RE: filmscanners: Re: looking at the Sun
Shough, Dean wrote:
From a prctical point of view, I seriously doubt that glass absorbs a
heck
of lot of UV, certainly not over a long term.
Correct. Most glass readily transmits near UV quite well. For example,
a
very common optical glass, BK7 transmits 80% of light at 340 nm and 5%
at
300 nm. This is typical for many glasses. It is mainly the heavy
flints
and rare earth glasses that tend to absorb the near UV.
Hi Dean,
I realize that your reference to optical glass is due to the discussion
of camera lenses and the risk of looking through the viewfinder at the
sun, but since this seems to be an area you have explored in some depth,
would you know how regular window glass or picture frame glass do in the
UV screening process? Also, I imagine certain wavelengths of UV are
more damaging to things like ink dyes than others, so does the
"bandwidth" of wavelengths the glass screens make a big difference in
things like inkjet prints fading?
Art
Actually, I wrote about BK7 glass because I had the information readily
available. All glasses strongly absorb UV radiation. Ordinary crown
glasses, e.g. BK7, pass near UV. The flints and especially the heavy flints
do not pass the near UV and may even appear slightly yellowish.
Ordinary window glass is similar to BK7 in composition and properties with
the addition of contaminants due to the inferior manufacturing process.
These contaminants induce scattering centers that increase absorption in the
blue and red, giving window glass its characteristic green cast. I have no
information about what happens to the near UV absorption. Higher quality
glass ("white window glass") is processed without the contaminants and
should have optical properties similar to BK7.
Re: filmscanners: Re: looking at the Sun
All glasses strongly absorb UV radiation Oh good. That's what I was trying to tell people. Thanks. We could also mention the effect of path length, i.e., a window pane vs a 14-element lens. Alan T - Original Message - From: Shough, Dean [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Tuesday, February 06, 2001 1:59 PM Subject: RE: filmscanners: Re: looking at the Sun
RE: filmscanners: Re: looking at the Sun
All of which doesn't change the fact that the level of UV absorption isn't
nearly enough to make it safe to look at the sun through the glass.
Hersch
At 05:59 AM 02/06/2001 -0800, you wrote:
Shough, Dean wrote:
From a prctical point of view, I seriously doubt that glass absorbs a
heck
of lot of UV, certainly not over a long term.
Correct. Most glass readily transmits near UV quite well. For example,
a
very common optical glass, BK7 transmits 80% of light at 340 nm and 5%
at
300 nm. This is typical for many glasses. It is mainly the heavy
flints
and rare earth glasses that tend to absorb the near UV.
Hi Dean,
I realize that your reference to optical glass is due to the discussion
of camera lenses and the risk of looking through the viewfinder at the
sun, but since this seems to be an area you have explored in some depth,
would you know how regular window glass or picture frame glass do in the
UV screening process? Also, I imagine certain wavelengths of UV are
more damaging to things like ink dyes than others, so does the
"bandwidth" of wavelengths the glass screens make a big difference in
things like inkjet prints fading?
Art
Actually, I wrote about BK7 glass because I had the information readily
available. All glasses strongly absorb UV radiation. Ordinary crown
glasses, e.g. BK7, pass near UV. The flints and especially the heavy flints
do not pass the near UV and may even appear slightly yellowish.
Ordinary window glass is similar to BK7 in composition and properties with
the addition of contaminants due to the inferior manufacturing process.
These contaminants induce scattering centers that increase absorption in the
blue and red, giving window glass its characteristic green cast. I have no
information about what happens to the near UV absorption. Higher quality
glass ("white window glass") is processed without the contaminants and
should have optical properties similar to BK7.
Re: filmscanners: Re: looking at the Sun
Alan Tyson wrote: All glasses strongly absorb UV radiation Oh good. That's what I was trying to tell people. Thanks. We could also mention the effect of path length, i.e., a window pane vs a 14-element lens. Alan T I'm convinced some camera lenses are made of 14 element window pane ;-) Art
RE: filmscanners: Re: looking at the Sun
All of which doesn't change the fact that the level of UV absorption isn't nearly enough to make it safe to look at the sun through the glass. Hersch Quite true. Uh, what was the original question? :-) Just kidding - time to let this topic die.
