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Seeable Lumens – Light That The Eye Can See

"Seeable Lumens" measure the Full Spectrum, which is both photopic (yellow light) seen by the cones, and scotopic (blue light) seen by the rods of the eyes.

When selecting fluorescent& compact fluorescent or CFL light bulbs and other color sensitive lamps, you will want to consider the CRI (Color Rendering Index), Kelvin/Color Temperature and "Seeable Lumens" output.

Understanding these key terms and reviewing this Seeable Lumens Table will help you determine which lamp type will best meet your needs. But first, lets review a few Key terms that you will need to know.

Color Rendering Index (CRI) is a percentage of measurement of how closely an artificial light source approximates natural sunlight when the sun is emitting a particular color temperature known as Kelvin.

For example, the Spectra-COLWITE T8 fluorescent lamps have a CRI of 75 at 4150Kelvin and the VITALUX T8 Full Spectrum fluorescent lamps have a CRI of 94 at 5400 Kelvin and natural sunlight has a CRI of 100 at any Kelvin.

The higher the CRI the closer the lamp color matches the sun at that Kevlin temperature, providing more true-to-life colors. Maintenance Engineering's balanced spectrum and full spectrum lamps closely match natural sunlight higher Kelvin temperatures increasing see-ability and visual comfort for your eyes.

Kelvin/Color Temperature - This refers to the quality of light emanating from a light source and is represented by the number of degrees Kelvin based on the Kelvin Color Temperature Scale.

Outside light varies from 2800K to 6500K, depending on season, time of day, latitude, and weather.

The higher the color temperature of a fluorescent lamp, the "bluer" it's appearance, the lower the color temperature, the "redder" it's appearance. See Kelvin Temperature Chart below.

Lumens – This refers to the amount of light emitted from a light source as measured by a light meter developed in the 1920′s to measure incandescent light.

* LIGHT METER LUMENS – STANDARD LIGHT METER READINGS. This is the light seen predominately by the cone receptors in the eye, which sense the yellow light of the photopic spectrum. This method of light measurement was developed in 1924 and was generally used to measure incandescent lighting. Even though this type of measurement is inaccurate, it is still widely used in the lighting industry today.

The typical light meter are most sensitive to the yellow-green part of the color band. This is the light that is seen by the cone receptors in the eye and is called the "photopic lumens". However, the rod receptors in the eye also receive light but it is the light rich in the blue portion of the spectrum. This light isn't measured by the typical light meter.

The combination of the light received by the rods and cones is called the "seeable lumens". Therefore, the photopic lumens could be misleading when comparing different colors of light. Even though a similar lumen reading is obtained with a full spectrum bulb vs. and cool white bulb, the full spectrum bulb will produce more seeable light.

For example, M.E.'s F40T12 4FT fluorescent Spectra-COLWITE light bulb will have 2850 photopic lumens and 4055 seeable lumens, whereas the F40T12 4FT fluorescent XTRABRITE ALPINEWITE will have 3700 photopic lumens and 6050 seeable lumens.

**SEEABLE LUMENS – LIGHT THAT THE EYE CAN SEE. A new method of measuring light was developed by Lawrence Berkeley Labs, Berkeley CA in 1992. Maintenance Engineering uses this new and more accurate method to measure called "Seeable Lumens." Seeable Lumens measure the full spectrum, which is both photopic (yellow light) seen by the cones, and scotopic (blue light) seen by the rods of the eye. "Seeable Lumens" give us a more accurate measurement of the usable light given off by a lamp.