# Laser Range Finder? **URL:** \ **Category:** Factual Questions\ **Created:** [December 4, 2025, 12:05am UTC](https://boards.straightdope.com/t/laser-range-finder/1025382 "2025-12-04T00:05:02Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![19USMC69](https://avatars.discourse-cdn.com/v4/letter/1/ac91a4/32.png) [@19USMC69](https://boards.straightdope.com/u/19USMC69)\ **Post date:** [December 4, 2025, 12:05am UTC](https://boards.straightdope.com/t/laser-range-finder/1025382/1 "2025-12-04T00:05:02Z")
I’ve seen these used by golfers and at shooting ranges. The terminal end of the laser is always a hard object. I want to measure the distance from the deck of a ship to the water. Obviously there’s a difference between the surface of water and a hard object. So, will a range finder work for my intended purpose? Thx. ---
**Author:** ![Reply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/reply/32/15952_2.png) [@Reply](https://boards.straightdope.com/u/Reply)\ **Post date:** [December 4, 2025, 12:47am UTC](https://boards.straightdope.com/t/laser-range-finder/1025382/2 "2025-12-04T00:47:23Z")
You’d have to find some wavelength that would reliably bounce off the water instead of scattering or absorbing. The angle hitting the water surface would matter too. I don’t think it would be easy or very reliable unless you could float some sort of reflective surface on the water, like some sort of buoy. Radar altimeters might be worth considering? They make some for drones, but I’m not sure how well they work over water. ---
**Author:** ![Reply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/reply/32/15952_2.png) [@Reply](https://boards.straightdope.com/u/Reply)\ **Post date:** [December 4, 2025, 12:55am UTC](https://boards.straightdope.com/t/laser-range-finder/1025382/3 "2025-12-04T00:55:15Z")
There are also ultrasonic rangefinders that as marketed as sensing water level (see diagram at the bottom of the page)… > **[ToughSonic® 100 50' Range, 2.5" NPT front & rear](https://senix.com/toughsonic-100-50-2-5-npt)** > > ToughSonic® 100.50 Ultrasonic Sensor, 2.5" NPT, 50' Range [![](https://cdn11.bigcommerce.com/s-ncrfq5jlmz/images/stencil/original/image-manager/senix-ts-100-product-photo-new.jpg) ](https://cdn11.bigcommerce.com/s-ncrfq5jlmz/images/stencil/original/image-manager/senix-ts-100-product-photo-new.jpg) ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [December 5, 2025, 2:14pm UTC](https://boards.straightdope.com/t/laser-range-finder/1025382/4 "2025-12-05T14:14:14Z")
I have a golfer’s laser rangefinder. I also live in a high rise overlooking a golf course with many ponds. My balcony experiments conclude that even at look angles as shallow as ~5 degrees I get reliable returns off smooth pond water as far away as ~450 yards _that is reflecting something_. IOW what I see when I look at that aimpoint is the reflection of trees or buildings or … OTOH, even at look angles as steep as 40 degrees (best I can do from home), I get no return at all if the background reflection off the water is blue sky. Further testing reveals that when I’m aiming at reflections I’m not measuring the distance to the reflections. But rather the two-part distance to the point on the water where the reflection is, then up to whatever is casting that reflection. Tree crown, building etc. Which demonstrated to me that the water provides an adequate reflective surface to handle 2 bounces round trip and still give a reading. Suggesting that a straight 90 degree look angle into the same water will produce a reliable reflection and therefore a reliable reading. I’ll be out later today and can take some straight down readings off a drawbridge into water 10 or 20 feet below. But based on my early returns (heh) I expect no problem with the OP using a golf rangefinder for his purpose over the distances that a ship deck might provide above the water. Now this isn’t the crystal clear transparent water found in tropical travelogues. But neither is it lentil soup. ---
