How to Read Blurred Barcodes: The Synergy Between Shutter Speed and Lighting

Barcode and datamatrix reading on moving parts is one of the most challenging illumination problems in industrial machine vision. The fundamental issue is motion blur: when a barcode moves during the camera exposure, the image of the barcode bars and spaces is smeared in the direction of motion. Smearing reduces the edge contrast between bars and spaces. Below a critical contrast level, the barcode decoder fails to read the symbol. The result is a missed read, which halts production or forces manual recovery.

The solution is to freeze the motion optically, not mechanically. Reducing the camera exposure time to the point where the barcode moves less than a small fraction of a bar width during the exposure eliminates visible motion blur. For a barcode moving at typical conveyor speed with typical bar widths, this requires exposure times of 50 to 500 microseconds. At these short exposures, the illumination must deliver sufficient photons to the sensor in the available time. Standard continuous-mode illuminators cannot provide adequate intensity for such short exposures. Pulsed strobe illumination is the engineering solution.

The Physics of Motion Blur in Barcode Reading

Motion blur is directly proportional to the product of the object velocity and the exposure duration. A barcode moving at 1 m/s with an exposure time of 1 ms moves 1 mm during the exposure. If the narrowest bar width in the barcode symbol is 0.5 mm, the barcode has moved by two bar widths during the exposure. This produces severe smearing and guaranteed read failure.

The acceptable blur limit for barcode reading is typically defined as a fraction of the minimum bar width, known as the X-dimension. ISO barcode quality standards define bar edge contrast requirements that must be met for a symbol to achieve a given quality grade. Motion blur reduces bar edge contrast in proportion to the ratio of the blur distance to the bar width. For reliable reading, the motion blur must be kept below 10% to 20% of the X-dimension.

Calculating the Required Exposure Time

The required exposure time is calculated from the object velocity, the barcode X-dimension, and the allowed blur fraction. For a barcode with a 0.33 mm X-dimension moving at 0.5 m/s, the allowable blur is 0.033 to 0.066 mm (10-20% of X-dimension). The required exposure time is 0.033 mm divided by 500 mm/s, which equals 66 microseconds maximum. For a barcode on a high-speed conveyor running at 2 m/s with the same X-dimension, the required exposure time falls to 16 microseconds.

These exposure times are well within the capability of modern machine vision cameras with global shutter sensors. The challenge is entirely on the illumination side: providing enough light for a correct exposure in 16 to 66 microseconds requires illumination intensities that are one to two orders of magnitude higher than what standard continuous-mode LED illuminators provide.

Strobe Illumination: How Peak Intensity Compensates for Short Exposure

LED illuminators can be operated in two fundamentally different modes: continuous mode and strobe mode. In continuous mode, the LEDs are driven at a constant current and the illuminator produces a steady, constant output. The maximum continuous current is set by the thermal capacity of the illuminator and its heatsink. Exceeding this current causes the LED junction temperature to rise to levels that permanently degrade the LED performance.

In strobe mode, the LEDs are driven at a much higher peak current for a very short duration, then switched off to allow the junction to cool before the next pulse. The peak optical output during the pulse is proportional to the peak drive current and can be many times higher than the continuous mode output. The duty cycle — the fraction of time the illuminator is on — determines the average power dissipation in the LEDs. With a low duty cycle of 1% to 5%, the peak current can be 10 to 50 times the continuous current without exceeding the average thermal rating.

Pulse Duration and Synchronisation

The strobe pulse must be synchronised with the camera exposure. The standard approach is to use the camera’s trigger output to initiate the strobe pulse. The pulse duration is set to match the camera exposure time. The illuminator must respond quickly to the trigger signal: the rise time from trigger input to full optical output must be shorter than the exposure time. For 50-microsecond exposures, the rise time must be less than a few microseconds.

RODER Vision DL5 and DL6 series illuminators are designed for strobe operation with very short pulse durations. The electronic driver circuits achieve rise times of less than 2 microseconds to full peak output. The strobe trigger input accepts both 5V and 24V logic signals and is directly compatible with the output formats of standard machine vision cameras and frame grabbers.

