Aug. 11, 2026
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If you are asking which side of a plano-convex lens should face the light source, the practical rule is simple: for a collimated beam being focused to a point, place the curved surface toward the incoming light and the flat surface toward the focus. This plano-convex lens orientation for collimated light usually reduces spherical aberration compared with reversing the lens. It is also the usual best lens orientation for focusing a laser beam. The result depends on the lens’s radius of curvature, the paraxial approximation, and the system’s numerical aperture, so the correct orientation should be checked against the beam diameter, wavelength, focal length, and working distance.
A plano-convex lens has one spherical convex surface and one plane surface. Both orientations have approximately the same paraxial focal length, but they do not produce the same off-axis and marginal-ray performance. The difference becomes noticeable when the beam fills a large portion of the clear aperture, when the numerical aperture increases, or when the application requires a small focused spot.
When the curved side faces a collimated input beam, the first refracting surface gradually bends the rays toward the optical axis. The plane surface then sends the converging rays into the image space without adding another curved-surface refraction. This arrangement generally lowers longitudinal spherical aberration for a positive plano-convex lens used to focus parallel light.
If the flat side faces the incoming collimated beam, the rays remain parallel while crossing the first surface and are refracted more strongly at the second curved surface. Marginal rays then experience a larger angular change than paraxial rays, causing them to intersect the optical axis at a different position. The focal region becomes elongated, and the measured spot may be larger than expected from the diffraction limit.
| Application | Recommended orientation | Reason |
|---|---|---|
| Collimated light focused to a point | Curved side toward the collimated source | Usually reduces spherical aberration |
| Point source collimated into a parallel beam | Curved side commonly toward the point source | Places the stronger refracting surface near the shorter conjugate |
| Low-NA, nearly paraxial imaging | Either direction may be acceptable | Orientation has a smaller effect when the beam uses only the central aperture |
| High-NA laser focusing or precision imaging | Use the manufacturer’s ray-trace recommendation | Orientation, lens spacing, wavelength, and aperture jointly determine aberration |
A plano-convex lens is a positive lens that converges light. Its flat surface has an effectively infinite radius of curvature, while its convex surface has a finite radius. For a thin lens in air, the focal length can be estimated with the lensmaker equation:
1/f = (n - 1)(1/R1 - 1/R2)
For a plano-convex lens, one radius is effectively infinite, so the relationship becomes approximately:
f ≈ R/(n - 1)
Here, f is focal length, R is the convex surface radius, and n is the refractive index at the operating wavelength. For example, N-BK7 has a refractive index of approximately 1.5168 at the Fraunhofer d-line of 587.6 nm. A 25 mm focal-length plano-convex lens would therefore require an approximate radius of:
R ≈ 25 × (1.5168 - 1) ≈ 12.92 mm
The real focal length can differ because of lens thickness, wavelength dispersion, coating design, manufacturing tolerance, and the reference plane used for measurement.
Paraxial rays travel close to the optical axis and usually follow the thin-lens approximation. Marginal rays pass near the edge of the clear aperture and encounter stronger refraction. In a spherical surface, these rays do not converge at exactly the same axial position as paraxial rays. This longitudinal separation is spherical aberration.
The effect is controlled by the aperture ratio, often expressed as the ratio between focal length and illuminated diameter. A 25 mm focal-length lens illuminated by a 5 mm beam has an approximate f-number of 5. A 20 mm beam produces an approximate f-number of 1.25 and places far greater demands on aberration control. The same lens orientation that performs adequately at f/5 may produce an unacceptable focal spot at f/1.25.
First determine whether the source produces a collimated beam, a diverging beam, or a converging beam.
Do not judge collimation by eye. Measure the beam diameter at two or more distances. If the diameter changes by less than the tolerance required by your application, the beam can be treated as approximately collimated.
For a collimated beam focused by a positive plano-convex lens, orient the curved surface toward the incoming beam. For a point-like diverging source that must be collimated, place the curved surface toward the source in the common arrangement.
