In ophthalmology, the well-known method of eye-sight diagnostics is the usage of special test charts[
Acta Photonica Sinica, Volume. 48, Issue 12, 1211001(2019)
Experimental Study on Contract Sensitivity for Fresnel Microprisms
Optotype test method in the field of ophthalmic diagnosis was simulated. The contrast sensitivity of visual images through Fresnel microprisms was investigated. An imaging system for stroke test objects through Fresnel microprisms was set up. Experimental data on contract were obtained using new electronic method for registering the resolution of test stroke images. The experimental data obtained by the imaging system is consistent with the visual acuity data obtained by the traditional optotype test method. It was stated that applying microprisms noticeably reduce the contrast of test stroke optotypes which is diminishing proportionally to the prism strength and to the initial contrast value of the optotypes. The main reason for the reducing of the contrast and for the worsening of the resolution of optical images when using prisms is the white light chromatism and the diffraction of light beams at the microrelief of microprisms with different prismatic strength. The practical ways for the diminishing effect of reducing the contrast of the test optotypes are proposed by adopting the neutral glass filters with a sprayed layer of chromium.
0 Introduction
In ophthalmology, the well-known method of eye-sight diagnostics is the usage of special test charts[
One type of low-contrast charts has eight lines of letters. The letters at the top of the chart are dark and then gradually become lighter until they are almost impossible to visualize. Scoring is based on the ability to see the letters. Patients fail when they have guessed incorrectly two of the three letters out of a combination of three letters in vertical stripes of decreasing shades of black to grey. Usually, the optotypes are printed at 25.0%, 10.0%, 5.0%, 2.5% and 1.25% contrast. The most often used chart is the 2.5% chart because at 2.5% contrast visual acuity values are close to one half of the visual acuity at the full contrast.
Testing is identical to the measurement of visual acuity at high contrast level, i.e., we measure the smallest size (lightest) of the optotypes that the patient can recognize. The threshold is defined as the line on which at least 3 out of the 5 optotypes are correctly recognized. The 2.5% test is the most practical test in clinical use. The main aim of this diagnostics is to find necessary refractive optics. If the person has severely impaired vision, the test must be quite close, which may require use of reading lenses.
In the case of patient′s strabismus, the prisms or microprisms should be used additionally to the refractive lenses.The Fresnel microprisms are the microstructured optics with the Fresnel prism structure that allows precise control of light refraction [
The application of Fresnel microprisms in ophthalmology always reduces the resolution of the formed optical images [
Studies[
1 Experiment setup
The decrease of the resolution of test images through microprisms was obtained earlier[
where θ is the angular resolution (minimum distance), λ is the wavelength, and D is the diameter of the entrance aperture of the optical system, which usually coincides with the diameter of the lens. According to Eq. (1), two neighboring spectral lines are resolved if the maximum of certain line coincides with the minimum of the other one.
In this paper, the contrast of the images kA is used for the determination of the resolution for the test ophthalmic charts.
where IF is the intensity of the background of the test chart, IA is the intensity of a certain optotype.
The scheme of the setup is shown in
Figure 1.The scheme of experimental setup
For simulation of the microprism influence on the contrast kA, the special experimental setup was created using the test stroke object Mira. The test strokes were projected to the screen with necessary magnification and these images simulated the optotypes of ophthalmic charts. The setup was based on the certain similarity criterion. This criterion stated the identity of the visual acuity data obtained in the traditional way using the test ophthalmic charts with the results of the measurements by the electronic method [
The luminous flux passes through the tested microprisms (5) that were placed at a certain distance LE from the screen. This value LE should be stated from the similarity criterion [
On the screen (7), the intensity of Mira images (8) with prisms and without prisms were recorded by a photodetector (6) for determining the microprism influence on the resolution of optotypes γS. In fact, the determination of the resolution γS is carried out through the registration of changes in the contrast kA of the stroke optotypes. Therefore, the influence of prisms can be obtained directly from the contrast of the images of test Mira optotypes. The contrast values kA were measured depending on the prismatic strength (PD) for a certain test stroke element MKHJ having a linear period of strokes DA, which corresponds to angular resolution γS of its image on the screen.
2 Experimental data on contrast sensitivity
Experimental data obtained for modified Fresnel microprisms with strengths ranging from 0.5 to 30.0 PD by electronic method [
Figure 2.The effect of microprism on the optotypes contrast
These contrast values were obtained for the distance between the prisms and the screen LE=51 cm. For convenience, curves 2~6 are normalized to the initial contrast value kА0=100%. The best correlation with data [
To determine the effect of diffraction on the contrast kA, the similar studies were conducted for monoprisms "Gulden Ophthalmics" (USA), for which there is no diffraction. These data are also shown in
It is advisable to carry out direct measurements of the microprisms influence on the contrast of test charts under minimal chromatism. Really, using test strokes with observation angles γS smaller compared to the chromaticity zone ΔγS, it is difficult to avoid the influence of white light dispersion, even monoprisms. Therefore, studies have been conducted for wider optotypes, the angular period of γS on the screen is substantially larger than the chromaticity zone ΔγS.
