Telecentric Measurement Lighting: How to Choose Backlight, Coaxial and Dark-Field Illumination

A telecentric lens removes perspective error and magnification drift. It does not create the edge you are about to measure — illumination does. On a telecentric dimensional station the lighting choice decides whether an edge exists as a geometrically defined boundary or only as a soft brightness transition, and that difference survives every later stage of the measurement chain. This page sets out the three illumination families, the selection table by measurement task, the beam-diameter and working-distance matching calculation, the fields you must record with any lighting condition, and four worked selection examples. All POMEAS figures quoted here are taken from published product specifications.

Reviewed by Tiger Xie · Published 2026-10-14 · POMEAS optical application team

1. Why illumination decides the edge in a telecentric measurement

An object-side telecentric lens accepts only the ray bundle that is essentially parallel to the optical axis. That property is what removes the perspective term from the measurement: a feature does not change its apparent size when the part moves up and down inside the depth of field. The same property is what makes the measurement depend on illumination, because the lens is no longer free to integrate light arriving from a wide range of directions.

The consequence is a clean division of labour:

Stage	What it fixes	What it cannot fix
Optics (telecentric lens)	Perspective error, magnification consistency across the field, depth-of-field behaviour	Whether the target feature produces contrast at all
Illumination	Which physical boundary of the part becomes the bright-to-dark transition that the edge detector finds	Optical distortion or calibration error
Algorithm	Sub-pixel position of the transition	A transition that is wide, noisy or intensity-dependent
Calibration and environment	Scale factor, traceability, thermal stability	An edge whose position changes with light level

The practical reading of that table is simple: the illumination decides which edge gets measured. On a chamfered part, a transmitted setup sees the outer silhouette, a bright-field reflected setup sees the chamfer face, and a low-angle setup sees the chamfer edge as a bright line. All three are “the edge”, and they sit at different coordinates. Choosing the lighting is therefore part of defining the measurement, not a lighting detail to be settled after the optics.

2. The three illumination families and what each one can measure

Machine-vision lighting is usually catalogued by geometry — ring, bar, dome, coaxial, backlight. For a telecentric measurement station the more useful classification is by where the contrast comes from, because that is what determines whether the detected edge is a dimension or a surface feature.

Family	Light path	Edge origin	Measures	Typical hardware
Transmitted (backlight)	Source behind the part, lens in front	Geometric silhouette — the part blocks the light	Outer profile, hole diameter, pitch, slot width, gap	Diffuse backlight; telecentric / collimated parallel backlight
Reflected bright-field	Source on the lens side; the part reflects light back into the lens	Reflectance change across the surface	Printed characters, pads, flat surfaces, machined faces, transparent-part interfaces	Coaxial (through the lens), ring at high angle, bar, dome
Reflected dark-field	Source on the lens side at a low angle to the surface	Surface relief — slopes and edges scatter light into the lens while the flat background scatters away from it	Scratches, burrs, engraving, edge chips, texture, dents	Low-angle ring, low-angle bar, spot / collector light

A single station may legitimately use two families at two camera positions — for example a transmitted view for the outer dimension and a dark-field view for a burr check on the same part. What must not happen is to mix them in one image and then treat the resulting edge as one measurement.

Where the three families sit relative to each other on accuracy is not a property of the lighting. It is a property of the whole chain — telecentricity, calibration, algorithm, fixturing and environment. Section 11 of this page states this explicitly, and it is the reason this page never ranks the families by accuracy.

3. Transmitted illumination: the default for dimensional measurement

For external profile, hole position, pitch and slot width, a transmitted setup is normally the first configuration to evaluate, because the contrast mechanism is a blocking/unblocking of light rather than a reflectance difference. The part is opaque; the background is a uniform emitting surface. Nothing about the part’s surface finish, colour or oxidation enters the measurement.

3.1 Diffuse backlight versus telecentric (collimated) parallel backlight

Both illuminate from behind. They differ in how the emitted light is distributed in angle.

