1. Brightness
(1) Brightness and Visual Luminance.
Brightness is the perception of the degree of lightness or darkness of a light source or object surface by the eyes, primarily determined by the intensity of light.
Generally, the stronger the light, the brighter it appears; the weaker the light, the darker it appears.
Most light is reflected from the surface of objects into the eyes, rather than directly from the light source.
Therefore, brightness is determined not only by the intensity of illumination but also by the reflectance of the object's surface. The higher the intensity of the light source and the larger the reflectance of the object's surface, the brighter it appears.
(2) Brightness and Wavelength.
Within the visible spectrum, the human eye has different sensitivities to light of different wavelengths.
Cone cells can absorb light of all wavelengths in the visible spectrum, but they are most sensitive to the central part of the spectrum and much less sensitive to wavelengths below 500nm and above 625nm.
The entire curve of rod cells shifts about 50nm towards the shorter end of the spectrum, making them more sensitive to the shorter wavelengths and almost insensitive to red light with wavelengths exceeding 620nm.
Therefore, when transitioning from cone vision (daytime vision) to rod vision (nighttime vision), the maximum sensitivity of the human eye to the spectrum shifts towards the shorter wavelengths, resulting in a change in brightness perception.
For example, under sunlight, a red flower and a blue flower may appear equally bright; but when night falls, the blue flower may appear brighter than the red flower, which is known as the Purkinje effect (also known as "Purkinje shift").
This phenomenon indicates that the visual mechanism of people is different under different lighting conditions (day or night).