Science Buddies: "Ask an Expert"

My daughter is doing a science fair project titled, "Are the SPF's from different brands of sunscreen equally as effective in blocking UV rays from the sun?" She is testing 3 different brands of SPF 8, SPF 15, SPF 30, and SPF 50. We have a UV meter, and purchased a UV bulb. Because we live in the Northeast the rays are not strong enough to conduct the experiment outside, so we purchased the meter and bulb. We were initially going to coat a layer of each sunscreen on saran wrap and place it underneath the UV lamp to get a reading. However, the teacher suggested applying lotion to different colors of paper. So we cut out 96 blocks of paper (used 8 different colors and put one of each of the 12 brands of lotion a given block). The UV meter gets up to about a 5 without any paper. I don't find the reading that reliable. We placed it in a box to keep the light concentrated in a small area. However, when we put the paper underneath the bulb, with the lotion side facing the bulb, the UV meter reading was 0. When we placed paper with no lotion under the bulb, the reading was also zero. So we can't get a reading on any of the 96 blocks of paper. We are desperate for suggestions. I was wondering whether going to a tanning salon might be a good option. Perhaps the UV would be stronger. We could take the UV reading inside the tanning bed, and then see if we can get a reading from the blocks of paper. I'm just not sure we can get a reading through paper - it seems to block the UV. Thank you for any suggestions you may have!!!

Hi,

I think you had the right idea first using the plastic wrap. You really need to have as little as possible between the UV lamp and the meter, except the sunscreen. The one suggestion I would make is to try several kinds of plastic wrap to makes sure the plastic itself does not absorb the UV.

One other thing i just thought of is the distance between the lamp and the meter. The strength of the UV decreases as the square of the distance from the source so try putting the lamp as close as possible to the meter. You could put the plastic wrap on the surface of the meter, apply the sunscreen and then position the lamp close to it. Remember that you must have the distance and orientation of the meter and lamp identical for all measurements. Also, try to get the film of sunscreen the same each time. Do at least three repeats for each SPF so you can average them.

I hope this helps.

Let us know if you have more questions.

Sybee

Hi,
Sybee's answers were very good. I would like to see if I could add to them a little. The distance between the UV lamp and the UV meter is critical as is the distance of the film from the UV lamp, and the distance of the film from the lamp. If possible you need to mount hem inside a wooden box or something similar. This would not only control the distance and the height of both the light an meter, but also mask out any UV sources in the room where you are doing the experiment. You want to have the height of the lamp, the film, and the meter constant every time or you will get a lot of variation.

If you use plastic wrap, you can get some brands of these in different colors (red, yellow, blue, etc.). Most cake decorating places have these if you cannot find them in a grocery store. You could use two colored films or one color side and one clear side. You could also combine colors such as blue and red and note the variations.

As Sybee said the thickness of the film is very important. You might want to use 2 pieces of plastic and sandwich the film in between them. You can use a rolling pin with a wooden spacers on each side to roll out a thin consistent film thickness.

Good luck with your experiment and be sure to let us know how it turned out!

How did your experiment go?