**Author:** ![Reply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/reply/32/15952_2.png) [@Reply](https://boards.straightdope.com/u/Reply)\ **Post date:** [December 5, 2025, 2:17pm UTC](https://boards.straightdope.com/t/laser-range-finder/1025382/5 "2025-12-05T14:17:05Z")
Cool experiments! I’d love to hear how they go, and if it differs depending on salinity, turbidity, waves, etc. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [December 5, 2025, 3:37pm UTC](https://boards.straightdope.com/t/laser-range-finder/1025382/6 "2025-12-05T15:37:35Z")
Well, that was disappointing. I visited the first drawbridge. About 20 feet straight down into salty baywater with light wind & current ripple on it. Visibility in the water was probably 6" or so, maybe a foot. Far from clear, but not lentil soup either. Result: No laser return. I tried the sunny & shady sides of the bridge. No return. Crap. Disregard my earlier optimism. Later today I will hit a different bridge with clearer water & a bit more range. I _think_ I was beyond the rangefinder’s minimum range, but I’m sure I will be on the other bridge. Still, we’re doing Science! [![](https://img.youtube.com/vi/V83JR2IoI8k/maxresdefault.jpg "Thomas Dolby - She Blinded Me With Science (Official Video - HD Remaster)") ](https://www.youtube.com/watch?v=V83JR2IoI8k) ---
**Author:** ![jnglmassiv](https://avatars.discourse-cdn.com/v4/letter/j/f07891/32.png) [@jnglmassiv](https://boards.straightdope.com/u/jnglmassiv)\ **Post date:** [December 5, 2025, 3:43pm UTC](https://boards.straightdope.com/t/laser-range-finder/1025382/7 "2025-12-05T15:43:48Z")
> [@Reply](#): > > ultrasonic rangefinders that as marketed as sensing water level Radar versions also exists for tank & hoppers for materials not suitable for sound or light based techniques like grain or airy powders. I recall some consumer grade sonic tape measures before the laser ones were available, I still see some supercheapy ones online. As an engineering student, we had little ex-Polaroid camera transducers (so we were told) in labs for interrupt timing and arithmetic programming on microcontrollers. They were really impressively accurate on the bench and, as a student, accuracy wasn’t even the objective. ---
**Author:** ![Reply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/reply/32/15952_2.png) [@Reply](https://boards.straightdope.com/u/Reply)\ **Post date:** [December 5, 2025, 3:47pm UTC](https://boards.straightdope.com/t/laser-range-finder/1025382/8 "2025-12-05T15:47:49Z")
> [@jnglmassiv](#): > > transducers What is that, and how is it different from a radar altimeter? ---
**Author:** ![Reply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/reply/32/15952_2.png) [@Reply](https://boards.straightdope.com/u/Reply)\ **Post date:** [December 5, 2025, 3:49pm UTC](https://boards.straightdope.com/t/laser-range-finder/1025382/9 "2025-12-05T15:49:54Z")
> [@LSLGuy](#): > > Result: No laser return. > > I tried the sunny & shady sides of the bridge. No return. > > Crap. > > Disregard my earlier optimism. Later today I will hit a different bridge with clearer water & a bit more range. I _think_ I was beyond the rangefinder’s minimum range, but I’m sure I will be on the other bridge. > > Still, we’re doing Science! Still, though, thanks for validating the hypothesis with real-world experiments! I’ll try some too at our local rivers later. Anyone have a radar or ultrasound one they can try? Or maybe you’ve just had a lot of practice with echolocation? ---
**Author:** ![Reply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/reply/32/15952_2.png) [@Reply](https://boards.straightdope.com/u/Reply)\ **Post date:** [December 5, 2025, 3:51pm UTC](https://boards.straightdope.com/t/laser-range-finder/1025382/10 "2025-12-05T15:51:42Z")
I was also thinking… for measuring water with laser… you should be able to just have some sort of floating mirror (or white plate of some sort on a rope, almost like a [secchi disk](https://www.google.com/search?hl=en&site=imghp&q=secchi+disk&udm=2)) that you can lower onto the water surface to measure its distance. But then you could easily just get a rope with built-in measurements on it and just use it to measure the distance directly… ---