This rule is not a substitute for optical modeling. If the object is at a finite distance, calculate the conjugates using:
1/f = 1/s + 1/s′
where s is the object distance and s′ is the image distance. When the object distance is not large compared with the focal length, the system is no longer a simple collimated-beam case.
Measure the beam diameter at the lens. A beam should not normally use the full mechanical diameter because edge bevels, mounting hardware, and aperture tolerances can clip the beam. For Gaussian laser beams, specify whether the diameter is measured at the 1/e2 intensity level, the 1/e level, or another definition.
For a Gaussian beam with a 1/e2 radius w, a thin-lens estimate of the diffraction-limited waist is:
w0 ≈ λf/(πw)
This estimate assumes a clean, nearly ideal beam and does not include spherical aberration, astigmatism, thermal lensing, surface irregularity, or alignment error. A larger input beam can produce a smaller theoretical waist, but it also increases the influence of marginal-ray aberration.
Refractive index changes with wavelength. A lens specified at 587.6 nm may have a different focal length at 405 nm, 532 nm, 635 nm, or 1064 nm. This chromatic change is called focal shift or longitudinal chromatic aberration.
For visible applications, N-BK7 is commonly used because of its transmission and optical homogeneity. Fused silica is often selected for ultraviolet transmission, low thermal expansion, and high laser-damage resistance. The correct choice depends on wavelength, pulse duration, power density, environmental conditions, and coating requirements.
Assume a 532 nm laser has a 6 mm 1/e2 beam diameter and is focused with a 50 mm focal-length plano-convex lens. The approximate input radius is 3 mm, giving an f-number near 8.3. Under ideal Gaussian-beam conditions:
w0 ≈ (532 × 10-9 × 0.050)/(π × 0.003) ≈ 2.8 µm
This is a theoretical waist estimate, not a guaranteed measured spot size. Surface quality, beam quality factor M2, lens centering, coating performance, and spherical aberration can increase the actual spot diameter. Mount the lens with its curved surface facing the laser and inspect the focal profile using a beam profiler or knife-edge measurement.
If the lens is reversed, the paraxial focal length may still appear close to 50 mm, but the focal spot can become asymmetric or elongated, particularly when the beam diameter is increased. A useful comparison is to record the 1/e2 spot diameter in both orientations while keeping lens position, input power, and beam diameter unchanged.
When the source is close to the focal plane and emits a diverging cone, orient the curved surface toward the source in the usual plano-convex configuration. The lens then produces a more nearly collimated output than the reversed orientation for many finite-conjugate setups.
However, an LED is an extended source, not a point source. Each point on the emitter creates a different output angle, so the final beam may remain divergent even when the central ray is collimated. In this situation, an aspheric collimator, condenser system, or multi-element lens may outperform a single plano-convex lens.
The paraxial focal length is similar in both directions, which can make the difference seem unimportant during a basic alignment test. At higher aperture, the ray distribution changes substantially. A system that only needs a broad illumination pattern may tolerate the reversal, while a microscopy, spectroscopy, or laser-marking system may not.
A single spherical surface is not an ideal solution for every high-NA application. If the lens is used near its full aperture, spherical aberration can dominate the error budget. Consider a best-form lens, aspheric lens, achromatic doublet, or custom multi-element design when the required spot size is close to the diffraction limit.
The specified focal length is usually measured from a principal plane rather than from the physical front or rear face. Therefore, positioning the workpiece exactly one nominal focal-length value from the lens housing may not place it at the actual focus. The center thickness and refractive index move the effective principal planes.
An antireflection coating designed for 532 nm may not provide the same reflectance at 405 nm or 1064 nm. For a high-power laser, confirm the coating’s wavelength range, angle of incidence, polarization dependence, continuous-wave rating, and pulsed-laser damage threshold.
The better orientation is normally the one that produces the smaller and more symmetric focal distribution. For a low-power visible beam, a camera-based profiler may be adequate. For ultraviolet, infrared, or high-power laser systems, use a detector rated for the wavelength and power.
