By Wu Peng, Senior Process Instrumentation Engineer · Last reviewed October 15, 2026
An industrial laser level sensor measures the distance from the top of a tank or silo to the surface by timing short pulses of infrared light. ABB’s LLT100, for example, uses a 905 nm Class 1 laser. It reads liquids to 30 m and solids to 100 m with a beam that spreads less than 0.3 degrees. This is a process instrument, not the laser level used on a building site to set a horizontal line.
The main reason to choose laser is the very narrow beam: it fits through a small nozzle and aims at one point in a tall, cluttered vessel. Light also has limits that radar does not. Clear liquids, dust, fog and dirty windows all affect a laser, and manufacturers disagree on how far each limit goes. The sections below give the numbers and each vendor’s position, and show where radar is the better choice.
Contents
- How it measures
- Beam and spot size
- Laser vs radar
- Clear liquids
- Bulk solids and silos
- Dust, fog and windows
- Mounting and eye safety
- Where laser fits
- Radar alternatives we build
- FAQ
How it measures
A laser level transmitter sends a pulse of light down to the surface, detects the reflection and measures the round-trip time. The working principle is time of flight:
Distance = c × t / 2 Level = H − Distance
Here c is the speed of light, about 299,792 km/s, and H is the distance from the sensor to the tank bottom. A 30 m (98 ft) distance gives a round trip of about 200 ns. The 5 mm resolution ABB quotes for the LLT100 corresponds to about 33 ps.
Unlike ultrasonic, the speed of the signal hardly depends on the gas above the surface, so temperature and vapor composition do not shift the reading. Unlike radar, a laser does not need a liquid with a high dielectric constant. It needs a surface that sends enough light back to the sensor, and most of its limits come from that requirement.
Beam and spot size
Spot size follows from the beam angle: at a distance d, a beam with full angle θ covers a circle about 2 × d × tan(θ/2) wide. With the LLT100’s divergence of under 0.3 degrees, the spot at 30 m is about 0.16 m (6 in) across. ABB’s manual gives 20 cm by 3 cm at 30 m as an example, so the real spot is elongated rather than round; treat the divergence figure as a guide.
For comparison, an 80 GHz radar with a 3 degree beam, which is narrow for radar, covers a spot about 1.57 m (5.2 ft) across at 30 m. That is roughly eight to ten times wider than the laser spot. In a silo with stiffening rings, ladders or a fill pipe, the laser beam can pass between obstructions that a radar beam would hit.
A calm, clear liquid returns only a small part of the light, so the tank bottom can return the stronger echo. The narrow spot is the laser’s main advantage over radar.
Laser vs radar
The table compares a named laser transmitter with our own 80 GHz radar, so every value has a datasheet behind it. For radar against ultrasonic, see our ultrasonic vs radar level sensor comparison.
| Point | Laser (ABB LLT100) | 80 GHz radar (FMW) |
|---|---|---|
| Signal | 905 nm infrared pulses | 80 GHz FMCW microwave |
| Beam | Under 0.3° | 3° to 8° by lens |
| Range | 30 m liquids, 100 m solids | Up to 150 m by model |
| Accuracy | ±11 mm typical | ±1 mm typical |
| Dielectric | Not a factor | Weak echo on low-εr liquids |
| Clear liquids | Special mode; vendors disagree | Water reflects strongly |
| Dust and foam | Dust tube advised; light rarely passes foam | Passes dust; foam can cut range |
| Process | −40 to +60 °C (280 °C with cooling tube); −1 to +2 bar base model | −40 to +260 °C, −0.1 to 2.5 MPa by model |
Sources: ABB LLT100 data sheet (Rev. F); Instranova FMW datasheet. The LLT100 also has a pressure-rated model to 49.6 bar, depending on the flange.
On these two datasheets, laser leads on beam width and is unaffected by dielectric constant. Radar leads on accuracy, range and dust, and handles water without a special mode.
Clear liquids
Manufacturers disagree on clear liquids. A calm liquid surface acts like a weak mirror. For water, with a refractive index of about 1.33, the Fresnel equation gives about 2% of the light reflected straight back at normal incidence. The rest goes into the liquid, and if the liquid is clear, it reaches the tank bottom and can return a stronger echo than the surface.
| Source | Position on clear liquids |
|---|---|
| ABB LLT100 manual | Supported with a clear liquid mode, which adds a bottom-of-the-tank detection algorithm |
| ifm (O1D) | Not appropriate for clear liquids; opaque liquids and solids only |
| Novus (TL400) | At low levels the bottom may reflect more light than the surface; a floating white foam target is suggested |
| BinMaster (LL-100) | Listed for opaque liquids |
The four positions agree on one point: clear liquid is possible only with an instrument designed to tell the surface echo from the bottom echo, and the risk is largest when the level is low. For water and clear water-based liquids, radar is the simpler choice, since water reflects microwaves strongly. Our radar level sensors range lists the options.
Bulk solids and silos
Solids suit laser well. A pile of pellets, grain or aggregate scatters light in all directions, so some light returns to the sensor at almost any surface angle. ABB rates the LLT100 to 100 m on solids, more than three times its liquid range. BinMaster lists the LL-100 for low-dust bulk solids, pellets and granular materials up to 50 m.
