Photopic vs. Scotopic
Ever wondered why lumens vs HID and LED never match up? That is because we are only looking at photometric reports. When we view general lighting, our perception is viewed photopically and scotopically. However Photopic readings are usually only considered when making a lighting choice, especially pertaining to a lighting layout. Photopic light is measured with a meter. The meter does not take into consideration a scotopic reading which is what we see using rods in our eyes. Rods are extremely active in bright light and contribute to the perception of brightness. SP ratios are important to factor into your lighting choice. Simply put an SP ratio is a combination of scotopic & photopic. Based on scientific test there have been formulas used to create an SP ratio for different lighting technologies. For example 250w high pressure sodium lamp has an SP ratio of 0.62 while a 140w 5000k LED has an SP ratio of 1.96. To figure out your scotopic lumens, you take the photopic lumens and multiply by your SP ratio. For example a 250w HPS lamp has a photopic lumen value of 24,500 lumens, and a SP ratio of 0.62. Take 24,500 x 0.62 and that gives you 15,190 scotopic lumens. On the other side, let’s look as what this means for LED. Taking a generic LED board of 140w at 5000k giving you 8,400 photopic lumens. Take the 8,400 lumens and multiply by the SP ratio of the LED (1.96). 8,400 X 1.96 gives you 16,464 scotopic lumens.
Delivered Lumens vs Total Lumens.
Are you comparing apples to apples when measuring the light output of a fixture? More often than not, the answer is no. For years people have been conditioned to refer to the illuminance of lamps and luminaires by their wattage rather than their lumens; this had often times lead many people to over light an entire area simply because they could not adequately light a targeted location.
One effect of the increased awareness of consumers and market share of LED lamps and LED luminaires is that people are now starting to refer to lumens instead of wattage when trying to determine replacements for existing luminaires & new construction. However, with this new discussion of lumens a new question arises; are all lumens the same? No, not all listed lumens are the same; take for example Total Lumens Vs Delivered Lumens, which one more accurately reflects perceived brightness (i.e Luminous Flux)? The answer is Delivered Lumens.
Delivered Lumens represent the actual amount of light that is being projected onto a surface or rather its Illuminance discounting any light that may be lost due to dispersion within the fixture housing, or is discharged in a direction away from the task area, or it can be lost through filtering in lensing, fixture positioning, or any of a number of other factors relevant to a specific installation.
Total Lumens on the other hand is the total amount of light that is emitted from a source, but does not fully represent what the Brightness will be of a targeted task light.
The illuminance of a surface can quantitatively be measured with a photometer and its measurement is represented in foot candles. All in All, when you want to effectively design and light a specific area you need to look at the luminaires Delivered Lumens and not its Total lumens.