Optical lenses used to change the focal length of another lens are transparent optical components used to converge or diffuse light emitted from surrounding objects. The transmitted light then forms a true or false image of the object.
Lenses are a good example of transmission optical components, which means that they allow light to pass through or be transmitted. Other transmission components include filters, windows, planes, prisms, beamsplitters and waveplates, while the opposite category - reflectors (which reflect rather than transmit light) - includes optics and backward-reflecting mirrors.
Important specifications for optical lenses include wavelength and material.
After selecting the best lens type to fit their application, buyers should analyze the wavelength range of the application. When specifying a lens, manufacturers typically provide a range of electromagnetic radiation that the lens is designed to transmit. The wavelengths can be grouped into three broad groups: infrared, visible, and ultraviolet. A lens might not be limited to a single spectrum, and may be able to transmit wavelengths from both the infrared and visible range, or visible and ultraviolet range, etc.
Infrared lenses are designed to operate within the 750 to 2500 nm wavelength range.
Lenses designed for use on the visible spectrum can transmit wavelengths within the 380 to 750 nm range.
Ultraviolet lenses can transmit wavelengths between 4 and 380 nm.
Historically, optical lenses were constructed from transparent glass, but are now made from other materials — acrylics, polymers, and minerals — as well. Lens material is determined by the raw material's dispersion and wavelength characteristics. For example, a lens designed for applications demanding low dispersion might be made of crown glass. Acrylic and polymer lenses are best suited to transmission within the visible spectrum, while minerals such as germanium and sapphire are suitable for a very wide range of wavelengths but particularly excel within the infrared spectrum.
Spherical lenses (or singlets) have curved surfaces which converge or diverge rays. All of the lens cross-sections are spherical lenses.
Unlike spherical lenses, cylindrical types have curved faces which can be considered part of a cylinder shape. This causes them to focus transmitted light to a line instead of a single point. Cylindrical lenses are commonly used to change an image's aspect ratio or shape a laser beam.
Achromats (also known as achromatic lenses) are used to minimize a special type of image distortion called chromatic aberration. This distortion occurs when a lens fails to focus all color wavelengths to the same convergence point, resulting in blurred contrast and color fringing. At least two separate lens elements —one high-dispersion concave and one low-dispersion convex — to achieve their corrective effect.
Fresnel lenses consist of thin, lightweight plastic sheets marked with a series of concentric grooves. Each groove serves as an individual refracting surface; the series of grooves bends collimated light to a common focal point. Fresnel lenses are a compromise between efficiency and optical quality: because the lens material is very thin, a very small amount of light is lost in the transmission process.
Gradient index (GRIN) lenses are simple planar lenses which continuously bend light rays within the lens until they finally converge on the focal point. This contrasts with conventional lenses, which primarily bend light abruptly when it exits the back of the lens material. GRIN lenses are therefore cost-effective and simple to employ. Additionally, the ability to precisely manufacture the length of the plane results in an enormous flexibility to fit application parameters.