S. O. Rykov, E. E. Antonov, V. B. Mellina, N. N. Aleeva


Summary. The infl uence of modifi ed Fresnel microprism to contrast sensitivity and visual acuity were investigated.
The conditions of the decrease of test optotype contrast due to the diffraction and chromaticity of white light while using prisms of different prismatic strength were obtained. It is necessary to consider above-outlined data when conducting eye diseases’ researches by vizocontrastometry methods.
The aim: Microprism Fresnel elements are now widely used in the ophthalmology for the treatment of human eye disorders [1, 3, 9]. For the diagnostics of strabismus, in recent years, the device KK-42 is used [6, 8]. Above-mentioned set consists of 42 microprism elements with prismatic strength (PD) of 0.5 to 30.0 prism diopters (Δ) and is suitable for measuring strabismus angles in the range of up to 60 Δ. The aim is to study the infl uence of microprisms on the resolution ability and contrast sensitivity while using prisms for diagnostics of patients’ vision state.
Method: For measuring vision acuity VС the electronic method was developed using special test object Mira.
The images of Mira stroke elements in the white light are formed at the screen thus modeling the optotypes of test ophthalmic charts. Luminous fl ux is passing through the studied microprisms that are placed at a certain distance L from the screen. In the area of the screen the intensity of test Mira strokes without prisms and with prisms are registered electronically by photodetector. Obtained detector data can determine the microprisms infl uence on the resolution ability of test optotype images with the angular size γС and the chromaticity zone ΔγС.

Results: All microprisms from the diagnostic set KK-42 were investigated for vision acuity and contrast decrease while using microprisms. The criterion of the identity for visual acuity data, which are received by the traditional way with test charts, and for the measurement results received by electronic method with Mira strokes, is the equality of the relative chromatic expansion kС = ΔγС / γС for the optotype at the screen and for the relative chromatic expansion kO = ΔγO / γO for the eye retina for the same prism, where ΔγO is the chromaticity area at the retina, γO is the average resolution ability of the human eye ≈ 1.0 arc min (‘).
Patients did not notice the deterioration of optotype images while using prisms till. The angular expansion ΔγO of test image on the retina is less comparative to the angular distance γO between the individual elements of ophthalmic chart optotypes.
With the growth of prismatic strength Δ the chromaticity value ΔγO increases due to chromatic aberration. When the chromaticity zone ΔγO becomes close to the vision acuity γO ≈ 1’ the value of patients’ visual acuity VС begins to decline.
Conclusion: Obtained data have shown that microprisms did not change the vision acuity only for little prismatic strength with chromatic zone ΔγO < γO. For stronger prisms the resolution ability and contrast sensitivity are markedly decreased depending on the prismatic strength Δ due to diffraction and chromaticity of white light. These data have to be taken into account under any vizocontrastometry investigations.


microprisms, contrast, visual acuity test optotype, chromaticity


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