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Dichroic Gobo Filters


eye4invisible

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eye4invisible

Hello all,

Just wondering if anyone has ever tried using dichroic gobo filters (the ones typically used in stage lighting and architectural illumination) in front of the lens on a full spectrum camera.

 

What I am wondering is how a yellow gobo might differ from, say, a Tiffen Yellow #8 filter. And would such a filter yield discolouration around the edge at wider angles, like the Baader-U does?

 

Any experiences to share?

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eye4invisible
Lee do indeed make gobo filters, but that thread seems to be more about the gel filters?
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Well I don’t know if they have been tried. cadmium will probably know. All dichroic filters will discolor on wide angle lenses because they work by interference of light, so the distance the light travels affects whether the internal reflections in the filter cancel the ray or reinforce it. When light comes in at a steep angle, it travels further than the spacing of the layers was designed for, so it doesn’t interfere in the desired manner.
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I have the Lee C47 Turquoise 7(Dichroic) Glass, Non frosted version. (part number C47-LD-MR16)

It is the closest glass filter they make to their #729 filter (is why I got it). It looks cerulean/turquoise blue when viewed straight on, and color shifts to blue then pink when viewed at an angle.

Thus they have some strong color shift when viewed at an angle.

http://www.leefilters.com/architecture/colour-details.html#C47

 

The C47 filter doesn't work like the 729 filter. It works more like the B-410 and more orange than red, like the B-410 does.

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Here is the Lee C47 viewed at different angles. Also notice the few slight scratches in the coating, this is how it came. $15 for the filter (special order based on size/thickness) and +$15 for shipping.

post-87-0-23679200-1566344711.jpg

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eye4invisible

The C47 filter doesn't work like the 729 filter. It works more like the B-410 and more orange than red, like the B-410 does.

Steve, thanks for the response.

 

When you say it doesn't work like the 729 filter, what kind of effect do you get when it's placed in front of the lens on a full spectrum camera? Does the turquoise C47 act like a hot mirror filter, after white balancing? Or does it just give a turquoise tint to the full spectrum image (eg. muddy brown foliage, as opposed to green leaves)?

 

Edit: I looked at the data sheet PDF at the link you posted, but it only covers the transmission between 400nm and 700nm - no data for infrared.

 

There's a vendor on Aliexpress who sells 54mm diameter dichroic glass in various colours @ USD 10.00 for 3 pieces. I'm tempted to order 1 lot in yellow, blue and deep red versions, hence my question here first.

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The reason for my post in this topic was to respond to the question about the dichroic nature of 'lighting filters'. Given the only lighting filter I have experience with is this one Lee dichroic filter.

The only reason I got this filter is because I wanted to find something that might be close to the Lee polyester filter Scuba Blue #729.

Both of those filters are what I call a minus red type filter, but they are not identical as far as what they transmit and don't transmit, they are close,

but the Scuba Blue #729 works better for blocking more of the visual red than the C47 does.

Lee graphs online don't show transmission about 700nm, but their swatch books have graphs that show from 200nm to 800nm, however there dichroic filters don't have any graphs that come with them,

so I am just guessing that they have similar up-to-800nm transmissions to the polyester Scuba Blue.

On a full spectrum camera they suppress visual red, so the sensor RED is only exposed to IR above 700nm, and any extended upper IR is then attenuated by stacking with KG3,

this leaves the lower part of the IR range which the sensor RED is a bit more sensitive to than GREEN and BLUE (color sensitivity in IR evens out more at about 850nm)...

The colors are compared here:

https://www.ultravio...__fromsearch__1

 

However, none of that has much to do with your questions about the dichroic nature of lighting filters, and I am not familiar with the exact lighting filters you are asking about.

So my main idea was to show that the lighting filter I have has a very strong angle of view color shift.

I don't really see it in the example shots I made comparing it to the 729 stack, but I wasn't really trying to test that at the time.

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eye4invisible
Thanks again, Steve. I'm going to order some pieces of various colours and see what results I can get (with and without stacking other filters).
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Good idea. :-)

I don't think it is about stacking or not. The color shift (center to sides) of that particular dichroic filter doesn't show up in the example I show in the link above.

However, I don't know off hand what focal length I shot that pic at.

I will see if I can find that info...

 

OK, it looks like I shot those at 26mm, using 18-55mm VR.

https://www.ultravio...__fromsearch__1

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