What causes the "star effect" glare of bright lights?

Question

When you look at a bright light through a window (or lense), what causes that light to appear to give off a star-like glare?

I've been searching the internet for the answer, but I haven't found much because I am not quite sure what that phenomenon is called. And so far, all of the keywords I have tried using haven't provided much help.

The only thing I really know about it is that, in photography, if you take a picture of a bright light using a small aperture, you can capture this effect. However, I noticed that you can also observe this effect when looking at a bright light through a window (specifically a windshield).

I want to figure out how/why this happens because I am trying to work out an art project (involving glass panels) in which I may want to intentionally use this effect.

Any help would be appreciated. I think that perhaps microscopic imperfections in the glass may have something to do with it, but I am not sure.

Thanks!

Answer 1

Lens Flare can cause streaking. Imperfections in the lens cause the different frequencies of light to bend at different angles causing abberations or iperfections in the image.


Diffraction will cause points of light to take the general shape of the apperture placed in front of the lens. This effect is well used in photography .

If you view lights through a piece of window screen you will see crossed lines from each point light source you view. You can rotate the screen and rotate the crosses. It you take two pieces of screen at 45degrees to each other you will see twice as many lines ( 4 point crosses will becom 8 point crosses)

You can buy special filter attacments for SLR camera to do the same thing. Pros who use medium format cameras with Matte boxes sometimes use custom cut outs infront of the lens to make shapes such as hearts circle a candle flame in a wedding picture etc.

The effect you see through a windshield is mosy likely caused by a combination of diffraction and flare. Water droplets create apatures and oil/dirt film causes flare and refraction.

Answer 2

its a refraction of light-the glass isn't always flawless. with your eyes its basically the same