A knife-edge scan can estimate beam radius when a beam profiler is unavailable. Move a sharp edge across the beam while recording transmitted power. The derivative of the power curve provides the transverse intensity profile. Repeat the scan at several axial positions to identify the smallest beam waist.
This method is sensitive to vibration, detector noise, edge quality, and power fluctuations. It is useful for comparison, but it should not be treated as a complete measurement of wavefront quality.
For focusing a collimated laser to a point, the curved convex side should normally face the laser. The plane side faces the focused output. This arrangement generally reduces spherical aberration compared with placing the plane side toward the collimated beam.
When a point-like source is placed near the focal position and the goal is collimation, the curved side commonly faces the source. The exact result depends on source distance, aperture, wavelength, and the required beam divergence.
The paraxial effective focal length is approximately the same in either direction for the same lens and wavelength. Reversing it mainly changes aberration, principal-plane location, and practical imaging performance.
Yes, if the beam occupies only the central portion of the aperture and the application accepts the resulting aberration. At low numerical aperture, the performance difference may be too small to matter. For a tightly focused or high-power beam, the curved-side-first orientation is the safer default.
It can be suitable at low to moderate numerical aperture with careful alignment and a properly selected diameter. At high numerical aperture, spherical aberration may prevent diffraction-limited performance. A precision asphere or multi-element objective may be more appropriate.
Inspect the reflected image of a straight object or light source. The curved surface produces a curved or magnified reflection, while the plane surface produces a simpler reflection. A mechanical drawing, optical inspection tool, or surface-sag measurement provides a more reliable identification.
For systems with large beam diameters, finite object distances, tilted elements, or strict spot-size requirements, model both orientations in optical design software. Enter the actual glass type, center thickness, wavelength, aperture stop, coating, and surface irregularity. Compare transverse ray aberration, wavefront error, modulation transfer function, and encircled energy.
The ideal Gaussian-beam equation assumes M2 = 1. Real laser beams often have M2 values above 1, increasing the focused waist approximately in proportion to M2. A lens orientation change cannot correct poor beam quality, astigmatism from a diode source, or clipping from an undersized aperture.
A larger diameter is not automatically better. Oversized lenses increase cost and may introduce unnecessary alignment sensitivity. Undersized lenses clip the beam and create diffraction rings. Select a clear aperture that provides practical margin without allowing the beam to interact with the bevel or mount.
For imaging and metrology, surface quality and transmitted wavefront error may matter more than the basic focal-length specification. A lens marked 25 mm focal length can still produce different results depending on centering tolerance, wedge, surface power, and coating uniformity. Request an inspection report when the application requires repeatable micrometer-scale positioning or low wavefront error.
Sunday Optics can help select a plano-convex lens according to wavelength, focal length, clear aperture, substrate, coating, surface quality, and intended beam geometry. When requesting a quotation, provide the light-source type, wavelength, optical power, beam diameter, desired spot size, working distance, and whether the system is focusing or collimating.
For standard laboratory focusing, a coated N-BK7 plano-convex lens is often a practical starting point. For ultraviolet work, fused silica may be preferable. For high-NA or near-diffraction-limited designs, ask whether an aspheric lens, achromatic doublet, or custom optical assembly would reduce the total aberration more effectively than a single spherical element.
For the common case of focusing a collimated beam, place the convex curved surface toward the light source and the flat surface toward the focus. For collimating a diverging point-like source, the curved surface commonly faces the source. The final choice should be confirmed using the beam diameter, conjugate distances, wavelength, aperture ratio, and measured spot profile rather than focal length alone. By comparing plano-convex lens orientation for collimated light, laser focusing lens selection, and spherical aberration control with the actual radius of curvature, effective focal length, and numerical aperture, you can achieve a more predictable optical system. For custom specifications, coating selection, or volume production, contact Sunday Optics, an experienced optical spherical lens manufacturer.
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