now will be beneficial to discuss certain characteristics of these 
holographic phase gratings that are particularly desireable. First, the 
exposure and development times and the emulsion have been chosen to 
produce "thin" gratings. As a specific example, I have made 393.7 
line-per-millimeter gratings on Kodak 131-01 plates according to the 
foregoing processing procedure using an average exposure of 200 
ergs/cm.sup.2 and a development time of 15 seconds in standard Kodak D-19 
developer at 80.degree. F. Uniform development is achieved by using a 
large development tank and rapid manual agitation of the plate. After 
complete processing in accordance with the steps of FIG. 2, the resulting 
thin phase grating diffracts both input spherical waves as well as input 
plane waves; as previously stated, a thick grating diffracts only input 
plane waves incident at a particular angle with respect to the grating. 
Measurements have shown that a thin phase grating manufactured according to 
the foregoing process has a pure sinusoidal phase transmission function 
whose peak-to-peak phase delay produces equal strength zero and .+-.1 
diffraction orders. The 200 ergs/cm.sup.2 exposure produces an average 
amplitude transmission of approximately 0.45 for the developed, but 
unbleached, Kodak 131-01 plates. Experimental data has confirmed that a 
pure sinusoidal phase transmission function is maintained when the thin 
grating has an average amplitude transmission of 0.5 or less in its 
developed but unbleached state. The strength or peak-to-peak phase delay 
of the final phase grating is adjusted by controlling the initial exposure 
(Step 1, FIG. 2) within the limits set by an average amplitude 
transmission of 0.5 (measured after Step 7 in FIG. 2). A very weak phase 
grating produced with low exposure levels exhibits a strong zero order 
diffraction, a weak first order, and an even weaker second order 
diffraction. Stronger gratings produced with higher exposure levels 
exhibit increasingly more powerful first and second order diffractions and 
decreased zero order diffraction. Equal strength zero and .+-. 1 
diffraction orders or equal strength zero and .+-.2 diffraction orders are 
achieved by a trial and error adjustment of the initial exposure. 
The advantages of such a thin phase grating that produces two different 
diffraction orders of equal strength will now become apparent in the 
following discussion of an interferometer that utilizes such a phase 
grating. 
B. Interferometer 
Referring now to FIG. 3, an interferometer is depicted in schematic form 
that includes a helium neon laser 30 which directs light along an axis 31 
to a negative lens 32. The negative lens 32 expands the beam slightly so 
that it completely fills a microscope objective 33. The microscope 
objective 33 focuses this light at a focal point FP displaced a distance 
Z.sub.1 from a holographic grating 34 constructed as described above. The 
laser 30, negative lens 32 and microscope objective 33 constitute a source 
of a quasi-monochromatic diverging spherical wave that emanates from the 
focal point FP. In one embodiment, the cone from the focal point FP is an 
f/2 cone. 
When the spherical wave from the point source at the focal point FP strikes 
the grating 34, it produces a number of cones of diffraction. According to 
scalar diffraction theory, the strength of the diffracted cones is 
governed by the Bessel function [J.sub.n (m/2)].sup.2 where n is the 
diffraction order number and m is the grating transmission function 
peak-to-peak phase delay in radians. The previously specified exposure and 
development times for a Kodak 131-01 plate yield a value of m=2.870 at 
.lambda.=6328 .ANG.. The zero and first order diffraction cones are of 
equal intensity because [J.sub.0 (1.435)].sup.2 =[J.sub.1 (1.435)].sup.2. 
Moreover, the diffraction angles are such that the zero order cone 
overlaps both first order cones, while the first order cones merely abut 
each other. At some point at a distance Z.sub.2 from the grating 34, an 
output such as is shown in FIG. 3 is produced. The zero order cone appears 
as planar circle 35; first order cones appear as planar circles 36A and 
36B. Areas 37A and 37B are areas of overlap and the fringes are produced 
in those areas. Moreover, the fringes in the areas 37A and 37B are out of 
phase with each other. Thus, if the centrally located fringe in area 37A 
is a dark band, the corresponding fringe in area 37B is a light, or 
bright, band. By "light" and "dark" bands, I do not mean bands having the 
same intensity across the band, as the bands are shown in the drawings. 
The fringe intensity actually varies smoothly and is proportional to the 
square of a sine function, although the eye may perceive distinct 
alternating bands under some illumination conditions. 
The 180.degree. phase shift between the fringes in areas 37A and 37B is a 
direct result of having a pure sinusoidal phase transmission function 
associated with grating 34. When the phase transmission function of 34 
departs from a pure sinusoid, the fringes in the areas 37A and 37B will 
have some other phase relationship not equal to 180.degree.. The 
180.degree. phase shift is not essential to the production of high 
contrast fringe patterns; but it is important in a position detecting 
application where quadrature electrical signals are derived from the 
central fringes. Control of the grating transmission function form is 
achieved by selecting the proper combination of emulsion, developer, 
exposure and development time as previously discussed. 
The interferometer shown in FIG. 3 has several properties. If the distance 
Z.sub.1 is varied, the number of fringes within the overlap areas 37A and 
37B changes. Specifically, decreasing the distance Z.sub.1 decreases the 
number of fringes that appear in the overlap areas. As Z.sub.1 is varied, 
fringes "flow" into or out of the areas 37A and 37B. Although this "fringe 
flow" may cause the central fringes to widen or narrow, it does not move 
the central fringes; they remain located at the centers of their 
respective areas. The importance of this central fringe behavior with 
Z.sub.1 variations will be discussed later. If the grating 34 is moved in 
a plane that is normal to the axis 31 and perpendicular to the direction 
of the fringes, all the fringes in the areas 37A and 37B appear to slide 
through those areas, but the number of fringes in those areas remains 
unchanged. If the distance Z.sub.2 varies, the number of fringes also 
remain the same, but in this case their sizes change, the fringe widths 
becoming smaller as Z.sub.2 decreases. The interferometer parameters are 
related by the equation: 
EQU T=(Z.sub.2 +Z.sub.1)/.delta.Z.sub.1 (2) 
Where T is the fringe period in overlap regions 37A and 37B, .delta. is the 
spatial frequency of the grating 34 defined by equation (1) and Z.sub.1 
and Z.sub.2 are the positive distances shown in FIG. 3. 
The holographic grating interferometer in FIG. 3 is very stable and free of 
fringe distortion from outside influences because it is essentially a 
common path interferometer. Atmospheric changes, air currents and thermal 
instabilities do not distort the fringes. Moreover, the intensity of the 
light in each of the diffraction cones is much greater than that usually 
obtained from amplitude gratings because the phase grating essentially is 
transparent and relies entirely on time delays within the grating 34 to 
produce the diffraction cones. As a result, the overall brightness of the 
fringe pattern is increased. Moreover, as the intensities in each of the 
zero-order and first-order cones are equal, the destructive and 
constructive interferences tend to be complete so the dark bands are 
essentially black, while the bright bands are essentially twice as bright 
as the average light. Thus the grating enables the production of a simple 
common path interferometer that produces bright, high-contrast fringes. 
The foregoing properties lay a basis for understanding the application of a 
bleached phase grating in a retinal acuity tester. In this tester, the 
distance Z.sub.1 is intentionally varied to change the number of fringes 
that appear in a given area and impinge the retina. Apparatus that is 
particularly adapted for use in a retinal acuity tester is discussed with 
reference to FIGS. 4 through 7B. 
