About a month and a half ago I ordered some "Eclipse Viewing Glasses" from Amazon - these being those cardboard things with plastic filters. When I got them, I looked through them and saw that they were very dark - and in looking briefly at the sun through them they seemed OK.
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| Figure 1: The suspect eclipse viewing glasses. These are the typical cardboard frame glasses with very dark plastic lenses. Click on the image for a slightly larger version. |
I was surprised and chagrined when, a few days ago, I got an email from Amazon saying that they were unable to verify to their satisfaction that the supplier of these glasses had, in fact, used proper ISO rated filters and were refunding the purchase price. This didn't mean that they were defective - it's just that they couldn't "guarantee" that they weren't.
I was somewhat annoyed, of course, that this had happened too soon prior to the event to be able to get some "proper" glasses, but I then started thinking: These glasses look dark - how good - or bad - are they?
I decided to analyze them.
WARNING - PLEASE READ!
What follows is my own, personal analysis of "potentially defective" products that, even when used properly, may result in permanent eye damage. This analysis was done using equipment at hand and should not considered to be scientifically rigorous or precise.
DO NOT take what follows as a recommendation - or even an inference - that the glasses that I tested are safe, or that if you have similar-looking glasses, that they, too, are safe to use!
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| Figure 2: The 60 watt LED light used for testing. This "flashlight" consists of a 60 watt Luminus white LED with a "secondary" lens placed in front of it. The "primary" lens (a 7" diameter Fresnel) used to collimate the beam was removed for this testing. Click on the image for a larger version. |
YOU HAVE BEEN WARNED!
White Light transmission test:
I happen to have on hand a homemade flashlight that uses a 60 watt white LED that, when viewed up close, would certainly be capable of causing eye damage when operating at full power - and this seemed to be a good, repeatable candidate for testing. For measuring the brightness I used a PIN photodiode (a Hammatsu S1223-01) and relative measurements in intensity could be ascertained by measuring the photon-induced currents by measuring that current with and without the filter in place.
Using my trusty Fluke 87V multimeter, when placed 1/4" (about 6mm) away from the light's secondary lens I consistently measured a current of about 53 milliamps - a significantly higher current than I can get from exposing this same photodiode to the noonday sun. In the darkened room I then had the challenge of measuring far smaller current.
Switching the Fluke to its "Hi Resolution" mode, I had, at the lowest range, a resolution of 10 nanoamps - but I was getting a consistent reading of several hundred nanoamps even when I covered the photodiode completely. It finally occurred to me that the photodiode - being a diode - might be picking up stray RF from radio and TV stations as well as the ever-present electromagnetic field from the wires within our houses so I placed a 0.0022uF capacitor across it and now had a reading of -30 nanoamps, or -0.03 microamps. Reversing the leads on the meter did not change this reading so I figured that this was due to an offset in the meter itself so I "zeroed" it out using the meter's "relative reading" function. Just to make sure that the all of the current that I was measuring was from the front of the photodiode I covered the back side with black electrical tape.
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| Figure 3: A close up of the S1223-01 photodiode and capacitor in front of the LED. The bypass capacitor was added to minimize rectification of stray RF and EM fields which caused a slight "bias" in the low-current readings. Click on the image for a lager version. |
I then placed the plastic film lens of the glasses in front of the LED, atop the flashlights secondary lens - and...
It melted.
Drat!
Moving to a still-intact "unmelted" portion of the lens I held it against the photodiode this time, placing it about 1/4" away from the LED as well and got a consistent reading of 0.03-0.04 microamps, or 30-40 nanoamps. Re-doing this measurement several times, I verified the consistency of these numbers.
