Light and photobiomodulation
Photoreceptor
A photoreceptor is a cell or molecule whose function is to absorb light and convert it into a biological signal. In the eye these are the rods, cones and light-sensitive ganglion cells. In photobiomodulation the word is also used loosely for molecules inside ordinary cells that absorb light, more properly called photoacceptors.
Also called photoreceptor cell, photoacceptor
Two different uses, and the confusion between them.
In the eye. Photoreceptors are specialised retinal cells. Rods handle dim light, cones handle colour, and a third type — intrinsically photosensitive retinal ganglion cells containing the pigment melanopsin — do not contribute much to vision but feed the body’s master clock, which is why the timing of light exposure affects sleep. These are true photoreceptor cells with a dedicated signalling job.
In photobiomodulation literature. The word is often stretched to mean any molecule that absorbs light in that setting, most commonly cytochrome c oxidase inside the mitochondrion. The more careful term for that is photoacceptor or chromophore. Cytochrome c oxidase is an enzyme of cellular respiration that happens to absorb red and near infrared light; it is not a sensory receptor and it does not exist in order to detect light.
Why the distinction is worth keeping. The plainer statement is that some molecules in tissue absorb light at certain wavelengths, and that absorption is the first step in a proposed chain of effects.
The eye point that follows. Because the retina is packed with photoreceptors, any light-emitting device raises an eye exposure question, and the answer depends on measured irradiance and geometry rather than on how bright the light feels.
Why it matters here
The phrase "activates photoreceptors" turns up often in material about light devices. Knowing which sense is meant — cells in the eye, or light-absorbing enzymes inside ordinary cells — tells you whether the subject is eye safety or mechanism.