Selective surface
In solar thermal collectors, a selective surface is chosen based on the ratio of radiation-absorption (alpha) to radiation-emission (epsilon), with a higher ratio meaning better performance. A black chromium nickel-plated copper ("black chrome") panel is very durable, highly resistant to humidity or oxidizing atmospheres and extreme temperatures while being able to retain it's selective properties - but expensive. Ordinary steel or aluminum plate can be spray-painted black for a cheaper (but less effective) collector plate.