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#131
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If our eyes were made out of paper, I might agree, but as Chris notes
they have substantial ability to remove heat. Have you ever boiled water in a paper cup? Ever heard of it? Could it be that's why a GLP held point-blank against one's cheek creates no perceptible increase in temperature? Nobody said they are harmless toys. Just that the alarmist viewpoints being adopted by a small minority are not based in fact. Tim Killian wrote: Paper ignites at a delivered energy density 10W/cm^2, so even you will agree that at 1W/cm^2, some eye damage is _possible_. A 5 mW pointer beam 1mm in diameter has an energy density of about .64 W/cm^2. As I said earlier, imperfect optics can easily increase the energy density of a pointer above 1W/cm^2 because they only have to shrink the beam by 25%! These are not harmless toys. |
#132
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Have you ever boiled water in a paper cup? Ever heard of it?
You can do that in a microwave oven pretty easily. Water boils well=20 below the flashpoint of paper. Can you do it over a flame? -Florian |
#133
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On Sun, 09 Jan 2005 20:38:44 GMT, "Florian"
wrote: Have you ever boiled water in a paper cup? Ever heard of it? You can do that in a microwave oven pretty easily. Water boils well below the flashpoint of paper. Can you do it over a flame? Yes. Fill a paper cup with water and put it on a gas stove. It will boil without burning the paper. The paper cup is thin enough that the outside temperature doesn't ever get much above the inside water temperature. _________________________________________________ Chris L Peterson Cloudbait Observatory http://www.cloudbait.com |
#134
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Chris L Peterson:
Regarding retinal damage I can offer some general remarks, however, scientists have given these matters an enormous amount of thought and scientific work. I would defer to their science. Casual remarks won't suffice in the realm of that topic I am sure that you will agree. Chris L Peterson wrote: [clip] You can have much higher energy densities at the retina if the spot size is small True. For example, the inverse square law will provide measurement information based on facts. because both the vitreous body and the retinal circulatory system efficiently carry heat away. The latter part of what you say doesn't follow logically from the first part of the sentence. There seems to be an error of logic there. You definitions need checking. The vitreous humour is in the lens. The aqueous humour is within the eye. The ability of any of the materials of the eye to carry heat away is determined by several factors. For example, the frequency of the light. Photons in the high frequency range, e.g., some X-rays or Gamma radiation may pass right through the materials of the eye without being absorbed. On the other hand, photons in the IR or radio frequencies may be absorbed and raise the temperature of the materials. Also, some visible frequencies will pass through the lens without being absorbed while some visible frequencies are absorbed by the aqueous humour causing chemical changes and darkening of the aqueous humour. The heat produced is a result of the frequency of the photons, and that is but one of the causes that are simultaneously operative. The materials of the eye, speaking of elements and molecules, absorb, reflect, or transmit various frequencies of photons according to their atomic properties. IR frequencies, for example, are readily absorbed by water raising the temperature of the water. Visible light striking the retina results in complex chemical changes, and there is not that much of a resulting increase of temperature. The photons go into combination with the rods and cones and electrons result. The eye is not really structure as an efficient heat dissappation device, and the nerve cells have specific capacities as electrochemical transmitters. The tolerance range of the eye to various photon energy levels is limited to very small amounts, and the eye is easily damaged. Thermal damage to the retina by a 5mW visible light laser is impossible, pure and simple. You have to specify the area that receives the light, the frequency, and the time duration of the light, as well as to specify the receiving biological materials and their sensitivities and abilities to accommodate the light. Human structures are finite in their abilities to absorb, transmit, reflect, or convert or re-radiate incoming frequencies. The rods and cones have specific limits in their ability to convert photons of certain frequencies into changed molecules and electrons. Beyond a certain point the body simply absorbs the photons and their energy, the cells die or become inoperative, and the body becomes a spent charcoal briquette. The rare (and largely anectodal) reports of injury from low power lasers are presumably associated with phototoxicity. [clip] I think that the reason you stated as "presumably associated" is an insufficient answer in the light of the enormous amount of science that has been generated on the numerous subclassifications of the subject. You may want to come up with a little more information on the matter. I would ask the question whether photons are toxic. Ralph Hertle |
#135
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On Sun, 09 Jan 2005 21:53:50 GMT, Ralph Hertle wrote:
because both the vitreous body and the retinal circulatory system efficiently carry heat away. The latter part of what you say doesn't follow logically from the first part of the sentence. There seems to be an error of logic there. You definitions need checking. The vitreous humour is in the lens. The aqueous humour is within the eye. The aqueous humor is in the anterior chamber of the eye- between the cornea and the lens, and is basically water with some electrolytes. The vitreous humor (also called the vitreous body) is in the anterior chamber of the eye, behind the lens and in front of the retina. It is in contact with the retina, and provides a primary mechanism for carrying heat away from it. The ability of any of the materials of the eye to carry heat away is determined by several factors. For example, the frequency of the light... Which is why I am careful to talk about radiation in the visible range. The eye is not really structure as an efficient heat dissappation device... Actually, it is quite good. Consider for example that brief exposure to the Sun is rarely harmful. Exposure to the Sun through a telescope, however, almost instantly destroys the retina. The energy density on the retina is the same in both cases, but the latter overwhelms the ability of the retinal blood supply and the vitreous body to remove heat. As long as the energy density is low, or the spot size is small, the eye will do a good job of protecting itself. Thermal damage to the retina by a 5mW visible light laser is impossible, pure and simple. You have to specify the area that receives the light, the frequency, and the time duration of the light, as well as to specify the receiving biological materials and their sensitivities and abilities to accommodate the light. You do not need to specify these things. 5mW of visible light, delivered to the retina through the cornea, will not cause thermal damage. This has been verified experimentally. Given a long enough exposure- minutes probably- there may be damage that is non-thermal, however. The rare (and largely anectodal) reports of injury from low power lasers are presumably associated with phototoxicity. I think that the reason you stated as "presumably associated" is an insufficient answer in the light of the enormous amount of science that has been generated on the numerous subclassifications of the subject. You may want to come up with a little more information on the matter. I would ask the question whether photons are toxic. Retinal exposure to high light intensities is damaging. This is a major concern when performing ophthalmic surgery, as the light source in the microscope can be bright enough to cause macular problems. Note that this is not a thermal effect. It is an area of active study, because phototoxicity is not well understood. Some people with certain macular problems can have an acute reaction to bright light. There are cases of partial vision loss following nothing more than routine examination with a slit lamp or ophthalmoscope. However, these conditions are rare, and often result in early vision loss just from normal exposure to daylight conditions over a period of years. _________________________________________________ Chris L Peterson Cloudbait Observatory http://www.cloudbait.com |
#136
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Florian wrote:
Have you ever boiled water in a paper cup? Ever heard of it? You can do that in a microwave oven pretty easily. Water boils well below the flashpoint of paper. Can you do it over a flame? -Florian Yes. Just ask any Boy Scout. Been there, done that, drank the coffee.... Dave N |
#137
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Yes. Just ask any Boy Scout. Been there, done that, drank the coffee
Did you use a microwave to boil the water |
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