Journal of Undergraduate Research
Volume 2, Issue 2 - November 2000

Ultraviolet Transmittance in Eyeglass Materials

Cross Reardon

ABSTRACT

The objective of this research has been to investigate whether or not additional coatings on eyewear are necessary to protect the wearer against harmful ultraviolet (UV) radiation from the sun. The spectral transmittance of visible as well as of UV light was measured for a variety of eyewear lenses made out of glass and plastic. For comparison the transmittances of ordinary soda-lime silicate glass and of borosilicate glass plates were also measured. Finally, the spectral transmittance of various sunglasses was investigated. It was found that the best UV protection is provided by plastic lenses (with or without additional UV protective coatings) and by plastic sunglasses. In contrast, eyewear made of glass had only reasonable UVB protection. Differences seem to exist between various anti-glare coatings which are occasionally added to those lenses. The results suggest that additional UV protective coatings especially on plastic lenses and sunglasses are, in general not necessary.

INTRODUCTION

The media have been inundated with advertisements for ultraviolet (UV) protection in eyewear. Spectacles (with plastic or glass lenses), contacts, and surgical lens implants are being advertised based on the ability to block ultraviolet radiation. In some cases this protection takes the form of a coating added to the lens for an additional fee. While it has been well known in the scientific community that glass offers a certain degree of UV protection without any additives or films, the possible necessity of additives or films for plastic eyowear has not been well documented.

Several companies have published information regarding UV protection of eyewear. However, these companies are primarily interested in selling products. Thus, their data should be treated with some caution. One source has produced a web page containing a series of reference spectral transmittance graphs for various materials used for ophthalmic purposes. While these "standard" graphs compare well with the data presented here, no effort is made by that source to differentiate UV-coated materials from untreated raw materials.

Ultraviolet radiation represents that part of the electromagnetic spectrum which has wavelengths immediately shorter than visible light. In other words, UV spans the region from approximately 50 to 400 nanometers [nm]. Within this range UV is divided into UVA, UVB, and UVC. The precise extent of wavelengths included in each region varies according to different sources as does the relative hazard represented by each. This may be partially due to the difference in human sensitivity to light. Table 1 lists UV ranges as reported by various sources.

Table 1
Ranges of various ultraviolet designated by UVA, UVB, and UVC
Source UVA UVB UVC
Environmental Protection Agency 400-320 nm 320-280 nm 280 nm
Corning Glass Works 380-315 nm 315-290 nm 290 nm
Government Standard ANSI Z87.1
380 nm <--------------------------------------> 50 nm

All three regions of UV radiation are included in the spectrum of radiation given off by the sun. The data published in the open literature seem to agree that UVC is completely absorbed by ozone and oxygen in the earth's atmosphere. Therefore this region is only dangerous near powerful UV sources (e.g. arc welders or UV lamps). In such cases the use of UV protective eyewear is imperative.

UVB is the region of ultraviolet light responsible for sunburn and is of primary concern. Although much of the UVB radiation is absorbed by the atmosphere a significant amount is still transmitted, especially in regions of ozone depletion. This range is the one most cited in advertisements for UV protective eyewear (i.e., glasses, contacts, and sunglasses). While "normal" (soda-lime silicate) glass is known to absorb some UVB, the precise spectral transmittance has not been well documented for ophthalmic purposes. The Environmental Protection Agency (EPA) suggests 99-100% UVB absorption for protection in bright sunlight. Most sunglasses meet this requirement.2

The range of ultraviolet closest to visible light, UVA, seems to be the most controversial. Most sources agree that it is transmitted through the atmosphere and absorbed by the lens within the eye, preventing it from reaching the retina. This UV region is chiefly the concern of companies vending lenses for surgical implants during cataract surgery. While the effects of UVA are still a matter of research, the effects of sunlight UVA are not known to be detrimental to the eye.3 Common practice seems to be the combination of the UVA and UVB regions into the "near-UV region.3" A great deal of research is being performed on the effects of this segment of UV radiation to verify the cause and severity of UV induced damage.4-10

Upon consideration of the possible risks represented by UV radiation, the spectral transmittance of eyeglass materials becomes an important issue. This paper presents measured transmittance spectra of glass and plastic lenses--both with and without added UV protective films. It should be emphasized that the present paper does not intend to address itself to medical questions such as the minimum permissible exposure of UV radiation to different parts of the eye, the UV effects for those patients with IOL implants, or the dilation of the pupil. Its main purpose is instead to elucidate whether or not UV protective films on eyewear give added UV protection, assuming that all other physical parameters such as diameter, thickness, shape, position of the lens with respect to the eye etc., remain alike.

