Diffuser film - shiny or matt side towards light

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Beatsy
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Joined: Fri Jul 05, 2013 3:10 am
Location: Malvern, UK

Diffuser film - shiny or matt side towards light

Post by Beatsy »

I added some extra air-gapped diffusion in front of my (already diffused) twin macro flashes today. It's a thin sheet material from a flat-screen TV, glossy on one side and matt on the other. It transmits light very well with no colour shift (unlike thicker diffusers) but needs two or three sheets with a small gap between each to ensure good softening (3-5mm is enough with 2 layers - more is better). I didn't think it would matter which side faced the light, but light seems slightly more diffuse (fewer specular highlights) when the rough sides of all layers face the light. Why?

Is there a "correct" orientation, or is it an "it depends" variable thing?

Thanks

ray_parkhurst
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Re: Diffuser film - shiny or matt side towards light

Post by ray_parkhurst »

I always face the matte side of diffuser material toward the subject to minimize re-reflection. For the same reason I put masking on the subject side of the diffuser. I get a lot of re-reflection color shifts when using short FL objectives, and need to keep the light apertures and reflectivities to a minimum.

Beatsy
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Joined: Fri Jul 05, 2013 3:10 am
Location: Malvern, UK

Re: Diffuser film - shiny or matt side towards light

Post by Beatsy »

Local reflections onto the subject were my first thought too Ray, and I always put the matt side to the subject based on that assumption (when there's a glossy side). Until I consciously tried it the other way around and noticed the extra diffusion (and loss of more light) with smooth side to the subject. Definitely a difference there - I checked several times.

I have a theory. When light hits the glossy side, what isn't reflected away travels straight through the material, and is scattered by the rough surface as it exits. Average light path length in the diffuser is about 1.0 (the thickness of the diffuser).

When light hits the rough side first, it is immediately scattered and travels through the material at a different angle (mix of small and moderate deflections). Average light path length in the diffuser is greater than 1.0 and it is scattered and attenuated more than the light that goes straight through (shorter path).

Feasible?

ray_parkhurst
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Joined: Sat Nov 20, 2010 10:40 am
Location: Santa Clara, CA, USA
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Re: Diffuser film - shiny or matt side towards light

Post by ray_parkhurst »

Beatsy wrote:
Wed Apr 28, 2021 10:59 pm
Local reflections onto the subject were my first thought too Ray, and I always put the matt side to the subject based on that assumption (when there's a glossy side). Until I consciously tried it the other way around and noticed the extra diffusion (and loss of more light) with smooth side to the subject. Definitely a difference there - I checked several times.

I have a theory. When light hits the glossy side, what isn't reflected away travels straight through the material, and is scattered by the rough surface as it exits. Average light path length in the diffuser is about 1.0 (the thickness of the diffuser).

When light hits the rough side first, it is immediately scattered and travels through the material at a different angle (mix of small and moderate deflections). Average light path length in the diffuser is greater than 1.0 and it is scattered and attenuated more than the light that goes straight through (shorter path).

Feasible?
Well, since you're using multiples of them, how about pointing all but one with rough side up to improve diffusion, and the final one down to minimize reflections?

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