Solar Filters
In 1994, a disucssion arose on a Usenet newsgroup involving safe materials for observing ths sun. Several materials were suggested for blocking the sun’s harmful light and a few were tested in the lab. Results are presented that show the differences between some common materials. The photometer used did not have a large dynamic range, but enough information was gathered to show the differences between aluminised mylar for astronomy purposes and the other suggeted materials (mylar potato-chip bag, floppy disk, welders’ glass). It is concluded that the only safe solar filter materials is aluminised mylar made specifically for filtering the sun’s light.
On 10 May 1994, on the sci.astro Usenet newsgroup (Subject: eclipse-view with floppy), a post was written describing the use of a floppy disk for visual observation of the sun. Further posts in the thread described the use of mylar bags for food, and compact disks. In my work at the time, I had access to a spectrometer which could be used to investigate the light transmission of these different ’filters’. The message which started everything follows:
A safe way to view the eclipse if it hasn’t already passed you by is to look through a floppy disk. If you hold the media, the disk itself not the case, to your eye it will filter out most of the light, giving you a red image. This works much better than the projection method.
My response was quick:
When I saw the post suggesting that a floppy disk is a safe solar filter, I thought this guy was _CRAZY_ ! So, being reasonable, i thought I would verify before saying any more. Below are the results of a quick measurement I made. I used an HP-8452 UV-Vis Spectrometer to check the %-Transmission of light through a 5.25 inch floppy disk in the 190 to 820 nm range.
A Hewlett-Packard HP-8452 UV-Vis Spectrophotometer was used to collect the data. The following settings were used:
Wavelength Range190 nm to 820 nm
Integration Time10 Seconds
Output Mode% Transmittance
Each material was installed in the spectrometer using the standard mounting mechanism. The entire spectral range of the instrument was used. In all, 4 samples were analysed: mylar from a potato-chip bag, floppy disk, #12 welders’ glass, aluminised mylar for astronomy use.
On 10 May 1994, on the sci.astro Usenet newsgroup (Subject: eclipse-view with floppy), a post was written describing the use of a floppy disk for visual observation of the sun. Further posts in the thread described the use of mylar bags for food, and compact disks. In my work at the time, I had access to a spectrometer which could be used to investigate the light transmission of these different ’filters’. The message which started everything follows:
A safe way to view the eclipse if it hasn’t already passed you by is to look through a floppy disk. If you hold the media, the disk itself not the case, to your eye it will filter out most of the light, giving you a red image. This works much better than the projection method.
My response was quick:
When I saw the post suggesting that a floppy disk is a safe solar filter, I thought this guy was _CRAZY_ ! So, being reasonable, i thought I would verify before saying any more. Below are the results of a quick measurement I made. I used an HP-8452 UV-Vis Spectrometer to check the %-Transmission of light through a 5.25 inch floppy disk in the 190 to 820 nm range.
A Hewlett-Packard HP-8452 UV-Vis Spectrophotometer was used to collect the data. The following settings were used:
Wavelength Range190 nm to 820 nm
Integration Time10 Seconds
Output Mode% Transmittance
Each material was installed in the spectrometer using the standard mounting mechanism. The entire spectral range of the instrument was used. In all, 4 samples were analysed: mylar from a potato-chip bag, floppy disk, #12 welders’ glass, aluminised mylar for astronomy use.
The first graph shows the response of the alumised mylar, the floppy disk, and the potato-chip bag. Clearly, much more light is transmitted through the sub-standard filters.Note that while the floppy disk does seem to attenutate light in the visible region, it has increasing transmission in the infrared region - which will cause eye damage.
|
It is obvious that the welders’ glass is much better than the previous filter materials. It must be noted here that the spectrometer has a dynamic range of about 12000 to 1. This prevents measurements of the true response of the alumised mylar (and welders’ glass).
The response to the measurements being posted was positive.
Hooray! The Net being used the way it should be! Someone posts a dumb idea, someone shows that it is a dumb idea by actually checking it out, and then shares the information with us. Well done!
Of course, there was the possibility of misunderstanding the original post - a helpful fellow tried to clarify things for me:
Obviously, he didn’t mean just using the magnetic media as a filter - you should have measured it through the cardboard body of the disk, in its protective cardboard sleeve, in series with 9 other disks in a shrinkwrapped carboard box... (_And_ the price sticker on the outside. :).
It is generally accepted that mylar solar film for visual use passes approximatley 1.0E-6 of incident light. And mylar solar film for photographic use passes between 1.0E-5 and 1.0E-4 of incident light, depending on the manufacturer.
The dynamic range of the spectrometer prevented measuring transmissions this small. For reference, #14 welders glass, which is typically recommend due to avilability and low cost, passes about 1.0E-7 of incident light.
I strongly recommend using solar filter materials sold by a telescope shop. Don't take chances with your eyes. Have a look at Ralph Chou's web page if you need more convincing.
The response to the measurements being posted was positive.
Hooray! The Net being used the way it should be! Someone posts a dumb idea, someone shows that it is a dumb idea by actually checking it out, and then shares the information with us. Well done!
Of course, there was the possibility of misunderstanding the original post - a helpful fellow tried to clarify things for me:
Obviously, he didn’t mean just using the magnetic media as a filter - you should have measured it through the cardboard body of the disk, in its protective cardboard sleeve, in series with 9 other disks in a shrinkwrapped carboard box... (_And_ the price sticker on the outside. :).
It is generally accepted that mylar solar film for visual use passes approximatley 1.0E-6 of incident light. And mylar solar film for photographic use passes between 1.0E-5 and 1.0E-4 of incident light, depending on the manufacturer.
The dynamic range of the spectrometer prevented measuring transmissions this small. For reference, #14 welders glass, which is typically recommend due to avilability and low cost, passes about 1.0E-7 of incident light.
I strongly recommend using solar filter materials sold by a telescope shop. Don't take chances with your eyes. Have a look at Ralph Chou's web page if you need more convincing.