Glossary: H

The layer of optical coating that is equal in optical thickness to one half the design wavelength. No change in reflectance/transmittance results from layers of this thickness. 

A type of glass filter that absorbs, rather than reflects, energy associated with heat.

A gear with angled teeth used in helicoid movement components. It is superior to a rack and pinion because of the smoothness of operation due to tightness of the mesh between gear and rack.

One of the most common types of lasers, the HeNe laser uses a combination of helium and neon gas as its laser medium. Although the most common output wavelength is 632.8nm, HeNe lasers are available in several different wavelengths. 

A type of grating in which a sinusoidal "blaze" pattern is generated on the surface by exposing the grating (covered with photoresist) to an interference pattern. The sinusoidal variations in the interference pattern lead to the groove profile for the holographic element.

In analog imaging, the number of individual pixels that can be distinguished in a horizontal scanning line. Expressed in terms of TV lines.

A mirror-like filter which reflects infrared light while transmitting visible light.

An eyepiece having two plano-convex lenses of the same glass separated by half the sum of their focal lengths. This eyepiece is free of lateral chromatic aberration, but is not suitable for use with reticles. 

A type of manufacturing technique that starts with a standard spherical surface, such as an achromatic lens, which is pressed onto a thin layer of photopolymer in an aspheric mold to create an aspheric surface.  After the surfaces are compressed and UV cured at room temperature, the end result is an aspherized achromatic lens that has chromatic and spherical aberration correction properties.

The (usually long) working distance beyond which all objects are at an acceptable focus.