Ординатура / Офтальмология / Английские материалы / Progress in Lens and Cataract Research_Hockwin_2002

Hockwin O, Kojima M, Takahashi N, Sliney DH (eds): Progress in Lens and Cataract Research. Dev Ophthalmol. Basel, Karger, 2002, vol 35, pp 93–103

The Effects of Protective Eyewear on Glare and Crystalline Lens Transparency

Yasuo Sakamoto, Kazuyuki Sasaki, Masami Kojima, Hiroshi Sasaki, Akiko Sakamoto, Masumi Sakai, Akiyo Tatami

Department of Ophthalmology, Kanazawa Medical University, Kahoku-gun,

Ishikawa-ken, Japan

Abstract

Purpose: Sunglasses have generally been used to protect against glare. Various kinds of sunglasses which correspond to the visual environment are on the market (e.g. for driving, fishing, skiing, etc.). As for the spectral transmission factor of sunglasses, the differences that occur in user’s eyes with aging have not been fully considered. We investigated the relationship between different levels of crystalline lens transparence and the effects of glare protection using two kinds of filters.

Subjects and Methods: A Tri-Blocker® filter (TB) and general driving filter (ActiveDrive®, ADR) were used. The TB absorbs three spectral wavelengths (below 400 nm, blue light, 575 nm) and can be transparent for other visible light. The ADR reduces the light below 650 nm. TB and ADR transmit 52.5 and 29.0% of the visible light, respectively. Twentyfive normal volunteers with transparent lenses (n 48 eyes, aged from 22 to 68 years) and 10 cortical cataract patients (n 18 eyes, aged from 48 to 71 years) were selected. The visual acuity of all subjects was 1.0 or better with the best correction. Contrast sensitivity function (CSF) was measured in four simulated light conditions (daylight, daylight with peripheral glare, twilight, twilight with central glare) by MCT8000 (Vistech). The light scattering intensity of the crystalline lens was measured by EAS-1000 (Nidek).

Results: The TB improved the CSF of the elderly volunteers under daylight conditions and of 1 of the cataract patients under all conditions. In the younger group, the CSF did not change under daylight conditions and deteriorated under twilight conditions. Although the ADR was effective for glare protection in the young volunteers, the protective effects of the TB were better than those of ADR for the middle-aged group.

Conclusion: Sunglasses not only protect against glare but also stabilize visual quality under various light conditions (e.g. passing through a tunnel while driving). Aging changes in lens transparency should be specially considered when developing protective eyewear.

Copyright © 2002 S. Karger AG, Basel

Introduction

Sunglasses with photochromatic lenses have generally been used to protect against glare. Various kinds of sunglasses which correspond to the visual environment are on the market (e.g. for driving, fishing, skiing, etc.). The transmittance of visible light through sunglasses ranges widely, from 10 to 90%. In the spectral transmission factor of sunglasses, the absorption bands are roughly divided into four wavelengths (neutral gray, yellow: 420 nm absorption peak, red: 510 nm and blue: 630 nm) [1]; however, combinations of these bands and the transmittance are manifold. Recent sunglasses also remove the ultraviolet (UV) rays (under 380 or 400 nm) which are harmful for the eyes. Formerly, D-line (Na-589 nm in Fraunhofer’s lines)-cut sunglasses devised for glassworkers were not difficult to obtain [2]. Although different types of sunglasses abound, absorption bands for the most effective kind of glare protection have not been clarified.

Sunglasses are not generally used for scotopic vision. The visual environment changes constantly when driving (e.g. passing through a tunnel), and it is essential that protective eyewear does not lower visibility when light conditions change. However, most sunglasses, except those for protection against glare for patients with retinitis pigmentosa, do not sufficiently consider this point [3, 4]. Aging changes in crystalline lens transparency are not specially considered when developing protective eyewear.

Recently, driving glasses with protective effects against glare [Tri-Blocker® (TB) Yamamoto Opt. Co., Japan] have been developed according to the concept that ‘although glare is reduced, the visibility does not decrease’. This lens was evaluated for its contrast sensitivity function (CSF) at various transparencies of the crystalline lens.

Materials, Subjects and Methods

Materials

Characteristics of the TB are: (1) absorption of UV rays under 400 nm, (2) reduction of blue light under 450 nm and (3) absorption of 575 nm rays.

TB is the filter for the driving glasses and these three wavelengths are regarded as the cause for glare. The other visible rays penetrate as much as possible to stabilize visibility. For the prevention of eye injury and autofluorescence to the eye by UV, UV under 400 nm was cutoff [5, 6]. Blue light was reduced in order to prevent scattering in the eye and light injury to the retina (peak 435–440 nm) [7, 8]. The 576-nm wavelength is a peak of relative luminous efficiency in photopic vision based on the color opponency theory (Hering’s theory) [9]. Since the production of filters that absorb 576 nm is especially difficult, 575 nm was absorbed in this filter. Although the light from 555 nm (maximum of standard photopic relative luminous efficiency, CIE: Commission International de l’Eclairage, 1924) generally seems to be a cause for glare, that light was not absorbed by this filter. In addition, for safety concerns the

Wavelength (nm)

Fig. 1. Spectral transmission factors of TB and ADR. CIE-sc Maximum of standard scotopic relative luminous efficiency (CIE, 1924); CIE-ph maximum of standard photopic relative luminous efficiency (CIE, 1924); WV-ph maximum of photopic relative luminous efficiency based on Hering’s theory (CIE Commission International de l’Eclairage).

