Annah,
You have asked the million dollar question. What do light measurements mean? Well, the first factor is that many wavelengths of light exist. The general convention is that wavelengths above 400 nanometers (it may be easier to use the word descriptions, rather than the symbol descriptions) are in the visible light spectrum, up to about 800 nanometers. Light immediately above 800 nanometers is infra-red (heat), and immediately below 400 nanometers is considered ultraviolet. As per your question, ultraviolet runs from about 100 nanometers to 400 nanometers. Daylight has all of these wavelengths, although not in the same amount at each wavelength. UVC runs from 100-280 nanometers. This is the UV that give us vitamin D through our skin. UVB runs from 280 - 315 nanometers, and is responsible for sunburn. UVA runs from 315-400 nanometers, and it, along with UVB, can be responsible for skin cancers. Generally, the shorter the wavelength (smaller the nanometer), the higher the energy of the UV light. However, all UV light can not be seen by the human eye, and all is harmful to historic objects. Therefore, eliminating all UV is recommended preservation procedure.
If the glass manufacturer is measuring transmittance only at 300 nanometers, they are not measuring anything in the rest of the UV range. It is possible that significant amounts of UVA and UVC are allowed through the glass. What you want is a specification that shows the TOTAL transmittance of UV through the entire 100-400 nanometer range. The transmittance at any single wavelength is meaningless.
Your meter appears to be measuring in micro-watts, which should also list per unit of area, such as square meters or square centimeters. This measures the total UV falling on an area. As with visible light, the further the surface is from the light source, the lower the amount of UV radiation falling on it (inverse square of the distance). Understanding this measurement is difficult unless you also measure the unfiltered light at the window exterior and the filtered light at the immediate window interior. Then, you can divide the filtered reading by the unfiltered reading to get a percentage of UV reduction. However, it is possible that your specific meter is reading at a particular wavelength, and not across the whole UV spectrum, which will not give you the full amount of filtering. The more common measurement for museums is micro-watts per lumen, which indicates the amount of UV in a given unit of total visible light. This is a better indicator of UV-filtering effectiveness, as it eliminates the effect of distance from the light source, provided that the meter is reading across the entire UV range. Generally, a reading of less than 75 micro-watts per lumen is considered acceptable for preservation, although lower is always better. Many meters can switch between the two measuring systems.
Unfortunately, it is very difficult to get an inexpensive UV meter that measures across the entire UV spectrum. And yes, if you want to get a full spectral curve at every wavelength, you will have to use the million dollar instrument. Therefore, the best alternative is to have the manufacturer provide you with a spectral absorbance chart (curve) for their product that runs from the far UV through the visible spectrum (100 nanometers through 800 nanometers), showing all the wavelengths. All the reputable manufacturers have them, although it can take quite a bit of effort to find them. You have to take on faith that the product that you receive is accurately represented by the spectrum that they supply, but the alternative of testing yourself to verify is much more complicated and expensive that it may seem at first due to the factors I have listed. Good luck.
Marc
American Conservation Consortium, Ltd.
4 Rockville Road
Broad Brook, CT 06016
www.conservator.com
860-386-6058
Marc A. Williams, President
MS in Art Conservation, Winterthur Museum Program
Former Chief Wooden Object Conservator, Smithsonian Institution
Fellow, American Institute for Conservation (AIC)
American Conservation Consortium, Ltd.
4 Rockville Road
Broad Brook, CT 06016
www.conservator.com
860-386-6058
*Collections Preservation Consultation
*Conservation Assessments & Surveys
*Environmental Monitoring & Low-Tech Control
*Moisture Management Solutions
*Collections in Historic Structures
*Collections Care Grant Preparation
*Conservation Treatment of:
Furniture
Painted Wood
Horse-Drawn Vehicles
Architectural Interiors
Decorative Objects & Folk Art
Marc A. Williams, President
MS in Art Conservation, Winterthur Museum Program
Former Chief Wooden Object Conservator, Smithsonian Institution
Fellow, American Institute for Conservation (AIC)
----- Original Message -----
From: Stepping Stones
To: [log in to unmask]
Sent: Wednesday, July 28, 2010 2:47 PM
Subject: [MUSEUM-L] UV Light Filtering
Hi Everyone - Stepping Stones, the historic home of Bill and Lois Wilson in Bedford, NY, recently took a big plunge and had UV/Solar laminated glass installed as storm windows. It was meant to filter out 99% of harmful UV rays. Now that the storms have been custom made and installed, I bought a UV light meter to measure. It is entirely possible that I am not reading the measuring equipment correctly, but so far I get something around 60%, and can't make heads or tails of the user manual.
Saflex, the company that manufactures the interlayer says that they measure against 300nm wavelength and the handheld equipment I bought measures in the UV unit (upsidown hW). I called the Saflex company and they said they could send me a statement guaranteeing their product but that the only way to accurately measure is to send a sample of the glass to somewhere that puts it into a million dollar machine. Also, the company I bought the glass from says that I am the only person in the history of his company who has ever wanted to measure the UV light.
Yet, I know that historic house museums are measuring UV and daylight all the time, so I am hoping that someone will share their light measuring procedures with me, and any ideas of how I might start accurately measuring the environment inside the historic house museum. Has anyone had a similar experience? Does anyone have a UV513AB Digital UV AB Light Meter and know how to use it, or could you recommend a better, affordable light meter?
Thanks in advance.
Annah Perch
The Stepping Stones Foundation
62 Oak Road
Katonah, NY 10536
(914) 232-4822
[log in to unmask]
www.steppingstones.org
========================================================= Important Subscriber Information:
The Museum-L FAQ file is located at http://www.finalchapter.com/museum-l-faq/ . You may obtain detailed information about the listserv commands by sending a one line e-mail message to [log in to unmask] . The body of the message should read "help" (without the quotes).
If you decide to leave Museum-L, please send a one line e-mail message to [log in to unmask] . The body of the message should read "Signoff Museum-L" (without the quotes).
=========================================================
Important Subscriber Information:
The Museum-L FAQ file is located at http://www.finalchapter.com/museum-l-faq/ . You may obtain detailed information about the listserv commands by sending a one line e-mail message to [log in to unmask] . The body of the message should read "help" (without the quotes).
If you decide to leave Museum-L, please send a one line e-mail message to [log in to unmask] . The body of the message should read "Signoff Museum-L" (without the quotes).
|
