Sortation tunnels and parcel read stations point machine-vision lights at labels from 800 to 1500 mm away, with belts running 2-3 m/s. At those distances the lens angle on the light — not the LED count alone — decides whether the camera collects enough photons in a 100 µs exposure. The choice reduces to two numbers: working distance from lens to label, and the field of view (FOV) the camera must cover. This page takes a 1000 mm mount as the reference case, gives the spot-size relation, the brightness penalty for mounting height, the strobe duty arithmetic, and a decision path for the three common tunnel cases, so you can fix the lens angle before the fixtures are ordered.
The spot a lens makes at distance
A lensed LED light does not throw a cylinder of light. The cone widens with distance, and a data-sheet lens angle is the full cone width at half of peak intensity. That half-max width is the useful spot: bright in the centre, fading smoothly toward the edge. The geometry is one line of trigonometry:
D is the distance from lens to label surface and θ is the full beam angle. At D = 1000 mm and θ = 21°: θ/2 = 10.5°, tan(10.5°) = 0.185, spot = 2 × 1000 mm × 0.185 = 371 mm. The same arithmetic for common lens angles at that reference distance:
| Full beam angle θ | Spot (half-max) at D = 1 m | FOV it overfills (spot ÷ 1.5) |
|---|---|---|
| 21° | 371 mm | about 250 mm |
| 32° | 574 mm | about 380 mm |
| 45° | 828 mm | about 550 mm |
| 60° | 1155 mm | about 770 mm |
Irradiance scales with 1/spot² when the LED flux is equal. The 32° spot has (574/371)² ≈ 2.4 times the area of the 21° spot, so the narrower lens puts about 2.4 times the brightness onto the same pixels. If two fixtures also differ in rated power, multiply by the power ratio as well. The mechanics behind focused versus diffused outputs are covered in LED lens beam shaping: focused versus diffused illumination.
What mounting height costs you
Inside the cone the spot grows in proportion to distance, so brightness at the parcel falls with 1/D². One fixture with a 32° lens shows it across the common heights: 500 mm gives a 287 mm spot, the reference metre gives 574 mm, 1500 mm gives 860 mm, and relative brightness at those three heights is 4, 1 and 0.45. A light chosen on the bench at 300 mm (172 mm spot) delivers about a ninth of that brightness from the reference mount, because (172/574)² = 0.09.
Height spread adds variation of its own. Labels 100 mm above or below the design plane sit at 900 or 1100 mm, and 1/0.9² = 1.23 while 1/1.1² = 0.83: roughly +23% and −17% brightness across one tunnel. Mixed carton heights therefore smear image brightness before exposure is even set — another reason to overfill the FOV so pixels sit in the flat centre of the cone.
The strobe arithmetic that sets exposure
Belt speed sets the exposure budget. Hold motion blur to 0.25 mm at 2.5 m/s (150 m/min) and exposure is 0.25 mm ÷ 2500 mm/s = 100 µs. Tighten the budget to 0.125 mm and the pulse halves to 50 µs, doubling the irradiance the label needs. This is where the beam angle earns its keep: at 100 µs the sensor integrates only 100 µs of light, so the 2.4× irradiance of a narrow lens turns into signal level instead of camera gain.
The duty-cycle headroom is larger than it looks. Parcels every 400 mm at 2.5 m/s trigger 2500 ÷ 400 = 6.25 reads per second. At 100 µs per pulse the light is on for 6.25 × 100 µs = 625 µs each second — a 0.06% duty cycle. Driving the LEDs at 60 times their continuous current for those pulses still leaves average electrical power near 4% of the continuous rating. The binding limit is the peak pulse current the LEDs accept, which each light's data states; stay inside it. The same flux-to-brightness budgeting appears in how many lumens a machine vision light needs, and the pulse itself must switch fast enough to fit a 100 µs window, as sized for high speed barcode verification strobe pulses.
The overfill rule: angle follows field of view
A beam that exactly covers the FOV puts its half-intensity edge on your pixels. Overfill by about half again so the FOV sits inside the bright core:
For a 400 mm FOV at the reference mount height: θ_min = 2 × atan(300/1000) = 33.4°. A 32° lens (574 mm spot) therefore overfills a FOV up to about 380 mm wide; a 21° lens covers one about 250 mm wide, both straight from the table above. That is the whole decision for a single overhead read station: read the FOV width off the camera set-up, measure the mount height to the label plane, and take the narrowest angle whose spot is at least 1.5 times the FOV. Narrower than that buys brightness you cannot use; wider spills light into the neighbouring lane and dilutes it.
Fixtures for the three tunnel cases
One camera, one FOV. A lensed spot light per camera is the clean fit, and the spot lights category lists the lens angles to choose from. The LS-SPT-0024 pairs a 32° lens with 455 nm blue LEDs over a 49.9 × 66.6 mm lit area. The LS-SPT-0037 runs a 21° lens at 505 nm cyan across a 99.9 × 141.6 mm lit area.