**Author:** ![Dorjan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dorjan/32/12440_2.png) [@Dorjan](https://boards.straightdope.com/u/Dorjan)\ **Post date:** [December 5, 2025, 3:54pm UTC](https://boards.straightdope.com/t/laser-range-finder/1025382/11 "2025-12-05T15:54:03Z")
It should work just fine from deck to water. laser range finders are tuned to and are very sensitive to a very specific wavelength (that of the laser itself, ovbs). If you can see the laser on the target, more than enough is being returned to the rangefinder for it to calculate the distance. ---
**Author:** ![Marvin\_the\_Martian](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/marvin_the_martian/32/2898_2.png) [@Marvin\_the\_Martian](https://boards.straightdope.com/u/Marvin_the_Martian)\ **Post date:** [December 5, 2025, 3:54pm UTC](https://boards.straightdope.com/t/laser-range-finder/1025382/12 "2025-12-05T15:54:49Z")
> [@LSLGuy](#): > > My balcony experiments conclude that even at look angles as shallow as ~5 degrees I get reliable returns off smooth pond water as far away as ~450 yards _that is reflecting something_. IOW what I see when I look at that aimpoint is the reflection of trees or buildings or … > [@LSLGuy](#): > > Well, that was disappointing. I visited the first drawbridge. About 20 feet straight down into salty baywater with light wind & current ripple on it. Visibility in the water was probably 6" or so, maybe a foot. Far from clear, but not lentil soup either. You are running into Snell’s Law and the critical angle. > **[Snell's law | Total internal reflection and critical angle](https://en.wikipedia.org/wiki/Snell%27s_law#Total_internal_reflection_and_critical_angle)** > > When light travels from a medium with a higher refractive index to one with a lower refractive index, Snell's law seems to require in some cases (whenever the angle of incidence is large enough) that the sine of the angle of refraction be greater than one. This of course is impossible, and the light in such cases is completely reflected by the boundary, a phenomenon known as total internal reflection. The largest possible angle of incidence which still results in a refracted ray is called the cri... At a shallow angle (as you define the angle) the angle of incidence (which is 90 degrees minus your angle) is greater than the critical angle so the water acts as a mirror. From directly overhead (or nearly so) the angle of incidence is less than the critical angle so the light passes _into_ the water, where it is scattered or absorbed before it can make it to the bottom of the pond and back to the surface. So no return. ---
**Author:** ![Reply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/reply/32/15952_2.png) [@Reply](https://boards.straightdope.com/u/Reply)\ **Post date:** [December 5, 2025, 3:55pm UTC](https://boards.straightdope.com/t/laser-range-finder/1025382/13 "2025-12-05T15:55:42Z")
> [@Dorjan](#): > > If you can see the laser on the target But doesn’t that depend a _lot_ on the water conditions? If the laser is refracting, reflecting at an angle, or hitting the ground below the water… what are you actually measuring? It doesn’t seem easy to reliably bounce it back off the surface in particular (as in LSLGuy’s experiments). Edit: Nevermind, ninjaed by @Marvin_the_Martian with a much better explanation. ---
**Author:** ![Dinsdale](https://avatars.discourse-cdn.com/v4/letter/d/97f17d/32.png) [@Dinsdale](https://boards.straightdope.com/u/Dinsdale)\ **Post date:** [December 5, 2025, 3:57pm UTC](https://boards.straightdope.com/t/laser-range-finder/1025382/14 "2025-12-05T15:57:21Z")
> [@19USMC69](#): > > I want to measure the distance from the deck of a ship to the water. Just curious. You mean you want to measure straight down, how high the deck is? Which would vary depending on how heavily it was laden? What sort of ship? Don’t many ships have plimsoll lines/depth marks to show that sort of thing? Sorry if that is a hijack. Or if it is an additional hijack to express the joy of golfing with someone who makes a point of scoping every shot, when you both know he lacks a reproducible swing and has no consistent yardage for any of his clubs? 🙄 ---
