Work has been in progress at the Bureau for several years on a reference standard lamp for testing colorfastness to light of textiles, paper, and similar materials, and on means for calibrating, in terms of this standard, the lamps used in laboratories throughout the country." The purpose is to provide a reliable method for expressing colorfastness to light in "standard fading hours."
Recently a survey was made of the relative fading abilities of some 46 lamps in 2I different laboratories, in order to arrive at an average value and to demonstrate the need for calibration. The standard lamp and proposed method of calibration are described briefly in the Bureau's Letter Circular LC785, and the results of the survey are given.
The survey was made by distributing test strips of paper dyed blue which were exposed for exactly 2o hours in each unit, and were then measured to determine the change in reflectance. The variation of from 0.405 to 0.092 found, reveals the great need for a standard means of calibrating the lamps.
In the proposed method of calibration, paper dyed blue made in the Bureau's paper mill is used. A strip of the paper is exposed in the lamp for a suitable period of time, say for 20 hours. The strip is then compared visually with a strip of the same calibration paper which has been exposed in the reference standard lamp in steps of x6, t8, 20, and 22 standard fading hours. The time of exposure in the lamp being calibrated that is equivalent to one of the steps of exposure in the reference standard lamp can be determined in this simple manner within IO per cent. or possibly better. Thus, the lamp can be calibrated in terms of the reference standard lamp and therefore in terms of any other similarly calibrated lamp. The calibration can be repeated as often as the particular installation requires. Laboratories desiring to try the paper will be supplied with unexposed test strips and with a book of strips which have been exposed in the reference standard lamp for I6, I8, 20 and 22 hours. They will be asked to comment on their experience with the paper to guide any further work that seems necessary. Requests for the paper, and for LC785, which are furnished without charge, should be addressed to the National Bureau of Standards,