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Monday, July 01, 2013

Ultraviolet light measurements 

Sper Scientific UVA/B Light Meter 85009.
Date: 2 July 2013.
Place: Canberra, Australia.
Conditions: Overcast, somewhat foggy.

Reading at 0930: 830 microwatts/sq. cm.

(A reading of 0 indoors.)

To me, a surprising amount of UV for such an overcast and foggy day.

Same details and approximate time:

Without a glass over the detector: approx. 900 microwatts/sq cm.
With a glass: 280 microwatts/sq cm.

These readings using the bottom of  a glass tumbler.

At 0940 hrs:

This time using glass from a small picture frame:

Still overcast and foggy.

No glass: 1035 microwatts/sq cm.
With glass: 615.

And do I need to buy a UVB meter like the one Frances Baines used? By the way, the suggestion here is that reptiles use UVA light to make vitamin D. Maybe Baines should have measured this for her reptile work, not UVB. "Reptiles need long wave UV light for de novo synthesis of vitamin D. Vitamin D is needed to metabolize calcium for bone and egg production. Thus, in a typical reptile enclosure, a fluorescent UV lamp should be available for vitamin D synthesis." However this site says that reptiles utilise UVB. So maybe Wikipedia is wrong.

Also, "Ordinary window glass passes about 90% of the light above 350 nm, but blocks over 90% of the light below 300 nm." UVA is 315 to 400 nm. UVB is 280-315 nm.

The Sper detector is most sensitive between 310 and 360 nm.

Conclusion: Even on an overcast foggy day, the UV reading was as high as 1000 microwatts/sq cm.

The Australian UV Index.

Try measuring the effects of various plastics, glasses and other materials on UV passage using the Sper instrument. It might be best to use it for experiment rather than observation. The instrument probably really just gives a rough idea of UVA levels.

Given that we (Baines and I) have established pretty clearly that UVB is reduced to one or two percentage points of the outside level in the rainforest, one thing to try might be to see what kind of percentage readings one gets with the Sper instrument, which mainly reads UVA. In fact, if plenty of UVA gets into such forests, it might explain why reptiles are OK for their calcium balance there.

UV and fish eyes.











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