Transmission of X-rays through Glass
(from a study performed by Schott Glass Technologies)

For many years, Schott's SF 6 high lead glass has been used in X-ray protection spectacles. Recently, questions have been raised concerning the possible use of other glass types in this application, therefore, the transmission of X-rays through four types of glasses produced by Schott was measured. The glasses chosen for this study are described in Table 1. Glass type S-1 is a standard white crown glass used in ophthalmic lenses. S-1005 which is also known as High-Lite® is a titanium silicate glass used in ophthalmic lenses requiring a high power prescription while still maintaining a thin cosmetically attractive appearance and light weight. Glass type SF 6, a dense flint containing over 70% by weight lead oxide is commonly used in X-ray protective spectacles. SF L6 is a lightweight version of SF 6 glass which maintains nearly the identical index of refraction and dispersion of SF 6 while reducing the density of the glass by 35%. This is made possible, in part, by the substitution of titanium oxide for the lead oxide in the glass. This glass was included in this study to demonstrate that a high index glass does not necessarily have a high lead oxide content.

The X-ray source was a Phillips Maximus M100 3 phase 12 pulse generator. A 30 cc Capentich ion chamber and model 192 Capentich electrometer were used to detect the X-rays. The X-ray beam was characterized prior to the experiment for values of peak kilovoltage of 70, 100, and 120 kV in terms of half value layers (HVL) of aluminum and effective beam energy (see Table 2). Polished plates of glass 10 cm x 10 cm with thicknesses between 1.05 and 6.95 were placed between the X-ray source and ion chamber. The transmission of glass plate is expressed as the ratio of the ion chamber readings with and without the plate in the beam.

Good beam geometry was maintained for each experiment so that a comparison of the transmission properties of the four glasses could be easily made. The raw data was corrected to standard dress eyewear, 2.2 mm thick. The transmission of X-rays as a function of kV for the dress thickness is shown for the four glass types in Figure 1. Due to the extremely low transmission (high X-ray absorption) for SF 6 glass, the experiment was repeated using a series of glass plates approximately 1 mm in thickness.

Please note that the absolute values of these transmission measurements should only serve as a guide to the consumer in ranking these four glass types in terms of their ability to block X-rays. Different experimental conditions, such as variations in inherent and added beam filtration may affect the absolute values of the transmission measurements. In addition, geometric factors such as the relative positions of the detector, glass plates and X-ray beam may also affect the transmission values. Studies (1-3) have indicated that in addition to the type of lens material used, the fit of the spectacle frame in which the lenses are mounted as well as the presence or absence of X-ray absorbing side shield can have an impact on the amount of X-rays reaching the eyes.

In conclusion, in terms of protection to the eyes from X-rays in the range of 70 to 120 kV, the glasses examined in this study are ranked:

Such that SF 6 blocks the most while S-1 blocks the least amount of X-rays.

1. A.J. Cousins, R.B. Lawdahl, D.P. Chakroborty and R.E. Koehler, "The Case for Radioprotective Eyewear/Facewear: Practical Implications and Suggestions", Invest. Radiology, 22: 688-692, (1987).

2. W.E. Moore, G. Ferguson and C. Rohrmann, "Physical factors determining the utility of radiation safety glasses", Med. Physics, 7 (1), Jan/Feb: 8-12, (1980).

3. A.H. Richman, B. Chan and M. Katz, "Effectiveness of Lead Lenses in Reducing Radiation Exposures", Radiology, 121: 357-359, Nov. (1976).