C. Retinal Acuity Tester 
The retinal acuity tester in FIG. 4 includes a laser 40 that can comprise a 
low-power TEM.sub.00 mode helium neon cylindrical laser, or other like 
laser. Light from the laser is directed along an axis 41A through a filter 
wheel 42. The filter wheel 42 contains a number of conventional metallic 
coated neutral density filters. These filters control the intensity of the 
light that is transmitted to the remaining elements in the retinal acuity 
tester. In this manner it is possible to control the brightness of the 
fringes eventually projected onto a patient's retina. 
A negative lens 43 and microscope objective lens 44 that are movable along 
the axis 41A focus the light at a focal point FP. The negative lens 43 
expands the beam from the laser slightly so as to completely fill the 
microscope objective lens 44 aperture with a uniform light distribution. A 
-4 mm focal length biconcave lens is a satisfactory negative lens. The 
microscope objective 44 is a conventional objective lens, a 10.times. N.A. 
0.25 objective lens being satisfactory. 
The grating 45 comprises a holographically recorded, single frequency phase 
grating that is produced as described earlier. The grating frequency is 
400 l/mm (lines per millimeter) to allow for ideal separation of zero and 
.+-.1 orders from the N.A. 0.25 objective input cone. The grating 45 also 
is optically thin, and the zero and first order diffractions have equal 
strengths. As becomes apparent later, there is no reason to control the 
phase of output fringes from the grating 45 when the grating is used in a 
retinal acuity testing apparatus. Therefore, the added constraints in the 
processing procedure associated with preserving a pure sinusoidal phase 
perturbation are eliminated. A convenient development time compatible with 
the requirement of producing an optically thin emulsion is chosen. Then 
exposure time is adjusted by trial and error until the desired strength of 
phase modulation is achieved. In this case, a modulation producing equal 
strength zero and .+-.1 orders is obtained. Thin, very clean, low noise, 
400 l/mm phase gratings for the retinal acuity tester can be produced on 
Kodak 120-01 plates using an average exposure of 1000 ergs/cm.sup.2 at 
6328 .ANG.. These plates are developed for 100 seconds Kodak D-19 
developer at 68.degree. F. (Steps 1 and 2 in FIG. 2). Steps 3 through 9 in 
FIG. 2 are used to complete the processing. 
The grating 45 produces the diverging cones of different order 
diffractions. More specifically, there is a zero order cone represented by 
circle 35 and first order cones represented by abutting circles 36A and 
36B. These cones are of equal strength so that they produce high contrast 
fringes as shown in areas 37A and 37B where the zero and first order cones 
overlap. In this specific embodiment, an axis 41B extends from the center 
of the grating 45 through the center of the area 37A. A dove prism 46 is 
positioned to receive the fringe field and is disposed with its 
longitudinal axis on the axis 41B. As the dove prism 46 is rotated about 
its longitudinal axis, the angle of fringe orientation within the fringe 
field 37A also rotates about the axis 41B through twice the prism rotation 
angle. 
The fringe field propagates through the dove prism 46 to an aperture wheel 
47. One aperture in the aperture wheel 47 is selectively aligned with the 
axis 41B by rotating the aperture wheel 47. An eyepiece 48 receives light 
transmitted through the selected aperture. This eyepiece 48 forms twin 
point sources within an eye pupil 49 of the patient. These point sources 
correspond to the point sources formed in FIG. 1 by the objective lenses 
and pinholes 16 and 21. The fringe field in the area 37A thereupon 
propagates through the eye and is projected onto the retina 50. 
During testing, a patient positions his eye pupil 49 on the axis 41B near 
the eyepiece 48 to intercept the twin point sources from the eyepiece 48. 
When his eye is in the proper position, the patient will sense or "see" 
the fringe pattern projected onto his retina 50. The cornea and eye lens 
have negligible optical power in such an arrangement and therefore have a 
negligible effect upon the fringe pattern projected onto the retina. 
The negative lens 43 and microscope objective 44 are positioned on a slider 
51 that can be moved along the axis 41A thereby to reposition the focal 
point (FP) with respect to the grating 45. As the slider 51 and focal 
point (FP) are repositioned, the number of fringes within fringe field 37A 
changes. The ability of the patient to see or discern a pattern of a given 
number of fringes within the field projected onto his retina is directly 
equated to standard measurements of acuity. 
During retinal examination the dove prism 46 and aperture wheel 47 play 
subtle, but important, roles because the retinal test is rather 
subjective. The examiner is able to control the orientation of the fringes 
by rotating the dove prism 46 thereby to determine whether a patient's 
claim of being able to see a pattern in a certain orientation is actually 
valid. To the extent that retinal response might exhibit orientational 
variations, the nature of such variations also can be evaluated. 
The diameter of the aperture selected by positioning the aperture wheel 47 
controls the size of the retinal area stimulated by the fringe pattern. 
This field control is important in determining the extent of any macular 
degeneration. The retinal fields offered by the various apertures in wheel 
47 range, in one specific example, from 20.degree. to 0.5.degree., these 
fields correspond to circular regions that are stimulated on the retina 
ranging from 5.0 to 0.155 mm. in diameter. 
FIG. 5 illustrates a number of different patterns as they will be perceived 
by a patient who is being examined utilizing the apparatus shown in FIG. 
4. If the slider 51 is located in an intermediate position, the patient 
could perceive the fringe pattern of alternate dark and bright bands that 
are shown as pattern A. If a laser that emits red light is used, the light 
areas are red and the dark areas are black. Thus, the patient perceives a 
series of straight red and black lines. If the slider 51 is moved along 
the axis 41A in FIG. 4 toward the grating 45, the number of fringes 
decreases and the patient perceives pattern B that contains fewer and 
wider fringes. Likewise, motion of the slider 51 in a direction away from 
the grating 45 beyond the intermediate position increases the number of 
fringes as shown in pattern C. If, on the other hand, the slider 51 is in 
the same position that produces pattern A, a 22.5.degree. rotation of the 
dove prism 46 in FIG. 4 rotates the fringes 45.degree. to an orientation 
shown in pattern D. 
Another embodiment of the retinal acuity tester is shown in FIG. 6. This 
tester differs from the retinal acuity tester shown in FIG. 4 by the 
addition of a viewing system for the examiner. This viewing system can be 
added because the common path principle applies to the overlapped orders 
producing the area 37A. Various viewing system designs could be used 
because the choice of specific components is not dictated by fringe 
distortion considerations. However, the components of the viewing system 
should be of reasonable quality to insure best viewing system performance. 
The viewing system as shown in FIG. 6 includes a beamsplitter 52 that is 
disposed between the aperture wheel 47 and the eyepiece 48. The 
beamsplitter 52 directs white light from a fiber optics light guide 53 
through the eyepiece 48 onto the eye. The source of light for the light 
guide can comprise a standard low power fiber optics illuminator (not 
shown). Light reflected from the eye passes through the eyepiece 48, the 
beamsplitter 52 and the aperture in aperture wheel 47 aligned with the 
axis 41B to another beamsplitter 54. Normally the largest aperture is 
aligned to provide the largest field of view. The beamsplitter 54 directs 
this light to a concave mirror 55 that forms a real image of the eye 
surface near the beamsplitter 54. Lens 56 relays the real image of the eye 
surface through a polarizer 57 to the focal plane of an eyepiece 58 for 
observation. The polarizer 57 coacts with another crossed polarizer 59 
between the dove prism 46 and the beamsplitter 54 to eliminate that 
portion of the fringe field reflected from beamsplitter 54 toward eyepiece 
58. Viewing system aberrations are reduced by locating the aperture wheel 
47 at the center of curvature of mirror 55 and using a symmetrical relay 
lens 56 at 1:1 conjugates. 