Illumination Geometry for Barcode Reading on Moving Parts

The geometry of the illumination affects barcode readability in several ways beyond simple intensity. The illumination angle, the uniformity across the barcode area, and the relationship between the illumination direction and the barcode orientation all influence the decoded image quality.

Direct Illumination for Flat Barcodes on Matt Surfaces

For barcodes printed on matt or satin surfaces such as paper labels, cardboard cartons, and uncoated plastics, direct front illumination from a matrix or ring illuminator provides the highest intensity and the best contrast. The illumination angle should be 30° to 60° from the camera axis to avoid the specular reflection that occurs at normal incidence on slightly reflective surfaces. A ring illuminator centred on the camera axis provides omnidirectional front illumination that is independent of barcode orientation on the label.

Diffuse Illumination for Gloss and Reflective Surfaces

Barcodes on gloss labels, metallised substrates, and shrink-wrapped products require diffuse illumination to avoid specular hotspots that saturate the sensor and mask the barcode bars. A flat dome illuminator provides diffuse illumination from a large solid angle. The barcode is imaged through the aperture in the dome. The diffuse illumination geometry eliminates specular reflections and produces a uniform, readable image of the barcode even on highly reflective substrates.

The trade-off with diffuse illumination is intensity: flat dome illuminators distribute their optical output over a large solid angle, resulting in lower intensity at the product surface compared to a direct illuminator of the same electrical power. For high-speed applications on reflective surfaces, a combination of moderate diffuse illumination for contrast and a very short strobe pulse for motion freezing is required.

Bar Orientation and Illumination Direction

For linear barcodes, the illumination direction relative to the bar orientation affects the edge contrast in the image. Illumination parallel to the bars (perpendicular to the scan direction) produces the maximum edge contrast at the bar boundaries. Illumination perpendicular to the bars (along the scan direction) minimises the edge contrast. In practice, the barcode orientation on the product is usually fixed, and the illumination geometry is designed accordingly. For applications where the barcode orientation is variable, a ring illuminator or a diffuse source is preferred because both are independent of barcode orientation.

Datamatrix and 2D Codes: Additional Illumination Considerations

Datamatrix codes, QR codes, and other 2D symbologies are increasingly used in industrial traceability applications because they carry more information per unit area than linear barcodes and can be read even when partially damaged. The cell size of a datamatrix code printed at typical industrial densities ranges from 0.25 mm to 1 mm per cell. The illumination requirements are similar to linear barcodes in terms of intensity and motion freeze requirements, but the 2D structure of the code imposes additional constraints.

Datamatrix codes are often marked directly on the part surface by dot-peen marking, laser engraving, or chemical etching rather than printed on a label. These direct part marking (DPM) techniques produce codes that are read under illumination conditions very different from those used for printed barcodes. Dot-peen marked codes on metal surfaces require directional illumination at a specific angle to create contrast between the indented dots and the surrounding flat surface. Ring illumination at low angles or coaxial illumination are the techniques most used for DPM barcode reading.

Practical Guidelines for Strobe Illumination in Barcode Reading Systems

Designing a strobe illumination system for barcode reading requires matching several parameters: exposure time, peak intensity, illuminator working distance, field of view, and trigger interface.

The exposure time is set first, based on the line speed and barcode X-dimension calculation above. The required surface irradiance is calculated from the exposure time, the camera sensor sensitivity, the lens f-number, and the target grey level in the barcode white spaces. The illuminator is selected to deliver this irradiance at the required working distance and field size. RODER Vision product datasheets provide peak irradiance specifications in strobe mode at defined working distances, allowing the system designer to verify that the required irradiance is achievable before system integration.

Products and Technologies

RODER Vision Illuminator Families for Barcode Reading

The following RODER Vision product families are optimised for high-speed barcode and datamatrix reading applications requiring strobe illumination.

Frequently Asked Questions