# Ways of achieving parallel rays of light?

I am thinking of ways of achieving parallel rays of light.

Let's say I have an object that emits light, for instance the display of my mobile phone. The display emits rays of light in every direction.

Is there a "filter" I can put in front of the screen that only lets rays of light traveling in a certain direction through - e.g. only rays of light traveling orthogonal to the screen?

Or what other ways are there to achieve parallel rays of light?

• Are you somehow trying to achieve parallel rays , or are you referring to plane wavefronts by saying parallel light waves ? – Rijul Gupta Oct 28 '13 at 11:16
• Thanks, I am trying to achieve parallel rays. I'm updating the question to clarify. – user2078515 Oct 28 '13 at 14:20
• You can possibly make a confinement with multiple mirrors inside and only a very small ( but not too small to cause diffraction) and let the light come out of it, as it would come out as a beam, hence it would be in form of parallel rays as a beam is a bundle or parallel rays – Rijul Gupta Oct 28 '13 at 14:24

## 5 Answers

The commonly available 'privacy screen' does this by blocking light beyond a certain angle off perpendicular. Typically the narrower the cutoff angle, the more expensive, but these work well for privacy.

They don't actually collimate light into an absolutely parallel beam, but in reality you wouldn't want this, as it would make the device quite unusable (you would only see a small area of the screen at any time), so what they actually do is allow between 15 and 30 degrees either side of 90 degrees, and heavily attenuate any light outside these angles.

• very clever answer +1 – Argus Oct 29 '13 at 6:19

All such methods are constrained by optical laws that ultimately are limited by the second law of thermodynamics.

If you have a light source of dimension H (one dimensional) in a medium of refractive index N, and it emits a beam over a range of angles from +U to -U (one dimensional), then the quantity NHSinU is invariant. In two dimensions, then (NHsinU)^2 is invariant. By invariant, we mean, under all geometrical optical transformations; reflection, refraction etc. This is true both for imaging optics, and non-imaging optics.

So if you try to reduce the beam angle (U), then the beam diameter must increase, assuming N is unchanged. So if you are talking about a single pixel on your screen, being made visible over only a small angle (in any direction), you can ONLY do this by throwing the rest of the light away.

Other ways: The classical way to make essentially parallel rays of light is with a parabolic reflector. A searchlight throws a beam of light that is nearly parallel, by placing the light source at the focal point of the parabola. Since the light source is bigger than a point, it isn't exactly at the focal point, but it's close, and the light coming from its surface is travelling from approximately the same direction as the focal point. A less perfect example is one of those "Cree" focusing flashlights. When you focus it right, it projects an image of the LED out into the world. The LED image is bigger than the original light source, so it is diverging some. Movie projectors are similar. More perfect is the light from a LASER, which is not only parallel (the rays are travelling in the same direction) but also coherent (all the rays have the same frequency and the same phase).

Love the question, have thought of it myself. I don't think it can be done.

'parallel light' is assumed to be light from infinity (a distance of approx 6m). Ever noticed how optometrists make you look at a mirror to the wall behind you when they're doing an eye check up? that is to achieve 6m, as the room is often too small.

I think that if, somehow, you did achieve parallel rays; only a small amount of pixels would be visible, those that are effectively orthogonal to the viewer.

think of the hoods on traffic lights. They are in place to obscure your view of the lights from traffic going other directions. Similar to the idea behind privacy screens on your phone. If you had parallel light, it would be light trying to view (from a close distance) a traffic light with a very long hood on it, and consequently you'd only see what is exactly orthogonal to your eye.

Beyond looking at how trying to collimate the beams would be done, there's also a fundamental optical limit on how angularly narrow the final beam can be due to diffraction effects - though in this case, it turns out to likely not matter.

For an approximate answer, I'll take a close-to-best-case set of assumptions: the beam is coherent and Gaussian, assuming a pixel size of 0.5 mm radius (roughly what you have for a standard ~500 DPI phone screen) for the $$1/e^2$$ intensity width at the waist, so $$w_0=0.5$$ mm. The worst angular spread will be for the longest wavelength of light, which will be the red pixels at approximately $$\lambda=700$$ nm (on an aside, for optimizing the spread, you could resize the size of the pixels so that the red ones are larger, shrinking the others so as the spreads end up similar). Final propagation will be through air, which is close to refractive index $$n=1$$.

This gives a limit of $$\theta=\lambda/\pi w_0\approx 0.03$$ degrees to either side of 90 degrees. Even if the details of the situation worsen this by two orders of magnitude, at worst, you'll have a few degrees of spread as your limit, which isn't be an issue for most practical applications. For example, the privacy screen mentioned by Rory Alsop has an angular spread of about three orders of magnitude above this limit.