Because the intensity of the light is proportional to the photodiode current, we can be reasonably assured that the ratio of the "with glasses" and "without glasses" currents are indicative of the amount of attenuation afforded by these glasses, so:
53mA = 5.3*10E-2 amps - direct LED, no glasses
40nA = 4.0*10E-8 amps - through the glasses
The ratio is therefore:
5.3*10E-2 / 4.8*10E-8 = 1325000
What this implies is that there is a 1.325 million-fold reduction in the brightness of the light. Compare this with #12 welding glass which has about a 30000 (30k)-fold reduction of visible light and the absolute minimum that is considered to be "safe" for direct viewing while #14 offers about a 300000 (300k)-fold reduction. According to various sources (NASA, etc.) a reduction of 100000 (100k)-fold will yield safe direct viewing. The commonly available #10 welding glass offers only "about" a 10000 (10k)-fold reduction at best and is not considered to be safe for direct solar viewing.
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| Figure 4: The typical spectral output of a "white" LED (blue line) and a typical silicon PIN photiode (black line.) The distinct peak is from the internal blue LED while the "yellow" Ce:YAG phosphors emit longer wavelengths to produce a "white" light. As can be seen, the sensitivity of the photodiode increases with longer wavelengths while the spectral output of a white LED drops. Click on the image for a larger version. |
This reading can't be taken entirely at face value as this assumes that the solar glasses have an even color response over the visible range - but in looking through them, they are distinctly red-orange. What this means is that the spectrum of the white LED - which is mostly red-yellow and some blue (because white LEDs use blue LEDs and a phosphor to produce the rest of the spectrum) and very little infrared - means that we are doing a bit of apples-oranges comparison.
In addition to this, the response of the photodiode itself is not "flat" over the visible spectrum, peaking in the near-infrared and trailing off with shorter wavelengths - that is, toward the blue end. Figure 4, above, shows the relative peak light outputs of a typical "white" LED overlaid with the response of the photodiode and once can see that they are somewhat complimentary.
To a limited degree, these two different curves will negate each other in that the sensitivity of the photodiode is a tilted toward the "red" end of the spectrum. With the inference being that these glasses may be "dark enough", I wanted to make some more measurements.
Photographing the sun:
As it happens I have a Baader ND 5.0 solar film filter for my 8" telescope to allow direct, safe viewing of the sun via the telescope. Because I'd melted a pair of glasses in front of the LED, I wasn't willing to make the same measurement with this (expensive!) filter so I decided to place each filter in front of the camera lens and photograph the sun using identical exposure settings as seen in Figure 5, below.
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| Figure 5: The Baader filter on the left and the suspect glasses on the right. These pictures were taken through a 200mm zoom lens using a Sigma SD-1 camera set to ISO 200 at F8 and 1/320th of a second. Both use identical, fixed "Daylight" white balance. Click on the image for a lager version. |
What is very apparent is that the Baader filter is pretty much neutral in tone while the glasses are quite red. To get a more meaningful measurement, I used an image manipulation program to determine the relative brightness of the R, G and B channels with their values rescaled to 8 bits: Because the camera that I used - a Sigma SD-1 actually has RGB channels with its Foveon sensor rather than the more typical Bayer CMY matrix, these levels are reasonably accurate. Note that the numbers below do not take "gamma" (discussed later) into account.
For the Baader filter:
- Red = 163
- Green = 167
- Blue = 162
- Red = 211
- Green = 67
- Blue = 0
What the eye cannot see:
It is not just the visible light that can damage the eye's retina, but also ultraviolet and infrared and these wavelengths are a problem because their invisibility will not trigger the normal, protective pupilary response. I have no easy way to measure the attenuation of ultraviolet of these glasses, but the complete lack of blue - and the fact that many plastics do a pretty good job of blocking UV - I wasn't particularly worried about it. If one was worried, ordinary glasses or a piece of polycarbonate plastic would likely block much of the UV that managed to get through.
Infrared is another concern - and the sun puts out a lot of it! What's more is that many plastics - even strongly tinted - will transmit near infrared quite easily even though they may block visible light. An example of this are "theater gels" that are used to color stage lighting: These gels can have a deep hue, but most are nearly transparent to infrared - and this also helps prevent them from instantly bursting into flame when placed in front of hot lights.
