How does the chromatic deviation of LED light influence a museum?

Author: Vicente Galdon
Date 01/09/2023

Introduction

A museography professional purchases an LED projector to illuminate a work of art in a museum. Many options come into play here, the usual thing is to keep in mind characteristics such as the color temperature of the light, the color rendering index, the angle of the projector, power, type of regulation, and any accessories that may be necessary, perhaps some fins or a honeycomb to reduce glare for those who will contemplate the works.

So far so good, but there is another parameter that is especially important in museums, and that is generally not reflected in the relevant information sections of the focus. However, neglecting this aspect can cause us to distort the work, “stain” the wall with a somewhat unpleasant tone, or simply cause an imbalance in the visual harmony in the exhibition room. Let’s talk about deviations. chromatic.

The CIE 1931

The most common color space we use in the lighting industry is CIE1931. It is convenient because it has a region where different shades of white can be located, more specifically, in the region that surrounds that small black curve that we call “Planck’s place.” As we go from one end to the other, we find different color temperatures, lower the more they tend towards red, and colder the more they tend towards blue.

Let’s focus now on that line, because we want to talk about white light.

The black line (which is red in the following image) represents an ideal color temperature, which according to theory would be the tone that a black body that is heated to that temperature would acquire. The black body, which is a theoretical object, when heated to 5000 Kelvin, would emit a glow whose color temperature would also be 5000K. Therefore, the regions of the color space that move away from that black line can be said to have a greater deviation.

Of course there are tolerances, and these are not established arbitrarily, but rather based on the perception capacity that humans have. “Steps” are established to indicate the probabilities that a human eye will perceive differences between two beams of light whose chromatic coordinates are separated in color space. A human eye does not notice differences between two coordinates separated by just one step, and virtually all of us will clearly distinguish two coordinates separated by more than three steps.

As we see the graph, we understand that drastic color differences can occur within two light beams whose color temperature is practically the same. In the image below, we have two points in color space, whose color temperature is practically the same, but whose color distances are enormous. The 4000K that the graphs show us, in one case it will have a very green tone, and in another case it will appear pink. Slide the center bar to compare the two images.

By now the reader will have already imagined the problem: that we can have 2 projectors whose color temperature is the same, but whose chromatic coordinates are a considerable distance away. This at the moment of projecting the beams of light in a work, will completely modify the meaning of it. By sliding the bar you will be able to see how the light completely modifies the perception of the work. Remember that the two light sources are at 4000K:

Why these differences between two projectors?

There are many reasons. Most of them are due to attributes of the production of semiconductors that are not the subject of this article, but which can be mitigated by grouping them. This means that when the company that manufactures the chips tests them, it leaves together those that have very similar characteristics, separating them into lots that can be sent to different countries or regions so that a lighting manufacturer can easily choose groups of components. similar so that their final products have homogeneous characteristics.

Another reason that is presented is quality, a manufacturer that cares about incorporating reliable components into its production will take into account these particularities of the industry, and will know which semiconductor manufacturers have the most optimal characteristics for their luminaires. A very direct way to lower production costs is to purchase batches of discontinued components, or from undesirable brands, decisions that have a direct impact on the level of quality of the final product.

The last reason, and not infrequent, is the deterioration of the LEDs that the projectors incorporate. LEDs use chemical components known in the industry as “phosphors” that degrade over the hours of use. As the light degrades, its chromatic coordinates are modified, leading to the perception of bluish, greenish, or pinkish tones in the most serious cases.

Of course, a quality LED that is working inside a luminaire with adequate thermal management will last unchanged for many more hours than a low-quality bulb, and although in the end they will all end up degrading, a well-designed museum bulb will be able to work For decades it presented practically unchanged photometric characteristics.

It is true that, in this article, and for educational purposes, we are slightly exaggerating the chromatic distances that two different luminaires with the same color temperature can present, at least if they are new, because on many occasions we have seen old LED projectors whose degradation is such that the light they emit cannot be considered “white light”

What to do about it?

The answer is very simple, having adequate lighting for your needs. A low-quality LED spotlight does not meet the parameters that a projector designed to work in a museum does. Chromatic degradation is not the only factor that threatens the exposure, so does flicker, optical quality, softness of the contours in the regions where the light ends, and so on. Many times it is not a question of price, because there are quality projectors manufactured by very solvent companies, but nevertheless their technical characteristics mean that they have no place in an exhibition that claims to be well lit.