Let There Be Dark
June 7th, 2009
Images (c) Dilip Krishnan & Rob Fergus
I was reading through the proceedings of this year’s SIGGRAPH (Special Interest Group on GRAPHics and Interactive Techniques) conference - as I’m sure we have all done at some point in our lives - when I came across a very interesting paper.
The conference generally concerns computer graphics and image analysis however the following title has a very practical application in photography: Dark Flash Photography. No we’re not talking about Joey Lawrence’s promotional images for the film Twilight, but instead a way to take flash photographs without people objecting, or even noticing!
The gist is that you can ‘hide’ the pulse of light outside the visible spectrum thus making it appear as much as 200 times dimmer than a regular flash. This requires an IR enabled camera and suitably modified flashgun. The use of flash for photography can range from an mild annoyance to a dangerous distraction in certain circumstances, which leads to the use of shutter speeds far slower than would be ideal. The dark flash solves this problem but creates a new one.
left to right, IR image, natural light image, combined image, long exposure natural light (for reference)
Capturing light in the IR part of the spectrum doesn’t allow natural looking colours to be recorded. Dilip Krishnan and Rob Fergus solved this problem by taking two exposures, one with the dark flash and one high ISO natural light photo immediately after it. The clever part is that the dark flash image can be used to guide to de-noise the high ISO image far more effectively that any other noise reduction technique. Simply put if you see an edge or spike in the natural light image and there isn’t a corresponding edge in the IR image then the point is probably the result of noise. The actual process is a little more involved, I’d recommend anyone technically minded to have a look at the original paper.
The result is a clean image with natural looking colours. The post processing required is complex, and there times when the approach wont work well if the subject moves between the two exposures (shooting fast paced sports for example). However if you have an IR enabled camera and like black and white images I can see this being a very valuable technique when in many circumstances, and indeed the only real solution to the problem of dazzling people with flash.
MG
June 8th, 2009 at 4:39 am
SIGGRAPH - yeah, we all read those conference proceedings! And attend the showing of every short film, too.
Thanks for posting this over on Strobist!
June 8th, 2009 at 9:17 am
This is such an incredibly stupid idea.
IR photography is used for forensics as is UV. It shows up details in the sub dermal (or whatever it is) layers of the skins. Bruises, and other exciting injuries. UV produces some really interesting portraits, but none are ‘everyday’ portraits. It shows up veins!
However… this brings us to point two. In order for it work practically you would need a modified sensor as the man says (expensive but doable) so that means for the second high ISO image you would need a separate camera.
Its fine if you have the man above (who looks very suspicious!) who stands still and doesnt move and you trigger both cameras together… but when would you want to use an IR flash? When you don’t want to use a real flash! So that would be events when you want to be discreet. Umm… those types of events do not have people standing still so you can take the same photo twice.
June 8th, 2009 at 9:20 am
Just had a read of their PDF. I disregard my previous comment based on my initial amusement
To be fair there system is clever. But not exactly accessible to anyone else without a modified camera
June 8th, 2009 at 11:12 pm
Their technique is mostly of theoretical interest to me, especially the concept of exploiting the [usually good] correlations between the colour channels (the existence of which makes Sigma’s foveon sensor fundamentally flawed in my eyes)
However I like the idea of shooting in IR with flash, although it has been pointed out to me that this has been done by other photographers already.