Question about laser rangefinders

[pmulcahy11b]

I'm watching a documentary-type show about the Battle of Fallujah. A tanker made a remark, "Sometimes the laser rangefinder wouldn't even acknowledge the target; it was too close."

What is the minimum range of a laser rangefinder? Is it just the M-1 series or do other vehicles have this problem?

[bobcat]

yes laser ranger finders have a minimum range. what this range is will vary based on the specific type of range finder it is. (oddly i don't recall what it is for most of the ones i used in the military.)

[leonpoi]

I can't find the source but from memory minimum range for M1a1 RF is 200m and something like a T72 uses is 500m. This I recall I read in some arms magazine years ago.

As another source ARMA3 has the following:

If the target is within the rangefinder limits, but beyond the maximum range of currently selected weapon, the display will flash. The weapon will be automatically set for its maximum range.
Minimum ranges:
T-90, T-72BA, BMP-3, M1A1, M1A2 = 200 m
T-72A, T-72B = 400 m

[Apache6]

Laser Range finders have a safety feature that prevent them from firing at short range. Intent is to keep crews from accidently blinding themselves or nearby friendlies.

[DigTw0Grav3s]

Possibly a stupid question, but how do rangefinders know the range they're firing at.. until they fire?

[StainlessSteelCynic]

Short answer - they don't. The whole idea of a rangefinder is to discover the range, it doesn't need to know the range before it fires.

Many early laser rangefinders fired a beam at the target and by measuring the flight time for the beam to hit the target and then be reflected back to the rangefinder's sensors, calculated the distance from that info. This is the Time Of Flight method, i.e. the amount of time a light beam will take to travel a certain distance (given that the speed of light is a known value).

Another method more commonly used these days is Multiple Frequency Phase Shift where a beam of known intensity/wavelength is measured upon it's return from the target so as to compare the difference in power modulation of the fired beam and the returned beam.
Or for a very crude explanation -- measuring the amount of energy/wavelength left in the beam when it returns compared to the amount of energy/wavelength used to fire it.

A third method is based on Interferometry where the laser beam is split into two distinct beams (that then share exactly the same wavelength/waveform) which are fired at the target but travel two slightly different paths to the target. Upon their return the two beams are recombined into a single beam before striking a detector. The difference between the two wavelengths, i.e. the interference, is noted and this is used to calculate the distance travelled.
Again, for a very crude explanation, an Interferometer laser rangefinder sort of combines the time of flight and phase shift principles in one, to calculate distance.

[DigTw0Grav3s]

Interesting. So how does a laser know not to fire at close range for eye-safe purposes?

[StainlessSteelCynic]

Originally Posted by DigTw0Grav3s

Interesting. So how does a laser know not to fire at close range for eye-safe purposes?

I don't actually know
Most manufacturers don't discuss the intricacies of their tech and a minimum safe distance limit was not something I ever encountered in some of the overlapping optical tech that I worked with
I have some ideas but they're educated guesses at best and "wild, shots in the dark" at worst
Note: LRF = Laser Range Finder

My first thought was that the focusing element might be set to disallow any action if the focal range is less than the minimum desired distance (with a lockout that prevents the laser from firing if the focal element is adjusted to equal/less than the minimum distance).
Keep in mind that the LRF is going to have some sort of integrated binocular or telescopic device so the operator can see the target (to allow them to line the LRF onto the target) so I'm thinking that at it's most basic, if the focus of the telescope is less than the minimum distance, then the LRF will not fire.

I suppose they could use a lower power beam to check target distance is greater than minimum safe distance before full power is applied but that just seems like "double-handling" and more complicated than necessary.
It's just as possible that they could fire a radio wave or IR beam first to check minimum distance before the main laser is engaged but again that seems like double-handling and extra complication (and just adds more stuff to break in the rangefinder!)
Personally I'm inclined to think they use the focusing system from the operator's optics.

[unkated]

I don't know but...

It is speed of light. Damage (to eyesight) is done over time - even if a short amount of time. If the response time is faster than the minimum range, it may be set to shutdown ASAP before calculating range (especially early models):

If time < .00002,
Then shutdown until new target
Else calculate range