Museums and light: How to avoid deterioration in photographic materials

Author: Vicente Galdon
Date 01/09/2023

Introduction

One of the most revealing parameters when evaluating the deterioration of a photograph is color. The effects of different lighting sources on light-sensitive objects are widely known. Thus, the most cited technical report on the relationship between lighting and preventive conservation is the one published by the Commission Internationale de l’Eclairage, CIE technical report 157-2004 “Control of damage by optical radiation to museum objects” (CIE 2004). Numerous protocols are defined here in which tests are found to evaluate color aging, a full-fledged aging test, very common in practically all industrial areas. The aforementioned report makes comparisons between LEDs and other sources, such as halogen lamps, but after a few years, we have certain technologies that allow us to spin much more finely and not only put the LED against other lamps, but the LED against other LEDs based on different technologies, or simply improved over time.

Some context

As most of us already know, white light is a spectral composition of many other colors. If we decompose two sources of white light, which are practically indistinguishable to our eyes, we can see great differences. For example, if we take two 5000K, CRI80 luminaires, but one whose source is mercury vapor, and the other is an LED, we will have two very different spectral compositions:

The longitudinal components of the spectra have been studied separately to determine which ones are more harmful to the materials used, so a coefficient “b” has been defined, which is an average value for each material that defines a relationship between the emitted and absorbed radiation ( Table 1), the level of potential damage in different materials exposed to different wavelengths is as shown in Figure 2

A very predictable conclusion is drawn from Figure 2: that, the shorter the wavelength, which also means the greater the radiated energy, the potential for damage to the materials under study is higher.

The led

Here one of the reasons why LEDs have always been controversial in certain academic environments comes to light: blue light. Blue light is to the right of the electromagnetic spectrum, with only purple light to its right, representing the most significant longitudinal component in most commercial LEDs.

Purple heart

The reason for this characteristic is that the LED cores actually emit blue, and a series of solid compounds play the role of fluorescence, and convert that band of blue light into bands of other frequencies, making up white light. The proportion and chemical composition of these compounds turn out to be the most decisive in the final photometric characteristics of the LED: color rendering index, color temperature, consistency, etc.

Until recently, practically all LEDs available on the market presented those same general characteristics: a blue peak, solid state phosphors, and one or another spectrum depending on the particular characteristics of the second, but technology has given us another milestone: the Purple LED.

The experiment

An experiment published by Hung-Wen Luo, Ming Ronnier Luo and Hung-Shing Chen and titled Museum lighting with LEDs: “Evaluation of lighting damage to contemporary photographic materials” shows a significant change in the behavior of the materials depending on the type of LED that we use to illuminate it. The experiment consists of intense exposure to controlled radiation to simulate the aging of the samples. The samples are three types of inks used in modern photography in different colors for subtractive synthesis: white, magenta, cyan, yellow. The researchers set up a small structure (figure 3) in which they arranged the color samples, and irradiated them with sufficient intensity and time following the criteria of the CIE 157:2004 publication to begin to detect changes in the chromatic integrity of the materials. .

By analyzing the changes that the materials suffered, it was possible to extract Table 2, which indicates the unit of illuminance per hour necessary for changes to begin to be noticed in the chromatic integrity of the samples. BP LED (blue pumped) is the typical blue core LED currently used in all museums, and PP (purple pumped) is the purple core LED. “HA lamp” is the halogen lamp with an integrated UV filter, a technology that is about to be completely displaced.

Marked differences can be seen in the deterioration of the samples, such as in the white chromogenic ink, where to obtain the same deterioration, the sample had to be exposed with the purple LED by a factor of 12.7 compared to the blue LED. Put another way, if lighting with a normal LED causes a change in chromatic integrity in 10 years, with the purple LED we would need 127 years to achieve the same negative effect in that specific ink. Most colors from the other samples and printing techniques present large differences in deterioration, with a couple of exceptions in which the difference against the purple LED is not very pronounced. In fact, the average shown at the end of the table tells us that we would generally need more than twice the radiation from a purple LED than from a blue one to cause the same effects.

Conclusions

Continuous spectrums that start from purple seem to be more suitable for lighting museum rooms and restoration departments, to the extent that their radiation is the one that causes the least damage to photographs from blue light. The original study, which I recommend reading, concludes that LED light is safer than other lighting sources. In the end, technological advances always open the doors to us being able to adopt new solutions to our problems.

Image 4 shows a comparison between the spectrum of two top quality LEDs. The one on the right, with a purple core, is the one we integrate into our luminaires for museums, exhibitions, and art restoration departments.

Bibliography

COMMISSION INTERNATIONALE DE L’ECLAIRAGE. (2004). Control of Damage to Museum Objects by Optical Radiation. CIE Technical Report 157:2004. Vienna: CIE.

HUNG-WEN LUO, MING RONNIER LUO, HUNG-SHING CHEN. (2018) Museum lighting with LEDs: “Evaluation of lighting damage to contemporary photographic materials” https://www.researchgate.net/publication/323979260