Datalogging in Science

Suntan cream blocks uv radiation

Overview
In this investigation, the levels of ultraviolet radiation that penetrate through a coating of various sun tan products are measured and compared. Students may be surprised at how much uv radiation penetrates even high factor sun lotions. The work may be conducted outdoors or in a classroom with the sensor facing out an open window. Alternatively an ultraviolet lamp can be used. If using a lamp read the safety note. Smears of different suntan creams are placed on several microscope slides and the penetration of uv radiation is measured using an uv sensor. The results are compared with those of a clean slide so as to calculate the Sun Protection Factor. (See note below).

Safety note
If using an Ultraviolet lamp, be sure that it is transmitting only in the UV A band.
Students should not look directly at the lamp for long periods.

Background
It is important to wear good protective sun cream in bright sunshine, so as to protect the skin from the dangers of ultraviolet light. The term Sun Protection Factor (SPF) is generally abbreviated to “factor 20” , etc. This gives an indication of how long one can safely spend in the sun.

Sun Protection Factor SPF
This is the industry’s agreed method of showing the relative effectiveness of the lotions sun blocking ability. To calculate the “factor number”:
1. Find the amount of ultraviolet that can be detected using no sun cream.
2. Find the amount of ultraviolet that can be detected with the sun cream.
3. Calculate the Percentage of the ultraviolet that is being blocked.
4. Take the difference (in percentages) of the ultraviolet blocked to unblocked and divide this into 100. The answer is the SPF factor.

Worked example
1. Ultraviolet measure without sun cream is 35 W/m2
2. Ultraviolet measure with sun cream is 30 W/m2
3. Percentage of Ultraviolet blocked is (30/35) x 100 = 86% (rounded).
4. 100 – 86 = 14
5. 100/14 = 7.14 = SPF 7.0

Apparatus
1. A data logger.
2. A Smart Q Ultraviolet sensor - set to 50w range.
3. Microscope slides.
4. A variety of suntan creams.
5. uv lamp (optional)

Procedure

1. Use a marker to label several microscope slides.


2. Take a small amount of the sun product on the end of a finger and gently rub onto a microscope slide. Cover it with a second slide (making a “sandwich” with the sun product as the filler). Bind them together with a pair of rubber bands.

3. Repeat for each of the different products to be tested.

4. Prepare a “reference” with a pair of clean slides and no sun product between them

5. Position the apparatus facing an open window or switch on the ultraviolet lamp.
(The Lamp should be allowed to ‘warm up’ for about 5 minutes before use and reach full output. Check the range of the sensor is correct for the lamp and adjust the range if necessary.)

6. Set the apparatus up as shown in the diagram, ensuring that the sensor is pointing at the light.

7. Connect the Ultraviolet sensor to the logger.

8. Launch the data logging software and use Snapshot mode.

9. Test each slide in turn and record the ultraviolet level.

10. Repeat until all the samples have been recorded.
Enter results in a table as shown

Results
The graph will have the appearance of a bar chart, with separate bars for each sun product. The height of the bar indicates how much ultraviolet light reached the sensor, short bar will mean more ultraviolet has been blocked; tall bars mean more ultraviolet has been let through.

Extensions

1. Does a quick swim really wash the lotion away?
2. Does T-shirt material protect you from the suns harmful rays?
3. Is all glass the same?

Questions
1. Why don’t you get sun burnt behind a glass window?

1. Connect the sensor to the datalogger and the logger to the computer
2. Then click the Set Up icon to the right of this message.
3. When the software opens, click the Play button.