Property	Diffuse backlight	Telecentric parallel backlight
Angular distribution	Broadband — light leaves every surface point over a wide range of angles	Collimated — light leaves the emitting surface close to the normal
Structure	Diffuser plate over an LED array	LED array with a collimating / condenser stage
What it pairs with	Any lens; commonly used with normal and telecentric lenses for silhouette work	Object-side telecentric lenses, particularly at high magnification
Edge transition	Wider transition, contrast falls off as the part leaves the diffuser	Narrow transition, contrast largely maintained over the rated working distance
Effect of part-to-source distance	Noticeable on transition width and contrast	Much smaller — this is the design purpose
Typical use	General silhouette inspection, lower-magnification overview	High-magnification dimensional measurement, thick parts, edge position that must not move

The reason a collimated backlight is preferred at high magnification is a matter of angular budget. A high-magnification telecentric lens has a small object-side numerical aperture — the manufacturer specification for LTCM2-110C, for example, states NA 0.0074. Such a lens accepts a very narrow cone of directions. A diffuse emitter fills that cone with light that has crossed the object region at many different angles, whereas a collimated emitter arrives already normal to the object plane. The practical difference shows up as the width of the bright-to-dark transition and as how much that transition responds to a change in light intensity or in part height.

3.2 How to find out whether the edge position is distance-sensitive

This is a measurement you run, not a number quoted from a catalogue. The procedure is short:

Step	Action	What it isolates
1	Set up the transmitted view with the part resting on a gauge block of known thickness; focus the lens; record the measured outer dimension	Baseline
2	Repeat with gauge blocks of two other thicknesses, re-focusing each time so the part is imaged in the same plane	The part-to-source distance term
3	At one thickness, run the measurement at three light levels (about 40 %, 60 % and 80 % of full scale) without changing exposure or gain	The intensity term
4	Compare the spread of the results with the tolerance you have to hold	Whether the lighting is good enough for the job, using your own data

Step 2 is where a diffuse and a collimated backlight usually separate; step 3 is where the edge detector’s threshold sensitivity shows up. Both are properties of the whole station, and both are worth recording alongside the measured value.

3.3 Matching the beam diameter to the field of view

The most common transmitted-lighting mistake is a backlight that is smaller than the field of view. The emitting area must cover the whole camera field plus the part-position tolerance, otherwise the corners of the image lose their reference and the edge detector is being asked to find a transition against a dark background.

POMEAS telecentric parallel backlights are specified by beam diameter and working distance. The published VP-GLTD series covers beam diameters of 30 / 60 / 100 / 145 / 189 / 265 mm with working distances from 45 mm to 500 mm depending on the model, in red / green / blue / white.

Beam diameter	Fits a field of view up to (with margin)	Typical application scale
30 mm	roughly 25 mm	Small precision parts, pins, small gears, connectors
60 mm	roughly 50 mm	Stamped parts, small brackets, thread profiles
100 mm	roughly 85 mm	Medium machined parts, multiple cavities in one view
145 mm	roughly 125 mm	Larger housings, multi-part trays
189 mm	roughly 160 mm	Large flat parts, gaskets, seals
265 mm	roughly 225 mm	Very large flat parts

The “fits a field of view up to” column applies a working margin of about 15 % so that part-position error does not walk the feature to the edge of the emitting area. Verify the actual illuminated field on the bench rather than assuming the nominal diameter equals the useful illuminated area, because the usable uniform region is smaller than the mechanical aperture.

3.4 Corner-cut (right-angle) backlights when mounting space is tight

A straight-through backlight requires a coaxial stack: camera, lens, part, backlight, all on one axis. When the machine frame or a conveyor occupies that space, a right-angle backlight moves the emitter axis off to the side and turns the beam through 90° with a mirror, while keeping the emitting surface parallel to the part.

The published VP-GLTDL corner-cut series offers 35 / 60 / 100 / 145 / 189 / 265 mm beam diameters with recommended working distances of 45–90, 90–180, 150–300, 100–300, 250–500 and 150–500 mm respectively, in red / green / blue / white. When the two series are compared, note that the same nominal diameter can carry a different recommended working distance range: always match the backlight working distance to the geometry of your fixture, not only to the diameter.

