Does the color rendering index matter in museums and exhibitions?

Author: Vicente Galdon
Date 01/09/2023

Who at this point in the lighting industry, or the museum sector, has not heard of the CRI? Yes, it’s a rhetorical question, we all know what it is.

But it is also true that very few of us have seen its effects realistically. Unless it is very obvious, of course. On a night walk we can see that a low-pressure sodium vapor lamp on a street lamp projects an infamous monochromatic band on a tree, turning it into a lifeless object close to dark brown, and if a few steps further on we see a luminaire of mercury vapor, or better yet, an LED, we will perceive the green very vivid and we will say to ourselves: -This is because of the CRI.

So it is easy to compare a CRI of 20 with another of 70 or 80 based on an eyemeter, the difficult thing is to perceive its effects in indoor lighting, because the truth is that there are practically no opportunities or means to make these types of comparisons in conditions. acceptable.

Limits of human vision

I have been in quite a few museums while they evaluated the benefits of my luminaires, with spectrometer in hand: Do you see the difference? -while showing them the result of the reading and/or pointing out the chart- this is what a CRI of 98 does.

My interlocutor nodded, more out of politeness than conviction, looking (but almost without seeing) the colorful graphs on the instrument’s screen, and focusing on the picture. It is a small farce that I have experienced many times, almost by inertia.

But neither my interlocutor nor I are to blame, because the truth is that, if you are not familiar with all those graphs, it is difficult to internalize the difference that I am proposing, since it is difficult for the appropriate conditions to be met for its correct observation.

It is not only the contaminating light that can come from other sources, but the fact that our vision system does not work like a static photograph that you can compare with another, but rather it works through reference points.

When we are watching a projection on the white wall, the black lines and very dark parts of the scene that we see are still the white wall that has practically the same light as the outside of the projection, the black light does not exist.

In that order of ideas, it is tremendously difficult to record on our retina the effect of the chromatic quality that light has on an object; we need a reference system or at least an environment that allows a more or less immediate comparison. There is also that you have to know where to look, CRI differences are not evident in all circumstances, nor are they perceived the same in all colors. That is what we are going to unravel in this article.

Surely you have attended several presentations or read several articles in which they showed “differences” between objects as a result of the CRI. Most of those images are manipulated.

In the Dark Ages, Lent was highly respected. Not only was meat not consumed, any product from meat-producing animals, such as milk, was also ruled out. Almond milk was made to make up for those long days, the thing is that there was not very sophisticated machinery and the almonds had to be crushed by hand, with the mortar. Like everything at that time, hard work. Until someone more ingenious than average discovered that by picking the almonds before they ripened, they were much more tender, so they were very easy to crush, and the result was very good. Green almonds are called almendrucos, and picking them green so as not to get tired making milk was called the almendruco trick.

The trick to represent CRI differences and put it in our presentations is to take an image editor and change contrast and saturation. We’ve all done it, including yours truly I’m afraid. Mea culpa.

Essay

There has been a Pink Lady apple in our company’s kitchen for about 2 months. It has no owner. The other day I noticed its beautiful color contrasts, it never fully matured, as it is already wrinkled and yet those red and yellow veins remained rooted in each other.

And here we are, a SLR camera with a tripod, a spectrometer, spotlights on the ceiling, a table with black Eva foam, a server, and the girl who is in charge of cleaning wondering why there is an old apple on a table in the dining area. exhibition of works of art.

As we pointed out before, it is easy to compare a CRI 20 to a CRI 70~80, all of which are typical of public lighting. But I am going to propose something more interesting, we are going to compare two high CRI values, one close to 87, and another close to 96. A CRI of 87 is very widespread in museums, which many manufacturers sell as an improvement to separate them from other product lines closest to retail, so we can consider that this comparison is very grounded in reality, we are studying a practical case of lighting that concerns today’s museum. Many may think that, between those very high values, 87 and 96, there is not going to be that much difference, in total, that there are 9 points, which is not that big of a deal. The difference will surprise you, I guarantee it.

Slide the center bar left and right to compare the two photos

indice de reproducción cromática elevado para luces de museos y exposiciones

They say that an image is worth a thousand words, once the image is placed the truth is that I no longer know very well what to write, because everything I write will pale (like the apple with CRI 87) before the images that speak for themselves. I just want to point out that without pretending to be an academic article, I have done it with the greatest possible rigor, using illuminances and other very similar conditions to rule out other conditions.

We can make it a little more interesting if we use the lights to compare a work of art, in this case a painting with a fairly noticeable predominance of warm colors, since these tones are a place to find differences that can be easily perceived with our senses.

The spectral composition of the two lights is not very similar. On the left we have that of CRI87, and on the right that of CRI96. We can appreciate that continuous and beautiful spectrum without peaks in cold components, this, my friends, is what makes a spotlight designed for museums.

The CRI under debate

Although the CRI is a standard that has been with us for many years, there are those who have been putting on the table for some time that it is necessary to change the standard, because it is becoming obsolete. Currently, the CRI consists of a calculation that is deduced from the comparison of 15 different color samples, in which a value is attributed that is at most 100 for each of them, with the most difficult to achieve satisfactorily being R9. , which is a somewhat saturated red, very important to reflect reality. Below, I show you the graph of values of the lights used for this test, at this point I know that we already understand each other so I trust that you will be able to deduce which is which.

There are numerous candidates to replace the CRI, I think the one with the most options is the TM-30-15 which, not to go into too much detail, is more precise because instead of 15 samples it uses 100, and instead of one value it presents us with two , range and fidelity. It is not the purpose of this article to introduce the aforementioned standard, so I leave you a link so that the most curious minds can delve deeper.

I suppose it is a matter of time, and that most laboratories and instrument manufacturers carry the TM30-15 within their standard measurements, for now the CRI continues to be the most used value to define chromatic quality in most applications. of lighting.

The images are free of rights, and you can use them in your presentations and articles, in fact, I would be very pleased to find them there, having the courtesy of citing their authorship will be enough