Why do we use patterned glasses and flash diffusers?

Question

I first noticed it on the xenon flash on my Nokia N8. When viewed from an angle you could easily see the glass on front of the flash tube which had clearly defined vertical cuts which made the glass look like its been made my gluing different glass shards.

When viewed from an angle you can see how each individual glass shard shines in order.

Then I saw things called flash diffusers for hot shoe xenon flashes.

So why does flashes have diagonal cut patterns on their glasses and what are these flash diffusers? How it would look like if these patterned glasses and flash diffusers didn't exist?

Answer 1

The patterns basically break up the surface of a concave lens into lots of pieces and remove the bulk of the lens in between, greatly reducing the weight and the possibility for chromatic aberration at the price of not seeing a nice image of the flash tube. Basically it is taking the compromise of a Fresnel lens one step further. Indeed the original concept of the Fresnel lens was for lighthouses, a similar light (rather than image) projection task that a flash has to perform. Discontinuities are pretty fine as long as they are distributed in a manner where there is no significant stackup of them at any point in the lightcone.

It's basically a size and weight (and dispersion/absorption) problem that those comparatively thin fragmented diffusor plates tackle better than continuous lenses would.