Aug. 14, 2026
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A bi-concave lens, also called a double-concave lens, is an optical element with two inward-curved surfaces. It is thinner at the center and thicker at the edge, so it causes parallel light rays to diverge as if they originated from a virtual focal point. In practical applications, a bi-concave lens is used for beam expansion, laser optics, myopia correction, image reduction, and controlling optical paths. For businesses, selecting the correct lens geometry, substrate, coating, and inspection method improves optical performance, reduces assembly errors, and supports reliable product development.
A bi-concave lens has two concave spherical surfaces. When a collimated light beam enters the lens, refraction changes the direction of the rays and produces a diverging output beam.
The lens does not normally create a real image on the opposite side. Instead, it forms a virtual, upright, and reduced image when used with a real object. Its focal length is negative, which is why it is often called a negative lens or diverging lens.
The approximate focal length of a thin bi-concave lens can be estimated with the lensmaker’s equation:
[ \frac{1}{f}=(n-1)\left(\frac{1}{R_1}-\frac{1}{R_2}\right) ]
where:
In production optics, the actual result also depends on center thickness, wavelength, material dispersion, surface accuracy, coating performance, and mechanical tolerances.
The most important use of a bi-concave lens is to control the direction and size of a light beam. Its negative optical power makes it valuable in systems that need controlled divergence rather than focusing.
A bi-concave lens can be paired with a positive lens to create a Galilean beam expander. In this configuration, the negative lens is placed before a positive lens. The first lens expands the beam, while the positive lens recollimates it.
This arrangement is common in:
A beam expander can reduce beam divergence and increase the beam diameter. For example, a negative lens with a focal length of -25 mm combined with a positive lens of +100 mm can produce an approximate four-times beam expansion, depending on the optical layout and effective focal lengths.
In eyeglasses and contact lenses, a negative lens corrects myopia, commonly known as nearsightedness. The lens diverges incoming light before it enters the eye, moving the focal point backward onto the retina.
Although many prescription lenses are meniscus-shaped rather than strictly bi-concave, the optical principle is similar. The required prescription is measured in diopters:
[ P=\frac{1}{f} ]
where (P) is optical power in diopters and (f) is focal length in meters. A lens with a focal length of -0.5 m has a power of -2.00 diopters.
A bi-concave lens can reduce the apparent size of an image when used in a controlled imaging system. It may be integrated into:
Designers often combine a negative lens with one or more positive elements to manage magnification, field curvature, and overall system length.
A negative element can help balance the aberrations introduced by positive lenses. In a compound lens group, it may contribute to:
This is one reason bi-concave elements are frequently found inside achromatic lens assemblies, beam expanders, and custom optical modules rather than used alone.
Some illumination systems require light to spread across a larger area. A bi-concave lens can assist with beam spreading before the light reaches a diffuser, detector, or inspection region.
It may also be used to keep a sensitive detector or imaging sensor away from a strong converging focus. However, the lens must be selected carefully to avoid excessive irradiance, ghost images, or coating damage.
Understanding the difference between lens profiles prevents common design and purchasing errors.
| Lens type | Optical power | Typical function | Common applications |
|---|---|---|---|
| Bi-concave lens | Negative | Diverges light | Beam expansion, myopia correction |
| Bi-convex lens | Positive | Converges light | Focusing, magnification |
| Plano-concave lens | Negative | Diverges light from one flat/one concave surface | Compact optical systems |
| Plano-convex lens | Positive | Focuses light | Condensers, imaging |
| Meniscus lens | Positive or negative | Controls aberration and focal length | Cameras, eyeglasses, relay optics |
| Cylindrical lens | Positive or negative in one axis | Shapes a line or elliptical beam | Laser line generation, barcode scanning |
A bi-concave lens is not automatically the best negative lens for every application. A plano-concave lens may be preferable when one surface must remain flat for mounting or when the optical design requires a specific aberration profile.
It does not. Divergence depends on the effective focal length, input beam diameter, wavelength, refractive index, lens spacing, and whether the incoming beam is truly collimated.
A laser beam entering a -50 mm Spherical Lens behaves differently from a beam entering a -200 mm Spherical Lens. The shorter focal-length lens generally produces stronger divergence.
A negative lens does not simply “scatter” light randomly. It produces predictable geometric divergence when the surfaces are accurately manufactured and properly aligned. In a Galilean beam expander, that divergence is intentionally converted into a larger collimated beam.
A bi-concave design can introduce spherical aberration, especially when used with a wide aperture or a fast optical system. Surface curvature, clear aperture, edge thickness, and material selection must be optimized through optical design software and tolerance analysis.
For demanding systems, an aspheric negative lens or a cemented achromatic group may offer better performance than a standard Spherical Lens.
Uncoated optical glass reflects a portion of incident light at each air-to-glass interface. For visible, ultraviolet, or infrared systems, an anti-reflection coating can significantly improve transmission and reduce back reflections.
The coating should match the operating wavelength, angle of incidence, laser power, and environmental conditions. For example, a visible broadband coating is not automatically suitable for a 1,064 nm laser system.
A clear technical specification helps an optical manufacturer quote the correct part and prevents costly revisions.
Material
Effective focal length
Diameter and thickness
Surface quality and flatness
Centering and wedge
Coating
Environmental requirements
A qualified optical spherical lens manufacturer should provide traceable inspection data rather than relying only on visual checks.
Depending on the product and application, the quality plan may reference:
A practical inspection process may include:
For production planning, buyers may request 100% inspection, a dimensional report with precision to 0.01 mm, and a documented 24-hour response for engineering questions. These are procurement requirements, not universal specifications; the correct values must be agreed upon according to the optical design.
Sunday Optics is positioned as a technical partner for custom and standard optical components, including negative lenses, optical windows, coated elements, and precision Spherical Lens products.
When evaluating an optical supplier, businesses should look for:
For example, a machine-vision integrator may need a negative lens to expand the field of view inside a compact camera module. The supplier can review the required focal length, clear aperture, distortion target, coating band, and mechanical envelope before recommending a bi-concave element or a custom negative spherical optical lens.
Similarly, a laser equipment manufacturer may require a low-absorption fused-silica element with an anti-reflection coating at 1,064 nm. In that case, substrate homogeneity, coating damage threshold, surface quality, and centering are more important than simply selecting the lowest-cost spherical glass lens.
Assume an optical designer needs to expand a 5 mm collimated laser beam before it enters a scanning system.
The design team might:
This process illustrates why lens selection should be based on the complete optical system rather than the lens shape alone.
A bi-concave lens is primarily used to diverge light, reduce image size, correct myopia, expand laser beams, and balance aberrations in compound optical systems. Its performance depends on focal length, material, coating, surface accuracy, centration, and system alignment.
For reliable commercial results, work with an experienced optical spherical lens manufacturer that can provide design support, documented inspection, and production consistency. Whether the requirement is a standard Spherical Lens, a coated bi-concave element, or a custom negative lens assembly, Sunday Optics can be considered as part of the supplier evaluation process.
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