Reflecting on times past

CUSTOMERS could, Henry Ford once quipped, have a car in any colour they wanted so long as it was black. In the end consumer choice got the better of him; cars now come in all manner of complexions. Black, nevertheless, remains popular. Some 17% of new cars sold around the world are black, second only to white, at 37%. White is favoured in hot countries because, along with silver, it is good at reflecting light—including the infra-red wavelengths that carry most of the sun’s heat. Black, grey and other dark colours, in contrast, absorb light, thereby warming cars painted in those hues.

Despite Ford’s equivocations, none of this has, until recently, presented carmakers with much of a problem. That is changing because, besides absorbing sunlight, dark shades also tend to absorb much of the signal transmitted from the increasing number of sensors being fitted to vehicles.

Radar sensors are used to operate safety systems such as automatic braking. They transmit radio waves and measure the time it takes for those waves to bounce back, and any changes in their frequency. From this it is possible to determine the range, position and velocity of objects around a vehicle. If some or all of the signal is absorbed instead of being reflected, though, radar sensors can miss things.

The same is true of lidar, which is similar to radar except that it employs infra-red laser light instead of radio waves. At present lidar is big and bulky (the blob-shaped roof racks on self-driving cars are lidar sets) but miniature versions are being developed. Though self-driving cars have suffered setbacks recently (in March a pedestrian was killed in Arizona by an Uber test vehicle fitted with lidar and a Tesla running on radar-assisted Autopilot crashed and killed its driver in California), both radar and lidar are likely to be used more widely as cars become more automated.

To improve the reliability of sensors it would help if all vehicles were painted in colours that are good at reflecting a wide range of wavelengths. But that is not going to happen, for the same reason Ford had eventually to introduce shades other than black—namely that colour is an essential part of vehicle design and marketing. But PPG, a firm in Pittsburgh that is one of the world’s biggest suppliers of paints and coatings, believes it has an answer. Its researchers are tampering with paint at a molecular level so that whatever colour a coat of it appears to be to the human eye, it will still be highly reflective to the signals from a car’s sensors.

Painting cars has become a high-tech process. Painted surfaces are now so good that cars barely rust as they once did. Arriving at this perfection involves the extensive cleaning and preparation of the surfaces in question, and the spraying of several layers of different formulations of paint in precise ways, usually by robots. As the layers dry, chemical reactions can change the size and spacing of pigments within them to produce such effects as vibrant colours or deep, rich tints. It is this sophistication that has allowed PPG’s paint technologists to make dark colours reflective to the signals from sensors.

The clue as to how to do this came in the form of an aubergine (or eggplant, as it is known in America). The aerospace division of PPG had already solved a different problem—that of keeping aircraft painted in dark colours cool—by basing the way they paint these aircraft on the dark-purple skin of aubergines. Instead of absorbing infra-red radiation, an aubergine’s skin permits such wavelengths to pass through. They are then reflected back out again by the vegetable’s white interior flesh. That way, an aubergine in a sunny field remains cool. By dispersing specially engineered pigments in a dark-coloured surface layer over the top of a reflective white underlayer, PPG was able to achieve the same thing for aircraft paint.

And, as David Bem, PPG’s chief technology officer, observes, what works for solar heat also works for the sensor radiation bounced back from cars. This permits a similar approach to be taken to the painting of vehicles in dark colours. Dr Bem also reckons that a reverse approach could be used to tone down, in relevant frequencies, road signs that are designed to be super bright to people but thus risk blinding lidars. In such cases, Dr Bem says, it is possible to engineer the pigments and layers of paint in ways that retain the brightness for human eyes but tone down the dazzling effect on artificial ones.