In lighting design there are two distinct kinds of lumen output produced by lamps. The first is called photopic or design lumens, which represent the relative sensitivity of the eye under intense lighting such as full outdoor sun. Photopic lumen output is the amount of light registered by the cones in the human eye and is measured by standard lumen and footcandle meters.
The second type of lumens are called scotopic, which represent the sensitivity of the eye under typical interior lighting conditions and cannot be measured directly with a standard light meter. Scotopic lumen output is the amount of light registered by the rods of the human eye and also controls pupil size directly effecting visual acuity for tasks.
For lighting interior spaces a correction factor may be applied to photopic lumen readings to find the usable light produced by a given light source, also called pupil lumens. As shown below, two different light sources having similar design lumen readings taken by a standard light meter can have drastically different usable light outputs and apparent brightness.
4000k MVR Metal Halide Ultralux™ High Bay w/ BlueMax™
85 Design Lumens/Watt 98 Design Lumens/Watt
107 Pupil Lumens/Watt 196 Pupil Lumens/Watt