Table 1. Light transmittance of the filters

light transmittance of the D-line (Na-589 nm) was set high, because tunnels are mainly lit by sodium vapor lamps (Na lamp) in Japan. There are two types of TB filters with a different visible transmittance: 52.5% (TB-50: 575 nm transmittance 10%) and 74.2% (TB-75: 575 nm transmittance 50%). A TB-50 filter was used in this investigation.

Ordinary driving sunglasses [ActiveDrive® (ADR) HOYA Co., Japan] were used to examine the effect of protection against glare on the aging changes of crystalline lens transparency. The performance of TB was compared with that of ADR. ADR’s spectral transmission factor was under 450 nm, which is almost equal to TB. At 29%, however, visible light transmittance was lower through ADR than TB (fig. 1, table 1).

Fig. 2. Typical examples in each group of subjects.

Table 2. Age and lens transparency of subjects (visual acuity: 1.0 or better with the best correction)

NS no significant difference by t test; cct computer compatible tapes: 8 bits gray scales.

Subjects

Twenty-five normal volunteers with transparent lenses (n 48 eyes, aged from 22 to 68 years) and 10 cortical cataract patients (n 18 eyes, aged from 48 to 71 years) were selected from persons who received health examinations at the Kanazawa Medical University Hospital. The visual acuity of all subjects was 1.0 or better with the best correction. Cases in which the cataract had progressed to the center of the pupil were excluded.

The normal volunteers were divided into three age groups (young: under 40 years old, middle-aged: 40–49 years old, elderly: over 50 years old). The two age groups of cataract patients were further divided according to opacification grade 1 (cortical 1) and over grade 1 (cortical 2 and 3, Jap-CCESG) [10]. As for the average of age and lens transparency, there was no significant difference between the elderly group and the cortical grade 1 group (t test, p 0.05, fig. 2, table 2).

In the normal volunteers, the effects of glare protection on various lens transparencies were examined using two kinds of filters (TB and ADR). The relationship between the different grades of cortical opacification and the protective effects of TB against glare were examined.

Methods

The transparency of the crystalline lens was evaluated from the backward scattered light obtained from Scheimpflug slit images using EAS-1000 (Nidek) [11]. The light scattering intensities of the lens layers in the adult nuclei were measured by the axial densitometry for peak height (EAS-1000 analysis software, version 1.21 for windows).

CSF was measured under four simulated light conditions by the MCT8000 (Vistech). The illumination levels were used in accordance with the manufacturer’s instructions. Daylight with peripheral glare simulates bright sunshine, and twilight with central glare simulates an oncoming car with a low-beam headlight at 100 ft:

(1)daylight: target illumination 40 footlambert (lm/ft2) (137 cd/m2),

(2)daylight with peripheral glare: 400 ft . cd at eye position (1,722 lx),

(3)twilight: target illumination 1 (lm/ft2) (3.4 cd/m2) and

(4)twilight with central glare: 30 ft . cd at eye position (129 lx).

TB and ADR filters were shaped into the trial lens frame, and the lens power was adjusted at 0 diopter. Worn both with and without filters, the CSF of the far focus was measured with correction giving the best visual acuity at 5 m.

Results

CSF of the Individual Group (without Filters)

Under daylight and twilight conditions without glare, CSF decreased with aging in the normal volunteers. This desensitization increased at a high-frequency optotype over 6 cycles/degree. In the cortical cataract patients, CSF decreased with an increase in opacification grade. Although there was no significant difference in age and lens transparency between the cortical cataract grade 1 group (C1) and the normal volunteers over 50 years old (N50 ), the CSF of C1 was lower than that of N50 (fig. 3, 4).

The CSF under glare light conditions also decreased in a similar way to that under nonglare conditions. In addition, the CSF differences between the groups extended into the high frequency optotype.

Variations in CSF with the Use of the TB Filter

The CSF of the normal volunteers did not change. Even when the TB was worn under daylight conditions without glare, the CSF of the cortical cataract patients markedly improved. Under daylight conditions with peripheral glare light, the CSF of all the subjects improved except in the 20s age group. The protective effects of TB increased with increased crystalline lens opacification.

Under twilight conditions, the CSF of the cataract patients also improved. The CSF of the normal volunteers, however, deteriorated when there was no glare light. The degree of deterioration tended to become more remarkable under the conditions without glare light, especially in younger subjects (fig. 5, 6).

