Fine Art and Photopic Vision

As we can see from this sumptuous frieze by Gustav Klimt (shown fully below), it is especially important to be able to view all colours and details on certain works of fine art.

Yet it is only with photopic (or daylight) vision that humans can view such splendid colours and details with clarity (also called visual acuity).

It is not the light incident on this piece which determines whether it can be viewed in photopic vision though, but the light reflected from the object and reaching your eyes.

When insufficient light enters the human eye, the photoreceptors in your eyes switch to mesopic vision (twilight vision) or even to scotopic vision (night vision), when light levels are very low.

In this article, we will explore how light levels can affect your ability to enjoy or appraise fine art.

How Much Light Do We Need for Photopic Vision?

As Christopher Cuttle states in his book [1], light levels of at least 10 lumens/m2 are needed to allow most people to discriminate fine details in artworks. This light emanating from the object is called the ‘exitance’.

The exitance is calculated simply as the product of the illuminance incident on an item and the reflectance of that item. 

In other words, if 100 lumens/m2 (or ‘lux’) shines on an artwork of 50% reflectivity, an exitance of only 50 lux will be reflected from it, hopefully reaching your eyes.

On top of that, objects located in display cases must also take into account the transparency of the glass which can reduce the exitance, either due to interior reflections from the glass or due to light absorbed by the glass itself.

Furthermore, in order to discriminate all colours faithfully, both the lighting and the display glass must have a sufficient colour rendering index to avoid any colour distortion.

How Does Light Affect Vision?

If we consider the human eye to be a camera, then the brain would represent the attached computer.

When light enters the human eye under photopic (daylight) conditions, vision is controlled by what are called ‘cone’ photoreceptors.

Under low-light conditions, vision is controlled by ‘rod’ photoreceptors.

The cone photoreceptors absorb and propagate colours differently depending on which photopigments they contain.

The so-called “S-type” photopigments are most receptive to ‘short’ wavelengths, specifically 420 nm (nanometres), which corresponds to blue light. 

The “M-type” photopigments respond best to medium wavelengths (534 nm, blue-green) and the “L-type” photopigments to yellow-green wavelengths (564 nm).

Under photopic vision, it is also true that visual adaptation (the ability of the retina to adjust to different levels of light) is also quicker. 

Visual adaptation unfortunately worsens both with age and with certain pathogens.

The Eye as a Logarithmic Sensor

First formulated by Gustav Theodor Fechner (1801–1887), Fechner’s law was the first in the nascent field of ‘psychophysics’, and refers to the logarithmic nature of human senses (including vision, sound, smell, touch and taste).

Fechner hypothesises that the perceived change in a sensory stimulus varies with the logarithm of the actual change.

In other words, an increase in the light intensity from 10 to 100 Lux would be perceived similarly to a change from 100 to 1000 Lux.

The human retina functions over twelve such orders of magnitude, from bright sunlight all the way down to weak starlight.

Our eyes allow us to see objects in a semi-dark indoor environment, while also letting us see the world outside through a window. 

Amazing, don’t you think?

How This Affects You

Whether you are an art collector or an artworld professional, remember to allow time for your eyes to adjust to the light levels in the room. Remember that this adaptation time will worsen with age.

Ensure also that the light reaching your eyes from the object is sufficient (at least 10 lumens/m2) so you are not missing out on colours and fine details.

And, last but not least, ensure that any glass between you and the artwork, and any interior lighting illuminating it, are not distorting the colours present on the object.

References

1. “Light for Art’s Sake”, Christopher Cuttle, ISBN 9781136387500
2. “The Visual System and Art”, Howard Howland, Cornell University, URL
3. “Colour and the Optical Properties of Materials”, Richard Tilley, ISBN 9781136387500
4. “Photopic Vision”, National Center for Biotechnology Information, URL
5. “Nightshades”, Esther Mathis, URL and Exhibition