- Size
- Under 50 mm
- Voltage
- 24 V
- Size
- 50-100 mm
- Voltage
- 24 V
At the reference mount height those two angles give the 574 mm and 371 mm spots tabulated above, so each fixture serves one camera's FOV without spilling into the next lane. For mixed cartons, white or red output gives the safest contrast on paper labels; the colour trade-offs are worked through in which LED colour gives the best contrast in machine vision.
Continuous coverage along the belt. Line scan cameras, or a read zone where parcels can sit anywhere along the belt, need light everywhere the sensor looks rather than one bright disc per FOV. A bar light along the tunnel does that. The LS-BAR-0069 has a 1097 mm lit length on 24 V, so one fixture spans more than a metre of belt.
- Size
- 400 mm +
- Type
- Standard
- Voltage
- 24 V
Short induct stations. At 250-300 mm working distance the logic reverses: a narrow lens makes a hot disc smaller than the FOV. Use a wide lens or a diffused fixture there; the options for cramped mounts are set out in compact machine vision lighting for tight spaces.
Driving it: channels, strobe and the PLC
Strobing needs a controller with a strobe input and one channel per fixture group; the reasons a dedicated unit beats a bench supply are in LED lighting controller benefits, and the light controllers category filters by channels, strobe feature and output type. The LS-CTL-0070 is a digital control box with 4 channels, 200 W, a strobe feature and RS-232 on a 24 V output. Four overhead spots fit it directly; a tunnel of eight fixtures takes two of them or an 8-channel unit.
- Output
- Constant voltage 24 V
- Channels
- 4 channels
- Control
- Digital
- Feature
- Strobe
When the PLC sequences fixtures zone by zone — induct, read, divert — the staging logic in a sequential tunnel-lighting ladder diagram for PLCs is the pattern to copy, with the vision trigger replacing the timer contacts.
When a narrow beam is the wrong answer
Glossy shrink wrap, poly bags and clear tape reflect the cone straight back down the camera axis, and a narrow beam makes that hot spot smaller and brighter, not better. Move the light off-axis so the specular reflection misses the lens, or diffuse it; the geometry choice is worked through in ring light vs bar light: choosing by part geometry and glare and in bright field vs dark field lighting.
Tall or irregular parcels break the single-plane assumption too. If labels can sit anywhere on a 400 mm tall carton under a metre-high fixture, no single cone lights them all at one brightness — either drop the fixture closer with a wider angle, or keep the narrow beam and let exposure and gain absorb the +23%/−17% spread computed above. Past that point, surface reflectivity rather than distance drives the geometry.
Checks before you order
- Measure D from lens to label plane, not to the belt: 200 mm of carton height on a 1000 mm mount is a 20% distance error and a 1.56× brightness error.
- Compute the spot as 2·D·tan(θ/2) and require spot ≥ 1.5 × FOV.
- When swapping angles, rescale expected brightness by (old spot ÷ new spot)².
- Multiply trigger rate by pulse time and confirm the duty cycle sits far inside the light's strobe rating.
- Count fixtures against controller channels and leave one channel spare for a future zone.
- Run a read-rate test at full line speed: a bench test at 300 mm tells you little about a tunnel mounted a metre up.
The lighting finder filters fixtures by size, colour and wavelength, and the light configurator walks fixture, controller and trigger selection in order. If the tunnel geometry is unusual, send belt speed, mount height and FOV through the contact page and the angle can be checked against your camera set-up before you buy.
Questions
Which working distance suits a 21° lens?
Long mounts where brightness is scarce. At 1000 mm a 21° lens gives a 371 mm spot covering about a 250 mm FOV. At 1500 mm the spot grows to 556 mm and covers about 370 mm, but irradiance falls to 0.45 of the design-height value. The 21° lens buys field of view at 1500 mm at the cost of brightness, which strobe overdrive within the light's pulse rating can recover.
How much light is lost by tilting the fixture off vertical?
Roughly the cosine cubed of the tilt. A fixture aimed 30° from vertical over a 1000 mm vertical height has a 1155 mm optical path and a 0.866 incidence factor, and 0.866³ ≈ 0.65, so about a third of the brightness is lost. Budget for that whenever lights are angled to dodge glare, and recompute the spot with the longer path.
Can one spot light cover a 600 mm conveyor belt from a metre above?
Optically yes, practically rarely. A 60° lens makes a 1155 mm spot at that height, enough to overfill a 600 mm belt, but its irradiance is about one tenth of a 21° beam's at the same flux. For strobed high-speed reads use two angled narrow fixtures per lane or a bar light along the belt, and keep the wide lens for slow or static stations.
What strobe pulse length does a 3 m/s belt allow?
Divide the blur budget by belt speed. At 3 m/s (3000 mm/s) with a 0.2 mm budget: 0.2 mm ÷ 3000 mm/s = 67 µs. Every shortening of the pulse raises the irradiance the label needs, which is the case for the narrowest lens angle that still overfills the field of view.