Re: filmscanners: Re: looking at the Sun
UV is dangerous through breaking chemical bonds directly; IR is dangerous through cooking (breaking chemical bonds by heating as in a grill or a toaster). The sun's radiant energy has lots and lots of both. Your retinal heat receptors (if any) won't be quick enough to prevent damage if you put a small solar image on your retina for long. If IR was nothing to worry about, fogged colour neg film would be fine for eclipse viewing, and it isn't, (see attached spectra from NASA's website). The 'greenhouse effect' with respect to the Earth is unfortunately misleading with respect to greenhouses. It's because the incident black body radiation from the sun is characteristic of intense 6500K and the reradiated energy is characteristic of weak 300-odd K. Not a lot of the sun's UV gets to the surface because it's absorbed in the ozone layer, but the energy does stay in the atmosphere and warm us up indirectly. The function of greenhouse glass and solar water heater coverings is to let IR in and keep draughts out. This is another divergent OT discussion, so I'm sorry for prolonging it, but it is a safety issue. When I can find an absorption spectrum of optical glass I'll send it you privately. Your greenhouse protects you surprisingly well from UV in temperate latitudes, as I recall (but not well enough for hanging up pictures of the plants). Regards, Alan T - Original Message - From: Frank Paris [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Sunday, February 04, 2001 10:13 PM Subject: RE: filmscanners: Re: looking at the Sun Infra-red is on the other end of the light spectrum and is of very low energy per photon compared even to light. It is manifested to us as heat. How is this dangerous? negabs01.gif
RE: filmscanners: Re: looking at the Sun
Infra-red is on the other end of the light spectrum and is of very low energy per photon compared even to light. It is manifested to us as heat. How is this dangerous? Have you ever taken a magnifying glass and used it to burn a leaf? Replace the magnifying glass with the lens in your eye and the leaf with your retina. If the light does not appear bright because the visible portion has been filtered out, then you will not know until too late that anything is happening.
RE: filmscanners: Re: looking at the Sun
From a prctical point of view, I seriously doubt that glass absorbs a heck of lot of UV, certainly not over a long term. Correct. Most glass readily transmits near UV quite well. For example, a very common optical glass, BK7 transmits 80% of light at 340 nm and 5% at 300 nm. This is typical for many glasses. It is mainly the heavy flints and rare earth glasses that tend to absorb the near UV.
RE: filmscanners: Re: looking at the Sun
I concur with you Hersch but would add that the danger is not from the brightness of the light but from the ultraviolet light rays that the sun emits and which are not screened out all that much by one-way mirrors and pentaprisms. -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Hersch Nitikman Sent: Saturday, February 03, 2001 11:17 PM To: [EMAIL PROTECTED] Subject: filmscanners: Re: looking at the Sun I'm sure others will chime in on this one, but I can't let that advice go unanswered. Just because the image in an SLR viewfinder is replected up through a pentaprism and a ground glass screen is no reason for complaisance about looking at the sun with such a camera. The efforts to make the screen view as bright as possible makes the light level in the eyepiece just about as dangerous as looking at the sun directly. True, there is some reduction, but in many cases, if not most, it is still bright enough to blind in a short time. Don't do it! Of course, a sunset may have the light attenuated enough by the atmosphere to make it safe. But, if it is uncomfortable to look with the unaided eye, don't gamble on looking through the viewfinder of an SLR. At 01:32 PM 02/03/2001 +, you wrote: In [EMAIL PROTECTED], Stuart wrote: But,of course ,no-one would do so while looking through the viewfinder as this would be extremely detrimental to ones eyesight and if the shutter was released would it not burn the blind ?? I don't think this is true of SLR's, as the image is formed on the ground glass screen and then the eye at the viewfinder looks at that image rather than the sun itself. In a viewfinder camera this might be different, as there is no ground glass screen; you look straight through the viewfinder lens(es). Also the mirror in such a camera covers the shutter blind until the last second, after which the blind moves very fast, I doubt if it would be focused on the blind of film for long enough to have any effect. Brian Rumary, England http://freespace.virgin.net/brian.rumary/homepage.htm
