Editorial Technical Reference

Interferometric optical head for absolute distance and thickness: fibre-coupled SLED probes with 1 to 19 mm stand-off, sub-nanometre resolution and multi-layer readout

Q: How far from the part does an interferometric head sit?

Micro-Epsilon heads start their range at 0.5, 1, 10 or 19 mm; Keyence's SI-F01 starts 0.05 mm from the head front and the SI-F10 at 11.3 mm.

Q: What resolution do these heads reach?

Micro-Epsilon gives below 1 nm for the IMS5400-DS and below 30 pm for the IMS5600-DS, both with 64-value averaging at 0.5 kHz; Keyence gives 0.25 µm with 4,096 averages.

Q: Can one head measure more than one layer?

Yes. Micro-Epsilon's multi-peak mode reads up to 14 distance values, and Precitec's IT family measures thickness from 4 µm to 17,700 µm depending on variant.

Q: What light source do they use?

Near-infrared superluminescent diodes: about 840 nm in Micro-Epsilon's controllers and 820 nm in Keyence's SI-F.

Fibre-coupled measuring heads for absolute low-coherence and spectral interferometers · stand-off and range of the head, the nanometre figures and what they were measured on, and multi-layer thickness

An interferometric optical head is the part of a non-contact interferometer that sits over the workpiece. It holds no electronics: a lens and fibre end bring broadband near-infrared light from a superluminescent diode in the controller to a small spot, and carry the reflections from every surface in the beam back to a spectrometer. Because the light is broadband rather than a single laser line, each reflecting surface gives an absolute distance rather than a count of fringes, so steps and the front and back faces of glass, wafers or coatings can be measured without moving the head. Choosing a head means fixing three things: how far it sits from the part and over what range, what its resolution and linearity figures really describe, and whether it must read one surface or a stack of layers.

Passive, fibre-coupled measuring heads (probes) for absolute low-coherence and spectral-interference distance and thickness sensors, with their stand-off, measuring range, spot size, tilt tolerance and vacuum versions, and the controller figures they are specified with. Lens form and centration interferometers are covered on the Precision optical lens inspection and measurement system page; Mirau and other interferometry objectives on the Objective lens page; spinning-disk and wafer defect scanners on the Optical surface analyzer page; rows of laser triangulation sensors on the Alignment sensor array page. Chromatic confocal sensors, displacement-measuring laser interferometers for stages, and white-light microscope profilers are outside this page.

CNFX editorial research · Sources checked October 13, 2026 · 4 sources

Match stand-off and range Read the nanometre figures

White and black optical measuring head with a lens barrel and side cable connectors mounted on a vertical stage on a granite column, its lens a few millimetres above a glass plate with a blue-lit spot on a round sample below — Fibre-fed measuring head on a vertical stage above a glass plate

Start with your task

Match stand-off and range0.5 to 19 mm stand-off. Read the nanometre figuresResolution, linearity, averaging.

Stand-off against range: a 4 mm head at 1 mm, a right-angle head at 0.5 mm, and long-distance heads at 10 to 19 mm

The head has to fit where the part is. In vacuum chambers, inside lens barrels or over a moving wafer, the distance from the head face to the start of the range and the head's diameter narrow the choice of model before resolution is compared.

Micro-Epsilon gives a maximum tilt of ±2° for a usable signal on polished glass (n = 1.5) in mid-range, and notes that accuracy falls towards that limit; its fibres have a bending radius of 30 mm static and 40 mm dynamic. [1] Non-transparent targets must be optically dense at the 840 nm working wavelength. [1]

Editorial calculation: the DS19 head's 2.1 mm range is about 11 % of its 19 mm stand-off (2.1 ÷ 19), while the SI-F10 covers 1.05 mm (12.35 − 11.3) at 11.3 mm, about 9 %. Long stand-off buys clearance, not range.

Head	Start of range (stand-off)	Distance range	Spot	Size and build
Micro-Epsilon IMP DS1/VAC	1 mm	1 mm	10 µm in mid-range	Ø4 mm × 23 mm, stainless, UHV sensor and fibre [1]
Micro-Epsilon IMP DS0.5/90/VAC	0.5 mm (90° beam exit)	1.5 mm	10 µm	Ø10 mm × about 78.1 mm, UHV [1]
Micro-Epsilon IMP DS10/90/VAC	10 mm (90° beam exit)	1.5 mm	10 µm	Ø10 mm × about 68.6 mm; titanium housing optional [1]
Micro-Epsilon IMP DS19	19 mm	2.1 mm	10 µm	Ø10 mm × 55 mm, IP65 (IP40 for vacuum option); Invar housing optional [1]
Keyence SI-F01	0.05 mm from the head front	to 1.1 mm	Ø20 µm minimum	IP67, about 24 g with cable [3]
Keyence SI-F10	11.3 mm	to 12.35 mm	Ø40 µm minimum	IP64, about 38 g; unit-to-unit difference up to ±0.2 mm [3]

Give the free distance between head and part, the space for the head (diameter, straight or right-angle exit), vacuum or cleanroom conditions, the target material and its tilt, and the cable route to the controller.

<1 nm, <30 pm, 0.25 µm: what each resolution and linearity figure was measured on

Interferometric heads are sold on very small numbers, and each one is tied to a footnote. The averaging and the target behind each figure matter more than the figure itself when comparing offers.