In these studies, the images of the widest optotypes M5H01were formed on the screen. Prisms were placed at a distance LE=35 cm from the photodetector for decreasing the linear zone of chromaticity ΔDS comparing to the width of registering slit SR that was unchanged and equal to 0.45 mm. Some of these data obtained for the standard initial contrast kA0=96.0% are shown in
Figure 3.The M5H01 intensity for different prismatic strength Δ
From the intensities of M5H01 images above, it is possible to determine directly the white light transmittance τ for certain microprisms, which should be at least 80% [
Figure 4.Dependence of contrast for test element M5H01 on prismatic strength for different
From the data for wide test strokes, it follows the initial contrast kA0=95.5% slightly decreases to kA=93.4% when microprism PD =0.5 Δ is set. While increasing the microprism strength, the chromaticity zone ΔDS on the screen is comparing with the linear period DS of the test images and the chromatic aberrations decrease the contrast of the optotypes. For microprism PD =30.0 Δ for distance LЕ=35 cm the period DS=1.5 ΔDS and the contrast kA is 87.0%. For the distance LE=51 cm the contrast kA=65.7%. This fact indicates a noticeable influence of chromatic aberrations on the contrast and it should be taken into account while low-contrast diagnostics because contrast is diminishing proportionally for Lea-symbol′s charts at any contrast level of optotypes.
While decreasing the intensity of the light flux, the initial contrast kA0 practically does not change. To reduce the illumination of stroke optotypes, neutral glass filters with a sprayed layer of chromium were used. The thickness of the chromium layer δ=10~35 nm, which ensured a change in the transmittance τ of the light flux for 25%~75%.
Figure 5.Normalized intensity distribution of M5H01 optotypes, 1: without filter; 2: with chrome filter (
Changing the lighting scheme for test M5H01 optotypes, the contrast of test strokes obtained by the method[
Similarly, a patient does not notice the change in the contrast through microprisms, until the size of the optotypes on his retinaγC exceeds the zone of chromatic aberrations ΔγC. Under the prismatic strength Δ increase, when ΔγC increases and approaches the value of γC, the contrast kAaccording to patient's feelings [
Therefore, for prisms of greater strength then PD > 14.0~20.0 Δ, when ΔγC > γC, the observed decrease in the contrast kA is caused not only by the disease of the patient's eye, but also by the prisms themselves. This effect should be taken into account by the special calibration tests with prisms before the contrast measurement in order to obtain reference data for a healthy eye.
The main reason for reducing the contrast and for worsening the resolution of optical images when using prisms is the white light chromaticity which occurs when the light rays of different wavelengths are refracting on the prism surfaces. To reduce the chromaticity, all microprisms in diagnostic bars unlike microprisms from the diagnostic set of KK-42 [
To eliminate the effect of diffraction, which decreases with the rise of the relief pitch W, all modified Fresnel microprisms [
3 Conclusion
In this paper, the contrast sensitivity of visual images through Fresnel microprisms was investigated. It was stated that applying microprisms noticeably reduce the contrast of test stroke optotypes which is diminishing proportionally to the prism strength and to the initial contrast value of the optotypes. The main reason for the reducing of the contrast and the worsening of the resolution of optical images when using prisms is the white light chromatism which occurs when the light rays of different wavelengths are refracting on the prism surfaces. The other reason for the reducing of the contrast of the test optotypes is the diffraction of light beams at the micropelief of microprisms with different prismatic strength.
The practical ways for diminishing the effect of reducing the contrast of the test optotypes are proposed. For prisms of greater strength then PD=20.0 Δ, the observed decrease in the contrast kA is caused not only by the disease of the patient's eye, but also by the prisms themselves. So, the special calibration tests with prisms should be taken before the contrast measurement in order to obtain reference data for a healthy eye.
The obtained data on the reduction of the contrast should be taken into account during ophthalmic diagnostics of eye-sight with high-contrast optotypes and when conducting the studies of visual defects using methods of visocontrastometry with low-contrast optotypes.
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Ming-lei FU, Ting-chao FAN, Antonov EUGENE, Chun-xiao LU, Wen-qi ZHANG, Jin-sheng LAI, Manko DMYTRO. Experimental Study on Contract Sensitivity for Fresnel Microprisms[J]. Acta Photonica Sinica, 2019, 48(12): 1211001
Received: Apr. 10, 2019
Accepted: Aug. 15, 2019
Published Online: Mar. 17, 2020
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