Lumafield takes a look inside Christmas tree lights with industrial CT scanning

Jon Bruner, former Product Lead and current Head of Marketing at Lumafield, posted an interesting, festive themed metrology thread on X (formerly Twitter) on December 14. Using Lumafield’s technology, Christmas tree lights were CT scanned and the team found that the insides were more complex and intricate than they initially seemed, and were able to showcase the capabilities of the CT scanning systems.

The first light used was a standard incandescent bulb, which has a tiny tungsten filament inside, similar to a regular bulb, but the CT scan also revealed something else in the image, an extra wire below the filament. Bruner commented that this solves a fundamental problem with Christmas lights: “They need to be wired in series to divide household voltage down to run across thin filaments in tiny bulbs, so a single burned-out bulb would darken an entire string.”

The loop is a shunt that allows current to bypass the filament when it is burned out according to Bruner. It has a thin insulating coating that burns away when the filament breaks. The shunt’s wire is 0.07mm in diameter (around 41 AWG), and is lower resistance than the filament.

The shunt wire is initially coated with an insulating substance, and if a bulb’s filament burns out and the shunt melts, turning it into a conductor, keeping the circuit closed, and the voltage across the rest of the bulbs increases slightly. This makes the other bulbs brighter, but also makes them burn out quicker.

Christmas tree lights often come with a replacement bulb that is tinted red. The Lumafield team also took a CT scan of this bulb and delved into how it works.

Bruner explained that if you replace one bulb in the string of lights with the red replacement bulb, the entire string blinks, and the red bulb uses a simple and entirely mechanical design to do this.

The CT scan showed that inside the blinker bulb is a bimetal leaf. When a current flows through it, the leaf heats up, and as it is made of two different metals with different thermal expansion properties, as it heats it bends.

Bruner explained that when the leaf bends, it breaks the circuit and begins to cool. As it cools, it returns to its original shape and closes the circuit again. As seen in the scans, a tiny spring keeps the leaf connected to the hot leg of the circuit, and this cycle repeats at a consistent rate.

Lumafield also CT scanned a more modern LED holiday light bulb, which has a unique lens shape that helps distribute the LED’s light uniformly to produce a twinkle effect. In the scans of this LED light, Lumafield industrial CT scanning let the team filter by material density. The plastic housing was removed and the metal internals were isolated.

Doing this makes visible the top of the cathode where the reflective cavity lies, which is where the semiconductor that generates light lives. This is connected to the anode and cathode by two tiny lead wires.

LED lights offer an energy-efficient alternative to incandescent lights, and can be programmed to create different types of displays. LED stands for Light Emitting Diode, a semiconductor device, and is designed to emit light when an electric current passes through it.

The Lumafield team also scanned a bubble light, which is a type of light typically consisting of a glass tube filled with liquid containing a small amount of methylene chloride, which has a low boiling point according to the company. 

When the light is turned on, an incandescent bulb below the tube generates both light and heat, causing the liquid to boil and produce bubbles. The vapour inside the bubbles is illuminated by the bulb, producing a warm glow.

Of course, none of the lights would work, or be safe without the fused AC plug. With the incandescent bulb, the shunted design could lead to a serious cascading failure according to Lumafield. As the blues start to burn out and the remaining bulbs burn out faster due to carrying more current at a higher voltage, the failure process is accelerate and the amount of power flowing through the string is rapidly increased.

The fused AC plug prevents this failure, and keeps Christmas trees from setting ablaze.

The full thread from Bruner can be found below, and the scans can be explored here.