4. The part-to-emitter distance is the variable most often left uncontrolled

In a transmitted setup the part almost never sits directly on the emitter. It sits on a fixture, on conveyor rails, on a glass plate, or on locating pins. Each of those adds a gap between the part and the emitting surface, and that gap is a parameter of the optical setup even though it is a mechanical feature.

Three consequences follow, and they are the reason the gap belongs in the drawing of the station:

It changes the illuminated field. The emitting area is not perfectly uniform, and a part lifted away from it samples a larger region of that area. A gap that varies with the part keeps the illumination condition from being constant.

It changes what a fixture window adds. A glass or acrylic support plate between the part and the emitter introduces its own refraction, its own surface reflections and its own scratches. At high magnification a window that is tilted by even a small angle will bend the transmitted beam and move the apparent edge. If a window is unavoidable, specify optical-quality material, keep it thin, keep it perpendicular, and keep it clean and inspectable.

It interacts with the illumination’s angular distribution. This is where a diffuse and a collimated backlight behave differently, and it is why the procedure in section 3.2 is worth running with the actual fixture in place. The result is a property of the assembled station, not of the backlight catalogue entry.

The engineering response is mundane but effective: fix the gap with a hard stop, put the gap value on the station drawing, and include it in the maintenance checklist. A gap that gets re-set by hand every time the fixture is cleaned is a gap that will eventually be set differently.

5. Coaxial illumination: when there is no silhouette to measure

A transmitted setup requires the part to block light. A polished metal cover, a glass panel, a wafer or a solder pad does not block usefully in the region you care about, because the feature is a surface condition rather than a hole. Coaxial illumination puts the light path on the lens axis, so that a flat surface facing the lens returns the light straight back and appears bright, while a scratch, a dent or a locally tilted region sends the light elsewhere and appears dark.

The published GC series coaxial light source is specified at 24 V DC with red 625 nm / white 6500 K / blue 470 nm as standard colours, optional green 525 nm and infrared 850–950 nm, a power range of 0.8–120 W depending on model, and a stated life of more than 30 000 hours at 50 % brightness for white. Its stated intended surfaces are mirror metal, glass, wafers and other highly reflective parts, for scratch, character, 2D-code and surface-defect detection.

Coaxial illumination has a structural requirement that is easy to miss: the light path through the lens has to exist. Either the lens is built with an internal coaxial port, or an external coaxial illuminator has to be introduced in front of a lens that is mechanically able to accept it. On POMEAS telecentric lenses the models with a built-in coaxial path include the LTCM high-resolution coaxial telecentric family, such as LTCM2-110C, LTCM1-110C, LTCM05-110C and LTCM03-110C. The choice between a built-in coaxial port and an external illuminator on a standard telecentric lens is covered in detail on the built-in coaxial telecentric selection guide and the coaxial illumination for reflective metal page; this page does not repeat that material.

What belongs here is the boundary. Coaxial illumination is a surface technique. It will not give you an outer dimension of a part whose edge is a silhouette, and it will not resolve a hole through an opaque part. Conversely, a transmitted backlight will not tell you whether the flat top of that part is scratched. If a drawing asks for both, the answer is two views — not one lighting that is hoped to do both.

6. Dark-field and low-angle illumination: surface relief, not dimension

In the dark-field configuration the light arrives at a shallow angle to the surface. A smooth flat region reflects the light away from the lens and stays dark; a scratch, a burr, a chamfer edge or a stamped character deflects part of that light into the lens and appears bright. The result is high contrast for relief features and very low contrast for the flat background, which is exactly the inverse of a bright-field setup.

