What is an S/P Ratio?
An S/P ratio is the ratio of light measured by
the scotopic and photopic methods. Some common S/P ratios are
listed below. Lamps with high S/P ratios (high scotopic content)
also improve one's ability to see both outdoors and indoors.
- The S/P ratio of 5000K daylight light sources is 2.1.
- The S/P ratio of 4000K light sources is 1.65.
- The S/P ratio of 3000K light sources is 1.4.
- The S/P ratio of yellow HPS light sources is 0.64.
What does the S/P Ratio Tell Us?
Lamps with high S/P ratios (high scotopic content)
also improve one's ability to see. Human studies have shown that
the S/P values, though previously applicable to only nighttime
conditions, can be used to describe the "effective lumens" of
indoor lighting as well.
examples describe the effect of S/P in relation to outdoor and
indoor lighting. Outdoor Lighting: The result is that a
4000K light source is at least twice as effective for viewing
at night as HPS sources. Similarly, a 5000K light source is more
than 3 times as effective as HPS Sources. Indoor Lighting:
If two rooms are lit separately with 4000K and 5000K lamps to
equal photopic luminance, the 5000K room will appear 13% brighter
[(S/P)0.5 = 1.13].
How does the S/P Ratio relate to
With its high S/P ratio (2.1) , Natural White lamps
provide customers with an:
- Improved ability to see indoors
- Improved nighttime vision
Natural White an excellent product for either indoor or
outdoor lighting applications. Combined with high CRI and high
CCT, the Natural White lamp/ballast system features superior
color rendering, as well as all the benefits of Uni-Form
arc tube and Opti-Wave ballast technology.
What are "Effective Lumens"?
defined by Dr. Ian Lewin, the term "effective lumens" describes the modified lumen output of a lamp, taking into account
the shifting color sensitivity of the eye at low light levels.
lumen rating of Natural White lamps reflects lumen output
relative to 4000K lamps. The formula used to determine this rating
indoors is (S/P)0.5. This provides the end
user, lighting designer or contractor with an accurate comparison
of traditional 4000K lamps to 5000K lamps.