EXPERIMENTAL METHOD

A transmittance spectrum is obtained by measuring that fraction of the light which is passed through a material as a function of the wavelength of the light. The transmittance is defined by

Reardon Equation 1

where I0 is the intensity of the light that falls unobstructed on a light sensitive device, and Itis the intensity of the light after it has passed through a transparent material such as glass. The source for UV radiation was a xenon high pressure lamp diffracted through a scanning double monochromator having a spectral range of 200-800 nm. The light was focused by a quartz lens on a photodetector which is sensitive in the 250-1100 nm range. The output of the photodetector (in nW) was measured by a digital meter. The experimental procedure included the measurement of I0 and It in 10 nm intervals in a darkened room. At critical points where changes in intensities were large, additional data points were taken using smaller intervals. The spectral transmission was then calculated from the ratio between I0 and It as stated above.

It is important to realize that the transmittance, as dehmed in equation (1) is calculated by relating the transmitted to the unobstructed light intensities. Thus, any spectral characteristics of the instrument used for the measurements is eliminated by the dividing process. In other words, the transmittance, which is given in percent of the irlcoming light, can be accurately given in the entire utilized spectral range, (see above), even though the output of the light source or the sensitivity of the photodetector may decrease towards the UV region.

It should also be noted that the total transmittance depends on the thickness of the material under consideration. However, 100% transmittance cannot be obtained even for very thin materials because of reflection of the light on the front as well as on the back surface of the material. The reflectivity, R, on one surface of a dielectric material (e.g. glass) depends on the index of refraction, n, as follows:

Reardon Equation 2

Knowing that n for glass is about 1.5 in relevant parts of the visible spectrum,11 one calculates an R of about 4%. Thus, the transmittance of untreated glass can essentially not exceed about 92% (front and back reflection). The transmittance value can be however, enhanced in certain wavelength regions by applying antireflection coatings to the lenses used for spectacles or cameras 11. This improvement is caused by interference effects which effect an increase in transmittance in some wavelength ranges and a decrease of transmittance in others, as can be seen in Fig. 2.

EXPERIMENTAL RESULTS AND DISCUSSION

Flat Glass

This section contains the experimental findings of the spectral transmittance data of randomly selected glass samples and eyewear lenses. No attempt has been made to systematically investigate and characterize the immense eyewear market. Nevertheless, the results presented here should be, characteristic for spectacles generally in use.

Common window glass typically consists of soda-lime silicate glass (73% Si02, 17% Na2O, 5% CaO, 4% MgO, 1% Al2 03) of which the major component is silica (Si02). Silica in its nearly pure form is mostly used for microscope slides, coverglasses, and optical lenses and windows for scientific instruments--but not for eyeglasses. Figure 1 depicts the spectral transmittance of two glass samples namely soda-lime silicate and borosilicate crown glass (69.5% Si02, 11.5% B2 03, 9% Na2O, 7% K2O, 3% BaO). Table 2 lists the thickness of these samples and the wavelength at which the transmittance was found to be 50% (l50). It is noted that the untreated soda-lime silicate glass absorbs all of UVB and UVC whereas borosilicate crown glass (used for optical instruments) transmits light much farther into the UV. Figure 1 also indicates the spectral ranges of UVA, UVB, and UVC. The range of particular interest in the present context (UVB) is shaded. Further, Figure 1 contains some transmittance date which have been extracted from the literature.12 The agreement between our and the literature data is quite good.

 

 

Table 2
Thickness and Wavelengths where Transmittance is 50% (l50) for Common Glass Materials
Sample Description
Thickness
l50[nm]
Soda-Lime Silicate Glass
0.17"
343
Borosilicate Crown Glass
0.02"
318

Figure 1. Transmittance spectra of soda-lime silicate glass and borosilicate crown glass having different thicknesses as listed in Table 2.
Figure 1. Transmittance spectra of soda-lime silicate glass and borosilicate crown glass having different thicknesses as listed in Table 2. The data points (D) are taken from the literature. 12 Note that the soda-lime silicate glass provides complete UVB and UVC protection.

Glass Lenses

Figure 2 depicts the spectral transmittance of three glass eyewear lenses. Two lenses had anti-glare coatings, the third one had no coating. It is observed that the anti-glare coating had some (probably unintended) impact on the onset of UV absorption. The lens that did not possess any coating has been found to yield less UVB protection. However, there seems to be a difference between the spectral transmission of the two anti-glare coatings chosen. One of them, designated as "Anti-glare coating 2" provides perfect UVB protection whereas the one labeled "Anti-glare coating 1" transmits some UVB light. This is interpreted to be caused by the above mentioned interference effect which suppresses certain wavelengths and enhances others. Table 3 summarizes the results.

