8 replies to this topic

[9TE-5]

What I am trying to ask here is will the filter be effective irrespective of which way the light travels through?

 

They seem to reflect red from both front and back surfaces. 

 

Don't know about the violet end.

[matt_astro_tx]

[KLWalsh]

What I am trying to ask here is will the filter be effective irrespective of which way the light travels through?

Yes, but…
The spectral transmittance will be the same, regardless of which way the filter is facing.
However, if you’re doing photography the configuration of your optical train may cause artifacts in the image due to reflections from the shinier side of the filter. When I can, I put my filters as far from the sensor as possible, with the shinier surface facing away from the sensor.
YMMV.

[matt_astro_tx]

Yeah, are you doing astrophotography or just visual observation?  I assume it matters less for visual.

[9TE-5]

Thank you Matt,

 

I am doing astro.

 

What are your images trying to show me please?

Edited by 9TE-5, 27 November 2022 - 08:38 PM.

[9TE-5]

Matt,

 

Very nice images, but what I meant was what are the images of the two filters trying to show me...

 

Sorry for the misunderstanding.

 

I see two types of filter structure, do they make a difference to the directionality of the filter?

 

Both sides of my filter are polished but I didn't remove it from the holder to observe the edge, should I do that to continue our chat?

 

I understand coating thicknesses and effects on various wavelengths.

[matt_astro_tx]

They are showing that one side will give multiple reflections, and the other will give a single reflection.  Supposedly if you hold a pencil just above the filter, you'll see this reflection affect.  I've never tried it though.

[briansalomon1]

In order to be certain you need to know what type of filter you have. If the UV/IR cut filter is an absorbance filter (works like a sponge) the filter will work regardless of orientation.

 

If it's a rejection filter (works like a mirror) it's very likely to need to be orientation sensitive.

 

Of course, as Matt pointed out, unless the filter has an AR coating on both sides, you will want the AR side facing your camera.

[9TE-5]

My cheapo filter is a FOTGA brand.  

 

Both front and back reflect the red part of any ambient light (eg light bulb) and I expect is the heat mirror effect.

 

So I also expect this means its a coated interference/rejection type filter.