Talk:Ultraviolet index

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Contents 1 'Media Color' column still needed? 2 The scale goes past 11 3 Still no clear answer on UV index 4 Cumulative damage 5 Is this a log scale? How is it defined in terms of Physics units and weighting curve 6 Explanation of units mW/m^2 nm

[edit] 'Media Color' column still needed?

Nice work on putting more color into the table, does this remove the need for a 'Media Color' column in the table? I say scrap it. IanMcGreene 19:20, July 21, 2005 (UTC)

I still like having the colors described in words, since the are part of the standard and bgcolors don't show up on text browsers. But the column does not need to be so prominent, I think. Note that in my last edit I changed the colors so that they exactly match those used on the EPA Sunwise site. hajhouse

[edit] The scale goes past 11

The article states that ratings beyond 11 are unlikely to occur. Not so - it is frequently 12 or 13 in Brisbane, where I live. In fact for a long time I wondered what the point of the scale was since the forecast here is always "extreme".

This site http://sedac.ciesin.org/ozone/maps/uvimap.shtml shows a range of values up to 21!

[edit] Still no clear answer on UV index

The article UV index is reiterating what we all know from generic, weather-report statements about the topic. I searched this article wanting to find out how long a person can stay unprotected in sun. For example, which duration of exposure at UV index of 4 (the minimal index that can burn skin, I gather) will result in a burn for pale skin? Then, if the same pale person were instead exposed to UV index of 5, how much less time could the person be in unprotected sunlight. Is it a linear or a log function? i.e. does UV index of 5 burn twice as fast as UV index of 4, or does it burn at a rate that is 1.25 (5/4) times faster?

My own understanding is that a pale person burns in about 20 min of summer sun -- what uv index might that be based upon? Inclusion of answers to these questions would bring the article a step beyond its current rather generic content.

I have heard that tanning can function like an SPF (sun protection factor) of 3. Does that mean it is equivalent to subtracting 3 from the UV index. In other words, how can one sensibly combine the net effects of sunscreen factors with UV index numbers? We keep hearing these numbers -- UV index and SPF -- all the time in summer, but no information source seems to be able to make much sense of them?

[edit] Cumulative damage

• Damage from sun exposure is cumulative over one's lifetime.

Who says so? And what do they mean by this? Will two days at level 4 equal one day at level 8? --Uncle Ed 15:47, 5 December 2006 (UTC)

Read the Definition: "it is reasonable to assume that one hour of exposure at index 10 is approximately equivalent to two hours at index 5, although other factors like the body's ability to repair damage over a given time period could detract from the validity of this assumption".

There are definite dermatological data proving that damage from sun exposure is cumulative. There's a sort of special ultraviolet picture you can take of your skin, that shows the level of damage, and people of increasing age show progressively more damage: look here.

D0nj03 (talk) 08:04, 11 May 2008 (UTC)

[edit] Is this a log scale? How is it defined in terms of Physics units and weighting curve

I came here looking for detailed facts. Is it a log scale or linear. How do permitted exposure and index relate (double time for each unit of index for example?). And the weighting (it has a name as do units of exposure I think). --Memestream 16:28, 20 August 2007 (UTC)

OK I've got to the bottom of this now, and added a definition section to the article. --Memestream 17:43, 20 August 2007 (UTC)

[edit] Explanation of units mW/m^2 nm

The following is a copy of the query and answer at my talk --Memestream (talk) 15:42, 14 April 2008 (UTC)

The new units are the same. But to me, they make no physical sense, either way you write them. What sources I found cited in units of power/area. Which in your terms, would be mW/m^2. The extra nm makes no sense. If it was supposed to be a capital N, it would stand for Newton, a unit of force. That doesn't parse, either. nm is correctly read as nanometer. mW/m^2 nm would be power/volume, which is not a unit of radiance. That would be a measure of delivered energy to a target region, but such a unit would typically be accompanied by a footnote to explain what region was selected and how (e.g. a particular sub-dermal strata, taking into account occlusion by surface layers). For general discussion, it is far simpler to speak of exposure in radiance terms: power/area. I was hoping you would provide the original reference so I could see for myself what the scientists were up to. MaxEnt (talk) 18:05, 11 April 2008 (UTC)

I see your problem now. The units do make perfect sense in that they refer to milliwatts (power or flow of energy), per square meter, per nanometre of wavelength (a way of defining an equivalent frequency band) across the frequency spectrum. These units are used when plotting the power spectrum, not when stating absolute power. It is perfectly possible to measure incident power, in mW/square meter, as you say, but only if you define the bandwidth of the electromagnetic spectrum within which the power is measured. The wider the bandwidth of the measurement the more power there is. While it is possible to quote mW/sq metre of 'ultraviolet', this leaves uncertainty as to what 'ultraviolet' is, ie whether we include what is commonly called UVA, B, C, and what exactly the frequencly limits for these are. Such a measure of incident total power is useless as a measure of sun damage though, because, for example, exposure to 1mW/m^2 taken in a location under the ozone hole could be a hundred times as damaging as the same 1mW/m^2 in a cloudy location, because in the former location we would be exposed mostly to UVC, while in the latter to UVA, since the shorter wavelengths are more readily filtered out by cloud and ozone, and it is the shorter wavelengths (UVC) that cause most damage. To get a measure of irradiance that corelates with damage we need to first know the spectral content of the UV, and then to weight each bit of spectrum according to its ability to damage. There is no such thing as the power content at one frequency - it's zero, since the power of any radiation that has a noise-like spectrum reduces to zero as you reduce the bandwidth to zero. Hence we arbitrarily choose the bandwidth corresponding to 1nm of wavelenght as the bandwidth of our measurement. Engineers face a similar problem when measuring noise in electronic circuits, and an op-amp is specified in terms of nV/sq.root Hz, based on the fact that for white noise the power in a given bandwidth is proportional to bandwidth in Hz, and power is proportional to the square of voltage. mW/Hz would be a suitable alternative unit to use for UV (though it's an awfully fine resolution at UV frequencies), but it doesn't matter what units you use so long as you are consistent when devising the weighting curve. Spectrum analysers involve the same consideration. They usually have selectable measurement bandwidth, and so the readings obtained are higher if the bandwidth is set larger. This makes the concept of 'spectral content' of, for example, music, meaningless, since the content at any given frequency on a random signal is zero. The concept of 'spectral content' only has meaning for a specified measurement bandwidth. The above is explained in the reference I cited inline [1] though this does not explain the rationale for the units used as I have tried to. --Memestream (talk) 15:55, 14 April 2008 (UTC)