Because the beam is narrow, it measures a single point. A silo surface typically forms a cone when filling and a crater when emptying, so the reading depends on where the beam lands. Aim it at a known, fixed point and accept that a single point is not the average level. For heavy dust, BinMaster points to 80 GHz radar or 3D acoustic scanners instead.
Dust, fog and windows
Anything in the air between the sensor and the surface scatters light. ABB lists dust and fog penetration among the LLT100 features. Its manual still strongly recommends a dust tube where dust is present and the heated window option in foggy conditions. BinMaster states that the LL-100 should not be used in dusty environments.
Foam and steam are harder. David Spitzer, writing in Control Global in 2006, notes that laser energy typically does not penetrate foam, and that steam is a vapor that reflects light. Novus lists dust, opaque smoke and dense foam as causes of incorrect readings. For any of these, guided wave radar or a pressure-based method is the more reliable route.
The optics need the same care. A coated or wet lens weakens the return, and the same applies to a sight glass, which can become dirty. Keep the optics clean, add purge air where the vessel allows it, and shield the sensor from direct sunlight; Novus warns that direct sunlight on its sensor can cause irreparable errors.
Mounting and eye safety
On liquids, mount the sensor so the beam is as close to perpendicular to the surface as possible. ABB states that a misalignment of up to 5 degrees may be acceptable in very good conditions. Over long distances, it says, a deviation of only 1 to 2 degrees may be needed for best performance. On solids the angle matters less, because the surface scatters light; BinMaster rates the LL-100 within ±1 in regardless of surface angle, slope, texture or color.
Keep the beam clear of the wall and of any fill stream. Novus recommends positioning its sensor away from the walls and a tank diameter of at least half the height. A small nozzle is enough for the beam itself, which helps on crowded tank roofs.
The sensors checked for this guide use eye-safe classes. The LLT100 is Class 1 at 905 nm, which is invisible, and BinMaster rates the LL-100 Class 1M. Spitzer notes that some sensors use Class 3 lasers, which can pose a risk of eye injury, so check the class on the datasheet before installation.
Hazardous-area versions exist. ABB lists the LLT100 as explosion proof for Class I, Division 1 (Zone 1), with flameproof, dust-ignition-proof and intrinsically safe versions. Confirm the approval against your area classification before ordering.
Where laser fits
A first screen by service, using the limits above:
| Service | Laser? | Better option |
|---|---|---|
| Low-dust pellets, grain, aggregate | Good fit | Radar for solids if dusty |
| Opaque liquid, crowded roof | Good fit | 80 GHz radar |
| Positioning with a target | Good fit | Not applicable |
| Clear water or solvent | Only with a clear liquid mode | Radar; check εr of solvents |
| Heavy dust during filling | Poor | 80 GHz radar |
| Foam or steam | Poor | Guided wave radar or differential pressure |
The ABB 200 m figure applies to positioning with a reflective target, not to level. For the other technologies in the right-hand column, see our level transmitter types overview and the guided wave radar level transmitter.
Radar alternatives we build
We do not list a laser level transmitter. If the reason for considering laser is a narrow beam through a small nozzle, narrow-beam radar covers much of that need without the clear-liquid, dust and window issues. These are our closest alternatives:
| Model | Range | Beam | Accuracy |
|---|---|---|---|
| FMW-13, 80 GHz | 0.3 to 150 m | 3°, 76 mm lens | ±1 mm |
| FMF-13, 120 GHz | 0.1 to 100 m | 3°, 78 mm lens | ±2 mm (±5 mm over 35 m) |
| SIRD-804, 6 GHz | Up to 35 m | Horn | ±20 mm |
From our datasheets. The SIRD-804 is for lumps and granules such as coal or clinker, not fine powder. Confirm the model for your vessel.
Full data is on the 80 GHz radar level transmitter, 120 GHz radar level transmitter and radar level sensor for solids pages. For hot, dusty vessels such as an iron furnace, our blast furnace level measurement guide covers the radar approach.
Application note
Choosing between laser and radar. Before a laser is specified, check four points: whether the liquid is clear or opaque, how much dust is in the air during filling, whether foam or steam forms, and how the window will be kept clean. If any answer is unfavorable, a narrow-beam radar, or guided wave radar for foam and steam, is usually the lower-risk choice.
FAQ
What is a laser transmitter?
In general, a laser transmitter is a device that emits a focused laser beam to send a signal or project a reference line. In level measurement it means a laser level transmitter, which times infrared light pulses to the surface of a liquid or solid and outputs the distance or level, typically as 4‑20 mA. The laser levels used on building sites to project a reference line are a different product.
Request a quote
Send the vessel height and nozzle size, the material and whether it is clear or opaque, any dust, foam or steam, and the process temperature and pressure. We will tell you whether a narrow-beam radar fits. Tell us the application and we configure one unit, not a shelf part. Reach our application engineers or use the form below.
Written and technically reviewed by Wu Peng and the Instranova engineering team.