Retroreflectors for remote detection

A method of detecting a chemical comprising:

• distributing an array of physically unconnected retroreflective elements over a region to map across the array the presence or absence of a chemical in an environment, each retroreflective element including a material having a chemical composition that changes its chemistry when exposed to the chemical in the environment, the material being subject to a change in light absorption of the material, on exposure to the chemical, that results in a change in transmission of incident radiation through the material having the chemical composition;
• illuminating the array of retroreflective elements with radiation at multiple values of an optical property of the radiation;
• and
• sensing radiation retroreflected from the retroreflective elements.

A detection device comprising:

• an array of physically unconnected retroreflective elements distributed over a region to map across the array the presence or absence of a chemical in an environment, each retroreflective element including:
• a retroreflector that can retroreflect light from any direction;
• and
• a layer of material over the retroreflector, the material having a chemical composition that changes its chemistry when exposed to the chemical in the environment, the material being subject to a change in light absorption of the material, on exposure to the chemical, that results in a change in transmission of incident radiation through the material having the chemical composition, the retroreflector and layer being exposed to light in all directions.