Light painting and the inverse square law

Author Muddy Funkster - Last updated: 27.09.2012

I tried a new technique for lighting a very dark space recently and thought it would make a good tutorial for those who wanted to try something like it.
When lighting gloomy or black environments I use a powerful LED light source with a very wide spread of light, I also really like the backlit look and ran into a few problems when trying to shoot one of the wind tunnels at Pyestock in this way a few months ago. On my last visit I returned to the same spot to try this new technique out.

My problem was I tried to back light the entire wind tunnel on a single set of brackets, resulting in lots of detail close to the light source but rapidly fading off a short distance down the tunnel. This is caused by ‘the inverse square law of light’ which goes something along these lines.
The intensity of a point light source is inversely proportional to the square of the distance from that light source. So if you stand 1m away from that light source its intensity will be ‘X’ and if you stand 2m away then the intensity is 4 x less intense than it was at 1m (2m squared). At 3 meters the intensity will be 9 x less intense that at 1m (3m squared) and so on.

The first attempt. The drop off in intensity can be seen fading almost to black.

Image

I could have put it on timer and sprinted down to stand behind the light as I shot this and with the light on spot, light painted in the tube giving the parts at the camera end more time to counteract this issue, but that would mean moving the light source about, creating undesired moving shadows.

On the recent trip I ended up using a similar technique but instead moved the light source for each set of new brackets until I had the entire tunnel illuminated well. These are the shot brackets.

Image

Once I had the brackets shot I ran each of them through Photomatix to get 5 HDR images of the tube lit from different places. I found an old pipe which I stood on end and placed the light source on top of and with each new set of brackets moved it another set distance down the tube. Creating these 5 images.

Image

From here it was a case of putting all 5 images into layers in Photoshop with the shot where the light source is closest to the camera on the top layer decending to the shot with the light source furthest away at the bottom. Then using a combination of a soft edged eraser and some radial gradient masks to cut away the dark part that shows up just behind the light source. This allows the layer beneath to show through, which has that missing bit of the tunnel exposed correctly on it. Care needs to be taken to get the blend between the two layers right. Repeat this for all layers until you have the entire scene lit correctly throughtout. My final shot was of the end of the tunnel with the light source facing away from the camera. When this was blended into the final shot it allowed me to remove any trace of flare that the light was creating in the back lit shots.

The finished result.

Image

I may add some more detail to this regarding the photoshop part but for now I’m off to bed.

4 comments

#1oldskoolSeptember 27, 2012, 6:27 pm

;-) like it cheers

#2AustadSeptember 27, 2012, 6:44 pm

This is real great! Awsome shit you are doing here. Fine picture too..
Love it!

#3MuddyOctober 1, 2012, 3:42 pm

Cheers Guys ;)

#4PlazzmiKOctober 8, 2012, 7:22 am

Great read! Haven’t heard from this way of working before. Thanks for sharing!

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