Is there a hot mirror that allows 900nm to pass?

[Christoph]

Does anyone know if this exists? An "inefficient" hot mirror, but one that is not working in the 700-750nm range, but further back, ideally starting at 900nm?

I ask because I think such a filter would be much better for Aerochrome emulations than others. 

 

As you may know, there is the Triple Bandpass Filter from Midwest Optics, which allows IR around 850nm to pass, in addition to red and green. Pretty good for Aerochrome variations, but still not good enough for me.

I recently tested with a DB940nm, which passes the full visible spectrum plus IR 940nm, and had better results.

I thus tried to make a "TB940" by additionally using a specific orange as well as a specific blue filter to get the two peaks at green and red. The result was - SOOC - a lot better than with the triple bandpass.

 

I think you could create the perfect Aerochrome filter. Sure, it wouldn't be SOOC (not possible), but it would be one which creates a SOOC image that can be turned into the perfect authentic Aerochrome image by a simple channel mixer change.

 

Blue (IR) -> Red
Red -> Green
Green -> Blue

 

Hence the question posed above: does anyone know of a hot mirror that allows IR to pass from 900nm, 950nm?

[Christoph]

This is the DB940 plus Orange 16 (Tiffen) plus Slate Blue by Lee Filters. Shot with e Full spectrum Nikon D600

1. SOOC 2. Channel mixer 3. White balanced

 

 

[Fedia]

I'm going through the LEE swatchbook right now. I see how you intend to reproduce the the two spikes of the TB with this gel. Yet I don't really understand why those spikes would matter... (maybe they really do, I just don't see why). Couldn't you leave the whole range from 550nm to 660nm untouched ? I suppose in this situation the TB940 would let in too little IR compared to visible, but you could easily fine tune the amount of visible you need by adding ND gels from the swatchbook as they should not affect the IR.

 

What I think your blue gel actually does here is play as a simple ND filter.

 

So the reciepe would be : orange 550nm + DB940 + a carefully selected ND gel

 

Concerning your question about a way to reduce IR at 940nm, the only thing I can think of is car windows... Basically what you are looking for is a sort of GRB3 but with a very very long tail.

[Christoph]

I have already tried various things and actually came up with this solution because I had used Half Mustard Yellow together with the DB940. This has - like the TB - transmission spikes in green and in red.

Only orange plus DB940 results in too uniform colors, and if the orange is too strong, the color of the sky is washed out as well. The Slate Blue takes out yellow. 

 

This solution would be very good, if not for the following:

1. the sky is still too magenta after white balancing. That was already the problem with the TB. How to improve that is beyond me. So before the channel mixer, the sky color would have to be more of a yellowish green, which would mean letting more yellow through again. Hmm...

2. My DB940 is diameter-wise to small and will give strange edges in combination with other filters, especially Tiffen 16, so I'm looking for a "cheaper" filter solution.

 

I like the ND idea though and will certainly try it out.

[colinbm]

Nice work Christoph.
With the last photo, could you white balance on the brightest cloud or the brightest road marking ?

[Christoph]

On whitest spot on the cloud--- sky is too magenta

[colinbm]

OK, that didn't get all the colour out of the cloud, but the road stripe is whiter.

[Christoph]

The contrast/saturation in the sky is pretty good with this combination. The problem really is the magenta coloration... 

[colinbm]

32 minutes ago, Christoph said:

The contrast/saturation in the sky is pretty good with this combination. The problem really is the magenta coloration... 

With the Sigma cameras in SPP with the Colour Adjustment Tool, if the cloud didn't WB to give a nice sky, I used to click around the WB tool to reduce the magenta or other cast ?
Mover the colour clicker into the green area ?

[Christoph]

Yes, I do that too. But that's unsatisfactory, isn't it?

[colinbm]

I made an edit to my last comment.
I find it very satisfactory.

[Christoph]

Yes, but then you have more cyan in the neutral tones. It's partly the same with Aerochrome - it just bothers me.

[Fedia]

11 hours ago, Christoph said:

the sky color would have to be more of a yellowish green, which would mean letting more yellow through again

As you said the problem of the magenta sky is probably due to the fact that the red channel of the camera isn't getting enough of yellow and orange light. Most of what it captures is deep red, resulting in darker skies in this channel.

 

Replacing the blue gel with a flat ND would solve this.

 

A photo from yesterday with a simple setup : orange filter + DB850 on my canon 1200D. The color of the sky was very satifactory on all the pictures.

 

 

 

[Christoph]

Does not work for me, unfortunately. With increasing ND strength the blue and later red tones become more and more uniform