Objective lens for microscopes, inspection and interferometric heads

01

Set numerical aperture for resolution and working distance

Nikon's MicroscopyU defines numerical aperture as NA = n × sin(α), where n is the refractive index of the medium between the front lens and the specimen and α is half the angular aperture. NA measures the objective's ability to gather light and resolve fine detail at a fixed object distance; MicroscopyU states that NA above 0.95 is difficult to reach with dry objectives, and that water (n = 1.33), glycerin (1.47) and immersion oil (1.51) allow higher values. [1]

MicroscopyU gives resolution, the shortest distance between two points that can still be distinguished, as r = 0.61λ/NA among its formulas, and notes that shorter wavelengths resolve finer detail and that the condenser NA also affects total resolution in transmitted light. [2] For 10x plan objectives it gives NA 0.25 for a plan achromat, 0.30 for a plan fluorite and 0.45 for a plan apochromat, showing that higher correction comes with higher NA at the same magnification. [1]

Editorial calculation: at an assumed 550 nm wavelength, r = 0.61 × 0.55 µm / 0.30 ≈ 1.1 µm for an NA 0.30 objective and ≈ 0.67 µm for NA 0.50. [2] Leica's 10x/0.30 Mirau objective has 4.27 mm free working distance and its 50x/0.50 Mirau objective 2.52 mm, so the higher-NA objective in this pair leaves less room above the part. [5] [6]

Give the supplier the task (imaging, inspection, interferometric surface measurement), the smallest feature to resolve, wavelength band and light source, required magnification and field of view, minimum working distance, immersion or dry, cover glass or none, the microscope or head the objective must fit (tube lens focal length, thread, parfocal distance), environment, and test data required.

02

Match the objective to the optical system

MicroscopyU explains that in infinity-corrected systems the objective sends parallel light to a tube lens that forms the intermediate image, allowing prisms, polarizers and illuminators in the parallel path with minimal effect on focus. It lists tube lens focal lengths of 200 mm (Leica, Nikon), 180 mm (Olympus) and 165 mm (Zeiss), parfocal distances of 45 mm or 60 mm (Nikon), and M25 or RMS threads. [3]

MicroscopyU warns that infinity-corrected objectives “are usually not interchangeable” with those for finite 160 mm or 170 mm tube length microscopes, that infinity objectives suffer spherical aberration on a finite system, and that infinity objectives from different makers may not interchange because of tube length and thread differences. [3] [4] Editorial calculation: in an infinity system, magnification is the tube lens focal length divided by the objective focal length, so the same objective focused by a 180 mm tube lens instead of a 200 mm one gives 10% lower magnification.

Barrel engravings carry the key data. MicroscopyU's table of designations includes Plan (flat field), Apo (apochromatic), LWD/ELWD (long working distances), Corr (correction collar), NC/NCG (no coverslip), M25/M32 metric threads, and DI, MI, TI for interferometry objectives. [4]

03

Choose an interferometry objective for surface measurement

Leica lists Mirau objectives for reflected-light interferometry: the HC PL FLUOTAR 10x/0.30 Mirau with 4.27 mm free working distance, field number 22, M25 thread and no cover glass, and the N PLAN L 50x/0.50 Mirau with 2.52 mm free working distance, field number 25 and M25 thread, also without cover glass. [5] [6]

Leica's 50x Mirau page lists the immersion as dry in its specification table while its summary text mentions oil immersion; editorial note: confirm the immersion medium and working distance with the supplier for the exact part number. [6] For any interferometric head, ask whether the reference mirror is built into the objective, how it is adjusted, and which wavelength and light source the objective is corrected for.

04

OEM microscope objectives and custom instrument objectives from Hitronics and VY Optoelectronics

Listings include custom objectives for microscopes and instruments and OEM microscope objectives for biomedical and metrology modules; ask each for NA, working distance, focal length, wavelength range and thread.

05

Lens elements, front lens and medium, housing and thread, coating, Mirau reference path and correction collar

The housing-and-thread row decides whether an objective fits at all: MicroscopyU lists M25 and RMS threads and parfocal distances of 45 mm or 60 mm. [3]

06

Focal length, NA, working distance, wavelength band, transmittance and thread for an objective order

Working distance is the only row with part-number values, from Leica's two Mirau objectives; focal length matters because, with the tube lens, it sets magnification. [5] [6] [3]

07

Send the task, optics and system data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Task: [imaging; inspection; interferometric measurement]
Optics: [magnification; NA; field of view; wavelength band]
Distance: [minimum working distance; immersion or dry; cover glass]
System: [infinity or finite; tube lens focal length; thread; parfocal distance]
Interferometry: [Mirau or other; reference adjustment; light source]
Documents: [MTF or wavefront test; transmittance curve]

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