816 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS G!yc Para:-.,--•"•' Sea:: & Ski .86 c .98 •:•. R :V P C Ont ::• .,84 .:95 Figure 8. Test substances and control. achieved. Differences in density correspond to protection Peripheral gray halo is artifactual since the assembly ports could not be made "light-tight" without scratching the film plate The plates are developed for 30 seconds at 24øC in normal developing solution, fixed, washed, and air dried. The short development time precludes overdevelopment of either the control or experiment. The degree of blackening of the plate of the sunscreen-protected area is measured with a Densichron densitometer and compared with that of the control area (Fig. 8). As expected, protected areas are less dark than

SUNSCREEN TESTING METHODS 817 controls this difference provides the basis for computation of the amount of protection given by each sunscreen (Table II). Results and Discussion Photographic Method Most of the sunscreening agents showed similar capabilities when either artificial or natural sunlight was used. However, some differences were expected, since about 30% of the filtered energy emitted by the sun lamp (2600-4000 fk) was of wavelength less than 3000 • (5), while only a miniscule amount of the sun's energy reaching the earth is of wavelength less than 3000 A. Light of wavelength less than 2950 • is absorbed by the ozone of the atmosphere (6). There is a progressively greater amount of light in the range from 3000 to 4000 • in the atmosphere (7). Since some sun protectants screen over the entire ultraviolet range while others act only in the sunburn range (2900-3200 fk) (8), differences are expected when two such diverse light sources are compared. Harber studied protection afforded by six ultraviolet-absorbing compounds against sunlight and that from a Hanovia quartz mercury lamp (which has an output somewhat similar to that of the Burdick lamp) as light sources. His results showed identical rankings of protection with either light source (9). The fact that those sunscreens with "broad-spectrum" ultraviolet protection have their maximum absorption in the sunburn range may account for the unexpected similarity of the present results when comparing protection in artificial and natural light. It is emphasized that the rankings in Table II are not valid rankings of product efficacy, but are primarily presented to show the closeness of ranks when using either artificial or natural sunlight. The further development of this test to produce the results obtained in vivo will then also have the advantage of using artificial light, an agent far less capri- cious than sunlight. The method presented employs photography and densitometry to measure the differences between sun protectants and controls. The advantage of the system is that it gives results which can be permanently recorded, compared, and analyzed with the assurance that equal amounts of light bombard the sensor for an equal length of time, traversing through a uniformly spread-out barrier, to a film uniformly sensi- tive in the ultraviolet spectrum. The precision of photography and the accuracy of instrumental readings eliminate the errors inherent in visual evaluation in the density of products used and color changes in the skin. It is felt that this is the best method yet devised for measuring the effectiveness of sunscreens in vitro.