Filtering infrared

[gyro_gearloose]

I'm working on a project involving attaching infrared markers to performers and using their movement to create visuals which are then projected back over the performers. For example the most basic effect is to simply project a blob of light so that it lines up with the markers. That way the blob(s) will always illuminate wherever the markers are. Another example is using the performers position to create a water ripples or smoke trails as the performers move about on stage.

 

I've got the hardware and software side sorted out and so far I've been testing it in my front room. In the next week or so, I'd like to bring everything into work and try it out on our stage. My ultimate aim is to use this effect at some point during our pantomime this year, but I need to create a short video or take a few photographs showing everything in action.

 

This is where I think I will have problems. The markers I'm using are just some cheap torches which I've modified to emit infrared light only, and have made hemispherical diffusers to make the infrared light visible no matter how the torches are held. I posted a photo of a diffuser in the recent thread on 3D printing. The problem is that every camera I've ever seen will detect infrared energy to some degree or other, but if the markers are visible in my photos/videos that will spoil the effect somewhat.

 

How do I go about filtering out the infrared light? I tried Googling, but all I could find was information on how to filter everything except infrared!!

[alistermorton]

Use a film camera? Digital sensors are pretty sensitive to IR, but good old fashioned film less so.

[Brian]

The astronomy guys use quite a lot of filters.

 

IR/UV cut filter.

 

 

[E2A]

'IR cut filter' seems to be a good search phrase

[gyro_gearloose]

£43 for a filter? Thanks, but thats a bit steep.

[Dmills]

Have a flick through the swatch book paying attention to the transmission curves, you want to find something that stomps everything below about 750nm or so.

 

Heatshield possibly (I am unsure where the cutoff is)?

 

Regards, Dan.

[Brian]

£43 for a filter? Thanks, but thats a bit steep.

Have a look on eBay, they are some there for £3.99 but only a 10mm x 10mm but that might be enough depending on the sensor size of the camera.

[bigclive]

Most decent camera's will filter out most IR light. It's just the cheapies that don't filter it out effectively as a means of making the cameras "more sensitive". Have you considered the MOCAP technique as an alternative to the individual IR sources? You would have an IR sensitive camera for position monitoring, which would have an infra red ring-light round the lens. The markers would be small plastic balls with strips of reflective tape applied round them to create a reflective sphere. The IR camera would then see bright IR dots reflected back at it. The reflectors wouldn't be very visible to the audience unless they were very close to the visible light projection source.

[Dmills]

The markers would be small plastic balls with strips of reflective tape applied

 

I think that should read retroreflective, but yea, that works.

 

A trick is to gen lock the IR camera and pulse the IR LED ring synchronised to the camera 'shutter', then you can shoot your normal video with a half field delay line in its sync input and see nothing of the IR because it is switched off when the visable light camera shutter is open, possibly more hacking then you really want to get involved with...

 

Regards, Dan.

[gyro_gearloose]

I had considered doing that, but the method I'm using now allows for the performers to turn their lights on and off individually if they wish. Not sure how you could do that with passive IR markers. Passive markers only reflect IR back along the light sources axis, which would mean that I could film/photograph the stage without needing IR cut filters. However I have found that the camera I'm using appears to be very sensitive to IR and any IR light pointed at the camera tends to produce quite a bright flare which covers a good portion of the image, even from about 50 feet away - hence the reason for the hemispherical diffuser that I made. Having an IR light reflected back at my camera by some reflective markers might end up washing out the image completely.

[alistermorton]

£43 for a filter? Thanks, but thats a bit steep.

 

If it's a decent filter, that's not expensive.

[musht]

http://www.edmundoptics.com/optics/optical-filters/shortpass-edge-filters/ir-cut-off-filters/1328

[birt]

to light your performers; use a normal tungsten spotlight with a stack RGB filters in front of it. lighting filters are designed to let the heat go through so you block all the visible light but the IR will light the stage.

for your camera get a roll of slide film and get it developed as it is. (so it has never seen any light, just straight out of the box) the resulting black film is a cheap IR filter for your camera. depending on what you want to do you may have to stack 2 of them. (I have learned this from Ole Kristensen who works at Recoil Performance Group in Copenhagen)

[dbini]

to light your performers; use a normal tungsten spotlight with a stack RGB filters in front of it. lighting filters are designed to let the heat go through so you block all the visible light but the IR will light the stage.

for your camera get a roll of slide film and get it developed as it is. (so it has never seen any light, just straight out of the box) the resulting black film is a cheap IR filter for your camera. depending on what you want to do you may have to stack 2 of them. (I have learned this from Ole Kristensen who works at Recoil Performance Group in Copenhagen)

 

OP wants to do the opposite - filter out the IR, rather than filter out everything in the visible spectrum - which is what you're suggesting.

 

you need an IR cut filter - sometimes called a hot mirror. and they're relatively expensive. (compared to a stack of LX filters or unexposed slide film) like this: http://www.ebay.co.uk/itm/OPTICAL-HOT-MIRROR-1-DIAMETER-COATED-INFRARED-LASER-OPTICS-/261270291691?pt=LH_DefaultDomain_0&hash=item3cd4ec44eb or http://www.ebay.co.uk/itm/IR-UV-Cut-Hot-Mirror-for-Olympus-E-PL1-E-PL2-E-PM1-Infrared-filter-infra-red-/190892569479?pt=Camera_Filters&hash=item2c72157387 - maybe you could message the seller of this one - they seem to know what they're talking about - removing filters from Olympus cameras......

[birt]

sorry, I misread this.

 

how far can you go with a blue-ish filter (cut red and infrared to some degree) and later on correct the colors in photoshop?

 

or just fake your photo's and don't use torches. I know it's cheating but if you show a model to a director you're not going to use the exact same materials and techniques as you would with the full size stage version either.

[Superpants]

This is what the military use to shield lighting in vehicles to stop it flaring in the night vision goggles, It won't be cheap, but perhaps you'd be able to get a sample?

 

Korry