Black

It seems like ‘black’ was an important topic of much of my early technical career. It all began with a story told us by Dr. Seibert Quimby Duntley during my second job out of college as a research engineer at the Scripps Institute of Oceanography, Visibility Lab, in San Diego. Doc Duntley, the director of the lab, had been a research scientist at MIT during WWII doing military camouflage studies. This work continued after the war in California at what came to be known as “The Vis Lab.”

Duntley described the development of the “Black Widow” paint used on night fighter aircraft during the forties. Before the days of radar antiaircraft defense by the Germans consisted of sonic detection with big horns and search lights to spot the overhead aircraft. When pinned by the lights the planes, as Duntley described them, appeared as easily seen bright grey silhouettes – even though painted black. It turned out that the best black paints diffusely reflect at least 5 or 6 percent of the light that illuminates them. That’s enough to make them easily seen and identified. Take the moon for example. We are used to seeing it as a brilliant white object in the night sky. It’s not at all white. The Moon’s average albedo (reflectance) is only 12% and the mare, flat areas, are probably closer to 5 or 6% - like black paint.

To achieve a reflectance of less than one percent a priority wartime project was undertaken. A pharmaceutical firm came up with the solution to the problem – a paint that consisted of tiny exploded carbon particles similar to popcorn suspended in a glossy transparent durable binder. This top secret material was sent to England in a container locked to a courier’s wrist for trials as a coating on the North American P-61 night fighter, the Black Widow. It was highly successful. Even though the glossy binder gave a substantial specular reflectance there was practically no diffuse reflectance. As seen from the ground. Only a few sparkles might appear and identification as an aircraft was exceedingly difficult.

Cmdr. Dayton R. E. Brown, a mentor of mine and the Navy’s camouflage expert during the war, tells the story of designing the paint for aircraft in the pacific. The Navy had already rejected the idea of black – to much like a coffin – and had opted for a deep blue. Admiral John Sidney McCain, ComNavAirPac, was paying a quick visit to an island base in the Pacific during a period that coincided with Brown’s presence at the base. Brown wanted to sell his idea for painting Navy planes more effectively and tried to get the Admiral to give him some time. The Admiral was in a hurry so Dayton volunteered to paint a plane at the side of the runway with a fire hose while the Admiral’s plane was waiting to depart. McCain agreed and Dayton Brown succeeded in selling his paint scheme to the Navy. It wasn’t black but it was a good camouflage story.

Eliminating stray light inside of optical instruments is always a prime concern in the design process. Various techniques have been used in cameras, telescopes and the like. Usually it’s just black paint which in most cases serves well. One technique for absorbing light that is little used but is very effective is a stack of razor blades seen edge on. If they are brand new and have never been handled the edges are very sharp and the incident light vanishes down the interstitial cavities between the individual blades. It's an interesting technique but hardly suitable for coating the entire inside of an instrument. One needs a hundred or more to make a small black trap.  Over the years we came to use a 3M product called Velvet Black. It may no longer be available but it had a diffuse reflectance of about 2 or 3 percent. It produced a very matte surface and was fairly delicate so it would not weather well if used on the exterior surfaces. 

Our major efforts at instrument design were directed to the measurement of "meteorological range." Roughly, this is defined as the maximum distance at which one could detect a small  dark object seen against the horizon sky. There are exact mathematical formulas for this distance and any reader who wishes to pursue it can look up the article referenced at the end of this blog.

In particular it found a good home inside the Meteorological Range Meter we installed on the aircraft pictured below which required a phototube to measure the scattering of light by particles and air molecules of a one meter column of air at an altitude of 35,000 feet. The photo below shows the MR Meter installed on the upper fuselage of the B-29 that the Air Force assigned to the Vis Lab for research purposes.

The most interesting and “colorful” use of black surfaces, however, was the employment of a very large amount of heavy black velvet cloth in our research station at Point Barrow, Alaska. The station consisted of a well-insulated cubicle facility containing all the electronic and mechanical machinery along with a stool, a desk, and a coffee pot for the attending scientist. This facility was located on a low platform well out on the tundra away from the base and village itself. The MR Meter machinery, optics, and recording devices were safely enclosed in the cubical. Out on the distant tundra were two black cavities – the more distant a ten foot cube –and the nearer one a three foot cube. They were arranged so that they both appeared the same size to the meter’s optics. During recording the telescope was electrically driven to view one after the other of these cavities and then the horizon just above them. If the reader is interested in the details of these experiments and the theory behind the measurements see the reference below.

The scale of the distant black cavity is evident in this shot of the construction process.

The large cavity, complete, and braced for bad weather.

This view below is similar to what the MR Meter saw except for alignment and actual distances.

 

Many Inuit helped with this project, all under the direction of Chester, the village chief. When all was complete Chester instituted the penultimate use of our wonderful black fabric by asking if he could have some of the scraps to decorate his best parka. We, of course, gave him all he wanted. One of my prized possessions is a photo of him leading his people in some of their ceremonial dancing at the village hall.

 

John M.  Hood Jr., "A Two Cavity Long-Base Mode Meteorological Range Meter”,  Applied Optics, Vol. 3, P. 603-8, May 1964.