Even with good chin rests one of the most frequently encountered problems 
in ophthalmic examinations is the proper positioning of the patient's eye. 
With a properly aligned viewing system of the type disclosed in FIG. 6, 
the exact center of the image observed through eyepiece 58 is centered 
between the twin coherent point sources formed by eyepiece 48. Thus, when 
the examiner properly positions a patient's eye pupil to intercept the 
twin coherent point sources, he will observe a clear, centralized image of 
the eye pupil through eyepiece 58. This viewing system is especially 
valuable for testing cataract patients, because it enables precise 
location of the twin coherent point sources at any existing opening in a 
cataract. 
FIGS. 7A and 7B are two views of a retinal acuity tester constructed in 
accordance with this invention. This specific tester embodies the elements 
that are disclosed in FIG. 4. More specifically, the tester includes a 
housing 60 having a conventional laser unit 61 extending from one end 62 
of the housing 60. The laser 61 is connected to a conventional laser power 
supply 63. 
The various elements within the housing 60 are supported on a base plate 
64. A first element includes an upright stand 65 that supports the filter 
wheel 42. The examiner rotates a portion of the circumference of the 
filter wheel 42 that extends through a slot in a top plate 67 of the 
housing 60 to position the appropriate filter on the light axis. Although 
the angular position of the filter wheel 42 might be maintained by 
friction, a more positive positioning means would incorporate some detent 
indexing mechanism for interacting between the upright stand 65 and the 
filter wheel 42. 
The negative lens 43 and microscope objective lens 44 shown in FIG. 4 are 
mounted in a housing 70 carried on the slider 51. A rotary cam 71 has a 
shaft that extends through a side wall 72 of the housing 60 and is 
supported on a stand 72A. This shaft carries a positioning knob 73, a 
scale 74 and a detent mechanism that is not shown. The scale 74 is 
graduated directly in equivalent Snellen acuities ranging from 20/15 
through 20/400. As the examiner rotates the knob 73, the cam 71 rotates 
and longitudinally displaces the slider 51 and both the negative lens 43 
and the microscope objective lens 44 thereby to vary the position of the 
focal point FP shown in FIG. 4. In this embodiment, the slider 51 is 
constituted by a cam follower that contacts the cam 72 and is supported in 
a slide 75. The slide 75 also houses springs to bias the slider 51 against 
the cam 71. 
Another upright stand 76 is mounted to the base plate 64. This stand 76 
carries the grating 45. Thus, when the power supply 63 is activated, the 
light emanating from the laser 61 passes through the filter wheel 42, the 
negative lens 43, the microscope objective lens 44 to the grating 45 
thereby to produce zero and first order diffraction cones that have equal 
strengths and that overlap. In one specific arrangement the distance 
between the grating 45 and the focal point varies over a range from about 
0.6 mm to 25 mm. That range of distances enables the apparatus to produce 
fringe patterns that correspond to acuity measurements from 20/400 through 
20/15. 
There is also located at a fixed position on the base plate 64 another 
stand 77. This stand is skewed slightly with respect to the housing 60 in 
order to position the longitudinal axis of the dove prism 46 on the axis 
41 B shown in FIG. 4. The stand 77 carries a rotatable wheel 80. A portion 
of the wheel 80 extends through another slot in the top 67. The wheel 80 
carries the dove prism 46 so that rotation of the wheel 80 by the examiner 
rotates the dove prism 46 and changes the orientation of the fringes, as 
shown in pattern D of FIG. 5. 
The next element in the tester is an end wall 81 that supports the aperture 
wheel 47 and the eyepiece 48 on the axis 41B in FIG. 4. A portion of the 
aperture wheel 47 extends through a slot in wall 60 allowing the examiner 
to center the various apertures on axis 41B shown in FIG. 4. In addition, 
the end wall 81 contains two notches 82 and 83 in an exterior portion of 
the wall. These notches are offset on opposite sides of the eyepiece 48. 
They allow the patient to position his nose with respect to the housing 
during examination. For example, the patient would position his nose in 
the notch 82 during examination of his right eye. 
From the foregoing discussion, it will be apparent that the retinal acuity 
tester disclosed in FIGS. 7A and 7B is compact and easy to construct. All 
the optical elements, except the grating 45, are conventional elements 
that are readily available and relatively inexpensive. Such elements are 
used because the retinal acuity tester is an example of a common path 
interferometer and because the fringes are not subject to thermal 
variations, vibrations or other environmental perturbations. 
D. White Light Source Acuity Tester 
Another embodiment of an ophthalmic device that uses the basic grating in 
an interferometer as shown in FIG. 3 is disclosed in FIG. 8. However, 
unlike the previously described figures, this apparatus uses "white light" 
rather than energy from a laser. More specifically, the disclosed white 
light retinal acuity instrument operates with a small, low voltage, 
filament lamp, thus eliminating the need for a laser and power supply. 
Moreover, it can be constructed as a hand-held instrument, thereby 
eliminating the need for head-chin rests, tables and other ancillary 
equipment. In comparison to the FIG. 3 version employing a laser light 
source, however, this instrument is less effective in penetrating 
cataracts in the patient's eye. 
Referring to FIG. 8, a spatially coherent light source indicated generally 
at 90 illuminates a 350 l/mm holographic grating 91 to generate second 
order diffraction bands 92A, 92B, first order bands 93A, 93B and zero 
order band 94. The spatially coherent light source consists of filament 
lamp 100, collecting lens 101, and rectangular slit 102. Lamp 100 emits 
white light and lens 101 collects that light flux and maximizes the 
irradiance at slit 102. Slit 102 has a width that is compatible with the 
existence of spatially coherent light at aperture 104 of a microscope 
objective 105; a typical slit width is between 5 and 15 microns. Objective 
105 is operated at a numerical aperture of about 1.3 times the 
.delta..lambda. product of the grating 91, .delta. being the grating 
spatial frequency and .lambda..apprxeq.5500 .ANG. for white light 
operation. Such a choice of numerical aperture maximizes the fringe 
overlap area 107. In FIG. 8, the fields or bands of fringes 92A, 92B, 93A, 
93B and 94 are rectangular because of the presence of rectangular slit 
102. Filter 103 is a simple, colored glass optical filter that limits the 
spectral content of the white light radiation. Microscope objective 105 
forms a line image 106 of slit 102 which is aligned to parallel the 
grating structure. 
The combination of the white light source 90 and holographic grating 91 is 
effective when the grating 91 diffracts all of the input energy at a 
particular wavelength out of the zero order. When this occurs, the area 
107 is formed by the overlap of +1 and -1 diffracted orders. The physical 
dimensions of the area 107 are varied by adjusting the numerical aperture 
of objective 105 by means of aperture 104. Interference fringes formed in 
area 107 are achromatic since light of all wavelengths form fringes of the 
same spatial frequency. The angle between +1 and -1 order diffractions 
within area 107 is larger for longer (red) wavelengths and smaller for the 
shorter (blue) wavelengths. This effect is essentially counterbalanced by 
the requirement of larger interaction angles for longer wavelengths and 
smaller interaction angles for shorter wavelengths to produce fringes of 
the same spatial frequency. Also important is the symmetrical diffraction 
of +1 and -1 order energy about an optical axis 110, thereby causing the 
fringe patterns associated with each wavelength to be in register or in 
phase, as well as being at the same spatial frequency. 
