Light Fastness Evaluation of Colour Photographs

Abstract

The light fastness on the British Standard (B.S.) scale of a variety of examples of colour photographs, both transparencies and prints, has been determined and the possible lifetimes under normal conditions, before fading is noticeable, have been estimated.

In transparencies from reversal films there appears to be a correlation between emulsion speed and speed of fading: the slowest film gives the best fastness, its colours are graded about 4 (on the B.S. scale of J to 8) in medium shade depths. This is probably adequate for occasional projection, though poor by textile standards, corresponding with a possible useful life of about 4 hours under continuous exposure in a modern projector. The fastest reversal films (l00 ASA and above) are very poor indeed and are graded about J on the scale, corresponding with a possible useful life of only about 0.5 hours in a projector.

Colour-developed paper prints are graded between J and 2, and estimated to have a useful life of between 4 and 12 months under continuous exposure in an average room in daylight. Positive commercial colour films also have poor light fastness, though some are a little better than paper prints.

Transparencies and prints made by the silver-dye bleach process have good fastness, between 5 and 6 on the B.S. scale, representing a possible useful life of about 8 hours in a projector or 70 months in an average room.

Detailed examination of the three individual colours of one reversal film show that cyan is the most resistant to fading and magenta the least; in deep shades cyan has the highest possible fastness on the B.S. scale whereas in pale shades magenta has extremely poor fastness.

Abrégé

Les spectres de réflexion d’une série de triméthinecyanines adsorbées sur du bromure d’argent sont comparés avec les spectres d’absorption des mêmes colorants en solution dans l’ethanol.

Le déplacement de la longueur du maximum d’absorption provoqué par la formation d’agrégats J a été relié à la taille des molécules de colorant suivant la théorie de NORLAND et al. Les résultats sont qualitativement en accord avec cette théorie et ne confirment pas l’hypothèse :on laquelle l’agrégation des molécules de colorant adsorbé est influencée par des interactions spécifiques entre les colorants et lesions argent Superficiels.

Zusammenfassung

An einer Reihe von Trimethincyaninfarbstoffen wurden einmal die Reflexionsspektren bei Adsorption an Silberbromid, zum anderen die Absorptionsspektren in äthanölischer Lösung gemessen und miteinander verglichen.

Die beobachtete Verschiebung der Absorptionsmaxima durch die Bildung von J-Aggregaten wurde zur Größe der Farbstoffmoleküle in Beziehung gesetzt, entsprechend der Theorie von Norland et al.

Die Ergebnisse stimmten gualitativ mit der Theorie überein und unter-stützen damit nicht die Anschauung, dass die Packungsstruktur der adsorbierten Farbstoffmoleküle durch spezifische Wechselwirkungen zwischen Farbstoff und Silberionen auf der Kristalloberfläche influßt wird.

Riassunto

Gli spettri di riflessione di una serie di coloranti, della classe delle trimetincianine, adsorbiti su bromuro di Ag, sono stati paragonati con gli spettri di assorbimento degli stessi in soluzione di alcool etilico. Lo spostamento del massimo di assorbimento prodotto dalla formazione di aggregati J, è stato messo in relazione con la dimensione delle molecole die coloranti, in accordo con la teoria di Norland et al.

Qualitativamente, i risultatisono in accordo con questa teoria e, comunque, non convalidano il punto divista che gli aggregati di molecole di colorante adsorbito siano influenzati da una specifica interazione tra i coloranti e gli ioni Ag superficiali.