Why Do Lasers Look Grainy?

You’ve probably come across lasers that are used as pointers in presentations. Perhaps you even have a laser pointer that you use to entertain your pet. But why does this beam of light always appear granular? And more importantly, how exactly does laser light work?

A Primer on Lasers

Laser is an abbreviation that stands for Light Amplification by Stimulated Emission of Radiation.

In simpler terms, a laser is a beam of very concentrated light. This light is so powerful that it can be used to cut, engrave and drill metals.

While lasers seem like a recent discovery, they’ve actually been around for nearly a hundred years. The concept was invented in 1958 and the first functional laser was made in 1960. Back then, these light beams were seen as something out of this world.

Today, however, this idea is applied in a wide range of sectors. The Blu-ray discs in our homes use laser technology. We also have laser printers at our workplaces. Plus, barcode scanners that capture information from the goods we buy also utilize laser technology.

So how does it work? At its core, this technology involves focusing light on one area, repeatedly, until a powerful beam emerges.

To know how lasers are generated, you should first understand that each electron sits in its own orbit. Each orbit is associated with an energy band in the atom. These bands can be likened to the steps of a stairway.

Typically, every electron assumes the first step, and this is what is referred to as the ground state.

If you provide the correct quantity of energy to each electron, you cause it to move up into a higher energy level. This mechanism is called absorption, that is to say an electron takes up energy causing it to move one step higher.

The movement of electrons from one level to another causes them to release a photon of energy, which escapes into the atmosphere as a powerful and focused beam of light called laser.

Laser speckle.jpg
By The original uploader was Deglr6328 at English Wikipedia. – taken from user jurvetson on flickr: [1].
Originally from en.wikipedia; description page is/was here., CC BY 2.0, Link

Why do Lasers Appear Granular?

Lasers appear grainy, primarily due to interference.

The laser light that hits your eye does not display speckle characteristics. Speckling is due to scattering of light off random surfaces. A rough surface is known to cause wavelets, bounce off objects and merge in space. Individual wavelets have unique phase relationships with other wavelets.

When your eyes observe these wavelets, more overlapping occurs as they try to focus. As a result, what you get is a speckled or grainy pattern, which is only as large as the size of your pupil.

Usually, speckle size is much smaller than the eye’s focal spot. Sometimes it’s almost the same size as the focus spot as in the case of corrected vision. For this reason, what you’ll observe in a photograph are fine speckles or grains. This is often the case even when the object you’re observing is poorly focused.

The grainy pattern observed is unique from one person to another. For instance, if you’re a near-sighted individual and happen to tilt your head to the left when observing the laser beam, it will seem as if the speckle pattern is moving to the right.

On the other hand, if you’re far-sighted, the beam will appear as though it’s moving to the left. But with the right level of focus, you should be able to see a somewhat stationary speckle pattern.

The beam that you observe is what is known as subjective speckle pattern. It’s called subjective because it differs based on the type of camera and the on-looker.

Another point to note is that it’s nearly impossible to see direct speckle patterns on a wall. To illustrate this, assume you’re in an extremely dark room where the laser light has been cast on the wall. If a large spot is cast on the wall, the size of the speckles will be very small.

On the other hand, if a small spot is cast, the speckle sizes will be large. In a nutshell, the size of the speckle pattern is inversely proportional to the size of the spot.

Conclusion

Laser technology has evolved significantly since its invention in the 1960s. Today, laser light is prevalent in numerous sectors ranging from medical to telecommunications and manufacturing. Given its ubiquity, it's likely that you’ve used or seen a laser pointer or other laser-driven gadget. This technology functions on a very simple concept: that of concentrating light on a spot until a beam emerges. Now if you look closely, you’ll notice that this beam of light appears grainy.

The truth is, the light reaching your eyes is not speckled. What makes it appear so is the fact that it bounces off on certain surfaces, causing the light to get scattered. However, the way you observe this granular pattern will vary depending on the individual and the type of camera used.

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  • Updated August 17, 2019
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