How to distinguish laser from normal light? - Page 1
Forum section: electronics / beginners

author: Logan
author_rank: Frequent Contributor
timestamp: January 25, 2022, 04:53:19 pm
content: Hi.I think sometimes laser give a "sparkling" looking. But this is neither reliable nor safe (high power ones), or sometimes even not working (invisible wavelengths).Are there any way to check if a light is laser or "normal" light?Thanks.

author: jmelson
author_rank: Super Contributor
timestamp: January 25, 2022, 05:03:52 pm
content: Laser light is usually very collimated, so it doesn't spread much with distance. There are line generators that spread it intentionally, however.it is also VERY monochromatic, so the wavelength has a very narrow spread. The visible speckle is usually quite obvious.Jon

author: PartialDischarge
author_rank: Super Contributor
timestamp: January 25, 2022, 05:23:57 pm
content: What really distinguishes a laser is the fine spectral line, ie the purity of the color, a red laser has a purer red that a red led.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 25, 2022, 05:41:56 pm
content: Lasers have a few special characteristics, so they are used for different reasons.Sometimes to get well collimated light: think about individual rays. They are not radially pointing at every direction; instead, they are parallel. This allows you to get a small dot, which stays a small dot even if you move your target further away. HOWEVER, not every use case of lasers is because of that property, so you can make the laser light non-collimates as well.Sometimes to get very narrow wavelength peak at well predictable wavelength. So say you need just 650nm photons, not a mix of anything between 620nm and 680nm even shifting with temperature, like with an LED. And again, sometimes this property is utterly irrelevant.The third reason to use a semiconductor laser is that it can be turned on/off faster than an LED. For example, 3D time-of-flight sensors may use lasers just to be able to blink them at 50MHz or so. An LED would be limited pretty much at 20MHz. Otherwise than that, LED does well.Oh, and polarization is another reason. Sometimes it matters. You can use filters to make polarized light from a "normal" light source having random polarization in every angle, but filtering loses light; lasers produce polarized light natively.

author: David Hess
author_rank: Super Contributor
timestamp: January 25, 2022, 06:24:21 pm
content: Speckle is the only thing which would be readily apparent with the eye. If the laser light is coherent, then interference fringes may be seen, but most lasers are coherent only over an insignificant distance.

author: Rick Law
author_rank: Super Contributor
timestamp: January 26, 2022, 12:03:59 am
content: If you have a good glass prism, this would be a definitive test: shoot the light at the prism, does it spread out on exit? If no, you have a laser (single frequency) light. If it spreads, you do not have a laser light (mixed frequency).A less definitive method: You can also use a glass bottle filled with water, shoot at it at an angle and discern how much it spreads. Little or no spreading, you have laser (single frequency). Now compare it against a known laser shoot at the same angle and compare the spreading. That would be a good but not as good as prism method.EDIT: Since a picture is worth a thousand words... added the freshly made photos belowThe set up us a beam from real laser compared against a beam from a focused flashlight with a pin-hole exit. The water-bottle as prism beam spreader is just a glass bottle, with water, and added a little coffee to enhance the visibility of the beam of light.You can see how the laser maintained a confined beam where as the focused flash light beam is spreading where exiting beam side is clearly wider than the entry side of the beam.

author: David Hess
author_rank: Super Contributor
timestamp: January 26, 2022, 01:54:09 am
content: A CD or DVD might work as a diffraction grating, but LEDs have pretty narrow bandwidths anyway so I am not sure how easy it will be to distinguish them from LED lasers using this method or a prism.

author: fourfathom
author_rank: Super Contributor
timestamp: January 26, 2022, 02:08:17 am
content: Quote from: Siwastaja on January 25, 2022, 05:41:56 pmThis doesn't help the OP, but FYI We used on/off modulated lasers at 2.5 GHz in fiber optic systems. At 10 GHz and above we used optical modulators.

author: Rick Law
author_rank: Super Contributor
timestamp: January 26, 2022, 02:19:41 am
content: Quote from: David Hess on January 26, 2022, 01:54:09 amYou have a point there... LED's narrow bandwidth could make distinction difficult, particularly when laser's imperfection will result in some small spreading also. LED also could have a focusing lens of some sort on the emitter altering the spreading pattern. But my educatedguesswould be laser's very limited frequency spread would still hold a beam's confinement better than LED's narrow peak, the word "definitive" I used in the first reply is too strong a term. "Probably" laser is likely more achievable.Guess is one thing, certainty is another. Now I need to find my prism/diffraction grader and use it as frequency filter to do some hard experiment. I have both, somewhere, in my basement full of things that I know I have but I can never locate.

author: Logan
author_rank: Frequent Contributor
timestamp: January 26, 2022, 06:15:58 pm
content: Thank you everyone!

author: rpiloverbd
author_rank: Regular Contributor
timestamp: January 27, 2022, 11:43:09 am
content: Hello, you may find this write up helpful:https://diffzi.com/laser-vs-light/Laser light is very distinctive. They cover much more distance than any other common source of light. They are not scattered. You'll always see laser lights follow a specific pattern. Like a dot or straight line or zigzag. It is always one-directional.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 27, 2022, 12:51:06 pm
content: Laser light does not have to be parallel. Actually the laser diodes produce a quite deverging light pattern (e.g. 3 degree in on direction and some +-45 degree in the other direction.Still lasers are often used to create parallel light, because the lasers can have a very high luminosity, that is a lot of light from a small spot / small angle. So it is possible to collaminate laser light quite well. A LED is limited by the much larger dimensions of the emitter.The bandwidth of laser light is several orders of magnitude narrower than LED light and also narrower than the spectral lines of gas discharge lamps. This is at least true for the normal contineous lasers - the highly pulesed ps and shorter laser pulses get wider in bandwidth.Some lasers may emitt additional normal light and maybe a 2nd laser wavelength at the same time: laser diodes at low current start as a LED and than above a certain current level give extra laser light. The LED type action howver still contineous.The way to distinguish is usually from the interference patterns that can be obtained. Either as the fine speckle pattern of interference rings / fringes depending on the geometry. One can still get interference with normal light (e.g. spectral lines), but this is much more limited.

