Thursday, 13 November 2008 23:55
A lumen (luminous flux) is a measure of the power of light perceived by the human eye. Luminous flux differs from radiant flux, the measure of the total power of light emitted, in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light. The lumen is defined in relation to the candela by
1 lm = 1 cd-sr
That is, a light source that uniformly radiates one candela in all directions which radiates a total of 4π lumens. If the source were partially covered by an ideal absorbing hemisphere, that system would radiate half as much luminous flux—only 2π lumens. The luminous intensity would still be one candela in those directions that are not obscured.
Luminous flux is not used to compare brightness, as this is a subjective perception which varies according to the distance from the light source.
But lighting can be very subjective and depends on various other factors including personal preference. One important factor relates to what the eye sees vs what a meter reads. Issues such as the photopic and scotopic vision can make one type of lighting appear brighter or dimmer regardless of meter readings. To simplify this process, there are what are called rods and cons in the back of the eyeball. The rods (scotopic vision) are more sensitive to light in general, but they have a hard time responding to colors in the upper frequency range. In contrast the cons (photopic vision) respond well to colors, but not to low light conditions. (See illustration below). In addition, the opening size of the eye responds to the scotopic frequency range. By providing lamps in the scotopic range or full spectrum, you can have a lower lumen meter reading, but actually see more light. Since we are more concerned with the light that we actually see, the meter readings can often be misleading. In addition, the meter reading specs were created in 1955 and have not been updated since.
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