Spectral transmittance of the ocular media of the thirteen-lined ground squirrel (Spermophilus tridecemlineatus)

Abstract

Direct spectral transmittance data over the wave band 200–2500 nm were obtained for the ocular media of the thirteen-lined ground squirrel (Spermophilus tridecemlineatus) using a Zeiss (Oberkochen) DMR-21 dual-beam recording spectrophotometer. These data were used to calculate cumulative transmittance curves for each of the interfaces between optical media in the ground squirrel eye. Although radiation in the wave band 300–2500 nm can penetrate the cornea, our data show that the yellow pigment of the crystalline lens absorbs all wavelengths below 410 nm, while the vitreous humour absorbs most radiation longer than 1400 nm. The ground squirrel retina is, therefore, effectively protected from both near ultraviolet and infrared radiation, both of which are known to have deleterious effects on retinal photoreceptors.

Eye model for the ground squirrel

“…The experimental data on thirteen-lined ground squirrels (Spermophilus tridecemlineatus) was adopted from Chou and Cullen [12], who originally collected it in order to analyse the spectral transmittance of the squirrel ocular media. Our literature review suggests they were the only researchers to measure the ocular parameters in this species.…”

“…Chou and Cullen [12] used a refractometer to measure the indices of refraction of the different ocular media, and a pachometer along with cross-sectional images of the eye, made at 10 mm spacing, to measure axial lengths. This data is summarised in Table 1.…”

“…This data is summarised in Table 1. We adopted from [14] a cross-sectional image of the ground squirrel eye (Figure 1), made through the centre of the pupil along the optical axis. Once scanned and digitised, was used to compute the radii of curvature of the four visible ocular surfaces.…”

“…The model is mainly based on experimental, anatomical measurements of thicknesses and indices of refraction of the different ocular media. The values of the surface curvatures were extracted from a cross-sectional image of a squirrel eye [12].…”

“…The four visible ocular surfaces are (from right to left): the cornea, the front of the crystalline lens, the back of the crystalline lens, and the retina. Adapted from Chou[14]. Reproduced with permission.…”