and in human sight red light may have medium brightness between green and blue light

Yes, in human vision, red light is indeed often perceived as having an intermediate brightness between green and blue light. Here's why:

1. Wavelength Sensitivity: Human eyes have three types of cone cells sensitive to different parts of the visible spectrum – S-cones (most sensitive to blue), M-cones (most sensitive to green), and L-cones (most sensitive to a longer wavelength, which we associate with yellowish-red to some extent). Red light, though at the longer end of the visible spectrum, does not have the peak sensitivity that green does but usually is detected by more cones than blue due to its closer proximity to the peak sensitivity of both M and L cone types.

2. Luminance Contribution: The luminance, or perceived brightness, is influenced by the sum of contributions from all color-receptive cones. Red light contributes more to our perceived brightness because it is better detected by the L-cones, which are less abundant than the M-cones but more than the S-cones. This results in an intermediate level of perceived brightness compared to green and blue lights.

3. Duo-Chromatic Cells: A portion of color perception can be attributed to so-called duo-chromatic cells in the retina where signals from pairs of cones are combined to form perceptions. This is particularly relevant for red-green perceptions. If these cells combine signals effectively from M-cones (green) and L-cones (red), they might perceive the red as bright, given it is a mix of the sensitivity profiles of both cone types.

4. Adaptation: The visual system adjusts to the overall light context, and colorful illumination can alter the apparent brightness of objects. For instance, when yellow or red light dominates an environment, this might affect how other colors seem bright by comparison.

5. Cone Distribution: L-cones are somewhat less dense on the retina than M-cones but are present in amounts greater than S-cones. This places red light detection somewhere between that of green and blue.

6. Environmental Familiarity: Since many natural conditions exhibit warm hues such as red and orange due to sunsets or fires, humans might have adapted to perceive these colors as brighter to be more sensitive to warmth and potentially life-sustaining or alerting signals.

Therefore, in human vision, red light is generally seen as having moderately bright characteristics, falling between the higher perceived brightness of green and the lower perceived brightness of blue. Of course, there can be variations between individuals based on their specific retinal capabilities and individual sensitivities.