Fun With UVB

I get a lot of junk mail at work. Partly this is because companies think that I may use their product in my classes. Most of it isn't worth a second look and much of it goes into the garbage unopened. I recently got a flyer for a new and worthwhile product that is of some astronomical interest. It is a handheld meter to measure the amount of UVB radiation. Basically, it is a small UV photometer.

There are three widely-recognized regions of ultraviolet radiation. These are UVA (320-400 nm), UVB (290-320 nm), and UVC (<280 nm). UVA is at the very short wavelength end of the visible spectrum. The eye has some sensitivity down to about 390 nm which is where the calcium H and K lines are in the solar spectrum. UVB contains the ozone absorption band-head and is the region of the spectrum primarily responsible for both tanning and skin damage. UVC never makes it to the ground.

One of the goals of this hand-held unit is to allow ordinary folks without $500 meters to measure the UVB flux and to determine how long they can stay in the sun without burning. This is dependent on skin type and so the unit comes with a handy set of small cards for each of the six skin types that tells you how long you can stay out for a given meter reading. The meter has its own solar cells, so it never needs batteries or an on-off switch! The readings are taken on a two-digit LCD panel. The whole thing easily fits in your shirt pocket and weighs about one ounce!

A second goal of Sunsor is to establish a wide network of volunteer UV reading submitters to provide a daily UV index for local weather stations. As such, they would like anyone with a meter to submit a reading once per day, preferably before solar noon (about 1:20pm EDT in Hamilton during the summer).

I did a little experiment with the meter last weekend. I wanted to see how quickly the UVB flux rose with solar altitude. Said differently, I wanted to determine the extinction coefficient in the bandpass 290-320 nm. Saturday, April 26 was one of the first really nice clear warm days this year. From the time I first got outside, to close to solar noon, I made measurements. Here they are:

1997 April 26 (Hamilton, ON)

The sun was at declination 13.6 degrees, my latitude was 43.25 degrees, and solar noon was at 13:17 EDT in Hamilton. The altitude was calculated with:

sin(alt) = sin(lat)*sin(dec) + cos(lat)*cos(dec)*cos(hour angle)

If the absorption is occurring in a layer of finite thickness, then the path length through that layer should vary as the cosecant of the sun's altitude. This is sometimes referred to as the 'air mass' and is defined in such a way that the path length at the zenith is one. Obviously, at the horizon the path length would be infinite - which it isn't - but that is because we have only considered a plane-parallel atmosphere. However, the approximation is pretty good up to zenith angles of about 80 degrees! The amount of absorption increases exponentially with the path length. My job is to find out which exponential fits the data.

In the graph below, I plot the results of these measurements and calculations. The intensity of UVB radiation does indeed fall off rapidly with angle from the zenith. At an altitude of 30 degrees, the sun is 1.45 magnitudes fainter than it would be directly overhead! This compares to a change of about 0.25 over the same range of angles in the visual part of the spectrum.

The photometer unit is made by:
Sunsor, Inc.,
4361 Route 8,
Allison Park, PA 15101
Tel: (412) 492-9814;
FAX: (412) 492-9309;

It costs US$39.95 plus shipping and handling. They have a WEB PAGE and E-MAIL

Doug Welch

Hamilton Amateur Astronomers
Maintained by Grant Dixon