MIRROR COATING


When a beam of infrared radiation is injected into a multiple-pass cell, energy is absorbed at the mirror surfaces. For many years gold coatings were used exclusively because of their high reflection coefficient and because of the resistance of gold to chemical attack. Although the reflection coefficient of silver was higher than that of gold, silver was ignored as a coating because it would react with compounds like H2S, SO2 and HCl. However, overcoatings on silver make it corrosion resistant so that when a cell has more than about 50 reflections, silver becomes a better choice than gold. A good gold surface will absorb between 1 and 2 per cent of the incident energy at each reflection. For a cell with 80 reflections, for example, when the reflection coefficient is 0.985, the fractional transmission will be 0.985 raised to the 80th power. This equals only about 30%. Good protected silver will absorb only 0.5 per cent per reflection. With 80 reflections, the fractional transmission then will be 0.995 to the 80th power. This equals 67%--more than twice as high as in the case of gold.


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