The light sources are close: Micro-Epsilon uses a NIR-SLED at about 840 nm with a 635 nm pilot laser for alignment, and Keyence an infrared SLD at 820 nm, 0.6 mW, with a 655 nm guide laser, both Class 1. [1] [3] Micro-Epsilon's controllers run from 100 Hz to 6 kHz and are rated for operation at +15 to +35 °C, with temperature stability below 10 ppm in that band; its heads drift by 0.1 nm/K (typ.) in linearity. [1] Keyence gives a temperature fluctuation of 0.01 % of full scale per °C and a 200 µs sampling cycle. [3]

Editorial calculation: Keyence's 0.01 % F.S./°C on the SI-F01's roughly 1.05 mm range (1.1 − 0.05) is about 0.1 µm per °C, so a 2 °C swing adds about 0.21 µm, some 84 % of the 0.25 µm resolution. Precitec's rule gives about 53 nm for the IT family's 17,700 µm upper limit and about 1.5 nm for the TW variant's 500 µm limit (3 × 10⁻⁶ × upper limit). [3] [4]

System	Resolution	Linearity	Conditions in the footnote
Micro-Epsilon IMS5400-DS controller	< 1 nm	< ±50 nm (first distance); < ±150 nm for each further distance on the multi-peak model	24 ± 2 °C, 0.5 kHz, moving average over 64 values, measured differentially between front and back of a thin glass plate, 2 sigma; linearity against a reference system on an ND filter [1]
Micro-Epsilon IMS5600-DS controller	< 30 pm	< ±10 nm (first distance); < ±100 nm for each further distance	Same footnote conditions as the IMS5400 [2]
Keyence SI-F01 / SI-F10 heads	0.25 µm	±0.3 µm	Resolution on a glass plate at mid-range with 4,096 averages; linearity on a glass plate with 256 averages [3]
Precitec CHRocodile 2 IT	3 × 10⁻⁶ × upper range limit	Not on the product page	Rate up to 66,000 Hz for the family, 70,000 Hz for DW [4]

Ask for resolution and linearity at your measuring rate and averaging, on your material, and for the temperature band in which they hold.

Front and back faces in one reading: multi-peak heads for glass, wafers and coatings from 4 µm to 17.7 mm

Because every reflecting interface returns its own peak, one head can report a set of distances at once and the controller turns them into thicknesses.

Micro-Epsilon's multi-peak mode evaluates up to 14 distance values in the 2.1 mm range, which it uses for example to find the distance between a glass and a mask; the multi-peak controller handles up to 13 layers, and its heads cover thicknesses from 0.01 mm up to 0.7, 1.0 or 1.3 mm depending on model. [1] Keyence's SI-F80 is a thickness-type head whose 0.05–1.1 mm range is the gap between glass plates, read from a detection distance of 80 to 81.1 mm. [3]

Precitec gives interferometric ranges in air of 29 µm to 17,700 µm for the CHRocodile 2 IT family, 15 µm to 10,700 µm for the DW variant and 4 µm to 500 µm for the TW variant, and names wafers, solar cells and plastic products among its uses; each of its point sensors is a controller, a fibre optic cable and an optical probe chosen separately. [4]

The picture above shows a head of this kind on a vertical stage over a glass plate, with its fibre and signal cables entering from the side and a blue-lit spot on a sample below.

List the layers to be measured, their materials and approximate thicknesses, which faces reflect, and whether the head looks through air, vacuum or another medium.

Data for an interferometric head enquiry

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

Measurement: [distance; thickness; number of layers]
Stand-off and range: [mm]
Space for head: [diameter; straight or 90° exit]
Target: [glass, Si, metal, coating; reflectivity; tilt]
Environment: [vacuum/UHV; temperature band]
Rate and averaging: [Hz; values averaged]
Resolution and linearity needed: [nm]
Fibre: [length; routing; feedthrough]
Controller interface: [Ethernet, EtherCAT, analog]

See manufacturer records Send an enquiry through CNFX

Buyer questions

How far from the part does an interferometric head sit?

Micro-Epsilon heads start their range at 0.5, 1, 10 or 19 mm; Keyence's SI-F01 starts 0.05 mm from the head front and the SI-F10 at 11.3 mm. [1] [3]

What resolution do these heads reach?

Micro-Epsilon gives below 1 nm for the IMS5400-DS and below 30 pm for the IMS5600-DS, both with 64-value averaging at 0.5 kHz; Keyence gives 0.25 µm with 4,096 averages. [1] [2] [3]

Can one head measure more than one layer?

Yes. Micro-Epsilon's multi-peak mode reads up to 14 distance values, and Precitec's IT family measures thickness from 4 µm to 17,700 µm depending on variant. [1] [4]

What light source do they use?

Near-infrared superluminescent diodes: about 840 nm in Micro-Epsilon's controllers and 820 nm in Keyence's SI-F. [1] [3]

Sources and what still needs a direct check

Sources checked 2026-10-13. Four documents from three sensor makers: Micro-Epsilon's interferoMETER 5400-DS and 5600-DS catalogue excerpts, Keyence's SI-F specification page, and Precitec's CHRocodile 2 IT product page.

Micro-Epsilon — interferoMETER 5400-DS, absolute distance measurement (catalogue excerpt, controller and sensor heads) ↗ Manufacturer catalogue excerpt Return to text ↑
Micro-Epsilon — interferoMETER 5600-DS, absolute distance measurement (catalogue excerpt) ↗ Manufacturer catalogue excerpt Return to text ↑
Keyence — SI-F series micro-head spectral-interference laser displacement meter, specifications ↗ Manufacturer specification page Return to text ↑
Precitec — CHRocodile 2 IT interferometric thickness sensor, product page ↗ Manufacturer product page Return to text ↑

Still to check yourself

Confirm stand-off, range and head size.
Ask for resolution and linearity at your rate, averaging and material.
Check tilt tolerance and target reflectivity.
Check vacuum and fibre routing.
Verify the legal supplier and factory.