Inconsistent Barcode Verifier Readings
Why would a barcode verifier produce inconsistent readings on the same barcode? One would expect readings with little or no variation. How can this happen?
Let’s start with the device itself. At the most basic level, a true verifier must be an ISO compliant device. Not all of the devices marketed as barcode verifiers are ISO compliant. Make this determination first. Contact the manufacturer and request a Certificate of ISO/IEC Conformance. If they hedge or offer anything other than the certificate you are requesting, your device is not ISO compliant. That alone could account for discrepancies in the report.
An ISO compliant barcode verifier will produce inconsistent results if it hasn’t been recently calibrated, or if it has not been re-certified to ISO/IEC conformance within the past 2 years (one year for some devices), or if you are not running the latest version of the manufacturer’s software.
The barcode you are testing can also be the cause of inconsistent verification reports. Laser and linear array verifiers examine a very narrow bath across a 1D barcode, and will detect nearly microscopic reflective and dimensional errors that can trigger the downgrading of certain ISO parameters such as Modulation and Decodability. Examine the verification report to see which ISO parameters are being downgraded. Although it is not an ISO parameter, Average Bar Gain is often shown in a verification report. When gain approaches or exceeds bar width tolerance, inconsistent verification readings can occur.
Even very slight movement of the verifier can present a dramatically different area of the barcode, resulting in significant variations in the report. Pay attention to which parameter(s) are involved.
Non-flat substrates are a frequent cause of inconsistent grades in 1D and 2D symbols. Very large ITF-14 barcodes on corrugated are engineered to tolerate the undulating substrate, but other symbologies, and especially smaller symbols are more sensitive to this type of surface.
2D or matrix symbols are verified with camera-based devices which capture the entire image rather than a thin path like 1D verifiers capture. Inconsistencies in the light and dark reflectance densities can make a 2D verifier struggle to determine the precise location of the light and dark elements or dots that comprise the 2D symbol. Gradient or unsharp images also make it difficult for the verifier to find the edges that distinguish light elements from dark elements. This can lead to inconsistent verifier readings. High Average Bar Gain can also be a factor with 2D symbols, as the printed elements spread and encroach upon elements represented by the unprinted substrate.
Variations—even subtle variations—in the light or dark reflectance values of a 1D or 2D symbol can cause inconsistencies in the verification report. A screened or patterned background will cause verifier report variations. Barcodes printed in highly reflective ink, or against a highly reflective background can also cause inconsistent grades in the verification report. Lamination can also cause grading inconsistencies.
When an ISO compliant verifier is producing inconsistent grades, the first thing to do is to examine the barcode closely with t a low power magnifier. Often the cause of the problem becomes immediately obvious.