Why Do Lens Coatings Look Green, Red, or Blue? The Optics Behind Reflected Colors

Hold a pair of binoculars up to the light and look at the objective lens, and you will see different reflected colors: some green, some blue, some with a reddish-purple cast. Many sellers turn color into a selling point: amber-gold coating, sapphire-blue coating. So what is the truth behind coating color?

First, the basic job of a coating. Every time light crosses a glass-to-air interface, roughly 4% of it is lost to reflection. A binocular with more than 10 air-to-glass surfaces would, without any coating, transmit only a little over 60% of the light, leaving an image that is dim and hazy. The essence of coating is interference-based anti-reflection: a thin film of a specific thickness causes reflected light to cancel itself out, letting light pass through instead.

So why is there color at all? Here is the key: a single layer can only perfectly cancel reflection at one wavelength, that is, one color. If a coating cancels green light most effectively, the light that remains reflected is a mix of red and blue. The reflected color you see is simply the leftover band that was not cancelled. A green reflection means the coating reflects red and blue strongly; a blue-purple reflection means it reflects red and green strongly.

The reflected color itself therefore says nothing about quality. It is just the outward sign of the band the coating was designed around. What really determines performance is the number of layers and how well they are matched. A single-layer coating lifts transmission to roughly 85-90%. Multi-layer coating (MC) stacks films of different thicknesses to cancel reflection across the whole visible band, pushing transmission above 95%, and the reflected color actually becomes faint and even.

How can you judge a coating's grade from its color? Look at evenness, not hue. A quality multi-coated lens shows a pale, uniform reflection, such as light blue or light green, and the color shifts gently as you tilt the binocular. A cheap coating shows a strong, saturated reflection with patchy unevenness, and different lenses do not match in color, which suggests the lenses came from different coating batches or different layer counts.

Now for the widely circulated tale of the magic red mirror. Red-coated binoculars, popular in the 1990s, were marketed as infrared night vision coating. In reality they were simply single-layer coatings that reflected away the middle band of visible light, which lowered transmission and tinted the image. Red coating as night vision is a complete marketing con. If you see a strongly red coating promoted that way, walk away.

To sum up: a pale, even color means a good coating, a strong and patchy color should make you cautious, and red coating night vision is a scam. The real science of coatings lies in the number of layers, not the color. On a spec sheet, look for FMC (fully multi-coated), which is far more reliable than judging by any color.