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Your objection applied to the backscatter X-ray machines that are no longer in use. It doesn't apply to the millimeter wave machines, which do not expose passen

by wl 13y ago

Your objection applied to the backscatter X-ray machines that are no longer in use. It doesn't apply to the millimeter wave machines, which do not expose passengers to any ionizing radiation.

lukifer 13y ago
Do you have a link that describes the exact difference? I'm still skeptical/squicked about these machines by reflex, but I'm willing to re-examine the issue.
- SamReidHughes 13y ago
  The radiation frequency is different.
- cheald 13y ago
  Wikipedia's a good start: https://en.wikipedia.org/wiki/Millimeter_wave_scanner https://en.wikipedia.org/wiki/Millimeter_wave_scanner https://en.wikipedia.org/wiki/Electromagnetic_spectrum https://en.wikipedia.org/wiki/Electromagnetic_spectrum Basically, millimeter wave radiation is ~6 orders of magnitude less energetic than x-rays and is non-ionizing (that is, it doesn't have the energy to knock electrons out of atoms). Ionizing radiation is the nasty stuff, and is what causes DNA damage - non-ionizing is generally safe, and includes things like light and radio waves.
  - lukifer 13y ago
    I'll read up on the subject. Thanks for sharing!
- dalke 13y ago
  There are two types of millimeter scanners: passive and active. The first only uses background sources, which is possible since things emit millimeter wavelengths naturally. This can have no additional physical effect on people. However, like passive infrared imaging, these can have poor sensitivity and low contrast. This is why people prefer to have a millimeter light source. (You see the same with infrared imaging. Passive IR uses natural heat sources, while active IR uses an infrared illumination source. For that matter, you see the same with visible light: the flash on a camera is an illumination source.) Active millimeter scanners use a source of millimeter waves. It's easy to figure out how much energy is in a mm wavelength photon: E = h * c/λ where h is Plack's constant, c is the speed of light, and λ is the wavelength. That's https://www.google.com/search?q=planck's+constant+*+speed+of+light+%2F+(1+mm)+in+eV https://www.google.com/search?q=planck's+constant+*+speed+of... = 0.001 eV. By comparison, a photon needs over 3 eV to break a chemical bond, which requires a wavelength in the ultraviolet range or lower. (This is worked out http://www.kwantlen.ca/science/physics/faculty/mcoombes/P1220_Solutions/ModernPhysics/P1220_05_Solutions.htm http://www.kwantlen.ca/science/physics/faculty/mcoombes/P122... ). There's not enough energy in a mm wavelength photon to cause a direct chemical change. I say "direct" because there are indirect possibilities that aren't relevant to this discussion. For example, you could use infrared radiation to heat something up and burn it. This happens because the energy in the photon is absorbed by the target, which can't be cooled off fast enough, so it gets hot. Or you could use microwave radiation in a microwave oven to do the same, even though there too the photon alone isn't enough to affect the bonds. BTW, the term "radiation" is used for regular light, infrared, ultraviolet, radio waves, microwaves, and gamma rays. A camera's flash is, in physics terms, a radiation source. As the wavelengths get smaller (eg, UV light), there is enough energy in a single photon to affect a chemical bond and cause it to split. This is why UV radiation exposure is a possible source for cancer, and why UV-B's shorter wavelengths are more dangerous than UV-A. At even smaller wavelengths (eg, X-rays), then there's enough energy in a single photon to force an electron off of an atom. The atom then has a positive charge, so it's called an ion. Ionized molecules are very reactive, and more likely to lead to chemical damage. Radiation with this level of energy is called "ionizing radiation." The confusing part is that people commonly use the term "radiation" to mean specifically "ionizing radiation", even though it can apply to things like a heat source. (Ie, the home radiator is an IR radiation source.)

by wl 13y ago

Your objection applied to the backscatter X-ray machines that are no longer in use. It doesn't apply to the millimeter wave machines, which do not expose passengers to any ionizing radiation.

lukifer 13y ago

Do you have a link that describes the exact difference? I'm still skeptical/squicked about these machines by reflex, but I'm willing to re-examine the issue.

SamReidHughes 13y ago
The radiation frequency is different.
cheald 13y ago
Wikipedia's a good start: https://en.wikipedia.org/wiki/Millimeter_wave_scanner https://en.wikipedia.org/wiki/Millimeter_wave_scanner https://en.wikipedia.org/wiki/Electromagnetic_spectrum https://en.wikipedia.org/wiki/Electromagnetic_spectrum Basically, millimeter wave radiation is ~6 orders of magnitude less energetic than x-rays and is non-ionizing (that is, it doesn't have the energy to knock electrons out of atoms). Ionizing radiation is the nasty stuff, and is what causes DNA damage - non-ionizing is generally safe, and includes things like light and radio waves.
- lukifer 13y ago
  I'll read up on the subject. Thanks for sharing!
dalke 13y ago
There are two types of millimeter scanners: passive and active. The first only uses background sources, which is possible since things emit millimeter wavelengths naturally. This can have no additional physical effect on people. However, like passive infrared imaging, these can have poor sensitivity and low contrast. This is why people prefer to have a millimeter light source. (You see the same with infrared imaging. Passive IR uses natural heat sources, while active IR uses an infrared illumination source. For that matter, you see the same with visible light: the flash on a camera is an illumination source.) Active millimeter scanners use a source of millimeter waves. It's easy to figure out how much energy is in a mm wavelength photon: E = h * c/λ where h is Plack's constant, c is the speed of light, and λ is the wavelength. That's https://www.google.com/search?q=planck's+constant+*+speed+of+light+%2F+(1+mm)+in+eV https://www.google.com/search?q=planck's+constant+*+speed+of... = 0.001 eV. By comparison, a photon needs over 3 eV to break a chemical bond, which requires a wavelength in the ultraviolet range or lower. (This is worked out http://www.kwantlen.ca/science/physics/faculty/mcoombes/P1220_Solutions/ModernPhysics/P1220_05_Solutions.htm http://www.kwantlen.ca/science/physics/faculty/mcoombes/P122... ). There's not enough energy in a mm wavelength photon to cause a direct chemical change. I say "direct" because there are indirect possibilities that aren't relevant to this discussion. For example, you could use infrared radiation to heat something up and burn it. This happens because the energy in the photon is absorbed by the target, which can't be cooled off fast enough, so it gets hot. Or you could use microwave radiation in a microwave oven to do the same, even though there too the photon alone isn't enough to affect the bonds. BTW, the term "radiation" is used for regular light, infrared, ultraviolet, radio waves, microwaves, and gamma rays. A camera's flash is, in physics terms, a radiation source. As the wavelengths get smaller (eg, UV light), there is enough energy in a single photon to affect a chemical bond and cause it to split. This is why UV radiation exposure is a possible source for cancer, and why UV-B's shorter wavelengths are more dangerous than UV-A. At even smaller wavelengths (eg, X-rays), then there's enough energy in a single photon to force an electron off of an atom. The atom then has a positive charge, so it's called an ion. Ionized molecules are very reactive, and more likely to lead to chemical damage. Radiation with this level of energy is called "ionizing radiation." The confusing part is that people commonly use the term "radiation" to mean specifically "ionizing radiation", even though it can apply to things like a heat source. (Ie, the home radiator is an IR radiation source.)