Achromatic fringes within fringe field 107 project to aperture 111 located 
at the front focal plane of eyepiece lens 112. Lens 112, acting via a 
reflection from cube beamsplitter 113 forms multiple chromatic images of 
the line source 102 in the pupillary region of the patient's eye 114. The 
achromatic fringe pattern then projects onto the retina, unaberrated by 
the eye's refractive error. Aperture 111 limits the fringe field size 
perceived by the patient. Beamsplitter cube 113 and a lens 115 allow the 
examiner to monitor penetration of light energy at the patient's pupil. 
Lens 115 acts as a simple magnifying glass focused at the patient's 
pupillary area. 
The spatial frequency of achromatic fringes perceived by the patient is 
controlled by varying the distance Z.sub.1 between the line image 106 and 
grating 91. Fringe orientation perceived by the patient is altered by 
rotating the entire light source 90, grating 91, aperture 111 and lens 112 
in unison about the axis 110 as shown by the double headed arrow A. 
As previously indicated, optimum results with the white light retinal 
acuity instrument are achieved when energy in the zero order diffraction 
area 94 is minimized. According to scalar diffraction theory, for any 
wavelength, an optically thin phase grating with a purely sinusoidal 
peak-to-peak phase delay of 4.8 radians extinguishes all energy in the 
zero order diffraction. However, for a given grating which has fixed 
characteristics, compete extinction of the zero order diffraction is not 
possible for a range of wavelengths since the phase delay of 4.8 radians 
is wavelength dependent. Therefore, the grating 91 is manufactured with a 
peak-to-peak phase delay of 4.8 radians at the wavelength associated with 
optimum eye sensitivity. 
To summarize, high contrast achromatic fringes are achieved when the 
grating 91 has the following characteristics: 
1. the emulsion is optically thin and conforms to scalar diffraction 
theory; 
2. the phase perturbation function is purely sinusoidal; and 
3. the peak-to-peak phase delay is 4.8 radians at the wavelength of maximum 
eye sensitivity. 
A grating with these characteristics can be exposed and developed according 
to the procedures of FIG. 2. 
The preferred embodiment of the hand held instrument pictured in FIG. 8 
includes a thin, holographic, single frequency phase grating which 
extinguishes zero order energy at about 5500 .ANG., the most sensitive 
region of the visible spectrum. Filter 103 is chosen to pass wavelengths 
near the wavelength of zero order extinction. Experiments on a prototype 
instrument have confirmed that even with a moderate passband of 1000 
.ANG., centered at the zero order extinction wavelength, nearly "white 
light" fringes of very high contrast, 0.9 or better, are typical. As the 
passband of filter 103 is increased, the fringe contrast diminishes due to 
an increase in zero order diffracted energy. However, even without a 
filter, fringe contrast is quite acceptable. 
A retinal acuity instrument utilizing a holographic grating interferometer 
can perform a variety of acuity measurements in addition to the previously 
described measurement of central field retinal acuity. FIG. 9 illustrates 
how optical components can be combined in a system to allow several modes 
of operation useful in the detection of various eye disorders. More 
particularly, an acuity perimeter section shown generally at 122 measures 
peripheral acuity for early detection of glaucoma, a variable contrast 
testing section indicated generally at 124 permits variable contrast 
testing to detect eye disorders which impair the ability of the eye to 
detect low contrast visual stimuli and a visually evoked response (VER) 
generation section shown generally at 126 helps to diagnose 
retinal-neurological dysfunction. 
Although visually evoked responses (VER), contrast sensitivity, and 
peripheral acuity have been measured previously, these measurements have 
been done by methods where the refractive power of the eye could 
significantly affect the results. In many cases, it is the general 
neurological retinal response which is impaired by eye diseases and it is 
necessary to measure the retinal function alone, without abnormalities and 
variations in the refractive properties of the eye interfering with 
accurate measurements. Thus, the holographic grating retinal acuity 
instrument can become a very useful diagnostic instrument because of its 
ability to by-pass the refractive influence of the eye and to project an 
interference pattern directly onto the retina. In cases where optical 
properties of the eye have a negligible effect on measurement of retinal 
response, laser interference acuity may offer only a convenient method of 
evaluation. However, in cases where measurement of retinal response is 
severly impaired by an opacity or refractive abnormality, laser 
interference acuity becomes a very unique and useful diagnostic tool. 
E. Acuity Perimeter 
The instrument of FIG. 9 can measure peripheral acuity by projecting 
interference fringe patterns onto those portions of the retina associated 
with peripheral vision. Acuity perimetry is particularly useful in the 
early diagnosis of glaucoma, an eye disorder characterized by 
deterioration of retinal regions responsible for peripheral vision. Today, 
as discussed at the outset, testing of peripheral vision is done by 
flashing light at a variety of locations oblique to the patient's direct 
line of sight. The patient's ability or inability to detect these flashes 
of light at different points within a peripheral field of view is directly 
related to the size of the patient's visual field, but not necessarily to 
his peripheral acuity. 
Wide field retinal acuity testing or acuity perimetry, allows a more 
quantitative analysis of peripheral vision by flashing a calibrated 
interference fringe pattern onto those segments of the retina associated 
with peripheral vision. Natural aberrations of the human eye inhibit the 
formation of detailed images on peripheral areas of the retina, but the 
formation of fine interference patterns is unaffected by the eye's 
refractive errors. Thus, acuity perimetry by means of interference fringes 
permits quantitative measurement of retinal acuity at eccentric field 
locations. The ability to resolve peripherally located interference 
patterns of a particular fineness is a more sensitive test than detecting 
oblique light flashes because many eye disorders, including glaucoma, 
cause a loss of peripheral acuity before an actual loss of peripheral 
sensitivity to light stimulus is evident. 
In the FIG. 4 apparatus embodiment described above, a dove prism 46 limits 
the size of overlap regions 37A and 37B at the aperture wheel 47, thus 
limiting the maximum field of view provided by the combination of aperture 
wheel 47 and eyepiece 48. Some relief from field limiting properties of a 
dove prism can be attained by using a large aperture, high refractive 
index dove prism, but such a prism is very bulky, expensive, and doesn't 
offer an optimal solution. 
Important differences between the acuity perimeter of FIG. 9 and the FIG. 4 
tester are the absence of a dove prism, the addition of collimating lens 
148A, andimplementation of fringe field rotation by means of a rotating 
grating-wedge prism assembly 151B. 
Coherent light from a laser 140 propagates along axis 141 and is reflected 
by a mirror 184 along axis 141A through expander optics to assembly 151B. 
Housed within assembly 151B are a 400 l/mm optically thin holographic 
phase grating 145 made as described herein, and a wedge prism 146. The 
wedge prism simply deviates the fringe field axis, 141B, causing axes 141A 
and 141B to intersect at the plane defined by aperture wheel 147. Fringe 
field orientation is changed by rotating assembly 151B about axis 141A, a 
360.degree. rotation of that assembly producing a 360.degree. rotation of 
fringe field 137A. The grating and wedge prism are juxtaposed within the 
assembly so that axes 141A and 141B are nearly coincident along the entire 
distance Z.sub.2. For all practical purposes, then, the FIG. 9 system is 
an in-line optical system. 