Fig. 3. CSF under daylight conditions (without protective filters). N20 Normal volunteers under 40 years old; N40 40–49 years old; N50 over 50 years old; C1 cataract patients with cortical opacification grade 1; C2 cataract patients with cortical opacification over grade 1.

Fig. 4. CSF under twilight conditions (without protective filters).

Fig. 5. Effects of the CSF in the TB being worn under daylight conditions.log (CSF) CSF differences between worn both with and without TB filters (CSF converted to logarithmic value).

Fig. 6. Effects of the CSF in the TB being worn under twilight conditions.

Fig. 7. Comparison of the protective effects between the TB and the ADR in the normal subjects.

A Comparison of the Protective Effects of TB and ADR in Normal Subjects (Younger vs. Middle-Aged Volunteers)

In the younger subjects, there was no difference between the effects of the two filters under daylight conditions with and without glare. The contrast sensitivity was higher with the TB than the ADR under both daylight conditions in the middle-aged subjects.

Under two twilight conditions, although ADR was effective to protect against glare in the younger subjects, the protective effects of the TB were greater than those of the ADR for the middle-aged subjects (fig. 7).

Discussion

Under photopic conditions (daylight conditions without glare light), TB improved the contrast sensitivity of the cataract patients because the veiling glare was suppressed by UV rays and blue light absorption. While the general driving filter absorbs the green-yellow light (under 600 nm) in order to further suppress the peripheral glare, the TB can transmit more blue-green light from 450 to 555 nm. It seems that the absorption of the 575-nm rays by TB did not decrease the CSF under daylight conditions with peripheral glare. TB, however,

did not effectively protect against glare under this same condition in younger normal subjects. The high transmission of the blue-green light of the spectral transmission factor of TB seems to be related to this result.

Twilight vision has both photopic (cone vision) and scotopic (rod vision) visual functions. The visual acuity with scotopic vision is generally lower than that with photopic vision. The CSF of normal subjects decreased under twilight conditions, because TB extinguished half of the visible rays. In other words, with TB the twilight vision approximated scotopic vision. The CSF of the cataract patients improved because TB decreased the scattered light from the lens opacification under twilight conditions. Under twilight conditions with central glare light (like the headlights of a car), the twilight vision approximated photopic vision. Therefore, the CSF under twilight conditions with glare light was better than that without glare light.

The following results seem to correlate with the spectral transmittance of the filters and the crystalline lens at the blue-green band:

(1)with TB the CSF of younger subjects was lower than that of the elderly subjects under twilight conditions,

(2)the ADR improved the visual quality of the younger subjects and

(3)the TB was effective for subjects over 50 years of age.

The TB allows too much blue-green light to pass through for the younger subjects, and the ADR absorbs too much of this band for the elderly subjects. In short, the quantity of light which reaches the retina is influenced by the user’s age, even when the same filter is worn. Light transmittance at the 505-nm wavelength was over 60%, when a 30-year-old subject wore TB. The transmittance, however, decreased to 50% or less in a 50-year-old subject. Therefore, the filter that absorbs the blue-green light is useful for younger subjects, while the passing filter improves the visual quality of elderly subjects [12, 13] (fig. 8).

In a visual environment with low light intensity, sunglasses are not normally used. The user’s manual advised that the use of sunglasses should be avoided in this kind of visual environment. However, in a visual environment which rapidly changes like when driving (e.g. passing through a tunnel, weather, driving time), the putting on and taking off of sunglasses is complicated. Therefore, sunglasses which can deal with a changing visual environment are necessary. The sunglasses generally used for driving protect against glare with a lower setting of light transmittance under 600 nm. Although this filtering of the spectrum is effective to protect against glare in younger drivers, this setting is not safe at all for elderly drivers and early cataract patients.

In conclusion, it was confirmed that the transmission of 575-nm rays influences the protection against glare; however, the degree of the transmission could not be determined. For middle-aged and elderly people (including cortical cataract patients), eyewear which protects against glare and does not absorb the following

Fig. 8. Simulated spectral transmission factors of the noncataractous lens [12, 13] and those of the light which passes the lens with each filter.

three wavelengths is useful: scotopic vision peak (505 nm) and photopic vision peak (555 nm) and D-line (589 nm). Since sodium vapor lamps are mainly used to light tunnels in Japan, the absorption of the D-line should be avoided in order to maintain visibility in the tunnel. In this investigation, the visual environment of the clinical examination differed a little from that outdoors, because a contrast sensitivity tester (MCT8000) equipped with a tungsten lamp was used for the evaluation of visual function. Headlights with the luminance of blue light are also common in Japan. More investigations are necessary to further examine the different light environments.

Finally, not only protection against glare but also the stability of visual quality are necessary for effective sunglasses. Therefore, the transparency of the crystalline lens should also be considered for the design of protective eyewear.

References

1Atsumi K: Protect glasses after cataract surgery. Ganka 2000;42:1137–1141.

2Japanese Industrial Standards Committee: Eye Protectors for Radiations; JIS T 8141. Tokyo, Japanese Standards Association, 1980.