Lighting Fundamentals

Below we show how different light sources Design Lumen readings compare when read by a standard light meter and measured in Conventional Photopic Lumen values.

For lighting design that wishes to maximize energy efficiencies by specifying light sources with both high Scotopic and Photopic Lumens, a Correction Factor (S&P Ratio) must be applied to the Photopic Lumen per Watt readings.

When applying this correction factor you will notice drastically different usable light outputs as measured in Pupil Lumen per Watt. Higher Pupil Lumens per Watt will significantly reduce the amount of energy necessary to satisfy maximum visual acuity within the optimal yellow-green regions of the spectrum. In other words; the higher the Pupil Lumen/Watt the less energy will be required of the lamp for the eye to accurately see what it's observing.

To illustrate this you can see by the charts below that the LPS (Ugly Yellow Street Lighting) lamp is more efficient from a conventional efficacy (Lm/W) perspective. However the LPS has a very low S/P ratio and poor pupil lumens per watt when compared to induction. Now the CRI and the VEL would indicate poor visual acuity. What this means is that while there may be a high lumen per watt when using LPS, the ability to accurately gauge the color of what we are observing is extremely poor.

Induction Lamps: Why They Appear Brighter:

Below we show how Photopic and Scotopic Values vary between different lamp types and how bright they will then appear to the eye. This is known as Apparent Brightness and is not measured in the conventional Lumens, Lux or Foot candle readings.

There are a number of terms engineers use that reference Apparent Brightness; Visually Effective Lumens (VEL), Spectrally Effective Lumens (SEL) or Pupil Lumens as this measurement, but whatever phrase you use, they all refer to the same thing: 

Standard Units of Measurement for Vision:

When taking into account the standard photo metric measurements of light for human vision the system of units we measure would be the LUMEN which measures the total amount of light emitted from a light source.

This light is then distributed over an area and the illuminated area is measured in LUX. LUX is measurement of intensity as perceived by the human eye. It is a way of measuring how many LUMENS fall within a square meter of an illuminated surface.

The difference between the LUX and the LUMEN is that a LUX measures the area over which the LUMEN is distributed. These levels are inversely proportional to the area being lit. The larger the area the lower the intensity of the LUX levels. For example a reading of 1000 LUMENS would correlate to 1000 LUX at a 1 meter area however the LUX illumination levels would fall to 100 LUX over a 10 meter area.

In the United States you'll often hear light measurements in FOOTCANDLES. This term is used alot in construction related projects and by engineers who deal with US Standards of measurement. LUX and FOOTCANDLES are different units of the same quantity in that FOOTCANDLE will measure the amount of LUMEN PER SQUARE FOOT whereas LUX measures the LUMENS PER SQUARE METER. Other then in the United States you will not usually hear light measured in FOOTCANDLES.

Since all light is emitted in wavelengths, and we know that the human eye can see certain wavelengths better then others, with the peak being measured @ 555 nanometers, we can now determine a given lamps source LUMENS PER WATT.

The LPW measurement adjusts for the spectral wavelengths the lamp produces. So when determining a task level of illumination for human eyesight, we can decide which lamp will best suit the task for the least amount of wattage, with factored depreciation, and how important the color, as measured in the CRI, to best match the task.