Fringe field 137A propagates along axis 141A, 141B, and fills the aperture 
of collimating lens 148A. After passing through collimating lens 148A, the 
fringe field is eventually bounded by one of the apertures in aperture 
wheel 147. Although the formation of interference fringes on the patient's 
retina is unaffected by refractive errors of the eye, resolution of a 
small aperture boundary surrounding the fringe pattern requires nearly 
normal refraction. Correction of a patient's abnormal refractive error is 
accomplished by adjusting the position of eyepiece 148 along axis 141A, 
141B. The collimating lens 148A insures invariance of the aperture field 
position during adjustment of element 148 by maintaining a constant chief 
ray angle at the patient's pupil. 
As shown in FIG. 10, the arrangement of lenses 148, 148A and eye pupil 149 
constitute an arrangement known as a Maxwellian view. Accurate collimation 
of spherical waves emanating from the focal point (FP) requires that FP 
remain at the front focal plane of lens 148A. Therefore, fringe spacing 
within fringe field 137A is varied by moving assembly 151B to change 
distance Z.sub.1. 
In the absence of a dove prism, the dimensions of overlap area 137A are 
sufficient to fill a large aperture in wheel 147A, or a small aperture 
located a large distance from axis 141A, 141B. The off-axis position or 
peripheral field location of the various apertures can be varied by moving 
the entire wheel assembly in a plane perpendicular to axis 141A, 141B. The 
maximum distance allowed between that axis and center of an aperture is 
determined by the f/number of eyepiece 148. Equivalently, the maximum 
field angle of an aperture presented to a patient is limited by the 
f/number of eyepiece 148. 
The aperture spacing in wheel 147 is chosen to allow only one aperture 
(target) to appear within the eyepiece field of view at any wheel assembly 
position. Acuity perimetry requires projection of patterns on small, well 
defined regions of the retina. Therefore, a small 1.degree. target 
aperture is most often used during acuity perimetry. Of course, wide field 
central acuity can be measured by simply locating the aperture wheel 
assembly at a position which centers the various apertures on axis 141A, 
141B. 
During peripheral vision testing, the subject sees in Maxwellian view a 
uniformly illuminated round background field. The background illumination 
is produced by a white light source 154 which is focused in the subject's 
entrance pupil, 149. Thus, the amount of background flux entering the eye 
is independent of pupil diameter. The size of the background is limited by 
the f/number of the eyepiece 148. Brightness of the background can be 
varied, allowing acuity testing to be done under either scotopic or 
photopic conditions. Background source 154 is any convenient, small white 
light, a preferred embodiment consisting of a pinhole backed by a piece of 
ground glass and transilluminated by an incandescent lamp. 
Still referring to FIG. 9, lens 157 forms an image of source 154 near 
beamsplitter 160. Lens 163 recollimates the energy passing through 
beamsplitters 160 and 161, and after propagation through beamsplitters 162 
and 152, the aperture of eyepiece lens 148 is filled with a uniform light 
distribution. Element 148 then refocuses the background flux into the eye 
pupil 149. The color of the background flux is controlled by color filter 
158, a standard colored glass filter being preferred. Background color can 
be changed by inserting color filters with a variety of bandpass spectra. 
A preferred method of controlling background intensity is to insert, 
between the source 154 and lens 157, a variable neutral density wedge 159 
driven by a rack and pinion (not shown). 
Proper visual fixation is attained by asking the patient to stare at a 
small fixation source 153, apparently located at the center of the 
eyepiece field of view defined by axis 141A, 141B. Proper apparent 
location of source 153 is produced by beamsplitter 152 which directs light 
from source 153 along axis 141A, 141B into eyepiece 148. Light from source 
153 is collimated by lens 167 and passed through linear polarizer 165. 
After redirection by cube beamsplitter 160, the fixation beam passes 
through a beamsplitter 161 and is focused by lens 163 into the focal plane 
of eyepiece 148. Redirection of fixation source energy by beamsplitter 152 
locates the apparent position of the fixation source on axis 141A, 141B, 
in the same plane as the target apertures. A fixed linear polarizer 166 
acts in conjunction with linear polarizer 165 which is rotated to adjust 
the fixation source intensity observed by the patient. Source 153 can be 
any convenient small bright source, such as a light-emitting diode, "grain 
of wheat" bulb or a pinhole transilluminated by an ordinary incandescent 
light bulb. 
After adjustment of a patient's eye position, fixation source intensity and 
background flux, acuity measurements are initiated by activating the 
shutter mechanism 180. The shutter action projects a laser interference 
fringe pattern onto the retina for a controlled period of time. As long as 
the patient fixates upon source 153, the retinal region, stimulated by the 
flash of interference fringes, is precisely determined by the eccentricity 
and meridian coordinates of the target aperture in wheel 147. If a patient 
resolves the fringe pattern by recognizing the orientation of fringes 
within the off-axis target aperture, a specific visual acuity at a 
particular field location is verified. 
One of the more obvious sources of error in acuity perimetry is associated 
with improper eye pupil position just prior to and during target 
presentation. During testing, the patient's eye pupil should be at the 
rear focal plane of eyepiece 148 and centered on the axis 141A, 141B to 
intercept twin coherent point sources. 
In an attempt to make appropriate eye position unambiguous, light from the 
fixation source is constrained by the aperture of lens 167 and enters the 
eye pupil as a 2 mm collimated beam along the optical axis 141A, 141B. In 
order to see the fixation source, the subject must adjust the lateral 
position of his eye pupil to the optical axis 141A, 141B. As already 
mentioned, background flux is brought to focus at the rear focal plane of 
eyepiece 148. Therefore, longitudinal adjustment of the eye pupil to the 
new focal plane of that element is required to accept background flux 
without vignetting. The criteria for proper eye position are unambiguous, 
and most subjects naturally seek the correct eye position. 
In spite of unambiguous eye position, uncooperative patients should have 
eye position and fixation monitored by an observation system. Lenses 148 
and 170, acting through beamsplitters 152 and 162 constitute an imaging 
system which forms a real image of the eye surface at image plane 172. A 
magnified image of the eye surface, including the pupil, can be observed 
by looking through eyepiece lens 171. Illumination of the eye surface for 
observation purposes can be generated by illumination source 155. 
A preferred embodiment of source 155 is a fairly large ground glass 
surface, transilluminated by a powerful incandescent lamp. Flux from the 
groung glass surface of source 155 is approximately collimated by lens 169 
and strikes a second ground glass surface 156 which is a ground face of 
beamsplitter 161. Lenses 163 and 148 collect diffused light from surface 
156 and project it onto the eye. A polarizer 168 can be rotated and thus 
operates in conjunction with fixed polarizer 166 to control the amount of 
illumination flux reaching the eye. 
All of the sources, 153, 154 and 155 produce light which is linearly 
polarized after passing through linear polarizer 166. Experience has shown 
that retroreflections from the surfaces of components 162, 152, and 148 
can be very distracting to the observer. Therefore, polarizer 166 is fixed 
with its plane of polarization either perpendicular to or parallel with 
the plane defined by axes 141A, 141B and 141C. With such an orientation, 
retroreflections are plane-polarized and can be eliminated by selecting 
the crossed orientation for linear polarizer 173 in the observation 
subsystem. Light scattered from the eye surface is in general, randomly 
polarized and only partially absorbed by polarizer 173. 