Because of this I decided to include near-infrared in my measurements. In addition to my Sigma SD-1, I also have an older SD-14 and a property of both of these cameras is that they have easily-removable "hot mirrors" which double as dust protectors. What this means is that in a matter of seconds, one can adapt the camera to "see" infrared. Using my SD-14 (that camera is mostly retired, and I didn't want to get dust on the SD-1's sensor) I repeated the same test with the hot mirror removed as can be seen in Figure 6.
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| Figure 6: The Baader filter on the left and the glasses on the right showing the relative brightness when photographed in visible light + near infrared. This camera, a Sigma SD-14, was set to ISO 100 at F25 and 1/400th of a second using the same 200mm lens as Figure 5. Click on the image for a larger version. |
According to published specifications (see this link) the response of the red channel of the Foveon sensor is fairly flat from about 575 to 775 nanometers and useful out a bit past 900 nanometers while the other channels - particularly the blue - have a bit of overlapping response while the hot mirror itself very strongly attenuates wavelengths longer than 675 nanometers. What this means is that by analyzing the pictures in Figure 5, we can get an idea as to how much infrared the respective filters pass by noting the 8-bit converted RGB levels:
For the Baader filter:
- Red = 111
- Green = 0
- Blue = 62
- Red = 224
- Green = 0
- Blue = 84
(Follow this link for a comparison of the transmission spectra of common filter media and follow this link for a discussion about the Baader filter in particular.)
What is clear is that the glasses let in a significant amount more infrared than the Baader filter within the response curve of the sensor - but by how much?
The data indicates that the pixel brightness of the "Red+IR" channel of the glasses is twice that of that of the Baader filter, but if one accounts for the gamma correction applied to photographic images (read about that here - link) - and presume this gamma value to be 2 - we can determine that the actual differences between the two is closer to 4:1.
What does all of this mean?
In terms of visible light, these particular "fake" glasses appear to transmit about the same amount of visible light as the known-safe Baader filter - although the glasses aren't offering true color rendition, putting a distinct red-orange cast on the solar disk. In the infrared range - likely between 675 and 950nM - the glasses seem to permit about 4 times the light of the Baader filter.
At this point is is worth reminding the reader that this Baader filter is considered to be "safe" when placed over a telescope - in this case, my 8" telescope, as the various glass/plastic lenses along the optical path (e.g. corrector lens, eyepiece, etc.) will adequately block any stray UV. What this means is that despite the tremendous light-gathering advantage of this telescope over the naked eye, the Baader filter still has a generous safety margin. (It should be noted that this Baader film is not advertised to be "safe for direct viewing". Their direct-viewing film has a stronger blue/UV and IR blocking.)
What may be inferred from this is that, based solely on the measurements that obtained with these glasses it would seem that they may let in about 4 times the amount of infrared (e.g. >675nm) light as the Baader filter.
Again, I did not have the facility to determine if these glasses adequately block UVA/B radiation - but the combination of these glasses and good-quality sunglasses will block UV A/B - and provide additional light reduction overall.
Will I use them?
Based on my testing, these particular glasses seem to be reasonably safe in most of the way that matter, but whatever "direct viewing" method that I choose (e.g. these glasses or other alternatives) I will be conservative: Taking only occasional glances.
(I will acquire some "bona-fide" glasses and analyze them when I get a chance.)
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Once again:WARNING - PLEASE READ!
What preceded was my own, personal analysis of potentially defective products that, even when used properly, may result in permanent eye damage. This analysis was done using equipment at hand and should not considered to be scientifically rigorous or precise.
DO NOT take what follows as a recommendation - or even an inference - that the glasses that I tested are safe, or that if you have similar-looking glasses, that they, too, are safe to use!
YOU HAVE BEEN WARNED!
After the eclipse:
As it turns out, I did use these glasses - at least part time. There turned out to be an excess of eclipse glasses on-hand, many of them being verifiably "genuine". With these on-hand I'll do an update and make a comparison between the two.
[End]
This page stolen from "ka7oei.blogspot.com".
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