Lens
What is a Lens?
A lens is an optical component made from a transparent material that has at least one curved surface. Its primary function is to refract (redirect) transmitted light rays, either converging them to a focus or diverging them to spread light out. The applications of lenses are extraordinarily diverse, spanning everything from eyeglasses, cameras, and automotive headlamps to laser systems, virtual reality goggles, and fiberoptic networks.
Lenses are fundamental optical components crafted from various transparent materials, such as glass (for visible light) or ZnSe (for infrared radiation), that manipulate light through refraction. This interaction changes the propagation direction of light rays as they pass through the lens, thus causing them to converge or diverge.
The shape of a lens – whether convex, concave, or a more complex form – dictates its specific effects on light. Typically, this means either gathering rays to a single point for imaging or beam concentration or spreading them apart to diminish intensity or enlarge the field-of-view. This unique ability to direct and focus light makes lenses indispensable in a myriad of optical devices, from the simple magnifying glass to the complex assemblies found in advanced scientific instruments.
Basic Operating Principles of Lenses
Lenses work using refraction, which is an optical phenomenon that occurs whenever light passes from one material into another material having a different refractive index. The index of refraction of a material quantifies how much light ‘slows down’ when it travels within it. Specifically, refractive index is defined to be the ratio between the speed of light in vacuum to the speed of light within the material.
This ‘slowing down’ causes light rays to change direction (bend or refract) when they enter the material at any angle other than perpendicular to the surface. The extent to which the light is redirected depends on the angle at which it hits the boundary between the two media and their respective refractive indices. This relationship is quantified by an equation called ‘Snell’s Law,’ which is illustrated in the figure.
The refraction (change of direction) of a light ray going from one material to another is governed by the simple equation known as Snell’s Law. On a curved surface, the orientation of an imaginary line perpendicular to that surface varies with position. Thus, the angles at which the light rays are refracted varies with position, too, and always in accordance with Snell’s Law.
Snell’s Law not only explains how light refracts, but also underpins the design and functioning of lenses. By shaping a lens with a particular curvature, optical engineers can control the path of light through the lens, focusing or dispersing the light rays as needed for various applications. The ability to manipulate light in this way is foundational to optical technology and a wide array of scientific and everyday applications.
Common Lens Types
While there are a great variety of lens types, the majority of them can be classified into a few broad groups. The most basic distinction is whether each surface is convex or concave. A convex surface curves or bulges outward, while a concave surface curves or recedes inward.
The three surface shapes – convex, concave, or flat – can be combined in a total of six different ways, as shown in the diagram (assuming that at least one is curved). If the combination produces a lens that is thicker in the center than it is at the edges, then it is a positive lens. A positive lens converges light – it focuses light down.
If the lens is thicker at the edges than at the center, then it is a negative lens. A negative diverges or spreads light.
The six basic lens shapes are shown here. The positive lenses converge light rays to a focal point. The negative lenses diverge light rays, causing them to spread.
Lens Shapes
The next distinction (after whether the surfaces are convex or concave) is the form of the curve. Specifically, this means whether each surface is spherical, aspherical, cylindrical, or something even more complex, like a freeform. The graphic illustrates this.
Each surface of a lens can be a section of a sphere, an aspheric shape, or a cylinder, or can be flat (plano).
Why do we need all these various lens shapes? One reason is because the earlier statement about a spherical lens directing all rays to a common focal point isn’t exactly true. In fact, when parallel rays encounter a spherical lens, those that enter towards the edge of the lens are focused to a slightly closer point those that enter near the center. As a result, the focused spot isn’t a perfect point. This problem – called ‘spherical aberration’ – lowers resolution in imaging systems and limits the ability to focus lasers to very small spot sizes.
A spherical lens doesn’t focus all rays at exactly the same point which limits performance. An aspherical lens can avoid this problem. But no lens can achieve a perfect point focus because of diffraction, the effects of which aren’t shown here.
There are a couple of ways to solve this problem. The first is to use a lens that doesn’t have a spherical surface; an asphere. These don’t have spherical aberration.
Another solution is to combine multiple lenses together, rather than using just a single component. Creating a lens system that has multiple surfaces better enables the optical designer to minimize spherical aberration and various other performance limiting aberrations.
Combining multiple lenses together can address another problem that occurs with any single element lens, whether spherical or aspheric. This is the tendency for the lens to focus off-axis light on to a curved surface, rather than a plane. Since most image sensors are flat, and the many materials processing applications also require focusing on to a flat surface, this ‘field curvature’ is a commonly encountered issue.
Multiple lens elements can be combined to eliminate field curvature as well as correct for many other aberrations and performance issues.
Cylindrical lenses act in the same manner as the spherical and aspherical surfaces just described, except only in one dimension. So, a positive cylindrical lens doesn’t focus light to a spot, it focuses it to a line.
A cylinder lens focuses in only one dimension and is often used for forming line beams.
Cylinder lenses have many applications. For example, they are used as laser line generators – the Powell Lens is one type of aspheric cylindrical lens shaped specifically to produce a laser line having a uniform intensity distribution. Cylinder lenses can also be used to convert the asymmetric output of most diode lasers into a circular beam.
Cylindrical lens elements are also widely employed in anamorphic lenses. These are used in cinematography (movie photography) to capture a widescreen image on a standard film frame or digital sensor. The anamorphic lens squeezes the wide field of view onto a narrower recording medium. The wider format is then stretched back to its original aspect ratio during projection or digital post-processing.
Lens Materials
Lenses are made from materials that transmit light, and there are many such optical materials currently in use. Each is chosen for a particular application based on its specific combination of optical, mechanical, thermal, and sometimes even chemical characteristics.
Optical properties are often the most critical factor, and this is usually the starting point for choosing a lens material. In particular, transmission range is frequently a key factor. Because if the material doesn’t transmit the desired wavelength, you can’t make a lens from it.
Optical glasses are the most widely used materials for precision lenses (such as for laser or instrumentation applications) operating in the visible and near infrared. ZnSe is the most popular material for CO₂ laser lenses, and for many other infrared applications, as well.
Plastics have become extremely common in consumer eyewear and contact lenses. This is because they are lightweight, impact resistant, easy to make in virtually any shape, and relatively low cost. But they do scratch much more easily than glass lenses. Polycarbonate and a polymer called ‘CR39’ are used for most eyeglasses, and ‘Hydrogel’ is the primary material for soft contact lenses.
Coherent produces numerous different lens types for precision applications, from single components through complex multi-element systems, such as f-theta scan lenses and IR thermal imaging lenses. Learn more about Coherent optics.