to the plants since this is also used as a fertilizer or magnesium chloride which is more expensive but more soluble so as to reduce the possibility of crystalization from the solution any combination of the salts such as a 1 1 mixture of nacl and kcl may have some advantage in obtaining higher chloride concentrations with reduced danger of crystallization it is even contemplated that carnalite kcl mgcl 2 6h 2 o which is an inexpensive intermediate in the production of kcl sea water or from the dead sea may also be used to provide an optical filter solution of minimum cost at least in that area the ph of the solution should be lower than 5 5 to avoid precipitation of copper hydroxide an optimum ph for many applications is about 4 0 a buffer such as an acetate buffer could be included so as to stabilize the ph it has been found that the optical filter solutions in accordance with the present invention even without the buffer experience a very small ph change even after standing for months in direct sunlight they are also stable and do not change their optical absorption properties even after such extended periods following are a number of examples of optical filter solutions made in accordance with the present invention in all the following examples the final concentration of copper ions was 0 05 m and the chloride concentration was 2 0 n according to the specific application one may use.