Re: filmscanners: Re: looking at the Sun
on 2/3/01 11:50 PM, Laurie Solomon at [EMAIL PROTECTED] wrote: I concur with you Hersch but would add that the danger is not from the brightness of the light but from the ultraviolet light rays that the sun emits and which are not screened out all that much by one-way mirrors and pentaprisms. -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Hersch Nitikman Sent: Saturday, February 03, 2001 11:17 PM To: [EMAIL PROTECTED] Subject: filmscanners: Re: looking at the Sun I'm sure others will chime in on this one, but I can't let that advice go unanswered. Just because the image in an SLR viewfinder is replected up through a pentaprism and a ground glass screen is no reason for complaisance about looking at the sun with such a camera. The efforts to make the screen view as bright as possible makes the light level in the eyepiece just about as dangerous as looking at the sun directly. True, there is some reduction, but in many cases, if not most, it is still bright enough to blind in a short time. Don't do it! Of course, a sunset may have the light attenuated enough by the atmosphere to make it safe. But, if it is uncomfortable to look with the unaided eye, don't gamble on looking through the viewfinder of an SLR. At 01:32 PM 02/03/2001 +, you wrote: In [EMAIL PROTECTED], Stuart wrote: But,of course ,no-one would do so while looking through the viewfinder as this would be extremely detrimental to ones eyesight and if the shutter was released would it not burn the blind ?? I don't think this is true of SLR's, as the image is formed on the ground glass screen and then the eye at the viewfinder looks at that image rather than the sun itself. In a viewfinder camera this might be different, as there is no ground glass screen; you look straight through the viewfinder lens(es). Also the mirror in such a camera covers the shutter blind until the last second, after which the blind moves very fast, I doubt if it would be focused on the blind of film for long enough to have any effect. Brian Rumary, England http://freespace.virgin.net/brian.rumary/homepage.htm Although some would be filtered by UV filter on the lens -Berry
Re: filmscanners: Re: looking at the Sun
At 21:16 03-02-01 -0800, you wrote: I'm sure others will chime in on this one, but I can't let that advice go unanswered. Just because the image in an SLR viewfinder is replected up through a pentaprism and a ground glass screen is no reason for complaisance about looking at the sun with such a camera. The efforts to make the screen view as bright as possible makes the light level in the eyepiece just about as dangerous as looking at the sun directly. True, there is some reduction, but in many cases, if not most, it is still bright enough to blind in a short time. Don't do it! Of course, a sunset may have the light attenuated enough by the atmosphere to make it safe. But, if it is uncomfortable to look with the unaided eye, don't gamble on looking through the viewfinder of an SLR. Thanks Herschfor supporting my views on this -There are enough warning given out at Eclipse times for folks to realise the dangers of looking at the sun -we all know how it hurts just glancing at the sun with the naked eye stuart
RE: filmscanners: Re: looking at the Sun
True, but only a very small amount. UV filters filter only enough to keep the image from displaying haze and color shift effects but not enough to protect eyes from the harmful effects over any prolonged period of time like more than 5 minutes duration or great accumulations of short duration periods ( Here I am speculating since I do not know if the harmful effects are cumulative). -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Berry Ives Sent: Sunday, February 04, 2001 9:22 AM To: [EMAIL PROTECTED] Subject: Re: filmscanners: Re: looking at the Sun on 2/3/01 11:50 PM, Laurie Solomon at [EMAIL PROTECTED] wrote: I concur with you Hersch but would add that the danger is not from the brightness of the light but from the ultraviolet light rays that the sun emits and which are not screened out all that much by one-way mirrors and pentaprisms. ...Snip Although some would be filtered by UV filter on the lens -Berry
