Measured light output is the quantifiable value of light (measured in foot-candles) by a light meter or similar device. This is the dimming ratio percentage indicated on LED product specification sheets. Perceived light is the amount of light that your eye interprets as a result of pupil dilation. The eye's pupil dilates at lower light levels, causing the amount of light to be perceived higher than measured. For instance, a measured light intensity of 10%, will be perceived as 32%, if 1% measured light, will have a perceived brightness of about 10%, and 0.1% measured light, will have a perceived brightness of about 3.2%.
In figure below, the horizontal axis shows light output measured by means of a light meter and the vertical axis shows the light output as we perceive it.