such as these three color codes ff0000 00ff00 0000ff is not look as equal brightness in human sight

You are correct that colors represented by hexadecimal codes like #ff0000 (red), #00ff00 (green), and #0000ff (blue) can appear with different levels of brightness or intensity to human eyes, even if they are displayed at the same physical brightness. This discrepancy arises from the properties of the human visual system. Here's why:

1. Spectral Sensitivity: The human eye's photoreceptors (cones) respond differently to different wavelengths of light. As mentioned earlier, green light (around 555 nm) is perceived as brighter than red (around 655 nm) and blue (around 470 nm) light when they have equal physical intensity.

2. Luminance Function: The "luminous efficacy" or sensitivity function of human vision peaks in the green region of the spectrum. This means that at equal physical intensity, green light produces a higher luminance (perceived brightness) than red or blue light.

3. Display Technologies: Most screens, including CRTs, LCDs, and OLEDs, do not emit light in a purely monochromatic fashion. Instead, they produce colors through additive color mixing using red, green, and blue subpixels. The intensity of light emitted depends on how much each subpixel is driven. To produce pure RGB colors, these subpixels would need to be driven at full intensity, but they often vary slightly between devices.

4. Gamma Correction: Color displays and image processing typically use gamma correction to convert linear color data into nonlinear light intensity values that look more natural to the human eye. This nonlinear relationship between digital values and actual light intensity can make colors with the same hex code appear differently in terms of perceived brightness across different screens or viewing conditions.

5. Environmental Factors: The appearance of brightness can be affected by surrounding colors, the background color, and overall lighting conditions. These factors influence contrast perception, further altering our impression of the brightness or intensity of a color.

In summary, while #ff0000, #00ff00, and #0000ff represent maximum saturation and full color intensity at their respective channels, the human visual system perceives them with different levels of brightness due to varying spectral sensitivities and environmental adaptations. That's why, even at equal physical intensity, green appears brightest, blue appears dimmest, and red lies in between when viewed under normal conditions.