Perforation Gauge Project

Background

I have become interested in collecting the stamps of Guatemala, having visited there several times. There are quite of few overprints on older stamps, made when there wasn't enough money to order new stamps for rate changes, etc. It is also evident that many forgeries were made of these overprints, both for postal use and for use in attempts to defraud stamp collectors. Careful measurement of the overprint is necessary to tell genuine from bogus stamps. Perforation varieties of the same stamp issue are also collected, with differences of gauge 0.1 – 0.2 at times being important.

The International Society of Guatemala Collectors (ISGC) published a reference text called Guatemala, A handbook on the postal history and philately of Guatemala, in two volumes, referred to as G-1 (1969), and G-2.(1974), both now out of print. Many additions and corrections to these texts have been published the Society's quarterly newsletter, El Quetzal. Perforations of the stamps in these texts were measured with the first Stanley Gibbons Instanta Gauge, which has since been shown to provide slightly different perforation gauge measurements than some modern gauges, including a newer version of the Instanta Gauge, and there is concern that there is variation among several of the modern gauges as well. The Society plans to update the above texts, and would like to use an accurate modern gauge for perforation measurement in this update.

I became interested in using high-resolution scanned images of the gauges and stamps for perforation and overprint measurement.  This study was undertaken in order to validate this measuring method and to compare the various gauges. Ken Lawrence, a Vice President of the American Philatelic Society, and Jack Jonza, of the ISGC, have each loaned me out-of-print gauges, and I purchased several others.


Methods

The gauges, rulers, and stamps were scanned in at 1016 ppi, using a Hewlett Packard ScanJet 5300C scanner, which has an optical resolution of 1200 ppi. The rulers, stamp perforations, and perforation gauge marks distances were then measured in pixels, using the Measure Tool in Photoshop (Version 6.0). The pixel measurements can then converted to metric. This allows more precise measurements, as the finest metric measurement using the Tool is 0.1 mm.  At the above resolution 0.1 mm = 4 pixels.

Upon measuring the rulers for calibration of this method, it was found that the images were about 1% longer in measurement along the horizontal axis than the actual object. Along the vertical axis, the accuracy was to within about .05%. Thus, for this study, objects were placed on the scanner bed such that the areas to be measured were oriented vertically. Also of note is that the gauges were placed on the scanner bed with the printed material towards the scanner to avoid refraction through the gauge material; the resultant images were then manipulated digitally to correct the mirror image appearance.


The Gauges

The scans on these pages are for illustrative purposes, and are not the scans used for the study.  Quotes are from the StampChat philatelic discussion board.

Instanta

Scott/Linn's Multi-Gauges

Sonic Imagery Labs Precision United States Specialty Multi-Gauge (USS)

German Philatelic Society's Building Series Study Group Perforation Gauge (BSG)


Results

The data and calculations from the study are noted in the table linked below. The accuracy of the gauges was assessed by measuring the distance over 10 perforation lines at each of the whole number perf lines, and then calculating the measured perf gauge for that line.  Fractional perf lines were used in measuring the USS gauge due to the black circles (to facilitate a visual match with the perforation holes on the stamp being measured)  placed on the whole number perf lines.

Table

Conclusions

The Old Instanta gauge was the least accurate, with the Old Linn's Multi-Gauge a close second.  Both of these gauges had enough inaccuracy to be wrong by gauge 0.1 at all perforation dimensions, and up to 0.2 in the case of the Old Instanta at gauge 18.  The New Instanta was more accurate than Old, but was still not accurate enough to be sure of measuring perfs to within gauge 0.1 over the entire range of the gauge.

The German Building Series Study Group gauge was the most accurate, with measurements within the confidence interval of the method used over the entire range of the gauge.

The New Linn's Multi-Gauge, Scott/Linn's Multi-Gauge, and the Sonic Imagery Lab's Precision U.S. Specialty Multi-Gauge were nearly as accurate as the BSG gauge, and any of the three may be used with confidence when measurement of a stamp's perforation to within gauge 0.1 is necessary.

Concern has been raised about the stability of the gauges over time.  The makers of the BSG and USS gauges have addressed this issue in the choice of materials and methods used in manufacturing their gauges.  I have received information that at least one of the BSG gauges has retained its accuracy over a 15 year period.  The long-term stability of the Instanta gauges and the Old LMG appears to be poor, contributing to significant inaccuracy.  The long-term stability of the other gauges is unknown at this time.  The scanned virtual digital versions of the gauges are not subject to physical instability over time, and perhaps may be used for comparison purposes in the future. 

As long as care is take to note and avoid introduction of any distortion error during scanning, the measurement of the gauge of stamp perforations and of the dimensions of stamps and overprints by using the Photoshop Measuring Tool on high-resolution scans of stamps and perforation gauges is accurate enough for the purposes of the revisions planned by the ISGC, and for most stamp collectors in general.  The extremely fine measurements needed by some specialists and expertizers require other methods.

Top     Stamps     Home