The human eye actually has various types of retinal ganglion cells, one of which does not process visual stimuli but responds to light. When light enters the eye, these cells—called intrinsically photosensitive retinal ganglion cells (ipRGCs)—trigger non-visual biological effects that significantly impact the release of hormones like melatonin and cortisol.
When ipRGCs are exposed to high-frequency, high-energy light, they suppress the production of melatonin, making the person feel alert and energized. Conversely, when ipRGCs are exposed to low-energy light, the melatonin levels in the body gradually rise, making the person feel relaxed and sleepy.
Thus, light is an important external factor affecting the human circadian rhythm. Modern people spend a lot of time indoors, so how does indoor lighting impact the human body? As technology advances, people increasingly recognize the importance of lighting on comfort and health. In recent years, the lighting industry has proposed health-oriented human-centric lighting standards. CAF and EML are two important indicators used to measure how light affects the human body!
Circadian Action Factor (CAF)
The Circadian Action Factor (CAF), also known as the "physiological stimulation value" or "circadian rhythm factor," is a parameter proposed to assess the circadian rhythm-stimulating ability of light sources and fixtures. The higher the CAF value, the greater the physiological stimulation from the light, helping a person feel more alert. The lower the CAF value, the less stimulating the light is, making the person feel relaxed.
The International Commission on Illumination (CIE) released the CIE S 026 standard, which defines methods and principles for non-visual light effects (i.e., the impact of light on physiological and psychological needs) and includes CAF value measurements. CAF is calculated based on circadian rhythm sensitivity and photopic sensitivity to the light spectrum, as well as the distribution of the light source’s spectrum. It is primarily used to measure the degree of stimulation a fixture provides to the human body.
Equivalent Melanopic Lux (EML)
Equivalent Melanopic Lux (EML) is used to quantify the biological effects of light on humans and is one of the standards recommended by the WELL Building Standard for human-centric lighting planning. Different daily activities require different EML values: during the day or when focusing, a higher EML value is needed, while at night or during relaxation, a lower EML value is recommended.
The calculation of EML is based on multiplying the vertical illuminance in a space by the ratio of the light source, measuring how light in a given environment stimulates the body's physiological cycle.
Differences Between CAF and EML
The difference between CAF and EML lies in what they measure: CAF measures the stimulation caused by the lighting fixtures, while EML measures the stimulation caused by the entire environment. However, both share the same principle: the higher the value, the greater the stimulation, which can boost alertness; the lower the value, the lesser the stimulation, aiding relaxation.
If you're installing lighting, it’s worth having your designer ensure the lighting meets the standards of human-centric lighting planning to make your living and working environment more comfortable, as these spaces are used for extended periods!