The Benefits & Challenges of Using UV Lenses
Ultraviolet (UV) is the region of the electromagnetic spectrum with wavelengths from 10 nm to 400 nm, shorter than that of visible light, but longer than X-rays.
Traditionally UV lenses were predominantly used in scientific institutions for optical research. However, many molecules contain chromophores which will absorb specific wavelengths of UV light which has led to a rapid expansion of the use of UV lenses in industrial and commercial applications. Notable areas that use UV lenses include pharmaceutical, life science and cosmetic research, material science, petrochemicals, forensic analysis, authenticating documents/artwork to determine forgery and in the nuclear industry.
Unfortunately, many optical materials which are transparent to visible light become strongly absorbing in ultraviolet spectral region. This is essentially because the photon energy then becomes comparable to the band gap energy; a single photon is then sufficient for exciting a carrier from the valence band to the conduction band.
Also due to the shorter wavelengths – scattering effects are very strong in the UV region. Consequently, to minimise these effects, manufacturing UV lenses with tight tolerances and high surface quality (λ/10 p-v or better) is much more critical than with visible or IR optics.
In the UV, many optical materials exhibit substantially stronger chromatic dispersion than in the visible or infrared regions. This is particularly a problem when using lenses for focusing broadband ultraviolet light. In such circumstances using achromatic UV lenses can then be particularly important.
Your target application’s analytical waveband will be the primary deciding factor in choosing the most suitable UV optical material. Most materials will have a sharp cut-off wavelength, where absorption begins, and how far into the UV the cut-off goes depends on the material type and purity.
Highly purified Calcium Fluoride (CaF2) is a popular material for UV lenses. This is because of its very low UV absorption, high homogeneity, low birefringence, relatively high hardness (compared with other fluoride materials), high physical stability, and high optical damage threshold. Calcium Fluoride can be used down to around 160 nm and is thus suitable for use with argon fluoride (ArF) excimer lasers. However, CaF2 is brittle, naturally anisotropic, and hygroscopic. Similar properties are obtained for other purified fluorides such as Magnesium fluoride (MgF2) and Lithium Fluoride (LiF2) which can be used down to 110 nm.
Fused Silica, also known by trade names including Suprasil, Spectrosil and Lithosil is the most common of the UV grade transmissive materials. It is very popular due to cheap production, as it is made from sand, very good thermal dimensional stability, and its durability. Unfortunately, cheaper standard-grade fused silica has significant attenuation below 260 nm, necessitating the use of UV-grade fused silica which offers good performance down to ~200nm.
Case Study: click here to learn about how a UV zoom lens is helping verify spent fuel rods in the nuclear industry.
For further information on UV lenses: click here.