Y value D65/10° - Luminance (%) ISO 2471
Y value C/2 - Luminance (%) ISO 2471
The Y value is an indicator of the perceived lightness, measured with an instrument whose response to the wavelengths of light corresponds to the response of a healthy eye, i.e. the full spectrum of visible light, which means all colours from blue to red and in between of the “rainbow”.
The Y value is measured with an instrument that emits D65 light, i.e. a light that is related to daylight with the same amount UV light as in a winter day. The reflected light are measured in a different wavelength compared with Brightness. The sample is illuminated vertical to its surface and the detector is located 10° from the vertical axis.
Y values are percentages. The Y value of a black non-reflecting material is 0% and the value of the perfect diffuser is 100%. Y value is also named as luminance, especially in the past.
Luminance measurement on newsprint and improved newsprint is done by using indoor illumination, C/2.