(This article is a supporting part of our ongoing testing of low-light camera metering reliability)
An integrating sphere is used for measuring emission or detection of light. It is a sphere coated on the inside with diffuse white paint, and this scatters light uniformly and thereby eliminates directionality effects. To state simply, an integrating sphere has very even internal illumination, and in the case of this project, it doesn’t matter how a camera is pointed inside of it.
A vacuum-rated integrating sphere useful in cryogenic temperatures (which I used extensively in grad school for characterizing infrared detectors) costs tens of thousands of dollars. If you are alright with ambient temperature and normal air pressures, an integrating sphere can be 3D printed for a few hundred dollars. I could have done that, but part of the fun of this project was to do everything as cheaply as possible. I decided to papier mâché one.
I bought a beach ball at Walmart for $2 and collected the Penny Saver newsprint coupons that came in the mail for a few weeks. I read that flour and water papier mâché molds, so I used wood glue.
I did the first layer with white paper to make it easier to paint. The rest used newsprint.
After three couple-hour papier mâché sessions, separated by a day or two of drying, I had a newfound respect for just 3D printing these. Oh well.
Finally, I cut it in half and removed the beach ball. I cut a hole at one end for the lens.
I painted the inside with flat ultrawhite house paint and then sanded it to improve the smoothness. This image was after the first sanding but before the second coat.
Most integrating spheres use many LEDs for even illumination, but I only used two in order to achieve the dim lighting conditions.
I put a 1-stop neutral density and ¾ color temperature orange (CTO) gel over each LED to make it dimmer and warmer in color.
The center ring bounces the LED light back, thereby forcing at least two reflections before entering the lens. I later put a sheet of paper over this in order to dim it further.
The initial fit test, before additional painting, and work to make it light-tight.
Integrating sphere in use.
Here’s the Arduino (lower) and my interface board (on top). The Arduino is the Duemilanove I originally bought for my timelapse motion project a decade ago. The knurled knob in the top center of the photo is a variable resistor which controls the voltage to the LEDs. This enabled me to adjust the max brightness level to 0 EV, and then I glued it to avoid further brightness changes. The dual-pin female plug at right is where the LED connects. The 2.5mm connector at bottom connects to the wire for triggering the camera, and the two integrated circuits alongside it are optocouplers for triggering focus and exposure (equivalent to a half-press and full-press of the shutter button). The pushbutton on the left enables me to interrupt the code if I want to go faster. The Arduino provides a USB interface to the user, and the code it runs can be viewed here.
Project price list:
|3 bottles wood glue||$12|
|Flat ultrawhite paint – sample size||$3|
|Black spray paint||$4|
|Opaque black fabric||$4|
|Arduino Duemilanove||$20 if bought new|
|Paper folder (thin plastic for baffles)||$2|
|LEDs||$5 for 30, used 2|
|Optocouplers||$5 for 50, used 2|
|Variable resistor||$4 for 3, used 1|
Integrating Sphere & Camera Metering Test Project
So, How Did You Build an Integrating Sphere, Anyway?
(YOU ARE HERE)