SLM-based interferometer for assessing the polychromatic neural transfer function of the eye

Abstract

A novel interferometric instrument for measuring neural transfer function (NTF) of the eye is presented. The device is based on a liquid-crystal-on-silicon spatial light modulator (SLM), which is used to create two laterally separated wavefronts in the pupil plane of the eye that interfere on the retina. The phase mask on the SLM, consisting of two diffraction gratings mixed in a checkerboard pattern and acting as a shearing interferometer, allows independent control of spatial frequency, orientation, and contrast of the fringes, as well as the field of view in a wide polychromatic spectrum. Coupled with a supercontinuum source, the system is able to produce achromatic fringes on the retina. The instrument was successfully tested in six normal subjects in four light conditions: polychromatic light and monochromatic blue, green and red light respectively (central wavelengths - 450, 550 and 650 nm). On average, the NTF in polychromatic light was approximately 20% higher than for green and red light, although not statistically significant due to high intersubject variability. Due to all-digital control of the interference fringes, the device is optically simple and virtually unsusceptible to vibrations, allowing its use in a non-laboratory environment. The study also contributes to color vision research, allowing to evaluate contrast sensitivity function without monochromatic or chromatic aberrations.

Fig. 1.

Simplified schematic of the system. Red dashed line denotes the conjugated plane of the SLM. Only the first diffraction order path is shown. The subset image in the top right schematically shows the checkerboard patterned mask on the SLM within a circular pupil. The non-coherent background path is omitted, further description is in the text.

Fig. 2.

Spot formation in the image plane of the system (corresponding to interference fringes at ${45}^{\circ }$). Green circle denotes the optical axis of the system, which corresponds to the common shift. Formation of the "chromatic lines" formed by beams $-\mathrm{\Delta }$ and $+\mathrm{\Delta }$ is described in text in more detail.

Fig. 3.

Normalized spectra of the four spectral conditions used in the experiment.

Fig. 4.

Images produced by the system in the field stop plane (panels A, B, C) and in retinal plane (panels D, E, F). In the top row zeroth, first and second diffraction orders are visible. Red dashed line shows field stop area. Chromatic lines shown in the top row result in fringes shown in the bottom row (A corresponds to D – vertical fringes, B to E – diagonal fringes, C to F – horizontal fringes). For clarity reasons (for better sampling with a color camera), different spatial frequencies were used for panels A, B, C (30 cycles/degree) than for panels D, E, F (6 cycles/degree). Further description is given in the text below.

Fig. 5.

Control of the interference fringe contrast using change in modulation depth. Averaged fringe profile is shown in the second row without any data smoothing. The grey level values correspond to modulation depth coefficient ($m$) values of 0 to 0.5. Wavelength of 550 nm was used for the measurements.

Fig. 6.

Average neural transfer function for 6 subjects measured in 4 conditions. Error bars indicate the standard deviation. Data at 36 cycles per degree is omitted for subjects S2 and S5 as they weren’t able to identify fringes at the highest available contrast levels (14%).