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D5626-94 – Standard Test Method Technical Guide
📐 Test Methodology and Specimen Requirements
These test methods, designated D5626-94, are specifically designed to evaluate optical properties critical for U.S. Postal Service (USPS) automatic mail handling equipment such as optical character recognition (OCR) readers and barcode readers. The standard focuses on measurements using the Envelope Reflectance Meter Model 2 (ERM-2), which examines small areas measuring 0.152 by 0.254 mm (0.006 by 0.010 in.). This precision allows for detailed analysis of regions as small as a single printed character or a barcode bar.
The ERM-2 instrument is mandatory for these tests, ensuring consistency with USPS optical measurement standards.
⚙️ Instrument Configuration and Measurement Properties
The ERM-2 employs a directional optical geometry with illumination provided from two sources at ±50° from the vertical and detection at 0° from the vertical. This configuration is specified to ensure consistent measurement of relative reflectance factors. The standard covers four optical properties, as outlined in the table below.
| 🔍 Property | 📏 Definition | 🎯 Measurement Focus |
|---|---|---|
| Background Reflectance (BR) | Relative directional reflectance factor of unprinted material | Base material reflectivity without printing |
| Print Reflectance Difference (PRD) | Difference in reflectance between envelope material and printed ink | Print quality and contrast |
| Print Contrast Ratio (PCR) | Ratio characterizing reflectance difference between printed and unprinted areas | Barcode and OCR readability |
| Print Show-Through (PST) | Reflectance difference due to printing on backing or reverse side | Show-through from inserts or backside printing |
📊 Core Optical Properties Defined
Background reflectance (BR) is defined as the relative directional reflectance factor, expressed as a percent, from a portion of material that has not been printed on either side and does not show printing from any backing material. Print Contrast Ratio (PCR) characterizes the difference in reflectance between the envelope material and the printed ink, essential for barcode readability. Print show-through measures the unwanted visibility of printing from the reverse side or from an underlying insert.
The standard also references other ASTM standards for consistency, including D585 for sampling, E284 for terminology, and E1347 for color measurement. These related standards provide a framework for test material selection and definition of optical terms.
| 📚 Standard | 📖 Title |
|---|---|
| D585 | Practice for Sampling and Accepting a Single Lot of Paper, Paperboard, Fiberboard, and Related Product |
| E284 | Terminology of Appearance |
| E1347 | Test Method for Color and Color-Difference Measurement by Tristimulus Colorimetry |
These test methods are limited to the ERM-2 instrument; alternative devices may not produce comparable results for USPS optical measurements.
❓ Frequently Asked Questions
🔍 What is the primary instrument used in these test methods?
The U.S. Postal Service Envelope Reflectance Meter Model 2 (ERM-2) is the sole instrument specified.
💡 How is background reflectance defined?
As the relative directional reflectance factor, expressed as a percent, from a portion of material that has not been printed on either side.
⚡ What are the key optical properties measured?
Background reflectance, print reflectance difference, print contrast ratio, and print show-through.
📌 Why is the small area measurement important?
It allows evaluation of individual printed characters and barcode bars, which is critical for USPS automatic processing.