Diffraction

Treasure Hunt: Grate;

I have been hunting high and low for this old filter! it does not even fit any of my current lenses so I had to hold it over my phones camera!

How does this fit the theme you ask...?
The filter splits the light with precise lines scratched on to the optical surface of the glass and is known as a Diffraction Grating! This type of filter is used all over the place and you'd never notice.
Does anyone have a Fresnel Lens to widen the view from the back window of the car or how about one of those adapters for the headlights that deflect the main beam for when you drive in countries, as I have just done, on the 'other' side of the road... all the same principle! you can even get them in the form of reading lenses for books, like a magnifying glass with a light!
Photographically, these went through a massive 'in' phase but I bought mine, used it once and only got it out again today!! The only one I used regularly was an 8 point star filter which I used to give a nice soft focus effect with and avoided the starbursts where possible! :-)