Further experience has revealed that the large amount of illuminating flux 
required to observe the eye disturbs the patient's observation of 
interference patterns. To circumvent this problem, a cut-off filter 164 
which blocks visible light and transmits near infrared energy can be 
placed in the illumination flux subsystem. Subsequently, an infrared image 
of the eye pupil, free from retroreflections, is formed at plane 172, but 
illuminating flux is invisible to the patient. Proper fixation can be 
detected by placing a small infrared sensor 174 at the image plane 172. If 
the eye is improperly positioned, infrared radiation from the iris and/or 
sclera will be detected by the infrared sensor. When the patient's pupil 
is properly positioned, an image of the relatively dark pupil will fall on 
the detector with the absence of a detector signal indicating proper eye 
pupil location. 
A magnified infrared image of the pupil formed by lens 170 can be formed 
directly on the detector 175 of a commercially available TV camera at the 
location of detector 174. Fixation can then be monitored directly by 
observing the patient's pupil on a television screen. 
F. Variable Contrast--Mode I 
It is a well known fact that many eye disorders impair the ability to 
resolve low contrast visual stimuli. Since the FIG. 9 instrument provides 
a visual stimulus directly to the retina, the ability to control the 
contrast of the stimulus would greatly increase the sensitivity of various 
diagnostic procedures, including acuity perimetry and central field 
response. Section 124 of that instrument provides that capability. 
Variable contrast interference patterns can be generated by using a 
linearly polarized laser 140 and exploiting the invariant response of the 
optically thin holographic grating 145 to coherent radiation in different 
states of polarization. 
Still referring to FIG. 9, a linearly polarized laser beam from the laser 
travels through an open shutter 180 and through a neutral density filter 
wheel 142 to a "half wave plate" or half wave retarder 182. The arrows 
extending from axis 141 indicate rotation of the polarization plane after 
passage through half wave retarder 182. The optical axis (a) of plate 182 
is indicated by two dots and as known in the art, the output plane of 
polarization is rotated through twice the angle between the input plane of 
polarization and the retarder optical axis. Polarizing beamsplitter 183 
splits the incoming linearly polarized beam into two, orthogonally 
polarized components .pi. and .sigma.. The .pi.component of the input beam 
is transmitted to mirror 184, while the .sigma. component is reflected 
along axis 141E, through compensating filter 185 and to mirror 186. The 
relative intensity of the .pi. and .sigma. beams is governed by the 
optical axis orientation of half wave plate 182. If the input beam to 
beamsplitter 183 is in a pure .sigma. polarization state, the transmitted 
beam intensity will be effectively zero and reflected beam intensity will 
be a maximum. Similarly, if the input beam at splitter 183 is in a pure 
.pi. polarization state, the transmitted beam intensity will be a maximum 
and reflected beam intensity will be effectively zero. 
The .pi. polarized beam proceeds through the acuity perimeter section 122 
via mirror 184 and is responsible for the fringe field 137A and the 
interference pattern eventually formed on the patient's retina. Cube 
beamsplitter 181 transmits a significant portion of the polarized fringe 
field to collimating lens 148A and aperture wheel 147. Because of the 
common path nature of the holographic grating interferometer, fringe field 
137A suffers insignificant distortion during transmission to the aperture 
wheel. Of importance is the preservation of .pi. polarization after 
diffraction by the optically thin, holographic grating 145. Eyepiece 148 
finally focuses the .pi. polarized waves in fringe field 137A and forms 
the coherent .pi. polarized sources within the patient's eye pupil 149. 
The .sigma. polarized beam eventually reaches the refractive element 187 
which is properly located to form point source FP.sub..sigma. at the 
front focal point of lens 148A. Element 187 can be either a positive or 
negative lens, the primary requirement being sufficient power to 
eventually diverge the .sigma. beam enough to fill the apertures within 
wheel 147 with a uniform wavefront. Cube beamsplitter 181 redirects the 
.sigma. polarized wave toward collimation lens 148A and aperture wheel 147 
without altering the state of polarization. Eyepiece 148 focuses the 
.sigma. polarized wavefront to a point source at the eye pupil. 
In general, the eye pupil now contains three point sources, two from .pi. 
polarized waves and one from a .sigma. polarized wave. The .pi. polarized 
wavefronts propagating back through the eye 150 toward the retina, 
interfere and are responsible for the fringe pattern projected onto the 
retina. The .sigma. polarized wavefront also propagates back through the 
eye, but fails to interact (interfere) with the orthogonally polarized 
.pi. wavefronts. Therefore, the .sigma. polarized wave provides a 
background irradiance which can be simply added to the fringe pattern 
irradiance. 
When the relative intensity of the .sigma. polarized wavefront is high, 
background illumination dominates and the fringe pattern seen by the 
patient has low contrast. If the .pi. polarized wavefronts are dominant, 
the fringe pattern seen by the patient is superimposed upon a relatively 
low level background, thus producing a high contrast pattern. Fringe 
pattern contrast is controlled simply by rotation of the half wave plate 
182. Element 185 is a variable neutral density filter which preserves the 
.sigma. polarized state of the background beam and equalizes the maximum 
average retinal irradiance from the .sigma. and .pi. polarized wavefronts. 
With the filter 185 correctly adjusted, the average retinal irradiance I 
from both .sigma. and .pi. wavefronts remains constant at any fringe 
pattern contrast. The standard definition of fringer pattern contrast C 
is: 
##EQU2## 
When filter 185 is adjusted to maintain a constant average retinal 
irradiance, the contrast C varies according to the formula: 
##EQU3## 
where .delta. is the rotation angle of the half wave plate 182. 
G. Variable Contrast--Mode II 
The instrument depicted in FIG. 11 can produce a variable contrast fringe 
pattern in another mode of operation which uses polarized light and the 
invariant response of an optically thin holographic grating 145 to 
coherent radiation in different states of polarization. Many of its 
components are common to the instrument shown in FIG. 9 and these carry 
the same identifying numerals. 
Operation of the FIG. 11 embodiment relies upon a double refraction element 
such as a Rochon or Wollaston prism 192 to generate orthogonally polarized 
.sigma. and .pi. beams. The Wollaston or Rochon prism 192 deviates the 
.sigma. and .pi. laser beam components at slightly different angles and 
the expander assembly 151A forms two spatially separated point sources 
FP.sub..sigma. and FP.sub.90. In both the FIG. 9 and FIG. 11 embodiments, 
the action of half wave plate 182 controls the relative intensity of 
sources FP.sub..sigma. and FP.sub..pi.. 
Holographic grating 145 generates two independent fringe fields and wedge 
prism 146 directs these fields into the overlap area 137A. One fringe 
field in area 137A is .pi. polarized, emanating from FP.sub..pi., while 
the second fringe field in area 137A is .sigma. polarized, emanating from 
FP.sub..sigma.. The two fringe field irradiances are superimposed as shown 
and fail to interfere because of their mutually perpendicular states of 
polarization. The angle between the .sigma. and .pi. beams is determined 
by the refracting angle of the Wollaston or Rochon prism 192. 
