【单选题】
Laser Pointers
A、laser pointer is a small (often battery-operated) laser device designed for pointing at objects by illuminating them with a collimated visible laser beam. Most laser pointers, particularly the cheap ones, contain a small GalnP/AIGaInP laser diode operating somewhere in the red spectral region. Some significantly more expensive pointers emit green or even blue light and normally contain a small diode-pumped solid state laser with a nonlinear crystal for frequency doubling.
A、typical use of a hand-held laser pointer is to point at some screen or chart during a presentation, e.g. a conference talk. This is convenient because it can be done from a large distance and requires only a rather small device. However, the visibility of the generated spot on the screen is often poor (particularly for red laser pointers with relatively long emission wavelength), and a fast moving light spot can have a somewhat nervous appearance. Therefore, some people would prefer an old-fashioned telescopic pointing device for presentations. Laser pointers can be useful e. g. for aligning some machinery, or for certain optical distance measurements.
The apparent brightness of the illuminated spot strongly depends on the wavelength of the emitted light. Most devices operate in the red spectral region, where the sensitivity of the eye rapidly decreases with increasing wavelength.Devices with 650-nm output appear about twice as bright as those emitting the same power at 670 nm, and 635-nm devices still about two times brighter. However, the shorter-wavelength laser pointers are typically more expensive. This is particularly true for green lasers, which are quite significantly brighter than their red counterparts, but still cost quite a lot. They involve a diode-pumped solid state laser and a frequency doubler.Due to the typically poor conversion efficiency of the frequency doubler at low power levels, hundreds of milliwatts of infrared (typically 1064-nm) light are required to generate a few milliwatts in the green, and the batteries will accordingly not last very long, except if they are comparatively heavy.
Lay persons often ask what is the range of a laser pointer, and responding to this interest some producers specify some more or less questionable number.
If the question is meant to be how far the light of a laser pointer can propagate, the answer is of course that there is no limit, as long as the light is not absorbed or scattered away in the atmosphere.
Of course, much of that light can get from earth to the moon without problems.
However, the beam area will eventually become larger due to the beam divergence, so that the intensity e.g. on a screen will be reduced even if the overall power stays constant.
One may also understand the range of a laser pointer as the maximum distance over which one can send the beam to some screen while still seeing the illuminated spot from the distance. That kind of range is normally not limited by the beam divergence but by the overall optical power (apart from the wavelength and level of ambient light), since the issue is not the comparatively minor divergence on the way from the laser pointer to the screen, but rather the huge divergence of the scattered light on the way back. Therefore, someone standing next to the illuminated screen would easily see the spot when it is already hardly perceivable from the position of the laser pointer.
There have been extensive debates on laser safety issues associated with laser pointers. Typical output powers are a few milliwatts--normally below 5 mW in order to comply with safety cla
网考网参考答案：A
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A、laser pointer is a small (often battery-operated) laser device designed for pointing at objects by illuminating them with a collimated visible laser beam. Most laser pointers, particularly the cheap ones, contain a small GalnP/AIGaInP laser diode operating somewhere in the red spectral region. Some significantly more expensive pointers emit green or even blue light and normally contain a small diode-pumped solid state laser with a nonlinear crystal for frequency doubling.
A、typical use of a hand-held laser pointer is to point at some screen or chart during a presentation, e.g. a conference talk. This is convenient because it can be done from a large distance and requires only a rather small device. However, the visibility of the generated spot on the screen is often poor (particularly for red laser pointers with relatively long emission wavelength), and a fast moving light spot can have a somewhat nervous appearance. Therefore, some people would prefer an old-fashioned telescopic pointing device for presentations. Laser pointers can be useful e. g. for aligning some machinery, or for certain optical distance measurements.
The apparent brightness of the illuminated spot strongly depends on the wavelength of the emitted light. Most devices operate in the red spectral region, where the sensitivity of the eye rapidly decreases with increasing wavelength.Devices with 650-nm output appear about twice as bright as those emitting the same power at 670 nm, and 635-nm devices still about two times brighter. However, the shorter-wavelength laser pointers are typically more expensive. This is particularly true for green lasers, which are quite significantly brighter than their red counterparts, but still cost quite a lot. They involve a diode-pumped solid state laser and a frequency doubler.Due to the typically poor conversion efficiency of the frequency doubler at low power levels, hundreds of milliwatts of infrared (typically 1064-nm) light are required to generate a few milliwatts in the green, and the batteries will accordingly not last very long, except if they are comparatively heavy.
Lay persons often ask what is the range of a laser pointer, and responding to this interest some producers specify some more or less questionable number.
If the question is meant to be how far the light of a laser pointer can propagate, the answer is of course that there is no limit, as long as the light is not absorbed or scattered away in the atmosphere. Of course, much of that light can get from earth to the moon without problems. However, the beam area will eventually become larger due to the beam divergence, so that the intensity e.g. on a screen will be reduced even if the overall power stays constant. One may also understand the range of a laser pointer as the maximum distance over which one can send the beam to some screen while still seeing the illuminated spot from the distance. That kind of range is normally not limited by the beam divergence but by the overall optical power (apart from the wavelength and level of ambient light), since the issue is not the comparatively minor divergence on the way from the laser pointer to the screen, but rather the huge divergence of the scattered light on the way back. Therefore, someone standing next to the illuminated screen would easily see the spot when it is already hardly perceivable from the position of the laser pointer.
There have been extensive debates on laser safety issues associated with laser pointers. Typical output powers are a few milliwatts--normally below 5 mW in order to comply with safety cla
网考网参考答案：A
网考网解析：
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