Illumination	Appears bright	Appears dark	Use it for
Coaxial (bright field)	Flat surfaces facing the lens	Scratches, dents, tilted regions	Surface quality on mirror-like parts; characters on flat metal
Ring, high angle (bright field)	Diffusely reflecting surfaces	Dark or absorbent features	General surface inspection, character and mark reading
Low-angle ring or bar (dark field)	Edges, burrs, scratches, relief	Smooth flat surfaces	Burr and chamfer inspection, engraving, edge defects
Dome / diffuse (bright field)	Curved and multi-orientation surfaces uniformly	Deep features that receive no light	Complex curved housings, highly reflective curved parts

POMEAS publishes a parallel surface source family in the 27 × 27 mm to 200 × 200 mm size range at 0.8–29 W, a bar light family at 50 / 100 / 150 / 200 / 250 / 300 mm lengths, a ring family with outer diameters from about 25 mm to 350 mm and a diffuse shadow-free ring family at 80 / 116 / 132 / 166 / 196 mm outer diameters, and a dome (shadow-free) family spanning GR70 to GR325. The standard white output is quoted at approximately 6500 K, red at approximately 625 nm and blue at approximately 470 nm, with green 525 nm and infrared 850–950 nm available depending on series.

The dark-field family should not be used to set a dimensional limit, for a specific reason: the bright line that appears at a burr or a chamfer edge is the image of a surface feature whose width and position depend on the illumination angle, on the scatter properties of the material and on the roughness of the edge. It is an excellent presence/absence signal and a poor coordinate reference. If a drawing specifies a dimension to a chamfer, that dimension needs a transmitted silhouette or a calibrated bright-field view with a clearly named edge definition.

7. Transparent, multilayer and chamfered parts

These parts break the simple assumption that a feature is either opaque or reflective, and they are where most lighting mistakes survive all the way to production.

Transparent parts. A clear part transmits most of the light, so a transmitted setup shows the outer moulding as a faint edge while internal interfaces, thickness variation and dirt on the far surface also appear. The useful configurations are usually: a bright-field transmitted setup with the light level set so that the edge is defined but internal refraction does not saturate the region, or a combination in which a coaxial or oblique reflection channel defines the top surface interface while a transmitted channel defines the silhouette. Which one measures what must be written down, because the two channels will not agree to the last digit and that difference is not an error.

Multilayer parts. Laminated stacks, coated glass and bonded assemblies have several interface planes inside the depth of field. A transmitted view in which the interfaces are all equally sharp means the depth of field is being used to integrate information from more than one plane. Reduce the aperture and the depth of field until only the interface of interest is sharp, or define the measurement on a different feature.

Chamfered and rounded edges. A chamfer is a face, so it behaves like a surface: whether it appears bright, dark or neutral depends on the illumination direction. The silhouette behind the chamfer still exists, and in a transmitted setup the detected edge will be the outermost extent of the part, which is the theoretical sharp corner position rather than the start of the chamfer. If the drawing dimension is to the theoretical corner, this is what you want and it should be stated. If the drawing dimension is to the shoulder, the transmitted silhouette is measuring the wrong feature and no amount of lighting adjustment will change that.

The general rule for all three cases is the same: name the geometric feature on the drawing first, then choose the lighting that makes that specific feature a transition. Selecting a light and then deciding which edge looks cleanest is how a station ends up measuring something that is not on the drawing.

8. Lighting selection table by measurement task

The table below is the operational form of the discussion above. Read it from the left column: identify the measurement task, then take the illumination family in the second column as the first configuration to evaluate. The third column states the reason, and the fourth lists what to watch.

Measurement task	First choice	Why	Watch out for
Outer profile / overall dimension	Transmitted — telecentric parallel backlight	Edge is defined by blocking, not by reflectance; stable edge for thick and thin parts	Beam diameter must cover the field; part-to-emitter gap must be fixed
Hole diameter, pitch, slot width	Transmitted — telecentric parallel backlight	Through-features produce clean silhouettes	Burrs at the hole edge add a false outer boundary; check both edges
Gear tooth profile, connector pitch	Transmitted — telecentric parallel backlight	Many edges in one view, all must be equally defined	Field uniformity across the whole tooth pattern
Precision dimension on a thick or tall part	Transmitted — collimated / telecentric backlight	Edge position is least affected by the part’s height and by light level	Confirm the rated working distance covers your gap
Flat polished / mirror-metal surface defects	Coaxial through the lens	Only a surface normal to the axis returns light to the lens	Lens must have a coaxial path; dust on the internal optics shows up directly
Glass panel, wafer, transparent interface	Coaxial, often combined with a transmitted channel	Interface location must be resolved, not only the silhouette	Multiple interfaces inside the depth of field
Printed characters, marks, 2D codes	Coaxial, ring or bar depending on print contrast	The mark is a reflectance feature	Specular highlight from a curved or angled surface
Burrs, chamfer edges, engraving, scratches	Low-angle ring or bar (dark field)	Relief scatters light into the lens; flat background stays dark	Not a coordinate reference — use for presence/absence
Curved or complex reflective housings	Dome / diffuse shadow-free, or a multi-direction combination	Uniform illumination from many directions avoids a moving specular highlight	Dome aperture limits the usable field and working distance
Small local feature, local height difference	Spot / collector light at a controlled angle	Concentrated light creates a deliberate shadow that makes the local step visible	Angle sensitivity — the result is easy to disturb during maintenance

