Niels Noordhoek’s weblog

A very simple but quite surprising experiment:

- take a sugar cube to a very dark room
- smash it with a hammer (which is not an easy task in the dark!)
- look at the sugar cube and see what happens

You should be able to see a clear blue flash “inside” the sugar cube.
Though the effect is clearly visible, taking a photo is quite hard.
I used a very light sensitive lense (f/1.8) and the camera at the most sensitive mode (ISO 3200) and the blue light was still in the lower part of the histogram:

Above: hammer and sugar cube in normal light, below the sugar cube in the dark while being smashed with the shutter open for 3 seconds at f/1.8 and at ISO3200. The room was not perfectly dark, so besides the blue glow, you can still see the vice and the remains of the cube.

The effect is called triboluminescence, an apperently not-so-well understood optical phenomenon that occurs when crystals are rubbed. So when you use more force/energy in hitting the sugar, you should get more light. Therfore shooting a sugar cube may give better photos:

Sugar cube shot from the side with an air pistol, indeed producing more blue light than when crushed with a hammer.

A sugar cube, shot from the direction of the camera. The triboluminescent light actually becomes a high speed flash itself. The dark spot is the pellet hitting the sugar.

I got to this subject, because a similar (though yellow) effect appeared in the mirror we shot with the .308 Winchester. However, when I smashed a piece of mirror with a hammer, no optical effect like in the sugar was visible at all. Some scientific papers seem to suggest that triboluminescence occurs in glass and that it has a Boltzmann like spectrum (similar to a glowing hot object). A more appropriate term may be fractoluminescnce in the case of glass: before it shatters, glass breaks at a very high speed, and the energy dissipated in a very small region of the material at the crack tip heats it up, and thermal photons are emitted (as confirmed by Elisabeth Bouchaud).

Light effect in the glass of a mirror. The fast flash has actually captured the bullet tip right before it impacted the mirror. The muzzle is in the center of the orange muzzle flash. The yellow light really seems to originate from the (cracks) in the mirror.

Flash photography lends itself perfectly for 3D photography: the flash ensures that the two photos that make up the anaglyph (stereo image) are taken at exactly the same moment in time. Simply place 2 cameras with the same focal length (preferably identical cameras with identical settings) and point them at the same scene.
By using a simple Matlab script, the red and green+blue channels are mixed to make a 3D image that must be viewed with red-cyan (or red-blue) glasses.
When you first make black and white images of the two photos, you can make a black and white anaglyp. For some situations this is the only solution: take for example the bright red tomato. You simply cannot expect that to work with red-blue glasses (nothing will pass the blue filter).

Chalk in 3D

Exploding orange about 4 milliseconds after impact. The bullet has long gone.

Same scene in black and white.

Tomato anaglyphs only work in black and white.

Some photo’s taken of the .308 caliber (DPMS LR-308, 150 grain Full Metal Jacket) at a muzzle velocity of about 860 m/s (3100 km/hour or 2820 feet per second), roughly 2.5 times the speed of sound. The light source was a home grown air-gap flash, triggered with a simple piezo microphone.

.308 exiting the barrel, photographed via a mirror

The obligatory business card.

Lightbulb shattered by .308

Clear lamps are bullet proof.

.308 tumbling through chalk (backside of bullet towards camera)

Jus de tomate preparation .308

Jus d’orange a la .308

.308 round tumbling after passing through an apple

Note that vaporized tomato, orange and apple juice all are white instead of red, orange and green!

The inner glass tube of the air-gap flash so far was made of Pyrex. The spark has a temperature of about 30,000 degrees Celsius, so the flash gives the pyrex a horrible temperature shock (more than 30,000,000,000 degrees Celcius per second). Pyrex is a good choice, since it has a very low thermal expansion coefficient, causing a minimum of stress in the material when heating up. For the same reason Pyrex is also very popular in kitchens (glass that does not break in an oven) and telecopes (mirrors that need to keep their shape extremely accurately at a range of temperatures, for the Hale telescope they even tried quartz, but gave up and used Pyrex).

Still, after a random number of flashes the Pyrex tube tends to shatter to small pieces.
In comes quartz: it has an even lower thermal expansion coefficient (4e-7 m/mK, about four times lower than that of Pyrex).

To my surprise the quartz tube has a beautiful blue afterglow for several minutes, which reminds me of Cherenkov radiation as seen in nuclear reactors. After some searching I found it is blue phosphorescence, caused by impurities in the quartz, cuased by the UV in the spark. It shows nicely how the spark went over the surface (different every flash).

Upper half shows the flash tube, with the inner tube being made of quartz. The lower half shows the phosphorescence that lasts several minutes after the flash.

Bet you never took the time to look at this happening!

Cork leaves the bottle at about 70 km/h.

Here is a 100% crop of the bubbles.

Click to see 100% crop.

I wanted to see what the flash produces before and during the flash.
While waiting for the trigger the cathode and anode produce a lot of corona and corresponding ultraviolet light.
During the flash a spectrum of oxygen and nitrogen is produced and it seems some copper (green) as well.
The spark also contains a lot of infrared radiation, which causes the red tail in the highlights of some fast moving objects. To my surprise the trace of the spark was very different each flash. It seems Edgerton cut a groove in the inner glass tube to guide the spark.

The gap has 20.000 Volts and a capacitor of 0.05 micro Farads. That equals an energy of 0.5*C*V^2=10 Joules. Since all of that energy is released in about 0.5 microseconds, this means a the flash has a peak power of 20 MegaWatts. Imagine twenty thousand floodlights of 1000 Watts, all illuminating a gap of 20 mm…. That also gives an impression how incredibly short half a microsecond is.