author: Cerebus
author_rank: Super Contributor
timestamp: January 27, 2022, 02:27:03 pm
content: I can see from what some people have written that there is some misunderstanding what 'coherence' means with respect to laser light. We say that laser light is coherent, and becuase the everyday meaning of 'cohere' is stick together, some people misinterpret that usage with lasers to mean that thebeam'sticks together', that is that it doesn't diverge. That is not the sense that 'cohere' is being used here. Coherence in laser light means that the photons 'stick together' when they come out of the laser or, to view the light as a waveform, that the waveshave the same phase. So simplycoherent light involves light waves that all have the same phase, non-coherent light involves waves that haverandomphase with relation to one another.The important property of laser light is not the monochromaticity - i.e.same frequency(although that is a necessary pre-condition) - but having thesame phase- i.e. coherent.Thus LEDs producenon-coherentlight that is monochromatic or near monochromatic, lasers producecoherentlight that is monochromatic (or in some edge cases near monochromatic).So if you take a collimated light beam from a good monochromatic LED and a collimated light beam from a laser source both beams will be bent as one by a prism and won't spread significantly on passing through the prism. If you bounce both of them off a diffraction grating (or pass them through a pair of slits) thelaserwillproduce adiffraction patternbecause the identical phase of photons/waves on different paths allows them to undergo constructive/destructive interference, whereas theLED light (although monochromatic)won'tproduce a diffraction patternbecause the phase relationship of the interacting photons/waves is random.It is the coherent property of laser light that produces the characteristic speckling that people have referred to. The specking is interference patterns being formed as the laser light follows paths of slightly different length as it reflects off microscopically uneven surfaces and so the slightly divergent reflections set up constructive and destructive interactions between the variously delayed waves.

author: emece67
author_rank: Frequent Contributor
timestamp: January 27, 2022, 02:46:16 pm
content: .

author: ejeffrey
author_rank: Super Contributor
timestamp: January 27, 2022, 05:33:56 pm
content: At the risk of sounding flippant, the way you tell if light is laser light is to look around and see if it comes out of a laser. A laser is a light source based on stimulated emission. They come in many varieties and are used in many ways so it is quite hard to come up with a single optical property that is both unique and universal to lasers. So this is fundamentally an XY problem, once you specify what you actually care about rather than "is from a laser" its easy to figure out.For safety purposes the most important factor is etendue, which is the product of the beam size at the waist with the far field divergence. It represents the ability to be focused to a small point and many but not all lasers can be focused down to a diffraction limited spot, such as for a single mode fiber. A beam with a lot of power in a small etendue can be focused to a small spot on the retina and cause eye damage. But the sun, most decidedly not a laser is so bright and far away that with a suitable aperture has a large power in a small etendue and can cause eye damage. On the other hand a laser producing a highly diverging beam can still be focused to a small spot on your retina but only a very small amount of power will get through the aperture of your eye and so the potential for damage is considerably less.Speckle in particular is not a reliable indicator of laser light. Many types of gas lasers produce extremely pronounced speckle patterns but most diode lasers produce very little speckle.

author: Gyro
author_rank: Super Contributor
timestamp: January 27, 2022, 06:26:59 pm
content: Quote from: emece67 on January 27, 2022, 02:46:16 pm... Unless you have uncorrected defective (near-sighted or far-sighted) eyesight. Then the speckles move in the opposite direction.

author: Rick Law
author_rank: Super Contributor
timestamp: January 27, 2022, 07:49:40 pm
content: Quote from: Cerebus on January 27, 2022, 02:27:03 pmYou are quite right there. For definitive test, I would prefer the interference pattern over diffraction. A double slit screen will do nicely and the interference pattern will tell you all you need to know. While that is easy to do in a physics lab (where you have all the necessary stuff), but not as easy without all the toys.Without a proper double slit screen, we are limited to use secondary characteristics like beam confinement. The spreading of the beam which really only test single frequency by how it diffracts, but not the beam being in-phase.

author: ejeffrey
author_rank: Super Contributor
timestamp: January 27, 2022, 11:06:45 pm
content: Quote from: Cerebus on January 27, 2022, 02:27:03 pmThis is actually not true at all. LEDs will produce perfectly nice diffraction patterns if you collimate it into a beam and shine it on a diffraction grating. So will sunlight for that matter, although you will get a rainbow effect since the different colors will diffract by different amounts. Mostly the issue with LEDs is whether they can create a small enough spot: LEDs can't normally be focused to a diffraction limited spot like a laser and if the spot size is larger than the spacing between the diffraction peaks the pattern will be washed out.You can try this yourself with as little as a hair. Just hold it up in front of your eye and look past it at a small but bright light (not too bright, don't look at the sun!) source. Even a bare fluorescent tube is usually good enough if it is reasonably far away. The rainbow effect you see is a single slit diffraction pattern created by the light source.

author: David Hess
author_rank: Super Contributor
timestamp: January 27, 2022, 11:09:33 pm
content: Quote from: Cerebus on January 27, 2022, 02:27:03 pmI agree with everything you write however on a practical level, most lasers have very short coherence lengths so the light might as well be non-coherent. This is especially true for LED lasers but also applies to most gas lasers. Special tuning and construction is required to extend the coherence length for use in applications which rely on interference.Even past the coherence length, some coherence remains so the speckle effect is still present.