9. Worked examples: four lighting selections

Four short examples, each showing the arithmetic that the selection actually requires. The numbers are from published POMEAS specifications and from the station geometry stated in each example.

Example 1 — 20 mm gear blank, outer diameter, on a telecentric station

Given: the outer diameter is the controlled dimension and the part is opaque. The camera field must hold the gear plus positioning tolerance, so a 24 mm field is required.

Lighting: transmitted, because the controlled feature is a silhouette. Beam diameter: 24 mm field + 15 % margin ≈ 27.6 mm, so the 30 mm class is the smallest suitable beam diameter. Working distance: the backlight sits behind the fixture, so its working distance is the distance from the emitter face to the part. If the fixture raises the gear 60 mm above the emitter plane, a model rated 45–90 mm covers the gap; if the fixture raises it 120 mm, the same 30 mm-diameter class in the straight-through family may not be rated that far, and either the fixture height is reduced or the next model up in rated distance is selected. What to record: beam diameter, emitter-to-part gap, light colour and drive current, and the result of the gap-variation check from section 3.2.

Example 2 — polished stainless cover, surface scratch check

Given: the surface is mirror-like and the defect is a shallow scratch. An outer dimension is taken on a separate view.

Lighting: coaxial, because the contrast mechanism must be the surface normal. A transmitted setup is useless here — the part is opaque and the scratch does not change the silhouette. Lens: one that provides a coaxial path, for example the LTCM2-110C built-in coaxial telecentric lens, or an external coaxial illuminator in front of a lens that accepts one. Light colour: for a scratch, wavelength affects the scatter and the diffraction limit; short wavelengths resolve finer relief, while the contrast of a shallow scratch often depends more on the illumination aperture and on whether the source is collimated than on colour alone. Test with the available colours on a known defective sample and keep the winner. Watch: a mirrored part also mirrors the environment — a bright object reflected in the part can look like a defect.

Example 3 — transparent tube, outer diameter and wall thickness

Given: a clear extruded tube where both the outer envelope and the wall thickness matter.

Lighting: two channels. A transmitted collimated backlight gives the outer envelope as a silhouette. A coaxial or oblique bright-field channel gives the top surface interface. Reason the transmitted channel alone is not enough: in a clear tube the silhouette edge is formed by refraction as much as by blocking, so the apparent outer edge depends on the wall curvature and on the refractive index; the outer-extent reading is a good repeatable reference but its absolute relation to a physical surface must be established on a known part. Record: which channel produced which reported dimension, and the calibration artefact used for each.

Example 4 — burr check on a stamped bracket edge, with the dimension measured in the same station

Given: the bracket outline is a controlled dimension and the burr height is a pass/fail check.

Lighting: two views. Transmitted collimated backlight for the outline, low-angle bar or ring for the burr. Do not attempt to take the outline dimension and the burr check from the same image: the dark-field configuration that makes the burr visible simultaneously makes the outer silhouette edge unreliable as a coordinate reference, because the bright line is generated by relief rather than by blocking. Record: separate lighting records for the two views, even when they share the same camera and lens.