Re: filmscanners: Re: looking at the Sun
Infrared is also a serious, if not worse, hazard. Glass is fairly transparent to it, as shown by greenhouses, passive solar panels, the burning of holes with magnifying glasses, and the feasibility of IR photography with ordinary lenses. Most glasses absorb UV much more strongly than IR. Most of the materials used for sun viewing and photography (eclipse goggles) have a (log10) density of 5-8 for UV and visible, and less than 5 for IR. The worst of the lot is fogged colour negative film, which is fine in the UV visible, but lethal to eyes because it's transparent to IR. NASA's web site has lots on this under solar eclipses. Regards, Alan T - Original Message - From: Laurie Solomon [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Sunday, February 04, 2001 5:13 PM Subject: RE: filmscanners: Re: looking at the Sun True, but only a very small amount. UV filters filter only enough to keep the image from displaying haze and color shift effects but not enough to protect eyes from the harmful effects over any prolonged period of time like more than 5 minutes duration or great accumulations of short duration periods ( Here I am speculating since I do not know if the harmful effects are cumulative).
RE: filmscanners: Re: looking at the Sun
Infra-red is on the other end of the light spectrum and is of very low energy per photon compared even to light. It is manifested to us as heat. How is this dangerous? It is only when you get into microwaves on the wide end of the spectrum that electromagnetic radiation becomes dangerous. Since infra-red is manifested as heat, our heat receptors will protect us long before any damage is done, unless of course we are caught in a fire. The reason a greenhouse builds up heat is that the light coming in is absorbed by surfaces and then re-radiated as infrared, which can't get out. It is not dangerous to spend all day in a greenhouse, tending your plants. Frank Paris [EMAIL PROTECTED] http://albums.photopoint.com/j/AlbumList?u=62684 -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Alan Tyson Sent: Sunday, February 04, 2001 11:53 AM To: [EMAIL PROTECTED] Subject: Re: filmscanners: Re: looking at the Sun Infrared is also a serious, if not worse, hazard. Glass is fairly transparent to it, as shown by greenhouses, passive solar panels, the burning of holes with magnifying glasses, and the feasibility of IR photography with ordinary lenses. Most glasses absorb UV much more strongly than IR. Most of the materials used for sun viewing and photography (eclipse goggles) have a (log10) density of 5-8 for UV and visible, and less than 5 for IR. The worst of the lot is fogged colour negative film, which is fine in the UV visible, but lethal to eyes because it's transparent to IR. NASA's web site has lots on this under solar eclipses. Regards, Alan T
Re: filmscanners: Re: looking at the Sun
From a prctical point of view, I seriously doubt that glass absorbs a heck of lot of UV, certainly not over a long term. Case in point: my house is on a hill, facing southeast. The roof overhangs 3 feet on all sides and the windows facing over the valley are all floor to ceiling glass (they're standard sliding glass door sections, 6'8" high by 4' wide -- or about 3 meters high and about 1 1/3 meters wide) fixed in place. The interior roof (all recycled 90 year old redwood fir) slants upwards from there to the peak of the house at about 15 feet. These glass panels are all double paned (the inner one made of tempered glass as in autos) with a dead air space between for insulation purposes. Our carpeting is royal blue and was first installed in 1975. We replaced it in 1995 but the closer you got to the windows, the lighter colored it became until the last few feet of carpet (a meter maybe) were bleached absolutely white by the UV in the sunlight, which had to pass through two sheets of glass to do the bleaching. These windows have now been replaced by sealed glass panels of the same contruction, including the dead air, excpt that the inner tempered glass panel now consists of two tempered glass panels with a special absolutely clear anti-UV filtering layer between them. Also, all framed photos, art prints, etc. in the areas where these windows are use special anti-UV glass which also has a UV filter layer on its back side. None of the images are within range of direct sunlight nor are any of the books on the bookshelves. Nonetheless, the spine portion of all of those books with so much of a trace of red in them are bleached out. So I doubt the the ability of glass to absorb UV in any serious way! Hart Corbett -- From: "Alan Tyson" [EMAIL PROTECTED] To: [EMAIL PROTECTED] Subject: Re: filmscanners: Re: looking at the Sun Date: Sun, Feb 4, 2001, 11:52 AM (snip) Most glasses absorb UV much more strongly than IR. Most of the materials used for sun viewing and photography (eclipse goggles) have a (log10) density of 5-8 for UV and visible, and less than 5 for IR. (snip Alan T
