2.0 What is a Chrome Ball?
This post will discuss the science behind why the chrome ball works for visual effects
What exactly is a chrome ball, and how can you use it?
A chrome ball, or mirrored ball, is a perfectly round sphere that reflects all the surrounding light. It is used in visual effects to capture lighting information from a set.
A professional chrome ball will perfectly reflect light and be spherical in shape. Chrome balls used on sets with many of visual effects can get quite pricey. Since this was my thesis project and I was a broke college student I had to find an alternative, cheaper ball. I purchased a garden sphere and was unsatisfied with the warping and distortions seen in the reflection. The second ball I bought had fewer scratches but still wasn't ideal for capturing good lighting information. Fortunately, I had a mechanical engineering friend who was willing to help me polish the ball for a better reflection!
The second ball highlights a lot of the scratches on the surface. Note how the sun flare spreads out through the scratches. This can be seen in the first ball as well, but not just in the highlights. It is extremely difficult to see the warping on each ball, because I was trying to hide it. The first ball had a significant amount of warping on the surface, it was after all a garden sphere. The second ball has only one major warp in the sphere around the equator of the ball. For shooting purposes the ball is placed on this equator to hide the warping and produce a minimal affect on the lighting information.
Polishing the ball in shop
More reflectivity and an overall smoother surface was achieved after polishing the ball. The warping around the equator was even reduced!
The next challenge was to figure out a way to mount the ball for shooting purposes while minimizing its impact on reflections and lighting information. Some professional chrome balls are mounted on a monopod or a similar device. I initially considered using magnets but struggled to find a powerful enough magnet that would conform to the ball's shape. Instead, I placed the ball on a tripod – not the best method for capturing accurate lighting profiles, but a compromise given the ball's imperfections.
I also considered measuring the ball's reflectivity in an Imaging Science lab; but due to time constraints this was something I would revisit later if there was any spare time. This is a recommended step to take if you have the equipment. Most of my time was taken up on the photogrammetry section of my thesis.
If I had measured the efficiency of reflectivity on the ball with a goniophotometer, I could compensate for it later in post to get a more accurate representation of the actual light on set.
A little ‘light’ background
Light can be measured and there are several equations that represent this. Light irradiates* from a source and travels to the set, interacts with objects and reflects into the camera. Objects have different attributes that absorb, reflect, or diffuses the light.* If you could place an object that could reflect 100% of the light from every direction and capture it; then you could have a fairly accurate representation of how light would interact with a 3D model at that particular spot. This is where the chrome ball comes into play! Unfortunately the sphere does not capture a full 360 degrees* it actually measures roughly 270 degrees (Bourke, 1999). I wanted to test this.
I placed a green object directly behind the ball, and unwrapped the ball to test whether or not the object would be seen. As you can clearly see, there is no green object in the unwrapped image. Also take notice of the color chart. The part seen behind the ball does not appear in the unwrapped image. Well, part of it does, but it gets pinched into a very tiny row of pixels.*
This is why it is recommended to take two separate images from different angles measuring approximately 90 degrees apart.
How Is This Unwrapped Image Useful?
Radiance is a term used to describe the power of light per solid angle per area. Through the radiance invariance principal it can be proven that radiance has the same value at any point along a ray in the absence of losses from scattering, absorption, or reflection. Basically this principal states that distance is no longer a factor in the measurement of light. This allows the camera to capture the exact values onto the mirror ball.
This information can be used in postproduction to provide lighting information at an exact point in the scene; and also be used as a reflection map, which can be useful for shiny or reflective objects.
Distance is no longer a factor in the measurement of light.
This was the key piece of information that was missing from most guides at the time of research. Why do chrome balls work? Light is a huge topic of study for my program, but it wasn’t until I sat down with my radiometry professor that it started to make sense. It was one of those things that was right in front of you, but never stated! Once you see it, it clicks and everything suddenly starts to make sense!
By measuring radiance, distance no longer has an affect on the measurements, like irradiance does with the inversely squared relationship it has with distance? Light intensity falls off exponentially as distance increases. (Inverse Square Law) If you double the distance, the intensity of the light is 1/4 the intensity it was at the original distance. For the longest time I was getting confused by luminance and illuminance and how they relate to radiance and irradiance. Have I lost you yet with terminology?
To avoid confusion, luminance and illuminance will not be discussed.
“Radiance is energy emitted from, or reflected from a specified point
Irradiance is the total power incident on an area. It varies as a function of distance.”— IENTILUCCI, E. (2015). LECTURES. LONG, D. (2012-2016). LECTURES.
For my particular thesis project I used the ball on set where I would place my 3D model in the scene. I shot night scenes and day scenes, in the studio, and on location at various times throughout the year.
The next post will go over How to shoot a chrome ball.
Notes
Yes, there are more attributes such as thermal transfer, but that was not the scope of this project
Irradiates is not a typo, it actually has a very specific definition in radiometry. This was part of the thesis paper that was not included in this online format to avoid confusion, however, irradiates is the proper term in this case.
A sphere is not actually 360 degrees since it is a 3D circle.
A Sphere is measured in steradians, which is degrees squared.
The tiny chrome ball was loaned to me for the duration of the year by my thesis advisor. Thanks Mark!
I purchased my chrome ball from CB2, which they no longer offer. It is a 10 inch S/S sphere. SKU 641-804
Keep an eye out on their products as they may bring it back
In my research I came across discussion on gathering lighting information from a glass ball
I do not have experience with this or how it would work for a 3D pipeline
This may require some investigating, which I do own a Lensball now and can play around with this
Lensball makes a suction cup to tripod mount that works fairly well as a mount on the chrome ball, however I do wish it was bigger to properly hold this larger chrome sphere