10. What to record with every lighting condition

A lighting condition is part of the measurement; if it is not recorded it is not reproducible. The following fields are the minimum set. They mirror the illumination requirements of the POMEAS TR-001 performance test method reference, which requires illumination type, geometry, spectrum, uniformity and stability to be recorded for any reported measurement.

Field	Example of what to write	Why it matters
Family and geometry	Transmitted telecentric parallel backlight, 30 mm beam, coaxial to the lens axis	Determines the edge origin
Source model and colour	Red, nominal 625 nm	Contrast and diffraction behaviour; colour is part of the condition, not a preference
Drive condition	Constant current, 350 mA, continuous	If PWM dimming is used, its frequency interacts with short exposures
Emitter-to-part distance	60 mm, set by the fixture hard stop	Changes illuminated field and transition behaviour
Peak signal level	65–75 % of full scale	Keeps the detector away from clipping and away from the low-signal noise region
Uniformity over the region of interest	±5 %	Non-uniformity changes effective edge contrast across the field
Stability check	Reference frame at the start and end of the series; mean grey value difference within 2 %	Detects drift that would otherwise appear as repeatability error
Filtering	Band-pass filter at 0.55 µm for MTF work, or “none”	White-light MTF measurement broadens the edge and depresses the measured value
Attachment	Diffuser, polariser, protective window — with their part numbers	These change the light and are easy to forget in a later rebuild

11. Illumination does not make a lens more accurate

This section exists to close a common expectation. Changing the lighting does not improve the accuracy of a telecentric lens. What it changes is whether the feature you intend to measure produces a transition that the lens and the algorithm can locate consistently.

Accordingly, this page does not state that any illumination family is more accurate than another, and it does not publish a ranking of backlight versus coaxial versus dark-field. Accuracy on a measurement station is the combined result of the lens telecentricity, the calibration and its traceability, the edge-detection algorithm and its thresholding rule, the fixturing, and the environmental stability — all of which are treated in the TR-001 reference and on the telecentric lens selection guide.

The defensible statement is narrower and more useful: a lighting configuration that produces a narrow, high-contrast, stable transition for the specific feature on the specific part makes it easier to reach a given measurement uncertainty, and a configuration that produces a wide or intensity-dependent transition makes it harder. Whether a particular pair of backlights differs enough to matter is decided by the check in section 3.2, performed on the actual station, and not by a general claim about lighting types.

12. Common mistakes

Mistake	What it looks like	Correction
Backlight smaller than the field	Corners of the image go dark; measured width depends on where the part sits	Size the beam to field plus part-position tolerance, with margin
Brightness used as the adjustment	Operators re-tune the light every shift; measured values drift with it	Fix the drive condition and the peak signal level; treat any adjustment as a change to the measurement
Part-to-emitter gap left to the fixture	Two identical stations disagree; the difference appears after maintenance	Put the gap on the drawing with a hard stop and include it in the checklist
Dark-field used for a dimension	The “edge” moves when the part surface changes	Use dark field for presence/absence; take the dimension from a transmitted or calibrated bright-field view
Ring light used with a telecentric lens as a default	Convenient, but the edge definition is now reflectance-based and surface-dependent	Start from the measurement task table in section 8, not from the light that is already on the shelf
Coaxial assumed to work on curved parts	A moving specular highlight walks across the surface	Dome or diffuse illumination, or a multi-direction combination
Lighting changed without updating the record	The old measurement and the new one are silently different measurements	Any lighting change re-opens the lighting record and the stability check

13. Frequently asked questions

What illumination should be used with a telecentric lens for dimensional measurement?

For an outer profile, hole position, pitch or slot width on an opaque part, start with a transmitted backlight — a telecentric parallel backlight when the part is thick or the edge position must not vary with part height. Use coaxial illumination when the controlled feature is a surface condition on a reflective part, and low-angle lighting when the feature is relief such as a burr or an engraving. The selection table in section 8 lists this task by task.

Do I need a telecentric backlight, or is a diffuse backlight enough?