**Author:** ![Reply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/reply/32/15952_2.png) [@Reply](https://boards.straightdope.com/u/Reply)\ **Post date:** [December 5, 2025, 4:02pm UTC](https://boards.straightdope.com/t/laser-range-finder/1025382/15 "2025-12-05T16:02:26Z")
> [@Marvin\_the\_Martian](#): > > From directly overhead (or nearly so) the angle of incidence is less than the critical angle so the light passes _into_ the water Some laser rangefinders (like [on Sony cameras](https://old.reddit.com/r/woahdude/comments/15qi0ss/my_sony_camera_has_an_innovative_laser_grid/)) use a Kinect-like point cloud or grid: [![](https://fightingignorance.org/5b89b873-d358-429a-890c-6892ac0049db-my-sony-camera-has-an-innovative-laser-grid-autofocus-system-which-sony-calls-hologram-af-to-help-focus-in-dim-light-woahdude-december-05-08-01-37-am.jpg) ](https://fightingignorance.org/5b89b873-d358-429a-890c-6892ac0049db-my-sony-camera-has-an-innovative-laser-grid-autofocus-system-which-sony-calls-hologram-af-to-help-focus-in-dim-light-woahdude-december-05-08-01-37-am.jpg) I wonder if that would be better & sufficient for this? Or would even the outermost points still be less than the critical angle? ---
**Author:** ![Reply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/reply/32/15952_2.png) [@Reply](https://boards.straightdope.com/u/Reply)\ **Post date:** [December 5, 2025, 4:12pm UTC](https://boards.straightdope.com/t/laser-range-finder/1025382/16 "2025-12-05T16:12:13Z")
Could you could use a laser pointer on a stick to bypass the critical angle limitation and then do some trig? Something like: [![](https://fightingignorance.org/e49b7ead-6998-4438-a339-16f74ae5e785-untitled-figma-december-05-08-11-31-am.jpg) ](https://fightingignorance.org/e49b7ead-6998-4438-a339-16f74ae5e785-untitled-figma-december-05-08-11-31-am.jpg) (oops, the pointer angle shouldn’t matter as long as the stick is long enough) ---
**Author:** ![Dorjan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dorjan/32/12440_2.png) [@Dorjan](https://boards.straightdope.com/u/Dorjan)\ **Post date:** [December 5, 2025, 4:20pm UTC](https://boards.straightdope.com/t/laser-range-finder/1025382/17 "2025-12-05T16:20:11Z")
> [@Reply](#): > > Could you could use a laser pointer on a stick to bypass the critical angle limitation and then do some trig? Something like: As long as you know the length and angle of the stick and the angle of the rangefinder, sure it should work. Simple Trig. ---
**Author:** ![Reply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/reply/32/15952_2.png) [@Reply](https://boards.straightdope.com/u/Reply)\ **Post date:** [December 5, 2025, 4:25pm UTC](https://boards.straightdope.com/t/laser-range-finder/1025382/18 "2025-12-05T16:25:33Z")
> [@Dorjan](#): > > As long as you know the length and angle of the stick and the angle of the rangefinder, sure it should work. Maybe @LSLGuy has some sticks lying around…? Golf clubs? Tourist whackers? > [@Dorjan](#): > > Simple Trig. Nothing about trig is simple 😅 I can barely remember how to calculate the other sides of a square, much less a triangle… ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [December 5, 2025, 4:41pm UTC](https://boards.straightdope.com/t/laser-range-finder/1025382/19 "2025-12-05T16:41:02Z")
> [@Dorjan](#): > > It should work just fine from deck to water. laser range finders are tuned to and are very sensitive to a very specific wavelength (that of the laser itself, ovbs). If you can see the laser on the target, more than enough is being returned to the rangefinder for it to calculate the distance. Golf laser rangefinders are IR. There is no seeing the laser spot. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [December 5, 2025, 4:44pm UTC](https://boards.straightdope.com/t/laser-range-finder/1025382/20 "2025-12-05T16:44:13Z")
> [@Marvin\_the\_Martian](#): > > From directly overhead (or nearly so) the angle of incidence is less than the critical angle so the light passes _into_ the water, where it is scattered or absorbed before it can make it to the bottom of the pond and back to the surface. So no return. Darn. But thanks for the explanation. _More Science!_ I was expecting the 90 degree incidence case to be some combo of specular reflection and penetration / absorption. Net of the undulating surface. But evidently there’s just not enough (or maybe any?) reflection going on to be detected on the way back up to the rangefinder. Oh well. [Next page](https://boards.straightdope.com/t/laser-range-finder/1025382.md?page=2)