The lateral distance X between FP.sub..sigma. and FP.sub..pi. can be 
controlled by adjusting the spacing d between negative lens 143 and 
objective 144. In a situation similar to the one described in my U.S. Pat. 
No. 4,265,534 with respect to FIG. 11A therein, the .sigma. and .pi. 
polarized fringe fields are shifted with respect to each other as the 
lateral distance X between FP.sub..sigma. and FP.sub..pi. varies. For 
variable contrast operation, the shift between .sigma. and .pi. fringe 
fields is set and locked at 180.degree. by an adjustment of spacing d and 
distance X. It is important to note that the phase shift is a function of 
distance X only and is independent of Z.sub.1. 
Changing the fringe field orientation by means of rotating the assembly 
151A will change the phase shift between the .sigma. and .pi. fields 
within area 137A unless the separation X between FP and FP remains 
constant and perpendicular to the grating structure within holographic 
grating 145. Invariance of the FP.sub..sigma., FP.sub..pi. separation, 
perpendicular to the grating structure is achieved by rotating element 192 
about axis 141. Rotation of element 192 is simply geared at a 1:1 ratio 
with the rotation of assembly 151B to maintain proper orientation between 
FP.sub..sigma., FP.sub..pi. and the structure of grating 145. With a 
180.degree. phase shift between the .sigma. and .pi. fringe fields, the 
irradiance distribution of the fringe pattern projected out to the 
aperture wheel and eventually onto the retina becomes: 
EQU I=I.sub.o (sin 2.delta.).sup.2 sin.sup.2 x+I.sub.o (cos 2.delta.).sup.2 
cos.sup.2 x (5) 
where I.sub.o (sin 2.delta.).sup.2 and I.sub.o (cos 2.delta.).sup.2 are the 
intensities of the .sigma. and .pi. beams as controlled by the half wave 
plate 182. 
When the half wave plate is rotated through .delta.=22.5.degree., I.sub.o 
(sin 45.degree.).sup.2 I.sub.o (cos 45.degree.).sup.2 and the fringe 
pattern shows no spatial modulation or zero contrast. When either I.sub.o 
(sin 2.delta.).sup.2 or I.sub.o (cos 2.delta.).sup.2 is equal to zero, the 
pattern seen by the patient has a maximum contrast of 1. The formula 
relating contrast to rotation angle .delta. of the half wave plate is: 
EQU C=(sin 2.delta.).sup.2 -(cos 2.delta.).sup.2 (6) 
H. Visually Evoked Response Pattern Generation--Mode I 
The variable contrast control provided by rotation of the half wave plate 
182 in FIG. 11 produces a phase reversing fringe pattern when the half 
wave plate is rotated at a constant angular velocity by a synchronous 
motor acting through a belt drive (not shown). During one revolution of 
the half wave plate 182, the fringe pattern appears as a cos.sup.2 X 
irradiance distribution at .delta.=0.degree., 90.degree., 180.degree. and 
270.degree.. The fringe pattern appears as a sin.sup.2 X distribution at 
.delta.=45.degree., 135.degree., 225.degree. and 315.degree.. 
Neurological response to a time varying, phase reversing stimulus can be 
measured with the aid of electronic equipment. Measurement of evoked 
potentials (EP) is very useful in the diagnosis of retinal-neurological 
dysfunction. The response to a time varying visual stimulus is an 
electrical signal sent to the brain via the optic nerve. The electrical 
signal or evoked potential EP is detected by electrodes (not shown) 
attached to the subject's scalp. Synchronization of the visual stimulus 
with a signal averager (not shown) permits extraction of the very weak EP 
signal from other "brain waves" . The EP measurements can also be made on 
newborn infants and unresponsive patients because confirmation of pattern 
recognition does not require communication with the patient. 
Today, the most common time varying visual stimulus presented to a patient 
is a phase-reversing checkerboard pattern or a phase-reversing bar pattern 
on a television screen. Research has shown that human visual evoked 
responses (VER) produced by patterned stimuli are highly dependent upon 
retinal image quality. In other words, VER is highly dependent upon how 
well the patient can see the television screen that is generating the 
visual stimuli. Therefore, the instrument shown in FIG. 11 modified to 
rotate plate 182 to generate phase reversing patterns is an ideal 
generator of patterned stimuli, remembering that the clarity of laser 
interference fringe patterns projected onto the retina is unaffected by 
refractive errors of the eye. 
I. Visually Evoked Response Pattern Generation--Mode II 
Another method of generating well synchronized, time varying visual stimuli 
on the retina involves insertion of an acousto-optic deflector 192 along 
axis 141A of the instrument depicted in FIG. 9. The acousto-optic 
deflector, a commercially available device, deflects the laser beam back 
and forth in the X direction at a frequency determined by an electrical 
waveform generator 193 which controls the deflector by way of a 
commercially available driver 194. Acousto-optic beam deflection changes 
the X coordinate of FP.sub..pi. and causes fringe field 137A to shift. A 
thorough explanation of pattern motion within field 137A caused by 
changing the relative position of FP and the holographic grating can be 
found in my U.S. Pat. 4,265,534. 
The temporal characteristics and magnitude of the fringe field shift are 
controlled by the waveform generator 193 and modulator driver 194. The 
visual stimulus produced by the FIG. 9 instrument operating in this Mode 
II, constitutes a pattern phase shift, while the stimulus produced by 
rotation of the half wave plate 182 in the FIG. 11 instrument in Mode I, 
is a pattern phase reversal. From a medical point of view, the advantages 
and disadvantages of Mode I vs. Mode II stimuli are unclear at the present 
time. 
J. Visually Evoked Response Pattern Generation "Laser Checkerboard". 
The general consensus of opinion is that a phase shifted or phase reversed 
rectilinear pattern provides less visual stimulation than a phase reversed 
or phase shifted "cross-hatched" or checkerboard pattern. Therefore, a 
preferred laser visually evoked response (VER) instrument should be 
capable of producing a two dimensional, phase shifted interference 
pattern. 
Operating in accordance with the principles explained in the description of 
Variable Contrast Mode I, and Visually Evoked Pattern Generation Mode II, 
a laser checkerboard (VER) stimulator shown in FIG. 12 can generate a 
phase shifted, laser checkerboard pattern, i.e., a pattern of superimposed 
horizontal and vertical fringes. Here again, this unit has many elements 
in common with the earlier described ones and carries the same 
identifiers. 
In the FIG. 12 apparatus, the .sigma. polarized beam passes to mirror 186, 
through an acousto-optical deflector 195, through lenses 143A and 144A and 
eventually forms source FP.sub..sigma. at the front focal plane of a 
collimating lens 148A. Holographic grating 145B and wedge prism 146B are 
located at the same variable distance Z.sub.1 from FP.sub.94 , thereby 
producing .sigma. polarized horizontal fringes of the same spatial 
frequency as the .pi. polarized vertical fringes. Deflector 195, grating 
145B, and wedge prism 146B are properly oriented to accommodate shifting 
of the .sigma. polarized horizontal fringes in the vertical direction. 
Again, since thin holographic gratings 145, 145B and cube 181 preserve the 
polarization state of original .sigma. and .pi. beams, the superimposed, 
horizontal and vertical fringes at aperture wheel 147 propagate without 
interaction. Both acousto-optical deflectors 192 and 195 can be driven by 
the function generator 193, thus shifting the horizontal and vertical 
fringe pattern in a synchronous manner to provide a phase shifted "laser 
checkerboard" visual stimulus. 
