First, adding diffusion to a light doesn't technically change the "look" of a video the way most people think of that.
Diffusion changes how shadows - particularly the edges of shadows - appear because diffusion changes the geometry of the light emitting surface (typically making it relatively larger and therefore emitting light from a larger area and increasing the variety of angles of incidence of the photons hitting the surface of the object being lit.)
But diffusion is just ONE thing that determines the "softness" of such shadows. Distance from the light to the object being lit is another critical factor. So you can add diffusion to a light and that alone might make absolutely NO difference because of other factors.
The classic example is that sunlight casts sharp shadows - even tho the sun is kinda big (!). The reason is distance. Soft lights - placed too far away from any object - look hard.
So you can use 10 different kinds of diffusion, and if the light is placed at distance, there will be no functional difference in the lighting.
Think about diffusion in three terms - how much light it allows to pass - how the surface spreads the light over a larger area than the initial instrument - and whether or not the material can absorb any heat being generated by the instrument.
Hope that helps.
Oh, and Bill Holshevnikov has some nice videos on lighting that I can happily recommend.
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