In recent years small pulse semiconductor laser rangefinder only developed rapidly, in traffic, measurement, entertainment and other aspects of use more and more widely. Compared with other types of laser rangefinders, semiconductor laser rangefinders only have simple structure, good reliability and high pulse repetition rate.
Since most small laser ranging only uses the wavelength of 905nm semiconductor laser, although the output energy is small compared to other lasers, the laser beam is still harmful to the human eye.
The human eye has different transmittance and absorption characteristics for different wavelengths of light radiation, and Figure1（1）shows its spectral transmittance and absorption curves. It can be seen that in the 0.4-1.4 μm band, the transmission rate of crystals and glass is high, and the bands on either side of it can rarely be transmitted. The optical medium of the eye has a strong focusing effect, converging the incident light beam into a very small spot, thus making the light energy received by the retina per unit area 105 times higher than that of a person shooting into the cornea.
A laser ranging method that is safe for the human eye is to move the wavelength of the laser to a wavelength band that is safe for the human eye, mainly mainly near the infrared 1.5 μm, which has the highest safety for the human eye, but whether it is using a Larmann frequency shift, an erbium laser, a 1.5 μm semiconductor laser or a semiconductor-pumped laser, etc., it is more complex and expensive and not suitable for small handheld laser rangefinders.
ERDI designed the laser rangefinder by controlling the laser output parameters of the rangefinder, making it safe for the human eye while improving the rangefinding capability and measurement accuracy.
2.Distance measuring equation of the rangefinder（3）
Laser ranging is a typical photonic radar system. Based on the optical time-domain reflection technique, the ranging equation takes into account the atmospheric attenuation, the transmittance of the optical system and the reflective properties of the target as follows
where NE ,NR are the number of photons emitted and received by each laser pulse of LD in the laser rangefinder;ττ，τRare the transmittance of the transmitting and receiving objectives of the laser rangefinder; μ, p are the attenuation coefficient of the atmosphere and the target reflectance; D, L are the aperture of the objective lens and the target distance, respectively.
It is derived from the ranging equation that the number of photons emitted by a semiconductor laser can be increased in the process of designing new rangefinders to improve the measurement distance and measurement accuracy, but at the same time the laser energy is increasing the hazard to the human eye. For most applications of small rangefinders, their safety is of concern.
3 Laser safety standards for pulsed semiconductors（4）
According to the national standard GB7247-1995 “radiation safety of laser products, equipment classification, requirements and user’s guide”, the classification of laser hazards for four categories, the higher the level, the greater the radiation hazards of laser, its safest standard for Class I lasers – inherent safety, that is, in any case will not exceed the maximum allowable exposure (AEL) or engineering design to ensure safety. In the 700-1050nm wavelength range there are two groups of AEL (radiation power or radiation energy) boundaries, at least one of them, can be classified as inherently safe, the following table.
Table 1 Ⅰ number of light products can reach the emission limit
|1011C4 .m-2 .ST-1||3.9×104τ0.75C4J.m-2. βT-1|
where C4＝10(λ-700)/500 is the wavelength correction factor. (1)
The laser light emitted by the semiconductor laser is focused and emitted through the optics of the rangefinder, so the laser energy reaching the human eye is also related to the optics of the rangefinder. We measure safety in terms of maximum permissible exposure (MPE) with values based on available experimental studies below known hazard levels, as shown in Table 2
Table 2 Maximum permissible exposure (MPE) to direct primary laser radiation to the eye
|Launch time||<10-9ns||<10-9 ~<10-7ns||10~103ns|
For the pulsed rangefinder we designed, the light source is a pulsed semiconductor laser, and the most restrictive requirements are utilized when determining the maximum allowable exposure (MPE) for repeated exposures, as follows.
a. The irradiation of any single pulse in the pulse column should not exceed the MPE of a single pulse.
b. The average irradiance of a pulse column of duration t should not exceed the MPE of a single pulse with pulse width t of Table 1.
c. The irradiation of any single pulse in the pulse column should not exceed the MPE of a single pulse and the product of the negative 1/4 power of the total number of pulses N expected during the irradiation time.
