prk@charm.UUCP (Paul Kolodner) (07/16/85)
Did someone on this net really say that cd-player optics don't need cleaning because the laser burns the dust off the optics? How can this be? To do that you need megaWatts of power, and you have to focus. The diddy little diode in there certainly won't do that. Also, the dust is on the optics, where the light is not focussed. Also, if there's enough power to burn dust off a lens, then that litte explosion leaves a defect on the glass surface with a lower breakdown threshold, which then gets damaged itself. Also, ....HELP!.....urp .... (plop)
frodo@wcom.UUCP (Jim Scardelis) (07/17/85)
> Did someone on this net really say that cd-player optics > don't need cleaning because the laser burns the dust > off the optics? How can this be? To do that you need > megaWatts of power, and you have to focus. The diddy little > diode in there certainly won't do that. Also, the dust > is on the optics, where the light is not focussed. Also, > if there's enough power to burn dust off a lens, then that > litte explosion leaves a defect on the glass surface with > a lower breakdown threshold, which then gets damaged itself. > Also, ....HELP!.....urp .... (plop) ...and imagine the effect on the lowly plastic surface of the CD itself as the now focused power of the laser sizzles it away.... -- Jim Scardelis uucp: {vax135|ihnp4}!timeinc!wcom!frodo ARPA: 1891@NJIT-EIES.MAILNET@MIT-MULTICS.ARPA "The opinions expressed herein are those of my computer, and not necessarily those of myself, Warner Computer Systems, or any other computer or company along the line. "
prk@charm.UUCP (Paul Kolodner) (07/20/85)
Private mail confirms that the original posting was indeed a joke. The laser diode does not clean its own optics, by a democratic vote of two of us. The laser only has a few mW of power, which is safe to look in the eye. Only on Star Trek are little lasers powerful enough to really blow things away. A good point: the plastic of the the cd is weaker than dust. Think about it.
karn@petrus.UUCP (Phil R. Karn) (07/21/85)
> The laser only has a few > mW of power, which is safe to look in the eye. Only on Star > Trek are little lasers powerful enough to really blow > things away. A correction on this point: milliwatt lasers are definitely NOT safe to look in the eye! A compact disc player is generally a Class 1 Laser Product. While this is low power as lasers go, the energy densities that can be achieved by focusing even a weak laser are spectacular. Since the laser operates in the infrared, its radiation is invisible and therefore even more hazardous. Fortunately, the depth of focus of a compact disk laser is extremely small (typically 4 micron -- this is how surface scratches are defocused), small in diameter (typically 1 square micron) and very close to the lens. In addition, the physical design of a typical player, along with various interlocks, makes it rather unlikely that any light will leak out of a typical player, so in practice there's nothing to worry about unless you disassemble your player, defeat the interlocks and hold your eyeball up to the lens. Phil
gvcormack@watdaisy.UUCP (Gordon V. Cormack) (07/21/85)
> a democratic vote of two of us. The laser only has a few > mW of power, which is safe to look in the eye. Only on Star I would not recommend looking ANY laser in the eye. While it obviously will not burn a hole in your hand, a small amount of laser light can still damage your retina. My CD player (a Yamaha CDX2) has dire warnings not to operate it without the cover. I intend to heed those warnings. -- Gordon V. Cormack CS Department, University of Waterloo gvcormack@watdaisy.uucp gvcormack%watdaisy@waterloo.csnet
ark@alice.UUCP (Andrew Koenig) (07/21/85)
> While this is low power as lasers go, the energy densities that can be > achieved by focusing even a weak laser are spectacular. Since the laser > operates in the infrared, its radiation is invisible and therefore even more > hazardous. When my CDP-101 is switched on, I can peer into the drawer and see red light. Are you sure that the laser operates in the infrared? (actually, I suppose the frequency is up to the manufacturer...)
karn@petrus.UUCP (Phil R. Karn) (07/22/85)
> When my CDP-101 is switched on, I can peer into the drawer > and see red light. Are you sure that the laser operates > in the infrared? (actually, I suppose the frequency is > up to the manufacturer...) Depending on which article you read, the wavelength of the laser in a CD player is 780 or 800 nm. This is dictated by the depth of the pits in the metal layer of the disc (.11-.13 micron) and the refractive index of the polycarbonate plastic (1.55). In the plastic, the wavelength becomes 500 nm, and the pit is one quarter this value. Light traveling down the pit is still reflected, but when it arrives back at the metal surface it has travelled an extra 1/2 wavelength and is therefore 180 degrees out of phase with the light being reflected from the area around the pit (the beam is larger than the pit.) The two reflections interfere with each other, so that less light is able to travel back up to the photodetector than when the beam hits an unpitted area. Technically, the visible spectrum ends at 750 nm, so I suppose it's possible for some people to see the CD laser as a deep red. I can't see any visible light out of either of my players, nor can I see the IR LEDs in my VCR remote control (although they probably operate at a different wavelength). Phil
karn@petrus.UUCP (Phil R. Karn) (07/22/85)
Just after posting my last note, I found the following in Reference Data for Radio Engineers (p 22-5, 1977 printing): "The lower limits of visual response at the extremities of both the photopic [cones] and scotopic [rods] are not necessarily restricted to the range of wavelengths normally defined as visible light. For example, many observers can see the 905 nm radiation from a GaAs injection laser or the 1.06 micron radiation from a Nd3:YAG laser if the intensity is sufficiently high. Fig. 6 is an extension of the absolute scotopic luminosity curve showing that a GaAs injection laser emitting at 905 nm will elicit a visual response in the mind of an observer whose eyes are sufficiently sensitive to respond to ~1e-6 lm/W. --> Since moderately high energy densities at the retina may be required to elicit visual response at near-infrared wavelengths, the safety factor must not be discounted." <-- [emphasis mine] The item was quoted from F. J. Gardner, "Effectiveness of IR Covert Illuminators," RCA Lasers, 1974, pp. 178-181. From the accompanying table, it looks like the rods have about 1.39e-7 of the efficiency at 780 nm that they do at the peak sensitivity of 510 nm (green). This is something near 70 dB down. Phil