Table 3
Thickness and Wavelengths where Transmittance is 50% (l50) for Various Glass Lenses
Sample Description Thickness l50[nm]
Glass lens without coating 0.17" 330
Glass Lens with anti-glare 1 0.17" 344
Glass Lens with anti-glare 2 0.17" 380

 

Spectral transmittance of two glass lenses with anti-glare coatings and a third glass lens without coating.
Spectral transmittance of two glass lenses with anti-glare coatings and a third glass lens without coating.

Plastic Lenses

Figure 3 displays the spectral transmittances of various plastic eyewear lenses that were made of a thermoset polymer (allydiglycloe carbonate) most commonly used for eyeglass lenses. One of these lenses had UV protection as well as a light brown tint. The second had only UV protection added. The other two lenses had no coatings. It is observed that the plastic lenses (with or without coatings) provide complete UVB protection and almost perfect UVA protection. In other words, there seems to be little difference in UV protection between UV-coated and uncoated plastic lenses. However, the tint provides a somewhat lower transmittance in the visible range which is probably intended. Table 4 summarizes the results.

Table 4
Thickness and Wavelengths where Transmittance is 50% (l50)for Various Plastic Lenses.
Sample Description Thickness (l50)
Plastic lens with UV coating and Tint 0.13" 400
Plastic lens with UV coating 0.09" 406
Plastic lens without coating 1 0.08" 402
Plastic lens without coating 2 0.085" 409

 

Spectral transmittance of four plastic eyewear lenses with and without various coatings as listed on the curves.Spectral transmittance of four plastic eyewear lenses with and without various coatings as listed on the curves.
Spectral transmittance of four plastic eyewear lenses with and without various coatings as listed on the curves.

Sunglasses

Figure 4 depicts the spectral transmittances of three arbitrarily selected sunglasses, one of them being a photogrey glass lens that darkens in sunlight. It was tested under "dark" conditions. The other two sunglasses were plastic "clip-ons" which are worn in addition to corrective eyeglasses. It is observed that all sunglasses tested provide excellent UVB and UVA protection (taking into account that the photogrey lens would darken more in bright sunlight).

 

 

Transmittance spectra of three sunglasses. Note that the maximum value in the graph is 70% transmittance.
Transmittance spectra of three sunglasses. Note that the maximum value in the graph is 70% transmittance.

CONCLUSION

It is concluded that plastic lenses intrinsically provide complete UVB protection and even some UVA protection without additional enhancements. On the other hand, glass lenses provide some, but noticeably less, protection in the UVB range. Further, any style of sunglasses can be assumed to provide all necessary UVB and UVA protection. While the investigation of a greater number of samples would have increased statistical accuracy, one nevertheless can safely assume that the results presented here are characteristic for most of the eyewear industry. In summary, additional films and additives to enhance UV protection in spectacles appears to be unnecessary for most cases. The additional expense of costly enhancements does not compare to the minimal (in some cases negligible) increase in protection from harmful UV radiation.

REFERENCES

  1. See for example: EPA--http://www.epa.gov/ozone/defns#UV
  2. Semes L. UV-A absorbing characteristics of commercial sunglasses intended for recreational and general use. J. Am. Optom. Assoc. 1991; 62:754-8.
  3. Zigman S, Environmental near -UV radiation and cataracts. Optom. Vis. Sci 1972; 72:899-901.
  4. Rafferty NS, Rafferty KA, Zigman S. Comparative response to UV irradiation of cytoskeletal elements in rabbit and skate lens epithelial cells. Curr. Eye Res. 1997; 16:310-19.
  5. Forker C, Wegener A, Grae J. Effects of UV-B radiation on hereditary suture cataract in mice. Exp. Eye Res. 1997; 64:405-11.
  6. Schwartz LH, Ferrand R, Buelle PY, et al. Lack of correlation between the location of choroidal melanoma and ultraviolet-radiation dose distribution. Radiat. Res. 1997; 147:451-6.
  7. Harris MG, Dang M, Garrod S, Wong W. Ultraviolet transmittance of contact lenses. Optom. Vis. Sci. 1994; 71: 1-5.
  8. Bergmanson JP. Protection from harmful UV radiation by contact lenses. J. Am. Optom. Assoc. 1998; 59:178-82.
  9. Quesnel NM, Simonet P. Spectral transmittance of UV absorbing soft and rigid gas permeable contact lenses. Optom. Vis. Sci. 1995; 72:2-10.
  10. Sturat DD, Cullen AP, Sivak JG, Doughty MJ. Optical effects of UV-A and UV-B radiation on the cultured bovine lens. Curr. Eye Res. 1994; 13:371-6.
  11. Rolf E. Hummel, Electronic Properties of Materials, 2nd Ed. Springer-Verlag New York 1993.
  12. CRC Handbook of Chemistry and Physics 74th Edition, CRC Press Boca Raton (1994) page 10-319.

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