A Puzzle

I mentioned that our mothing light had a shield on it in a prior post. Shields limit the spread of light. Good for not blinding humans. Not so good for bringing in the greatest number of moths.

The shield on our lamp is also colorful.

I noticed these colors when we first had the light on this year, the Wednesday prior to the big moth night. A dry run to make sure the equipment was in good working order. Note that when the light is off there are no colors other than the silver of the aluminum shield.

But when I first tried to take a picture the result was this:

Curious. The colors are there. But what are those bands? They were not visible to the naked eye.

And the closer I got to the light, the more bands there were in the image.

The above images were taken with an iPhone 6 Plus. By the time I had retrieved another camera to see if I got the same results, the light had completely warmed up and was so bright the colors were completely washed out.

So on the big moth night I was ready with my Sony RX100 II camera. I chose this camera because Sony makes the camera module in the iPhone.

The results:

As with the iPhone, from a distance no bands. Although the color palette is limited to greens and blues.

But as I got closer ...

... no bands either. no matter how close I got. Hmmm, very curious.

I emailed the images to Dr. Les Cowley, he of the awesome Atmospherics Optics and OPOD sites. I had thought they might be diffraction effects. He had (no surprise!) explanations for the colors.

The colours are produced in two ways

  1.  by micro grooves on the metal shade.  The grooves act as a diffraction grating. 

  2.  interference across the anodised film if aluminium - see http://www.atoptics.co.uk/fz801.htm

Could be both.

You can see the groves on the shield, and that's what made me think diffraction for the colors. But I'd not thought of thin film interference.

As to the bands,

The bands are puzzling.   Any filament lamp powered by AC fluctuates in light intensity at twice the supply frequency.    The bands 'might' be due to the intensity fluctuations if the iPhone camera scans its sensor in a peculiar way.  The test of that would be to image a dim surface illuminated by a fluorescent lamp.

It's an iPhone issue rather than a lamp one!  What's puzzling is that they are so strong and are not produced by another camera.

So time for more experiments in iPhone photography. Stay tuned.