Below David Dilaura shares his knowledge about the history of various lighting units and terms explaining where they come from and what they mean.

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 we lighting professionals use day in and day out: The lumen.

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Photopic and Scotopic Lumens - 2: A Bit of History

Is there a good reason to consider the historical development of our current definition of the lumen? The answer is not obvious. In our hurtling, warp-speed, instant-this and instant-that world, it may seem that we have no time for history; it is all we can do to come to grips with the latest whiz-bangery.

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Photopic and Scotopic Lumens - 3: The Modern Lumen

The definition of the lumen that we now use was established in 1924 by international agreement that was obtained by delegates to the CIE meeting of that year.

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Photopic and Scotopic lumens - 4: When the Photopic Lumen Fails Us

When should we use scotopic lumens? Never. There are no practical situations involving lighting or lighting design where the occupants or users of a space are sufficiently dark adapted and using only their peripheral vision so that the scotopic luminous efficiency function would be a useful predictor of the visual effect of radiant power.

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