It depends on how much the edge reading is allowed to vary with part height and with light level. A diffuse backlight is usually adequate for overview silhouette work. Where the part is thick, the magnification is high, or the tolerance is a small fraction of a millimetre, a collimated telecentric backlight removes the part-to-emitter distance from the list of influencing factors. Decide with the check in section 3.2 rather than from a general rule.

How do I know if my backlight is large enough for my field of view?

The emitting area must cover the camera field plus the part-position tolerance, with a margin — about 15 % is a practical starting figure. A 24 mm field therefore asks for roughly a 28 mm beam, so the 30 mm class is the smallest suitable size. Verify the actual uniformly illuminated area on the bench, because the usable uniform region is smaller than the mechanical aperture.

Can I use a ring light with a telecentric lens?

Mechanically yes, and it is sometimes the right answer for surface and character inspection. But it converts the measurement from a silhouette to a reflectance measurement, so the detected edge now depends on the surface finish, the angle and the light level. For a dimensional feature on an opaque part, a transmitted backlight remains the configuration to evaluate first.

When is coaxial illumination the right choice?

When the feature you must measure is a surface condition on a part that reflects specularly: a scratch, a dent, a printed character on polished metal, a glass or wafer interface. Coaxial illumination requires a light path through the lens, so either the lens has a built-in coaxial port or an external illuminator must fit in front of it.

My part is polished metal — why does backlighting not work?

Backlighting needs the part to block light. If the controlled feature is a surface mark on an opaque polished part, the silhouette does not contain that feature at all, so no transmitted configuration can reveal it. The dimension and the surface check need two different views.

Can a telecentric lens measure a transparent part?

Yes, but the transmitted silhouette of a transparent part is formed by refraction as much as by blocking, so the relation between the apparent edge and the physical outer surface has to be established on a known part. Transparent and multilayer parts commonly need a second channel — coaxial or oblique bright field — to define the interface that the drawing actually calls out.

Does changing the backlight change the measured dimension?

It can change the reading, and that is useful diagnostic information rather than a defect. A change in beam size, part-to-emitter gap, light level or colour alters the transition that the edge detector locates. The remedy is not to re-tune until the number matches the drawing; it is to fix the lighting condition, record it, and re-verify the measurement with a traceable artefact.

What is a corner-cut backlight for?

It turns the beam through 90 degrees so the emitter can be mounted to the side while its output remains parallel to the part. It exists for cases where the machine frame, conveyor or tooling occupies the space behind the part and a straight-through stack cannot be built. The POMEAS corner-cut family covers 35 to 265 mm beam diameters.

What illumination information should be recorded in a test report?

At minimum the family and geometry, source model and colour or spectrum, drive condition, emitter-to-part distance, peak signal level as a percentage of full scale, uniformity over the region of interest, the stability check result, any filter or diffuser used, and whether the illumination itself is telecentric. This is the field set required by the POMEAS TR-001 reference for any reported measurement.

Does a brighter light make the measurement more accurate?

No. Brightness changes the signal level and therefore the operating point of the edge detector, but it does not change the optical or calibration contributions to accuracy. The useful target is a stable, unclipped signal level — commonly kept in the 60–80 % of full-scale range — held constant across the run, not the brightest image available.

Can I use the same lighting for a zoom lens at every magnification?

The lighting condition has to be re-checked at each magnification you actually measure at, because the field of view, the object-side numerical aperture and the visible depth of field all change with magnification. A backlight that covers the field at low magnification may vignette the field at high magnification, and the working magnification range of a motorized zoom lens is wide enough that this is a real risk. Record the lighting condition per magnification.

14. Get a telecentric lighting and lens selection review

Send the part drawing, the feature that must be measured, the tolerance, the camera and lens you intend to use, and a photograph of the space available behind the part. We will come back with the illumination family, the beam diameter and working distance, the lens that provides the required coaxial path if one is needed, and the fields to record so that the station is reproducible.

Contact: [email protected] · WhatsApp +86 185 9810 2007

Reviewed by Tiger Xie. Published 2026-10-14. Parameters quoted on this page are taken from POMEAS published product specifications at the date of publication; verify the current specification before design freeze.

Knowledge

Recent Knowledge

Produuct Categories

8618598102007