The Eye Does More Than See
Understanding Rods, Cones, and ipRGCs: Why Light Affects the Whole Body
Author | Lawrence Lin
Chairman, GLGA | Board Member, GLG | IWBI WELL Light Concept Advisor | LRS Founder & CEO
An architect once asked me, “If light affects the biological clock, why not simply measure how bright it is?” That question gets to the heart of the matter. We tend to think of the eye as a camera, but overlook the fact that it is also the body’s gateway for sensing the day–night cycle.
Rods and cones help us form visual perception. Meanwhile, intrinsically photosensitive retinal ganglion cells (ipRGCs), which contain the photopigment melanopsin, transmit information about environmental light to brain regions involved in circadian rhythms, pupil responses, and alertness.
These systems do not operate as isolated switches. Instead, they work together as an interacting system, allowing light to influence not only what we see, but also how our bodies respond.
Three Types of Light-Sensitive Cells, Each With a Distinct Role
Rods are sensitive to dim environments, helping us perceive shapes and movement under low light levels. Cones enable us to distinguish colors and see fine details in brighter conditions. ipRGCs, meanwhile, contain melanopsin and also receive input from both rods and cones.
This means that “non-visual responses” do not occur entirely outside the visual system, nor are they driven by a single wavelength alone. This is why CIE S 026 established five categories of α-opic metrics, each corresponding to the spectral sensitivity of the S-cones, M-cones, L-cones, rods, and melanopsin-mediated pathway.
In practical lighting applications, we most often discuss melanopic stimulation, meaning stimulation related to melanopsin. But “most commonly used” does not mean “the only one that matters.” Reducing the body’s response to a single blue-green wavelength range is still an oversimplification.
Why the Same Spectrum Doesn’t Always Feel the Same
The light entering the eye passes through the cornea, lens, and vitreous humor. As we age, the lens typically absorbs more short-wavelength light. Pupil size, field of view, and the direction from which light enters the eye can also change the effective stimulus. Two people of different ages standing in the same space may measure the same ambient illuminance, but that does not mean their retinas receive exactly the same signal.
Another often-overlooked factor is the visual field. A bright ceiling with a dark field of view in front of you can provide a very different stimulus from having a bright window directly in front of you—even when the illuminance measured on the desk is identical.
This is why health-oriented lighting design needs to move beyond “lighting the desk” toward “lighting the person.”
Two Sets of Tasks in the Same Space
In a classroom, students need to see their books, the board, and screens clearly—that is the visual task. They also need an appropriately bright environment in the morning to support alertness and a stable daily rhythm—that is the circadian-related task. The two can work together, but they are not always achieved through the same measurement point, the same light source, or the same control strategy.
Good lighting design should first address glare, uniformity, color rendering, and visual performance, and then consider light exposure in the direction of the eyes. A new metric should not be used to replace all the established requirements.
What We Can Say Today / What We Cannot Yet Say
What we can say:
The human eye contains multiple types of photoreceptors involved in both visual and circadian-related responses. The spectrum, intensity, direction, and duration of light exposure all influence the signals received by the body.
What we cannot yet say:
ipRGCs are not a standalone “biological clock button,” nor can we assume that simply increasing a particular band of blue light will produce a predictable or identical physiological response in everyone.
Three Things You Can Do Today
- When assessing a space, look at both the work plane and the directions people commonly look toward.
- When discussing melanopic metrics, clarify that they are one part of the five α-opic photometric quantities.
- For children, older adults, and night-shift workers, document the specific user conditions rather than applying a generic “average adult” model.
Evidence status: Established consensus.
Multiple retinal pathways contribute to both the visual and non-visual responses to light; specific health outcomes are influenced by multiple factors.
References
- CIE S 026:2018 ：https://www.cie.co.at/publications/cie-system-metrology-optical-radiation-iprgc-influenced-responses-light-0
- ISO/CIE TR 21783:2022 ：https://www.iso.org/standard/71623.html
- CIE TN 015:2023：https://www.cie.co.at/publications/second-international-workshop-circadian-and-neurophysiological-photoreception
In the next article, we continue the question: Once light enters the eye, how exactly does it influence alertness, sleep, and the biological clock?
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