‘Reverse’ IBL

I watched ‘Beyond the Black Rainbow’ last weekend and put together this test video,

This weekend’s tests were all about getting brainstorming on ways to improve Cambot and doing a bit of shooting in the studio. The studio time definitely helped to see what is working on Cambot and where it can use some improvements. The goal of my shoot was to experiment with lights.

I’ve spent some time before working with traditional Image based lighting, where I basically used a reference image from a location to digitally light a 3D model.

The reference image is typically a panoramic image unwrapped from a photo of a chrome sphere. The chrome sphere reflects a full 180 degree image of the reflection and in doing so the reflected light which would be cast upon the object. Typically a gray sphere is photographed at the same time for comparison with the in-progress/finished model.

I want to begin the same way, background locations photographed with reference spheres, then I want to photograph practical scale models to place in them. Typically this is achieved by attempting to mimick the lighting conditions, i.e. “the sun was here so we place a key light here, with a fill light on this side and…”. Then through a process of ‘match-lighting’ a cinematographer/Director of photography can reproduce the location’s lighting.

Why ‘Reverse’?
Now I want to turn this all around. In theory by using these reference images it should be possible to recreate the environment lighting on demand when photographing a practical model or actor. All that would need to happen is for a directional light source to project onto the surface of the model with the same hue and intensity as the reference.

This definitely isn’t a new idea, Paul Debevec, developed this years ago through ICT with his lightstage, pictured above, which I’ve linked here rather than attempting to further summarize it;

My approach is through the use of DMX stage lighting. There are multi colored lights capable of mixing Red Blue and Green in real time. These are also programmable via DMX512 protocol, so they can be set up to run through pre-set lighting configurations.

My Stage
I’ve been using these slimpar 64 RGB LED lights from Chauvet. I’ve currently arranged five of these in a half ring around my stage all pointing inward.

I plan to eventually upgrade this to a more automatic solution. There are a lot of software packages designed for stage techs, in fact many concerts and night clubs use these systems. There is also computer soft/hardware solutions more designed for filmmakers and animators like this card for Kuper which fits in with a Kuper motion control system. Or the DDMX-S2 from Dragonframe, which allows stop-motion playback control for incremental programs. For now I’m controlling these via the Chauvet Obey-10 mixing board, which allows me to set sliders for each of the color channels of the lights independently.

What I’d really like is for it to be able to process a ref image or video and reproduce it automatically, or use a video clip and essentially ‘play it back’. It makes sense that through software I could take a reference image and sample the quadrant’s hue and value, and route that into a DMX controller, then those values could be used for control of the lights.

The Science
The theory sounds great, but first I have to figure out the physical lighting limitations of this rig and about LEDs in general. I’ve often been warned about color temperature in photography. The difference between Tungsten (3200K), Daylight (5700K), and Fluorescent (4000k). However in attempting to get a clear answer to the temperature of LEDs I went down a rabbit hole. It seems this all goes out the window the moment you start color mixing. It is completely variable, which means it could be anywhere. Added to this LEDs typically have a more limited spectrum, take a look at

these graphs;

LEDs are often assigned a CRI which as I understand it, is how well they can reproduce the sun’s light, and thus how balanced a color will appear when illuminated by it. The other thing I’ve discovered about LEDs is the pulse width of the lights themselves. They aren’t actually constantly on, the light blinks on and off at arate so fast we can’t detect it. For many lights this is slowed down for the dimming feature/effect. This pulse width modulation which our naked eye cannot detect, even at lower frequencies will be detected by the camera when set to a high shutter speed.

I found it really interesting in this test to see the way the light’s wavelengths interacted with the shutter speed of the camera. It seems there are ways to work around it, selecting a lower shutter speed for example, but I haven’t quite figured out the science for it. Looking over forums shooting around PWM seems to be an increasing problem, especially for venue/location photographers;
Shutterspeed and flickering hmis
PWM is not your friend
LED flicker on camera