Medicion de la luz

Photopic and Scotopic Lumens - 1: An Introduction
This is the first in a series of articles on the basic unit of light that is the foundation of the
wide range of concepts and calculations that welighting professionals use day in and day
out: The lumen. There is renewed interest in the fact that there are two definitions of the
lumen: one for daytime, high light level conditions, and one fornighttime, low light level
conditions. These are photopic and scotopic lumens. Virtually all lighting applications
today involve photopic lumens. Should they? What are the practical alternatives?How
important are the differences? When, if at all, should we be using scotopic lumens as the
basis for our work as lighting professionals? This series of eight articles will attempt to
answer thesequestions by exploring the development of the lumen, the technical issues
involved in its definition and use, and some recently proposed changes to our
professional practice that are based on andinvolve the differences between photopic and
scotopic lumens.
The most common use of the lumen is to express the total light emitting power of a light
source and to express its efficacy. And so wespeak of a 100 watt incandescent lamp
being rated at 1690 lumens and having an efficacy of 16.9 lumens per watt. As a practical
matter, we most often use units that are derived from the lumen, likeluminous intensity
and illuminance. These derived units are used to evaluate lighting equipment and what
that equipment does to architectural spaces. Luminous intensity, measured in candelas
andoften called candlepower, describes how densely a source emits lumens in a
particular direction. All the directions can be considered together and we have the
candlepower distribution of a source.Illuminance, measured in footcandles or lux,
describes how densely a source puts lumens onto a surface. Intensity is the universally
used method to describe what luminaires can do and Illuminance is...