MPEL——The amount of exposure to any single pulse in the pulse train.
MPED——MPA of single pulse;
N—— the total number of pulses expected during the irradiation time.
The corresponding another practical parameter is the nominal eye hazard distance (NOHD), i.e., the distance whose irradiance or radiation is under the corresponding MPE under ideal conditions, expressed as r (ignoring atmospheric attenuation, optical system attenuation, etc.).
P0——is the average radiated power of the pulsed laser (W).
a——is the diameter of the emitted laser beam (m).
r——Distance from the laser to the observer (m).
Φ——Emission beam divergence (rad).
This assumes that the laser is a Gaussian beam, and for laser systems with multimode laser beams P0 is increased by a factor of 2.5, thus.
To make the rangefinder safe, the nominal eye hazard distance should be zero, then.
4. Limit launch parameters calculation
The small handheld semiconductor laser rangefinder uses a 905nm infrared laser, which is not visible. Therefore, the target is aimed at ranging with an auxiliary observation aiming system, there are two main ways: telescope aiming and visible laser indication aiming.
4.1 Telescopic sighting system
Telescope aiming is to determine the position of the target through the dividing plate in the telescope, and its observation distance is long. The maximum allowable exposure at this point is the maximum allowable exposure of the semiconductor laser through the rangefinder optical system.
In the designed rangefinder, the semiconductor laser is collimated through the transmitting objective lens, and the diameter of the laser transmitting objective lens is 25 mm, while the maximum diameter of the human pupil is about 7 mm. Because the measurement speed of the rangefinder is very fast, 10 s is set as the maximum measurement time, ignoring the divergence of the beam, the attenuation of the optical system and the air, etc.
Semiconductor laser selected EG & G’s PGEW, whose wavelength is 905nm, pulse power is 20W, and pulse width is 10ns. Because small ranging only utilizes techniques such as multi-pulse repetitive measurement, the laser emitted is a repetitive pulse, so the first thing is to estimate the single pulse radiation entering the eye, followed by the superposition and averaging effect of multi-pulse irradiation.
The specification of the laser from Table 2 is 700nm < λ < 1050nm, its pulse width is 10ns, the single pulse MPE of this radiation is 5×10-3C4J.m-2, at this time C4＝10(905-700)/500＝2.57. The single pulse MPE of this radiation is.
Again, since it is a multi-pulse laser beam, the corresponding MPE has to be reduced. Assuming that the highest repetition frequency is f and the total number of pulses is t × ʄ, the correction factor is （10×ʄ）–1/4” and the reduced single-pulse MPE is
When NOHD is zero, equation (4) reads
This gives ʄ = 2.7kHz
Here it is concluded that irradiation in any range of the laser beam at 2.7 kHz is safe on the basis of the single pulse broadening.
Then the estimation of the average irradiance is carried out, and the radius of action distance if the maximum irradiance produced by each single pulse at the zero point is E。Then the average irradiance of the pulse train is
At the maximum repetition frequency of 2.7 kHz calculated above with a duration of 10 s, in this case.
According to Table 2, it is safe to observe laser radiation emitted at this frequency with the naked eye during the ranging only typical measurement time.
4.2 Laser indication targeting system
Visible laser indication generally uses 635nm or 650nm semiconductor laser, this method is mainly used in short distance or indoor and other dark environment.
At this time to calculate the maximum allowable operating frequency to consider the indication laser continuous laser radiation.
As seen from the above analysis, in a small LD pulsed laser rangefinder, if the system uses a non-emitting objective lens diameter of 25 mm, a semiconductor laser of EC&G PGEW, a power of 20 W and a pulse width of 10 ns, the maximum laser emission frequency is 2.7 kHz. This means that the rangefinder is safe over the entire measurement range.
In practice, the design has to be fine-tuned according to the optical system attenuation and other factors. If the safety of observation with a telescope system is then considered, further corrections can be made.
With this data makes it possible to develop high-precision or other types of semiconductor laser rangefinders with a good guarantee of safety for the human eye, thus expanding their range of use and creating conditions for such laser rangefinders to be used in everyday life.