K. Focimeter 
The optical system which constitutes the basic retinal acuity tester 
depicted in FIG. 3 can also be used as a focimeter; i.e., an instrument 
which detects the focal point of, and measures the focal length of, a 
lens. The basic phase grating interferometer shown in FIG. 3 produces 
fringes in the areas of overlap whose spacing is governed by the Equation 
(2) above. As seen from this equation, T.fwdarw..infin. as Z.sub.1 
.fwdarw.0. Therefore, a very broad or null fringe appears in the overlap 
areas when the focal point FP lies on the grating surface. Once the focal 
point is located on the grating surface, a straightforward measurement of 
the distance between the grating and the lens housing yields the back 
focal length of the lens. 
One practical approach to the design of a phase grating focimeter involves 
the construction of a reference optical system with minimal spherical 
aberration, well specified foci, and a numerical aperture compatible with 
grating frequency. Once the reference optical system is built, the foci 
can be located by means of the holographic phase grating. The test lens 
can then be inserted into the reference optical system at a specified 
plane and the resulting change in focal distance of the reference system 
plus the test lens can be measured with the phase grating. Since all 
parameters of the reference system are specified, the measured change in 
focus can be used to calculate the focal length of the test lens. 
A specific embodiment of a phase grating focimeter is shown in FIGS. 3 and 
3A, the optical elements shown in FIG. 3A being placed on the optical axis 
31 ahead of grating 34 as depicted in FIG. 3. As seen there, the 
microscope objective 33 forms a point source FP at the unit magnification 
plane, which is a distance of 2 focal lengths from lens 38. A test lens 39 
is placed between objective 33 and lens 38. Lens 38 is a four-element lens 
which is designed to image FP at FP' with minimal spherical aberration. In 
one embodiment, lens 38 is a 30 mm focal length symmetrical lens 
consisting of two optimized achromats mounted "back to back" to minimize 
spherical aberration. Lens 38 should also possess significantly greater 
power than the test lens 39 to prevent the test lens from forming image 
FP' in front of lens 38. 
When a spherical wave from source FP' strikes the grating 34, a number of 
cones of diffraction are produced. Of particular interest are cones 35, 
36A and 36B which overlap and produce fringes in areas 37A and 37B. The 
fringe spacing in overlap areas 37A and 37B can be calculated from 
Equation (2). 
The point FP' can be located on the surface of the holographic grating 34 
by translating the grating and observing the fringe patterns in areas 37A 
and 37B. When Z.sub.1 .fwdarw.0, T.fwdarw..infin. and a very broad or null 
fringe is observed. With the grating 34 located at the null fringe 
position, a test lens 39 can be inserted at the front focal plane of lens 
38. Placing the test lens at the front focal plane of lens 38 leaves the 
optical power of the system unchanged, but alters the back focal length 
and therefore the position of FP'. The change in Z.sub.1 caused by 
insertion of a test lens is: 
##EQU4## 
where f is the focal length of lens 38 and f.sub.t is the focal length of 
the test lens. 
The focal length of the test lens can be determined simply by measuring the 
subsequent grating displacement .DELTA.Z.sub.1 required to regenerate a 
null fringe pattern. Since f is known and .DELTA.Z.sub.1 can be measured 
directly by measuring grating 34 translation, the focal length f.sub.t can 
be calculated directly from Equation (7). 
One particularly useful modification of the FIGS. 3 and 3A apparatus 
involves testing of aspheric lenses. Reference lens 38 can be constructed 
of basically simple spherical elements with radii of curvature and 
spacings selected to image FP at FP' with a calculable amount of 
aberration. Initially, the fringes within areas 37A and 37B will depart 
from a straight line or null pattern due to the aberrations of reference 
system 38. However, if the aberrations introduced by lens 38 are of 
opposite mathematical sign to those generated by an aspheric test lens 39, 
the fringes within areas 37A and 37B will revert back to a null pattern 
upon insertion of the aspheric test lens. Such a null system is 
particularly useful for testing intraocular lenses. The ideal intraocular 
lens implanted into the human eye is an aspheric element with a modest 
amount of asphericity, easily "nulled" by a relatively simple reference 
lens 38 in the FIGS. 3 and 3A apparatus. 
In another mode of operation, grating 34 is positioned and remains 
stationary at an original null fringe position. Insertion of a test lens 
39 results in the generation of a fringe pattern in overlap areas 37A and 
37B. The fringe spacing depends upon the power of test lens 39 as related 
by 
##EQU5## 
where T is the fringe period associated with insertion of the test lens 
39, .xi. is the grating frequency, f is the focal length of reference lens 
38, f.sub.t is the focal length of the test lens, and Z.sub.2 is the 
projection distance shown in FIG. 3. Typical values of the quantities in 
Equation (8) are f=30 mm, .xi.=200 l/mm; Z.sub.2 =100 mm. These values 
yield T=0.116 mm, when f.sub.t =200 mm. The fringe period can be measured 
electronically with a scanning photodetector or visually with the aid of 
an eyepiece and reticle. 
Today, many focimeters yield data by relying on operator judgment to align 
and focus optical patterns. Data gathered from such measurement oftentimes 
lacks the desired accuracy and repeatability required in focimetry. 
Whatever method of fringe measurement chosen, the present grating 
focimeter offers the advantage of presenting an unambiguous pattern from 
which quantitative measurements can be made. 
Another important consideration in the operation of a phase grating 
focimeter is the testing of lenses which have considerable aberration. 
Lens aberrations result in the formation of an imperfect point source at 
FP' resulting in the formation of fringe patterns in areas 37A and 37B 
which depart significantly from straight lines, e.g. shearing 
interferograms which are patterns that can be deciphered to yield 
quantitative information about wavefront deformations associated with lens 
aberrations. 
With phase grating focimetry, the longitudinal adjustment of grating 34 
results in the generation of paraxial focus, mid-focus and marginal focus 
fringe patterns. The distance between grating positions for generation of 
paraxial, mid- and marginal focus interferograms is a direct measure of 
the test lens spherical aberration. The same procedure of moving the 
grating 34 to find various zonal foci can also provide a direct measure of 
the asphericity of the test lens. 
When an ophthalmic lens is attached to a rotating means, the axis of any 
cylinder power can be oriented with the grating to produce the patterns 
shown in FIG. 3. The focal length of a preferred axis can be measured by 
aligning the grating with the cylinder axis and "nulling" the fringes 
along the preferred axis as shown in that figure. The practicality of 
using a phase grating focimeter to test severely aberrated lenses is 
determined by the complexity of the generated fringe patterns and the 
method of fringe detection. 
In summary, there has been disclosed herein a basic interferometer 
construction that utilizes a holographically recorded grating for 
projecting stable, high-contrast fringe patterns with high efficiency for 
testing retinal acuity. It will be apparent from the foregoing discussion, 
however, that the specific embodiments of this invention that have been 
disclosed are merely representative. The basic principles can be employed 
in a wide variety of applications with the attainment of some or all of 
the advantages of this invention. One important point to remember is that 
the grating must be optically thin so that, when polarized light sources 
are used, the grating does not change the state of polarization. 
Therefore, it is an object of the appended claims to cover all such 
variations and modifications as come within the true spirit and scope of 
this invention.