Human Centric Lighting Dictionary

Here is a glossary with explanations to some usual words and terms in HCL

Circadian rhythm

A circadian rhythm is a natural, internal process that regulates the sleep-wake cycle and repeats roughly every 24 hour


Is also known as the stress hormone. It has a number of physiological effects including stimulating the metabolism and program the body for day mode.

Ganglion cells

A retinal ganglion cell (RGC) is a type of neuron located near the inner surface (the ganglion cell layer) of the retina of the eye. It receives visual information from photoreceptors via two intermediate neuron types: bipolar cells and retina amacrine cells.


The point during sleep when hormone production and body temperature is at the bottom.


is the SI derived unit of luminous flux, a measure of the total quantity of visible light emitted by a source per unit of time.


is the SI derived unit of illuminance and luminous emittance, measuring luminous flux per unit area. It is equal to one lumen per square meter. In photometry, this is used as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface. lx = lm/m2


Melatonin is the body's sleep hormone. Melatonin is formed by serotonin and is excreted by the epiphysis, also called the pineal gland. This occurs during darkness. Melatonin is also called the darkness hormone. When light hits the eyes, melatonin in the blood breaks down and excretion stops.


An average, empirical value for the color rendering of 8 specific test colors.


The retina is the innermost, light-sensitive layer of tissue of the eye.


Serotonin is a hormone that works as a stimulant and motivator. Among its functions is the regulation of mood and sleep.

Suprachiasmatic nucleus

Tiny region of the brain in the hypothalamus, situated directly above the optic chiasm. It is responsible for controlling circadian rhythms.