filmscanners: Re: looking at the Sun
I'm sure others will chime in on this one, but I can't let that advice go unanswered. Just because the image in an SLR viewfinder is replected up through a pentaprism and a ground glass screen is no reason for complaisance about looking at the sun with such a camera. The efforts to make the screen view as bright as possible makes the light level in the eyepiece just about as dangerous as looking at the sun directly. True, there is some reduction, but in many cases, if not most, it is still bright enough to blind in a short time. Don't do it! Of course, a sunset may have the light attenuated enough by the atmosphere to make it safe. But, if it is uncomfortable to look with the unaided eye, don't gamble on looking through the viewfinder of an SLR. At 01:32 PM 02/03/2001 +, you wrote: In [EMAIL PROTECTED], Stuart wrote: But,of course ,no-one would do so while looking through the viewfinder as this would be extremely detrimental to ones eyesight and if the shutter was released would it not burn the blind ?? I don't think this is true of SLR's, as the image is formed on the ground glass screen and then the eye at the viewfinder looks at that image rather than the sun itself. In a viewfinder camera this might be different, as there is no ground glass screen; you look straight through the viewfinder lens(es). Also the mirror in such a camera covers the shutter blind until the last second, after which the blind moves very fast, I doubt if it would be focused on the blind of film for long enough to have any effect. Brian Rumary, England http://freespace.virgin.net/brian.rumary/homepage.htm
RE: filmscanners: Re: looking at the Sun
The shots I mentioned where I do do this are always extreme wide angle which is no worse than looking up in the sky with the sun at the extreme periphery of our vision. Still, extreme care should be exercised, as you say. Usually what I do is compose with the sun just out of reach then shift slightly without looking in and hope for the best. Frank Paris [EMAIL PROTECTED] http://albums.photopoint.com/j/AlbumList?u=62684 -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Hersch Nitikman Sent: Saturday, February 03, 2001 9:17 PM To: [EMAIL PROTECTED] Subject: filmscanners: Re: looking at the Sun I'm sure others will chime in on this one, but I can't let that advice go unanswered. Just because the image in an SLR viewfinder is replected up through a pentaprism and a ground glass screen is no reason for complaisance about looking at the sun with such a camera. The efforts to make the screen view as bright as possible makes the light level in the eyepiece just about as dangerous as looking at the sun directly. True, there is some reduction, but in many cases, if not most, it is still bright enough to blind in a short time. Don't do it! Of course, a sunset may have the light attenuated enough by the atmosphere to make it safe. But, if it is uncomfortable to look with the unaided eye, don't gamble on looking through the viewfinder of an SLR. At 01:32 PM 02/03/2001 +, you wrote: In [EMAIL PROTECTED], Stuart wrote: But,of course ,no-one would do so while looking through the viewfinder as this would be extremely detrimental to ones eyesight and if the shutter was released would it not burn the blind ?? I don't think this is true of SLR's, as the image is formed on the ground glass screen and then the eye at the viewfinder looks at that image rather than the sun itself. In a viewfinder camera this might be different, as there is no ground glass screen; you look straight through the viewfinder lens(es). Also the mirror in such a camera covers the shutter blind until the last second, after which the blind moves very fast, I doubt if it would be focused on the blind of film for long enough to have any effect. Brian Rumary, England http://freespace.virgin.net/brian.rumary/homepage.htm
