Hard vs Soft Optical Coatings: The Choice

Quote a custom filter and the first real engineering question will not be about wavelength. It will be: hard coating or soft? The answer costs cents per piece and decides whether the part survives its first year in the field.

What "hard" actually means

Hard coatings are deposited with ion assist — bombarding the growing film so it densifies as it lands. The result adheres to glass like a ceramic skin: it survives alcohol wipes, salt spray, humidity soak and a decade of thermal cycling. Thermal-evaporated ("soft") coatings skip the bombardment; they are cheaper, faster and perfectly correct when the optic lives inside a sealed bench instrument and never sees a thumb.

When soft is the right answer

Indoor instrumentation, optical path components behind covers, prototypes where the coating only needs to survive measurement. Soft coatings on a bench AR are a legitimate production choice, not a downgrade — they just are not field coatings pretending to be hard.

When hard is not optional

Anything a human cleans, a weather seal forgets, or a shipping container cooks: outdoor sensors, machine-vision windows in factory washdown, astronomy filters that leave a dew cold into a warm car, rental photography glass. The failure mode of soft coatings outdoors is not dramatic — it is a slow haze at the layer interface that customers report as "the image went milky," months after your invoice was paid.

Ask for the test, not the adjective

Any supplier can type "hard coating" on a datasheet. The questions that separate factories: which adhesion standard was run on your lot (tape per ASTM, or a scratch tester)? Which humidity and thermal-cycle counts? Was the environmental qualification done on this design or on a witness coating? A coating factory that owns the coater answers with test numbers; a broker answers with the word.

The cost reality

On common sensing filters the hard-vs-soft delta is pennies per piece and a few days of campaign scheduling. On consumer effect glass and astronomical accessories, hard coats plus hydrophobic tops become brand claims — "scratch resistant," "fog proof" — worth more shelf dollars than the process cost. Specify the environment honestly, pay for the coating that survives it, and stop the field-return math before it starts.